ML19309G035

Comments on Review by Professor B Bresler of Evaluation of Tensile Bond & Shear Bond of Masonry by Means of Centrifugal Force,By M Hatzinikolas,J Longworth & J Warwaruk,Alberta Masonry Institute,Undated (1978-1979)

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Comments on Review by Prof.'B. Bresler of

" Evaluation of Tensile Bond and Shear Bond of Masonry by Means of Centrifugal Force," by M. Hatzinikolas, J. Longworth, and J. h'arwaruk Alberta Masonry Institute, Undated (1978 _ 1979) by i

Dr. James Colville, p,E, April 8, 1980.

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Comments on Review by Prof. B. Bresler of

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" Evaluation of Tensile Bond and Shear Bond of Masonry by means of Centrifugal Force,"

by M. Hat-inikolas, J. Longworth, and J. k'arwaruk Alberta Masonry Institute, Undated (1978-1979) l The following comments address several questions raised by Prof.

Bresler on hbrch 15, 1980 concerning the validity of the test procedure and test results described in Ref. (1) i Various items are discussed as follows:

(1)

The calculated ratio of mean value of shear bond to the mean value of tensile bond for type M mortar from Ref. (1) is 0.63.

The corresponding value for type N mortar is 0.77, which implies that this ratio depends on the mortar typs Prof. Bresler indicates that these values should exceed 1.0, since the shear bond strength should be greater than tensile bond strength.

This observation is presumably based on expected properties of the 7

mortar. However, the measures of strength of interest and the strengths presented in Ref (1) relate to the shear bond and tensile bond strengths of the mortar - concrete masonry interface.

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These latter strengths may be significantly different than those of the mortar itself. An attempt to clarify the validity of the results presented in Ref. (1) is presented in the following.

From page 68 of Ref. (2), a relationship between tensile bond strength and compressive strength of mortar is presented.

Using the^"best fit" curve shown, a tsnsile bond strength of 70 psi is obtained for type M mortar, assuming a compressive strength of 2500 psi (page 60 of Ref. (2)).

The mean value of tensile bond from Ref. (1) for type H mortar is 74 psi, which is within 6% of the value obtained from Ref. (2).

If the tensile strength of the mortar is assumed to equal 7.5v 7c'~, the strength F

of type M mortar in tension will be 375 psi.

Thus the tensile bond strength is only about 20% of the mortar tensile strength.

The shearing strength of mortar joints in masonry construction is a function of the compressive stress across the joint.

In the absence of axial compressive stress, page 11 of Ref. (2) indicates a shear strength of 30 psi for concrete masonry.

The corresponding value for brick masonry is 150 psi.

In Ref.(3), on page 87, the following values of shear bond strength in concrete masonry construction are presented:

(a) 25.6 psi for mortars with strengths from 610 psi to 2150 psi (from Meli and Reyes (Ref.4)).

(b) from pg. 90 of Ref. (3), values of 18 psi for 1:11 6 type M mortar and 48 psi for a 1:1:4h type M mortar. These values are from 1 test and an average of 3 tests respectively from Ref.(5,6)

(c) On pg. 93 of Ref.(3), reviewing results of tests by Copeland and Saxer (7), tensile bond of mortar to block is stated as varying from 4 psi to 175 psi, and shear bond from 24 psi to 100 psi. Hegemier (3) concludes "The test results clearly indicate the erratic and complex nature of mortar adhesion".

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Although tha ttst values givcn above result from a variety of test procedures, the range of shear bond strength of mortar bed joints i

appears to fall between 18 psi and 100 psi.

In particular, the mean value of 47 psi for type M mortar.obtained from Ref.(1) is centered within this range and is very close to the value of 48 psi obtained from Refs. (5,6) for a 1:1: 4 type M mortar.

In any event as is evident from the above values the shear bond strength of mortar is cuch lower than the mortar shear strength. For concrete, Ref. (11) states that the shearing strength will be between 35% to 80% of the compressive strength. Applying these values to type Jf ; mortar with a compressive strength of 25n0 psi, yields a shear strength' range of

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575 psi to 2000 psi.

Thus the shear bond strength is roughly 3% to 65 of the mortar shear strength.

The overall significance of this data for concrete masonry, is that although both the tensile bond and shear bond strengths are lower than the tensile and shear strengths of the mortar, the strength reductions are not the same.

Thus since the shearing bond strength is a much smaller percentage of the mortar shear strength than the tensile bond strength is to the mortar tension strength, the ratio of mean shear bond to mean tensile bond can be less than 1.0 This same effect may not necessarily apply to brick masonry, and attempts to use data from tests of brick-mortar bond strengths and apply them to concrete masonry should be avoided.

For example from Ref. (10), flexural tests on prisms constructed with type S mortar indicate a modulus of rupture for brick masonry of around 120 psi with a corresponding value for concrete masonry of 53 psi.

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Tensile bond strength from page 65 of Rtf.(2) for type S mortar (no = 1800 psi) with concrete masonry units indicates an average value of around 64 psi.

The mean value for Ref. (1) is 65 psi.

However, for brick masonry shear bond strength values of 150 psi have been presented (p II, Ref. (2)).

Thus for brick masonry it is possible that the ratio of shear bond to tensile bond is greater than 1.0 (2) The scatter of test results for both tensile bond and shear bond of type M mortar reported in Ref. (1) is considered to be due to the variability of the strength properties rather than due to the characteristics of the testing procedure. This is supported by data from Ref. (S) in which tensile bond strengths were measured for a grouted joint.

The measured mean tensile bond strength from 18 specimens was 139.3 psi. Results varied from 125 psi to 153 psi with a standard deviation of 6.8 psi and a coefficient of variation of 4.9%.

Also as mentioned above (Ref.(3)), a significant scatter in tensile bond and shear bond values for mortar joints is not unusual.

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l (3)

It is believed that the information presented by Prof.

Bresler concerning the size of the shear bond specimen and

'the radial distance from the center of the centrifuge to the I

center of,the specimen is correct.

(4)

Shear stresses were computed using the computed force at failure, based on the weight of separated portion, divided i

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by the measured shear area.

(9) (ie. shear bond strength = shear s

force / shear area.)

(5) Failure was not defined as initial slip on the shear plane.

Complete separation of the specimen constituted failure (9).

The mode of failure consisted of separation along the mortar-block interface although some mortar separation may also have occurred.

(ie, some mortar remained on the separated portion).

s An analysis of the weights of separated portions for type M mortar tests indicate a coefficient of variation of 4.25% for tensile bond and 8.5% for shear bond. These variations are not considered larEe as implied by Prof. Bresler.

Based on the information to date, the following conclusions are presented:

(1) Variability in the data is due to the erratic and complex nature

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of mortar adhesion.

The coefficient of variation for type M mortar tensile bond values is 22.9%.

The corresponding value using a grouted joint is 4.9%.

This indicates that the test method for l

a tensile bond is not the source of the variabilit/ of results.

The coefficient of variation for shear bond of 26.9% is close to that for tensile bond and suggests that the shearing bond test results are also a reliable indication of actual strengths.

l As noted above, strengths obtained by other testing methods also exhibit significant variation.

Therefore it does not seem justified to attribute the variability of the test results to the test method.

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) The data on tension and shear failures in cementitious materials which indicate that pure shear strength (in the abscence of secondary nor:a1 stresses) is equal to or greater than tension strength is not applicable to bond strengths of mortar.

As noted above the relationship between shear bond and tensile bond strengths of

( concrete masonry mortar joints are significantly different than the corresponding strengths of the mortar material.

In fact the mean values of 75 psi for tensile bond and 47 psi for shear bond are in reasonable agreement with other test values obtained for type M mortar and concrete masonry units.

(3)

Khile it is conceded that the centrifuge test method presented in Ref. (1) could be improved by increasing the radial distance to the specimen center of mass and that other refinement may also be suggested, the test method is considered to suffer from fewer deficiencies than other existing methods of evaluating tensile bond and shear bond strengths of mortar joints.

i (4) The existance of tension stresses at the mortar-block interface which would, as noted by Prof. Bresler, tend to reduce the shearing bond strength of the mortar joint, should be confirmed by a two-dimensional stress analysis of the test specimen. However, the support of the shear bond specimen was designed to minimize moments at the mortar joint and it is not anticipated that significant tensile stresses will be developed at these locations.

4 Prof. Sresler's prelir.inary

t would, '.:wecer, he useful to rev: -

c11:ulaticns 5:hi::- indicate sizeable tensien stresses at these i:.terfaces.

'5)

Finally it is cen:1uded that the ratio of mean shear bond to mean tensile bond of 0.6 for type M mortar provides a reasonable estimate of the relatier cf these strength values.

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REFERENCES (1) Hat:inikolas, ':., Longworth, J., and Warwaruk, J.,

" Evaluation of Tensile Bond and Shear Bond of Masonry by Means of Centrifugal Force",

Alberta Masonry Institute, Undated (1978-1979)

(2) Gensert, R.M.,

" Design and Detailing of Engineered Masonry with the New ACI Standard Building Code Requirements for Concrete Masonry Structures", ACI Seminar notes on Building Code Requirements for Concrete Masonry Structures, ACI Undated (1979-19S0)

(3) Heg emi er, G. A.,

" Mechanics of Reinforced Concrete Masonry: A Literature Study",

Report No. AMES-NSF-TR-7S-S, NSF Report, Sept. 197S (4) Meli, R.P., and Reyes, A.G.,

" Propiedades Mechanicas de la Mamposteria ",

Instituto de Ingenieria, Informe No. 288, Universidad Nacional Autonoma de Mexico, July 1971.

(S) Self, M.W.,

" The Structural Prop'erties of Load-Bearing Concrete Masonry ",

EIES Proj ect D-622, Engineering and Industrial Experitent Station, University of Florida, p. 67, March 1974 (6)

Balachandran, K.,

" An Investigation of the Strength of Concrete Masonry Shear WaII Structures ",

Ph.D. Dissertation, Univ.

of Florida, 1974.

(7)

Copeland, R.E.,

and Saxer, E.L.,

" Tests of Structural Bond Masonry Mortars to Concrete Block ",

Journal of the American Concrete Institute, 61, pp 1411-14S1, 1964 (6) Hat:inikolas, M, Longworth, J., and Warwaruk, J.,

" Concrete Masonry Walls ", Structural Engineering Report No. 70, The University of Alberta, Edmonton, Alberta, Sept. 197 (9)

M. Hat:inikolas, Private Communication, March. 19 (10)

Fattal, S.G.,

and Caltaneo, L.E.,

" Structural Performance of Masonry Walls Under Compressien and Flexure ", U.S. Dept.

of Commerce, National Bureau of Standards, Bldg. Science Series 73, June 1976.

(11)

Ferguson, P.M.,

" Reinforced Concrete Fundamentals ", Fourth Edition, John Wiley and Sons, 1979

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