International Concrete Abstracts Portal

The authors had the opportunity to apply partially prestressed concrete (PPC) members reinforced and prestressed with braided aramid fiber bars to an actual structure. The PPC members were used as beams for an upper foundation of a seismic base-isolated story. The PPC beams were precast, pretensioned, prestressed concrete members reinforced with braided aramid fiber bars manufactured by braiding aramid fibers and epoxy resin impregnation. Braided aramid fibers were used as main reinforcement, prestressing tendons, and transverse reinforcement of the PPC beams. Design, fabrication, and construction of the beams are reported. Full-scale flexural tests were also conducted to insure the safety margins of the beams. Two specimens were prepared using aramid fiber bars and carbon fiber bars, with cross sections similar to the actual beams. The flexural capacity of the beam at rupture of tendons shows good agreement with the calculated capacity, which uses the average tensile strength of the bars. Full-scale fire-resistance tests were also conducted on two specimens with the same cross sections as the specimens for the flexural tests, although the design did not require fire resistance of the beams because of their use in the foundation. From the test results, the beams were considered to resist fire for about 2 hr.

An experimental study of rehabilitation methods was investigated using artificially damaged concrete beams. The rehabilitation consisted of strengthening the damaged concrete beams by external post-tensioning, and some beams were not only externally prestressed but were also specially injected with epoxy resin to repair several sizes of cracks. Static as well as fatigue tests for three-point bending were conducted to investigate the effect of these rehabilitation methods. Fatigue tests of PRC strengthened by external cable were conducted at 2 million cycles, with a stress level of 33 percent of the ultimate static beam strength and cable tension force of 34 percent of tensile strength. From these test results, the static behavior of deformation and ultimate strength of the rehabilitated beams were confirmed as reasonably upgraded and strengthened by the proposed method. The results indicate that the deflection and ultimate strength of beams for the yield stage can be estimated by theoretical calculation. For the plastic hinge formation stage, deflection and ultimate strength were also evaluated by theoretical calculation. The change in beam rigidity was found to differ insignificantly before and after fatigue tests. In the same manner, ultimate bending strength of beams before and after fatigue tests was nearly the same. As a result of measuring the ratio of loss in the tension force of aramid rope, values of approximately 10 percent were obtained for all three stress states.

Reinforced concrete slabs on steel girder bridges on the Tokyo Metropolitan Expressway are generally strengthened by installing additional stringers or attaching steel plates. However, it is difficult to apply such methods to strengthening work in confined box girders or where there are obstructions. Carbon FRP plates (CFRP) have been selected as strengthening materials for their applicability to strengthening work in confined spaces, and because they can be bonded in lattice forms, allowing for bonding condition inspection on the lower faces of the slabs. The aim of CFRP strengthening is to reduce the reinforcing bar stress caused by excessive wheel loads. In Part 1, investigations are conducted on CFRP applicability to strengthening work through static loading and work efficiency tests. Reinforcing bar tensile stress intensity was reduced by 38 to 56 percent of the unstrengthened specimen. The loads at which the tensile stress intensity is 140 MPa (allowable reinforcing bar load) are around 1.3 times that of an unstrengthened specimen. It can be concluded from the preceding that reinforcing bar stress intensity can be reduced, confirming the possibility of strengthening.

Part 1 of this study concluded that reinforcing bar stress intensity can be reduced, thereby confirming the possibility of strengthening. In Part 2, CFRP is bonded by various methods on the lower faces of reinforced concrete slabs. To determine bonding methods in detail, CFRP is studied with respect to the confirmation test and strengthening design methods. Strengthening design and measurement plans are then made for reinforced concrete slabs on an existing bridge. In strengthening design, the main reinforcing bars are strengthened with three to five layers. It is confirmed that the reinforcing bar tensile stress intensity is reduced around 120 to 130 MPa. CFRP may be used as a strengthening material for reinforced concrete slabs. Measurements will be conducted in 1993. Bending resistance will be confirmed directly during existing bridge strengthening operations.

The three-dimensional fabric studied as reinforcement for concrete is a stereo-fabric made of fiber rovings, woven into three directions, and impregnated with epoxy resin. Fiber material, number of filaments, and distance between rovings can be varied easily. Efficient production is also possible, since three-dimensional weaving, resin impregnation, and hardening can all be done by an automatic weaving machine. The authors investigated the flexural and fire-resistance behaviors of three-dimensional fabric reinforced concrete (3D-FRC) toward applying the material to building panels. The fibers studied were carbon and aramid, and the matrix was vinylon short-fiber reinforced concrete. The results demonstrate that 3D-FRC panels have sufficient strength and rigidity to withstand design wind loads, and the fire resistance of 60 min was achieved. The 3D-FRC panels have been used for curtain walls, parapets, partition walls, louvers, etc., and installations amount to 7000 m 2.