INVESTIGATION OF STRENGTH-ENHANCEMENT MECHANISM AND SIZE EFFECT ON ULTIMATE SHEAR STRENGTH OF STEEL FIBER-REINFORCED CONCRETE (SFRC) SLENDER BEAMS

Abstract

This research study consists of two separate phases. In the first phase, an experimental study was conducted to identify the shear-enhancement and failure mechanisms behind the ultimate shear strength of steel fiber-reinforced concrete (SFRC) slender beams by utilizing 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 a range of heights including 12 in. (305 mm), 18 in. (457 mm), 24 in. (610 mm), 36 in. (915 mm), and 48 in. (1220 mm) were tested under monotonic point load. The greater shear strength in SFRC beams stems from the fiber bridging effect which delays the propagation of the cracks into the compression zone. In contrast to 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% as the beam depth increases from 12 in. (305 mm) to 48 in. (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 virtually unchanged at 20%, regardless of beam depth. In this study, the minimum shear strength obtained was in the range of 5 SQRT (f'c) psi (0.42 SQRT (f'c) MPa) for the beams with the greatest depth. This indicates that the maximum allowed shear stress limit of 1.5 SQRT (f'c) psi (0.125 SQRT (f'c) MPa) specified in ACI 318-14 is on the very conservative side. While the size effect on ultimate shear strength of plain concrete beams has been well researched in the past decades, limited tests were carried out to study the extent and mechanism of size effect in steel fiber-reinforced concrete (SFRC) beams. Current American Concrete Institute’s ACI 318 Building Code restricts the use of steel fiber as minimum shear reinforcement to beams with a height up to 24 in. (610 mm). In the next phase of the study, in addition to the analyzing of the current testing data, the laboratory test results from the first part of the study and the respective digital image correlation (DIC) images were examined to identify the underlying factors that cause size effect on ultimate shear stress of SFRC slender beams. Moderate size effect was observed in the beams tested in this study. Through the full field strains and a mechanical based analysis, it was found that the size effect is a function of both the beam height and the shear span length. In larger beams, due to the greater horizontal and vertical distance from the compression zone to the supports, the critical diagonal shear crack was able to propagate deeply into the top of the beams. As a consequence, the compression zone exhibits less contribution to shear resistance in larger size beams, and the dowel action becomes more critical. Therefore, a minor flaw in dowel zone such as lacking well-distribution of steel fibers results in early destruction of dowel resistance and shear failure.