Hollow sections have been increasingly applied in the construction of buildings, bridges, offshore structures, and towers for passing electrical and mechanical pipes or other utilities. Torsion caused by external force is a weakness of hollow sections that is rarely investigated. In particular, the behavior of hollow sections with high-strength concrete (HSC) and ultra-high performance concrete (UHPC) remains poorly studied. This study aims to examine the behavior of a reinforced concrete hollow beam with opening and compare it with a hollow beam without opening. The hollow beam with an opening is modeled using the finite element method and analyzed under torsional, flexural, and cyclic loading with HSC and UHPC materials. The effect of the opening section size on the behavior of hollow beam is also evaluated. The openings created in the web of hollow beams led to a decrease in beam capacity although the hollow beam with small opening can carry almost the same load as that of hollow beam without an opening. The result also shows that the capacity of UHPC beams for twisting is twice that of HSC beams.

Several stress-strain models were used to predict the strengths of steel fiber reinforced concrete, which are distinctive of the material. However, insufficient research has been done on the influence of hybrid fiber combinations (comprising two or more distinct fibers) on the characteristics of concrete. For this reason, the researchers conducted an experimental program to determine the stress-strain relationship of 30 concrete samples reinforced with two distinct fibers (a hybrid of polyvinyl alcohol and steel fibers), with compressive strengths ranging from 40 to 120 MPa. A total of 80% of the experimental results were used to develop a new empirical stress-strain model, which was accomplished through the application of the particle swarm optimization (PSO) technique. It was discovered in this investigation that the new stress-strain model predictions are consistent with the remaining 20% of the experimental stress-strain curves obtained. Case studies of hybrid–fiber–reinforced concrete constructions were investigated in order to better understand the behavior of such elements. The data revealed that the proposed model has the highest absolute relative error (ARE) frequencies (ARE 10%) and the lowest absolute relative error (ARE > 15%) frequencies (ARE > 15%).

The mechanical properties of concrete containing crumb rubber (CR) as a replacement for fine aggregate have been studied by many researchers. The consequence of these studies indicates that when CR is used as a substitute for fine aggregates in concrete, the properties in the fresh and hardened states are affected. Although the compressive strength of rubberised concrete decreased as the percentage of CR increased, rubberised concrete had better thermal resistivity. Therefore, this paper presents the results of research on the evaluation of conventional compressive strength (f _cu ) and non-destructive testing (NDT) at high temperatures for lightweight mortar made with CR and oil palm fruit fibre (OPFF). Sixteen mortar mixtures with 0-30% CR and 1-1.5% OPFF as a replacement ratio by weight of aggregate and cement were tested. All samples were subjected to elevated temperatures (ETs) of 200°C, 400°C and 600°C. The results show that both compressive strength and NDT decreased for all samples, especially samples containing 30% CR and exposed to 600°C. Despite this fact, the compressive strength of more than 97% of the samples fell within the range of moderate to structural lightweight concrete.