Experimental Study of Cotton Waste Fibre Effects on the Structural Performance of High-Strength Concrete Deep Beams with Light Shear Reinforcement

The structural performance of reinforced concrete (RC) deep beams in both ultimate and serviceability limit states (ULSs/SLSs) is influenced by beam size effects and the adopted reinforcement ratios. In this study, the effects of cotton waste fibres (CWFs) and beam depth were studied using a four-point load test on five sets of 400 mm and 500 mm CWF high-strength RC deep beams with a 0.3% web reinforcement ratio. Control unnotched and notched, and notched specimens with 0–0.75% CWF content in each specimen set, were studied. The failure modes, stirrup and strut-and-tie zones’ strains, shear capacity, and load-deflection were analyzed. It was observed that the beam failure evolved from shear-compression in unnotched specimens to the web-splitting failure phenomenon in notched ones, coupled by 27.44% and 0.17% decline in ultimate shear capacity (Vu) in notched 400 mm and 500 mm beam sets, respectively. The notched RC deep beam specimens, mimicking the SLS shear performance parameters, showed a 34.28% and 14.71% Vu increase in 400 mm and 500 mm depth beams, respectively. The crack opening load increased from 13.87 kN to 44.63 k and from 11.88 kN to 35.19 kN in these beam sets, respectively. In a similar analysis, 84.76% and 38.39% stiffness increase, and 49.15% and 39.95% shear ductility index increase in the outer-most stirrup, were observed in the respective specimen sets. The contribution factor of the shear reinforcement to Vu improved from 0.31 to 0.94, and from 0.87 to 0.97 in 400 mm and 500 mm, with 0% and 0.75% CWF, respectively. These experimental results confirm the contribution of beam size effect and fibres to the shear performance properties of RC deep beams. The feasibility of using CWF in structural concrete is confirmed by its improvement of the SLS properties of the studied RC deep beam specimens with light shear reinforcement.