ML19309G033

Comments on Documentation Substantiating 18 Psi Allowable Collar Joint Shear Stress

ML19309G033
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	04/08/1980
From:	Colville J AFFILIATION NOT ASSIGNED
To:
Shared Package
ML19309G031	List: ML19309G030 ML19309G033 ML19309G034 ML19309G035
References
TAC-12369, NUDOCS 8005020238
Download: ML19309G033 (6)

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i Comments on

,1 Documentation Substantiating 18 psi Allowable Collar Joint Shear Stress.

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Dr. James Colville, P.E.

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i April 8, 1980.

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• Comments on 4

Documentation Substantiating 18 psi Allowable Collar Joint Shear Stress These comments address the information submitted by A. Schwenler on March IS, 1980, Attachment 2.

(1) The question of the appropriateness of the ratio of mean shear bond strength to mean tensile bond strength of 0.6 addressed by Prof. Boris Bresler is reviewed and discussed separately.

(1)

At present, this ratio is still condidered appropriate for type M mortar' bed joints in concrete' marpnry construction.

(2)

As noted in the discussion presented in Ref (1) the use of

, data obtained from specimens having little similarity to the construction in question in the double wythe concrete masonry walls in the Trojan Plant may be hisleading. The

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tests performed by Testing Engineers of Oakland, California are in this category since a 1" mortar joint was used between two concrete surfaces. The ultimate shear strengths ranging i

from 200 to 286 psi are far in excess of values reported from i

l other sources which range between 24 psi and 100 psi, as discussed in Ref (1). Thus, these test values are not considered applicable to a 3/8" collar joint between concrete masonry units. Also the test specimen were unusual in that in one series of tests a support plate was apparently extended across the mortar joint.

In the second test procedure the load 1

was applied directly to the 1" mortar segment. Also no

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informatien is given concerning the mortar properties.

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(3)

A number of si=ilar comments apply to the tests by Benjamin and Willians. These tests were performed on brick-mortar couplets and in Ref (1) evidence is reviewed which indicates that much lower shearing bond strengths can be expected along concrete masonry - mortar interfaces.

(4) The tests performed by Stucco Stone Products of Napa, California by Structural Testing, Inc. of Santa Rosa, California involved mortar joints between. artificial stone veneer and concrete masonry. The reported shear strengths ranged from 56 psi to 519 psi. The standard deviation was 185.5 psi with a coefficient of variation of 70%.

The average value of 264.4 psi is also significantly in excess of values reported for shear bond in concrete masonry by a variety of investigators (1).

A finite element analysis is being performed on the specimen to ascertain the magnitude of normal compressive stresses present in the specimen which may account for the large variation in reported values. However, in those specimens in which block to mortar separation occurred the mean strength value from three tests was 67 psi. The excessive scatter of these test results and the fact that separation along the block mortar interfaces did not occur in all tests make these results suspect. These tests could, however, be interp-

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reted as indicating a shearing bond failure of around 67 psi.

The higher strengths indicated here may also be the result of

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q having only one potential failure interface associated with

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each joint, due to the " fir.ger" rolling of the Stucco elements.

(5) The Eldorado Stone Corporation tests also used dissimilar caterials along both sides of the cortar joint.

No details of the test procedure are given. However the mean shear bond strength of 120 psi is based on only two specimens.

In summary, the scattered test results submitted invcive a number of different test specimens, test procedures, and materials with a wide variation in reported rhear bond s~trengths. Also the strengths reported are significantly in excess of values obtained frca sources referenced in Ref (1). More importantly,none of the tests relate to collar joints as constructed in the Trojan Plant. For example, on page 65 of Ref (2), a discussion of the effect of methods of forming the mortar joint is presented. This indicates, for example, that tapping the masonry unit down to uniform joint thickness increases the bond strength from 50 to 100 percent over that obtained using hand pressure alone to bring the mortar joint to uniform thickness.

The difference in bond obtained in bed joints in which hand pressure and the weighting action of the unit are available to the bond in collar joints which do not benefit from such weighting actions can only be surmised due to the lack of test data on collar joints. However, in my opinion it is entirely misleading to state, as on page 3 of attachment 2, that the tests discussed above provide results of tests of mortar collar joints,

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n and that the minunum ultimate shear strength of a collar joint

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is at least 50 psi.

Thus it is concluded that the information submitted does not justify the statement that using a shear stress value of 18 psi provides a l

l facter of safety of about 3 for type M mortar collar joints, i

Thus, an ultimate shear bond strength of 12 psi is still considered l

to be a reasonable upper bound for the collar joints in the Trojan i

Plant.

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RyERENCES 1

(1) " Response to Co=ents on Appendix B of Dr. Colville's Renort of 2/13/80 en Trojan Masonry Walls", Submitted to NRC, DDR, dated A'ril 8, 1960 p

(2) R1traer, H.C., "Erick and Tile Engineering", Published by Brick Institute of America.

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