Effectiveness of ultra-high-performance concrete containing recycled tire steel fibers and high-volume scoria rock powder as natural pozzolan

This study investigated the efficacy of the production of ultra-high-performance concrete (UHPC) with optimized recycled tire steel fibers (RTSF) from various sources (D & R). The incorporation of high-volume scoria rock powder as a cement replacement made the mixtures more environmentally friendly. The two sources of RTSF have different geometries and aspect ratios. Accordingly, the early fresh and 28-day hardened properties of UHPC varied according to the type of RTSF. A hybridization of the two RTSF sources was performed and studied in their combinations. The best source was further sieved to obtain shorter fibers. For comparison, a reference mixture was prepared containing hook-end manufactured steel fibers (MSF). The compressive strength results of the UHPC mixtures with individual RTSF showed that both D and R improved the strength by 8 and 10%, respectively, compared to the mix with MSF. Moreover, the results showed that both the hybridized and non-hybridized RTSF mixtures had lower compressive strengths, ranging from 13 to 17 MPa, compared to the control MSF mixture, which had a compressive strength of 19 MPa. This represents a reduction of 31% to 10%, respectively. Furthermore, among these mixtures with RTSF, those hybridized ones with 50% D and 50% R in addition to 25% D and 75% R showed the highest performance (17 MPa). However, the mixtures with sieved RTSF fibers (R) of high volume (2.3%) showed higher flexural strengths of 21 MPa representing an increase of 10% compared to the reference MSF mixture made with 1.6% volume. Therefore, higher amounts of fibers, with properties improved by sieving, could be used to offset costs and achieve performance comparable to, or even better than, that of more expensive fibers.