Shear Strength Enhancement Mechanisms of Steel Fiber-Reinforced Concrete Slender Beams

摘要

An experimental study was conducted to identify the shear-enhance-ment and failure mechanisms behind the ultimate shear strength of steel fiber-reinforced concrete (SFRC) slender beams by using the full field-deformation-measuring capability of digital image correlation (DIC) technology. A total ,of 12 large-scale simply supported SFRC and RC beams with an overall height from 12 to 48 in. (305 to 1220 mm) were tested under monotonic point load up to failure. The greater shear strength in SFRC beams originates from the ability of the fiber bridging effect that delays the propagation of the cracks into the compression zone, whose shear strength is enhanced by the compressive stresses induced by the higher load. The slow progression of the cracks keeps the compression zone depth large, thereby enabling it to contribute to a higher shear resistance. In contrast with the traditional assumption for either plain concrete or SFRC beams, where the shear contribution resulting from dowel action is completely neglected, this research clearly shows that the dowel action has an appreciable effect on the ultimate shear strength. Its contribution varies from 10 to 30% when the beam depth increases from 12 to 48 in. (305 to 1220 mm). On the other hand, the compression zone's contribution decreases from 69 to 36% with the increase in beam depth. In addition, the shear contribution from the fiber bridging effect along the critical shear crack stays approximately unchanged at 20%, irrespective of the beam depth. In this study, the minimum shear strength obtained was in the range of 5√f_c' psi (0.42√f_c' MPa) for the beams with the greatest depth. This indicates that the maximum allowed shear stress limit of 1.5√f_c' psi (0.125√f_c' MPa) specified in ACI 318-14 is on the very conservative side.