International Concrete Abstracts Portal

A new approach for seismic retrofitting of unreinforced masonry structures is presented. Results from flexural tests of six masonry beams are reported. The beams were strengthened by epoxying a composite fabric to their tension face. The variables studied include the type of epoxy, fabric, mortar, brick, and the finished surface of the wall. It is shown that both the strength and ductility of the beams were significantly improved. When a sufficient amount of fabric is used to prevent premature tension failure, the integrity of the beam is maintained until the beam fails by reaching the compressive strength of the masonry.

Fatigue tests of pretensioned concrete beams with aramid fiber rods as prestressing tendons were carried out to investigate the effects of upper load ratio and initial tendon tension on the beam fatigue strength and deformation. The beams were 15 x 15 x 210 cm. Three levels of initial tendon tension were chosen in the lower beam tendons, approximately 40, 60, and 70 percent of the rod tensile strength. Symmetrical two-point loading was applied to the beam. The upper load in cyclic loading was varied from 45 to 80 percent of the static ultimate load, and the lower load was constant at 4.9 kN. Sinusoidal loading of 4 Hz was applied. The beams with aramid fiber tendons behaved well in fatigue performance, comparable to beams with prestressing wires. The effect of initial tendon tension on beam fatigue life was not observed; fatigue life was determined by the upper load. The fatigue strength at 2 million cycles of loading was more than 65 percent of the static ultimate beam strength.

Flexural and shear behavior of prestressed concrete (PC) beams using carbon or aramid fiber reinforced plastics (CFRP or AFRP) rods are investigated experimentally and compared with those using prestressing steel bars. Since moduli of elasticity of AFRP and CFRP are about 1/4 and 2/3 that of prestressing steel bars, respectively, the deflection of beams using FRP rods is larger and ultimate flexural and shear strengths of beams are smaller than those of PC beams using prestressing steel bars with similar tensile strength of tendons and web reinforcement. The ultimate flexural and shear strengths and deflection of PC beams using FRP rods are improved by an increase of prestress in beams. 229-493

Research and development of FRP bars and cables for reinforcement of concrete structures has recently been carried out. The basic behavior of concrete members reinforced with these FRP bars has become clear. However, there are still debatable points in terms of design concept, such as the recommended failure mode or required toughness and ductility. The authors carried out loading tests of 16 concrete beams reinforced with carbon FRP bars and cables to discuss both the serviceability and ultimate limit states. The specimens included RC, PC, and PPC members. The main factors were bond properties of the FRP reinforcement and prestress force. The experimental results show that cracking and deformation behavior vary with the prestress force and bond property of FRP bars, and that the reasonable serviceability condition will be achieved by controlling these factors. Also, failure mode was affected by these factors and the reinforcing systems, despite the fact that these specimens have almost the same reinforcement ratio. In relation to the failure mode, the energy absorption, which is defined as the area enclosed by the load-deflection curve, was measured to determine toughness and ductility in the ultimate limit state. The authors recommend that the design take into account the toughness based on energy absorbed before the maximum load.

Analysis of the experimental results obtained by testing 45 concrete specimens reinforced with fiber reinforced plastic (FRP) reinforcing bars is outlined. Theoretical correlations with experimental results are conducted in terms of elastic and ultimate bending moment, crack width, and bond and development length. Emphasis is placed on the beam bending analysis and design using regular as well as high-strength (4 to 10 ksi) concrete reinforced with FRP bars by modifying the state-of-the-art design per ACI 318-89 provisions applicable for steel reinforced beams. However, modifications (from the current ACI Building Code) for FRP reinforced beams in terms of ultimate moment capacity, crack pattern, and development length are made without deviating significantly from the design philosophy given in ACI 318-89. Equations for design loads and bending, resistance, bond and development lengths, and crack widths are developed in a simplified form for practical design applications. Similarities and parallels of these design equations with current ACI 318-89 equations are maintained when possible.