Flexural, shear, and mechanical behaviour of self-consolidating-rubberized concrete with/without steel fibres

This research program focuses on investigating the applicability of using crumb rubber (CR) as a replacement for fine aggregate in developing a novel eco-friendly concrete type suitable for structural applications, especially when self-consolidating concrete (SCC) is used. Four extensive experimental studies have been carried out on both small- and large-scale concrete specimens to accomplish the research objective. The first and second studies aimed to optimize a number of successful self-consolidating rubberized concrete (SCRC) and steel fibre SCRC (SFSCRC) mixtures with maximized percentages of CR and minimized reduction in the stability and strength. The first parametric study included 27 SCRC mixtures developed with different binder contents (500-550 kg/m³), supplementary cementing materials (SCMs) (metakaolin (MK), fly ash (FA), and ground granulated blast-furnace slag (GGBS)), coarse aggregate sizes (10-20 mm), and entrained air admixture. The second parametric study included 19 SFSCRC mixtures developed with different binder content (550–600 kg/m³), steel fibre (SF) volume fractions (0.35% and 0.5%), and lengths of SFs (35 and 60 mm). In this study (study 2), another three vibrated rubberized concrete (VRC) and four steel fibre VRC (SFVRC) mixtures were developed for comparison. In both study 1 and 2, the fresh and mechanical properties of the developed mixtures were evaluated using small-scale specimens. The fresh properties included flowability, passing ability, high-range water-reducer admixture (HRWRA) demand, coarse aggregate segregation, and CR distribution. On the other hand, the evaluation of mechanical properties in both study 1 and 2 included compressive strength, splitting tensile strength (STS), flexural strength (FS), modulus of elasticity, impact resistance, ultrasonic pulse velocity, and acoustic emission measurements. The third and fourth studies evaluated the structural performance (flexural and shear) of large-scale reinforced concrete beams made with SCRC, VRC, SFSCRC, and SFVRC. In these studies, a total of 36 optimized mixtures from study 1 and 2 were selected to cast 36 large-scale reinforced concrete beams to be tested in flexure and shear (24 beams in flexure and 12 beams in shear). The performance of some design codes and empirical equations in predicting the first cracking moment, flexural, and shear capacity of the tested beams was also evaluated in study 3 and 4. The results showed that using CR in SCRC helped to develop mixtures with improved impact resistance, acoustic absorption capacity, and lower self-weight, but their stability, fresh, and mechanical properties were decreased. However, using higher binder content, different SCMs, and entrained air in SCRC improved their fresh properties and allowed high percentages of CR to be used, successfully. Moreover, MK was found to be the most effective SCMs that could obviously improve the stability and strength of SCRC. Although using SFs in SCRC mixtures negatively affected the fresh properties of the mixtures, they proved to have a significant enhancement on the mixtures’ strengths, especially STS, FS, and impact resistance. Since the challenge to optimize mixtures with high flowability and passing ability was not a factor in developing vibrated mixtures, it was possible to develop SFVRC mixtures with higher percentage of CR and SFs. This high combination of CR and SFs provides a new concrete composite with further improvement in ductility, toughness, impact resistance, and with further reduction in self-weight. The results of the flexural testing conducted in study 3 indicated that increasing the CR appeared to narrow crack widths and improve deformability of SCRC and VRC beams at given load. The safe use of CR in structural applications was found to be 15%. Further increase in the CR content showed a significant reduction in the first cracking moment and ultimate flexural capacity of the tested beams, while the ductility and toughness did not show a confirmed effect for the higher percentages of CR. On the other hand, in SFVRC, the addition of 1% SFs (35 and 60 mm) helped to extend the possible safe content of CR to 35%, achieving successfully semi-lightweight concrete beams with a sufficient capacity, ductility, and toughness for multiple structural applications. In shear testing conducted in study 4, using CR in SCRC and VRC beams showed a reduction in their shear capacity, post-diagonal cracking resistance, and energy absorption. These reductions could be alleviated by inclusion of SFs. The composite effect of CR and SFs also helped to narrow the developed cracks and change the failure mode from a brittle shear failure to a ductile flexural failure, particularly for SFs volume of 1%. The comparisons between the predictions and the experimental results (obtained from study 3 and 4) indicated that most of the proposed equations can satisfactorily estimate the flexural and shear capacity, but the first cracking moment was overestimated.