International Concrete Abstracts Portal

The effects of carbonated aggregate and aggregate replacement ratio on the stress-strain behavior of recycled aggregate concrete (RAC) under uniaxial compression were studied, and based on Lemaitre's strain equivalence hypothesis and Weibull distribution, a damage constitutive model was proposed. The results showed that carbonated aggregate enhanced the peak stress. As the aggregate replacement ratio increased, the slopes of both the ascending and descending sections of the stress-strain curve gradually decreased, resulting in reduced peak stresses and decreased material brittleness. Besides, the damage constitutive model modified using linear regression analysis could describe the stress-strain curves well. As the aggregate replacement ratio increased, the slope of the “S” curve representing the damage variable evolution law gradually slowed down, and the corresponding strain gradually increased when the damage variable was 1. Meanwhile, the shape of the “parabola” curve representing the damage variable evolution rate became wider, and its vertex gradually decreased.

This study investigates the behavior of recycled steel fibers (RSFs) recovered from waste tires and industrial hooked-end steel fibers (ISF) in two single and hybrid reinforcement types with different volume content, incorporating microstructural and macrostructural analyses. Scanning electron microscopy (SEM) is used to study the microstructure and fractures, focusing on crack initiation in the fiber interface transition zone (FITZ). The macrostructural analysis involves using digital image correlation (DIC) software, Ncorr, to analyze the split tensile behavior of plain and fiber reinforced concrete (FRC) specimens, calculating strain distribution and investigating crack initiation and propagation. The SEM study reveals that, due to the presence of hooked ends, industrial fibers promoted improved mechanical interlocking; created anchors within the matrix; added frictional resistance during crack propagation; significantly improved load transfer; and had better bonding, crack bridging, and crack deflection than recycled fibers. RSFs significantly delay crack initiation and enhance strength in the pre-peak zone. The study suggests hybridizing recycled fibers from automobile tires with industrial fibers as an optimum strategy for improving tensile performance and using environmentally friendly materials in FRC.

To make full use of recycled aggregate concrete (RAC), carbon fiber (CF) and nano-metakaolin (NMK) were mixed into RAC to improve its mechanical properties and microstructure. The effects of NMK content, CF content, recycled aggregate (RA) replacement rate, and CF length on the compressive strength, splitting tensile strength, and tension-compression ratio of RAC were studied by the orthogonal test method; then, the test results were analyzed. The results show that the NMK content and RA replacement rate have significant effects on the compressive strength of RAC, while the CF content has significant effects on the splitting tensile strength and the tension-compression ratio. Through the synergistic effect of NMK and CF, the pore structure characteristics of RAC are improved, and the bond strength of the interfacial transition zone (ITZ) of CF-mortar is increased, which further enhances the strengthening effect of CF; thus, the mechanical properties of RAC are continuously enhanced.

Compared to external curing, internal curing enables the judicious use of available water to provide additional moisture in concrete for more effective hydration and improvement in the performance of concrete structures. However, certain challenges with the incorporation of internal curing materials (ICMs) still need to be addressed, as their effectiveness depends on several factors. Furthermore, sustainable construction demands the use of recycled materials, and this paper discusses the comparative evaluation of recycled aggregate (RA) as an ICM, along with two other types of ICMs, on various properties of high-performance concrete in the hardened state under two curing conditions. Concrete mixtures were prepared with pre-wetted RAs, superabsorbent polymers (SAPs), and pre-wetted lightweight volcanic aggregates (LWVAs) as ICMs. Concrete performance was compared through the investigation of the strength development, shrinkage, mass loss, and volumetric water absorption. In addition, the change in internal humidity of concrete with time at different stages of hardening was determined. The compressive strength results showed that RA and LWVA are more efficient in early days, and the performance of SAP is better in the later age due to its slow water releasing capabilities. Compared to the control mixture, the least reduction in strength of 4% and 8% at 28 days and 90 days, respectively, could be observed for the mixtures containing RA under both air and water curing.

The attached cement paste in recycled concrete (RC) aggregateleads to its potential reactivity against sulfate ions. Several testmethods were evaluated to find a suitable, reliable, and accuratemethod to evaluate the potential reactivity of aggregates. Differentquality RC aggregates were used to apply those methods. Thestudies included evaluations of concrete cores drilled from sourceconcrete (SC), RC aggregates, recycled mortar bars under differentexposures, and RC prisms exposed to external sulfate attack (ESA).The concrete core test allowed qualifying SC as potentially reactiveagainst sulfate in a short time. Tests on recycled aggregates andrecycled mortar bars showed variable sensitivity levels. Resultsfrom concrete prisms showed an effective reactivity of recycledaggregates when the replacement is higher.