Journal of Composites for Construction

This paper presents the results of an experimental study to investigate the role of each layer of reinforcement on the behavior of concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars. Four full-scale concrete deck slabs of $\mathrm{3,000}\mathrm{mm}$ length by $\mathrm{2,500}\mathrm{mm}$ width and $200\mathrm{mm}$ depth were constructed and tested in the laboratory. One deck slab was reinforced with top and bottom mats of glass FRP bars. Two deck slabs had only a bottom reinforcement mat with different reinforcement ratios in the longitudinal direction, while the remaining deck slab was constructed with plain concrete without any reinforcement. The deck slabs were supported on two steel girders spaced at $\mathrm{2,000}\mathrm{mm}$ center to center and were tested to failure under a central concentrated load. The three reinforced concrete slabs had very similar behavior and failed in punching shear mode at relatively high load levels, whereas the unreinforced slab behaved differently and failed at a very low load level. The experimental punching capacities of the reinforced slabs were compared to the theoretical predictions provided by ACI 318-05, ACI 440.1R-06, and a model proposed by the writers. The tests on the four deck slabs showed that the bottom transverse reinforcement layer has the major influence on the behavior and capacity of the tested slabs. In addition, the ACI 318-05 design method slightly overestimated the punching shear strength of the tested slabs. The ACI 440.1R-06 design method yielded very conservative predictions whereas the proposed method provided reasonable yet conservative predictions.

Due to concerns with corrosion, the use of fiber-reinforced polymer (FRP) as a replacement to conventional steel reinforcement has greatly increased over the last decade. However, when considering thermal gradients, many researchers have identified differential swelling between FRP reinforcement and concrete. This can lead to the development of splitting tensile stresses within the concrete cover surrounding the reinforcing bar and deterioration of the bond between the materials. A total of ten beams were therefore tested in flexure to investigate bond strength deterioration arising from thermal cycles ranging between $-40$ and $+40°\mathrm{C}$ . Beams were cast at $+20°\mathrm{C}$ and prestressed with either glass FRP (GFRP) or carbon FRP (CFRP) tendons. Additionally, the level of damage expected in the concrete cover of the specimens was predicted on the basis of linear elastic and nonlinear finite element analyses of thermally induced stresses. Although damage was theoretically expected, experimental results showed excellent bond performance of GFRP and CFRP tendons in concrete based on material properties and concrete cover requirements adopted for the project.

An innovative wedge-anchor system with a longitudinal circular profile has been designed for gripping carbon-fiber-reinforced-polymer rods. The anchor system, consisting of a barrel, a number of wedges, and a sleeve, is capable of carrying the ultimate tensile strength of the rod. Three-dimensional (3D) and axisymmetric finite-element models have been developed and applied in order to determine the stresses inside the anchor system. Using the 3D model, the overall performance of the anchor represented by the tensile load-displacement relationship compared favorably with the experimental results. A comparison study between the 3D and axisymmetric models revealed that the axisymmetric model underestimated the contact pressure on the rod surface. Using the force-fitting principle of thick cylinders, a mathematical model was also developed, and under specific assumptions, the mathematical solution compared favorably with finite-element model results.

A fundamental understanding of fiber-reinforced polymer (FRP) laminate bonding behavior, including bond strength and effective bonding length, is of primary importance for the development of design guidelines and codes for concrete structures strengthened with externally bonded FRP reinforcement as a bond-critical application. However, the long-term serviceability of such FRP-strengthened structures is still a concern due to a lack of both long-term performance data and a suitable model to represent these performances. This study aims at presenting a viscoelastic model describing the time-dependent behavior of the FRP–concrete interface. The proposed model has been calibrated using strain measurements of the designed specimen for the experimental investigation of the time-dependent behavior of the FRP–concrete interface, including the development of the effective bonding length. Afterward, the proposed model satisfactorily predicts the time-dependent bonding length of the FRP sheet in comparison with the experimental results. The effects, both of creep of the adhesive layer and of creep and shrinkage of the concrete, on the changes in the effective bonding length of the PFRP sheet are also discussed.

Fiber reinforced plastics (FRP) are commonly used for the strengthening of concrete members. For shear strengthening of beams, FRP strips can be bonded to the sides of the member alone, to both the sides and the bottom (i.e., the U configuration), or wrapped around the whole beam. For the various strengthening configurations, empirical equations have been proposed for predicting the contribution of strips to the shear capacity of the member. However, for the same strengthened member, the equations recommended by different design guidelines (American Concrete Institute, International Federation for Structural Concrete, and Japan Society for Civil Engineers) predict different shear capacities. Moreover, as the equations were obtained through the fitting of laboratory data on relatively small beams, their applicability to beams of practical sizes have not really been assessed. In the present investigation, geometrically similar beams with depth of 180, 360, and $720\mathrm{mm}$ were retrofitted in shear with carbon FRP strips in both the U configuration and fully wrapped configuration. The retrofitted members were tested to failure to (1) provide data on beams of practical sizes for verification of design equations and (2) investigate if the strengthening effectiveness is similar for small and large beams. Measured FRP contribution to the shear capacity is also compared to predictions from equations in the various guidelines. Based on our findings, for beams retrofitted with strips in the U configuration, the strengthening effectiveness may significantly decrease with member size, and none of the available design equations can consistently provide conservative values for the shear capacity. For beams with fully wrapped strips, strengthening effectiveness is independent of member size, and the FIB equation appears to be most appropriate for practical design.

The results of an experimental and numerical study of the flexural behavior of reinforced concrete beams strengthened with glass-fiber-reinforced-polymer (GFRP) laminates are presented in this paper. In the experimental program, ten strengthened beams and two unstrengthened beams are tested to failure under monotonic loading. A number of external GFRP laminate layers and bond length of GFRP laminates in shear span are taken as the test variables. Longitudinal GFRP strain development and interfacial shear stress distribution from the tests are examined. The experimental results generally showed that both flexural strength and stiffness of reinforced concrete beams could be increased by such a bonding technique. In the numerical study, an eight-node interface element is developed to simulate the interface behavior between the concrete and GFRP laminates. This element is implemented into the MARC software package for the finite-element analyses of GFRP laminate strengthened reinforced concrete beams. Reasonably good correlations between experimental and numerical results are achieved.

The objective of this paper is to study and compare the performance of concrete beams strengthened with carbon fiber sheets bonded with inorganic and organic resin matrices. The experimental study consisted of testing two groups of steel-reinforced concrete beams. The first group of beams was strengthened with carbon fiber sheets bonded with an organic matrix, and the second with carbon fiber sheets bonded with an inorganic matrix. The first group of beams was strengthened with 2, 3, and 4 layers of carbon fiber sheets, while the second group was strengthened with 2, 3, 4, and 5 layers of carbon fiber sheets. Strength, stiffness, ductility, deflection, failure pattern, and cracking of beams strengthened with the two systems were compared. Results showed that the inorganic matrix system is as effective in increasing strength and stiffness of reinforced concrete beams as the organic matrix. The failure mechanism of the inorganic system, however, seems more brittle. The failure of beams strengthened with inorganic matrix showed crack formation in the composite and a minimum buildup of strain along the interface of the composite and concrete. Analytical models were proposed to predict deflection and moment capacity of the strengthened beams. The experimental values compared well with those predicted by the analytical models.

The potential for using textiles impregnated with cement as a means for in situ strengthening and retrofit of structural concrete was explored and compared with the conventional epoxy method. Fabrics of different geometries, yarns, and modulus values were evaluated. Simulation of repair and strengthening was carried out by wrapping fabric impregnated with cement and epoxy around damaged and undamaged concrete cylinders. The cylinders with the impregnated sleeves were loaded in compression, and their performance was compared with that of the “virgin” (control) concrete. In addition, the mechanical properties of the cement composite layer were evaluated, as well as the stiffness of the fabrics themselves, and correlated with that of the repaired system. Efficient repair with the cement binder was accomplished with the high modulus fabric (Kevlar) in particular. However, even the lower modulus (polypropylene) could be effective in repairing and inducing some postpeak resistance (although to a smaller extent than with the Kevlar). Improved behavior was also obtained with the nondamaged concrete cylinder when it was strengthened with the cement–fabric jacket, mainly at composite strain values of up to 0.2–0.3%.

This paper describes a parking garage retrofitting project where near-surface-mounted carbon fiber-reinforced polymer (FRP) bars were used to strengthen the reinforced-concrete decks. The garage reportedly exhibited numerous signs of deterioration, such as excessive deflections of the decks at bay midspans, extensive concrete cracking, concrete spalling, etc. The results of the structural analyses indicated that several negative-moment slab regions were deficient to support the design loads. The amount of overstressing was up to 50% in some areas. The structural analysis considered as-built conditions and showed that deficiencies were predominantly due to misplacement of the negative-moment steel reinforcement. The paper summarizes the design approach for carbon FRP strengthening of the concrete decks and describes a load testing program used to evaluate their performance. Finally, a description of parameters and considerations used in development and implementation of the adopted strengthening strategy is also presented.

Worldwide interest is being generated in the use of fiber-reinforced polymer composites (FRP) in the rehabilitation of aged or damaged reinforced concrete structures. As a replacement for the traditional steel plates or external posttensioning in strengthening applications, various types of FRP plates, with their high strength-to-weight ratio and good resistance to corrosion, represent a class of ideal material in externally retrofitting. This paper describes a solution proposed to strengthen the damaged reinforced concrete headstock of the Tenthill Creeks Bridge, Queensland, Australia, using FRP composites. A decision was made to consider strengthening the headstock using bonded carbon FRP laminates to increase the load carrying capacity of the headstock in shear and bending. The relevant guidelines and design recommendations were compared and adopted in accordance with AS 3600 and Austroads bridge design code to estimate the shear and flexural capacity of a rectangular cracked FRP reinforced concrete section.

A primary means of demonstrating the feasibility and effectiveness of fiber-reinforced polymer (FRP) composite bridge materials is via in situ bridge load testing. For this study, the prescribed or assumed design factors for each of the study bridges were compared to those exhibited by the performance of the bridge. Specifically, the wheel load distribution factors and impact factors as defined by AASHTO were considered in order to assess the load transfer and distribution in structures utilizing FRP panels. The in situ testing configurations for the study bridges are outlined, including the truck and instrumentation placement to obtain the desired information. Furthermore, comparisons were drawn between the design values for deflection and those experienced by the structures during testing. It was found that although the deflections exhibited by the bridges were well within the design limits, further research is needed to be able to prescribe bridge design factors for FRP panels.