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. Three fiber volumes of 0, 1.6 and 2.3 % used in this study. 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 control mixture (control) without fibers and a mixture containing hook-end manufactured steel fibers (MSF) were prepared. 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 control mixture. Moreover, the results showed that both the hybridized and non-hybridized RTSF mixtures had lower flexural strengths, ranging from 13 to 17 MPa, compared to the MSF mixture, which had a flexural 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 flexural 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 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 manufactured steel fibers.