Nominal maximum aggregate size (MAS) and particle distribution affect the performance of concrete significantly. However, their effect is influenced by the type of aggregate and the target concrete strength. This research investigates the effect of MAS on the mechanical performance of high-strength self-consolidating concrete (HSSCC). Two different types of coarse aggregates, natural quartz aggregate (NA) and recycled concrete aggregate (RA), were used in the evaluation. Compressive, splitting tensile, flexural, and shear strengths were tested and used as criteria for evaluation. Ultrasonic pulse velocity and rebound value were also used as nondestructive evaluation techniques. The results showed that compressive strength decreased when using a bigger MAS of NA, while it increased when using a bigger MAS of RA. However, this conclusion cannot be generalized to include all mechanical properties of concrete, as the failure mechanism for each test depends on the type and size of aggregate. In addition, finite and discrete element methods were applied to study the effect of MAS as well as to simulate the experimental performance of concrete. Following proper proportioning and mixing, RA could be used to produce HSSCC concrete.

The stability and structural buildup of concrete can be evaluated by understanding the nature of the corresponding cementitious suspension using the small-amplitude oscillatory shear (SAOS) test through the time of percolation and rigidification rate, respectively. In the present study, four different cementitious suspensions—namely, 100% ordinary portland cement (OPC), OPC with 70% replacement of slag, OPC with 25% replacement of fly ash (FA), and OPC with 8% replacement of microsilica (MS)—were investigated. From the results, for OPC-based suspensions, the percolation time decreases for increasing dosages of high-range water reducing admixture (HRWRA) at low water-binder ratios (w/b) due to their high reactivity. In contrast, the suspensions with FA and MS exhibit a higher time for the formation of the elastic network, leading to a higher time of percolation. Further, it was identified that the suspensions with slag have the highest affinity toward the HRWRA, resulting in higher dispersion and therefore higher time required for the formation of the initial elastic network. This confirms that the dispersion and reactivity of the particles in suspensions dictate the stability and the structural buildup.

Cement concretes modified with hydrothermal nanosilica and basalt microfiber were developed. The compressive strength Fcom, flexural strength Fflex, and characteristics of impact viscosity were determined: the number of blows before the first fracture Nff and before ultimate failure Ncd, the coefficient Niv = Ncd/Nff, and the specific energy of impact destruction Eim/Sc. The strong effect of SiO2 action and synergistic effect of the combined action of nanoparticles and microfiber on Ncd and Eim/Sc was revealed. Statistical correlations with high R2 values were obtained between the characteristics of mechanical strength and impact viscosity at different doses of SiO2 nanoparticles. Correlations obtained can be used for reduction of the cross section of concrete structures and cement consumption. The mechanism of the strong synergistic effect of the combination is explained by the enlargement of the volume fraction of the high-density (HD) phase of calcium-silicate-hydrate (C-S-H) gel with more packed nanogranules and an increase in the shear stress of C-S-H gel relative to the lateral microfiber surfaces inside the HD-phase volume. The reduction of the coefficient of water filtration Kf and an increase in the frost resistance were achieved.

The deterioration of concrete due to alkali-silica reaction (ASR) is a worldwide problem. Rocks close to shear and/or fault zones generally favor the occurrence of this reaction when used as aggregates in concrete. This study evaluated six coarse aggregates from quarries at different distances from shear and fault zones in the eastern part of the state of Pernambuco, Brazil, to verify the influence of tectonic deformation in the aggregate reactivity. Petrographic analyses and accelerated mortar bar tests were performed. The presence of microcrystalline quartz from dynamic recrystallization in samples collected closer to transcurrent shear zones provided higher reactivity. Therefore, a strong correlation between expansion/microcrystalline quartz content and distance from transcurrent shear zones was obtained. Thus, the geological mapping and characterization may provide an initial and fast indication of the reactive potential of aggregates, being a guidance for the choice of the extraction location or an initial indication of the need to use mitigation measures.

The durability of three repair materials was investigated under two exposures: freezing-and-thawing cycles in the presence of deicing salts, and substrate undergoing expansion due to alkali-aggregate reaction (AAR). The bond strength of the repairs under freezing-and-thawing exposure was evaluated using slant shear, splitting tensile, and pulloff tests. Additionally, the pulloff test was implemented to investigate the bond strength of repairs undergoing expansion due to AAR. Under freezing and thawing, the substrate surface roughness was evaluated and resulted in a higher bond strength under combined shear and compression forces (slant shear test). The results for both exposures showed that the efficacy of a repair could not only be explained based on the net unrestrained length change between the repair and the substrate. While significant autogenous shrinkage of ultra-high-performance concrete (UHPC) can increase the net unrestrained length change, the strength, fibers, and high paste content of such material enhance the bond strength.