Proportion of Shear Carried in Compression Zone

In order to definitively demonstrate the importance of aggregate interlock, the compression zone at the head of dominant shear cracks in several beams was instrumented with two columns of longitudinal concrete strain gauges. This was done with the intention of measuring the shear force carried by the compression zone.

As shown in Figure 8, the flexural strains measured by columns of gauges spaced at 80mm on the 365mm deep compression zone of Beam SB-10-N-2 allowed for the calculation of adjacent flexural stress profiles. Equilibrium requires that a horizontal complimentary shear stress of v = AC/(bw x 80) act at every level of the compression zone. Equilibrium further requires that the vertical shear stress equals the horizontal shear stress at every level of the compression zone. This allows for the vertical shear stress distribution to be calculated, as shown in Figure 8. The total shear force acting in the compression zone is therefore equal to V = JA v • dA. For the shear stress distribution shown in the figure, the shear force carried in the compression zone is 50kN, representing 24% of the total shear force acting at the section. Clearly, then, 76% of the remaining shear must be carried by aggregate interlocking along the dominant shear crack and dowel forces. However, the shear crack is vertical at the level of the steel, indicating that the dowel forces transferred by the longitudinal reinforcement are extremely small.