Journal of Composites for Construction

Concrete beams reinforced with glass-fiber reinforced polymer (GFRP) bars exhibit large deflections in comparison with steel-reinforced concrete beams because of the low modulus of elasticity of GFRP bars. This paper proposes new equations for estimating the effective moment of inertia of FRP-reinforced concrete beams on the basis of the genetic algorithm and experimental results. Genetic algorithm is used to optimize the error function between experimental and analytical responses. In the experimental part of the study, nine beam specimens were manufactured and tested. In addition, the results of 55 beam specimens tested by other researchers were also used. The effects of elastic modulus of FRP bars, reinforcement ratio, and the level of loading on the effective moment of inertia are taken into account. The proposed equations are compared with different code provisions and previous models for predicting the deflection of FRP-reinforced concrete beams. The values calculated using the proposed equations are also compared with different test results. The experimental results correlated well with the values predicted using the proposed equations, especially in the cases of high reinforcement ratios and high levels of loading.

Three exterior reinforced concrete beam-column joint specimens were tested under reverse cyclic loading. The joint region of these specimens suffered significant damage, whereas limited damage was observed in the beams. The damaged specimens were repaired and strengthened to prevent shear damage and strength deterioration inside the joint region and to achieve more ductile response. First, the damaged loose concrete was removed and replaced by high-strength nonshrink mortar. Then, fiber-reinforced polymer (FRP) strips were diagonally wrapped over the joint region, and longitudinal FRP strips were applied and anchored on the beams. The original strength was restored in all strengthened specimens under the application of the same loading history. Deformation capacities of the strengthened specimens were much larger than those of the original specimens.

In this paper, the effects of eccentricity on the effectiveness of carbon-fiber-reinforced polymer (CFRP) or glass-fiber-reinforced polymer (GFRP) in repairing the shear strength and ductility of nonseismically detailed beam-wide column joints have been evaluated. For this purpose, four (two concentric and two eccentric) full-scale nonseismically detailed interior beamwide column joints were used as control specimens. All four subassemblages were subjected to similar cyclic lateral displacement to provide the equivalent of severe earthquake damage. The damaged control specimens were then repaired by filling their cracks with epoxy and externally bonding them with CFRP sheets and GFRP sheets. These repaired specimens were then retested and their response histories were obtained. Hence, a total of eight specimens were tested: four control, and four repaired. The response histories of the control and repaired specimens were then compared. The results were compared through hysteretic loops, load-displacement envelopes, energy dissipation capacity, secant stiffness degradation, and shear strength and damage indices. Moreover, the effectiveness of the proposed repair schemes in repairing the eccentric and concentric joints was also compared. The present study demonstrates that proposed repair schemes can recover the performance of both damaged reinforced concrete (RC) concentric and eccentric beam-wide column joints effectively.

Passive confinement provided by fiber-reinforced polymer (FRP) jackets increases the compressive strength and axial deformation capacity of concrete. This study explains a normalized confinement stiffness approach to quantify the strength and strain increase of FRP-confined concrete using a previously proposed and most widely-used model for both active and passive confinement of concrete, and claims that these equations can still be used for FRP-confined concrete with very simple modifications. A comparison of the proposed model's accuracy to American Concrete Institute guidelines was made using experimental results reported in the literature. The proposed modified model was shown to be quite effective in predicting the increased strength and strain values of FRP-confined concrete, and was also modified for FRP-confined hollow concrete cylinders.

The embedded through-section (ETS) technique was recently developed to avoid the debonding failure that occurs with other fiber-reinforced polymer (FRP) strengthening techniques, such as the externally bonded (EB) and near-surface-mounted (NSM) methods. The method offers greater confinement and leads, therefore, to a substantial improvement in bond performance. In addition, it requires less concrete preparation than the EB strengthening technique. In this study, experimental results from 13 direct-shear test specimens are reported. The influence of the following major parameters on the bond behavior of FRP-strengthened reinforced concrete beams is examined: concrete strength, hole diameter, bar diameter, bar surface area, and bar bond length. The experimental results show that debonding can be avoided by providing a sufficient bar length and high concrete strength. Among the analytical models suggested in the literature for the bond behavior of FRP reinforced concrete, the Eligehausen, Popov, and Bertero (BPE) modified model and the Cosenza, Manfredi, and Realfonzo (CMR) model are compared with the experimental results obtained here. The CMR model with new fitting parameters is found to be an accurate way of simulating the experimental results. A new equation that accounts for concrete compressive strength and bar diameter is provided to estimate the development length.

This paper presents results of an analytical and experimental investigation on RC T-beams retrofitted in shear with embedded through-section (ETS) fiber-reinforced polymer (FRP). The ETS FRP rod method is a promising method to increase the shear strength of RC beams. As this method develops, the structural behavior of RC beams strengthened with the ETS method needs to be thoroughly characterized and the influencing parameters addressed. In this research study, nine tests were performed on 4,520-mm-long RC T-beams. The parameters of this study are (1) the effect of the surface coating on the FRP bars, (2) the effect of internal transverse steel reinforcement on the FRP shear contribution, (3) the effect of FRP bar spacing, (4) the effect of FRP rod diameter, and (5) the efficiency of the embedded through-section FRP rod method. The main objective of the study is to analyze the behavior of RC T-beams strengthened in shear with ETS FRP rods by varying the parameters just mentioned. New design equations are proposed to calculate the shear contribution of FRP for beams strengthened using the ETS FRP method. The design equations are validated against results collected from the experimental part of the current research study. The proposed model shows an acceptable correlation with the experimental results.

The main objective of the experimental work reported herein is the comparative evaluation of steel-reinforced polymers (SRPs) and carbon-reinforced polymers (CFRPs) used as externally-bonded reinforcement in strengthening of reinforced-concrete (RC) members. Tensile stress strain as well as bond constitutive laws for these materials were first derived from 16 tests and are summarized here. Results are then reported from four-point bending tests of five full-scale RC beams strengthened at their span using SRP and CFRP strips. The bond tests have shown that by providing a bond length greater than the effective one, neither the bond strength nor the deformation capacity are increased, whereas by increasing the width of the strip the bond strength is increased. From the bending tests of beams it was found that the use of both SRP and CFRP strips resulted in a significant increase in strength (up to 92%) with respect to the strength of the initial specimen. The experimentally measured strengths were estimated analytically using both the experimental measurements of the specimen deformations and the pertinent provisions of standards from the American Concrete Institute and the European Committee for Standardization.

Interface debonding can grow slowly within the FRP-to-concrete interface in aggressive environments, even though the energy release rate at the crack tip is only a fraction of the critical energy release rate of the interface. This slow debonding process is called environment-assisted subcritical debonding, which may be a dominant mechanism for the failure of the FRP-to-concrete interface under service loads in aggressive environments. In this study, environment-assisted subcritical debonding of the epoxy-concrete interface was first observed and characterized using wedge-driven testing. Aggressive environments have been found to substantially increase the debonding growth rate along the epoxy-concrete interface. Fracture surface analysis suggests that the debonding mode can change from the cohesive failure within the concrete in critical debonding to the adhesive failure along the epoxy-concrete interface in subcritical debonding. The proposed subcritical debonding testing closely simulates the failure occurring during the service life of the FRP-to-concrete interface, and allows interaction with environmental species during testing. Subcritical debonding testing provides a new approach to understand the degradation mechanism and to assess the long-term durability of the FRP-to-concrete interface.

A constitutive model is developed for fiber-reinforced polymer (FRP)-to-concrete bond for fatigue loading. The model is verified against available test data with a framework developed for further calibration as new data becomes available. The formulation is incorporated into a finite-element program and the fatigue behavior of FRP-repaired reinforced concrete (RC) beams is analyzed. Comparing the finite-element model results with test data shows that the model is capable of accurately predicting the overall cyclic fatigue loading response of RC members strengthened with FRP composites. The model is also able to capture the change in the peak deflection, the FRP plate strain development and distribution with increasing load cycles, and the number of load cycles to failure. The need for more tests on FRP-to-concrete bonded interfaces subjected to fatigue loading is identified, as is the need to measure key modeling parameters from such tests.

Fiber-reinforced polymers (FRP) composites are used as supplementary reinforcement to increase the in-plane shear capacity or to provide out-of-plane load-carrying capability of masonry walls and to modify the collapse mechanism in arches and vaults. In these applications, the efficiency of load transfer is limited by the debonding of FRP from the masonry substrate. In this paper, the debonding mechanism of FRP-masonry is experimentally studied. Experimental procedures and test specimens are designed to investigate the progressive debonding of FRP from brick and mortar substrates and relate it to the response obtained from the FRP-masonry interface. Surface displacements during debonding are obtained using digital image correlation. A one-dimensional numerical model is developed for predicting the fracture behavior along the FRP-masonry interface using the cohesive fracture parameters from the brick and mortar interfaces obtained from the computed strains.

Many masonry structures have cracking problems after earthquake exposure. Rapid and reliable rehabilitation of such masonry structures is necessary for disaster relief purposes. To improve current methods for strengthening masonry structures with cracking problems using fiber-reinforced polymer, an investigation of the strengthening of a scaled five-story masonry structure built with concrete perforated bricks was conducted. The structure was initially tested on a shake table, from which its displacement, acceleration responses, and cracking were recorded. The partially damaged structure was subjected to rapid strengthening (in less than 3 days) using basaltic fiber-reinforced polymer (BFRP) because of its low elastic modulus and high strength (good compatibility with masonry substrate). The strengthening was carefully designed, and special anchorage measures were used on the first floor of the structure to ensure the strengthening effect of the externally bonded BFRP stripped sheets. The strengthened structure was then retested on the shake table to quantify the strengthening effect. By comparing the performance of the masonry structure during the two shake table tests, it was found that the externally bonded BFRP sheets worked well with the masonry structure, and the seismic performance of the masonry structure was improved significantly.

A hybrid system, concrete-filled fiber-reinforced polymer (FRP) tube (CFFT)-encased steel I-section, is introduced. The embedded steel section enhances flexural strength and stiffness, and provides a pseudoductile behavior. It also facilitates connection of the CFFT member to footings or other members. Phase I of the experimental program addresses the flexural behavior of the system through ten beam specimens, including steel and CFFT control specimens. The glass fiber-reinforced polymer (GFRP) tubes varied in thickness and laminate structure. The steel section enhanced performance considerably, especially pseudoductility, in tubes with cross-ply laminates, in which a significant sustained reserved strength remains stable over large deflections on fracture of the tube. CFFTs with angle-ply tubes showed a considerable inherent ductility on their own, in which case, adding the steel section enhanced strength and stiffness only. Phase II addresses a moment connection through five cantilever tests. The connections consist of steel base plates welded to the steel sections, which are embedded into the CFFT members at various length ($L_s$)-to-span-length ($L$) ratios ranging from 0.1 to 1.0. Three distinct failure modes are observed. At ($L_s/L$) ratios of up to 0.17, premature bond failure occurs. At ratios of 0.17 to 0.47, the CFFT member achieves flexural tension failure of the tube just beyond the free end of the steel section. Beyond a 0.47 ratio, the full plastic hinge capacity is developed at the fixed end. A simple design-oriented model that predicts strengths at the full range of ($L_s/L$) ratios is developed and validated.