Journal of Composites for Construction

Recent earthquakes have produced extensive damage in a large number of existing masonry buildings, demonstrating the need for retrofitting masonry structures. Externally bonded carbon fiber is a retrofitting technique that has been used to increase the strength of reinforced concrete elements. Sixteen full-scale shear dominant clay brick masonry walls, six with wire-steel shear reinforcement, were retrofitted with two configurations of externally bonded carbon fiber strips and subjected to shear loading. The results of the experimental program showed that the strength of the walls could be increased 13–84%, whereas, their displacement capacity increased 51–146%. This paper presents an analysis of the experimental results and simple equations to estimate the cracking load and the maximum shear strength of clay brick masonry walls, retrofitted with carbon fiber.

In recent years, numerous investigations have addressed the shear strengthening of reinforced concrete (RC) beams with externally bonded fiber-reinforced polymer (FRP) composites. Despite this research effort, the mechanisms of shear resistance that are developed in such a strengthening system have not yet been fully documented and explained. This clearly inhibits the development of rational and reliable code specifications. This paper aims to contribute to the understanding of the shear resistance mechanisms involved in RC beams strengthened in shear with externally bonded FRP. It is based on results obtained from an experimental program, involving 17 tests, performed on full size T beams, and using a comprehensive and carefully optimized measuring device. The resistance mechanisms are studied by observing the evolution of the behavior of the strengthened beams as the applied loads are increased. The local behavior of the FRP and the transverse steel, in particular in the failure zones, are thoroughly examined. The operative resistance mechanisms are also studied through the load sharing among the concrete, the FRP, and the transverse steel, at increasing levels of applied load.

The cracking characteristics of fiber-reinforced polymer (FRP) strengthened reinforced concrete (RC) beams in both the short- and long-term is addressed in this paper. First, an empirical equation based on regression analysis of test results obtained from 36 beams was derived for the evaluation of crack widths in FRP-strengthened RC beams under short-term loading. The equation accounts for the effective concrete area in tension, steel stress, proximity of tensile longitudinal reinforcement, and primary crack height. Next, the long-term crack widths of glass FRP-strengthened RC beams under sustained loads were studied. Beams strengthened with glass FRP laminates showed improved cracking characteristics with smaller crack widths compared to conventional RC beams. Based on the investigation, two empirical equations are presented to compute the long-term crack widths in FRP-strengthened beams.

The use of fiber-reinforced polymers (FRPs) to poststrengthen concrete structures started to be investigated in the mid-1970s and today is widely recognized as an attractive technique to be used in civil structures, especially when aggressive environments prevent the use of materials that are susceptible to corrosion, such as steel. Different FRP poststrengthening techniques have been developed and applied in existing structures, aiming to increase their load capacity. Most FRP systems used nowadays consist of carbon fibers embedded in epoxy matrix. Regardless of the advantages and the good results achieved using carbon fiber-reinforced polymers, some new possibilities, such as the use of prestressing and lower cost fiber materials, are being analyzed in an attempt to provide viable alternatives for a more efficient, safe, and rational use of FRP systems. The main purpose of the present work was to make a comparative analysis of the behavior of reinforced concrete beams poststrengthened with carbon, aramid, and glass FRP subjected to static loading tests. Experimental results were evaluated against theoretical ones obtained through an analytical model that considers a trilinear behavior for the load versus displacement curves. The experimental results indicate that all FRP systems applied have appropriate structural performance for use in poststrengthening applications of RC. The choice of the more suitable system would, therefore, be strongly influenced by circumstances regarding cost limitations and level of reinforcement required.

Numerical analyses are performed to predict the stress–strain behavior of square reinforced concrete columns strengthened by fiber-reinforced polymer (FRP) sheet confinement. The research focuses on the contribution of FRP sheets to the prevention of elastic buckling of longitudinal steel bars under compression, in cases of inadequate stirrup spacing. A new Drucker–Prager-type plasticity model is proposed for confined concrete and is used in constructed finite-element model. Suitable plasticity and elasticity models are used for steel reinforcing bars and fiber-reinforced polymers correspondingly. The finite-element analyses results are compared against published experimental results of columns subjected to axial compression, to validate the proposed finite-element model. Stress concentrations in concrete core and on FRP jacket are investigated considering circular or square sectioned, plain or reinforced concrete columns. Geometry of the section as well as the presence of steel bars and stirrups affect remarkably the variation and magnitude of stress on FRP as percentage of its tensile strength.

One disadvantage of most available stress–strain models for concrete confined with fiber-reinforced polymer (FRP) composites is that they do not take into consideration the interaction between the internal lateral steel reinforcement and the external FRP sheets. According to most structural concrete design codes, concrete columns must contain minimum amounts of longitudinal and transverse reinforcement. Therefore, concrete columns that have to be retrofitted (and therefore confined) with FRP sheets usually contain lateral steel. Hence, the retrofitted concrete column is under two actions of confinement: the action due to the FRP and that due to the steel ties. This paper presents a new designed-oriented confinement model for the axial and lateral behavior of circular concrete columns confined with steel ties, FRP composites, and both steel ties and FRP composites. Comparison with experimental results of confined concrete stress–strain curves shows good agreement between the test and predicted results.

The use of externally-bonded fiber-reinforced polymers (FRPs) to strengthen reinforced concrete (RC) structures is now widely recognized. However, a concern that continues to discourage the use of FRPs in many applications is their susceptibility to high temperature and fire. Although recent studies have shown that the fire endurance of appropriately designed and insulated FRP strengthened RC members is satisfactory, the specific performance of FRP systems at, and after exposure to, high temperature remains largely unknown. The results of tests on the residual properties after high-temperature exposure of various available FRP strengthening systems for concrete are reported; these include: tension coupon tests, single-lap FRP-to-FRP bond tests, direct tension FRP-to-concrete bond tests, and pull-apart FRP-to-concrete shear bond tests after exposure to temperatures up to $400°\mathrm{C}$ . The data show that the allowable exposure temperatures for residual performance of externally bonded FRP systems lie between the glass transition temperature $\left(T_g\right)$ and the thermal decomposition temperature of the resin systems used. The potential consequences for fire-safe design of FRP strengthened RC members are discussed. Material properties during a fire event are not specifically addressed.

This paper presents an experimental and computational study of the buckling behavior of steel members strengthened with carbon fiber-reinforced polymer (CFRP) wraps. In the proposed strengthening system, steel members are first sandwiched within a core comprised of mortar or PVC blocks and then the entire system is wrapped with CFRP sheets. A matrix of specimens is tested under monotonic compression to investigate the parameters that influence system response. Test results show that the proposed strengthening method can provide enough lateral support to a steel bar member to allow it to reach yield in compression and to continue deforming inelastically beyond. Key failure modes are identified in the test program. Important parameters that influence behavior are also pinpointed and studied in more detail through a computational simulation model that is validated using the test data. Parameters identified as influential in the experimental and computational studies include: number of CFRP layers, core thickness, bond between CFRP layers and the core, bond between the core and the inner steel member, and strength of transverse sheets at the member ends.

Precast/prestressed concrete sandwich panels consist of two concrete wythes separated by a rigid insulation foam layer and are generally used as walls or slabs in thermal insulation applications. Commonly used connectors between the two wythes, such as steel trusses or concrete stems, penetrate the insulation layer causing a thermal bridge effect, which reduces thermal efficiency. Glass fiber-reinforced polymer (GFRP) composite shell connectors between the two concrete wythes are used in this research as horizontal shear transfer reinforcement. The design criterion is to establish composite action, in which both wythes resist flexural loads as one unit, while maintaining insulation across the two concrete wythes of the panel. The experiments carried out in this research show that hybrid GFRP/steel reinforced sandwich panels can withstand out-of-plane loads while providing resistance to horizontal shear between the two concrete wythes. An analytical method is developed for modeling the horizontal shear transfer enhancement using a shear flow approach. In addition, a truss model is built, which predicts the panel deflections observed in the experiments with reasonable accuracy.