The early-age material parameters of three-dimensional (3-D)-printable concrete defined under the umbrella of printability, namely, pumpability, extrudability, buildability, and the “printability window/open time,” are subjective measures. The need to correlate and successively substitute these subjective measures with objective and accepted material properties, such as tensile strength, shear strength, and compressive strength, is paramount. This study validates new testing methodologies to quantify the tensile and shear strengths of printable fiber-reinforced concretes still in their fresh state. A tailored mixture with high sulfoaluminate cement and nonstructural basalt fibers has been assumed as a reference. The relation between the previously mentioned parameters and rheological parameters, such as yield strength obtained through International Center for Aggregates Research (ICAR) rheometer tests, is also explored. Furthermore, in an attempt to pave the way and contribute toward a better understanding of the mechanical properties of 3-D-printed concrete, to be further transferred into design procedures, a comparative study analyzing the work of fracture per unit crack width in three-point bending has been performed on printed and companion nominally identical monolithically cast specimens, investigating the effects of printing directions, position in the printed circuit, and specimen slenderness (length to depth) ratio.

This research examines the performance of quality-controlled recycled concrete aggregates (QRAs) with fly-ash-based cement. Compared to concrete made from untreated recycled concrete aggregates (URC), quality-controlled recycled aggregate concrete (QRC) has superior physical, mechanical, and durability properties. Except for sorptivity, the physical, mechanical, and durability properties of QRC are almost identical to those of natural aggregate concrete (NC). The compressive strength, splitting tensile strength, flexural strength, fracture energy, and modulus of elasticity of QRC are higher than those of URC by 18.0%, 16.8%, 60.0%, 27.17%, and 43.46%, respectively. The abrasion resistance of QRC is approximately 60% higher than URC. Scanning electron microscope (SEM) image and energy-dispersive X-ray (EDX) analysis prove that quality control produces denser old interfacial transition zones (OITZ) with fewer microvoids. The QRA improves not only the pore structure but also the weak mortar structure attached to the aggregate. There is also a strong correlation between the compressive strength and splitting tensile strength, flexural strength, fracture energy, and modulus of elasticity of QRC. QRA can be used to compute the mixture proportions for concrete (certainly up to medium-strength concrete) according to either the Indian standard or the international standard. It is challenging to improve the sorptivity of recycled concrete aggregates closer to NC. In addition, QRC has an initial sorptivity of two times (initial) and a final sorptivity of 1.8 times higher than NC, whereas URC has an initial sorptivity of 3.5 times (initial) and a final sorptivity of 2.35 times higher than NC.

This paper describes an approach to predict the mechanical and fracture behavior of cement-based systems by combining thermodynamic and finite element analysis models. First, the reaction products in a hydrated cementitious paste are predicted using a thermodynamic model. Second, a pore partitioning model is used to segment the total porosity into porosity associated with gel pores and capillary pores. A property-porosity relationship is used to predict the elastic modulus, tensile strength, and fracture energy of the hardened cement paste. The paste’s modulus, fracture energy, and tensile strength, along with information on the aggregate properties and interfacial transition zone properties, are used as inputs to a finite element analysis model to predict the flexural strength and fracture response of mortars.

Corrosion of reinforcing steel increases the probability of the fracturing of longitudinal reinforcing bars and leads to the loss of load-carrying capacity in reinforced concrete (RC) members. Twenty-four reinforcing steel bars were subjected to the buckled bar tension (BBT) test, and the critical bending strain was obtained at different corrosion levels. The specimens were passivated reinforcing steel bars that were corroded through accelerated electrolytic corrosion. The results show that the critical bending strain decreases as the corrosion level increases. The critical bending strain influences the post-buckling bar fracture limit state and reduces the displacement capacity of columns as the corrosion level in the longitudinal reinforcing bar increases. In addition, the degradation of yield strength, ultimate strength, and uniform axial elongation for corroded reinforcing steel bars were observed.

In this study, carbon nanofibers (CNFs) were uniformly dispersed into cement-based materials by the ultrasonic dispersion method to prepare cement-based materials modified by CNFs, and the deformation performance and fracture toughness of the CNF-modified cement-based materials were studied. The results showed that CNFs could inhibit the autogenous shrinkage and drying shrinkage of cement paste, and significantly delay the cracking time of cement mortar. When the dosage of CNFs was 0.05 wt%, the cracking time of cement mortar was extended by nearly 1.5 times longer than that of the control group. The fracture performance of the CNF-modified cement mortar was studied by the fracture test of a three-point bending beam, and the fracture toughness of the mortar was evaluated by double-K fracture parameters. The results showed that at 0.05 wt% dosage of CNFs, the unstable fracture toughness and fracture energy of the mortar increased by 53.5% and 17.2%, respectively. Scanning electron microscopy images illustrated that CNFs could produce bridging and pullout effects in the cement-based materials and retard crack propagation, thus reducing shrinkage deformation and improving fracture toughness.