Spatial correlation of flexural bond strength for masonry walls: An experimental and statistical study

The unit flexural bond strength in clay brick unreinforced masonry (URM) walls is a key material property that affects their lateral load capacity. It is well known that the flexural bond strength of individual units (one unit is defined as a brick and the underlying mortar bed that brick is attached to) varies considerably between units, and that this variability might significantly affect the structural performance and reliability of URM walls in flexure. The paper describes an experimental program where six full sized clay brick URM walls were constructed to replicate field conditions. The walls were constructed by four masons to represent a range of mason workmanship. The timing and placement of different batches of mortar were closely monitored. The flexural bond strength of each unit in each wall was obtained using the bond wrench test. This provided the data for a statistical analysis to assess the spatial correlation of unit flexural bond strengths – i.e. the degree of correlation between units within and between courses, and how such correlation may depend upon the placement of each mortar batch and mason workmanship. It was found that there is little correlation between units either within or between courses. It is thus recommended that flexural bond strengths between units are statistically independent. The study also found that the clay brick wall unit flexural bond strengths best fit a truncated Normal probability distribution.