This study evaluates the impact of incorporating polypropylene fibers on the endurance of 100% ground granulated blast furnace slag (GGBFS) based alkali-activated composite (AAC) exposed to elevated temperatures. In addition, the impact of critical variants, like polypropylene fiber volume fractions, fiber length, alkaline solution monomer ratio, and curing regime, on the overall performance of the fibrous composites under high temperatures was evaluated. While the polypropylene fiber content was varied at 0, 0.15, and 0.3%, two polypropylene fiber lengths, 12 and 20 mm, were examined to assess their influence on the composite thermal stability. The alkaline activating solution monomer ratios (Na₂SiO₃/NaOH, denoted as SS/SH ratio) was varied at 2.5 and 3.0, and two different curing regimes were adopted; the ambient curing regime, with temperature 24±2°C and 65% RH, and the heat curing regime, with 24 h heat curing in the oven at 65°C, for all mixes. The specimens were subjected to high-temperature effects at 150, 300, 600, and 800°C, and the results of residual compressive strength, residual density, and ultrasonic pulse velocity (UPV) were examined, before and after the exposure, to assess the impact of high thermal conditions. In addition to the visual inspection, SEM analyses were carried out to evaluate the effect of different parametric variances on the microstructure properties of the fibrous AACs under high-temperature exposure. The fibrous composite variants with short (12 mm) polypropylene fibers length and most of the variants with 0.15% long (20 mm) polypropylene fibers exhibited high thermal stability and better residual compressive strength than the equivalent control non-fibrous mixes when exposed to temperatures of 150 and 300°C. Further, the increase in alkali solution monomer ratio (SS/SH) from 2.5 to 3.0 has negatively influenced the overall thermal resistance of composites across all variants.

Reinforced concrete (RC) hollow cylindrical members were numerically investigated to understand the impact of the radiation-induced volume expansion of aggregates on the seismic performance of the member. The rigid body spring network model was used in this analysis, and the proposed constitutive laws and used parameters were validated by comparing two horizontal loading experiments for two RC members: a reference experiment and one in which a temperature gradient developed in wall of the members. The resultant volume expansion of concrete was confirmed. After validating the methodology, different degrees of aggregate expansion strain were applied to the aggregate elements considering the reduced temperature and neutron fluence distribution inside the wall, assuming the real size of the biological shielding concrete. The RC members were then loaded horizontally. It was confirmed that the stiffness and maximum bearing capacity decreased slightly with an increase in the neutron fluence, and the deformation at the maximum bearing capacity increased slightly. Based on the rigid body spring network model calculation results, a simplified analytical model that can reproduce the shear deformation–horizontal load relationship was proposed based on the model proposed by Inada (1987).

In this paper, the mechanical properties of 84 different hybrid fiber recycled aggregate concrete under cyclic compression are designed and studied. The modification effect of four kinds of hybrid fiber combinations of steel fiber (SF) and carbon fiber (CF), glass fiber (GF), polyvinyl alcohol fiber (PVA) and polypropylene fiber (PF) on recycled aggregate concrete (RAC) is analyzed from macro and micro point of view. The fracture interfacial inclination angle model considering fiber content was proposed, and the strengthening mechanism of fibers under cyclic loading was discussed The results show that the addition of fiber changes the failure mode of recycled aggregate concrete, SF has a great influence on the main fracture inclination angle of the specimen, while the fracture mode of carbon fiber makes it have a good crack resistance effect, and the combination of SF/CF can significantly increase the peak stress of RAC. SF/PVA combination has remarkable toughening effect and plastic strain inhibition effect. The stress degradation process model of hybrid fiber recycled aggregate concrete was proposed. Finally, according to the experimental results, the uniaxial cyclic compressive stress-strain constitutive equation of hybrid fiber concrete considering the fiber characteristic index was established.