International Concrete Abstracts Portal

SP-188 This volume presents 24 papers from the Fourth International Symposium and represents a significant expansion in the state of knowledge that has occurred since the First Symposium in 1993.

In this paper, a new method to use Carbon Fiber Sheets(CFS) which is prestressed before they are bonded to the concrete surfaces based on the concept of prestressing technique is developed. A prestressing system is first designed to be suitable for strengthening existing concrete structures. An experimental program is carried out to verify the reinforcement effects of beams such as on the improvements of flexural strength, ductility, stiffness and crack resistance. To avoid the debonding failure near the ends upon releasing the pre-tensioned force, the anchorage zone are dealt with by several proposed reinforcing methods and the effects of anchorage treatments are also discussed. An effort is also made to investigate the determination of appropriate presressing stress level of CFS and structural optimization of reinforcement.

In this paper, uniaxial tension tests on CFS(carbon fiber sheet)-strengthened concrete specimens with and without steel bar reinforcement and bending tests on CFS strengthened RC beams were conducted to examine the crack behavior in the concret. The testing showed that the crack spacing of the uniaxial tension members strengthened with CFS was only slightly affected by the diameter of the reinforcing steel bars, the thickness of the concrete covering and the stiffness of the CFS. The crack spacing and the crack width both for tensile and flexural members was significantly smaller when the CFS was used. And also the cracks were distributed in the plain concrete tension member without steel reinforcement strengthened with CFS. Finally, it was recognized that the tension stiffening effect is realized to be improved by the non-liner behavior of the CFS-concrete interface.

An experimental study was conducted to determine the transfer length, development length and flexural behavior of fiber-reinforced polymer (FRP) tendons in prestressed concrete beams. Three kinds of nominally 5/16 in (8 mm) diameter FRP tendons were included in the study: Carbon Leadline, Aramid Technora and Carbon Strawman. Thirty beams were pretensioned using a single FRP tendon. In addition, twelve control beams were pretensioned with a seven-wire steel strand (ST). Transfer length observations from this study were based on concrete strain measurements with a DEMEC gage system. Development length observations were based on three-point flexural tests. Four-point flexure tests were also performed on each material to gain additional understanding of the bond behavior between concrete and the PC reinforcing materials. The "95% average plateau strain" method of using concrete strain results was shown to be an effective way to determine transfer length. By using an appropriate flexural model and extrapolating results from over-reinforced tests to situations where the tendon would actually fail, it was possible to determine development length in this investigation. Despite differences in tendon material properties and prestressing forces, both the measured transfer lengths and the development lengths were almost identical for all tendon materials tested. The development length for FRP tendons was reasonably predicted by the ACI design equation, although transfer length appears to be underestimated.

The bond behavior of carbon FRP tendons for concrete is characterized with an interface model. In particular tendons with a surface structure that produce significant mechanical interlocking with the adjacent concrete are considered. This type of mechanical interaction can produce damage in the adjacent concrete and within the surface structure of the reinforcing element. The combination of these mechanisms is characterized with an elastoplasticity model that fully couples the longitudinal and radial response; the model calibration is based upon a series of bond tests under differing stress states. The model does not provide a detailed description of the underlying mechanics associated with the progressive bond failure, and it will generally require recalibration when applied to significantly different FRP bars or tendons. However, using a calibration for a GFRP bar, the model gives acceptable estimates of the bond strength for several tests of a particular CFRP tendon, even though the specimens have significantly different attributes. Additional validation tests (using data with measures of the experimental scatter) are needed to define the predictive limits of the model; nonetheless the transfer length problem further demonstrates the potential application of the model to help predict and understand the behavior of FRP-reinforced structural components.