Journal of Composites for Construction

The long-term behavior of glass fiber-reinforced polymer (GFRP) reinforcing bars is one of the most critical issues for the acceptance of these materials as reinforcement for concrete structures. There is a high demand for experimental studies to investigate the stability of the tensile strength, ultimate elongation, and elasticity modulus. GFRP reinforcing bars inherently have a low elasticity modulus, which must not significantly decrease over time under loading or the serviceability behavior of the concrete element containing them will be jeopardized. This paper evaluates the residual tensile properties of three sizes of sand-coated GFRP reinforcing bars in alkaline and water environments combined with sustained loading and elevated temperature. Bar diameters of 15.9 (No. 5), 12.7 (No. 4), and $9.5\mathrm{mm}$ (No. 3) were loaded for different durations, then tested in axial tension for residual tensile properties. The test periods varied from $1\text{to}4\text{months}$ under elevated temperature to hasten degradation and simulate extended service periods. The reduction in tensile strength was found to be 7–13% of the guaranteed strength for the three bar sizes under elevated temperature, which is at least 26% higher than the specified design strength as recommended by ACI 440.1R-03. More importantly, no significant change in the elastic modulus was observed.

The local bond mechanics of glass-fiber reinforced polymer (GFRP) bars in normal strength concrete was investigated through experimental testing and analytical modeling. The experimental program was comprised of 30 direct tension pullout specimens with short anchorages. A novel test setup, specially designed so as to minimize the spurious influence of testing conditions on measured bond properties was adopted in the study. Parameters considered were the bar roughness and diameter, the size effect expressed by the constant cover to bar diameter ratio, and the external confining pressure exerted over the anchorage length by transverse externally bonded FRP sheets. Results of the study were summarized in the form of local bond-slip curves, whereby performance limit states were quantified by the amount of loaded end slip and bond strength. An analytical model of the bond stress-slip response of a GFRP bar was derived from first principles and calibrated against the test data of the present investigation. Using the calibrated model, design values for bond and slip were estimated with reference to the code limit state model for bond.

The demand for a rapid technique to strengthen existing wood bridge structural elements is evident in military and civilian sectors. An experimental program was undertaken to evaluate the feasibility of increasing the flexural strength of Southern Pine wood members using mechanically fastened fiber-reinforced polymer (FRP) strips. Three control specimens and twelve composite members were fabricated using two FRP material types with varying fastener spacing and tested to failure. The experimental results show that the proposed strengthening technique induced a gradual failure of the composite members and increased ultimate moment, initial stiffness, and ductility over that found for the control specimens. Increased fastener spacing decreased member ultimate moment, initial stiffness, and ductility ratio. The moisture content of the wood material greatly affected the ductility ratio of the wood members.

The results of an experimental investigation undertaken to evaluate a new technique for strengthening interior slab–column connection in combined flexural and shear modes are presented. The technique consists of using a combination of shear bolts inserted into holes and prestressed against the concrete surface for improving the punching shear capacity, and external [fiber-reinforced polymer (FRP)] reinforcement bonded to the tension face of the slabs in two perpendicular directions for increasing the flexural strength of the slabs. Square slab specimens of $670\times 670\mathrm{mm}$ dimensions were tested and the main test variables included the ratio of steel reinforcement (1.0 and 1.5%), span–depth ratio or thickness (55 and $75\mathrm{mm}$ ) of the slabs, area, and configuration of steel bolts, and area of FRP reinforcement. It was found that the use of shear bolts alone improves the punching shear strength and increases the ductility of failure by changing the failure mode from punching to flexural. However, the use of a combination of shear bolts and a moderate amount of FRP reinforcement increased the flexural strength and resulted in a substantial improvement of the punching shear capacity of the slabs. The corresponding increases attained levels between 34 and 77%. A design approach is presented for evaluating the ultimate capacity of the slab–column connections when strengthened using the proposed strengthening technique. Strength results predicted by the proposed approach were in good agreement with the experimental results.

This paper presents the results of an experimental investigation studying the effect of retrofitting interior slab–column connections against punching shear failure with externally bonded carbon fiber reinforced polymer (CFRP) strips. Six full-scale, $2000\mathrm{mm}\text{-}\mathrm{square}\times 150\text{-}\mathrm{mm}\text{-}\mathrm{thick}$ slab specimens were constructed. The effect of varying the CFRP strengthening amount and configuration on the load-carrying capacity of the slab specimens was investigated. Specimens were supported along their edges and tested to failure. Strengthened slabs showed an increase in stiffness between 29 and 60% and in punching capacity between 6 and 16% with respect to the control unstrengthened slab. An analytical model was refined to predict the punching shear capacity of the specimens strengthened with CFRP strips. The model takes into account both the configuration and amount of CFRP strips. The proposed model shows good agreement with the experimental results.

Controlling the prestressing strand-stress range in precracked prestressed concrete girders is critical in the FRP strengthening process to avoid long-term fatigue failures. This paper will address the details of a design procedure that was developed to satisfy target-strengthening requirements while imposing stress range serviceability limits. Two main CFRP flexural strengthening designs were established for use in the experimental program herein. In the first, the amount of CFRP was designed to limit the average strand-stress range to $125\mathrm{MPa}$ $\left(18\mathrm{ksi}\right)$ , as per AASHTO requirements, under service live load while maintaining the service-ultimate moment relationship constant. The second design was intended to double the strand-stress range under service live load while keeping the same service-ultimate moment relationship. This was accomplished with iterative cycles of nonlinear sectional analysis to determine the amount of external CFRP reinforcement needed to yield both the targeted stress range and ultimate capacity. The girders were overly reinforced for shear with internal steel stirrups. However, external CFRP stirrups were used to prevent the longitudinal CFRP from premature separation and to develop full flexural capacity. The ACI 318-05 model for shear friction was used for this purpose. The paper also presents analysis results to qualify the experimental behavior of the tested girders. Load-deflection, load-strain, and moment-strand stress variations are seen to have excellent correlation with corresponding experimental curves. CFRP is shown to develop higher strains across cracks relieving strand stresses at these critical locations.

Rehabilitation of existing structures with carbon fiber reinforced polymers (CFRP) has been growing in popularity because they offer resistance to corrosion and a high stiffness-to-weight ratio. This paper presents the flexural strengthening of seven reinforced concrete (RC) beams with two FRP systems. Two beams were maintained as unstrengthened control samples. Three of the RC beams were strengthened with CFRP fabrics, whereas the remaining two were strengthened using FRP precured laminates. Glass fiber anchor spikes were applied in one of the CFRP fabric strengthened beams. One of the FRP precured laminate strengthened beams was bonded with epoxy adhesive and the other one was attached by using mechanical fasteners. Five of the beams were tested under fatigue loading for two million cycles. All of the beams survived fatigue testing. The results showed that use of anchor spikes in fabric strengthening increase ultimate strength, and mechanical fasteners can be an alternative to epoxy bonded precured laminate systems.

This paper presents the results of an experimental study on the flexural behavior of a new type of hybrid FRP-concrete-steel member as well as results from a corresponding theoretical model based on the plane section assumption and the fiber element approach. This new type of hybrid member is in the form of a double-skin tube, composed of a steel inner tube and an FRP outer tube with a concrete infill between the two tubes, and may be employed as columns or beams. The parameters examined in this study include the section configuration, the concrete strength, and the thicknesses of the steel tube and the FRP tube, respectively. The results presented in this paper show that these hybrid beams have a very ductile response because the compressive concrete is confined by the FRP tube and the steel tube provides ductile longitudinal reinforcement. The beams' flexural response, including the flexural stiffness, ultimate load, and cracking, can be substantially improved by shifting the inner steel tube toward the tension zone or by providing FRP bars as additional longitudinal reinforcement. The predictions from the theoretical model are in reasonably close agreement with the test results. Differences between the test and predicted results arise from factors not considered in the theoretical model, including the existence of a strain gradient in the confined concrete, concentrations of cracks and the slips between the concrete and the two tubes; these are issues to be accounted for in the development of a more accurate model in the future.

The use of fiber-reinforced polymer (FRP) composites for new construction is still limited primarily due to high costs and designs based on shapes best suitable to conventional materials. The layered and fiber dominated structure of FRP composites is most efficient when used under in-plane stress demands. Structural forms that most efficiently use tensile stress carrying capacity are membranes or thin shells. An analytical investigation is presented where an innovative optimization algorithm for the integrated shape and laminate optimization of free-form FRP shells was developed and used to generate two new bridge systems that efficiently use FRP composites in membrane-based forms. One concept makes use of a FRP membrane working compositely with a concrete slab to resist flexure while the second uses the FRP membrane in a dual suspension system supporting a deck through diaphragms. The resulting designs and a discussion on their performance under optimal and nonoptimal loading are presented. While practical issues are still to be resolved, the presented approach and concepts can promote and provide insight to efficient structural forms for FRP composites.