Experimental determination of double-K fracture parameters for self-compacting concrete with waste tire rubber and silica fume

In order to address environmental issues caused by the large amount of waste tire rubber, which is often deposited uncontrollably, its use in concrete, by replacing a portion of natural aggregates with waste rubber particles, seems to be an effective approach. At the same time, it must be noted that such an action will likely change many of its mechanical parameters, as well as the fracture and cracking behavior of self-compacting concrete. With that in mind, mechanical and fracture properties of self-compacting concrete with crumb rubber and silica fume were investigated in this study. A total of 14 self-compacting concrete mixtures were prepared with constant water to binder ratio of 0.4 and a cement content of 450 kg/m³. Chemical admixtures were used to maintain the desirable fresh properties of self-compacting concrete and to achieve adequate aggregate distribution during casting of the test samples. The experimental program focused on determining the fracture toughness of self-compacting rubberized concrete (SCRC). Fracture properties were determined using the wedge splitting test (WST) as an alternative to three-point bending test (3PBT) and were subsequently described using the double K-fracture model. Test results also indicated that mixtures containing up to 15 % of crumb rubber yielded satisfactory results for both initial and unstable fracture behavior. Up to the 15 % replacement level, K_Ic,ini remains approximately equal to the reference value, with a maximum reduction of 16 %. Considering fracture process, the addition of rubber alters the fracture mechanism of SCC from brittle/quasi-plastic to quasi-plastic. In addition, scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to analyze the interfacial transition zone (ITZ). Microstructural analysis confirmed that the effect of silica fume on improving interfacial bonding with crumb rubber is limited due to the inherent chemical incompatibility and hydrophobic nature of rubber particles.