International Concrete Abstracts Portal

Corrosion of prestressing steel in precast concrete is a significant problemfor coastal bridges in Florida. Replacement of prestressing steel with carbon fiber-reinforced polymer (CFRP) reinforcement provides a potential solution to this costlyproblem. The Florida Department of Transportation (FDOT) structures research centerhas teamed with the University of Florida (UF) to evaluate CFRP reinforced piles thatemploy two types of carbon reinforcement: (a) CFRP reinforcing bars and (b) CFRP grid.The CFRP bars act as flexural reinforcement while the CFRP grid provides confinement tothe concrete core. The focus of this paper is on the confinement provided by theembedded CFRP grid, which is tied into a circular shape and cast into the concrete in asimilar configuration to spiral ties. Existing confinement models are based onconfinement provided by FRP wraps. Consequently, their use in predicting confinementmust be validated with tests on embedded FRP grid. Standard (152 mm x 304 mm)concrete cylinders were cast both with and without the embedded CFRP grid. Thecylinders were tested in compression to determine the effect of the CFRP grid on theirstrength and ductility. A significant improvement in ductility was observed for thecylinders with the embedded CFRP grid compared to the control cylinders.

A constitutive strain-based confinement model is developed herein forcircular concrete columns confined with fiber reinforced polymer (FRP) composites. Aseries of relationships were developed from experimental data, which facilitated theestablishment of the strain-based model. The FRP-confined concrete constitutivemodel calculates the internal damage of the column by using the radial strain. Theradial and axial strains at zero volumetric strain were used to mark the beginning ofeffective dilation response of the FRP composite jacket. The secant concrete moduluswas used in the model and expressed as a function of the secant modulus softeningrate, which depends on the ultimate radial to axial strain ratio. The experimentalrelationship for the ultimate radial to axial strain ratio is a function of the normalizedeffective confining stiffness. The secant modulus softening rate is constant throughoutthe plastic stress-strain response until failure. The FRP-confined concrete constitutivemodel evaluates the ultimate radial strain, which was related to the FRP compositeeffectiveness. The FRP-confined concrete model predicts the stress-strain responseaccurately for any normalized effective confinement stiffness.

This paper presents design examples that illustrate the interaction of designparameters, to examine some of the more critical issues and challenges that arisewhen designing FRP-RC to satisfy ultimate strength and serviceability criteria using theACI 440.1R-03 Guide. A spreadsheet program was written for flexural and serviceabilityanalysis and design of concrete sections reinforced with a single layer of glass orcarbon FRP bars. Analysis and design examples were developed and design aids wereconstructed to assist in economically and efficiently sizing FRP-reinforced concretemembers. Potential difficulties that arise from the inherent nature of FRP-reinforcedconcrete failure modes have been identified and explored.

ACI Committee 440 has proposed a design approach for evaluating theconcrete contribution to the shear resistance of FRP-reinforced concrete beams thataccounts for the axial stiffness of FRP longitudinal reinforcement. Recent shear testsconducted on beams longitudinally reinforced with different types and ratios of FRPbars indicate that the current ACI 440.1R-03 shear design approach significantlyunderestimates the concrete shear strength of such beams. This paper presents aproposed modification to the ACI 440.1R-03 shear design equation. The proposedequation was verified against experimental shear strengths of 98 specimens tested todate, and the calculated values are shown to compare well. In addition, the proposedequation was compared to the major design provisions using the available test results.Better and consistent predictions were obtained using the proposed equation.

This paper presents an experimental study into the structural response ofGlass Fiber Reinforced Polymers Reinforced Concrete (GFRP-RC) tension members. Theinfluence of concrete strength, reinforcement ratio and bar diameter on tensionstiffening is investigated by testing elements in direct tension. Using bars speciallymanufactured with internal strain gauges, typical strain patterns occurring betweencracks during direct tension tests were measured and bond stresses derived, therebyobtaining the information for modeling tension stiffening behavior of GFRP-RC. Anincrease in the tension stiffening behavior with decrease in reinforcement ratio andincrease in concrete strength was observed. No appreciable change in tensionstiffening was recorded with changes in bar diameter at constant reinforcement ratio.This paper also discusses the limitations that may be encountered in modifying currentmodels to represent the tension stiffening effect of GFRP-RC.