Experimental Results

Experimental results are summarized in Table 1 and typical load deflection curves are presented in Figure 4. All beams failed in shear prior to reaching their flexural capacity. The large beams without stirrups exhibited extremely brittle behaviour, failing at ratios of mid-span deflection to span of 1/750 or less. Failure was sudden and was preceded by relatively little cracking. The large beam with stirrups (SB-10- H-S) failed after a significant amount of cracking and deflection. Prior to failure, the failure shear crack reached a width of 4mm, and the Д/L ratio was 1/300. The use of minimum stirrups and additional longitudinal steel increased the shear capacity by 2.9 times over the equivalent specimen without stirrups, providing a dramatic and beneficial effect on the shear strength of the section. The small beams all exhibited slightly greater ductility. The small beam with stirrups was the most ductile of all the specimens with a Д/L ratio of 1/200. Like the equivalent large specimen, this beam was extensively cracked prior to failure. However, the use of minimum stirrups and additional longitudinal steel increased the shear capacity by only 1.8 times over the equivalent beam without stirrups. Thus, stirrups are far more effective for deep members than for shallow members.

The results shown in Figure 4 also indicate that the failure load generally increased for increasing aggregate size. The most likely explanation for this result is the increased surface roughness of the failure shear cracks caused by the larger aggregate. That is, failure was initiated at a higher shear stress in the
beams with large aggregate due to enhanced aggregate interlock capacity at the cracks. Note that Specimen SB-10-H-1, constructed with high-strength concrete, had the lowest peak load of all the large beams tested. This was due to reduced aggregate interlock caused by aggregate fracturing. In this case the effective aggregate size was 0mm.

The typical progression of failure in a large beam without stirrups is illustrated in Figure 5. These photos are of the shear critical region on the east side of Specimen SB-40-N-1 and were taken with a high-speed digital camera. The cracks have been digitally enhanced for clarity. In Figure 5a, the progression of cracks at 98% of the peak load is shown, and it can be clearly seen that a dominant flexural-shear crack has formed. The cause of the size effect is clearly demonstrated, in that the average longitudinal spacing of the cracks increases from 150mm at the level of the steel to 900mm at the beam mid-depth. In order to maintain a linear strain profile, these cracks are wider near the beam mid-depth than they are at the level of the steel. As the load is increased to the peak load, the dominant flexural shear crack extends slightly (5b). After the peak load has been reached, the dominant shear crack extends towards the loading point and widens (5c, 5d). After the dominant crack extends towards the load point, it also extends back towards the support point (5e) due to dowel forces in the longitudinal steel. At final failure, shown in Figure 5f, the failure crack is very wide and the bottom cover has been ripped from the bottom steel.