Journal of Composites for Construction

The present study focuses on the mechanical response of concrete columns confined with fiber-reinforced polymer composites (FRP). Practical columns often deviate from axisymmetric conditions due to noncircular cross section, geometric imperfections, and loading eccentricities. This paper discusses these complicating effects on the mechanical behavior of columns confined with FRP. Experiments have been carried out to examine the effects of geometric and loading imperfections on columns of various shapes. A model originally developed for axisymmetric situations has been extended to include the complicating effects. An analytical study for the corner radius that avoids concentration of stress is carried out. The theoretical models have been verified with the present and published experimental results.

This paper presents a theoretical model, based on truss analogy, to analyze the structural behavior at failure of reinforced concrete beams with steel plates or fiber-reinforced polymer lamitates bonded to their tension faces. The analytical approach, incorporated in the framework of strut-and-tie models, takes into account the nonlinear behavior of materials and of the structural member. In addition, it includes the load transfer mechanism to reflect the plate-debonding phenomenon and associated cracking of concrete cover, both of which play a critical role in the failure process of plated beams. The model, which takes into consideration all the possible failure modes of plated beams, is capable of predicting the beam load-carrying capacity at ultimate and, also, of indicating the associated mode of failure. It aims to develop a rational engineering analysis in a field which until now has been studied with linear elastic approaches or empirical methods. The proposed model has been validated by comparing the results obtained in the present analysis with over a hundred experimental results available in published literature. Furthermore, the results obtained with the present analysis are compared with those obtained by two other models, and it is shown that the model proposed here provides a consistent and satisfactory correlation with a wide range of reinforced concrete beam tests strengthened with steel or polymer composite plates.

As existing structures age or are required to meet the changing demands on our civil infrastructure, poststrengthening and retrofitting are inevitable. A relatively recent technique to strengthen reinforced concrete (RC) beams in flexure uses fiber-reinforced polymer (FRP) strips or sheets glued to the tension side of the beam. A number of researchers have reported that the failure mode of an FRP-strengthened RC beam can change from the desired ductile mode of an underreinforced beam to a brittle one. This paper analyzes the effects of this strengthening technique on the response and failure modes of a reference RC beam. A nonlinear RC beam element model with bond-slip between the concrete and the FRP plate is used to study how the failure mechanism of simply supported strengthened RC beams is affected by the following parameters: plate length, plate width, plate stiffness, and loading type. The beam geometry is kept constant. The parametric studies confirm the experimentally observed results according to which the most commonly observed failure modes due to loss of composite actions are affected by the plate geometric and material properties. In addition, distributed loads (difficult to apply in an experimental test) may not be as sensitive to plate debonding in the region of maximum bending moment as are beams subjected to point loads.

Although there has been growing interest and field applications of poststrengthening concrete structures using carbon fiber reinforced plastic (CFRP) laminates, very little information exists regarding the flexural fatigue behavior of reinforced concrete beams strengthened with CFRP. This paper presents the results of an investigation into the fatigue behavior of reinforced concrete beams poststrengthened with CFRP laminates. The results of twenty 3 m and six 5 m beams loaded monotonically and cyclically to failure are discussed. Comparisons are made between beams without and with CFRP strengthening. The effect on fatigue life of increasing the amount of CFRP used to strengthen the beams is also examined.

Research to determine the shear modulus of standard pultruded fiber reinforced plastic (FRP) material is reviewed and appraised. It is found that different test methods have given shear moduli data in the range from 1.3 to 5.1 GPa, with varying degrees of scatter. Pultruded material is comprised of alternate layers of two distinct glass reinforcement types. By applying micromechanical modeling, it is shown that the in-plane shear modulus of the continuous unidirectional rovings layer is similar to that of the continuous filament (or strand) mat layer, and that these layer moduli, generally, lie in the range 3.5 to 4.8 GPa (depending on fiber volume fraction). This finding indicates that the significant difference (>1.3 times) between the in-plane (3 GPa or less) and the St. Venant torsion (always >4 GPa) shear moduli is likely to be due to the experimental test procedures and the physical interpretation of shearing, rather than the layer construction of the material. For structural profiles, it is seen that the shear modulus of 3 GPa in company design manuals is often less than measured. Researchers require correlated elastic constant data if elastic deflections and instability loads for structural members can be accurately predicted using elastic theory. Further work is, therefore, recommended to establish standard test and analytical methods for the determination of shear moduli of pultruded FRP material.

The paper present the results of extended (225 weeks) aqueous immersion of E-glass/vinylester composites, fabricated by the resin infusion process. Two different architectures (unidirectional and bidirectional) are tested to assess effects of temperature levels between 5 and 60°C on the short-beam shear strength. Tests show the competing effects of plasticization and postcuring balanced by hydrolytic degradation. The maximum reduction in performance is seen through immersion in deionized water at 60°C wherein pronounced interface and fiber level degradation is noted. Cycling between the two extremes of 5 and 60°C is also seen to cause acceleration of some interface and bulk resin related degradation phenomena. Experimental results obtained from hygrothermal aging at 23°C are compared with predictions based on an Arrhenius type model, and it is shown that good correlation can be obtained in sets where degradation mechanisms remain the same. Shortcomings of this type of model for life predictions are discussed to emphasize viability of use in design.

Research has shown that fiber-reinforced polymer (FRP) composites can increase flexural, axial, and shear capacity of beams, columns, and walls. The present paper describes both experimental and analytical programs focused on the torsional strengthening of reinforced concrete spandrel beams using composite laminates. The variables considered in this study included fiber orientation, composite laminate, and effects of a laminate anchoring system. The study proved that the FRP laminates could increase the torsional capacity of concrete beams by more than 70%. The analytical procedure developed revealed a good comparison between experimental and analytical results.

One significant cause of deterioration of steel bridge structures is the corrosion due to extensive use of deicing salts in winter weather. The investigation presented in this paper focused on the behavior of steel composite beams damaged intentionally at their tension flange to simulate corrosion and then repaired with carbon fiber-reinforced polymer (CFRP) plates attached to their tension areas side. Damage to the beams was induced by removing part of the bottom flange, which was varied between no damage and loss of 75% of the bottom flange. All beams were tested to failure to observe their behavior in the elastic, inelastic, and ultimate states. To help implement this strengthening technique, a nonlinear analytical procedure was also developed to predict the behavior of the section/member in the elastic, inelastic, and ultimate states. The test results showed a significant increase in the strength and stiffness of the repaired beams. Through the use of CFRP plates, all damaged beams were fully restored to their original (undamaged state) strength.

This paper examines the behavior of Eurocrete fiber-reinforced polymer (FRP) bars (glass, carbon, aramid, and hybrid) in concrete under direct pullout conditions. More than 130 cube specimens were tested in direct pullout where no splitting was allowed to develop. In normal concrete, the mode of bond failure of FRP bars was found to differ substantially from that of deformed steel bars because of damage to the resin rich surface of the bar when pullout takes place. Bond strengths developed by carbon fiber-reinforced polymer and glass fiber-reinforced polymer bars appear to be very similar and just below what is expected from deformed steel bars under similar experimental conditions. The load slip curves highlight some of the fundamental differences between steel and FRP materials. This paper reports in detail on the influence of various parameters that affect bond strength and development such as the embedment length, type, shape, surface characteristics, and diameter of the bar as well as concrete strength. The testing arrangement is also shown to influence bond strength because of the “wedging effect” of the bars.

This paper presents the experimental results from static and fatigue testing on a scale model of a hybrid fiber-reinforced polymer (FRP)–concrete bridge superstructure. The hybrid superstructure was designed as a simply-supported single span bridge with a span of 18.3 m. Three trapezoidal glass fiber-reinforced polymer (GFRP) box sections are bonded together to make up a one-lane superstructure, and a layer of concrete is placed in the compression side of those sections. This new design was proposed in order to reduce the initial costs and to increase the stiffness of GFRP composite structures. Static test results showed that the bridge model meets the stiffness requirement and has significant reserve strength. The bridge model was also subjected to two million load cycles to investigate its fatigue characteristics. The fatigue testing revealed that the structural system exhibits insignificant stiffness degradation.