Journal of Composites for Construction

Accelerated aging tests are being conducted on more than 20 types of glass fiber-reinforced polymer (GFRP) reinforcing bars, which are produced from different combinations of constituent materials, manufacturing parameters, sizes and shapes, and surface coatings. The specimens are being subjected to various sustained tensile loading (22 to 68% of ultimate strength) in three types of alkaline environments: NaOH, simulated pore-water solution, and embedded in concrete. Time to rupture or residual strength, as applicable, have been determined. Additionally, stress corrosion mechanisms were evaluated by various microstructural analyses. The results showed clearly that alkaline ions and moisture could penetrate or diffuse through the resin (or through cracks and voids) to the interphases and the fibers. For GFRP bars embedded in moist concrete under various sustained stress levels, three types of stress corrosion mechanisms have been identified: stress dominated, crack propagation dominated, and diffusion dominated.

The conventional design philosophy for reinforced concrete (RC) relies heavily on the ductile properties of steel. These ductile properties are used as a “fuse” and conceal the large uncertainty in the determination of modes of failure caused directly by concrete. Current design guidelines for fiber reinforced polymer (FRP) RC structures have inappropriately adopted the same design philosophy used for steel RC, leading either to the adoption of conservative safety factors or reduced structural reliability. A reliability-based analysis of FRP RC beams shows that the current, very conservative partial safety factors for FRP reinforcement on their own do not influence the structural safety of overreinforced concrete elements. Proposals are made for the modification of the material partial safety factors to achieve target safety levels.

The scope of this paper is to present a possible methodology for the calibration of partial safety factors for the design of strengthening measures of reinforced-concrete (RC) members using fiber-reinforced plastic (FRP). The methodology is general and can be used for any type of strengthening measure, e.g., in flexure, shear, ductility, or for the design of anchorage zones. The approach considers the problem of strengthening an RC member from a current unsafe situation, where all involved quantities are known from assessment, though only in probabilistic terms, to a target safe one, of which only the desired reliability is known. All relevant random variables are attributed a predefined probability distribution, based on a statistical survey separately conducted on geometrical and mechanical characteristics of old-style components. A first-order reliability method based optimization procedure is used to seek the solution of such a problem so that the target reliability is attained with the optimal FRP quantity, the appropriate collapse mechanism of the strengthened member, and the FRP design strength, with the associated partial safety factor. From Monte Carlo design simulations, the partial safety factor is probabilistically characterized, thus allowing one to select an appropriate fractile value for it.

This paper presents experimental results on the behavior in tension of reinforced concrete members strengthened with carbon fiber sheets (CFS). CFS reduced the crack spacing and brought about good crack width control as well as a significant change in the average bond stress and the average stress of steel reinforcing bars and concrete. The deterioration mechanism of the CFS bond properties near cracks was presented.

The strength enhancement of concentrically loaded columns bonded with external fiber-reinforced polymer (FRP) systems arises from two sources. First, the confinement effect of transverse fiber sheets leads to an increase in the uniaxial compressive strength of the confined concrete, resulting subsequently in an increase in the contribution of concrete to the load-carrying capacity of the column. Second, the longitudinal fiber sheets contribute directly to the load-carrying capacity of the column. The confinement effect of the externally bonded FRP systems in rectangular columns is known to be less than that in circular or square columns. This study specifically examines the case of rectangular reinforced concrete columns with a section aspect ratio of 3.65, which are typical in construction of monolithic housing apartments. A total of 52 half-scale, short columns were fabricated and tested to failure under a concentric load to investigate the effect of fiber type and configuration and fiber anchors on the strength enhancement of the columns. The effect of the presence of plaster finishes was also examined. A simple analytical approach is proposed for the evaluation of the axial load capacity of the strengthened columns.

The concrete-repair fiber reinforced polymer technology emerged in the United States during the past 10 years. Since 1995, carbon fiber reinforced polymer (CFRP) has been applied to strengthen concrete decks of a troubled posttensioned garage in Atlanta. During the construction of the garage, design deficiencies were found. A remedial repair, involving heavily reinforced, 7.6 cm (3 in.) thick Gunite (Shotcrete) beams, applied to the underside of the slab between drop panels in the east-west (E-W) direction, was developed in 1984. Since then, delamination of Gunite beams and other structural problems repeatedly occurred. Epoxy injection and other limited repairs were done over the years in an attempt to remediate the problems. In 2000, due to the growing delamination concerns, backed up with nondestructive impact-echo testing results, and due to the newest set of structural analyses that showed additional design deficiencies, Simpson Gumpertz & Heger developed a new and comprehensive remedial program. The first phase of this program included an in-depth mechanical in situ load test program to study the strength and stiffness performance of the existing typical slab spans, including the effects of Gunite beams, the loss of Gunite beams due to delamination, and the CFRP strengthing of spans. The tests showed that the CFRP repair of E-W spans with delaminated Gunite beams is warranted and that it performs well. The performance of the existing typical spans in the north-south direction was found to be acceptable without any repairs. Further monitoring of the performance of Gunite beams is required, and additional CFRP strengthening will be done periodically. In addition, structural deficiencies of the typical second-level end bays were repaired using additional steel framing supports.

The structural characterization of hybrid fiber-reinforced polymer (FRP)–glued laminated (glulam) panels for bridge deck construction is examined using a combined analytical and experimental approach. The structural system is based on the concept of sandwich construction with strong and stiff FRP composite skins bonded to an inner glulam panel. The FRP composite material was made of E-glass reinforcing fabrics embedded in a vinyl ester resin matrix. The glulam panels were fabricated with bonded eastern hemlock vertical laminations. The FRP reinforcement was applied on the top and bottom faces of the glulam panel by wet layup and compacted using vacuum bagging. An experimental protocol based on a two-span continuous bending test configuration is proposed to characterize the stiffness, ductility, and strength response of FRP-glulam panels under simulated loads. Half-scale FRP-glulam panel prototypes with two different fiber orientations, unidirectional (0°) and angle-ply (±45°), were studied and the structural response correlated with control glulam panels. A simple beam linear model based on laminate analysis and first-order shear deformation theory was proposed to compute stiffness properties and to predict service load deflections. In addition, a beam nonlinear model based on layered moment-curvature numerical analysis was proposed to predict ultimate load and deflections. Correlations between experimental results and the two proposed beam models emphasize the need for complementing both analytical tools to characterize the hybrid panel structural response with a view toward bridge deck design.