Effect of different transverse reinforcement confinements on circular columns cast with recycled concrete aggregates and treated wastewater.

The use of recycled concrete aggregate (RCA) and treated wastewater (TWW) in the concrete industry promotes sustainability, conserves potable water, reduces environmental impact, and minimizes waste, offering an eco-friendly alternative to conventional concrete production. However, RCA and TWW can negatively impact concrete performance, as RCA's high variability, porosity, adhered mortar, and crushing-induced defects weaken the microstructure, while TWW's dissolved salts and organic compounds interfere with cement hydration. Therefore, it is essential to fully understand their effects on concrete performance at both the material and structural levels and develop strategies to mitigate these drawbacks. Contributing to this, the present study investigated the effect of using RCA and/or TWW on the strength of concrete and the behavior of reinforced concrete (RC) circular columns. A total of 10 mixes were developed with RCA replacement levels of 0%, 30%, 50%, and 100%, and TWW replacement levels of 0%, 50%, and 100%. These concrete mixes were used to cast 24 RC columns, which were tested under monotonic concentric load. The columns were reinforced with different types of transverse reinforcement-stirrups at various spacings (200 mm, 150 mm, and 100 mm) and spiral reinforcement at different pitches (60 mm and 40 mm)-to provide varying levels of confinement, aiming to mitigate the negative effects of RCA and TWW on column capacity. The results indicated that the capacity of columns negatively affected by incorporating RCA and/or TWW. In columns with 200 mm spaced stirrups, as the RCA replacement increased from 0 to 30% and 50%, the failure loads decreased by 16.1% and 29.8%, respectively. The use of closely spaced stirrups or spiral reinforcement provided higher confinement, effectively alleviating the reduction in capacity caused by the use of RCA. Reducing the stirrups space to 100 mm allowed for up to 30% RCA replacement with no reduction in capacity, and up to 100% RCA replacement with a maximum capacity reduction of around 14.9%. Similarly, the use of spiral reinforcement at pitch of 40 mm enabled the use of up to 100% RCA with a reduction of 8.6%, compared to corresponding columns cast with no RCA. Higher reductions in capacity were observed when TWW was used, which further limits its application in concrete.