Journal of Composites for Construction

This paper presents results of an experimental investigation on half-scale concrete columns retrofitted with externally bonded fiber reinforced polymers (FRP). It then reviews six selected confinement models and three North American design guidelines for circular columns retrofitted with FRP and provides an in-depth comparison between experimental test results, models predictions, and values obtained using the guidelines. The comparison, which is carried out in terms of the confined to unconfined strengths, but also in terms of the gain in the load carrying capacity, considers the following parameters: The FRP confinement ratio, the unconfined concrete strength, and the elastic and mechanical properties of FRP. The results of this investigation indicated that for columns made of lower strength concrete, existing models and guidelines overestimate the confined concrete strength, and hence, the load carrying capacity of the columns.

The efficacy of fiber-reinforced polymer (FRP) confinement as a means of repairing/strengthening reinforced concrete members with nonductile details is explored in this paper with particular emphasis on the interaction between jacket and the embedded longitudinal compression reinforcement at the onset of bar buckling. To this end, a total of 27 short prismatic specimens were tested to failure under concentric compression subsequent to application of FRP jackets; four additional specimens were tested as controls. Specimens had a square cross section and most were detailed according to former practices with the exception of few specimens that contained the minimum transverse reinforcement as specified by modern provisions. Parameters of the experimental study were the extent of initial damage prior to jacketing, the jacket material, and the number of layers. Response variables included ductility at failure, deformation capacity, strength increase, and mode of failure of the repaired/strengthened member. Confinement effectiveness was quantified using the degree of lateral dilation as the primary performance index. Deformation capacity as limited by embedded longitudinal bar buckling and jacket rupture/debonding strains was evaluated from the experimental results and analytical considerations.

The application of carbon fiber reinforced polymers (CFRP) for the repair and strengthening of corrosion-damaged elements is relatively recent. Although several demonstration studies have been completed, no formal design method is available. This paper presents analyses developed for the repair and rehabilitation of four square prestressed piles exposed to an aggressive marine environment. The analysis considers both loss in strength and corrosion-induced expansion. Strength restoration is obtained from interaction relations using a strain compatibility analysis. The longitudinal fibers of the bidirectional FRP layer configuration were designed to restore the strength; whereas, the transverse fibers were verified that they provide the necessary confinement to withstand expansive strains caused by corrosion. Three models proposed by Caltrans, ISIS, and the writers are compared. The Caltrans model is found to be the most conservative. The ISIS and the writers’ model gave similar results. A field demonstration was carried out in which segments of four prestressed piles located in the splash zone supporting the Allen Creek Bridge in Clearwater, Fla., were wrapped. Two of these piles are instrumented for continuous monitoring of their corrosion performance.

The objective of this paper is to develop a new mechanistic understanding of moisture affected debonding failures in carbon fiber reinforced polymer (CFRP) plated concrete systems by mechanically testing accelerated moisture conditioned mesoscale peel and shear interface fracture specimens. Central to the investigation is the use of interface fracture toughness as the quantification parameter of the CFRP-epoxy-concrete trilayer system, which is considered a bond property, to analyze, compare, and correlate physical observations. Results have shown that fracture toughness of the CFRP bonded concrete systems significantly degrades, and its value becomes asymptotic with increasing moisture ingress. This asymptotic behavior is associated with certain moisture concentration levels as predicted by a three-dimensional moisture diffusion simulation. The generally observed debonding mode by concrete delamination for the dry specimens changes to an epoxy/concrete interface separation mode for the wet specimens. Finite element fracture computation, mixed-mode characterization, and kink criterion implementation synergistically suggest that the interface separation mode is attributed to an interfacial material toughening and an interface weakening mechanism as a consequence of moisture diffusion.

This study focuses on debonding failure in reinforced concrete beams with carbon fiber reinforced polymer composite bonded on the soffit using the wet lay-up method. An experimental study, which involved 26 tests, was carried out. The experiments showed two failure modes: Intermediate span debond and end debond. The first failure is the result of the high bond stress near the tip of a flexure-shear crack, whereas the second type of failure is due to the high shear stress developed in the weakest concrete layer at the tension reinforcement level. The experiments have shown that U-straps can be effective in preventing intermediate span and end debond. Based on experimental observations, two simple and practical theoretical models were developed and verified with the experimental data, together with a large database of other existing tests.

Retrofitting concrete structures with fiber reinforced polymer (FRP) has today grown to be a widely used method throughout most parts of the world. The main reason for this is that it is possible to obtain a good strengthening effect with a relatively small work effort. It is also possible to carry out strengthening work without changing the appearance or dimensions of the structure. Nevertheless, when strengthening a structure with external FRP, it is often not possible to make full use of the FRP. The reason for this depends mainly on the fact that a strain distribution exists over the section due to dead load or other loads that cannot be removed during strengthening. This implies that steel yielding in the reinforcement may already be occurring in the service limit state or that compressive failure in the concrete is occurring. By prestressing, a higher utilization of the FRP material is made possible. It is extremely important to ensure that, if external prestressing is used, the force is properly transferred to the structure. Most of the research conducted with prestressing carbon fiber reinforced polymer (CFRP) for strengthening has been on surface bonded laminates. However, this paper presents research on prestressed CFRP quadratic rods bonded in sawed grooves in the concrete cover. This method has proven to be an advantageous means of bonding CFRP to concrete, and in comparison to surface bonded laminates, the shear and normal stress between the CFRP and the concrete are more efficiently transferred to the structure. In the presented test, no mechanical device has been used to maintain the prestress during testing, which means that the adhesive must transfer all shear stresses to the concrete. Fifteen beams with a length of $4\mathrm{m}$ have been tested. The tests show that the prestressed beams exhibited a higher first-crack load as well as a higher steel-yielding load as compared to nonprestressed strengthened beams. The ultimate load at failure was also higher, as compared to nonprestressed beams, but in relation not as large as for the cracking and yielding. In addition, the beams strengthened with prestressed FRP had a smaller midpoint deflection. All strengthened beams failed due to fiber rupture of the FRP.

This paper addresses the debonding strength of partial-length, adhesively bonded carbon fiber-reinforced polymer (CFRP) plates that are used to strengthen steel beams. Bonded CFRP plates tend to debond under static and fatigue loadings because of the very high stress field at the plate end. Such failures limit the application of CFRP plates. Static and fatigue tests show that the stress intensity factor governs the debonding strength. The steel/adhesive corner was the locus of debond initiation. The effects of the following parameters on stress intensity factors are discussed: plate thickness, plate modulus, bondline thickness, adhesive modulus, and adhesive spew-fillet angle. The stress intensity factors are calculated using the Betti’s law-based reciprocal work contour integral method (RWCIM). The parametric study results indicate that the stress intensity factors cannot be used to represent the severity of the corner as the adhesive spew-fillet angle (and singularity) changes. Therefore, the use of stress intensity factors as a failure criterion for the purpose of predicting debonding strength is limited to the same spew-fillet angle.

Innovative fiber-reinforced polymer (FRP) composite highway bridge deck systems are gradually gaining acceptance in replacing damaged/deteriorated concrete and timber decks. FRP bridge decks can be designed to meet the American Association of State Highway and Transportation Officials (AASHTO) HS-25 load requirements. Because a rather complex sub- and superstructure system is used to support the FRP deck, it is important to include the entire system in analyzing the deck behavior and performance. In this paper, we will present a finite-element analysis (FEA) that is able to consider the structural complexity of the entire bridge system and the material complexity of an FRP sandwich deck. The FEA is constructed using a two-step analysis approach. The first step is to analyze the global behavior of the entire bridge under the AASHTO HS-25 loading. The next step is to analyze the local behavior of the FRP deck with appropriate load and boundary conditions determined from the first step. For the latter, a layered FEA module is proposed to compute the internal stresses and deformations of the FRP sandwich deck. This approach produces predictions that are in good agreement with experimental measurements.

Available guidance for mitigating debonding failure of externally bonded fiber reinforced polymer (FRP) composites applied to concrete is evaluated based on data obtained from large- and full-scale experimental programs. Current recommendations are inadequate to effectively mitigate intermediate crack-induced debonding in flexural members. An improved, although still simple to apply, equation for determining limiting FRP material properties to mitigate debonding is presented and evaluated. Additionally, the effect of fatigue loading on the debonding behavior of externally bonded FRP is not adequately addressed in practice. In the limited data presented, a degradation of bond behavior is observed even under fatigue loading conditions amounting to an FRP stress range of no more than 4% of the FRP capacity.