EVALUATION OF DUCTILITY AND STIFFNESS OF SHEAR-CRITICAL BEAMS INSTANT CONCRETE WITH f_c^' = 25 MPa (14-DAY TARGET) AND INSTANT CONCRETE WITH f_c^' = 25 MPa (28-DAY TARGET)

Abstract

Limited research on the performance of instant concrete in shear-critical beams, particularly at the early age of 14 days, highlights the necessity for further investigation. This study evaluates the ductility and stiffness of reinforced concrete shear beams utilizing instant concrete with a target compressive strength  of 25 MPa at 14 and 28 days. The primary objective is to analyze the structural capacity, ductility, stiffness, and the resulting crack patterns. Four specimens with dimensions of 120×200×2120 mm were designed without shear reinforcement to induce shear failure and were subjected to monotonic testing under symmetrical loading. Material testing results demonstrated that the actual average compressive strengths exceeded the design targets, reaching 32.69 MPa (14 days) and 35.50 MPa (28 days). Structurally, the 28-day beams exhibited higher experimental stiffness compared to the 14-day beams. All specimens displayed low ductility values according to the FEMA 356 classification, indicating brittle behavior attributed to the absence of shear reinforcement. Stiffness analysis confirmed that the 28-day beams possessed greater stiffness than the 14-day beams, correlating with the increase in concrete compressive strength. The failure mechanism was dominated by significant diagonal shear cracks extending from the supports to the loading points. The study concludes that while instant concrete offers the advantage of rapid strength gain, the inclusion of shear reinforcement remains essential to enhance ductility and prevent brittle failure in structural elements.