Enhanced mechanical and microstructural properties of sustainable hybrid fiber reinforced self-compacting concrete containing treated recycled aggregate

Recycled concrete aggregates have been developed as a replacement for decreasing natural sources of normal aggregates. Despite the use of numerous treatment strategies to improve the performance of recycled concrete aggregate (RCA), its properties remain inferior to those of natural coarse aggregate (NA), necessitating future treatment in this area. This study aims at investigating the following influencing parameters: (a) three types of coarse aggregate (natural coarse aggregate (NA), recycled concrete aggregate RCA, and treated recycled concrete aggregate TRA); (b) three types of steel fibers (Micro, Hooked, and Hybrid), (c) three volume fractions of steel fibers: 0%, 0.5%, and 1%; and (d) two treatment methods: (i) enhanced microstructure of RCA by cement silica fume slurry (CSFS) and (ii) enhanced structure of sustainable SCC mixes by steel fibers. Eight SCC mixes were utilized in the experimental work with reference to NA, RCA, TRA, and TRA + SF, reinforced with varying types of steel fiber amounts (0%, 0.5%, and 1%). Slump flow, T500, V-funnel, compressive strength ($${f}_{cu}$$), Splitting tensile strength ($${f}_{st}$$), and flexural strength ($${f}_{r}$$) tests were performed in accordance with the appropriate standards. The test results indicated that the fresh characteristics of SCC mixes were adversely affected by the use of untreated RA, particularly at higher replacement amounts. While a drop in concrete strength for untreated RCA mixes was observed compared to the reference mix. The treated RA enhanced the concrete mechanical properties by about 12%, 11%, and 10% for $${f}_{cu}$$, $${f}_{st}$$, and $${f}_{r}$$, respectively, compared to the untreated RCA. On the other hand, for mixes with steel fibers, it was noted that incorporating steel fibers led to a considerable increase in $${f}_{cu}$$, $${f}_{st}$$, and $${f}_{r}$$ values, particularly when 1% hybrid steel fibers (HYSF) was added. No Clinical trial was used for conducting this study.