ML20049J103

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Evaluation of Cracking in Svc Water Pump Structure at Midland Plant
ML20049J103
Person / Time
Site: Midland
Issue date: 02/28/1982
From: Corley W, Fiorato A
CONSTRUCTION TECHNOLOGY LABORATORIES, INC.
To:
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ML20049J102 List:
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NUDOCS 8203120066
Download: ML20049J103 (68)
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o s i Report to CONSUMERS POWER COMPANY Jackson, Michigan s EVALUATION OF CRACKING IN SERVICE WATER PUMP STRUCTURE AT MIDLAND PLANT \\ by W. G. Corley and A. E. Fiorato L i Submitted by' CONSTRUCTIfN TECHNOLOGY LABORATORIES A Division of the Portland Cement Association 5420 Old Orchard Road Skokie, Illinois 60077 February 1982 7

e e TABLE OF CONTENTS. Page INTRODUCTION 1 DESCRIPTION CF STRUCTURE 1 EVALUATION OF CRACKING 11 Bechtel Crack Mapping 11 CTL Observations. . ~. i 25 SIGNIFICANCE OF CRACKS 29 RECOMMENDED PROGRAM FOR MONITORING STRUCTURAL INTEGRITY 34 Displacement Monitoring 34 Crack Monitoring 37

SUMMARY

AND CONCLUSIONS 39 itEFERENCES 40 APPENDIX A - STRENGTH OF CRACKED REINFORCED CONCRnTE MEMBERS A-1 4 i ( -i-construction technology laboratories

a EVALUATICN OF CRACKING IN SERVICE ~ WATER PUMP STRUCTURE AT MIDLAND PLANT by W. G. Corley and A. E. Florato* INTRODUCTION This report presents an evaluation of the significance of cracks observed in the Service Water Pump' Structure located at Midland Nuclear Power Plant Units 1 and 2. Observed cracks in the structure are described and significance of the cracks with regard to future load carrying capacity is discussed. In addition, a program for monitoring structural integrity during implementation of remedial measures is described. Remedial measures include underpinning the north portion of the structure. DESCRIPTION OF STRUCTURE A site plan for the Midland Plant is shown in Fig. 1. The Service Water Pump Structure is located east of the turbine building adjacent to the emergency cooling water reservoir. The structure contains water-filled reservoirs and five pumps that provide water to cool equipment components during normal plant operation. These pumps also supply water to several

  • Respectively, Divisional Director, Engineering Development Division, and Director, Construction Methods Department, Construction Technology Laboratories, A Division of the Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077 construction technology laboratories

TITTABAWASSEE RIVER COMBINATION SHOP N EVAP AND AUX BOILER-BLDG O BORATED WATER O COOLING TWER TANK V O i SOLID RADWASTE

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AUX BLDG - 0,,,cyOR BtDG REACTOR BLDG 3 UNIT 1 7 1 O unit 2 ADMINISTRATION AND' SgavlCE WATER CONTROLTOWER SERVICE BLDG TURBINE BLDG PUMP STRUCTURE q DIESEL GENERATOR = DLDG s____ ~,'w IRCULATING WATER O INTAKE STRUCTURE s,' s OE 's \\ \\g EMERGENCY COOLING \\ f' \\ WATER RESERVOIR \\ AFFLE DIKE s I s,___,-,_/ l ... = COOLING POND FIG. 1 SITE PLAN

) safety-related cooling systems that are required to function during a design basis accident. Because of its safety-related function, the building is a Seismic Category 1 structure. As such it must maintain its integrity during and after a design basis accident including a postulated safe shutdown earthquake. A section through the Service Water Pump Structure is shown in Fig. 2. The lower foundation slab of the structure at eleva-tion 587.0 ft is 5-ft thick. This slab is founded on undis-turbed natural material. A second foundation slab at elevation 617.0 ft is 3-ft thick. This slab is founded on backfill soil. Both slabs are locally thickened near sumps. The south wall of the structure is adjacent to the cooling pond. Approximate high point elevation of the water level is 627.0 ft. Top of grade elevation on the north side of the structure is 634.0 ft. Roof elevation is 656.0 ft. A plan of the Service Water Pump Structure at elevation 634.5 ft is shown in Fig. 3. The structure is rectangular in plan with overall dimensions of 86.0 by 106.0 f t at elevation 634.5 ft. Figure 4 shows wall designations that will be used to describe the structure in this report. The east and west walls are main structural walls in the north-south direction. It should be noted that the center east wall extends f rom elevation 620.0 ft to the roof while the center west wall extends from elevation 634.5 ft to the roof. - construction technology laboratories

N -> TOP OF GRADE E L 6 3 4'- 0 " ROOF E L 65 6'-O'S t E L. .634'- 6" 7 ..= BO T. OF MAT E L 617'- 0 "') 6" \\ g 4* SACKFIL L [ x f ,3 w w r-- w :=- - o i SOT. OF M AT E L. 5 8 7'-O ' 71'-11" r,< 34*1" M NATURAL M ATERI A L Fig. 2 Service Water' Pump Structure Section

o 8 6'- O' y 43'-08 kl Ii --p_- a a i .i-:- M j i s i FOUNDATION WALL "o = _i tC -i O .N i I n Fig. 3 Service Water Punp Structure Plan at Elevation 634'-6" ~5-

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0 Exterior and interior walls of the Service Water Pump Structure are constructed of reinforced concrete. Figure 5 and Table 1 show details of selected walls in the structure. Table 2 contains a listing of drawings used to obtain data on member dimensions, and on amounts and arrangement of reinforcement. The east, west, and south walls of the structure are 3.25-f t thick below elevation 634.5 ft. Above this elevation the walls are 2.0-ft th i ck. The north wall of the structure is 2.75-ft thick below elevation 632.75 ft. Above this level it is 2.0-ft th ick. The center east and center west walls are 1.5-ft-thick. The reinforcement ratio in exterior walls above elevation 634.5 ft is 0,n041 or greater. Additional reinforcement details are given in Table 1 and in drawings listed in Table 2. Specified concrete strength for the Service Water Pump Structure is 4000 psi. Grade 60 reinforcement :s used in the structure. As indicated in Fig. 2, the north portion of the Service Water Pump Structure is founded on backfill. As a result of unsatisfactory backfill performance under other buildings at the Midland Plant, settlements occurring in the Service Water Dump Structure have been monitored. Although settlement measurements have not indicated the presence of significant dif ferential movements in the building, the observation of cracks in walls of the structure has led to questions regarding future structural integrity. The following sections provide data on observed cracks in the structure and an evaluation of the significance of those cracks with regard to future load carrying capacity. construction technology laboratories

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TABLE 2 SERVICE WATER PUMP STRUCTURE DRAWINGS Bechtel Drawing No. Revision No. Date Title C-80 4 11/7/78 Reinforcing Slab El 634'-6" C-86 6 12/8/79 Miscellaneous Steel and Con-crete Details - Sheet 2 C-94 12 6/25/81 Concrete Floor Plans at El 592'-0" and El 634'-6" C-95 3 9/20/78 Concrete Plan at El 656'-0* and Sections C-96 8 1/8/80 Concrete Sections C-97 5 1/8/80 Concrete Sections and Details, Sheet 1 C-99 7 5/19/80 Miscellaneous Steel and Con-r' crete Details - Sheet 1 C-140 14 2/14/81 Reinforced Concrete General Notes and Details - Sheet 1 construction technology laboratories i

EVALUATION OF CRACKING It has been hypothesized that cracking observed in the walls of the Service Water Pump Structure is rela *.ed to two factors. The first is normal cracking that can occur 2 rom restrained volume changes in reinforced concrete. The second is cracking that could occur if the north portion of the structure, which is founded on fill, settles more than the south portion of the structure, which is founded on natural soil. In this report, evaluation of cracking is based on review of crack mapping reported by Bechtel and on inspection of selected areas by personnel of the Construction Technology Laboratories (CTL). Areas inspected by CTL personnel were selected because of a need to obtain data that would clarify if cracking is related to hypothesized differential settlement in the structure. Therefore, primary emphasis was given to inspec-tion of the east wall and the west wall. These walls were selected because they are the only load-carrying walls that continue through the entire north-south length of the structure. Bechtel Crack Mapping Cracks in walls of the Service Water Pump Structure were mapped by Bechtel personnel at several stages of construction.* Figure 6 shows cracking observed in west and east walls during a survey conducted in October and November 1981. A key to wall

  • Crack widths shown in this report are from measurements made prior to installation of temporary post-tensioning which has subsequently been applied as part of the remedial measures to underpin the structure. construction technology laboratories w

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designations is shown in Fig. 4. Numbers on the figure show crack widths in inches. Maximum reported crack widths are 0.030 in. in the west wall and 0.025 in. in the east wall of the structure. Cracks above elevation 634.5 ft run primarily vertically and are distributed over the entire horizontal length of the wall. If cracks in the east and west walls were caused by settle-ment of the north end of the Service Water pump Structure, a fan-shaped pattern radiating from the foundation wall 36.1 ft from the north end would be visible. No fan-shaped pattern is apparent. Rather, the cracks tend to be vertical at all locations along the walls. Cracks observed in the center west and center east walls of the structure are shown in Fig. 7. These cracks were also mapped in October and November 1981. Maximum reported crack widths are 0.030 in the center west wall and 0.020 in the center east wall. The cracks are primarily vertical in direction and are distributed over the entire horizontal length of the center west and center east walls. The somewhat larger widths observed in the center west wall are consistent with the lower reinforce-ment ratio in this wall. Table 1 contains reinforcement details for both walls. Figure 8 shows cracks observed in the south wall of the Service Water Pump Structure by Bechtel personnel in October and November 1981. Cracks on the south face of this wall were not mapped below the water line of the cooling pond. No cracks construction technology laboratories

r r ) q c 1 t .02S h@ ~as OI Y .oso 0M .GS .om l I }? 1 CENTER WE5T WALL - EAST FACE Fig. 7 (a) Cracking in East Face of Center West Wall of Service Water Pump Structure from Bechtel Drawing C-2041 Revision A (Crack Widths in Inches)

l ~ l 7 025 ] CIS s %./ i g .020 40' .ots CW D N I I CENTER WEST WALL-WEST FACE Fig. 7 (b) Cracking in West Face of Center West Wall of Service Water Pump Structure from Bechtel Drawing g-2041 Revision A (Crack Widths in Inches)

\\ \\@ '@ $g \\ .ws .as a t.e qg h "[ ".=7 k/(ii)(i h E O Q .ms / CEN.TER E AST WALL-V/EST ' FACE Fig. 7 (c) Cracking in West Face of Center East Wall of Service Water Pump Structure from Bechtel Drawing C-2041 Revision A (Crack Widths in Inches)

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tE)l.oos O I .oos .oos U e e oc oos g I 'O G g.oio $2e, "']p 4 e .oos g e gI .=, f ) f.=., e~5 =s .oos .oo, .o.. s l .oo. .o o ? @2@ Yh ( @L @)@ =. O=ds ~ @@ D s 1 r 3 r ,cos' l 42) @ 8 .oo, I.o@oy f9) h' .oo's.oos(d).13h l '0* 8 i .00 I .ooS k L. SOUTH WALL - SOUTH FACE Fig. 8(b) Cracking in South Face of South Wall of Service Water Pump Structure from Bechtel Drawing C-2040 Revision A (Crack Widths in Inches)

larger than 0.010 in, were recorded on the north face of the south wall below elevation 634.5 ft. Maximum observed crack width in the south wall was 0.020 in. Figure 9 shows cracking in the roof of the Service Water Pump Structure as mapped by Bechtel personnel in October and November 1981. Maximum measured crack width reported for the underside of the roof is 0.020 in. If the north end of the Service Water Pump Structure had settled more than the south end, and if such settlement was sufficient to induce cracking, it would be expected that a definite pattern of east-west cracks would show in the roof at the line where the north overhang starts. This location coincides with the series of five openings in the roof of the Service Water Pump Structure. These openings are covered by precast concrete panels. Some cracking in the cast-in-place concrete around the openings was recorded in the Bechtel survey. However, there is no clear pattern showing that these cracks are related to hypothesized building settlement. In fact, many of the cracks at this location occur at the inter-section of members and at discontinuities in the roof. Based on overall review of Bechtel drawings, it appears that cracks shown can be attributed primarily to restrained volume changes that occur in concrete after final set, and during subsequent curing and drying. The possibility that some cracks formed because of hypothesized differential settlement of the building cannot be completely excluded. However, the construction technology laboratories

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pattern and size of reported cracks does not entirely support the conclusion that settlement related cracking occurred. CTL Observations Visual observations of cracking in the Service Water Pump Structure were made by Dr. W. G. Corley of CTL on October 20, 1981 and December 3, 1981. At this time Dr. Corley did not do detailed mapping of cracks. Rather, the inspections were made to obtain an overall view of cracking in the structure and to correlate this with impressions obtained from review of Bechtel crack mapping drawings. In general, impressions obtained from visual inspections at the site were consistent with those obtained from review of the Bechtel drawings. As noted earlier in this report, the east and west walls are the only continuous walls in the north-south direction of the building. Consequently, any cracks that could be attribute 7 to hypothesized differential settlement would appear in these walls. For this reason, interior surfaces of the east and west walls were inspected in detail by CTL personnel on November 4 and 5,

1981, In addition, a general visual survey was made of selected walls, floor slabs, and roof slab surfaces.

A general visual inspection of the exterior of the st'ructure was also made. It should be noted that the top surface of the roof had been finished and the concrete slab surface was not visible. General access to most areas inside the Service Water Pump Structure was difficult because of construction work in progress and because equipment already in place obstructed many areas. Inspection of wall areas was conducted from eye level ' constructlan technology laboratories

1 l at elevations 634.5 ft and 620.0 ft. Areas above eye level were inspected where access was available. General lighting in all areas of inspection was relatively poor. Therefore, primary light for inspection was provided by hand-held flashlights. In addition to visual observations, widths of selected cracks were measured using a 50 power crack measuring microscope with a manufacturer's rated sensitivity of 0.001 in. Approxi-mate crack locations were measured using commercial quality tape measures. These tapes provided accuracy of measurements well within that required to draw conclusions based on the results. Figures 10 and 11 show cracks observed by CTL personnel in November 1981.* Cracks in the west and east walls of the Service Water Pump Structure are shown. The patterns of cracking are similar to those observed by Bechtel personnel. As shown in Fig. 10, maximum measured crack width in the west wall was 0.025 in. Maximum measured crack width in the east wall was 0.020 in. Although cracks shown in Figs. 10 and 11 generally run in a vertical direction, there is no consistent trend of flexural cracking above the foundation wall where the north overhang intersects the remainder of the building. Larger measured widths in the vertical crack immediately above this foundation wall are attributed to the fact that there is a vertical con-struction joint at this location.

  • Crack mapping undertaken by CTL personnel in November 1981 was done prior to installation of temporary post-tensioning which has subsequently been applied as part of the remedial measures to underpin the structure. construction technology laboratories

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The locations of vertical cracks in the east and west walls above elevation 634.5 ft are similar throughout the entire horizontal length of both walls. No fan-shapped pattern was seen radiating from the foundation wall where the overhanging north portion of the building intersects the remaining portion of the building. The hypothesis of concentrated bending at this intersection cannot be supported in the absence of the characteristic pattern of flexural cracking. Based on crack data shown in Figs. 10 and.ll, it cannot be concluded that differential settlement related cracking occurred in the Service Water Pump Structure. However, since the structure will be underpinned it is not necessary to make a more detailed analysis to determine the precise cause of observed cracks. Qualitatively, it appears that the cracking can be attributed to restrained volume changes caused by temperature and shrinkage of wall concrete combined with restraining effects of floor slabs. SIGNIFICANCE OF CRACKS Cracks observed in the Service Water Pump Structure by CTL personnel are primarily attributed to volume changes that occur in concrete during curing and subsequent drying. No evidence of structural distress was observed. Although the possibility of stress related cracking because of differential building settlement cannot be completely eliminated, crack patterns do not support the conclusion that this mechanism was a primary cause of cracking. construction technology laboratories

As a measure of significance of observed cracks relative to future integrity of the structure *, the tensile stress that uncracked concrete may be assumed to carry was compared to availa le tensile capacity provided by structural reinforcement crossing the cracks. This calculation was made for sections in the vicinity of cracks that had a measured width of 0.010 in. or greater. Available structural reinforcement was determined from Bechtel drawings listed in Table 2. It should be noted that this calculation is not intended to imply a change in design criteria for the walls. Rather it is a means of esti-mating membrane capacity. Table 3 summarizes the comparison of " tensile capacity" for walls in which cracks larger than 0.010 in. were observed. In the calculation, concrete is assumed to carry a principle ten-silestressof4/f{wheref'isspecifiedconcretecompressive strength. This assumption is consistent with Section 11.4.2.2 of the ACI Building Code.( }** Resistance of reinforcement was calculated as A f where g A = area of reinforcement and g f = specified yield stress of reinforcement. y If calculated resistance provided by reinforcement crossing thecrackequals4/I}thereissufficient reinforcement to carry the stress that may be attributed to concrete. As indicated in

  • A general discussion of strength of cracked reinforced concrete members is given in Appendix A.
    • Superscript numbers in parentheses refer to references listed at the end of this report. construction technology laboratories

TABLE 3 - AVAILABLE " TENSILE CAPACITY" AT SELECTED CRACK LOCATIONS IN PRIMARY LOAD BEARING WALLS. 4/F A (kips) Af (kips) A,f (kips) c g sy a esignation Vertical Hor zontal

1. East and West Walls (a) North Portion Above 72.9 96.0 70.4 El. 620'-0" (b) South Portion Above 72.9 94.8 70.4 El. 634'-6"
2. Center East Wall (a) Above El. 634'-6" 54.6 96.0 52.8 (b) Below El. 634'-6" 54.6 203.2 52.8
3. Center West Wall 54.6 35.2 35.2 construction technology laboratories

Table 3 resistance provided by available horizontal reinforce-ment in upper levels of the east and west walls is only slightly less than the tensile stress assumed to be carried by the con-crete. Resistance provided by the vertical reinforcement exceeds the tensile stress assumed to be carried by concrete. Vertical and horizontal forces on the east and west walls would result in a principal stress direction that is inclined from vertical. Both vertical and horizontal reinforcement will provide tensile resistance across inclined cracks. A crack inclination of only 15 degrees from vertical would mobilize enough vertical reinforcement force to exceed the stress attri-buted to concrete tensile strength. Therefore, it is concluded that resistance provided by the reinforcement is sufficient. The comparison of reinforcement resistance to assumed concrete tensile stress in the center east wall is si6ilar to that observed for the east and west walls. Tensile resistance of the vertical reinforcement exceeds that attributed to the concrete while that in the horizontal reinforcement is slightly less. As was discussed for the east and west walls, it can be concluded that the reinforcement resistance is satisfactory. Horizontal and vertical reinforcement resistance in the center west wall is approximately 65 percent of the tensile stress assumed to be carried by the concrete. It should be noted, however, that the center west wall extends only from elevation 634.5 ft to the roof. This wall is not a primary load resisting wall for the structure. construction technology laboratories

5 As an additional check on influence of cracking on the center west wall, an analysis was made to estimate two limits on capacity. First the wall was assumed to act as a horizontal cantilever with a concentrated vertical foJce at its exterior end. Based on sectional analysis, flexural capacity of the wall was calculated. This flexural capacity defines an upper limit on the amount of vertical shear that can be induced in the wall. For assumed conditions, it was estimated that the maximum nominal vertical shear stress which can be induced in the wall is 1.8 /E[. Available shear capacity would be in excess of 2.6/E[ which is provided by the reinforcement. A Thus, available shea. capacity exceeds that needed to resist the potential maximum shear. It should be noted that assump-tions for this calculation are very conservative in that under-pinning support for the north end of the structure was neglected. A second analysis was made to check shear capacity in the horizontal direction. A hypothetical horizontal force correspondingtoanominalshearstressof2/f}wasappliedto the top of the center west wall.* The wall was assumed to be supported at its south end at elevation 634.5 ft. Based on building geometry, it was estimated that application of the hypothetical force would cause flexural distress or even uplift at the north end or the wall. Thus, shear could not limit wall capacity.

  • Note that available shear capacity exceeds the nominal stress of2.6i{providedbythereinforcement.

/ construction technology laboratories

) ] Based on a conservative analysis of limits on capacity, it can be concluded that observed cracks do not affect shear strength of the center west wall. RECOMMENDED PROGRAM FOR MONITORING STRUCTURAL INTEGRITY As part of remedial measures to eliminate the possibility of unsatisfactory foundation conditions, the north portion of the Service Water Pump Structure will be underpinned as shown in Fig. 12. During underpinning operations, movement of the structure should be monitored. Monitoring operations should include periodic measurement of structure displacements and periodic visual inspection for cracking. Displacement Monitoring A time history of displacements of the Service Water Pump Structure should be maintained during underpinning operations. It is recommended that displacement readings be taken at selec-ted construction milestones with a maximum interval of one week. Displacement measurements should be made to monitor absolute movement and relative distortions of structural elements. Fig-ure 13 shows approximate locations of recommended displacement measurement points. Designation of absolute and relative measurement points will be completed as part of the overall monitoring plan prior to start of underpinning operations. Displacement measurements should be recorded as a function of time for the duration of underpinning operations. Signifi-construction milestones should be identified at appropri-cant ate time intervals. Prior to start of underpinning, limiting distortion criteria should be selected so that critical defor-construction technology laboratories

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/7 m-s u a i I FOUNDATION l WALL "o = _ie T o E s 1 i \\ / l g, P 4 \\ n n /\\ X = MEASUREMENT POINT l Fig. 13 Service Water Puntp Structure Plan Showing Approximate Locations of Displacement Points . 1 6

mation limits of the structure are not exceeded. In this way the measured displacements will provide a warning of impending structural distress. If displacement limits are reached, under-pinning operations should be stopped until remedial measures are evaluated. It is also recommended that the time history of displace-ments be submitted on a regular basis to a. consultant familiar with reinforced _ concrete behavior and design. The consultant will provide recommendations on trends observed in the data. Prior to the start of underpinning operations and distortion monitoring, the consultant stould review details of the monitoring plan. Crack Monitoring As a supplement to the displacement monitoring program periodic visual inspections of the Service Water Pump Structure should be made to determine if new cracking has developed or if existing cracks have changed in width or length. Crack inspec-tions should be conducted on a periodic basis by qualified personnel. In addition a consultant knowledgeable in reinforced concrete design and behavior should inspect the Service Water Pump Structure at selected construction milestones. Personnel who monitor cracking should be instructed in crack mapping techniques by the consultant prior to start of operations. The following criteria should be used for evaluation of observed crack widths. 1. If a new crack develops that is wider than 0.010 in, a consultant should evaluate significance of the new construction technology laboratories

cracking. Within two hours after observation of the crack the consultant should provide a verbal report recommending whether underpinning operations should Stop or Continue. The verbal report should be con-firmed with a written report within five days. 2. If any crack exceeds 0.030 in. in width a consultant should evaluate significance of the cracking. Within two hours after observation of the crack the consul-tant should provide a verbal report recommending waether underpinning operations should stop or continue. The verbal report should be confirmed with a written report within five days. 3. If development of yield strain in the reinforcement is inferred from any observed crack, underpinning opera-tions should be stopped immediately. Individual criteria will be recommended by the consultant for the structure. If criteria are exceeded a consultant should evaluate significance of the cracking. Within two hours after observation of the crack the consultant should provide a verbal report recommending whether underpinning operations should resume. The verbal report should be confirmed by a written report within five days. The following criteria should be used in evaluation of the significance of cracks that develop in the Service Water Pump Structure: construction technology laboratories

a cracking. Within two hours after observation of the crack the consultant should provide a verbal report recommending whether underpinning operations should stop or continue. The verbal report should be con-firmed with a written report within five days. 2. If any crack exceeds 0.030 in. in width a consultant should evaluate significance of the cracking. Within two hours after observation of the crack the consul-tant should provide a verbal report recommending whether underpinning operations should stop or continue. The verbal report should be confirmed with a written report within five days. 3. If development of yield strain in the reinforcement is inferred from any observed crack, underpinning opera-tions should be stopped immediately. Individual criteria will be recommended by the consultant for the structure. If criteria are exceeded a consultant should evaluate significance of the cracking. Within two hours after observation of the crack the consultant should provide a verbal report recommending whether underpinning operations should resume. The verbal report should be confirmed by a written report within five days. The following criteria should be used in evaluation of the significance of cracks that develop in the Service Water Pump Structure: construction technology laboratories

1. Geometry of member 2. Amount and distribution of reinforcement in the member 3. Material properties of the member 4. Function of the member 5 Magnitude and distribution of loads on the member 6. Construction technique 7. Sequence of construction 8. Crack location and distribution 9. Crack size 10. Interaction of multiple cracks Basically, these criteria outline a procedure that requires the function and load carrying mechanism of the member or structure to first be defined. Then the influence of cracks on the path of load distribution is determined. In this way the cause of cracking is defined and the influence of crackina on future load carrying capacity of the structure can be evalusced. In evaluating cracks in reinforced concrete structures it is not sufficient to base conclusions on a single criteria cuch as crack width. The overall crack pattern including location I and direction of cracks, length and width of cracks, and inter-relationship between multiple cracks must be considered. Th% pattern of cracking provides significant clues with regard to causes of cracks and their effects on future performance.

SUMMARY

AND CONCLUSIONS This report presents an evalution of the significance of cracks observed in the Service Water Pump Structure at Midland Nuclear Power Plant Units 1 and 2. Cracks observed in this construction technology laboratories

structure by Bechtel personnel and by Construction Technology Laboratories personnel are attributed to restrained volume changes that occur during curing and drying of concrete. No indications of structural distress were observed during site visits. While occurrence of stress related cracking because of differential building settlement cannot be completely dismissed, it does not appear that such hypothesized settlements were a primary cause of cracks observed in'the structure. Calculations based on section geometry and macerial properties indicate that structural reinforcement provided in primary load carrying walls at selected crack locations has sufficient capacity to offset the loss of tensile stress attributed to concrete. A program for monitoring structural integrity of the Service Water Pump Structure during implementation of remedial measures to underpin the structure is described. It is recommended that measured displacements be used as the primary means of monitor-ing behavior of the structure. It is also recommended that periodic displacement measurements be supplemented with visual inspections to monitor cracking in the structure. Displacement and crack monitoring should be reviewed by a consultant knowl-edgeable in reinforced concrete behavior and design. REFERENCES 1. ACI Committee 318, " Building Code Requirements for Rein-forced Concrete (ACI 381-77)," American Concrete Institute, Detroit, 1977. r construction technology laboratories

APPENDIX A STRENGTH OF CRACKED REINFORCED CONCRETE MEMBERS Construction technology laboratories

APPENDIX A TABLE OF CONTENTS Page No. INTRODUCTION A-1 TESTS OF STRUCTURAL WALLS A-1 Tests of " Low-Rise" Structural Walls A-2 Tests of "High-Rise" Structural Walls A-4 TESTS OF BEAMS A-13 TESTS OF CONTAINMENT ELEMENTS A-15 SUM ARY AND CONCLUSIONS A-16 REFERENCES A-23 Construction technology laboratories

APPENDIX A STRENGTH OF CRACKED REINFORCED CONCRETE MEMBERS by A. E. Florato and W. G. Corley* INTRODUCTION Cracking is an inherent characteristic of reinforced con-crete structures. The existe,nce of cracks is not necessarily indicative of structural distress. The objective of this report is to clarify the relationship between cracking and strength of reinforced concrete members. The relationship will be demon-strated by examining the response of selected structural members that have been loaded to destruction in the laboratory. To provide a cross-section of data, results from tests on struc-tural walls, beams, and containment elements will be considered. TESTS OF STRUCTURAL WALLS Reinforced concrete structural walls are commonly used as lateral load resisting elements in buildings. Both " low-rise" walls, which act as deep beams, and "high-rise" walls, which undergo significant flexural yielding, have been tested in the laboratory. l

  • Respectively, Manager, Construction Methods Section and Divisional Director, Engineering Development Division, Construction Technology Laboratories, a Division of the Portland Cement Association, 5420 Old Orchard Road, Skokie, i

Illinois 60077. I A-1 construction technology laboratories

Tests of " Low-Rise" Structural Walls Figure 1 shows the test setup used to apply reversing loads to eight specimens representing " low-rise" structural walls with boundary elements. (1)

  • Principal variables in this test program included amount of flexural reinforcement, amount of horizontal wall reinforcement, amount of vertical wall reinforcement, and height-to-horizontal length ratio of the wall.

Flexural reinforcement was varied from 1.8 to 6.4% of the boundary element area. Horizontal and vertical wall reinforcement were varied from 0 to 0.5% of the wall area. Height-to-horizontal length ratio of the wall was varied from 1:4 to 1:1. The test program was designed to deter-mine effects of load reversals. Data obtained also provided information on the relationship between cracking and strength. Principal test results for the eight walls are shown in Table 1. For all specimens, except B5-4, the maximum nominal shear stress in the wall exceeded the stress at first observed shear cracking by a factor of at least 2.4. For Specimen B5-4, which contained no vertical reinforcement in the diaphram, the maximum nominal shear stress exceeded the stress at first shear cracking by a factor of 1.5. The ratio of maximum nominal shear force to first shear cracking even exceeded 2.5 for Specimen B4-3 which contained no horizontal reinforement. For each of the " low-rise" walls tested, measured capacity exceeded

  • The superscript numbers in parentheses refer to references listed at the end of this report.

A copy of each reference is attached. construction technology laboratories

s

  1. .0 Yao ties

( e. x~- essearv base Prestressed EM 4 to Test Fleer et a Lacetione he4' t.=75 x sa' N 4 / / 10'- 0* ,. 2 3.. y. s'.0 i'. o.soes = 4*.0-Fig. 1 - Getup for Tests of " Low Rise" Walls (1) TABLE 1 - Principal Test Results (1) First Sheer Cracking Ultimate Load End of Test Sheer Sheer Sheer stress ..E. Deflection d Deflection d strees i [" stress Specimen VariebleW d ,al h, af, h, r g r r, y, y m, poi in. psi in. pee 81 1 p = 12g2) 420 6.5 0.027 0.00072 1,010 15.5 0.23 0.0061 280 4.4 82 1 a = 6.4%(2) 240 4.9 0.016 0.00043 767 15.8 0.26 0.0069 270 5.5 832 Control 330 5.2 not measured 881 14.1 0.21 0.0056 190 3.0 8}2R Repair 190 3.3 0.020 0.00053 676 11.5 0.49 0.0130 230 4.0 843

  1. 4 = 0 320 6.1 0.015 0.00040 810 15.4 0.20 0.0053 160 3.0 85-4 a=0 330 5.2 0.012 0.00032 538 8.3 0.20 0.0053 280 4.3 n

864 p = 0.25% 280 5.0 0.013 0.00035 686 12.3 0.23 0.0061 190 3.5 87-5 A /f w = 1/4 330 5.4 0.006 0.00032 906 14.8 0.16 0.0085 350 5.7 88 5 A./1, = 1 200 3.5 0.027 0.00036 704 12.1 0.42 0.0056 150 2.6 11I E aceot as ind6cated below, all specirnens had the following characteristics: hw//w = t/2, #4 = 0.5%, pn = 0.5%, p

  • 4.1 A (2I pecimens subjected to static loading. All other noecimens suciected to load reversats.

S Note: 1 in. = 25.4 ram: 1.000 poi

  • 70.3 kg per square centimeter A-3

that calculated by American Concrete Institute Building Code Requirements for Reinforced Concrete. Figure 2 shows crack patterns in the " low-rise" walls at the ultimate load levels listed in Table 1. The inclined cracks are indicative of shear stresses that predominate in short cantilever members. It is apparent that the presence of cracks does not necessarily indicate loss of structural capa-city. Even with the extensive cracking shown in Fig. 2, the walls were carrying maximum applied loads. For a particular section geometry and applied loading, structural capacity is a function of the amount and distribution of reinforcement. There was no evidence that reversing loads caused residual stresses that reduced strength of the walls. Additional data on these tests are given in Reference 1. Tests of "High-Rise" Structural Walls Tests reported in References 2, 3, and 4 were conducted to obtain data on strength and deformation capacity of structural walls subjected to significant numbers of inelastic load rever sals. Effects of load history, section shape, vertical and horizontal reinforcement, confinement reinforcement, moment-to-shear ratio, axial compressive stress, and concrete l strength were considered. Figure 3 shows the setup used for tests of "high-rise" walls. The walls were tested as vertical cantilever members with forces applied through the top slab. The behavior of one of the test specimens is described in detail in the following A-4 construction technology labcratories

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paragraphs. This behavior illustrates the influence of cracks that developed during the tests. Additional data on other specimens can be obtained in References 2, 3, and 4. Figure 4 shows the measured load vs deflection relationship for Specimen B3. This was a barbell shaped specimen which represented a wall with column boundary elements at each end. As can be seen in Fig. 4, the wall was subjected to increasing levels of load reversals. The test consisted of 42 complete load cycles. Initial cracking was observed in the fourth cycle at a load of 28 kips. First yielding in the vertical flexural reinforce-ment occurred in Cycle 10.at a load of 45 kips. Maximum measured crack widths were 0.012 in. in tLe tension boundary element and 0.025 in. across a diagonal crack in the web. Figure 5 is a photograph of Specimen B3 at Load Stage 112. This load stage, which is marked on Fig. 4, represents a point in the test when the specimen was unloaded. There were no applied in-plane hori zontal forces. Figure 5 shows the inter-secting pattern of cracks in the lower six feet of the wall af ter the first 21 load cycles. From Load Stage 112, loads were increased in a positive direction until Load S tage 117 was reached. Figure 6 shows the l l condi tion of the specimen at Load S tage 117. A t Load S tage 117, maximum n easured crack width in the tension boundary element was 0.07 in, and maximum measured crack width in the wall web was approximately 0.16 in. It should be noted that, at this load stage, the wall had been pushed to a lateral deflection of more than three times its yield deflection. A-7 construction technologylaboratories 1

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After Load Stage 117 was reached, the wall was unloaded and pushed in the opposite direction until Load Stage 123 was reached. Figure 7 shows the condition of Specimen B3 at Load Stage 123. At this load stage, the maximum crack width measured in the tension column was approximately 0.07 in. and the maximum measured crack width in the wall web was 0.16 in. When the wall was again unloaded, to Load Stage 125, the crack pattern shown in Fig. 8 resulted. It is clearly evident from the behavior of Specimen B3 (and from other specimens tested) that the presence of cracks did not prevent the walls from main-taining their structural integrity and developing their nominal s treng th. Figure 9 shows Specimen B3 at Load Stage 196. This load stage is also indicated in Fig. 4. The cracking pattern in, Fig. 9 is indicative of severe distress in the member, yet at this stage the wall carried its maximum load which corresponded to approximately 3.1/T[. For purposes of comparison, the design strength this member calculated in accordance with the American ConcreteInstituteBuildingCodeis2.3vf[. A question that occurs in evaluating cracked reinforced concrete structures is whether residual stresses associated with the occurrence of cracks influence strength of the member. It is evident from the behavior of Specimen B3 that internally balanced residual stresses, such as those existing when the specimen was unloaded, did not influence strength. A-10 construction technology labe ~ tortes

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I TESTS OF BEAMS Background data on strength of cracked reinforced concrete members can also be obtained from tests on reinforced concrete 4 beams. Data from tests reported by Scribner and Wight are shown in Figs. 10 and ll.IN Figure 10 shows the load vs displacement curve for a reinforced concrete beam element that contained positive and negative steel. The beam was subjected to increasing levels of fully reversed load cycles. Yielding occurred in the first load cycle as indicated in Fig. 10. Figure 11 illustrates. crack patterns that developed during the first inelastic loading and during subsequent load rever-sals. As increasing numbers of load cycles were applied, the entire beam moment at the fece of the column was carried by a l force couple between the top and bottom layers of longitudinal steel. Thus, applied moments were primarily resisted by the positive and negative longitudinal reinforcement. Under load reversals a complete crack plane, labeled A-B-C in Fig. 11, formed through the beam. This crack plane did not prevent the beam from transferring load. During the final stages of the test, increasing numbers of inelastic load rever-sals caused concrete near the face of the column to abrade and eventually disintegrate. This resulted in a " slip plane" along the beam at the face of the column. The significance of such a slip plane is related to the. number of inelastic load reversals and the level of shear stress on the beam. The existence of A-13 construction technology laboratories

p A p Yield p ( E,10 B , PV K / l 5"-- lC of + P. A p 4 j Formation of Crocks during First inclostic Loading -l? / 0.5

p

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the crack plane did not become significant until repeated num-bers of inelastic cycles were applied. kdditional data on beam tests can be obtained from References 6 and 7. In addition, tests of beam-column joints reported in Reference 8 also provide useful information. Results shown in Fig. 10 indicate that beams can transfer flexural and shear loads even with the presence of cracks through their entire depth. Tests conducted at the University of Washington have shown that the effectiveness of web rein-forcement in resisting shear in reinforced concrete beams is not affected by axial force in the beam. OI These tests were conducted on beams subjected to combined axial tension, bending, and shear. Results indicated that effectiveness of web rein-forcement is not reduced by the presence of axial tension. In the tests, applied axial load was sufficient to cause cracking prior to the application of transverse load. For all beams with web reinforcement, measured load capacity of the precracked beams exceeded values calculated in accordance with the American Concrete Institute Building Code. TESTS OF CONTAINMENT ELEMENTS Another series of tests that can be used to damonstrate the strength of cracked reinforced concrete members is reported in an experimental program to investigate shear transfer in cracked containments without diagonal reinforcement. (10) The test setup was designed and constructed to simulate boundary conditions of a wall element of a pressurized containment sub-jected to tangential shear stresses. Forces on an element in A-15 construction technology laboratories

a containment wall are illustrated in Fig. 12. Figures 13 and 14 show the test setup used for the experiments. The experimental program included monotonic and reversing load tests on large-scale specimens subjected to biaxial tension and shear. Specimens were 5-f t square and 2-f t thick with No.14 and No. 18 reinforcement. This discussion includes a description of one of the test specimens. Additional data are available in Reference 10. Figure 15 shows the crack pattern observed in Specimen MBl after reinforcement in the element was loaded to obtain a ten-sion stress of 54 ksi in the steel. This stress corresponds to 90% of the yield stress of the reinforcement. Crack width measurements made on the specimen after biaxial tension was applied indicated a maximum width of approximately 0.036 in. Figures 16 and 17 show the crack pattern and nominal shear stress vs shear distortion relationship for Specimen MBl. Shear forces were applied while constant biaxial tension was main tained. It is evident from Fig. 17 that the reinforced concrete element was capable of transferring shear forces even though it was traversed by biaxial tension cracks through the con.plete thickness.

SUMMARY

AND CONCLUSIONS Test data presented in this report demonstrate that cracks in an adequately reinforced concrete member do not prevent the member from developing its expected strength. Adequate rein-forcement for the test specimens was determined in accordance with current code provisions. Data presented also indicate the A-16 construction technology laboratories

NUREG/CR-1374 \\ I / r \\ I / p / ,a s 1 Y ij ^ w,(n _,g \\ .N p. ..q 7 i'" - - 1 _ies I c A'4<_ l l j U.__p. - i i L_. L_ i N v I '/ b. . Q. 4 111llV4x x ELEMENT OF CONTAINMENT WALL o o o o f _ l k _ _1. _ _C _ !' _ I 3 .__(i I !/ q'.__7 _ _ _;fl,i t i t i ,1 i-I _~~~~~ V a o e a) Biaxial Tension Due to b) Shear Forces Due to Internal Pressurization Lateral Load Fig. 12 Forces on Element in Containment Wall (10) l A-17 l

+ --- 2 in. maximum distortion 1 ,i o a 2T l/2 in.mos. V M tronstation plus o g -h ~~~s elongatton r- - \\ i l V& I V.V t r i P >u 4 3 l I i I t e I Ng t y) ~ ~ 2Tg 't t r I4f /////////t /t / //// /f ili // /// / 4 li=0 to 280 kips sustained tension occa el8 bor T =0 to 160 kios 2susteined tensien each *l4 bcr V eo to 210 kios reversing shear capiled at 3 locations ecca fece Fig. 13 Loading System Capabilities (10) A Q[ ,b (y,.. .x-N p].

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Np y hr W 'W# \\p+ ?.,. -4.I,gg,iit - as {; ~ j ,,6 9 r- ..,j ' . '. ~ j '3 ., c. -= 4 .p g ,,3 'b b fn -Q y %: e 9-n i.. x ~., 4

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.ay u&&ft% %sn.ga Fig. 14 Test Setup for Containment Element (10) A-18

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==DQ fli y / L il T C ( zss3 / f N Y' i o> / i l @NE E ~ Cracks considered to Penetrate the Full Thickness of the Specimen. Cracks considered to Penetrate only the Cover Layer. Fig. 15 Crack Pattern After Biaxial Tension of 54 ksi in Containment Element Spr:cimen MB1 (10) A-19 e 9 c n

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\\ %. i ' a) Just Prior to Loss of Shear Capacity g i sg g .~ y i V' l L d l M,, A in, W _ A~.]__. t i -W C, N A 4 A -1 _ V_ P, 1 ~ l J' d% ik 4 .A h a=Ng s g-Ciccus existing prict to snect lacting. - Crects occ:mnq cfte-ccoucenen of sheer toed. b) Crack Pattern' Just Prior to Maximum Shear Load Fig. 16 Crack Pattern in Specimen MB1 (10) A-20 l 1

i Nominal Shear Stress, psi 300 200 MBI lIlI u 4; n l,' r" loo -- ~ L _ _ _ _.,%y I I il o O O.004 0.008 Shear Distortion, y, rad. Fig. 17 Nominal Shear Stress versus Shear Distortions for Containment Element Specimen M31 (After Ref. 10) A-21

level or severity of cracking associated with severe stress in reinforced concrete members. Obviously the presence of cracks in a reinforced concrete structure cannot be summarily dismissed as insignificant. The pattern of cracking and crack widths should be evaluated to determine their significance.

However, the mere presence of a crack does not necessarily indicate that the integrity of the structure is in jeopardy, or that its load-carr ying capacity has been reduced.

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JW RE FE RENCES 1. Barda, F., Hanson, J.M., and Corley, W.G., "S hear S trength of Low-Rise Walls with Boundary Elements," Special Publica-tion SP-53, Reinforced Concrete Structures in Seismic Zones, American Concrete Institute, Detroit, 1977, 496 pp. 2. Corley, W.G., Fiorato, A.E., and Oesterle, R.G., "S truc-tural Walls," Special Publication, C.P. Siess Symposium, American Concrete Institute, Detroit, 1979 (to be published). I 3. Oesterle, R.G., Fiorato, A.E., and Corley, W.G., " Rein-forcement Details for Earthquake-Resistant Structural Walls," Concrete International, December 1980, pp. 55-66. 4. Oesterle, R.G, Fiorato, A.E., and Corley, W.G., " Effects of Reinforcement Details on Seismic Performance of Walls," Proceedings of a Conference on Earthquakes and Earthquake Engineering: The Eastern United S tates, Vol. 2, Ann Arbor S ci ence P ubli s h e rs, I nc., 19 81, pp. 685-70 7. 5. Scri bner, C.F. and Wight, J.K., "A Method for Delaying Shear Strength Decay of RC Beams," Proceedings of a Workshop on Earthquake-Resistant Reinforced Concrete Building Construction, Vol. 3, University of California, Berkeley, June 1978, pp.1215-1241. 6. Wight, J.K. and Sozen, M.A., " Strength Decay of RC Columns Under Shear Reversals," Journal of the S tructural Division, AS CE, May 1975, pp. 10 53-1065. 7. Brown, R.H. and Jirsa, J.O., " Reinforced Concrete Beams Under Load Reversals," Journal of the American Concrete I ns t i tut e, Vol. 6 8, No. 5, May 1971, pp. 3 80 -3 90. 8. H a ns on, N.W. and Conner, H.W., " Tests of Reinforced Concrete Beam-Column Joints Under Simulated Seismic Loading," Research and Development Bulletin RD012, l Portland Cement Association, 1972, 12 pp. l 9. Haddadin, M.J., Hong, S.T., and Mattock, A.H., "S tirrup Effectiveness in Reinforced Concrete Beams with Axial l Force," Journal of the S tructural Division, ASCE, i September 1971, pp. 2277-2297. l 10. Oesterle, R.G. and Russell, H.G., " Shear Transf er in Large Scale Reinforced Concrete Containment Elements," Report No. 1, HUREG/CR-1374, Construction Technology Laboratories, a Division of the Portland Cement Association, prepared for U.S. Nuclear Regulatory Commission, Washington, D.C., April 1980. A-23 construction technology laboratories .}}

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