Experimental study of NaOH pretreated crumb rubber as substitute of fine aggregate in concrete

Accession of discarded scrap tires can cause severe environmental and health dangers through air, water, and soil pollution. This danger can be overcome by partially replacing crumb rubber in concrete with fine aggregate. Unlike fine aggregate, the surface of crumb rubber is smooth and non-porous due to the presence of oil stains and the chemical nature of rubber. Therefore, pretreatment with sodium hydroxide has been introduced to eliminate substances from the surface of crumb rubber, causing voids on the outer rubber surface, resulting in a coarse and more porous surface than untreated rubber. However, the effect of NaOH concentration, along with the duration of time, has not been discussed. Therefore, in this study, crumb rubber has been treated with different concentrated solutions of NaOH (0.1 mol, 0.5 mol, and 1 M) for different durations (2 h and 24 h). Then mechanical performance of hardened concrete (7 days, 28 days and 90 days compressive strength and 28 days split tensile strength) was used to study this impact by replacing 2 %, 5 %, 10 % and 20 % of fine aggregate with crumb rubber. The results reveal that pretreatment affects strength negatively for 2 % dosages of crumb rubber treated with 0.5 mol and 1 M solution for 2 h, as a 3.07 % decrease in 7 days compressive strength has been recorded compared to untreated. Meanwhile, pretreatment for a high 20 % dosage of crumb rubber with 1 M NaOH solution for 24 h, shows 19.86 % and 14.3 % recovery in lost 28-days compressive and split tensile strength compared to concrete prepared with untreated crumb rubber. A numerical relation in-between both mechanical characteristics has also been developed for crumb rubber concrete (CRC) with R-2 = 0.914, which has shown a strong correlation compared to previously developed numerical equations.