Journal of Composites for Construction

Fiber-reinforced polymer (FRP) composites have been examined as a convenient and cost-effective means of strengthening unreinforced masonry structures. Seismic design in the United States is almost entirely based on the assumption that the structural system provides a ductile failure mode. FRP strengthened masonry walls inherently have brittle failure modes due to the nature of the strengthening system. The concept explored in this article is the introduction of ductility using a hybrid strengthening system. This involves the placement of structural steel or reinforcing steel at critical locations in the lateral force resisting system. This article presents the testing and analysis of a ductile structural steel connection that can be used to strengthen the connection of FRP strengthened shear walls to the foundation. The connection also increases energy dissipation. Results indicate that a ductile failure mode can be attained when the connection is designed to yield prior to the failure of the FRP strengthening.

Repair and rehabilitation of infrastructure is becoming increasingly important for bridges due to material deterioration and limited capacity to accommodate current load levels. An experimental program was undertaken to study the flexural behavior of creosote-treated sawn Douglas fir timber beams strengthened with glass fiber-reinforced polymer (GFRP) bars. Twenty-two half-scale and four full-scale timber beams strengthened with GFRP were tested to failure. The percent reinforcement ratios were between 0.27 and 0.82%. Additional unreinforced timber beams were tested as control specimens. The results have shown that using the proposed experimental technique changed the failure mode from brittle tension to compression failure, and flexural strength increased by 18 to 46%. Research findings indicate the use of near-surface GFRP bars overcomes the effect of local defects in the timber and enhances the bending strength of the members. Based on the experimental results, an analytical model is proposed to predict the flexural capacity of both unreinforced and GFRP-reinforced timber beams. The article also reviews implementation of the proposed technique for strengthening a timber bridge near Winnipeg, Manitoba, Canada.

This report examines the effects of water and moisture on the durability of pultruded glass-fiber-reinforced polymers (GFRP) with vinylester resin for applications in normal air conditions. Deterioration tests, including both immersion and atmosphere conditions at various temperatures, were conducted to investigate the deterioration characteristics of pultruded GFRP after being permeated by water. The main findings from the tests were that cracks appeared on the surface of the GFRP and that the weight of the materials decreased, which may be attributed in the surface treatment oil for the glass-fiber cloth being dissolved away. The bending strength of the GFRP was also found to be reduced. The rate of weight decrease and the reductions in bending strength were greater in a 60°C water-immersion condition compared to both a 60°C moist-atmosphere condition and a 40°C water-immersion condition. This difference is due to the fact that the glass fiber and matrix resin separated in water at 60°C. Although the matrix resin changed color in air at 60°C, becoming noticeably brown, no chemical changes were found by infrared spectra.

Experimental determination of the full section flexural and torsional properties of pultruded fiber reinforced plastic I-profiles is described. Based on beam theory approximations, test configurations for determining the various section properties are established. Tests were conducted on three different I-profiles with a range of span-to-depth ratios. Major and minor axis flexural rigidities and flexural moduli, determined from three- and four-point bending tests, show close correlation. Major and minor axis transverse shear rigidities and shear moduli show significant variation, due to differing effective areas of the cross section resisting transverse shear and differing fiber content and orientation in the web and flanges. St. Venant torsional shear moduli, determined from uniform torsion tests, are consistent but significantly greater than the transverse shear moduli, which may be due to variations in fiber content, orientation, and lay up. Warping torsional rigidities, determined from nonuniform torsion tests, are consistent with values deduced from minor axis flexural rigidities, indicating that the influence of shear deformation on restrained torsional warping is insignificant.

Unidirectional carbon/vinylester composites and concrete cylinders wrapped with three layers of the same composite were exposed to −18°C (0°F) conditions both with and without prior saturation by moisture and to freeze-thaw cycling after saturation. All specimens show degradation in strength, with the maximum degradation being due to the saturated freeze-thaw condition caused by cyclic effects of absorption, subsequent crack-opening and fiber-matrix debonding. Analytical predictions, based on approximation of hygrothermomechanical response models for composites combined with a simple confinement model, are shown to correlate well with experimental data for most of the exposure conditions.

Analytical models are presented in this study for the analysis of reinforced concrete joints strengthened with composite materials in the form of externally bonded reinforcement comprising unidirectional strips or flexible fabrics. The models provide equations for stresses and strains at various stages of the response (before or after yielding of the beam or column reinforcement) until the ultimate capacity is reached, defined by concrete crushing or fiber-reinforced polymer (FRP) failure due to fracture or debonding. Solutions to these equations are obtained numerically. The models provide useful information on the shear capacity of FRP-strengthened joints in terms of the quantity and configuration of the externally bonded reinforcement and may be used to design FRP patching for inadequately detailed beam-column joints. A number of case studies are examined in this article, indicating that even low quantities of FRP materials may provide significant enhancement of the shear capacity. The effectiveness of external reinforcement increases considerably if debonding is suppressed and depends heavily on the distribution of layers in the beam and column. The latter depends on the relative quantities of steel reinforcement crossing the joint panel and the level of axial load in the column. Analytical shear strength predictions were in good agreement with test results found in the literature, thus adding confidence to the validity of the proposed models.

Analysis and design procedures are presented for a carbon-fiber-reinforced polymer (CFRP) composite seismic retrofit of a reinforced concrete (RC) three-column bridge bent. The CFRP jacket was designed using performance-based criteria to provide a target displacement ductility based on seismic retrofit measures for the columns, bent cap, and bent cap-column joints. In situ quasi-static cyclic tests of a bent in the as-built condition and a bent retrofitted with the CFRP jacket were carried out in 1998. The seismic retrofit was successful, and the bridge bent retrofitted with CFRP composites reached a displacement ductility level in excess of the target ductility and double the hysteretic energy dissipation of the as-built bent. A description of the CFRP composite layout and validation of the design assumptions from the experimental results is presented. Recommendations for improvement of the original CFRP composite seismic retrofit design are offered based on the lessons learned from the in situ tests.

The effects of immersing coupons of glass-reinforced epoxy in four different liquid media at two separate temperatures were investigated in this study which is aimed at examining the durability of fiber-reinforced plastics currently being used in the construction industry. A commercially available epoxy resin was reinforced with 47% by weight of an epoxy-compatible, E-glass woven fabric. Composite samples were soaked for up to 5 months in distilled water, a saturated salt solution (30g/100 cc NaCl), a 5-molar NaOH solution, and a 1-molar hydrochloric acid solution. Aging was conducted at room temperature and at 60°C. Samples were harvested periodically and their tensile and fracture properties determined. The fracture surfaces were also examined using scanning electron microscopy. Results show that commercial epoxy resins used in glass fiber-reinforced polymers are fairly durable. It was found that all the solutions marginally degraded the mechanical properties of the neat resin, especially at the higher temperature; this was mainly the result of polymer hydrolysis. The strength of the composites, however, was reduced by more than 70% by the acid at room temperature and by the alkali at the elevated temperature. Water immersion was less damaging than either acid or alkali soaking, and immersion in brine had the least effect on mechanical properties. As evidenced by SEM micrographs, the worst cases of damage involved attack on the glass fibers in acid at 60°C compared to room temperature. Therefore, reinforcing glass fibers have to be protected from attack by liquid media to improve the durability of composites.