Shear strength of longitudinally reinforced recycled aggregate concrete beams

The inferior qualities of recycled concrete coarse aggregates (RCA) relative to natural coarse aggregates (NCA) can unfavorably affect the properties of concrete made using RCA. The detrimental effects can be more pronounced in the resistance mechanisms which rely significantly on the properties of the coarse aggregates, such as the shear resistance. This paper presents the results of an experimental investigation of the effects of the use of RCA on the shear strength of longitudinally reinforced concrete (RC) beams. Thirteen lightly reinforced 35 MPa concrete beams were tested in a four-point loading test setup at a shear to span ratio of 3. In one series of specimens, the percentages of replacement (PR) of NCA with RCA were 0%, 10%, 20%, 35%, 50%, 75% and 100%, and the partial replacement affected all grades of the aggregates equally. It was shown that the use of RCA decreased the shearing strength by 13%-18%. In another series of specimens, the PR was 5%, 10%, 16%, 23% and 35% and the replacement affected the smaller grade of coarse aggregates only. The detrimental effects of the use of RCA were alleviated for PR ≤ 15%. The experimentally observed shear strengths from this study and from thirty six test data available in the literature were compared with the calculations of the ACI simplified method, the CSA General Method, and the critical shear crack method (CSCT). Based on the experimental results and the comparisons, it was shown that the “square root of the compressive strength of the concrete” term does not adequately account for the detrimental effects that the shearing strength undergoes when RCA are used. It is suggested that a 20% reduction is applied to the shear strength equations of longitudinally reinforced beams when RCA are incorporated in the concrete.