ML102871121

7.10 Hydrodemolition Induced Cracking

ML102871121
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Site:	Crystal River
Issue date:	02/19/2010
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Download: ML102871121 (80)

Text

1 7.10 Hydro-Demolition Induced Cracking

==

Description:==

The process of hydro-demolition exploits the existence of micro-cracks, voids, capillaries and cracks to enable concrete demolition using high pressure water jets. This raises the question of potential damage to concrete in adjoining area through direct pressure, vibrations, or crack propagation.

This document is intended to determine if hydro-demolition can cause cracking and if any occurred at CR3.

Note that issues of vibration induced by hydro-demolition are covered in depth in FM 7.2.

Data to be collected and Analyzed:

1. Review literature (FM 7.10 Exhibit 1 is a "Guide for the Preparation of Concrete Surfaces for Repair Using Hydrodemolition methods", FM 7.10 Exhibit 2 includes selected pages from ACI 546R-04 "Concrete Repair Guide", FM 7.10 Exhibit 6 includes selected pages from the book "Hydrodemolition of Concrete Surfaces and Reinforced Concrete" by Andreas Momber, FM 7.10 Exhibit 7 is a published article, and FM 7.10 Exhibit 8 is a research report from MoDOT);

2. Interview personnel involved in the Hydrodemolition at CR3 (FM 7.10 Exhibit 3 is a summary of interviews);

3. Review physical data collected from the structure (FM 7.10 Exhibit 4 and FM 7.10 Exhibit 5 are photos taken of areas impacted by the demolition process).

Verified Supporting Evidence:

a. Hydro-demolition works through the introduction of high pressure water into cracks (FM 7.10 Exhibit 6). If a delamination crack is encountered during demolition, the water will fill it and exert pressure internally, potentially accelerating propagation. Eyewitness report large volume of water exiting the wall through a moon-shaped crack that developed outside the demolition area (FM 7.10 2/19/2010 Page 1 of 2 Draft 1

Exhibit 5).

Verified Refuting Evidence:

a. Literature review shows a consensus that hydro-demolition does not cause significant damage to adjacent material (see highlighted sections of FM 7.10 Exhibits 1, 2, 7, and 8);

b. Hydro-demolition removes concrete not by impact, but by introducing high-pressure water into existing voids (FM 7.10 Exhibit 6).

The high internal pressure in these voids causes the concrete to fail, spalling the surface material;

c. Cores taken adjacent to the demolition area did not show degradation of strength properties (see discussion b. below).

Discussion:

a. Published information about the principles of hydro-demolition (FM 7.10 Exhibit 6, page 3 of 21) describes the three modes of failure established in quoted research. These include water flow into crack, creating stress at the tip, water flow into capillaries which result in internal pressure amplification, and water flow through open pore system creating friction forces to the material grains. The exhibit provides research information and formulas that may be used to estimate parameters of failure;

b. Nine (9) cores were taken through the edge of the SGR opening following the discovery of the delamination. Three (3) of the cores were tested for splitting tensile strength and averaged 615 psi (compared to average of 604 psi for fourteen (14) cores tested from the structure). Three (3) of the cores were tested in compression and averaged 7390 psi (compared to average of 7460 psi for fourteen (14) cores tested). These results confirm that there was no degradation to the concrete adjacent to the hydro-demolition area.

==

Conclusion:==

Hydro-demolition did not cause cracking and degradation of the concrete adjacent to the demolition area. However, it may have played a role in the propagation of existing cracks and may have been a contributing factor to the extent of the delamination.

2/19/2010 Page 2of2 DraftI

FM 7.10 Exhibit 1 page 1 of 16 TECHNICAL GUIDELINES Prepared by the International Concrete Repair Institute September 2004 Guide for the Preparation of Concrete Surfaces for Repair Using Hydrodemolition Methods Guideline No. 03737 Copyright © 2004 International Concrete Repair Institute All rights reserved.

International Concrete Repair Institute 3166 S. River Road, Suite 132, Des Plaines, IL 60018 Phone: 847-827-0830 Fax: 847-827-0832 Web: www.icri.org E-mail: info@icri.org 925

926 FM 7.10 Exhibit 1 CONCRETE REPAIR MANUAL About ICRI Guidelines page 2 of 16 The International Concrete Repair Institute (ICRI) was founded to improve the durability of concrete repair and enhance its value for structure owners. The identification, development, andpromotion of the most promising methods and materials is a primary vehicle for accelerating advances in repair technology.

Working through a variety offorums, ICRI members have the opportunity to address these issues and to directly contribute to improving the practice of concrete repair A principal component of this effort is to make carefully selected information on important repair subjects readily accessible to decision makers. During the past several decades, much has been reported in Technical Activities Committee Rick Edelson, Chair David Akers Paul Carter Bruce Collins William "Bud" Earley Garth Fallis Tim Gillespie Fred Goodwin Scott Greenhaus Robert Johnson Kevin Michols Allen Roth Joe Solomon Synopsis This guideline is intended to provide an introduction to hydrodemolition for concrete removal and surface preparation, the benefits and limitations of using hydrodemolition, and an understanding of other aspects to be addressed when incorporating hydrodemolition into a repair project. This guideline provides a description of the equipment, applications, safety procedures, and methods of water control and cleanup.

literature on concrete repair methods and materials as they have been developed and refined Neverthe-less, it has been difficult to find critically reviewed information on the state of the art condensed into easy-to-use Jbrmats.

To that end, ICRI guidelines are prepared by sanctioned task groups and approved by the ICRI Technical Activities Committee. Each guideline is designed to address a specific area of practice recognized as essential to the achievement of durable repairs. All ICRI guideline documents are subject to continual review by the membership and may be revised as approved by the Technical Activities Committee.

Producers of this Guideline Subcommittee Members Pat Winkler, Chair Don Caple Bruce Collins Eric Edelson Ken Lozen Bob Nittinger Steve Toms Contributors Scott Greenhaus Rick Toman Mike Woodward Keywords Bond, bonding surface, bruising, chipping hammer, coating, concrete, delamination, deterioration, full depth repair, hand lance, high-pressure water, hydrodemolition, impact removal, mechanical removal, micro-fracture, post-tensioning, rebar, reinforced concrete, reinforcing steel, robot, rotomill, safety, sound concrete, surface preparation, surface profile, surface repair, tendon, vibration, wastewater, and water jet.

This document is intended as a voluntary guideline for the owner, design professional, and concrete repair contractor. It is not intended to relieve the professional engineer or designer of any responsibility for the specification of concrete repair methods, materials, or practices. While we believe the information contained herein represents the proper means to achieve quality results, the International Concrete Repair Institute must disclaim any liability or responsibility to those who may choose to rely on all or any part of this guideline.

FM 7.10 ERREIRTION OF CONCRETE SURFACES FOR REPAIR USING HYDRODEMOLITION MEJIP, 3 of 16 927 Purpose This guideline is intended to provide owners, design professionals, contractors, and other interested parties with a detailed description of the hydrodemolition process; a list of the benefits and limitations of using hydrodemolition for concrete removal and surface preparation; and an understanding of other aspects to be addressed when incorporating hydrodemolition into a repair project. The guideline provides a description of the equipment, applications, safety procedures, and methods of water control and cleanup. This guideline is not intended as an operating manual for hydrodemolition equipment as that information is specific to each equipment manufacturer.

The scope of this guideline includes the use of hydrodemolition for the removal of deteriorated and sound concrete in preparation for a concrete surface repair. In addition, the use of hydrodemolition for the removal of coatings is discussed.

While the procedures outlined herein have been found to work on many projects, the requirements for each project will vary due to many different factors. Each project should be evaluated individually to ascertain the applica-bility and cost-effectiveness of the procedures described herein. Other methods of surface preparation are discussed in ICRI Technical Guideline No. 03732, "Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, and Polymer Overlays."

Introduction Hydrodemolition is a concrete removal technique which utilizes high-pressure water to remove deteriorated and sound concrete. This process provides an excellent bonding surface for repair material. First developed in Europe in the 1970s, this technology has become widely accepted for concrete removal and surface preparation throughout Europe and North America.

Hydrodemolition can be used for horizontal, vertical, and overhead concrete removals and surface preparation on reinforced and non-reinforced structures. It is effective in removing concrete from around embedded metal elements such as reinforcing steel, expansion joints, anchorages, conduits, shear connectors, and shear studs. Hydrodemolition can be used for localized removals where deterioration is confined to small areas and for large area removals in preparation for a bonded overlay. This technology can also be used to remove existing coatings from concrete.

Hydrodemolition has been used on the following types of structures:

" Bridge decks and substructures

" Parking structures

" Dams and spillways

" Water treatment facilities

" Tunnels and aqueducts

" Nuclear power plants

" Piers and docks

" Stadiums

" Warehouses

• Retaining walls The Effects of Mechanical Impact Techniques Mechanical methods such as chipping hammers,

, rotomills, scabblers, and scarifiers remove concrete by impacting the surface. These procedures crush (bruise) the surface, fracture and split the coarse aggregate, and create micro-fractures in the

'substrate (Fi g.d.saresult, tle abili~y of the fractured substrate to provide a durable

c. j: vamage createa Dy cnipping nammer Fig. 2: Damage created by rotomilling

928 FM 7.10 Exhibit 1 CONCRETE REPAIR MANUAL page 4 of 16 bond with the repair material is compromised, requiring a second step of surface preparation to remove the damaged region.

Furthermore, impact methods may damage the reinforcing steel and embedded items such as conduit, shear studs and connectors, and ex.pansion joint hardware. Impact methods transmit vibrations:

through the reinforcing steel, which may cause:

further cracking, delamination, and loss of bond between the reinforcing steel and the existing:

concrete. Vibration and noise created by the:

mechanical impact will travel through the structure, disturbing the occupants. During repair of thin slabs and precast tees, chipping hammers may shatter the substrate resulting in unanticipated full depth repairs.

For a discussion on surface bruising and the mechanics of concrete removal by impact methods, refer to ICRI Technical Guideline No. 03732, "Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings and Polymer Overlays."

Hydrodemolition Benefits and Limitations The benefits of hydrodemolition can be placed into two groups: structural benefits that improve the quality of the repair, and environmental benefits that improve the quality of the work place.

Hydrodemolition also has limitations, which need to be considered.

Structural Benefits A rough, irregular surface profile is created to provide an excellent mechanical bond for

... r.epýair materials;.....................

.. Surface.micro-frac is eliminated;,

" Exposed aggregates are not fractured or split;

" Lower strength and deteriorated concrete is selectivelk removedk, "t

.~r;Ywtt......

• ..v~i~brati.on. is minimal; Reinforcement is cleaned, eliminating the need for a second step of surface preparation; and 0 Reinforcing and other embedded metal elements are undama, ed.

During concrete removal, the water jet is directed at the surface, causing high-speed erosion of the cement, sand, and aggregate. The water jet does not cut normal weight aggregate which remains intact and embedded as part of the rough, irregular surface profile (Fig. 3). The aggregate interlocks with the.reair material to assist in developing a Fig. 3: Surface prepared by hydrodemolition has a rough irregular profile with protruding aggregate and is excellent for creating a mechanical bond mechanical bond and composite action between the substrate and the repair material.

The rough, irregular surface profile provided by hydrodemolition can result in bond strengths that equal or exceed the tensile strength of the existing concrete. The concrete surface profile can exceed CSP-9 (very rough) as defined in ICRI Technical Guideline No. 03732.

Rotomills and scarifiers remove concrete to a uniform depth and may leave deteriorated concrete below the specified depth. Alterna-tively, the waterjet moves in a consistent pattern over the surface and will remove unsound concrete even if it is below the specified depth.

m ice flewaterjetcr~o'esn'ot'craerneý;chan;Ica1:

impact, vibration is not transmitted into the structure:

from the hydrodemolition operation. Delami-nation beyond the repair area caused by vibration:

,of the reinforcing steel isgreatly reduced..

During hydrodemolition, sand and cement particles mix with the waterjet. The abrasive action of theses particles is usually sufficient to clean uncoated reinforcing bar and embedded metal items without damaging them. Corrosion material is removed from the reinforcing bar and metal items, allowing for easy inspection and identifi-cation of cross-sectional area loss. The reinforcing bar is cleaned without any loss of deformations.

Cleaning of the entire reinforcing bar, however, will not occur if the reinforcing bar has not been completely exposed during hydrodemolition.

Environmental Benefits

• Minimizes disruptions to users of occupied space by significantly reducing transmitted sound through the structure; Increased speed of concrete removal can reduce construction time; o Minimizes dust; and

FM 7. 10 EffiV8R11TION OF CONCRETE SURFACES FOR REPAIR USING HYDRODEMOLITION ME'[Wg 5 of 16 929

• Robotic units reduce labor and minimize injuries as compared to chipping hammers.

Concrete removal by hydrodemolition can take place inside an occupied structure, such as a hotel, apartment building, office building or hospital with minimal noise disruption to the occupants.

Hydrodemolition can quickly remove concrete.

As such, project duration can be reduced, mini-mizing the impact on the users of the structure.

During demolition, cleanup, and final wash down, the concrete debris and repair surface remain wet, minimizing dust in the work area. Since hydro-demolition cleans the reinforcing steel, the need to sandblast is eliminated unless additional concrete removal is required using chipping hammers. As such, silica dust in the work area is reduced, thereby providing a safer work environment.

The use of chipping hammers and other impact methods are labor intensive and physically demanding, which can cause injury to the employee.

Robotic hydrodemolition equipment reduces the use of these tools and the possibility of injury.

Limitations

• The hydrodemolition process consumes a significant amount of water (6 to 100 gpm

[25 to 380 1pm]). A potable water source must be available. The cost of the water should be considered;

" Wastewater containing sand and cement fines (slurry) must be collected, treated, and returned to the environment. Wastewater disposal may require a permit;

• Projects requiring total demolition can be done faster and more economically with crushers and similar equipment;

" Water can leak through cracks in the concrete and damage occupied space below the repair area. Hydrodemolition should not be used over occupied areas due to the risk of blow-through (unanticipated full-depth removal);

" Repair areas of varying strength will result in non-uniform removal. Areas of high strength may need to be removed using hand lances or chipping hammers;

" The water jet is blocked by reinforcing steel resulting in concrete shadows under the reinforcing bar that may need to be removed using hand lances or chipping hammers;

" Since the waterjet of a robotic unit is contained in a metal shroud, some robots are unable to completely remove concrete up to a vertical surface such as a curb, wall or column. The remaining concrete may have to be removed using hand lances or chipping hammers;

" The water jet will remove the sheathing from post-tensioning tendons and may drive water into the tendon;

• The hydrodemolition robot may be too large to access small or confined areas of the structure;

• The waterjet can damage coatings on reinforcing steel and other embedded items;

" The waterjet can introduce water into electrical system components, especially if embedded in the concrete and already deteriorated or not properly sealed; and

" If cleanup is not properly performed in a timely manner, further surface preparation may be required.

The Hydrodemolition System The hydrodemolition system consists of a support trailer or vehicle, high-pressure pump(s), a robotic unit to perform the demolition, and high-pressure hoses to connect the pump (s) to the robot. Hand lances are also available to remove concrete in areas inaccessible to the robot.

Support Trailer Hydrodemolition units are typically transported on 40 to 50 ft trailers (Fig. 4). The robot may C

Fig. 4: Hydrodemolition support trailer A self-contained unit transports pumps, robot, hoses, and spare parts be transported on the same trailer or separately on a smaller trailer. The support trailer usually contains a supply of spare parts, tools, maintenance area, fuel and water storage, supply water hoses, and filters. These units are designed to be self-sufficient on the job site with adequate spare parts to perform routine maintenance and repairs.

930 FM 7.10 Exhibit 1 CONCRETE REPAIR MANUAL page 6 of 16 High Pressure Pumps The high-pressure pumps used for hydrodemolition are capable of generating pressures from 10,000 psi to 40,000 psi (70 to 275 MPa) with flow rates from 6 to 100 gpm (25 to 380 1pm).

The pumps are driven by a diesel or electric motor, typically operating between 100 and 700 horsepower. The engine size will vary based on the flow and pressure rating of the pump. The pumps operate most efficiently at their design pressure and flow. High-pressure hoses connect the pumps to the robot. The pumps may be located a significant distance (500 ft [150 m])

from the actual removal area. However, due to a drop in pressure and flow through the high-pressure hoses, the pumps should be located as close as possible to the removal area, typically within 300 ft (100 m).

Robotic Removal Unit-Horizontal Surfaces The force created by the high-pressure pump(s) is controlled using a robotic removal unit (Fig. 5).

The robot is a diesel or electric powered, self-propelled, wheeled or tracked vehicle. It is used to uniformly move and advance the waterjet over the surface during concrete removal.

Fig. 6: Nozzle is mounted on a traverse beam Rotation Oscillation Fig. 7. Rotating or oscillating nozzles Fig. 8: Rotating nozzles are angled from center Fig. 5: Typical hydrodemolition robot The water jet is mounted on a trolley that traverses over the removal area along a cross feed or traverse beam (Fig. 6) perpendicular to the advance of the robot. The water-jet nozzle may either oscillate or rotate (Fig. 7). The oscillating nozzle is angled forward in the direction of the traverse. Rotating nozzles are angled from the center, creating a cone effect while rotating (Fig. 8 and 9).

The nozzle assembly is enclosed within a steel shroud with rubber seals around the perimeter to contain the debris during demolition (Fig. 10).

Fig. 9: Rotation of the angled nozzle creates a water cone

FM 7.10 ERJMPRPITION OF CONCRETE SURFACES FOR REPAIR USING HYDRODEMOLITION MEPDW' 7 of 16 931 Fig. 10: Nozzle is enclosed within a steel shroud The rotation/oscillation of the nozzle combined with the traverse and advance of the robot provide a uniform and continuous motion of the waterjet over the removal area (Fig. 11). Each of these functions is fully adjustable. The depth of concrete removal is determined by the length of time the waterjet is directed at the removal area.

3 seconds and the waterjet may traverse only one time before the robot advances 2 to 4 in. (50 to 100 mm).

The depth of concrete removal is controlled at the robot. Since the pumps are designed to operate at a specific pressure and flow rate, it is unusual to reduce the pressure (and subsequently the flow rate) to adjust the depth of removal.

Narrow areas may be removed by adjusting sensors that limit the movement of the water jet along the traverse beam. The traverse and advance functions limit the removal to a rectangular area along the advance path of the robot. Because the waterjet is contained within a steel shroud, most robots are unable to remove concrete within 3 to 6 in. (75 to 150 mm) of vertical surfaces.

Specialized Robotic Equipment-Vertical and Overhead Surfaces Various types of robotic equipment are available to perform removals on walls, soffits, substructures, beams, columns, and tunnels. These robots are often built on wheeled or tracked vehicles and have the ability to lift the traverse beam into the vertical or overhead position. The primary functions of traverse and advance are utilized in order to provide uniform concrete removal during vertical and overhead repairs.

As an alternative to the robot, the waterjet may also be attached to a frame that allows the jet to move in a two dimensional "X-Y" plane. The X-Y movement of traverse and advance are present in these units to provide uniform concrete removal.

The X-Y frames can be lifted and positioned over the removal area using a crane, backhoe, all-terrain forklift or other similar equipment.

Hand Lance Hand lances operate at pressures of 10,000 to 40,000 psi (70 to 275 MPa) while delivering approximately 2 to 12 gpm (8 to 45 lpm) of water.

Hand lances are not as fast or as precise for concrete removal as a programmed robot and are slower than chipping hammers. Hand lances are effective in performing light scarification and coating removals. It should be noted that the water jets on hand lances may not be shrouded, increasing the risk of debris becoming airborne.

Hand lances can be used for removal of:

" Concrete shadows below reinforcing bar;

" Concrete adjacent to walls, columns, curbs, and in tight and confined areas not accessible to the robotic equipment; and

" Coatings.

Fig. 11: The water jet traverses back and forth perpen-dicular to the forward advance of the robot Adjusting the following parameters will increase or decrease the depth of removal:

a. Total traverse time (time of each traverse x number of traverses); and

b. Distance of the advance.

Once these parameters are set, the robot will reproduce the settings in a programmed sequence to provide consistent removal of the concrete. For example, during deep removal to expose the reinforcing bar 3 to 4 in. (75 to 100 mm), the traverse speed may be 8 seconds (the time required for the water jet to move from one side of the traverse beam to the other) and the water jet may traverse 3 times before the robot advances forward i to 2 in. (25 to 50 mm). On the other hand, for light scarification 1/4 to 1/2 in. (6 to 13 mm) or coating removal, the traverse speed may be

932 FM 7.10 Exhibit 1 CONCRETE REPAIR MANUAL page 8 of 16 Safety Hydrodemolition involves the use of potentially dangerous specialized equipment. At all times, the manufacturer's instructions for the safe operation of the equipment and personal protective equipment should be followed, as well as all local, state, and federal regulations. Hydrodemolition units should be supervised and operated by qualified personnel certified by the equipment manufacturer.

Hydrodemolition employs high-velocity water jets to demolish concrete and perform surface preparation. Even though the waterjet is shrouded on robotic units, debris can be propelled from beneath the shroud with sufficient velocity to cause serious injury. Serious injury or death can also occur if struck by the water jet. Hand lances are typically not shrouded and care must be exercised to avoid injury when using these tools.

Workers, equipment operators, and any indi-viduals entering the work area are required to wear hard hats, safety glasses, hearing protection, safety shoes, gloves, long pants and long-sleeve shirts, and must be trained in the proper use of personal protective equipment. When using a hand lance, the operator should wear a full-face shield, rain suit, and metatarsal and shin guards.

Additional protective clothing may also be required for use with hand lances. Everyone involved with the hydrodemolition operation should receive specific training outlining the dangers associated with the use of high-pressure water.

Prior to starting demolition, an inspection of the area should be performed including the area under the work area. All barricades, partitions, shielding, and shoring must be installed and warning signs posted to prevent unauthorized entry into the work area. The area below the work area must be closed off and clearly marked "Danger-Do Not Enter." Electrical conduits or other electrical equipment in the work area should be deenergized to avoid electrical shock.

Special precautions are required for post-tensioned structures as referred to in the section "Considerations for Hydrodemolition Use."

Hydrodemolition Applications Scarification Scarification is performed to remove the surface concrete and provide a rough profile (Fig. 12 and 13). Scarification is often used in preparation for Fig. 12: Scarified surface with I in. aggregate Fig. 13: Scarified surface with 3/4 in. aggregate a concrete overlay. If the surface was previously rotomilled, the minimum removal depth using hydrodemolition should equal the size of the coarse aggregate to remove all concrete micro fractures and damaged or crushed aggregate.

Scarification may not remove all unsound concrete due to the rapid rate at which the water jet moves over the surface. It may be necessary to resurvey the scarified surface and identify delami-nated or deteriorated areas for further removal.

Partial Depth Removal Partial depth removal is commonly required if chloride contamination has reached the top mat of reinforcing steel or deterioration, delamination or spalling occurs within the top mat of reinforcing steel. Partial depth concrete removal can expose the top mat of reinforcing steel and provide clearance, typically a minimum of 3/4 in. (19 mm),

below the bottom reinforcing bar of the top mat (Fig. 14 and 15). Determining the reinforcing bar size and concrete cover are critical to determine the required removal depth.

Concrete removal using hand lances or chipping hammers may be required to remove shadows under the reinforcing bar, previously repaired areas

FM 7.10 EP**MAR1TION OF CONCRETE SURFACES FOR REPAIR USING HYDRODEMOLITION MEPW 9 of 16 933 used. Other structural elements such as shear connectors, shear studs, and steel beam flanges can be exposed without damage.

During full depth removal, the removal rate slows as the depth increases because the water jet stream dissipates as it moves away from the nozzle and the water jet must push more water and debris from its path prior to contacting the surface to be removed.

Full depth removal is often necessary on waffle or pan joist slab systems (Fig. 16).

Fig. 14: Partial depth removal Fig. 15: Partial depth removal on a retaining wall or high areas resulting from variations in the strength of the concrete. In addition, concrete removal may be necessary adjacent to vertical surfaces such as curbs, walls and columns. Saw cutting of the perimeter of the repair area, if required, should be performed after hydro-demolition to prevent damage to the saw cut. This will require additional concrete removal along the repair perimeter with hand lances or chipping hammers. If the saw cut is made first, the area outside the saw cut should be protected using a steel plate. The steel plate will allow the waterjet to slightly over run the saw cut without damaging the surface outside the saw cut while completely removing the concrete within the repair area.

Full Depth Removal Hydrodemolition can be used for full depth removal where delamination has occurred in the lower mat of reinforcing or chloride contami-nation exists throughout the entire thickness of the slab. Full depth removal can be performed along expansion joints and other areas where there is a high concentration of reinforcing steel that may be damaged if conventional removal methods are Fig. 16: Full depth removal-waffle slab Coating Removal Hydrodemolition can be used for the removal of epoxy, urethane, hot applied membrane, and other coatings from concrete surfaces (Fig. 17).

When performing coating removal, a multiple jet nozzle is used. The multiple jets allow the water to penetrate the coating without damaging the concrete. However, if the concrete below the coating is deteriorated, it may be removed along with the coating.

Fig. 17: Coating removal using a spinning, multi-nozzle spray head

934 FM 7.10 Exhibit 1 CONCRETE REPAIR MANUAL page 10 of 16 The Hydrodemolition Process Concrete removal by hydrodemolition is impacted by the following factors:

• Size and density of the aggregate;

• Concrete strength;

" Uniformity of concrete strength;

• Extent of cracking;

" Deterioration and delamination;

" Surface hardeners;

" Previous repairs with dissimilar strength material; and

" Size and spacing of reinforcing steel or other embedded items.

In sound concrete, the variation in the depth of removal will generally equal the size of the coarse aggregate (Fig. 18). For example, if the coarse aggregate is 1 in. (25 mm), D = 1 in.

(25 mm) and the specified depth of removal is 2 in. (50 mm), the range of removal will be 2 in.

(50mm) +/- D12 (1/2 in. or 13 mm), or 1-1/2 in.

(38 mm) to 2-1/2 in. (63 mm).

If the strength of the concrete increases or a high-strength repair area is encountered during hydrodemolition, the removal depth will decrease (Fig. 19). The decrease in depth may not be immediately detected by the operator, resulting in an area of shallow removal (Fig. 20). To obtain the required depth in higher strength concrete, the total traverse time is increased and the advance of the robot is decreased. If the high-strength repair area is large enough, it may be possible to set up the hydrodemolition robot over the area and remove to the specified depth. This DftWe C080W t-Nm D*

("O~.

),5OOw4,W0pw I

Fig. 18: The depth of removal depends on the size of the course aggregate During hydrodemolition, a high-pressure water jet is uniformly moved over the surface and, provided the concrete is sound and the strength does not change significantly, the removal depth will remain consistent. Depth variations occur when the concrete strength changes, cracking or delamination is present, the concrete is deteriorated or the surface has been previously repaired using a different type and strength of material. In comparison, rotomilling or dry-milling equipment can be set to a specific depth and the milling drum will mill the surface to that depth regardless of any variations in the concrete strength, quality or level of deterioration.

Fig. 19: High-strength concrete is removed at a slower rate than normal concrete, which can result in a non-uniform removal

FM 7.10 ERRtRITION OFCONCRETE SURFACES FOR REPAIR USING HYDRODEMOLITION MPjTiJ 1 of 16 935 Fig. 20: High-strength repair area within the hydro-demolition area procedure can be problematic for two reasons.

First, if the water jet overruns the high-strength repair area, it may result in a blow-through or full depth removal at the perimeter of the high-strength repair area. Second, since the water jet must be slowed significantly, it may cause excessive removal below the high-strength area once it is removed and the softer base concrete is exposed.

For these reasons, it is often preferable to use chipping hammers in high-strength repair areas.

The opposite effect is encountered if the concrete strength decreases or there is cracking, deterioration or delaminations (Fig. 21). Concrete that is deteriorated, low strength or delaminated is removed faster than the surrounding sound concrete by the water jet. For example, if the average removal depth is 2 in. (50 mm) and there is a delamination that is 2 in. (50 mm) deep, the actual removal within the delaminated area could be 3 to 4 in. (75 to 100 mm) deep. For this reason, removal in an area that is seriously deteriorated and delaminated may not be consistent.

This effect is often described as "selective removal of deteriorated concrete." While the water jet is traversing and advancing uniformly over the surface, it is removing unsound, delaminated, deteriorated, cracked, and low strength concrete selectively below the specified removal depth.

Selective removal is not without limitations.

For example, if the robot is traversing and advancing rapidly as during scarification, it may not remove deeper delaminations.

Size and spacing of the reinforcing steel will also influence the removal depth. The reinforcing steel blocks the waterjet and shields the concrete below, creating concrete "shadows" (Fig. 22 and 23). Removal of concrete shadows becomes more difficult as the reinforcing bar size increases and Fig. 21: Delaminated or deteriorated concrete is removed at a faster rate leading to non-uniform removal is most difficult at reinforcing bar intersections.

Increasing the specified depth of removal will minimize the amount of shadowing.

Pointing the water jet under the reinforcing bar can reduce concrete shadows. This can be accomplished by using a rotating or oscillating nozzle (refer to Fig. 7-9). Rotating nozzles are typically angled 10' and 30' from center. The nozzle rotates between 100 and 1800 rpm, creating a demolition cone that will undercut both the transverse and parallel reinforcing bar provided the specified removal depth is greater than the

936 FM 7.10 Exhibit 1 CONCRETE REPAIR MANUAL page 12 of 16 Fig. 23: "Shadow" under the rebar (note tie wire undamaged and in excellent condition) removal may result in the removal of sound concrete, it will minimize the need for concrete removal under the reinforcing bar with chipping hammers or hand lances.

Considerations for Hydrodemolition Use Issues that should be considered when evalu-ating the use of hydrodemolition for a repair project include:

Limited quantity of repair: Mobilization and set up of the hydrodemolition equipment can be expensive. If there are only minor repairs or a limited quantity of repairs, the mobilization cost may make the process uneconomical.

Increase in repair quantity.: The traverse and advance function of the hydrodemolition robot results in removal areas that are rectangular. The removal areas may have to be "squared up" in order for the hydrodemolition equipment to efficiently remove the concrete. "Squaring up" the repair areas may lead to an increase in the removal quantity and the cost of the project.

Reinforcing bar size and concrete cover.:

Partial-depth removal normally requires clearance below the bottom reinforcing bar of the top mat of reinforcing. The size and quantity of the reinforcing bar and the concrete cover over the reinforcing bar should be determined in order to specify the correct removal depth to achieve the required clearance.

Potential for full-depth blow-throughs: Hydro-demolition of severely deteriorated structures may result in full-depth blow-throughs. Blow-throughs may take place where full depth slab cracks occur, especially if deterioration is evident on the slab underside. Shielding may be required Fig. 22: Reinforcing steel blocks the water jet leaving a concrete "shadow" under the reinforcing. Increasing the removal depth will decrease the amount of shadowing depth of the reinforcing bar. Similarly, the oscil-lating nozzle moves from side to side as it traverses, directing the water jet at an angle to the surface, cutting under the reinforcing bar. The nozzle is angled forward as it traverses left, and at the end of the traverse, flips to face forward as it traverses right. To minimize concrete shadows, the required depth of removal should be at least 3/4 in. (19 mm) below a #5 reinforcing bar. Larger reinforcing bars will require a greater removal depth to minimize shadowing. While this additional

FM 7.10 ERMO'ITION OF CONCRETE SURFACES FOR REPAIR USING HYDRODEMOLITION MPTJI 3 of 16 937 to protect the area below from damage. Shoring below the blow-through may be damaged or destroyed. When the water jet is in the open air, as will happen when the waterjet blows through the deck, it is extremely noisy (may exceed 130 db) and dangerous. Sound resistant partitions should be installed to contain the noise within the structure if blow-throughs are expected.

Extent of previous repairs: Repair materials may have a different compressive strength than the original concrete. Since the hydrodemolition jet is set to move at a uniform rate, the presence of dissimilar strengths of material will result in a variation in the depth of removal. Higher strength areas may require further concrete removals using chipping hammers or hand lances to achieve the specified depth of removal. Lower strength areas may result in deeper removals and possibly full-depth blow-throughs.

Occupied areas adjacent to or under the repair area: Occupied spaces such as stores or offices may occur in the structure. It may not be practical to perform hydrodemolition adjacent to or over these areas. Water from the hydrodemolition may leak to the occupied level below. As such, the repair area should be protected to prevent water from entering the occupied area.

Shoring requirements.: During structural repairs, concrete may be removed from around the top reinforcing. An analysis of the structural capacity of the remaining slab section should be made by a qualified engineer to determine if shoring will be required. The weight of the hydrodemolition robot should be considered when determining shoring requirements.

Equipment location: The hydrodemolition equipment is transported on a trailer. If possible, the pumps should be located within 300 ft of the repair area. A suitable location next to the structure must be selected. Pump units that are powered by diesels engines should not be located next to the air intake of adjacent buildings. In congested metropolitan areas, the pumps may be removed from the trailer and placed within the structure. Diesel powered pumps will need to be located close to an exhaust shaft and the exhaust from the pumps piped to this location. A fuel tank will also have to be placed in the pump area and provisions made to fill the tank as required.

Although electric pumps may be used inside the structure eliminating the fueling and exhaust concerns, they have a substantial power requirement and will need an electrical service installed. Due to the weight of the pumps, they may need to be placed on the slab on ground or in a shored area of the structure. Temporary shoring may be needed to move the pumps into the structure.

Available water sources: Pumps used for hydrodemolition require a steady supply of clean water at a sufficient volume to perform the work.

Generally, local municipal water is used for hydrodemolition. Sources close to the work area, such as a nearby fire hydrant or water line feeding the structure, should be adequate. Specific water requirements will vary, depending on the hydro-demolition unit used for the project and the method of cleanup. Cleanup performed using a fire hose operating at 100 to 200 gpm (380 lpm to 760 1pm) will use substantially more water than an 8000 to 10,000-psi (55 to 70 MPa) water blaster operating at 8 to 12 gpm (30 to 45 1pm).

In remote areas, water can be drawn from wells, fresh water lakes, rivers, or streams. This water must be pre-filtered to remove any suspended solids to avoid damage to the high-pressure pumps. Recycled water has been used for hydro-demolition, however, it can add substantially to the cost of the project due to collection and filtration of the water and the added wear to the equipment caused by dissolved minerals in the recycled water. When available, potable water is used. Water may have to be trucked into remote locations.

Post-tensioned structures: The use of hydro-demolition on post-tensioned structures has potentially severe risks and must be carefully evaluated to maintain a safe working environment, maintain structural integrity, and to preserve the long-term durability of the structure. Sudden release of anchorages can result in dangerous explosive energy and flying debris capable of causing damage to equipment and serious injury or death to workers. Tendons should be de-tensioned prior to removing concrete from around anchorages to prevent the sudden release of the anchorages and loss of pre-stress forces. The loss of pre-stress forces may result in the loss of structural integrity and result in the need for shoring. Careful eval-uation must also be exercised when removing concrete around post-tensioning tendons. Removal of concrete around tendons can result in a change of tendon profile, which may also result in the loss of prestressing force and structural integrity.

The wires or strands of post-tensioning tendons are usually undamaged during hydrodemolition, however the sheathing and protective grease will be removed from unbonded tendons. In bonded post-tensioning tendons, the waterjet may penetrate the duct and remove the grout inside. In either case, the hydrodemolition water may enter the

938 FM 7.10 Exhibit 1 CONCRETE REPAIR MANUAL page 14 of 16 tendon at the edge of the repair area and can be driven into the tendon outside the work area.

Water remaining in the tendon can cause future corrosion affecting the long-term durability of the post-tensioning system. Each tendon must be carefully examined and any water that has entered the tendon removed. Both the grease and the protective sheathing must be restored.

It may not be possible to remove moisture that has entered the post-tensioning system during the hydrodemolition process. In addition, verification of the presence of moisture is difficult and may not be possible. Refer to ICRI Technical Guideline No. 03736, "Guide for the Evaluation of Unbonded Post-Tensioned Concrete Structures,"

for suggested procedures to detect water in post-tensioning tendons. Long term monitoring for future corrosion may also be prudent.

Conduit and embedded metal items: Embedded aluminum and steel conduit will not be damaged by hydrodemolition if they are in good condition.

However, deteriorated portions of aluminum and steel conduit will be damaged and water will enter the conduit system. PVC conduit will be damaged during hydrodemolition. As a safety precaution, all conduits should be deenergized during demolition. Other metal items within the removal area such as shear connectors, shear studs, and anchorages will not be damaged by hydrodemolition.

Noise limitations: Hydrodemolition does not produce sound that is transmitted through a structure, however, the noise from the hydro-demolition unit in the work area is sufficiently loud to be objectionable to the public. Further-more, noise can be excessive during full-depth repairs or blow-throughs. Sound reducing partition walls that separate the public from the work area may be required. Acoustical studies indicate that the sound waves created by hydrodemolition are low frequency and are best controlled using dense material such as sheet rock or concrete board.

There are a variety of sound deadening materials supplied by various vendors that have proven effective in controlling noise. Partition walls should be protected from moisture. If properly sealed at the base, a water resistant sound reducing partition wall will also assist in containing the water within the work area.

Protection of lighting, sprinklers, and other services: Light fixtures, fire protection systems, and other services may be damaged by airborne debris from the hydrodemolition or clean up operation. If full depth removal or blow-throughs are anticipated, light fixtures may need to be removed and stored and temporary lighting installed.

Sprinkler heads may need to be protected. Mist and high humidity in the work area could damage electrical panels and other services. Items remaining in the work area should be protected.

Temperature: When the temperature falls below freezing, the structure must be heated or the hydrodemolition stopped to prevent water from freezing in the work area.

Test Area A test area should be designated to establish the operating parameters and to demonstrate that the equipment, personnel, and methods of operation are capable of producing satisfactory concrete removal results. The test should include sound and deteriorated concrete areas, each a minimum of 50 ft2 (5 m2). First the robot is set to remove sound concrete to the specified depth. Once the operating parameters have been determined, the equipment is moved to the deteriorated area and a second test is performed using the same operating parameters. If satisfactory results are achieved, the quality and depth of removal will become the standard for the project. If hand lances are to be used to perform concrete removals, they should also be demonstrated to show satisfactory results.

It is noted that the hydrodemolition robot will move the waterjet over the surface in a constant motion and if the concrete is of uniform strength, the removal depth will be consistent. However, since concrete is seldom uniform, there will be variations in the removal depth on the project.

Other factors affecting the removal depth include the extent and depth of deterioration, the size and quantity of reinforcing bar, the concrete cover over the reinforcing bar, and the presence of surface hardeners. As the equipment is used, nozzles will wear, changing the force created by the water jet. As such, the hydrodemolition equipment operator must monitor the depth and quality of removal and adjust the parameters of the robot to provide consistent removal through-out the project.

Wastewater Control Controlling the wastewater has often been viewed as one of the more difficult tasks associated with the use of hydrodemolition. However, with pre-planning and proper installation of a wastewater control system, the water can be properly managed (Fig. 24). Hydrodemolition wastewater should be

FM 7.10 ERRBAR*,TION OF CONCRETE SURFACES FOR REPAIR USING HYDRODEMOLITION MPWI 5 of 16 939 II Fig. 24.: Typical wastewater handling system discharged to the storm or sanitary sewer or to the ground for absorption and/or evaporation under permit from the controlling authority.

Discharge into an existing storm or sanitary line may occur in the structure or to a nearby storm or sanitary line accessed through a manhole. A 4-in. (100 mm) connection should be adequate.

Wastewater may not be discharged directly to a wetland, stream, river or lake.

Hydrodemolition wastewater contains suspended particles and typically has a pH of 11 to 12.5. The wastewater is initially placed in settling tanks or ponds to reduce the suspended solids, The particles are heavy and settle out quickly as the water is allowed to stand. This can also be accomplished by allowing the water to pass through a series of berms that are lined with filter fabric or hay bales.

The controlling authorities for discharge have varying requirements for the level of suspended solids and the range of pH for discharge into their system. Typically the water should be clear and the pH range between 5 and 10. Ponding the water will clarify it, however, the pH of the wastewater may have to be reduced prior to discharge. This can be accomplished by the introduction of acid, CO2 or other pH reducing materials into the wastewater. Adding flocculants can assist in reducing suspended solids. A location for settling ponds or tanks and pH reducing equipment should be determined.

The cost to discharge wastewater ranges from the cost of a discharge permit to charges for the actual water consumed and discharged. The cost of water consumed is generally that of commercial water usage within the community. The controlling authority may require monitoring and testing of the wastewater. Local ordinance requirements must be reviewed and met prior to discharge, including the obtaining of proper permits.

Water containment and collection systems will vary depending on the structure. Where possible, it is best to take advantage of gravity to move the water to the treatment area. In many structures, the slab on ground can be used to collect and treat the water. The water may be allowed to flow through the structure to the lowest level or through the existing drains, which have been disconnected just below the underside of the first supported level. All slab-on-ground drains should be plugged and water should not be allowed to enter the drainage system prior to treatment. Once the water is clear and the pH adjusted, it can be pumped directly to the discharge point. Additional treatment capacity may be necessary if rainwater cannot be separated from the wastewater.

Floor slabs and decks are commonly crowned or sloped to provide drainage. Since water will run to the low area, a simple method of water control involves the use of hay bales or aggregate dams, which can be set up along curb lines or the perimeter of the work area. As the water ponds in front of the hay bales or aggregate dams, the suspended solids will settle out. In areas where the drains are plugged, the water is forced to pass through the hay bales or aggregate dams. Retention ponds can be built at the end of the structure and the water directed or pumped to these ponds.

Settling tanks can also be used and the water pumped from the structure to the tanks.

Debris Cleanup and Disposal Hydrodemolition debris consists of wet sand, aggregate, chips or chunks of concrete, and slurry water. Slurry contains cement particles and ranges from muddy water to a thick paste. Removal of the debris should occur as soon as possible to prevent the debris from solidifying and adhering to the surface, making cleanup more difficult.

Tools used for cleanup include: fire hoses, pressure washers, compressed air, sweepers, skid steer loaders, vacuum trucks, and manual labor.

The types of cleanup will vary based on the type of removal performed as follows:

1. Above the reinforcing bar-any removal depth above the top reinforcing bar of the top mat of reinforcing and the reinforcing bar remains supported by the concrete;

2. Below the reinforcing bar-any removal depth below the top mat of reinforcing bar in which the top reinforcing bar mat becomes unsupported by the original concrete; and

940 FM 7.10 Exhibit 1 CONCRETE REPAIR MANUAL page 16 of 16

3. Full-depth removal.

During above the reinforcing bar clean up, equipment such as skid steer loaders, sweepers, and vacuum trucks may be driven over the surface to assist with the cleanup (providing they meet the weight requirements of the structure). The debris can be swept, pressure washed or air blown into piles where it is picked up by a loader. A vacuum truck may be used to vacuum the debris from the surface. In all cases, the surface must be pressure washed to remove any remaining cement slurry.

If the removal is below the reinforcing bar and the reinforcing bar is unsupported, it is difficult and possibly unsafe to drive equipment into the removal area. The debris can be removed by washing with a fire hose (large water consumption),

pressure washing or blowing it onto the adjacent original surface where it can be picked up with a loader. A pressure washer operating at 8000 to 10,000 psi (55 to 70 MPa) and 8 to 12 gpm (30 to 45 1pm) is effective. Vacuuming has proven very effective in removing debris from around the reinforcing steel, however, the surface will require pressure washing to remove the cement slurry and paste.

During full-depth removal, the debris simply falls to the floor below where it can be picked up with a loader.

The debris, which consists of wet sand, aggregate, chips or chunks of concrete, and slurry is placed in dumpsters or hauled away in trucks and may be recycled or placed in a landfill in accordance with the requirements of the controlling authority.

Removal Depth Measurements Following hydrodemolition, the surface profile is very rough and three depth measurements are possible (Fig. 25):

1. Minimum removal-original surface to the shallowest removal point.

2. Maximum removal-original surface to the deepest removal point.

3. Average depth of removal-The difference between the minimum and maximum removal at the same location.

Measuring the depth of removal can be accomplished using:

1. A straight-edge placed on the original surface;

2. A string-line pulled over the removal area; and

3. A surveyor's level.

Fig.: 25.: Measuring depth of removal using a straight edge The most common practice of measuring the depth of removal is to place a straightedge on top of the original surface and extend it over the removal area. Measurements are taken from the bottom of the straightedge to determine the depth of removal. This quick and simple technique can only be used during the removal process and is not applicable for final measurements in large removal areas.

A string line may be pulled over the removal area and measurements taken below the string.

However, this method could provide incorrect results if slopes or crowns occur in the original surface. Surveying equipment may be used and is very accurate; however, to account for slopes, pitches and crowns in the original surface, a detailed survey must be made of the original surface prior to removal and measurements taken at the same locations after removal for comparison and determination of the actual removal depth.

Summary Effective concrete removal and proper surface preparation are key elements to a successful repair project. A surface prepared using hydrodemolition is rough, irregular, and is excellent in creating a mechanical bond with the repair material.

Hydrodemolition eliminates micro-fractures and damage to reinforcing steel, minimizes transmitted noise and dust, and cleans the reinforcing steel.

The use of hydrodemolition may not be appro-priate for every structure and a careful review of the benefits and limitations of the process relative to each structure should be undertaken. Proper safety procedures must be observed at all times when using hydrodemolition.

FM 7.10 Exhibit 2 page 1 of 8 ACI 546R-04:

Concrete Repair Guide Reported by ACI Committee 546 Jay H. Paul Paul E. Gaudette Chair Secretary James P. Barlow Paul D. Carter Michael M. Chehab Marwan A. Daye Floyd E. Dimmick Peter H. Emmons Michael J. Garlich Steven H. Gebler I. Leon Glassgold Yelena S. Golod Harald G. Greve Robert F. Joyce Lawrence F. Kahn Brian F. Keane Benjamin Lavon Kenneth M. Lozen James E. McDonald Kevin A. Michols Richard Montani Myles A. Murray Thomas J. Pasko Don T. Pyle Richard C. Reed Johan L. Silfwerbrand W. Glenn Smoak Joe Solomon Michael M. Sprinkel Ronald R. Stankie George I. Taylor Alexander Vaysburd D. Gerald Walters Patrick M. Watson Mark V. Ziegler This document provides guidance on the selection and application of materials and methods for the repair protection, and strengthening of concrete structures. An overtiew of,naterials and methods is presented as a guide for making a selection for a particular application. References are provided for obtaining in-depth information on the selected materials or methods.

Keywords: anchorage; cementitious; coating; concrete; joint sealant; placement; polymer; reinforcement; repair.

ACI Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in planning, designing, executing, and inspecting construction.

This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains.

The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom.

Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.

CONTENTS Chapter 1-Introduction, p. 546R-2

1. 1-Use of this document 1.2-Definitions 1.3-Repair methodology 1.4-Design considerations 1.5-Format and organization Chapter 2-Concrete removal, preparation, and repair techniques, p. 546R-6

2. 1-introduction and general considerations 2.2--Concrete removal 2.3-Surface preparation 2.4-Reinforcement repair 2.5-Anchorage methods and materials 2.6-Materials placement for various repair techniques 2.7-Bonding methods Chapter 3-Repair materials, p. 546R-20 3.1--Introduction 3.2--Cementitious materials 3.3-Polymer materials 3.4-Bonding materials 3.5--Coatings on reinforcement 3.6-Reinforcement 3.7-Material selection ACI 546R-04 supersedes ACI 546R-96 and became effective September 23, 2004.

Copyright © 2004, American Concrete Institute.

All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by electronic or mechanical device, printed, written, or oral. or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.

663 It is the responsibility of the user of this document to establish health and safety practices appropriate to the specific circumstances involved with its use. ACI does not make any representations with regard to health and safety issues and the use of this document.

The user must determine the applicability of all regulatory limitations before applying the document and must comply with all applicable laws and regulations, including but not limited to, United States Occupational Safety and Health Administration (OSHA) health and safety standards.

664 FM 7.10 Exhibit 2 page 2 of 8 CONCRETE REPAIR MANUAL Chapter 4-Protective systems, p. 546R-30

4. 1-Introduction and selection factors 4.2-Concrete surface preparation and installation requirements 4.3-Typical selection problems 4.4-Systems concept 4.5-Surface treatments 4.6-Joint sealants 4.7--Cathodic protection 4.8--Chloride extraction 4.9-Realkalization Chapter 5-Strengthening techniques, p. 546R-41 5.1--General 5.2-Internal structural repair (restoration to original member strength) 5.3-Interior reinforcement 5.4-Exterior reinforcement (encased and exposed) 5.5-Exterior post-tensioning 5.6-Jackets and collars 5.7-Supplemental members 5.8-Repair of concrete columns 5.9-Column repair parameters Chapter 6-References, p. 546R-48 6.1-Referenced standards and reports 6.2--Cited references CHAPTER 1-INTRODUCTION 1.1-Use of this document This document provides guidance on selection of materials and application methods for the repair, protection, and strengthening of concrete structures. The information is applicable to repairing damaged or deteriorated concrete structures, correcting design or construction deficiencies, or upgrading a structure for new uses or to meet more restrictive building codes.

This guide summarizes current practices in concrete repair and provides sufficient information for the initial planning of repair work' and for selecting suitable repair materials and application methods for specific conditions. Many of the topics covered in this guide are more extensively covered in other ACI committee documents. Readers of this guide should refer to the appropriate documents of other ACI committees and other industry resources for additional information.

1.2-Definitions corrosion-destruction of metal by chemical, electro-chemical, or electrolytic reaction within its environment.

dampproofing-treatment of concrete or mortar to retard the passage or absorption of water, or water vapor, either by application of a suitable admixture or treated cement, or by use of preformed film, such as polyethylene sheets, placed on grade before placing a slab.

excavation-steps taken to remove deteriorated concrete, sound concrete, or both, designated for removal.

nonstructural repair-a repair that addresses local deterioration and is not intended to affect the structural capacity of a member.

protection-the procedure of shielding the concrete struc-ture from environmental and other damage for the purpose of preserving the structure or prolonging its useful life.

repair-to replace or correct deteriorated, damaged, or faulty materials, components, or elements of a structure.

repair systems-the combination of materials and techniques used in the repair of a structure.

strengthening-the process of restoring the capacity of damaged components of structural concrete to its original design capacity, or increasing the strength of structural concrete.

structural repair-a repair that re-establishes or enhances the structural capacity of a member.

surface preparation-steps taken after removal of deteriorated concrete, including conditioning of the surface of substrate at bond line and the cleaning of existing reinforcing steel.

waterproofing-prevention of the passage of water, in liquid form, under hydrostatic pressure.

1.3-Repair methodology A basic understanding of the causes of concrete deficiencies is essential to perform meaningful evaluations and successful repairs. If the cause of a deficiency is understood, it is much more likely that an appropriate repair system will be selected and, consequently, that the repair will be successful and the maximum life of the repair will be obtained.

Symptoms or observations of a deficiency should be differentiated from the actual cause of the deficiency, and it is imperative that causes and not symptoms be dealt with wherever possible or practical. For example, cracking can be a symptom of distress that may have a variety of causes such as drying shrinkage, thermal cycling, accidental over-loading, corrosion of embedded metal, or inadequate design or construction. Only after the cause or causes of deficiency are determined can rational decisions be made regarding the selection of a proper repair system and the implementation of the repair process (Fig. 1.1).

1.3.1 Condition evaluation-The first step in concrete repair is to evaluate the current condition of the concrete structure. This evaluation may include a review of available design and construction documents, structural analysis of the structure in its deteriorated condition, a review of available test data, a review of records of any previous repair work, review of maintenance records, a visual inspection of the structure, an evaluation of corrosion activity, destructive and nondestructive testing, and review of laboratory results from chemical and petrographic analysis of concrete samples.

Upon completion of this evaluation step, the personnel responsible for the evaluation should have a thorough under-standing of the condition of the concrete structure and be able to provide insights into the causes of the observed deterioration or distress.

Additional information on conducting surveys can be found in ACI 201.IR, 222R, 224.1R, 228.2R, 364. IR, and 437R.

1.3.2 Determination of causes of deterioration or distress-After the condition evaluation of a structure has been completed, the deterioration mechanism that caused the

668 FM 7.10 Exhibit 2 CONCRETE "behavior to evaluate and design a structural repair, strength-ening procedure, or both. Some design considerations follow and are discussed throughout this guide.

1.4.1 Current load distribution-In a deteriorated state, a structural member or system distributes dead and live loads differently than first assumed when the structure was new.

Cracking, deteriorated concrete and corroded reinforcement alter the stiffness of members, which leads to changes in shear, moment, and axial load distribution. As concrete and reinforcement are removed and replaced during the repair operation, these redistributed forces are further modified. To understand the final behavior of the structural system, the engineer should evaluate the redistribution of the forces. To fully re-establish the original load distribution, a member should be relieved of the load by jacking or other means. The repaired member and the repair itself supports the loads differ-ently than would be assumed in the original or a new structure.

1.4.2 Compatibility of materials-If a repair and the member have the same stiffness-for example, modulus of elasticity-the analysis of the repaired member may be the same as a new section. If the stiffness varies, however, then the composite nature of the repaired system. should be considered. A mismatch of other material characteristics further exacerbates the effects of thermal changes, vibra-tions, and long-term creep and shrinkage effects. Different coefficients of thermal expansion of the repair and original material results in different dimensional changes. The engineer should design for the different movements, or the repair system should be similar to the thermal and dimensional characteristics of the original material.

1.4.3 Creep, shrinkage, or both-Reduction in length, area, or volume of both the repair and original materials due to creep, shrinkage, or both, affect the structures service-ability and durability. As an example, compared with the original material, high creep or shrinkage of repair materials results in loss of stiffness of the repair, redistributed forces, and increased deformations. The engineer should consider these effects in the design.

1.4.4 Vibration-When the installed repair material is in a plastic state or until adequate strength has been developed, vibration of a structure can result in reduced bonding of the repair material. Isolating the repairs or eliminating the vibration may be a design consideration.

1.4.5 Water and vapor migration-Water or vapor migration through a concrete structure can degrade a repair. Under-standing the cause of the migration and controlling it should be part of a repair design consideration.

1.4.6 Safety-The contractor is responsible forconstruction safety. Nevertheless, as the engineer considers a repair design, which may involve substantial concrete removal, steel reinforcing cutting, or both, he or she should notify the contractor of the need and extent of shoring and bracing. The local repair of one small section can affect a much larger area, of which the contractor may not be aware.

1.4.7 Material behavior characteristics-When new and innovative materials and systems are used for repair and strengthening, the structural behavior of the repaired section can differ substantially from the behavior of the original REPAIR MANUAL page 3 of 8 section. For example, if a beam's steel reinforcement has corroded extensively and lost part of its load-carrying capacity, the steel reinforcement may be replaced by carbon fiber-reinforced polymer (CFRP) applied to the external bottom face of the beam. The original yielding behavior of the steel bar is replaced by FRP that is stronger, but has a more elastic and brittle behavior. The behavior assumptions of codes like ACI 318 are no longer valid. The engineer should consider the behavior and performance of the new repair under the actual service and ultimate load, and design the repair to provide at least an equivalent level of safety to the original design. Such a design is outside the scope of ACI 318.

1.5-Format and organization Chapter 2 discusses removal of deteriorated concrete, preparation of surfaces to receive repair materials, general methods for concrete repair, and repair techniques for reinforcing and prestressing steel. Chapter 3 discusses various types of repair materials that may be used. The reader is urged to use Chapters 2 and 3 in combination when selecting the repair material and method for a given situation. Chapter 4 describes materials and systems that may be used to protect repaired or unrepaired concrete from deterioration. Chapter 5 provides methods for strengthening an existing structure when repairing deficiencies, improving load-carrying capabilities, or both. Chapter 6 provides references, including other appropriate ACI documents and industry resources.

CHAPTER 2-CONCRETE REMOVAL, PREPARATION, AND REPAIR TECHNIQUES 2.1r-introduction and general considerations This chapter covers removal, excavation, or demolition of existing deteriorated concrete, preparation of the concrete surface to receive new material, preparation and repair of reinforcement, methods for anchoring repair materials to the existing concrete, and various methods that are available to place repair materials. The care that is exercised during the removal and preparation phases of a repair project can be the most important factor in determining the longevity of the repair, regardless of the material or technique used.

Specific attention should be given to the removal of concrete around prestress strands, both bonded and unbonded. The high-energy-impact tools, such as chipping hammers, should avoid contact with the strand because this will reduce the strands' load-carrying capacity and may cause the wire(s) to rupture, which may lead to strand failure.

S2.2-Concrete removal A repair *projct uually in'volves removal of deteriorated, damaged, or defective concrete. In most concrete repair projects, the zones of damaged concrete are not well defined.

Most references state that all damaged or deteriorated material should be removed, but it is not always easy to determine when all such material has been removed or when too much good material has been removed. A common recommendation is to remove sound concrete for a defined distance beyond the delaminated area; thereby, exposing the reinforcing steel beyond the point of corroded steel.

FM 7.10 Exhibit 2 page 4 of 8 CONCRETE REPAIR GUIDE 669 Removal of concrete using explosives or other aggressive methods can damage the concrete that is intended to remain in place. For example, blasting with explosives or the use of some impact tools heavier than 12 kg (30 lb) can result in additional delamination or cracking. Delaminated areas can be identified by using a hammer to take soundings. In most cases, such delaminations should be removed before repair materials are placed.

small-scale microcracking damage (termed bruising) to the:

surface of the concrete left in place. Unle.ss this damaged.

layer is removed, a weakened plane may occur in the parent concrete below the repair material bondline. This condition can result in a low tensile rupture (bond) strength between the parent concrete and repair material. Thus, a perfectly sound and acceptable replacement material may fail due to improper surface preparation. All damaged or delaminated concrete, including bruising, at the interface of the repair and the parent concrete should be removed before placing the repair material. This may require one type of aggressive removal for gross removal followed by another type of removal for bruising.

a~l'a~sJsi'nNviZ w1iZcncrte haZ KWii We(Mea T'From 'aa a structure by primary means such as blasting or aggressivea '

impact methods, the concrete left in place should be prepared a

by using a secondary method. such as chipping, abrasive:

blasting, orahigh-Vressure water jetting; to remove any° aremalining danag:ed surface material. Careful visual ispectons of the prepared surfaces should be conducted before placing repair materials. Wetting the surface may help to identify the presence of cracking. Determination of the tensile strength (ACI 503R, Appendix A) by pull-off testing is advisable on prepared surfaces to determine the suitability of the surface to receive repair material.

Removal of limited areas of concrete in a slab, wall, or column surface, requires saw-cutting the perimeter of the removal area, providing an adequate minimum thickness of repair material at the edge of the repaired area, and mitigating the advancement of undetected incipient cracking. Feathering of repair materials generally should be avoided. The prep-aration for shotcreting is an exception. ACI 506R recom-mends tapered edges around the perimeter of such patches.

Saw cutting can also improve the appearance of the repaired area. The general shape of the repaired areas should be as symmetrical as possible (ICRI 03730). Reentrant corners should be avoided. Large variations in the depth of removal in short distances should also be avoided. The texture of the prepared surface should be appropriate for the intended

'repair material (ICRI 03732).

Every precaution should be made to avoid cutting under-lying reinforcement. Reviewing design drawings and using a covermeter or similar device provides data as to the location and depth of reinforcement. In addition, the removal of small areas of concrete is commonly used to confirm the location and depth of bars before saw cutting.

Sections 2.2.1 through 2.2.18 present descriptions of many of the commonly used concrete removal techniques to help in the selection process.

2.2.1 General considerations-Concrete removal addresses deteriorated and damaged material. Some sound concrete, however, may be removed to permit structural modifications and to ensure that all unsound material is removed. The effectiveness of various removal techniques can differ for deteriorated and sound concrete. Some techniques may be more effective in sound concrete, while others may work better for deteriorated concrete.

Concrete removal techniques selected should be effective, safe, economical, environmentally friendly, and minimize damage to the concrete left in place. The removal technique chosen may have a significant effect on the length of time that a structure will be out of service. Some techniques permit a significant portion of the work to be accomplished without removing the structure from service. The same removal technique, however, may not be suitable for all portions of a given structure. In some instances, a combination of removal techniques may be more appropriate to speed removal and limit damage to the remaining sound concrete.

Trial field testing various removal techniques can help confirm the best procedures.

In general, the engineer responsible for the design of the repair should specify the objectives to be achieved by the concrete removal, and the repair contractor should be allowed to select the most economical removal method, subject to the engineer's acceptance. In special circum-stances, the engineer may also need to specify the removal techniques to be used and those that are prohibited.

The mechanical properties of the concrete and the type and size of aggregate to be removed provide important information to determine the method and cost of concrete removal. The mechanical properties include compressive and tensile strengths. This information is also necessary for the engineer to specify the prepared surface condition and select the repair material, and it should be made available to contractors for bidding purposes.

2.2.2 Monitoring and shoring during removal opera-tions-It is essential to evaluate the removal operations to limit the extent of damage to the structure and to the concrete that remains. Structural elements may require shoring, removal of applied loads, or both, before concrete removal to prevent structural deformations, possible collapse, buckling, or slippage of reinforcement. Care should be used during removal of concrete to avoid cutting and damaging rein-forcing steel. Because reinforcement is often misplaced, unanticipated damage may occur when saw cutting, impacting, or removing concrete.

Careful monitoring is required throughout the concrete removal operation. This can be accomplished by visual inspection, sounding, use of a covermeter, or other means to locate reinforcement. The project specifications should assign responsibilities for the inspection of the prepared concrete.

Sounding is an excellent means to detect delaminated concrete adjacent to the outermost layers of reinforcing steel.

Subsurface cracks, the extent of deterioration, or other internal defects, however, may not be identified by this method alone. Other means of evaluation should be used to properly identify the extent of concrete to be removed. In

FM 7.10 Exhibit 2 page 5 of 8 670 CONCRETE REPAIR MANUAL addition, sounding usually does not indicate near-surface microcracking or bruising. Only microscopic examination or bond testing may disclose near-surface damage.

Subsurface evaluation (examination of the substrate) can provide valuable information about the condition of the concrete. This information may be obtained by the following methods before, during, or after concrete removal (ACI 228.2R):

a) Taking cores for visual examination, microscopic examination, compressive strength tests, and splitting-tensile strength tests; b) Pulse-velocity tests; c) Impact-echo tests; d) Bond tests (pull-off testing, ACI 503R Appendix A);

e) Covermeter or similar equipment to locate reinforce-ment and determine its depth below the surface; f) Infrared thermography; and g) Ground-penetrating radar (GPR).

Many of these methods are discussed in ACI 228.2R.

2.2.3 Quantity of concrete to be removed-In most repair projects, all damaged or deteriorated concrete should be removed; however, the quantity of concrete to be removed is directly related to the elapsed time between preparation of the estimate and actual removal. Substantial overruns are common. Estimating inaccuracies can be minimized by a thorough condition survey as close as possible to the time the repair work is executed. Potential quantity overruns, based on field-measured quantities, should be taken into account.

When, by necessity, the condition survey is done far in advance of the repair work, the estimated quantities should be increased to account for continued deterioration. Because most concrete repair projects are based on unit prices, repair areas should be accurately measured before forms are installed. This is usually done jointly by the engineer and the contractor. It is not uncommon for estimated quantities to increase significantly between the detectable quantities and the actual quantity removed. ICRI 03735 provides guidelines for methods of measurement for concrete repair work.

2.2.4 Classification of concrete removal methods-Removal and excavation methods can be categorized by the way in which the process acts on the concrete. These categories are blasting, cutting, impacting, milling, hydrodemolition, presplitting, and abrading. Table 2.1 provides a general description of these categories, lists the specific removal techniques within each category, and provides a summary of information on each technique. The techniques are discussed in detail in the following sections.

2.2.5 Blasting methods-Blasting methods generally employ rapidly expanding gas confined within a series of bore holes to produce controlled fracture and removal of the concrete. The only blasting method addressed in this report is explosive blasting.

Explosive blasting is the most cost-effective and expedient means for removing large quantities of concrete-for example, portions of large mass concrete foundations or walls.

This method involves drilling bore holes, placing an explosive in each hole, and detonating the explosive. Controlled-blasting techniques minimize damage to the material that remains after blasting. One such technique, cushion blasting, involves drilling a line of 75 mm (3 in.) diameter or smaller bore holes parallel to the removal face, loading each hole with light charges of explosive (usually detonating cord) distributed along its length, cushioning the charges by stemming each hole completely or in the collar with wet sand, and detonating the explosive with electric blasting caps. The uniform distribution and cushioning of the light charges produce a relatively sound surface with little overbreak.

Blasting machines and electrical blasting-cap delay series are also Used for controlled demolition and employ proper timing sequences to provide greater control in reducing ground vibration. Controlled blasting has been used success-fully on numerous repair projects. The selection of proper charge weight, borehole diameter, and borehole spacing for a repair project depends on the location of the structure, the acceptable degree of vibration and damage, and the quantity and quality of concrete to be removed. If at all possible, a pilot test program should be implemented to determine the optimum parameters. Because of the inherent dangers in the handling and usage of explosives, all phases of the blasting project should be performed by qualified, appropriately licensed personnel with proven experience and ability.

2.2.6 Cutting methods-Cutting methods generally employ mechanical sawing, intense heat, or high-pressure water jets to cut around the perimeter of concrete sections to permit their removal. The size of the sections that are cut free is governed by the available lifting and transporting equipment. The cutting methods include high-pressure water jets, saw cutting, diamond wire cutting, mechanical shearinp, stitch drilling,,and thermal cuttinZ............

a) High-pressure water jet (without abrasives)-A high-pressure water jet uses a small jet of water driven at high:

velocities, commonly producing pressures of 69 to 310 MPa (10,000 to 45,000 psi). A number of different types of water:

jets are currently being used. The most promising are the:

ultra high-pressure jet and the cavitating jet. Section 2.2.10 a describes using a water jet as a primary removal method.

Water jets used with abrasives are described in Section a

a a 2.2.11.

b) Saw cutting-Diamond or carbide saws are available in sizes ranging from small (capable of being hand-held) to large (capable of cutting depths of up to 1.3 m [52 in.]).

c) Diamond wire cutting-Diamond wire cutting is accomplished with a wire containing nodules impregnated with diamonds. The wire is wrapped around the concrete mass to be cut and reconnected with the power pack to form a continuous loop. The loop is spun in the plane of the cut while being drawn through the concrete member. This system can be used to cut a structure of any size as long as the wire can be wrapped around the concrete. The limits of the power source determines the size of the concrete structure that can be cut. This system provides an efficient method for cutting up and dismantling large or small concrete structures.

d) Mechanical shearing-The mechanical shearing method employs hydraulically powered jaws to cut concrete and reinforcing steel. This method is applicable for making cutouts through slabs, decks, and other thin concrete

FM 7.10 Exhibit 2 CONCRETE REPAIR GUIDE page 6 of 8 671 Table 2.1-Summary of features and considerations/limitations for concrete removal methods Category Features Considerations/Limitations 2.2.5 Blasting Explosives Uses rapidly expanding gas confined within a series of Most expedient method for removing large volumes Requires highly skilled personnel fordesign and execution.

boreholes to produce controlled fracture and removal of where concrete section is 10 in. (250 mm) thick or more.

Stringent safety regulations must be complied with regarding concrete.

Produces good fragmentation of concrete debris for easy the transportation, storage, and use of explosives due to removal.

their inherent dangers.

Blast energy must be controlled to avoid damage to surrounding improvements resulting from air blast

--.pnegurey g--u-d-ib----..a-d'li-debpi..........

I 2.2.6 Cutting High-pressure waterjet (without abrasives)

I Uses perimeter cuts to remove large pieces of concrete.

Applicable for making cutouts through slabs, decks, and Cutouts for removal limited to thin sections.

other thin concrete members.

Cutting is typically slower and more costly than diamond Cuts irregular and curved shapes.

blade sawing.

U Makes cutouts without overcutfing corners.

Moderate levels of noise may be produced.

Cuts flush with intersecting surfaces.

Controlling flow of waste water may be required.

• No telatvi%*auo*,lods io d

Additional safety precautions are required due to the high S'Handli ng of debtisis efficient because bulk of concrete is water pressure produced by the system.

a removed in large pieces.

S2.2.6Cutting.(continued.

Diamond saw Applicable for making cutouts through slabs, decks, and Cutouts for removal limited to thin sections.

other thin concrete members.

Performance is affected by type of diamonds and the dia-Makes precision cuts.

mond-to-metal bond in blade segments (segment selec-No dust or vibration is produced.

tion is based on aggregate hardness).

Handling of debris is efficient because bulk of concrete is The higher the percentage of steel reinforcement in cuts, removed in large pieces.

the sltwer and more costly the cutting.

The harder the aggregate, the slower and more costly the cutting.

Controlling flow of waste water may be required.

2.2.6 Cutting (continued)

Diamond wire cutting Applicable for cutting large and/or thick pieces of concrete.

The cutting chain must be continuous.

The diamond wire chain can be infinitely long.

Access to drill holes through the concrete must be available.

No dust or vibration is produced.

Water must be available to the chain.

Large blocks of concrete can be easily lifted out by a Controlling the flow of waste water may be required.

crane or other mechanical methods.

The harder the aggregate and/or concrete, the slower and The cutting operation can be equally efficient in any more costly the cutting.

direction.

Performance is affected by the quality, type, and number of diamonds as well as the diamond-to-metal bond in the chain.

2.2.6 Cutting (continued)

Mechanical shearing Applicable for making cutouts through slabs, decks, and Limited to thin sections where an edge is available or other thin concrete members, a hole can be made to start the cut.

Steel reinforcement can be cut.

Exposed reinforcing steel is damaged beyond reuse.

Limited noise and vibration are produced.

Remaining concrete is damaged.

Handling of debris is efficient because bulk of concrete is Extremely rugged profile is produced at the cut edge.

removed in large pieces.

Ragged feather edges remain after removal.

2.2.6 Cutting (continued)

Stitch drilling Applicable for making cutouts through concrete members Rotary-percussion drilling is significantly more expedient where access to only one face is feasible, and economical than diamond core drilling; however, it Handling of debris is more efficient because bulk of results in more damage to the concrete that remains, concrete is removed in large pieces.

especially at the point of exit from the concrete.

Depth of cuts is dependent on accuracy of drilling equip-ment in maintaining overlap between holes with depth and diameter of the boreholes drilled. The deeper the cut, the greater borehole diameter required to maintain over-lap between adjacent holes and the greater the cost.

Uncut portions between adjacent boreholes will hamper or prevent the removal.

Cutting reinforced concrete increases the cutting time and increases the cost. Aggregate toughness for percussion drilling and aggregate hardness for diamond coring will affect cutting cost and rate.

Personnel must wear hearing protection due to high noise levels.

members. It is especially applicable where total demolition of the member is desired. The major limitation of this method is that cuts should be started from free edges or from holes made by hand-held breakers or other means.

e) Stitch drilling-The stitch-drilling method employs the use of overlapping boreholes along the removal perimeter to cut out sections for removal. This method is applicable for making cutouts through concrete members where access to only one face is possible, and the depth of cut is greater than can be economically cut by the diamond-blade method. The primary drawback of stitch drilling is the potential for costly removal complications if the cutting depth exceeds the accuracy of the drilling equipment, so that uncut concrete remains between adjacent holes.

f) Thermal cutting-This method requires powder torch, thermal lance, and powder lance, which develop intense heat generated by the reaction between oxygen and powdered metals to melt a slot into concrete. The thermal device's ability for removing concrete from structures mainly depends on the rate at which the resulting slag can flow from the slot. These devices use intense heat and are especially effective for cutting reinforced concrete; however, they are considered slow, relatively expensive, and are not widely used.

PRtA 7 1 nl IF-vhihit 92

ý fQ 672 CONCRETE REPAIR MANUAL V

Table 2.1 (cont.)-Summary of features and considerations/limitations for concrete removal methods

• JI U

Category Features Considerations/Limitations 2.2.6 Cutting (continued)

Thermal cutting Applicable for making cutouts through heavily reinforced Limited availability commercially.

decks, beams, walls, and other thin to medium concrete Not applicable for cuts where slag flow is restricted.

members.

Remaining concrete has thermal damage with more extensive An effective means of cutting reinforced concrete, damage occurring around steel reinforcement.

Cuts irregular shapes.

Produces smoke and fumes.

Produces minimal noise, vibrations, and dust!

Personnel must be protected from heat and hot slag produced by cutting operation.

2.2.7 Impacting Hand-held breakers Uses repeated striking of the surface with a mass to fracture Applicable for limited volumes of concrete removal.

Performance is a function of concrete soundness and and spall the concrete.

Applicable where blow energy must be limited, aggregate toughness.

Widely available commercially.

Significant loss of productivity occurs when breaking action is other than downward.

Can be used in areas of limited work space.

ato sohrta onad Removal boundaries will likely require saw cutting to Produces relatively small and easily handled debris, avoid feathered edges.

Concrete that remains may be damaged (microcracking).

Produces high levels of noise, dust, and vibration.

Boomn-mounted breakers Applicable for full-depth removal from slabs, decks, and Blow energy delivered to the concrete may have to be other thin concrete members and for surface removal limited to protect the structure being repaired and the from more massive concrete structures, surrounding structures from damage due to high cyclic Can be used for vertical and overhead surfaces, energy generated.

Widely available commercially.

Performance is a function of concrete soundness and Produces easily handled debris, aggregate toughness.,

Damages remaining concrete.

Damages reinforcing steel.

Produces feathered edges.

Produces high level of noise and dust.

2.2.7 Impacting (continued)

Scabblers Low initial cost.

High cyclic energy applied to a structure will produce Can be operated by unskilled labor, fractures in the remaining concrete surface area.

Can be used in areas of limited work space..

Produces high level of noise and dust.

Removes deteriorated concrete from wall or floor surfaces Limited depth removal.

efficiently.

Readily available commercially.

2.2,8 Milling Scarifier Uses scarifiers to remove concrete surfaces.

Applicable for removing deteriorated concrete surfaces Removal is limited to concrete without steel reinforcement.

from slabs, decks, and mass concrete.

Sound concrete significantly reduces the rate of removal.

Boom-mounted cutters are 'applicable for removal from Can damage concrete that remains (microcracking).

wall and ceiling surfaces.

Noise, vibration, and dust are produced.

Removal profile can be controlled.

jed alval l

.slj.

....d le.b SII 12.2.9 Ilydmrdemnolhtion Applicable tor removal ot deteriorated concrete trom Productivity is significantly reduced when sound concrete is

• Uses high-pressure water to remove concrete, surfaces of bridges and parking decks and other deteriorated being removed.

• surfaceswhere removal depth is Iin. (150 mm) or less.

Removal profile will vary with changes in depth of Does not damage the concrete that remains. Is deterioration.

Steel reinforcing is left clean and undamaged for reuse.

Method requires large source of potable water to meet Method produces easily handled, aggregate-sized debris, water demand.

Waste water may have to be controlled.

An environmental impact statement may be required if waste water is to enter a waterwal.

1`Per-so-nnelmust wear hearing protection due to the high i I Flying debris is produced.

Additional safety requirements are required due to the high pressures produced by these systems.

2.2.7 Inpacting methods-Impacting methods are the most commonly used concrete removal systems. They repeatedly strike a concrete surface with a high-energy tool or a large mass to fracture and spall the concrete. The use of these methods in partial-depth concrete removal can result in microcracking on the surface of the concrete left in place.

Extensive microcracking results in a weakened plane below the bond line. Currently, the committee is unable to provide definitive guidelines to prevent such damage when using impact methods; however, factors such as the weight and size of the equipment should be considered to minimize microcracking. Determination of the tensile strength by pull-off testing is recommended to determine the suitability of the surface to receive repair materials. Additionally, after impacting secondary methods, such as sandblasting, abra-sive blasting, and water blasting, may be required to remove excessive microcracking.

a) Hand-held breakers-The hand-held breaker or chipping hammer is probably the best known of all concrete removal devices. Hand-held breakers are available in various sizes with different levels of energy and efficiency. These tools are generally defined by weight and vary' in size from 3.5 to 41 kg (8 to 90 lb). (Note: the larger the hammer, such as 14 kg

[30 lb] and larger, the greater the potential for microc-racking.) The smaller hand-held breakers, such as 7 kg (15 lb) and smaller, are used in partial removal of unsound concrete or concrete around reinforcing steel because they do little damage to surrounding concrete. Larger breakers are used for complete removal of large volumes of concrete or delam-inations. Care should be exercised when selecting the size of

674 FM 7.10 Exhibit 2 CONCRETE REPAIR MANUAL page 8 of 8

" (0.1 to 4 in.). Milling operations usually leave a sound surface with less microfractures than impact methods (Virginia Transportation Research Council 2001).

2.2.9 Scarifier-A scarifier is a concrete cutting tool that employs the rotary action and mass of its cutter bits to rout cuts into concrete surfaces. It removes loose concrete fragments (scale) from freshly blasted surfaces and removes concrete that is cracked and weakened by an expansive agent. It also is the sole method of removing deteriorated and sound concrete in which some of the concrete contains form ties and wire mesh. Scarifiers are available in a range of sizes. The scarifier is an effective tool for removing deteriorated concrete on vertical and horizontal surfaces. Other advantages include well-defined limits of concrete removal, relatively small and easily handled concrete debris, and simplicity of operation.

7~'~e~Ttion-High-pressure water Jett ing (hydrodemolition) can be used to remove concrete to preserve and clean the steel reinforcement for reuse and to aminimize mrocran to teremalnlnin

-ie

&crete The method also has a high efficiency. Hydrodemolition disintegrates concrete, returning it to sand and gravel-sized pieces. This process works on sound or deteriorated concrete a

and leaves a rough profile. Hydrodemolition punches through the full depth of slabs in small areas when the concrete is unsound or when full-depth patches are inadequately bonded flu

-*t

e.

n on rcrd*t-lje ued in a% structures with unbonded tendons, except under the direct

,a

• Sme.rvision of a structural engineer.

High-pressure water jets in the 70 MPa (10,000 psi) range require 130 to 150 L/min (35 to 40 gal./min). As the pressure increases to 100 to 140 MPa (15,000 to 20,000 psi), the water demand varies from 75 to 150 L/min (20 to 40 gal./min)

(Nittenger 1997). The equipment manufacturer should be consulted to confirm the water demand. Ultra-high-pressure equipment operating at 170 to 240 MPa (25,000 to 35,000 psi) has the capability of milling concrete to depths of 3 to 150 mm (0.1 to 6 in.). Containment and subsequent disposal of the water are requirements of the hydrodemolition process.

Many localities require this water to be filtered and then treated to reduce the alkalinity and particulates before the water can be released into a storm or wastewater system..

Water jet lances operating at pressures of 70 to 140 MPa (10,000 to 20,000 psi) and having a water demand of 75 to a 150 L/min (20 to 40 gal./min) are available. They are capable of cutting sections of concrete or selectively removing a surface concrete in areas that are difficult to reach with larger aenuinm ent (ICRI 2.2.11 Presplitting inethods-Presplitting methods use hydraulic splitters, water pressure pulses, or expansive chem-icals used in boreholes drilled at points along a predetermined line to induce a crack plane for the removal of concrete. The pattern, spacing, and depth of the boreholes affect the direction and extent of the crack planes that propagate.

Presplitting is generally used in mass concrete structures or unreinforced concrete.

a) Hydraulic splitter-The hydraulic splitter is a wedging device that is used in predrilled boreholes to split concrete into sections. This method has potential as a primary means for removal of large volumes of material from mass concrete structures. Secondary means of separating and handling the concrete, however, may be required where reinforcing steel is involved.

b) Water-pulse splitter-The water-pressure pulse method requires that the boreholes be filled with water. A device, or devices, containing a very small explosive charge is placed into one or more holes, and the explosive is detonated. The explosion creates a high-pressure pulse that is transmitted through the water to the structure, cracking the concrete.

Secondary means may be required to complete the removal of reinforced concrete. This method does not work if the concrete is so badly cracked or deteriorated that it does not hold water in the drill holes.

c) Expansive product agents-Commercially available cementitious expansive product agents, such as those containing aluminum powder, when correctly mixed with water, exhibit a large increase in volume over a short period of time. By placing the expansive agent in boreholes located in a predetermined pattern within a concrete structure, the concrete can be split in a controlled manner for removal.

This technique has potential as a primary means of removing large volumes of material from concrete structures and is best suited for use in holes of significant depth. Secondary means may be required to complete the separation and removal of concrete from the reinforcement. A key advantage to the use of expansive agents is the relatively nonviolent nature of the process and the reduced tendency to disturb the adjacent concrete.

2.2.12 Abrading blasting-Abrading blasting removes concrete by propelling an abrasive medium at high velocity against the concrete surface to abrade it. Abrasive blasting is typically used to remove surface contaminants and as a final surface preparation. Commonly used methods include sand-blasting, shotblasting, and high-pressure water blasting.

2.2.13 Sandblasting-Sandblasting is the most commonly used method of cleaning concrete and reinforcing steel in the construction industry. The process uses common sands, silica sands, or metallic sands as the primary abrading tool.

The process may be executed in one of three methods.

2.2.14 Dry sandblasting-Sands are projected at the concrete or steel in a stream of high-pressure air in the open atmosphere. The sand particles are usually angular and may range in size from passing a 212 to a 4.75 mm (No. 70 to a No. 4) sieve. The rougher the required surface condition, the larger the sand particle size.

The sand particles are propelled at the surface in a stream of compressed air at a minimum pressure of 860 kPa (125 psi).

The compressor size varies, depending on the size of the sand-blasting pot. Finer sands are used for removing contaminants and laitance from the concrete and loose scale from rein-forcing steel. Coarser sands are commonly used to expose fine and coarse aggregates in the concrete by removing the paste or tightly bonded corrosion products from reinforcing steel.

Although sandblasting has the ability to cut quite deeply into concrete, it is not economically practical to remove more than 6 mm (0.25 in.) from the concrete surface.

I..

FIVI 7. 10 Exhibit 3 Interviews at CR3 Page 1 of 4 11/11/09 Interview with ironworker crew tasked with cutting rebar and tendons Interviewers: Marci Cooper and Craig Miller Interviewees: Jonah Hovey, Jessie Sadler, Gary Deal, Randy Cleveland, Al Adams Q. What did the concrete behind the rebar mat look like when they arrived?

• When they came to cut the rebar, the concrete behind the mat looked solid and good, no cracks. Looked good, was hard, difficult to chip/remove around rebar Q. What was their thinking and perception of what occurred and what they were seeing?

• another surprise was the sheathing - unusual, rigid instead of normally see corrugated sheathing which crushes under the hydro-demolition. In this case, they theorized that the hydro-demolition could cause vibration of the tendon vs. crushing like normal 11/12/09 Interview with Progress Responsible Civil Structural Engineer Interviewers: Marci Cooper and Patrick Berbon Interviewee: John Holliday Q. Based on what you saw, what was your thinking at the time?

e Hydro-demolition resulted not in the small pieces would expect, big chunks falling off the wall Q. Hydro-demolition?

I went beyond the protective net. No huge vibration. Recalls 22,000 psi pressure, 300 gpm flow, 600 rpm.

Interview 11/10/09 CR-3 Civil Structural Engineering Supervisor responsible for EC (design change package) for SGR Interviewer: Marci Cooper Interviewee: Dan Jopling Q. At the time of execution in the field, what occurred and what was observed?

" He and John Holiday went to platform to observe - 8'Hxl 5'W exposed. On right hand side, saw a relatively tight (maybe 'A") crack along the tendon plane. There was some discussion but decided to continue. Thought was pre-existing condition, concrete coming off in sheets (he had been at Oconee w/ American Hydro -would wash away mortar leaving aggregate pieces; mac&mac expected a slurry of aggregate and cement, not what was seen). Perception of pre-existing crack and now being discovered

" Within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> there was a report of water coming out of the containment surface (at the moon-shape opening), Led them to further believe was due to pre-existing cracks as couldn't believe what they were doing could possibly cause a crack through containment wall roughly 30' away.

11/12/09 Interview with Bechtel Field Engineer Interviewer: Marci Cooper Interviewee: Richard lonelli Q. What was your perception of what you saw, how happened?

• Substantial crack all around the opening. Did not believe that the hydro-demolition caused it; however hydro could be opening it up due to erosion (was bloWing debris behind the hydro-blasting machine)

" The demolition was slow, had to reduce pressure as approached liner Q. How did the concrete appear to you, cracked, solid?

FIVI 7. 10 Exhibit 3 Interviews at CR3 Page 2 of 4

" Looked like good, solid concrete.

" Concrete looked like solid sheets, except in shadows behind the tendon. Before cut rebar mat, the concrete looked solid

• The tendon sleeves were unexpected, were expecting spiral, not sure why, maybe what was told or normally see.

Q. Did it appear that cracks changed any over time?

• Was there when moon-shape opening occurred - discussed that thought it just ran toward the buttress like might expect 0

11/16/09 Phone Interview with Construction Opening Task Manager Interviewer: Marci Cooper Interviewee: Jon Burchette Q. Tell me about your involvement in the SGR project and any concerns that you might have had relative to the plan?

" Has opinion why delaminated is different than popular wisdom of some people that think hydro-demolition caused it - was there for every pass

• Did the test cut, roughly 8'x5' (top 2 horizontal tendons were the only ones not completed detensioning, think they were in process of removing at this point); afterward did another small pass roughly 2'x3' within the test area turning pressure up a little to 16-17000 psi for about two minutes. On the bottom right comer this pass cut down to tendon sheaths. Were surprised to find rigid sleeves. Saw cracking in concrete and stopped - crack was already there. Discussed with John Franks. Single crack, maybe 1/4 to 3/8" by maybe 8" long.

" Rebar mat at about 3 V2" and horizontal tendons at 12". In 1 st. phase excavated to roughly 5" depth - as clearing rebar, looked like solid concrete, but as got to around 5" deep the concrete broke in large chunks and slabs and started to fall down - believe these chunks/slabs resulted from delamination existing behind this layer (around 7" thick) at plane of horizontal tendons Q. Do you think the detensioning could have caused the delamination?

• Again, when got to rebar mat, looked good for first layer, then big chunks; after this, for rest of demolition came off as rubble - process kind of melts cement and sand away from aggregate.

Was there for every pass.

Q. Do you think hydro-demolition would cause the crack to open more?

• No, the rebar mat holds in place - did not perceive that the cracks changed over time - measured by inserting hand. Was there every day.

11/17/09 Interview with Rick Kopicki (supporting SGR)

Interviewer: Marci Cooper Q. When did you become aware of the cracking problems?

• John looked at it and they decided to go ahead with the demolition. Then got a call that water was coming out of the spalling location - the hydro was forcing water out to the surface.

11/20/09 Interview with Jabari Jackson, Nova Technical, SGR Night Shift Civil Engineer Interviewer: Patrick Berbon Interviewee: Jabari Jackson, night shift civil engineer for SGR

6. Impressions from when you were on the chipping platform during hydro-lazing?

a I do not remember any vibration;

FM 7.10 Exhibit 3 Interviews at CR3 Page 3 of 4

• I was surprised by how quiet it was;

• I heard no "boom" from the wall; 12/05/09 Interview with John Piazza, Exelon Interviewer: Patrick Berbon Interviewee: John Piazza, Exelon, TMI nuclear plant

8. Tensile properties?

0 Split tensile properties done by CTL: 1,1 30psi; 10/28/2009 Interview Dave McNeill on phone Present: Dave McNeill (Mac&Mac, Vancouver), Craig Miller, Chong Chiu, Patrick Berbon 17,000-17,500 psi water pressure.

• Flow rate 40-45 gallons/minute/nozzle.

• Running 2 x 3 nozzles at the same time (3 nozzles per head, 2 heads).

" Nozzle rotates at 500 rpm and projects water at a 5 degrees angle around the axis of rotation.

Spin rate

" Can be adjusted from 250 to 1,000 rpm. Was fixed at 500 rpm here.

Cut about 4 feet wide per head (total width of cut about 8' per pass).

" Travels 1 ft/s along the beam, then move up 1.5 in, travels back 1 ft/s.

" Each pass cuts lin to 1.5in deep.

" Nozzles are 7 inches apart.

" Water is taken from Progress large holding tank. It is pumped up from the tank and filtered to 1 micron.

" It is recovered and re-filtered at the end (not recycled, dumped in settling pond after use).

" Supply water is usually around 60 F. it was 85 F in this case. The pumping added an additional 25 F so

• That the water exited the nozzle around 110 F.

" Mac&Mac works on many bridge decks. First nuclear project for the company.

Their in-house calculations for 17,000 psi, 500 rpm rotation, 40 gal/min water, 1 ft/s travel, shows a force on the wall of 100 psi.

" Nozzle is 1/8 inch.

" First nozzle is 2-4 in from the wall, second nozzle is 3-5 in, and third nozzle is 4-6 in (nozzles are in same plane, but distance increases as material is removed by each nozzle).

" Takes approximately 35 min for one pass.

• Pump is diesel powered (1,800 rpm), runs at 500 rpm.

• Pistons are 1.5 in diameter, and move back-and-forth at 500 rpm.

11/20/09 Interview with Domingo Correira, Sargent & Lundy (retired)

Interviewer: Patrick Berbon Interviewee: Domingo Correira, Sargent & Lundy (retired) 19 years at S&L, now consultant

2. Did you design for 7,000 psi?

" Not selected for compressive strength;

" Selected for low creep and high early strength;

" Planned to re-tension after 7-10 days;

FM 7.10 Exhibit 3 Interviews at CR3 Page 4 of 4

6. Where did you select the new coarse aggregates from?

• Do not like Florida aggregates;

7. Can you say more about Florida coarse aggregates?

" Concrete for bridges, cannot use Florida CA;

" Suspect corrosion in Keys bridge related to them;

8. Creep, strength, chemical reaction issues?

• Tensile strength too low;

9. Any worries in combining with the new superior concrete?

• Original concrete is very old so the creep from now will be very small;

10. Please speculate on what happened here o Radial tension in the concrete plus low tensile strength;

11. Do you think the delamination was there before doing the opening?

• Hydro-cutting has nothing to do with it; 11/20/09 Interview with CN Krishnaswamy, Sargent & Lundy Interviewer: Patrick Berbon Interviewee: CN Krishnaswamy (Krish), Sargent & Lundy

6. From your level of knowledge at this point, what is your thinking regarding possible contributors?

" Found out high-Pressure hydrolazing;

" Most of the industry uses collapsible sheathing, CR3 has rigid conduits. These could vibrate or cause additional stresses;

" My feeling is that the rigid conduits are a bigger factor. I heard from Dr. Singh that the concrete was loose behind the rebars 10/30/2009 Interview Rich Kopicki (Progress Energy contractor)

Present: Rich Kopicki, Craig Miller, Patrick Berbon Rich Kopicki and Richard lonelli were inspecting the liner because paint flakes were reported. They were inside the RB while hydrolazing was taking place outside.

1-Liner plate o

The bulges on the inside of the liner have been attributed to pulling a vacuum inside the containment.

o Have been there a long time. Present from floor to top of vessel at various locations.

o On 10/5, paint scaling was initially reported as being due to scaffold rubbing against it, as a large scaffold was erected in this area. The main flaking point was 1.5ft above scaffold board.

2-Vibration and heat o

The next morning (10/6), the paint flaking was clearer and more extensive.

o The hydrolazing was reaching the last 3in of concrete and getting down to the liner.

o They were on the scaffold and observed the following:

• Liner plate was vibrating (radially);

• Liner plate was warm to the touch;

• Could see the paint shaking and flaking off; o The paint scaling was now observed to follow the location of the vertical stiffeners (present on the other side of the plate). Several areas were vibrating the most, mostly vertically, with at least one instance of a horizontal line.

FM 7.10 Exhibit 4 U?

U?

c Stiffeners (typ) too page 2 of 2 414ll 9'- 1 Coating removed from perimeter (typ)

Panel Joints Liner Plate - View from Outside RB (Coating on Opposite Side)

"io

T

FIA 7 1n If:vl

FM 7.10 Exhibit 6 page 1 of 21 Hydrodemolition of Concrete Surfaces and Reinforced Concrete, Andreas Momber 2005 CHAPTER 2 Fundamentals of Hydrodemolition 2.1 Properties and structure of high-speed water jets 2.1.1 Kinematics of high-speed water jets 2.1.2 Structure of high-speed water jets 2.1.3 Water drop formation 2.2 Material loading due to stationary jets 2.2.1 General loading modes 2.2.2 Material response 2.2.3 Material resistance parameters 2.3 Process parameter effects on material removal 2.3.1 Parameter definitions 2.3.2 Pump pressure effects 2.3.3 Nozzle diameter effects 2.3.4 Stand-off distance effects 2.3.5 Traverse rate effects 2.3.6 Traverse increment effects 2.3.7 Impact angle effects 2.3.8 Nozzle movement effects 2.4 Concrete parameter effects on material removal 2.4.1 Material failure types 2.4.2 Compressive strength effects 2.4.3 Aggregate fineness effects 2.4.4 Aggregate sort effects 2.4.5 Porosity effects 2.4.6 Steel bar reinforcement effects 2.4.7 Steel fibre reinforcement effects 2.5 Hydrodemolition model copyrighted material, do not reproduce or distribute

FM 7.10 Exhibit 6 page 2 of 21 Fundamentals of Hydrodemolition 33 1

CL C.

0 02 0.8 0.8 0.4 0.2 0

0 200 400 800 800 Relative jet length Figure2.8 Orifice type effects on stagnation pressure profiles (Leach and Walker, 1966)

Ps(O X)

(2.23) for x< 3 "xc: ' K*=O.27+O.075"(x/xc) 2.

for x> 3 -xc: K*=0.3.

The loading duration is given through the exposure time which is for a moving jet:

tE dN VT (2.24)

The difference between the stagnation pressure at the surface and the pressure inside the target material forces a certain volume of water to penetrate the structure. This volume is Q S='M*-0,tE (2.25) where w*=0 is the limiting case for a completely non-permeable material, and w*= 1 is the limiting case when the whole volume delivered by the nozzle penetrates into the material. For co*>O, the following three cases can be distinguished:

M

FM 7.10 Exhibit 6 page 3 of 21 34 Hydrodemolition of concrete surfaces and reinforced concrete structures b..m~~

(i) the water flows into a crack and creates a corresponding stress at the crack tip; (ii) the water flows into a capillary which results in pressure amplification;

• (iii) the water flows through an open pore system and creates friction forces lI to the structural elements (e.g. grains).

Qqasg J(owas experimentally investigated by Mazurkiewicz et al. (1986) whose results are illustrated in Fig. 2.9. Although the experiments were restricted to comparatively low water pressures, a linear relationship between pump pressure and pressure developed at the crack tip could be noted. If jet velocity is considered instead of pump pressure, the following relationship is valid:

2 PR = C 1 'V, (2.26) 10 M

0.

o.P 1pw=0.218pr IA P6 0

8 16 24 32 40 Stagnation pressure in MPa Figure 2.9 Water flow into a crack (Mazurkiewicz et al., 1986)

The constant was found to be C1=0.22 which corresponded closely to values estimated by Momber and Kovacevic (1995) who found 0.19<C1<0.21 based on an LEFM-model. Lin et al. (1996) used a finite element code to investigate the influence of the water jet velocity on principal stresses as well as stress intensity at the crack tip. Some of their results are shown in Fig. 2.10. The calculated points (filled circles) can be approximated by a square-root relationships which verifies Eq.

(2.25). The open circles in Fig. 2.10 are experimental points estimated by Witzel (1998) on rocks. However, the calculated points in Fig. 2.10 can be approximated by an almost linear function in the range of low jet velocities up to 300 m/s. A

FM 7.10 Exhibit 6 page 4 of 21 Fundamentals of Hydrodemolition 35 IB0 crack length 0,2 mm nmbar of cracks: 3 material: basalt 120 E

E 0

E80 0 point for Basatt I

40 o Jtrassic limestone grinite concrete 855 concrete o result VMtzel (19981 845 0

200 400 6O0 80 Jet velocity in mfs Figure 2.10 Relationship between jet velocity and stress intensity (Lin et al., 1996) mercury intrusion study performed by Momber (1992) on cementitious composites showed clearly that the fracture started in the interfacial zone between cement matrix and aggregate which is known to be the weakest link in conventional concrete.

Moreover, fracture propagation was mainly affected by aggregate size and distri-i!.sAicye&~rcT T Rio 'ga "es r

i~r 55 sein concretW 1

samples eroded by water jets were made by Momber (2003b) who found clear evidence of crack deflection, crack stopping, crack tip bluntness, but also of crack

,bridpinjand crack face friction. Some of these features are illustrated in Fig. 2.11.

Cam 141).reo

.p~aprt-li.e r

A model for pressure intensification in "blind", air filled tubes was developed by Evers et al. (1982). A Figure 2.11 Microscopic features of concrete eroded by high-speed water jets (Momber. 2003b)

FM 7.10 Exhibit 6 36 Hydrodemolition of concrete surfaces and reinforced concrete structures page 5 of 21 20 16 j12 E

4 0

0 200 400 600 800 1,000 Pore slenderness Lpldp Figure 2.12 Pressure amplification in a pore subjected by a water jet (Evers et aL., 1982) displaces the air. A force balance as shown in Fig. 2.12 delivers the following:

PI "Ac =s "

5 Pr'x+P2 -Ac (2.27)

Thus, pressure intensification depends on shear stress, pore geometry and perimeter of the liquid column. The approach was later modified by Evers and Maoangfela I T*5,*)

-o-n-srlo~ermng compressibility el'lects.hne capfaUry moder was verified experimentally for rather large pores and low pressures. For a pore with a diameter of 0.2 mm and a length of 38 mm where a water jet with a velocity of 71 m/s traversed over with a speed of 6.0 m/s (corresponding exposure time would be 3.3 10-5 s), a pressure intensification of 3.5 was estimated.

Case (flA)wgs in, detail investi tedbRehbinder, 19 77) for porous solids. Based on a known pressure gradient, the speed, the liquid penetrates the pore system at, can be estimated with Darcy's Law:

VF kp gra p, 11W (2.28)

The frictional force acting on an individual grain due to the liquid flow can be approximated for low Reynolds numbers and spherical particles as follows:

FM 7.10 Exhibit 6 page 6 of 21 Fundamentals of Hydrodemolition 37 FF =C'dm '[iwVF (2.29)

Further treatment - especially the replacement of the constant C - delivers the following relationship (Rehbinder, 1977):

FF =

V

-gradps 1 M (2.30)

.n.

u If this frictional force exceeds the cohesion force to neighbouring grains, the:

_,,grain in question will be removed.

m...m...m mm.m 2.2.2 Material response Important information about the response of concrete to water jet loading is stored in the structure of fracture faces. Depending on loading regime and material structure, two general types of macroscopic failure can be distinguished:

type I: sections without brittle fracture features; type II: sections with dominant brittle fracture features.

Both types, that were probably first distinguished by Nikonov (1971) for coal cutting with water jets, are illustrated in Fig. 2.13. It can be noted that type-Il failure occurred always near larger aggregate particles. Investigations on rock materials have shown that the transition from type-I to type-I1 failure depended on jet velocity and exposure time. The transition jet velocity was a function of the Figure 2.13 Failure types in concrete during hydrodemolition (Momber 2004a)

FM 7.10 Exhibit 6 page 7 of 21 38 Hydrodemolition of concrete surfaces and reinforced concrete structures 10 I

V 4

2 0

rrmcroscopic off ects of"istical range threshold Velfocity macrocopic, effects 0

100 200 300 400 Jet velocity in m/s Figure 2.14 Effect of Jet velocity on kerf width variation in concrete (Momber and Kovacevic, 1995)

Figure 2.15 Fracture surface of a cement sample cut with a high-speed water jet (Momber and Kovacevic, 1994)

(I FM 7.10 Exhibit 6 page 8 of 21 Fundamentals of Hydrodemolition 39 tensile strength, whereas the transition exposure time. followed a more complex relationship (Sugawara et al., 1998). Momber and Kovacevic (1995) have investigated the influence of jet velocity on the fracture statistics of concrete surfaces. The results illustrated in Fig. 2.14 show that the standard deviation of the kerf width started to drop at a certain jet velocity (at about 300 m/s). If this velocity was exceeded the failure process was more homogeneous. Effects due to the microstructure (e.g. microcrack distribution) were eliminated, and a macroscopic material property, lets say tensile strength, determined the material response. The average kerf width was always larger for a plain matrix material (cement matrix) than for a composite (mortar or concrete) due to the rather unrestrained fracture propagation in the matrix. This is illustrated in Fig. 2.15 showing the very smooth fracture face in a cement matrix. It may, however, be noted that the roughness of the surface increased as the fracture propagated (from top to bottom). Such effects are known from other brittle material as well (Hull, 1999: Schbnert, 1972) and is considered to be a result of crack acceleration. If two concrete materials were compared, a water jet formed wider kerfs in the material with the coarser aggregates (Momber, 1998b; Werner, 1991a).

2.2.3 Material resistance parameters Conventional properties of concrete, namely strength parameters, can not characterise the resistance against water jet erosion. This was found in very detailed studies performed by Kauw (1996) and Werner (1991a); an illustrative example is shown in Fig. 2.16. Figure 2.17 shows the situation if the compressive strength in Fig. 2.16 is replaced by the characteristic length. The characteristic length is a fracture parameter originating from a fracture model introduced for concrete by Hillerborg et al. (1976); see Section 1.4.2. The relationship between volumetric erosion rate and characteristic length is:

VM o, L,, =Cm-(2.31)

CYC whereby the very right term expresses an empirical relationship between aggregate size, compressive strength, and characteristic length (Hilsdorf and Brameshuber, 1991). It was shown that Eq. (2.31) also holds for other impact situations, namely for the comminution of concrete in a jaw breaker (Momber, 2002b). The sup-porting effect of coarse aggregates on concrete hydrodemolition, as expressed in Eq.

(2.31), was verified by Werner (1991a). The proportionality coefficient CM is considered to be a machine parameter.

Rehbinder (1978) defined a so-called 'specific erodability' to evaluate the resistance of porous solids against water jet erosion. This parameter is defined as follows:

RE_=

kp

_hM. 1 (2.32) liW " dm AP tE

FM 7.10 Exhibit 6 page 9 of 21 40 Hydrodemolition of concrete surfaces and reinforced concrete structures 1.2

.3 0

E CE

.5 C

CC Ii 0.8 0.4 I-0 0

20 40 80 Compressive strength in MPa Figure 2.16 Relationship between removal rate in concrete and compressive strength (Werner 1991) 1.2 V '

T5 E

E 2

T XI 0:8 V

A 0.4 -

Max. aggregate size in mm:

v 16 8

A 4

0 I

I I

I.

0 500 1,000 1.500 Characteristic Length in mm Figure 2.17 Relationship between removal rate in concrete and characteristic length (Momber, 2003c); values correspond to Fig. 2.16

FM 7.10 Exhibit 6 page 10 of 21 Fundamentals of Hydrodemolition 41 The higher specific erodability, the lower the resistance. The physical unit of this parameter is [m3/N-s]. The right term of Eq. (2.32) allows the experimental estimation of RE, whereby AhM/Ap is simply the progress of an erosion depth-pressure function. Specific erodability increases as grain size or viscosity decreases, and as permeability increases. It was in fact shown by Kolle and Marvin (2000) that the resistance of rock materials was higher for water as a liquid compared to liquefied carbon dioxide (having a lower viscosity). If, however, viscosity is a constant value, erosion resistance depend only on pore structure (Rehbinder, 1980):

RE 0C "M do (2.33)

Thus, resistance is proportional to pore slenderness and it decreases if - for a given pore slenderness - grain size decreases. These results are partly in agreement with experimental results obtained by Evers et al. (1982) on rocks.

2.3 Process parameter effects on material removal 2.3.1 Parameter definitions Basic target parameters include thickness of removed layers (hM), volume removal (VM), volumetric removal rate (V7M), and removal width (wM). They are illustrated in Fig. 2.18. For the erosion with a stationary water jet, these parameters are related through the following approximation:

VM-M (2.34) 4 For a given removal width, a certain concrete volume must be removed to completely erode a layer of given thickness. A maximum volume removal is desired.

The energy efficiency of the demolition process is given by the specific energy:

Ej Es - Ei (2.35)

VM This parameter should be as low as possible; its physical unit is [kJ/m 3]. The volumetric removal rate is the mass removed in a given period of time:

VM (2.36) tE

FM 7.10 Exhibit 6 page 11 of 21 42 Hydrodemolition of concrete surfaces and reinforced concrete structures p

Figure2.18 Target and process parameters for hydrodemolition Volumetric removal rate should also be maximum; its physical unit is [m3/h].

Other target parameters that may focus on the surface quality, such as roughness or cleanliness, are not considered in this paragraph. Hydrodemolition process parameters are summarised in Fig. 2.18. They can be subdivided into hydraulic parameters and performance parameters. Hydraulic parameters characterise the pump-nozzle-system; they include the following:

operating pressure (p):

volumetric flow rate (OA);

nozzle diameter (dN).

Typical relationships between these parameters are described in Chapter 3.

Performance parameters are more related to the process and include the following:

stand-off distance (x);

traverse rate (vT):

traverse increment (y);

impact angle (#);

nozzle guidance.

The traverse rate covers additional parameters, such as the number of cleaning steps, ns, and the exposure time tE.

FM 7.10 Exhibit 6 page 12 of 21 56 Hydrodemolition of concrete surfaces and reinforced concrete structures 2.4 Concrete parameter effects on material removal 2.4.1 Material failure types Compressive strength is the standard strength parameter of concrete that can be evaluated under site conditions as well. The most common method is to use cylinder cores drilled off the structure. Momber (1998b) was probably the first to suggest to use the way how a cylinder fails during the compression test as a criterion of the material behaviour during hydrodemolition. The studies showed that two general types of failure, type I and type II as listed in Table 2.5, can be distinguished during the compression testing (see Momber (20001) for more information about the testing of testing of concrete cores). If the failure type I is observed, the following features can be expected for the hydrodemolition process (compare Fig. 2.3 Oa):

the predominant material, removal mode is intergranular erosion of the cement matrix; the aggregates are completely exposed; the eroded surface is cleaved and uneven; a comparatively large number of small, regular debris is generated; the generated kerfs are deep but small; Table 2.5 Failure types during cylinder core compression testing of concrete (Momber, 1998b, 2000f)

Feature Type I Type I1 Failure mode Slow crumbling Rapid crushing Primary debris Two; symmetric More than two; asymmetric Secondary debris Many small debris; round Several larger debris; irregular Debris surface Aggregates exposed, undamaged Aggregates fractured If the failure type II is observed, the following features can be expected for the hydrodemolition process (compare Fig. 2.30b):

the predominant material removal mode is transgranular spalling of the concrete structure; the eroded surface is even; just a few gaps appear; the eroded surface preferably contains broken aggregate grains; a comparatively low number of large, irregular debris is generated; the generated kerfs are shallow but wide.

Type II-response could be observed usually with concrete mixtures containing large, irregular (broken) aggregate. It is assumed that the transition criterion, derived in Section 7.1.2 is partly responsible for this behaviour. Rather hard materials, such as many aggregates, respond elastic, whereas softer materials, such as cement matrix, show plastic response (see Table 1.3 for typical hardness values).

FM 7.10 Exhibit 6 page 13 of 21 Fundamentals of Hydrodemolition 57 2.4.2 Compressive strength effects The influence of the compressive strength on the relative erosion rate is already illustrated in Fig. 2.16. There is no general trend between both parameters and it seems that standard compressive strength is not a useful parameter to evaluate concrete resistance. An equal trend was reported by Kauw (1996). However, the figure changes if maximum aggregate diameter is considered as done in the graph in Fig. 2.35. Under this circumstances compressive strength can characterise the efficiency of hydrodemolition processes. For large aggregate diameters (16 mm) removal rate increases if compressive strength increases, whereas the opposite trend can be observed for small aggregate diameters (4 mm). For a given compressive strength, removal rate is always higher for a concrete made with coarse aggregates. The reasons for this behaviour are already outlined in Section 2.2.2. A strong and coarse concrete enables the formation of rather large radial fractures in the structure. Based on fracture mechanics arguments, Momber (2003b) introduced the following semi-empirical relationship:

dA V.3 M GC (2.48)

Note the agreement with the trends in Fig. 2.35 at least for the medium-grained and fine-grained concrete mixtures.

1.2 0

E 4)

E 0

0.8 I-C816 SAJB8 Bice

• A184 0.4 0

0 20 40 60 Compressive strength in MPa Figure 2.35 Effects of compressive strength and aggregate size on removal rate (Werner, 199 Ia)

FM 7.10 Exhibit 6 page 14 of 21 58 Hydrodemolition of concrete surfaces and reinforced concrete structures 2.4.3 Aggregate fineness effects Aspects of aggregate fineness are already illustrated in Fig. 2.35 showing that concrete mixtures with fine aggregates are more resistant against water jet erosion.

A design parameter that characterises aggregate fineness is the k-number (graining number). The larger the k-number the higher the amount of coarse particles. In Fig. 2.36, the relative removal rate is plotted against the k-numbers for certain sieve lines. A linear relationship, that proves the results obtained in Section 2.4.2, can be noted between both parameters. The coarser the mixture the higher the hydro-demolition efficiency. Another design parameter for concrete manufacture is the flour particle content which is the sum of cement and very fine aggregate particles.

It was proven by Werner (1991a) that this parameter did not affect removal rate notably.

1.2 S

0 [.4 aggregate: rhine gravel aggregate size: 0-8 mm 0

2.25 2.5 2.75 3

3.25 k-number Figure 2.36 Effect of aggregate fineness on removal rate (Werner, 199 1a) 2.4.4 Aggregate sort effects Two basic types of aggregates can be distinguished in concrete. The first type is fine aggregate which is often referred to as 'sand' only. In fact, fine aggregate consists usually of rounded river (quartz) sand. The second type is coarse aggregate which is often referred to as 'gravel'. Coarse aggregate material include in fact gravel (rounded river gravel), but also broken rocks, namely basalt or limestone. It is known that the sort of coarse aggregate affects the response of concrete to hydrodemolition. This includes not only the resistance of the material but also its failure behaviour. Figure 2.3 7 shows the effects of coarse aggregate sort and sieve

F M 7.10 Exhibit 6 page 15 of 21 Fundamentals of Hydrodemolition 59 line on the relative removal rate. Notably effects can be noted. The difference between maximum and minimum removal rate is about 470%. Removal rate is maximum for a limestone-based concrete under all circumstances, followed by the gravel-based concrete. The basalt-based concrete has the highest hydrodemolition resistance. It also seems that the effects of aggregate sort become more important for the coarser mixtures. The corresponding eroded surfaces are rather even and characterised by always broken aggregates in case of limestone, whereas they are very uneven and characterised by mainly (but not exclusively) broken aggregates in case of basalt. In case of river gravel the amount of broken aggregate was less than 30% (Werner, 1991a). Numerous aspects cause the different behaviours of the materials, among them morphology and surface energy of the aggregates and the compositions and properties of the aggregate-matrix interfaces.

15 12 0,9~1 I

I 03 0

li mestone Rhine gravel basalt Aggregate type Effect of aggregate type on removal rate (Werner, Figure 2.37 1991a) 2.4.5 Porosity effects The influence of cement paste porosity on the relative removal rate are illustrated in Fig. 2.38. The tendencies visible in that graph also apply to the relationship between capillary porosity and removal rate (Werner, 199 1a). Porosity parameters alone can not characterise the response of concrete but only if they are combined with another parameter, namely the aggregate size. For coarser concrete mixtures removal rates decrease slightly if porosity increases. The opposite trend is valid for fine concrete mixtures. However, the effects of aggregate size are much more pronounced than those of porosity.

FM 7. 10 Exhibit 6 page 16 of 21 60 Hydrodemolition of concrete surfaces and reinforced concrete structures 1.2 dA=16 mm (max.)

0.8 E

6 dA474mm (max.)

S 0.4 0

40 50 60 Cement matrix porosity in vol.-%

Figure 2.38 Effect of cement paste porosity on removal rate (Werner, 1991a) 2.4.6 Steel bar reinforcement effects Many practical hydrodemolition applications include reinforced concrete structures. Effects of steel bar reinforcement on volumetric removal rate are shown in Fig. 2.39a where the influence of the depth of reinforcement is illustrated as well. A distinct drop in efficiency can be noted if the depth of reinforcement exceeds a value of 100 mm, which applies to a plain non-reinforced concrete. Thus, reinforcement supports the removal process. The thickness of the concrete layer that covers the reinforcement does, however, not play any role. The same relationship is valid for the removal depth (Kauw, 1996). The reason for the increase in hydrodemolition efficiency due to reinforcement is the installation of weak zones in the interface between concrete and reinforcement bars (Balaguru and Shah, 1992). If a single steel bar is replaced by a bar bundle, as shown in Fig.

2.39b, removal rate drops slightly. However, if a dense reinforcement bar net exists, the concrete removal process is disturbed and 'shadows zones' form at the lower surface of the steel bars. A typical practical example is shown in Fig. 2.40. These Ishadow zones' can be avoided by using complex nozzle guiding systems that include angled jets. The effect of reinforcement becomes stronger if the structure is damaged through chlorides. In that case rust grows at the corroded steel and the stresses generated due to volume expansion form cracks in the surrounding concrete. These cracks are exploited by the water jet. An increase in efficiency of about 15% was estimated if chloride-corroded reinforced concrete was treated instead of non-corroded reinforced concrete (Kauw, 1996).

FM 7.10 Exhibit 6 a

4.8 W

E 4.4 4

\\corresponmdsto uflrekfwced cuicre~te E

4 2

b a

page 17 of 21 Fundamentals of Hydrodemolition 61 bar bar bundla Degree of reinforcement 0

0 40 80 Depth of reinforcement In mm 120 Figure 2.39 Effect of steel bar reinforcement on removal rate (Kauw, 1996) a - single reinforcement bar b-bar bundle Figure 2.40 Shadow zones, formed under reinforcement bars during hydrodemolition (Rosa, 1 991)

FM 7. 10 Exhibit 6 page 18 of 21 62 Hydrodemolitlon of concrete surfaces and reinforced concrete structures 2.4.7 Steel fibre reinforcement effects Hydrodemolition of steel fibre reinforced concrete plays a role if industrial floors or, respectively, hydraulic structures are maintained. Effects of steel fibre reinforce-ment on hydrodemolition processes are investigated by Hu et al. (2004) who pointed out that impact angle determines the influence of reinforcing fibres. At low impact angles (15°) the fibres form 'shadow zones', as illustrated in Fig. 2.41, that prevent the concrete behind the fibres from being eroded. For this reason, removal rate drops. However, the addition of fibres to a concrete also adds weak interfacial zones between matrix and fibres (Balaguru and Shah, 1992). The pressurised water penetrates these zones and causes a separation of the fibres. Therefore, these zones deteriorate the erosion resistance especially if the material is impinged by jets at vertical angles. Under such conditions, a steel fibre reinforced concrete can even more efficiently be removed by water jets than a corresponding plain concrete; this conclusion is proven in Fig. 2.42.

Figure 2.41 Shadow zones, formed behind reinforcement fibres during water let erosion (Hu et al., 2004)

FM 7.10 Exhibit 6 page 19 of 21 Fundamentals of Hydrodemolztion 63 V

U U-

.2 I'1

• 1 I

150 120 90 30 0 U+/-

Figure 2.42 et al., 2004) 44 99 140 197 243 Water jet uelocity in m/s Effect of steel fibre reinforcement on mass removal (Hu 2.5 Hydrodemolition model Labus (1984) developed models for estimating depth of cut as well as material removal rate for hydrodemolition applications. The model for calculating depth of cut is based on non-dimensional terms; it has the following structure:

r 0K, M =K

(_p_). ( dN ). L_)

Ilk (3c I '

l VT (2.49)

Figure 2.43 shows a plot of this relationship as applied to experimentally estimated data. The correlation fits the data quite well with a correlation coefficient of 0.88, and the constants in Eq. (2.49) can be estimated to K0=9.515, and K]=0.355. The model was expanded to a rotating nozzle carrier; structure and geometry are illustrated in Fig. 2.44. The final model reads as follows:

VMu

- 2 VT -K, -cosY.-(b+ s -tanY).{(

• ).(dM (1VT-OT b, *dfl)TtE)' +(

1.owT t)15

-b (2.50)

FM 7:10 Exhibit 6 64 Hydrodemolition of concrete surfaces and reinforced concrete structures page 20 of 21 100 2

.r 0 10 x= (p /or)(dN/X)(VJ'VT )1/2 Figure 2.43 Verification of Labus' (1984) hydrodemolition model VT Figure 2.44 Parameters used in Eq. (2.50)

FM 7.10 Exhibit 6 page 21 of 21 Fundamentals of Hydrodemolition 65 In contrast to the depth of cut, volumetric material removal rate is a function of time, since the relationship is an instantaneous rate. By integrating over the time, it takes for one revolution of the nozzle carrier head, the average volumetric material removal rate can be estimated. Calculations based on the model showed some good agreements with trends from experimental results; this applies in particular to nozzle diameter, pump pressure, and stand-off distance. However, from the results discussed in the previous Sections, it is clear that the model simplifies the effects of material parameters. Compressive strength alone can not determine the resistance of concrete to hydrodemolition. Aggregate type and size are more important, and at least one of these parameters must be included into Eq. (2.50).

FM 7.10 Exhibit 7 HydrodemolitionC for Removi~ng Concrete:

I page 1 of 4 6b lms anr-H ydrodemolition is a relatively new technology for the re-moval of concrete. First used in Italy in 1979 -

with proto-type equipment -

to remove concrete on the Viadotto del Lago, its develop-ment was slow until 1984, when it was introduced in Canada and Sweden.

Only within the last decade has it been commonly used. Hydrodemoli-tion involves impingement of a discrete blast of water under very high pressure in a controlled manner. The concrete is disintegrated into generally small rub-ble as a result of both the impact of the water on to the surface and the pressur-ization of internal pores, cracks, and voids.

Advantages of hydrodemolition The procedure offers many significant advantages. It is much faster than tradi-t'ý Ma i~ij `ith i

pneumatic toofs. ft is fairly quiet, is free from dust and vi-,

Sbrationand results in small size rubble that can be easily handled and reused for surface cover or base material, often without additional processing. Hydro-demolition is selective in that it will generally find and disintegrate all con-crete below a given porosity or strength level.

Bond surface The greatest advantage, however, is the provision of a superior quality surface upon which to bond new concrete. Hy-drodemolition does not "bruise" the re-suTing Isurfce" -

T tJoes n7ot cause 1a network of microcracks in the parent concrete near the bond line, as do other removal methods that involve direct impacton the surface. 1-4 It results in a very clean, rough, undulating, high am-plitude surface profile that provides the maximum obtainable bond surface area.

It generally will not damage existing reinforcing steel and will, in fact, clean the reinforcing of all corrosion prod-ucts and other contaminates.

Selective removal In addition to removing all porous or low-strength concrete, it will tend to open and rout out remaining cracks that extend into competent material. And when properly controlled in concrete with uniform porosity and strength, it will not damage or remove excessive amounts of good quality concrete that will be left with a well-prepared sur-face for immediate placement of the new overlay.

With all these advantages, one might quickly conclude that the introduction of hydrodemolition is truly a blessing.

And in most instances it is, but like all good things, there are disadvantages and limitations that sometimes con-found its use.

Disadvantages Large amounts of water are required that not only must be obtained, but han-dled and disposed of as well. The water must be directed to appropriate areas for collection and controlled so that it does not flow into areas that must re-main dry. The spent water is usually very alkaline and contains a large amount of suspended solids that will often require neutralization and separa-tion before it can be run into sanitary or storm sewers. This can be readily han-dled by the use of settling and neutral-ization tanks.

Recycling At least one hydrodemolition contrac-tor is recycling the water on some projects,5 and because it must be very clean and completely free of even minute suspended solids, an extensive reclamation plant is required. Even with the use of a polishing filter that re-moves the smallest particles (I to 5 mi-crons in size), the service life of the nozzle and some pump components is shortened due to the high abrasiveness of even very small particles when trav-eling at very high velocities that are in-herent with the procedure.

Frame assembly for hydrodemolition on inclined/vertical surfaces.

Removal The demolished rubble is usually pushed into piles on the adjacent unde-molished surface, where it can be han-dled with a small loader. This is generally accomplished with the water pressure of a fire hose or pressure washer. A fine cementitious slurry will remain long after the larger particles have been removed. If allowed to dry, a very fine powder residue will result. If the residue is not removed, it will have a deleterious effect on the bond of the repair material. It is very hard to re-move once it has been allowed to dry, but its drying can be precluded by ex-tended washing of the wet surface im-mediately after hydrodemolition with clean water until the wastewater runs clear.

The process will find and pulverize all low strength or deteriorated con-crete, and unfortunately, even compe-Au in. tIIQQR A7 Avvrtaift QQR A7

FM 7.10 Exhibit 7 page 2 of 4 tent concrete that is very porous, regardless of depth. There are many projects where the removal of all ques-tionable concrete is not acceptable due to economic or other considerations, and the removal of significant amounts of competent concrete is seldom de-sired. This is a frequent problem on structural slabs in which reinforcing is not uniformly distributed, or where po-rosity and/or strength are highly vari-able.

This can be a particular problem where a minimum thickness of removal below the reinforcing is required. To obtain the required reinforcing clear-ance in those areas of relatively high competency or strength, those areas of lower strength or lesser quality might be cut excessively deep and in extreme situations, disintegrated for their full depth.

The importance of the strength of the concrete is widely recognized in the hydrodemolition industry; however, the significant influence of porosity or microcracking is not. Terms widely used in the hydrodemolition industry to describe concrete are "sound" and "un-sound." Virtually all concrete that is disintegrated in the process is described as low strength or unsound, which of-ten is not accurate because concrete that is porous or contains microcracks can still be competent and of adequate strength.

Application Typical water pressures used in hydro-demolition will generally be within a range of 55 to 350 MPa (8000 to 50,000 psi) or more, with water flow rates on the order of 19 to 300 L (5 to 80 gal.) per minute. The amount of work accomplished is dependent on the hydrodemolition energy that is basical-ly the product of the pressure and the flow rate. Also of some influence are the nozzle design, trajectory, and dis-tance from the impingement surface.

Low flow rates generally require rela-tively high pressures, whereas a similar amount of removal can be accom-plished at lower pressures combined with higher flow rates. There are mini-mum pressures required, however, that are dependent on the concrete strength and condition.

Rock mechanics research has shown that where a minimum threshold pres-sure is required to cause erosion, the rate, and thus depth of removal, is de-pendent on rock permeability, which is a cross-sectional area of the pores. 6 As a general rule, the strength of concrete is directly related to its porosity. Lower strength concrete is usually more po-rous than high-strength concrete, and because the pressurization of pore space has a great influence on the re-moval operation, hydrodemolition effi-ciency is thus directly related to the volume of pore space and thus, con-crete strength. Rules of thumb regard-ing the required water pressure to demolish good quality concrete have been offered by various hydrodemoli-tion equipment manufacturers and con-tractors. These are generally within a range of 3 to 3-1/2 times the compres-sive strength of the concrete.

Almost from the original develop-ment of the process, there have been two different philosophies as to optimal pressure and flow rates. Equipment commonly used in the United States operates either at a pressure of about 117 MPa (17,000 psi) and a flow rate of 190 to 300 L (50 to 80 gpm) per minute, or so-called ultra-high pressure, about 230 MPa (34,000 psi) and flow rate of about 120 L (32 gal.) per minute. Con-crete can be disintegrated equally well with either set of pressure/flow param-eters, although the lower flow rates that are inherent with the ultra-high pres-sure are more gentle on the concrete.

The author suggests that whereas use of the higher flow rate equipment is prac-tical on open structures such as bridge decks, applications in parking struc-tures and other enclosed areas (where both water control and debris removal are difficult), might best be accom-plished with the ultra-high pres-sure/lower flow rate equipment.

Primary use Considerably less energy is required to disintegrate concrete that contains large and extensive voiding, such as cracking and microcracking caused by sulfate attack, alkali silica reaction, and similar mechanisms, or delamination or spalling as a result of corrosion of embedded ferrous metals or reinforcing steel. For this reason, hydrodemolition is particularly suited for removal of such deteriorated concrete, and this is its primary use.

Rate of removal Concrete removal is dependent on not only the water volume, pressure, and nozzle factors, but also on the rate of movement of the nozzle over the surface.

To maintain control of the concrete removal, all of these parameters must be set and consistently maintained. Al-though a hand-held lance can be used for small areas, hydrodemolition of large planar areas is generally facilitated AR t~nnE~rp$pInt.rnRtinnRI AR r~nnerpfai nfrnatinnnl

FM 7.10 Exhibit 7 by the use of a controlled "robot" or frame assembly onto which one or more oscillating or rotating nozzles are mounted.

Typical robots are self-propelled, hy-draulically powered vehicles. The noz-zles are mounted on a transverse bar that is commonly about 6 ft (2 m) long.

They traverse back and forth on the bar at a uniform rate that can be adjusted from about 1 to 60 seconds. In opera-tion, the robot moves away from the demolished area in discrete "steps," of-ten referred to as "indexing," which is variable from about 0.25 to 6 in. (6 to 150 mm). The traverse speed, number of traverses, and length of the step can be set into the controller, which then uniformly manages the operation.

Uniformity of removal can be en-hanced when several rapid traverses are made at a given step, instead of a sin-gle, slow traverse. Computers are used to control the parameters on some ma-chines, while manual settings are used on others. To maximize the efficiency of the diesel engines that power the high-pressure pumps, hydrodemolition equipment is generally run at a constant pressure and flow rate.

When hydrodemolition is executed in good quality concrete of uniform strength and free of cracks or micro-cracks, removal will generally be to a reasonably uniform depth. Depending on the strength of the concrete and the operating parameters that have been entered into the on-board controller, re-moval depths can be set from a few mil-limeters to about 20 cm (8 in.) in a single pass. Obviously, under similar operating parameters, much greater depths will be removed if the concrete is deteriorated or contains extensive cracks, microcracks, or other internal imperfections.

The operating parameters will thus vary from one project to another, and often vary between different areas of a given structure. Where hydrodemoli-tion is used on a repair project, typical practice is to initially set the operating parameters by trial and error prior to the production work.

A small test area 3 to 5 m2 (30 to 50 ft2) of apparently undamaged concrete will be disintegrated, with the equip-ment controller set to remove to a depth of about 12 to 25 mm (0.5 to 1 in.). If the expected results are not achieved, the settings will be adjusted and further trials completed until satisfactory re-sults are obtained. The equipment will then be relocated to a typical area of damaged concrete and a similar small area removed. Any further adjustments required will be made until the desired result is obtained. Ideally, this will completely remove all damaged con-crete. However, in some cases, this may not be desired or practical.

Concrete of varying strength or a nonuniform distribution of internal de-fects will result in removal to varying depths, and in some cases, the entire thickness of the section. Where dam-aged concrete exists only on the surface and is underlain by uniformly good quality concrete, the parameters can be set to remove only defective materials.

However, if the thickness of the defec-tive concrete or the porosity or strength of the underlying undamaged concrete varies, periodic adjustment of the pa-rameters will be required and consider-able variation of the depth of cut will result. In extreme cases, hydrodemoli-tion may prove to be an inappropriate removal method.

page 3 of 4 Case studies In one instance, hydrodemolition was performed to remove the deteriorated surface concrete of a steel girder con-crete deck bridge. Following the typi-cal robot trial and error period, an initial production pass was made be-tween the girders. Removal was as de-sired and hydrodemolition appeared to be a wise choice. However, when the next pass was made, which was over a beam, the concrete was blasted away for the full depth of the deck.

The excessive removal was due to the fact that the concrete contained many cracks for its full depth over the steel girders as a result of corrosion of the closely spaced shear pins, which were attached to the top of the steel sections.

It would have been desirable to remove the faulty concrete to full depth, but practicality did not allow that. If all concrete bearing on the girders had been removed there would not be any support for the remaining deck sec-tions. Furthermore, environmental re-strictions prohibited the discharge of demolition debris under the bridge. Hy-drodemoliton was not an appropriate removal method for this project.

In another case, hydrodemolition was specified as the removal method for de-fective concrete on the decks of a park-ing structure. On commencement of the work, it was found that the depth of de-terioration varied greatly, and in many areas the concrete was demolished full depth (blow through). While it was ad-vantageous to find and remove all of the deteriorated concrete, a consider-able amount of competent concrete was removed as well. The contractor's shoring method had to be changed, and extensive formwork was required, AimenemqtQQR A

FM 7.10 Exhibit 7 page 4 of 4 greatly increasing the cost of the work.

It is believed by many in the industry that the removal depth is related strictly to concrete strength, and that all con-crete disintegrated by the water jet is thus either low strength or deteriorated.

While that is sometimes the case, it is not true in all instances. Very compe-tent and even high-strength concrete can have a system of microcracks, the distribution of which is not necessarily uniform. Also, significant variation of strength or content of entrained or en-trapped air can exist even in very corn-porosity is far greater from the stand-point of the concrete's susceptibility to disintegration by high pressure water t1ýaD.t iýýbV.i.*s. ýtjejngtl,.Taat*

fect of high pressure water can be quite variable even on concrete of high and generally uniform strength. To com-pensate for these factors, the settings of the robot may require frequent adjust-ment. Accordingly, the quality of the finished cut relies heavily on the ability and attentiveness of the hydrodemoli-tion operator.

Due to these factors, cost overruns and claims have occurred on several hydrodemolition projects due to exces-sive removal quantities. It is thus im-portant that designers and specifiers be aware of possible variability of the con-crete porosity and/or strength when considering hydrodemolition. Budget and contract documents should allow for appropriate additions and adjust-ments as applicable.

Conclusions The advantages of hydrodemolition are many, and the ability to remove all con-crete that has been subjected to deterio-ration, or that which is below an approximate strength level is certainly beneficial. However, when considering

-WYy(7ce~iTion,kVeep Ni~n mnd tn~tat t

'water will pressurize all voids it is able:

to access, even those that might exist ats

.a greater depth than the planned remov-:

al. Likewise, where the compressivel strength, porosity, or defect level of the:

'concrete is variable, the depth of re-:

,moval will likewise va '..

Any areas that have cracking or ex-cessive porosity for full depth will likely be completely penetrated. This should be considered prior to hydrodemoli-tion. Consideration must be directed to the likelihood of such occurrences, so that both the budget and timely remedi-al action can be taken if necessary.

References I. Hindo, K. R., "In-Place Bond Testing and Surface Preparation of Concrete," Concrete In-ternational, V. 12, No. 4, April 1990, pp. 46-48.

2. Silfwerbrand, J., "Improving Concrete Bond in Repaired Bridge Decks," Concrete Interna-tional. V. 12, No. 9, Sept. 1990.

3. lngvarsson, H., and Eriksson, B., "Hydro-demolition for Bridge Repairs," Nordisk Betong, No. 2-3, Stockholm, Sweden, 1988, pp. 49-54.

4. Warner, J., et al.. "Surface Preparation for Overlays," Concrete International, V. 20, No. 5, May 1998, pp. 43-46.

5. Morin, M., and Tuttle, T., "Recycling Hydro-demolition Wastewater -

The Hidden Advan-tage," Concrete Repair Bulletin, V. 10, No. 2, March-April 1997, International Concrete Repair Institute, Sterling, Va.

6. Rehbinder, G., "Theory About Cutting Rock With a Water Jet," Rock Mechanics, V. 12, 1980, pp. 247-257.

Selected for reader interest by the editors.

The topic covered by this article was present-ed at the ACI Seattle,'WMsh., con-ention in, April 1997..

ACd Fellow James, Warnier is a consult-ing engirieer based' in Mariposa, Califor-nia. His international practice involves analysis and solu-tion of foundation, structural, and material problems.,,

He is a member of ACI'Committee 364, Rehabilitation.

rnnr-rPfP1nfPrn=tinnn1

FM 7.10 Exhibit 8 page 1 of 18 MriIODOT Research, Development and Technology RDT 02-002 Hydrodemolition and Repair of Bridge Decks RI 97-025 December, 2002

FM 7.10 Exhibit 8 page 2 of 18 TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

2. Government Accession No.

3. Recipient's Catalog No.

RDT02-002

4. Title and Subtitle

5. Report Date Hydrodemolition and Repair of Bridge Decks December 16, 2002

6. Performing Organization Code

7. Author(s)

John D. Wenzlick, P.E.

8. Performing Organization Report No.

R197-025

9. Performing Organization Name and Address

10. Work Unit No.

Missouri Department of Transportation Research, Development and Technology

11. Contract or Grant No.

P. 0. Box 270-Jefferson City, MO 65102

12. Sponsoring Agency Name and Address

13. Type of Report and Period Covered Missouri Department of Transportation Final Report Research, Development and Technology

14. Sponsoring Agency Code P. 0. Box 270-Jefferson City, MO 65102

15. Supplementary Notes The investigation was conducted in cooperation with the U. S. Department of Transportation, Federal Highway Administration.

16. Abstract The use of modem hydrodemolition equipment for removal of deteriorated concrete and preparation of bridge decks for concrete repair. Comparison of hydrodemolition to conventional sawing and jackhammer removal concerning cost and harm to remaining concrete.

17. Key Words

18. Distribution Statement Hydrodemolition, hydroblasting, concrete removal, concrete repair, No restrictions. This document is available to the public bridge decks through National Technical Information Center, Springfield, Virginia 22161

19. Security Classification (of this report)

20. Security Classification (of this page)

21. No. of Pages

22. Price Unclassified Unclassified 36 Form DOT F 1700.7 (06/98)

FM 7.10 Exhibit 8 page 3 of 18 R197-025 Hydrodemolition and Repair of Bridge Decks Final Report MISSOURI DEPARTMENT OF TRANSPORTATION RESEARCH, DEVELOPMENT AND TECHNOLOGY BY: John D. Wenzlick, P.E.

Acknowledgment to:

Pat Martens, Dave O'Connor JEFFERSON CITY, MISSOURI DATE SUBMITTED: December 16, 2002 The opinions, findings, and conclusions expressed in this publication are those of the principal investigators and the Missouri Department of Transportation; Research, Development and Technology.

They are not necessarily those of the U.S. Department of Transportation, Federal Highway Administration. This report does not constitute a standard or regulation.

FM 7.10 Exhibit 8 page 4 of 18 EXECUTIVE

SUMMARY

It was the intent of this study to prove that hydrodemolition is a better alternative to removing deteriorated concrete form bridge decks than conventional mechanical methods such as jackhammers. Jackhammers can cause microfracturing of the concrete left in place.

Microfractures in the remaining deck can cause premature loss of bond in the patches or the overlaid surface to which a large investment has been applied in hopes of getting a rehabilitated bridge deck that will last another twenty to thirty years. MoDOT over the past ten years has experienced extensive cracking and debonding of its dense concrete bridge overlays leading to premature deterioration of the rehabilitated decks, well before the end of their design life.

Hydrodemolition could help solve these problems in future bridge rehabilitation projects.

Additionally, after the hydro-blasted material is removed, hydrodemolition leaves the substrate deck clean, it removes all corrosion from the rebar, and the deck is ready for new concrete to be poured. Additional mechanical cleaning and sandblasting of the concrete surface and rebar is needed with mechanical removal methods. Hydrodemolition has generally been bid cheaper than conventional mechanical methods but is overall more expensive because of mobilization costs and limited availability of hydroblasting equipment and hydrodemolition contractors close to Missouri. Other items like traffic control and staged construction can be an extra cost because it is necessary to have larger areas of bridge deck closed to do hydrodemolition than it is for mechanical methods.

The practices of Missouri's adjoining states were surveyed pertaining to the use of hydrodemolition. Most state specifications use it as an equal alternative to mechanical methods.

This study looked at hydrodemolition projects done in Missouri, first by maintenance starting in 1996 in the St. Louis area and continuing on maintenance projects there through 1999. It also looked at the first, and so far only, project designating hydrodemolition as the only method of concrete removal let by construction contract on Route 1-44 near Springfield in Green County in 1998. Costs for all the projects done by MoDOT are presented. Costs for hydrodemolition ranged from $ 1.25 to $ 3.50 per square foot ($3.50 bid on the 1-44 construction project mentioned above) compared to $ 28.79 to $ 32.99 per square foot for conventional removal. A study of the relative damage done to the concrete left in place was done using direct tension or pull off tests. Generally the testing showed pulloff strengths around 150, psi (pounds per square inch) versus 125 psi for the mechanically prepared concrete. This was not as high as expected since a Swedish study had shown strengths up to 300 psi.

The limited bond testing done did not show large gains in strength over conventional removal but it is believed further testing would show better results. However, hydrodemolition does provide less damage to the remaining concrete and a cleaner surface ready for the patching or overlay concrete to stick to in a third of the time as conventional removal. It is recommended that more maintenance and construction contracts be advertised designating hydrodemolition as the only option for removing deteriorated concrete. It is proposed that, on all bridges that meet the criteria for ease of hydrodemolition, all contracts in 2003 be let specifying hydrodemolition exclusively. (It is estimated this would be about twenty five percent (25%) of bridges contracted to be rehabilitated or widened.) This will foster more availability of this equipment and contractors using it. A report on costs savings and life-cycle costs would be prepared from these 2003 jobs to verify how superior and cost effective hydrodemolition is compared to mechanical methods in ensuring long lasting concrete repairs.

FM 7.10 Exhibit 8 page 5 of 18 TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES INTRODUCTION AND OBJECTIVES TECHNICAL APPROACH RESULTS AND DISCUSSION (EVALUATION)

HYDRODEMOLITION, FALL 1996 HYDRODEMOLITION, 1997 HYDRODEMOLITION, 1998 HYDRODEMOLITION, 1999 BID PRICES FOR HYDRODEMOLITION TESTING PROCEDURES CONCLUSIONS RECOMMENDATIONS PRINCIPAL INVESTIGATOR AND PROJECT MEMBERS AFFECTED BUSINESS UNITS AND PRINCIPAL CONTACT TECHNOLOGY TRANSFER BIBLIOGRAPHY II II 2

2 2

4 5

6 7

7 10 11 11 11 11 12 APPENDIX A APPENDIX B APPENDIX C WORK PLAN HYDRODEMOLITION SPECIFICATION - JOB J810674 "GENERAL SPECIAL PROVISIONS - BRIDGES" FOR REPAIRING CONCRETE" i

FM 7.10 Exhibit 8 page 6 of 18 LIST OF TABLES Table 1: Hydrodemolition Specifications of Other States 1

Table 2: Bid Prices for Hydrodemolition 7

Table 3: Pull-Off Strength - Hydrodemolition, 8

Bridge L-868, St. Mary's Way /1-44, Franklin Co.

Table 4: Pull-Off Strength - Hydrodemolition, 9

Bridge A-174, 1-44 EBL, Greene Co.

Table 5: Pull-Off Strength - Hydrodemolition, 9

Bridge A-135RP, 1-70 WBL, St. Louis Co Table 6: Pull-Off Strength - Mechanical Methods, 10 Bridge A-241, 1-270 WBL, St. Louis Co.

LIST OF FIGURES Figure 1: Hydrodemolition machine in action 3

Figure 2: Filtering waste water 3

Figure 3: Self contained vacuum truck 3

Figure 4: Vacuum truck with hose on boom 3

Figure 5: Traffic control and containment of blast water 4

Figure 6: Finished hydrodemolition surface 5

Figure 7: Poor concrete left after hydrodemolition at 13,000 psi 5

ii

FM 7.10 Exhibit 8 page 7 of 18 Introduction and Objectives Hydrodemolition is a faster, cleaner and better way to remove deteriorated concrete from bridge decks in order to patch or rehabilitate the driving surface. The basic steps in the hydrodemolition of concrete bridge decks are as follows. First scarifying of the original bridge deck is required before hydrodemolition of the surface. Hydrodemolition is done with a computerized, self-propelled robotic machine utilizing a high pressure water jet stream in the range of 15,000 to 20,000 PSI and usually removes all unsound concrete in one pass. If required, hand held high pressure wands or 35 lb. maximum jackhammers shall be used in areas inaccessible to the hydrodemolition equipment. The contractor removes the hydrodemolition debris with vacuum equipment before the debris and water is allowed to dry on the deck surface. The contractor will take steps to prevent damage to existing reinforcing steel and not place wheels from heavy equipment, such as vacuum trucks, on areas where the top layer of slab reinforcement has been left unsupported by the hydrodemolition process. After debris is removed the deck surface and patches and the exposed reinforcing steel is usually clean and ready for concrete placement.

MoDOT's Job Special Provision (JSP) allows in areas where removal of unsound concrete does not expose the bottom mat of reinforcing in the deck to be patched with latex modified concrete and placed monolithically with the concrete wearing surface.

The hydrodemolition process allows several steps needed in conventional removal to be eliminated. Sounding and marking of delaminated areas is not necessary because after the hydrodemolition equipment is correctly calibrated it will automatically remove any delaminated or deteriorated concrete. This eliminates the need to saw cut around patching areas as needed with conventional jackhammer methods. Sandblasting of rusty or dirty reinforcing steel is not needed because it is cleaned at the time of hydrodemolition. Because of the very good bonding surface left by the hydrodemolition patches are allowed to be placed, if the bottom reinforcement hasn't been exposed, at the same time as the wearing surface concrete. This step alone eliminates the time and labor needed for a separate patching operation and the time to wait for patches to cure before being able to place the wearing surface. The only additional needs for hydrodemolition are a large water supply and the control of runoff water.

It was intended to prove that hydrodemolition is a more efficient and less destructive method than using jackhammers for removing deteriorated concrete from reinforced concrete bridge decks. In hydrodemolition all the deteriorated concrete is removed, the reinforcing steel is cleaned, and the remaining concrete is not left with micro-fractures as it is when jackhammers are used. MoDOT has had a problem over the last ten years or so with premature failures of rehabilitated bridge decks using dense concrete overlays. There have been problems with excessive cracking and with debonding of the overlay from the original deck concrete. These problems have occurred with all types of overlays, latex modified concrete, low slump concrete and silica fume concrete. Curing of the concrete and other factors are causing the cracking problem, but loss of bond could be alleviated by using hydrodemolition instead of conventional mechanical methods of removing deteriorated concrete. Hydrodemolition is more expensive at this time because of the expense of the equipment and the short supply of contractors doing this kind of work. For this reason mobilization costs are high, however, these costs have come down recently with more equipment being manufactured and more contractors now getting into this type of work.

I

FM 7.10 Exhibit 8 page 8 of 18 A review of concrete removal practices of the adjacent states was made. Table 1 below lists the states contacted All the states that specify hydrodemolition, allow either it or conventional mechanical methods except Kansas. If a bridge deck is to receive a concrete overlay, Kansas DOT only allows hydrodemolition. Two of the states don't specify hydrodemolition at all.

Table 1: Hydrodemolition Specifications of Other States State Hydrodemolition Specifications Missouri Special Provision if an overlay is involved (includes hydrodemolition as alternate for conventional)

Kansas Specification 724 (hydrodemolition only for bridge overlays)

Illinois In Deck Slab Repair Specification (includes hydrodemolition and conventional both)

Iowa Specification 2413 (includes hydrodemolition and conventional both)

Arkansas Nothing found Nebraska Does not indicate use of hydrodemolition Technical Approach This study was set up to observe the hydrodemolition process and become more familiar with the equipment and its operation. Through pull-off or direct tension testing before and after removal of the deteriorated concrete and after patching, this study was designed to determine the effectiveness of hydrodemolition over conventional jackhammer methods in leaving a better substrate on which to apply new concrete. Hydrodemolition can reduce micro-fracturing while removing all of the deteriorated concrete. Also price comparisons between the two methods were made using costs from several maintenance projects and also one bridge rehabilitation construction project and previous maintenance and contract work.

Results and Discussion (Evaluation)

Hydrodemolition, Fall 1996 The Missouri Department of Transportation, MoDOT, first tried hydrodemolition for the repair and concrete overlay of a bridge by maintenance contract on St. Mary's Way over 1-44 in Franklin County just southwest of St. Louis. (Figure 1) The cost was $ 12,000 for one pass of 2

FM 7.10 Exhibit 8 page 9 of 18 the hydroblast machine over the whole bridge, 5,800 square feet, or $ 2.06/sf. This price included vacuuming up the debris and dumped on site. MoDOT maintenance forces were used to haul the rubble away. MoDOT also had to set up straw bail dams to catch the solids in the water before it was allowed to enter the roadway ditch. The effluent was checked by MoDOT to supply information on turbidity to the Missouri Department of Natural Resources to make sure it passed clean water standards.

Figure 1: Hydro machine in action. (Note the rubble in Front, compared to the milled deck behind.)

Figgure 2: Note tfe straw balls covered wI the end of the bridge to filter waste water.

The biggest concern from this first project was about the vacuum truck backing onto the rebar mat and bending it down where concrete was removed below the top mat. The heavy truck (Figure 3) worked all right here. However, if a lot of reinforcing steel is showing, plywood would need to be placed under the truck tires to distribute the load better. Alternately, a hand guided vacuum, or one with a boom (Figure 4), which didn't have to travel over the rebar could be substituted for the truck.

Figure 3: Heavy, self contained vacuum truck.

Note: vacuum nozzle located in front of the rear wheel works very well to pick up debris.

Figure 4: Vacuum truck with hose on boom; can stay off rebar mat but is slower picking up debris.

3

FM 7.10 Exhibit 8 page 10 of 18 Hydrodemolition, 1997 A second hydrodemolition project was done again by maintenance contract in the summer of 1997 on Bridge L-896, Franklin County, Rt. 100/1-44 only about a mile from the first bridge.

Hydroblasting of 5,800 sf. was done for $ 1.25/sf. or a total price of $ 7250. This bridge received a full concrete overlay like Bridge L-868. It demonstrated that MoDOT could extend the life of a second bridge deck by relatively low cost hydrodemolition and repair with a dense concrete overlay. On both the 1996 bridge and this one, one step in the repair process, pouring concrete patches before overlaying, was also eliminated by pouring the patches and overlay at the same time (a monolithic concrete placement).

Under the same bid, hydrodemolition of unsound concrete and patching of 6 other bridges decks on the aging 1-70 corridor in St. Louis was completed in a third of the time of conventional jack hammer repair done by MoDOT maintenance crews. (Figure 5) Prices were bid lump sum for each bridge and depended on the amount of square feet patched, they ranged from $ 1.33 sf.

($ 12/sy.) to $ 8.33/sf. ($ 75/sy.).

Repairing these 7 bridge decks (the complete overlay of Br. L-868 plus patching of 6 others) was done in 20 working days using hydrodemolition. It would have taken 60 days by normal hand methods.

Figure 5: Shows traffic control and containment of blast water while hydroblasting for patches in two center lanes of a four lane bridge.

4

FM 7.10 Exhibit 8 page 11 of 18 Hydrodemolition, 1998 In 1998 the first construction contract specifying use of hydrodemolition was let for bridges AO 1741 E and AO 1741 W on Route 1-44, Greene County near Springfield. This was also MoDOT's first contract allowing a monolithic pour after removal of deteriorated deck with a Latex Modified Concrete overlay. This eliminated the usual patching step in between by filling of excavated areas and overlaying with new concrete at the same time.

Because of staged construction, this project required two mobilizations of the hydrodemolition equipment. The westbound bridge was closed to traffic in 1998. The whole deck of the westbound structure, 7,100 sf., was completed. The hydrodemolition contractor returned in early 1999 to do the 7,100 sf. of the eastbound bridge (Figure 6). Even though two trips were required, it is believed the bid was lower than expected due to being able to hydroblast a fairly large amount of surface each trip. Also, no traffic control was needed since the bridges were Figure 6: Finished hydrodemolition of half (background) of Bridge A0174 E. In the foreground, new latex modified concrete overlay. Note: 2" core holes in the overlay are where pull-off tests for direct tension were taken.

5

FM 7.10 Exhibit 8 page 12 of 18 Hydrodemolition 1999 Bridge A-185R Ramp on Route 1-70, St. Louis City was shut down due to construction in the area. MoDOT maintenance forces took this opportunity to again use hydrodemolition work to repair this bridge deck. A maintenance contract was let for hydrodemolition. The cost was

$29.09/sy ($ 3.23/sf) which compared well with the only construction project MoDOT had let with hydrodemolition, discussed above, which bid at $ 31.50/sy ($ 3.50/sf ). Poor concrete and a thin 6 1/2" upper deck on this type box girder bridge made it necessary to make two passes with the hydrodemolition machine set at 13,000 psi. (see Figure 7) One pass at the normal setting of 17,000 - 18,000 psi would have blown through the poor quality concrete.

Hydrodemolition makes it easier to regulate than conventional methods with regards to how much concrete is removed when it's necessary to patch and keep open a badly deteriorated deck.

Figure 7: Poor concrete and a thin 61/2" upper deck made it necessary to do hydrodemolition at 13,000 psi. This is after the first pass (Note how clean the re-steel is in the foreground on the left). A second pass with the hydro-blaster was necessary to remove the island of unsound concrete left over the rebar in the center of the photo.

6

FM 7.10 Exhibit 8 page 13 of 18 Bid prices for Hydrodemolition On maintenance contracts the bid prices have stayed consistently low $ 1.25/sf. to $3.23/sf.

Only one construction project has been let and the price was $ 3.50/sf, this is almost an order of ten times less than mechanical removal, which bid for $ 28.79/sf for partial depth and $ 32.99/sf for full depth removal. The limiting factor in getting hydrodemolition bid in Missouri has been the lack of contractors and hydro machinery and the need for numerous mobilizations of the equipment on most projects. It should be noted that allowing larger areas of deck to be opened up for hydrodemolition may cause additional traffic control costs. A summary of the costs of hydrodemolition for the study projects is listed in Table2 below.

Table 2: Bid prices for Hydrodemolition Location Date Total Area Bid Price Total Cost Bridge L-868, St. Marys/I-44, Franklin Co.

Fall 1996 5,800 sf.

$ 2.06/sf*

$12,000 Bridge L-896, Rt. 100/1-44, Franklin Co Summer 1997 5,800 sf.

$ 1.25/sf.*

$7,250 Patching of several bridges

$12/sy to $ 75/sy**

on 1-70, St. Louis Summer 1997

($ 1.33/sf to $ 8.33 /sf)

Bridge A01741 E&W, 1-44, Greene County 1998 14,220sf. (1580sy.)

$ 3.50/sf($ 31.50/sy.)

$49,770 1 st construction contract specifying use of hydrodemolition.

Bridge A-I85R Ramp 1-70, 1

F St. Louis City 1999

$ 3.23/sf. ($ 29.09/sy.)

• One pass over whole bridge, vacuumed up and dumped on site. Maintenance hauled away rubble.

• • Prices ranged from $ 12/sy to $ 75/sy depending on the amount of area contracted TESTING PROCEDURES

_Hvdrodemolition does not cause damage to the rood concrete left in pnlace Milling and jack hammering leave micro-fractures in the surface of the concrete, which can cause poor bond to patching or overlay material. Note: during surface preparation the milling step cannot be excluded if specifying hydrodemolition because the hydroblasting requires a rough concrete surface to initiate the removal process. Milling is a separate bid item and no savings with regard

, to "TlT1]ig le"~ea1A'z~e~fl'*7**ng F de, J

ihion" over'j ack hammring]." "lIo~v

,a'lT micro-*

fracturing caused from milling is later removed by hydrodemolition leaving a more sound Ssubstrate.

Direct tension or pull off strength testing was done on each project using the ACI-503R, Field Test for Surface Soundness and Adhesion, method. Testing was performed on the original concrete after milling and either hydrodemolition or jackhammer removal. Additionally, direct tension tests were taken through the overlay and patch material into the original deck after the new concrete reached required strength.

The limited testing performed on these bridges showed hydrodemolition resulting in average pull 7

FM 7.10 Exhibit 8 page 14 of 18 off strengths of the bond between the overlay and the hydrodemolition prepared deck to be 151 psi and 166 psi on maintenance work on bridges L-868 and A-135 Ramp respectively. (These pull off tests were taken after milling, hydrodemolition and the deck overlay was placed - see Notes: on Table 3 and Table 5) On the one construction contract using hydrodemolition, the average pull off strength was 121 psi. on Br. A-174. (see Notes: on Table 4) This compares to 80 psi pull off strength on Br. A-241 using jackhammer removal for patching and 140 psi pull off strength on a milled only area. (no jackhammer or hydrodemolition done in this area - see Notes:

on Table 6) It was expected to get higher pull off strengths using hydrodemolition as the literature said strengths were up to twice as strong as surfaces using mechanical methods. It is believed that with a larger number of tests and with a more agile testing machine better results wouldl be obtained. The base plate of the tester used is very large (1 ft. x 1 ft) and testing on rough surfaces and around rebar made it hard to always ensure it was normal to the surface.

Sweden has obtained pull off strengths up to 300 psi on testing of over 300 hydro blasted decks.

(Improving Concrete Bond in Repaired Decks. Concrete International, September 1990)

Values for MoDOT testing are included in the tables below.

Table 3: Pull-Off Strength - Hydrodemolition Bridge L-868, St. Mary's Way/I-44, Franklin Co.

Tested: 10/2/97 Core No. Tension, # Pull Off, psi Location of Failure 1

745 237 1/4" into overlay 2

805 256 Interface, 50% old deck, 50% in overlay 3

1080 344 1/4" into overlay 4

230 73 Interface, only small part of overlay attached 5

500 159 Middle of overlay, 1 3/4' down into overlay 6

390 124 Interface about 75% old concrete Avg. Pull Off Strength = 199 psi Note: ACI calls for a minimum PO strength of 100psi. Only cores that break off at the interface give a true bond strength; Average of cores 2, 4 & 6 = 151 psi.

8

FM 7.10 Exhibit 8 page 15 of 18 Bridge A-174, 1-44 EBL, Greene Co Tested: 07/16/1999 (Constructon hydrodemolitrion contract with 1.75 in. latex modified concrete overlay.

Location No.Core No. Tension, #Pull Off, psi Location of Failure 1

1 180 57 100% in base 1

2 1020 325 100% in base 1

3 420 134 100% interface Avg. Pull Off Strengt = 172 psi 2

1 340 108 100% interface 2

2 520 166 Not recorded 2

3 120 38 50% old patch/50% interface Avg. Pull Off Strength = 104 psi 3

1 320 102 100% in base 3

2 420 134 100% in base 3

3 960 306 100% in base Avg. Pull Off Strength = 180 psi Note: ACI calls for a minimum PO strength of 100psi. Only cores that break off at the interface give a true bond strength; location 1, core 3 and location 2 core 1: average 121 psi.

Table 5: Pull-Off Strength - Hydrodemolition Pull-Off Strength Bridge A-135RP, 1-70 WBL, ST. Louis Co Tested: 3/01/00 Location No.Core No Tension #Pull Off, psi Location of Failure 1

1 760 242 100% in epoxy 1

2 640 204 Interface, 50% in base 1

3 400 127 100% interface 1

4 980 312 100% in epoxy Avg. Pull Off Strength = 191 psi Note: Only cores that break off at the interface give a true bond strength; location 1, core 2 and location 1, core 3 average 166 psi 9

FM 7.10 Exhibit 8 page 16 of 18 Table 6: Pull-Off Strength - Mechanical Methods Stage 3 - Silica Fume Overlay poured March 22, 2000, Control: Mechanical equipment used for concrete removal Location: Bridge A-241, 1-270 wbl, St. Louis Co.

Tested: 05/24/2000

{

Pul Of sLocation of Failure Location No. Core No. ýension, # Pull Off, psi I Pull Off, Psi Milinnf"M nm arnx=l~ nn irnf nf tr-hl broke in orig. concrete-1 7/8" thick, 1 (sf/patch) 3 120 38

_sf patch 2 1/4" thick 1 (sf/patch) 4 380 121 80 broke at epoxy, 2" sf & 2 1/4" limestone patch broke @ interface of overlay & orig. deck, 1 (sf/patch) 1 100 32

-no patch-2" thick sf 1 (sf/patch) 2 400 127 broke at interface-2"sfno patch Silicafume overlay on Do of milled surface only)

{broke R interface w/deck, 2 (sound sf) 5 360 115

_very smooth-2 1/16" sf overlay

[broke @ interface w/deck, 2 (sound sf) 6 400 127 140 jInterface rough 2 1/2" thick sf

[broke 100% A, interface w/orig. deck, 2 (sound sf) 7 560 178 linterface smooth surface-sf = silica fume overlay Note: Only cores that break off at the interface give a true bond strength; For the cores over patches, coreland core 2 average 80 psi, Conclusions The follow findings were made from monitoring of various maintenance and construction contracts using hydrodemolition:

1. Cost can range from $ 12/sy ($ 1.33/sf) to $ 75/sy ($ 8.33/sf) depending on the amount of area contracted.

2. IHydrodemolition does not cause damage to the good concrete left in place. Milling and jack hammering leave micro-fractures in the surface of the concrete, which can cause aI_..

poor =bonu to.!paten~ing or overiay= material=.........

3. In direct tension or pull off testing, limited field data has shown pulloff strengths between overlays or patches and surfaces prepared by hydrodemolition of (121 -161 psi) slightly higher than a jack hammered surface (80 psi) or a milled only surface (140 psi).

4. Pulloff strengths for hydrodemolition prepared surfaces averaged 150 psi, which was not as high as expected. It is believed a bigger sample is needed and that with more testing the average would rise. Also there were problems keeping the pulloff tester at a perfect right angle to surface, which would cause lower readings. Sweden claims of pull off strengths for hydrodemolition prepared decks at least twice as strong as those od decks prepared using conventional methods. Sweeden has used hydrodemolition on over 300 bridges. 1

5. Hydrodemolition leaves the rebar and deck ready in one operation.

10

FM 7.10 Exhibit 8 page 17 6f 18 Recommendations 1.)

Results of this study show that hydrodemolition should be used on all construction projects where the cost of mobilization isn't prohibitive. Costs can become prohibitive because of many small spread out work zones caused by zoned repairs on structures with concrete superstructures integral with the deck. Costs also go up because of staged construction and difficult traffic control plans, or because hydrodemolition equipment isn't available in the area.

However, the advantages gained from not damaging the remaining concrete as well as the speed of preparation of the existing reinforced concrete will far outweigh any additional costs and can save MoDOT and the contractors money. It is estimated that at least one-quarter of the bridge decks contracted by MoDOT for rehabilitation each year meet the criteria that could use hydro demolition and even be more economical than conventional jackhammer methods. It is believed that equipment and the number of contractors available to do hydrodemolition should increase and the bid prices go down as this new technology establishes itself.

2.)

Maintenance bridge repair crews statewide should try to employ hydrodemolition whenever possible on bridge decks with good service ratings that are expected to remain in use for a long time. A video recording of the process was made on the first bridge using hydrodemolition in the St. Louis district and has been distributed to all district maintenance units to let them familiarize themselves with the process.

Principal Investigator and Project Members John Wenzlick was the principal investigator for RDT with help in reporting by Anika Careaga and field testing by Steven Clark. Hydrodemolition work in the St. Louis area was initiated and coordinated by Pat Martens, District Bridge Inspection Engineer. Testing done on 1-44, Greene County project was coordinated through Jim Blackburn, Resident Engineer in Buffalo, Mo.

Testing done on the 1-270, St. Louis County was coordinated through Lucy Smith, Senior Construction Inspector.

Affected Business Units and Principal Contact All district maintenance and design personnel as well as Bridge Design should consider the use of new hydrodemolition techniques for repair of bridge decks. John 'JD' Wenzlick in Research, Development and Technology or Pat Martens of District 6 Maintenance can be contacted for further information.

Technology Transfer Designers should use this report to promote the use of hydrodemolition in areas where it can be expected that the bid price will be close to that of conventional mechanical repair methods.

Contractors should be more receptive when they find how much quicker it is and also the little or no preparatory work needed before pouring new concrete. Reduced time and preparation costs should outweigh the higher capital equipment costs as more subcontractors get into the hydrodemolition business.

1I

FM 7.10 Exhibit 8 page 18 of 18 Districts wanting to do hydrodemolition with their own maintenance forces already have an excellent videotape describing the process that was distributed statewide back in 1997. The video covers all steps in the hydrodemolition process, just as they were done on the St. Marys/I-44 bridge in Franklin County.

Bibliography

1. Silfwerbrand, Johan Improving Concrete Bond in Repaired Decks. Concrete International, September 1990

2. American Concrete Institute, P.O. Box 9094, Farmington Hills, MI 48333, ACI Manual of Concrete Practice, 1997 12