Masonry Magazine October 1995 Page. 21
unit, the head mortar joint is subject to tensile stresses. Bond at this interface will reduce vertical cracking. If clay masonry units with slight expansive characteristics constitute the major portion of the masonry, the head mortar joint will be in compression and bond strength demands are minimal.
Horizontal (bed) mortar joints bond adjacent courses of masonry together. As masonry is subject to volume change (expansion or shrinkage) from moisture or temperature change, the bed joint resists shear longitudinally. When subject to wind loads (pressure or suction), the mortar joints are subject to flexural stresses. The outer surface of the mortar joint receives the greatest stress and, when failure occurs, the stress is direct tension. In addition to the above conditions, there is an imposed load on the masonry that can stress both joints, albeit, minimal.
During the development of a bond test method, the above stress conditions should be considered. It should also be recognized that bond strength is less than the tensile strength of the mortar, elsewise tensile failure will occur within the mortar and not at the interface. Thus bond strength will be a percentage of the mortar tensile strength, directly dependent on the compatibility of the combined masonry.
Two bond test methods will be considered here: the crossed brick couplet test method and the bond wrench test method. The crossed brick, bond test method involves two masonry units bonded together such that the units cross each other at their centers. The unit after conditioning is subject to tension by transferring the downward force of a compression machine to tension throughout the test specimen cross sectional area using a jig. The ease of constructing and testing specimens promoted its early acceptance. More recently it has been shown that the elasticity of the units affects the measured tensile bond strength. Additionally, the large joint to bonded surface area and corner loading cause the method to be in dis favor.
The bond wrench test method, ASTM C 1072, more realistically subjects test specimens to those flexural stresses the masonry walls encounter. Tension at the extreme fiber of the bed joint is measured. The major disadvantages of the test method is the atypical moisture expo-
TENSILE BOND
STRENGTH,
HALF-BRICK COUPLETS,
DIRECT TENSION, psi
300
250
200
150
100
50
0
50
Line of Equality
100
Clay Brick
Expanded Shale
Concrete Brick
150
200 250
TENSILE BOND STRENGTH,
CROSSED-BRICK COUPLETS,
TRIPODS, psi
FIGURE 2 - Using essentially the same test method and a direct tension test mehod,
data showed poor correlation between crossed brick and direct tension results.
sure of test assemblies during conditioning compared to mortar and units in a masonry segment. Whichever bond test method is used, it should be recognized that all masonry mortar is prone to shrinkage tensile stresses when allowed to air dry. Thus when partially dried specimens are subjected to additional tension, the measured apparent load does not indicate the total tensile strength.
Bond Test Results
The crossed brick test method allowed developing certain existing recommendation. The present recommendation to prewet clay masonry units is, no doubt, traceable to this research study [1]. As depicted, tensile bond is optimum when the IRA of the unit is approximately 20 grams per minute per 30 square inches. A less well promoted interpretation of these depicted data is that respectable bond strengths can be obtained using the same units while altering the mortar composition. Using essentially the same test method and a direct tension test method, Kuenning's [2] data showed poor correlation between crossed brick and direct tension results. The reason is related to the bending of the clay masonry units during testing. The bond strength test method applies a tensile force throughout the tensile zone of the assemblage. The tensile force is maximum at the extreme fiber (outer surface) of the mortar joint. Failure than progresses throughout the interface. Once failed the mortar and unit separate.
Tests using the bond wrench apparatus show the omnipresent high coefficient of variation common during any tensile or flexural test. Using laboratory prepared concrete, "standard brick and portland cement-lime mortars for preparing the test assemblages and a bond wrench test apparatus, lower coefficient of variations (14 to 44%, within laboratory) were realized [3]. Under the tight laboratory control
MASONRY-SEPTEMBER/OCTOBER, 1995 21