Journal of Composites for Construction

Use of carbon fiber reinforced polymers (CFRP) reinforcement for prestressing concrete structures introduces a promising solution for deterioration of concrete structures due to corrosion of steel reinforcements. Due to the low elastic modulus and limited strain at failure of CFRP reinforcement, partial prestressing could be the most appropriate approach to enhance deformability and reduce the cost in comparison to fully prestressed concrete structures. For members reinforced or prestressed with fiber reinforced polymers reinforcements, serviceability requirements may be the governing criteria for the design; therefore, deflection under service loading conditions should be well defined. This paper introduces simplified methods to calculate the deflection of beams prestressed by CFRP reinforcement under short-term and repeated loading. It also examines the applicability of current approaches available to calculate the deflection. Based on an experimental program undertaken at the University of Manitoba, bond factors are introduced to account for tension stiffening of concrete beams prestressed by CFRP. A procedure to determine the location of the centroidal axis of cracked prestressed sections is also proposed. The proposed methods for deflection calculation are calibrated using the results obtained from different experimental programs. Design guidelines are proposed to predict the deflection of beams partially prestressed by CFRP reinforcement.

Recent increases in bridge design loading requirements have highlighted the need for fast, efficient, and durable strengthening methods. External steel plate bonding provides a satisfactory solution, but carbon fiber reinforced plastic (CFRP) offers the added advantages of resistance to corrosion, low weight, and high mechanical strength. This paper examines the fatigue performance of CFRP-strengthened concrete beams as part of a project investigating the use of CFRP as an alternative to steel. Five reinforced concrete beams were tested in fatigue; two control beams and three strengthened with externally bonded CFRP plates. Three loading options were used: (1) apply the same loads to both plated and unplated beams, (2) apply loads to give the same stress range in the rebar in both beams, and (3) apply the same percentage of the ultimate load capacity to each beam. Fatigue fracture of the internal reinforcement steel would appear to be the dominant factor governing failure, and it would appear reasonable to expect the same fatigue life for plated and unplated beams with comparable values of stress range in the steel bar.

The bond properties of fiber reinforced polymer (FRP) reinforcing bars (rebars) at temperatures ranging from room temperature (20°C) to high temperatures of up to 250°C are discussed in this paper. The bond properties in this temperature range were studied for a number of commercially produced rebars, where different bond “treatments” were applied to FRP rebars. Test results showed a reduction of between 80 and 90% in the bond strength as the temperature increased from 20 to 250°C. In comparison, ordinary deformed steel rebars showed a reduction of only 38% in the same temperature range. In addition, a reduction in the bond stiffness, which was determined from the slope of the ascending branch of the pullout load versus slip curve, was seen as the temperature increased. At elevated temperatures the postpeak bond decrease was gradual as compared with the instantaneous drop at room temperature. Greater sensitivity to high temperatures was seen in FRP rebars, in which the bond relies mainly on the polymer treatment at the surface of the rod.

Experiments on commercially-produced composite reinforcements have shown that the transverse coefficients of thermal expansion (CTE) of these rebar are 3–5 times higher than the CTE of concrete. In this paper, thermoelastic solutions of plain and spirally wrapped composite rebar, embedded in concrete and subject to a uniform temperature increase, are presented. Comparisons are made between bars with no spiral wrapping and those produced with a spiral wrap of unidirectional rovings. Results show that the spiral wrapping, included primarily to improve the bond between the concrete and the composite rebar, also aids in the reduction of overall thermal expansion of the rebar. Thermoelastic solutions of unidirectional composite rods and concrete predict that the tensile stresses in the concrete surrounding the rebar will exceed the tensile strength of the concrete for relatively small temperature increases. The extent of this cracking cannot be determined from the analytical predictions. To predict the extent of the cracking, nonlinear thermoelastic simulations, completed using the COSMOS-M finite-element code, are used to determine the extent of concrete cracking. Results show that cracking decreases with a decreasing rebar diameter and increasing confining pressure of the concrete.

The results of bond and tensile strength investigations of hollow glass-fiber reinforced plastic rebars are presented. Rebars with similar geometries but different microstructures were executed. Bond characteristics of on- and off-axis specimens, in terms of maximum sustained load, bond strength and stress, concrete compressive strength, and initial bond stress, are reported. It appears that the compressive strength of concrete has a minor influence on bond strength and stress performance of the composite reinforcement investigated herein. The bond between concrete and reinforcement is controlled by the geometry of the rebar, the embedment length, and the clear cover provided by the concrete. To confirm the experimental results, a computer simulation of the rebar's behavior was performed. Stresses and strains were computed for each layer of both rebar structures. The theoretical analysis coincides with the experimental results. The unidirectional bars exhibited higher longitudinal stresses in all three simulated loading schemes. The off-axis rebars showed higher transverse strains and stresses. The results of this study suggest that the microstructure of the composite bars is a significant factor influencing the bond performance. The specimens reinforced with unidirectional glass fibers showed higher bond strength than the off-axis fiber-reinforced specimens.

There is a need to develop innovative means for improving the performance and service life of wooden railroad crossties. This paper discusses the experimental testing and finite-element (FE) modeling of wood and polymer-reinforced wood samples for future application to the development of a prototype composite-reinforced wood railroad crosstie. Using glass fiber rovings, epoxy resin, and a resorcinol formaldehyde primer, wood cores are reinforced with a relatively thin layer of glass fiber–reinforced plastic (GFRP) composite by the filament winding process. Both wood and GFRP-wood samples are tested under combined static and moisture loading. The experimental results indicate that significant increases in stiffness (about 21% for dry and 25% for wet conditions) and strength (about 28% for dry and 70% for wet conditions) are achieved by wrapping wood cores with a relatively thin layer [1.78 mm (0.07 in.)] of composite. The effects of moisture on deflection and ultimate load were less pronounced (about 47%) for GFRP-wood samples than for similar control wood samples. The reinforcement provided a confinement effect, which resulted in a much more consistent and favorable failure mode for the reinforced samples compared with the wood samples. A 3D FE model of a beam is used to predict the linear response of the samples, and the predicted displacements and strains correlate well with experimental results. The experimentally verified FE model is subsequently used for a parametric study and design optimization of an actual full-size GFRP-wood crosstie. The experimental evaluation of test samples and design modeling study presented in this paper is used to provide design recommendations for the future development of full-size GFRP-wood crossties.

An 18.3 m (60 ft) pedestrian I-girder bridge was analyzed, designed, tested, and constructed. The main load-carrying members for the bridge are two 61-cm (24-in.) deep composite fiberglass I-girders with embedded carbon fibers in the flanges. Laboratory testing quantified the increased stiffness of the hybrid glass/carbon fiber-reinforced polymer beams over those manufactured solely with glass fibers. Typical joint connections were tested to investigate their performance, and a dynamic analysis was performed to check the bridge's vibration under pedestrian load.