Ultimate Bearing Capacities of Hybrid Fiber Reinforced Lightweight Aggregate Concrete Beam with Membrane Effect

Lightweight aggregate concrete (LWC) offers some advantages like low density, thermal insulation, heat resistance, fire resistance, and excellent seismic performance. The incorporation of hybrid fibers further enhances its toughness, durability, and crack resistance, resulting in a composite material known as hybrid fiber reinforced-lightweight aggregate concrete (HFR-LWC). In protective engineering, deformations induce a "membrane effect" at the end restraints of structures, altering their ultimate resistance and failure modes. Conventional load-bearing components are often simplified as simply supported or fixed in mechanical analyses, neglecting the influence of the membrane effect. This leads to conservative designs for ultimate bearing capacity, preventing the full utilization of component strength. Therefore, investigating the membrane effect’s contribution to improving component bearing capacity is of significant importance. This paper was to design and fabricate a set of support restraint devices for horizontal stiffness and rotational stiffness to simulate the membrane effect in beam structures. Four-point bending tests were conducted on ten HFR-LWC beams under static loading. The contributions of restraint conditions, reinforcement ratios, and the presence of fibers to the ultimate bearing capacity of HFR-LWC beams were analyzed. The variation patterns of mid-span strain, crack distribution, and ultimate bearing capacity of the HFR-LWC beams were examined. In addition, the influences of horizontal stiffness and rotational stiffness on the membrane effect were also quantitatively analyzed.

Based on the Technical Specification for Lightweight Aggregate Concrete (JGJ/T 12—2019), this study developed a hybrid fiber reinforced lightweight aggregate concrete (HFR-LWC) with high strength, low density, and good workability through multiple mix proportion trials. Ten concrete beams with the dimensions of 200 mm×100 mm×1500 mm were cast, featuring a symmetrical reinforcement with A6 deformed longitudinal bars and A6@100 plain stirrups at different reinforcement ratios (i.e., ρ of 0.42% and ρ of 0.28%). After 28-d curing under identical conditions, the compressive strength and elastic modulus of concrete specimens were tested according to the standard GB/T 50081—2019 by a universal testing machine. A specialized support restraint device was designed and fabricated to account for both horizontal and rotational stiffness. The experimental program included three simply supported beams and seven boundary-restrained beams tested under four-point static bending. Five displacement transducers were evenly spaced along the beam span to measure deformation patterns under various load levels and record the deflection at ultimate load. Five strain gauges were installed at mid-span to monitor neutral axis variations and strain distribution under different support conditions and deflection levels. Additional strain gauges were placed at beam ends and mid-span to analyze strain variations caused under different restraint conditions. Through the quantitative analysis of experimental data, this research systematically investigated the influences of support constraint, reinforcement ratio and fiber incorporation on the membrane effects. The findings could elucidate the mechanisms by which these factors affected the ultimate load-bearing capacity of HFR-LWC beams, providing valuable insights for the structural design.

Based on the experimental conditions, the test beams are grouped to investigate the influences of restraint condition, reinforcement ratio and fiber incorporation on the load-bearing capacity. The analysis of test groups under different restraint conditions reveals that the presence of tie rods significantly enhances the ultimate load-bearing capacity of the beams, and variations in both the number and position of tie rods have substantial effects on this capacity. The comparative analysis of test groups at different reinforcement ratios demonstrates that for membrane effects within a certain range of reinforcement ratios, there exists an inverse relationship between reinforcement ratio and the increase in load-bearing capacity. Specifically, a lower reinforcement ratio results in more pronounced improvements in ultimate load-bearing capacity under end restraints. The examination of test groups with and without fiber incorporation shows that fiber materials effectively participate in the crack propagation process and improve the deformation performance of the members. The analysis of strain and crack development in the test beams indicates that the mechanism by which membrane effects affect a load-bearing capacity primarily involves restricting concrete cracking and crack propagation, thereby increasing ultimate capacity, while simultaneously reducing structural deformation performance. The relationship between tie rod force and deflection shows a synchronous variation with beam load-bearing capacity. In the initial deformation stage, these two parameters exhibit the similar results. However, the tie rod force surpasses the peak load-bearing capacity of the beam, with its maximum value occurring later than that of the beam’s capacity as the deflection increases.

Unlike conventional calculation methods that could simplify supports, the influence of end restraints on the ultimate bearing capacity of reinforced concrete members could reflects actual engineering conditions. The membrane effect increased the ultimate bearing capacity of beams by approximately 1.5 times to 5.5 times and effectively controlled the crack propagation. Compared to the influence of horizontal stiffness on the ultimate bearing capacity, the membrane effect showed a greater sensitivity to variations in rotational stiffness. The hybrid fibers could significantly improve the cracking performance of concrete, resulting in a more uniform crack distribution, which could facilitate the development of membrane effects.