Shear Strength of Stirrup-Free Recycled Aggregate Concrete Beams Reinforced with Steel Fibers
Abstract
The shear strength of recycled aggregate concrete (RAC) beams is usually lower to that of counterpart natural aggregate concrete (NAC) beams. The use of steel fibers can recover some of the shear strength, but research is needed to develop practical design tools for RAC beams reinforced with steel fibers. This article studies experimentally and analytically the shear strength of stirrup-free RAC beams reinforced with commercial steel fibers. Sixteen beams were tested in four-point bending using a shear-span-to-depth ratio (a/d) of 3.2. Fourteen of such beams were cast with RAC using 100% recycled concrete aggregates (RCA) reinforced internally with different fiber volume fractions (Vf = 0%, 0.5%, 0.75%, 1.0% or 1.5%), types of steel fibers (hook ends or double hook ends), and longitudinal reinforcement ratios (rw = 1.6% and 2.5%). Another two control beams were cast with NAC without steel fibers. It was found that adding a minimum Vf = 0.5% of steel fibers to the RAC beams improved the shear strength by more than 51%. Steel fibers significantly improved shear behavior at volume fractions >0.75% and prevented shear failures in RAC beams. A shear stress limit of square root of f'c, as recommended by ACI 318-19 for normal beams with steel fibers used as shear reinforcement, can be conservatively adopted for the design of RAC beams with steel fibers and without stirrups. A new modification factor to the ACI 318-19 shear equation is also proposed to explicitly account for the effect of steel fibers and longitudinal reinforcement ratios on the shear strength of RAC beams. The findings in this article contribute towards the development of practical guidelines for the design of RAC elements with steel fibers subjected to shear.