Effect of concrete cover on the pure torsional behavior of reinforced concrete beams

Abstract

Torsional behavior of RC members is dependent on various factors including the amount of rebar, concrete strength, member cross-section, depth to width ratio, and concrete cover. Although there is an enormous amount of knowledge on the effect of the above-mentioned factors on torsional behavior, studies pertaining to the effect of concrete cover is scarce. In the current study, a total of nine RC beams with clear concrete cover thickness ranging from 16 mm to 46 mm were experimentally investigated for pure torsion. Coupled with concrete cover depth, the effect of transverse reinforcement spacing and compressive strength of concrete were systematically investigated. Moreover, the experimental results were used to examine the prediction of commonly used codes including ACI, CSA, and Eurocode. To have a holistic view, the reliability of existing prediction graphical methods developed based on the well-known and accepted theories of MCFT and STM as well as recently developed empirical equations were also examined. The experimental results indicated the presence of relatively larger cover greatly affects both the ultimate capacity and overall torsional behavior of RC members. When compared with the experiments, ACI provides a highly conservative estimate, particularly with the increase of the cover depth. On the contrary, CSA and Eurocode provide unsafe predictions. Although the prediction is on the unsafe side, CSA and Eurocode gave a consistent prediction with smaller scatter for the different concrete depths. The prediction of the models based on the graphical methods and the empirical equation were also found to be unsafe with the effect becoming highly pronounced for cases with relatively large concrete cover. Overall, the experimental investigation showed the thickness of cover concrete has a significant effect on the pre and post-peak torsional behavior of RC members.