U-shaped steel−concrete composite beams (USCCBs) have been widely used in civil engineering due to their numerous advantages, including high load-bearing capacity, high rigidity, good ductility, short construction periods, and compatibility with the development of prefabricated buildings. In particular, USCCBs have been increasingly applied to super high-rise buildings and extra-large span bridges. Over the past decade or so, many new types of shear connectors and structural forms for USCCBs have been developed. Meanwhile, significant progress has been achieved in research on the flexural, shear, torsional, and fire-resistance performance of USCCBs, the seismic behavior of beam−column joints, and the strengthening of concrete beams with U-shaped steel casings. Nevertheless, challenges and limitations remain in both experimental research and practical applications. This paper presents a systematic review of recent research advances in USCCBs. Existing problems, development prospects, and future research priorities are comprehensively summarized and discussed, with the aim of further promoting the development and engineering application of USCCBs.

Waste glass powder (WGP) is a promising recycled constituent for sustainable cementitious materials, yet its effectiveness depends strongly on the packing characteristics of the fine-aggregate skeleton. This study investigates the coupled effects of WGP replacement ratio and fine-aggregate gradation on the mechanical performance and microstructural evolution of cement mortar. Five aggregate gradations and four WGP replacement levels (0%, 8%, 10%, and 12% by mass) were evaluated through compressive and flexural strength tests at 7, 14, and 28 days, yielding 180 prism specimens in total. Scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were used to identify the governing microstructural mechanisms. Four regression models—linear regression (LR), support vector regression (SVR), random forest (RF), and extreme gradient boosting (XGBoost)—were compared for compressive strength prediction under identical data partitioning. XGBoost achieved the highest accuracy (R² = 0.9346, RMSE = 0.9860 MPa, MAE = 0.6943 MPa), outperforming RF (R² = 0.8896), LR (R² = 0.8845), and SVR (R² = 0.8612), and was subsequently coupled with particle swarm optimization (PSO) for constrained mixture design. Appropriate aggregate packing substantially improved WGP utilization by combining micro-filling, secondary pozzolanic reaction, pore refinement, and interfacial transition-zone densification. The best experimental combination (GCS10-M3) increased 28-day compressive and flexural strengths by 14.0% and 42.9%, respectively, relative to the reference mortar with the same gradation. The XGBoost-PSO framework identified a theoretical optimum containing 14.86% WGP, beyond the tested range, in a fines-rich bimodal aggregate gradation, with predicted 28-day compressive and flexural strengths of 41.10 MPa and 8.23 MPa, respectively. The results establish a mechanism-informed, data-driven route for designing resource-efficient mortar for building engineering applications.

U-shaped steel strengthening can substantially enhance the mechanical performance of reinforced concrete (RC) beams while having minimal impact on their cross-sectional dimensions and self-weight. However, the influence of strengthening RC beams with U-shaped steel casing on the overall seismic performance of frame joints remains insufficiently investigated through experimental studies. To address this gap, four exterior beam-column joint specimens were tested under quasi-static cyclic loading without column axial load. The specimens comprised one reference specimen and three specimens with U-shaped steel-strengthened RC beams, with the U-shaped steel plate thickness as the primary variable. The failure modes, hysteretic behavior, load-carrying capacity, deformation response, stiffness degradation and energy dissipation performance were evaluated. The results showed that all specimens exhibited plastic-hinge-dominated failures at the beam ends, while U-shaped steel strengthening provided only limited improvement in the overall lateral load-carrying capacity. Compared with the reference specimen, the strengthened specimens exhibited fuller hysteretic loops and improved energy dissipation performance at large drift ratios. At a drift ratio of 6%, the energy dissipated per hysteretic loop by specimens SJ1, SJ2, and SJ3 increased by 6.5%, 14.3%, and 8.1%, respectively, compared with the reference specimen NJ1. The corresponding equivalent viscous damping coefficients increased by 15.4%, 16.5%, and 8.9%, respectively. The average test-termination-based displacement ratios of the strengthened specimens exceeded 3.2, indicating considerable deformation capacity up to the prescribed termination drift ratio of 8%.