Bond strength performance characteristics of brick-mortar interfaces.

This work uses a direct tensile test approach, developed in-house, to measure the bond strength of brick-mortar interfaces. The thesis postulates that direct tensile bond strength values can be used as a means of assessing compatibility of brick and mortar properties which directly influence the structural and durability performance of brickwork. The work identifies that direct forms of tensile testing are suited for comparative research more readily than flexural testing techniques, which induce inherent variability to the test system due to sample format and by application of the associated flexural bending theory. It is maintained that flexural bending tests reflect parameters which influence disproportionately the compressive strength of the mortar and the joint periphery. The work uses traditional volume ratios for mortars, proportioned to a constant mass of sand, to identify the effect that discrete changes in cement and lime content have upon bond strength performance of the mortar. The results show that cement content of mortar has no significant influence on bond strength, provided that the combined proportion of cement and lime maintain a 1:3 ratio by volume with the sand. Furthermore, it has been shown that the volume of the mix water should match the volume of cementitious material, in order to achieve suitable workability. The work produces a bond strength development curve for samples aged between 5-minutes and 2-years of age and concludes that bond strength does not develop in the same manner to compressive strength and that bond strength may decline post 28-days. Samples up to 2-years in age can demonstrate up to 40-percent loss of bond due to the effects of sustained drying shrinkage. Consequently the work questions the value of using 28-day strength tests as a means of predicting future bond strength performance. It is identified that the controlling parameter which effects bond strength development is the removal of the excess mix water from the mortar by brick suction forces. The work examines unit water absorption characteristics and identifies that the initial rate of suction test is not sufficiently representative of a unit's ability to remove water from a retentive mortar bed. In response a unique method, which measures the continuous water uptake of the brick bed-face is presented. The resulting water absorption profile identifies the rate of change of flow and the resulting force function, with which water is potentially extracted from the retentive mortar bed. Results show that a good correlation between a unit's suction force and bond strength can be attained. It is presented that initial bond strength is developed by volumetric plastic shrinkage of the mortar bed, induced by rapid removal of the excess mix water by brick background suction, which generates a mechanical lateral gripping action to the undulations of the brick bed-face.