Journal of Composites for Construction

Very limited studies exist on the durability of “green” biocomposites, particularly resins, in the context of structural engineering applications. This paper investigates the durability of concrete cylinders wrapped with glass fiber reinforced bioresin made from furfuryl alcohol extracted from renewable sources such as corncobs and sugar cane. Conditioning was achieved by immersion of various groups of glass fiber–reinforced polymer (GFRP)-wrapped $153\times 305\mathrm{mm}$ cylinders in saline solutions with 3% salt concentration, at 23, 40, and 55°C, for up to 300 days. The groups included control specimens fabricated with conventional epoxy resin for comparison and plain concrete cylinders. Specimens were then tested in axial compression, periodically, at 29, 97, 182, and 300 days. It was shown that control unconditioned epoxy-GFRP and bioresin-GFRP wrapped cylinders had the same strengthening ratio ($f_{cc}^{\prime }/f_c^{\prime }$). After conditioning for 300 days, the ($f_{cc}^{\prime }/f_c^{\prime }$) ratio retentions for the bioresin-GFRP was 73% at all temperatures, and were 81, 76, and 76% for the epoxy-GFRP at the 23, 40, and 55°C, respectively. Statistical analysis of variance (ANOVA) was carried out and showed that the small difference in the 300 days’ strength between the bioresin-GFRP and epoxy-GFRP specimens is statistically significant at the 23°C only, but not at 40°C and 55°C.

Flexural retrofitting of reinforced concrete (RC) beams using fiber-reinforced polymer (FRP) plate is a common way to increase the flexural capacity. There is a lack of rational design method to determine where the strengthening plate can safely be curtailed. As a result, retrofitted beams commonly fail by debonding of the FRP plate, which occurs well before the target flexural capacity. Debonding is brittle failure; thus, ductility of the retrofitted beam needs to be ensured by debonding prevention. With sufficient ductility the ultimate strength can continuously increase beyond steel yields with the elastic behavior of the FRP strengthening plate. Debonding prevention has been accounted for empirically in most design approaches so far. The global energy balance approach (GEBA) using fracture mechanics has been proposed to determine the debonding load of an FRP-RC beam that is affected by the plate curtailment location. The GEBA results for various FRP-RC beams can be summarized using debonding contours on plots of moment capacity against the safe plate curtailment locations. The debonding contours constructed in this way for the beams with the same ratio of depth to fracture energy are virtually the same. This paper shows how GEBA can be incorporated into the design process to prevent premature debonding of the FRP plate. The method makes use of the debonding contours and derives from these simplified design charts that could be made available to designers. The retrofitting design consideration and the theoretical background of this unified design method are first explained, followed by the derivation of the conceptual design charts. Numerically correct design charts are then constructed for a wide range of design cases. Finally, a worked example is used to explain the way to apply the unified design method using design charts.

For reinforced concrete (RC) beams and slabs under-reinforced with steel and strengthened in flexure with carbon fiber-reinforced polymers (CFRPs), the yield plateau is eliminated and post-yield flexural capacity increases linearly by elastic straining in the CFRP. At the ultimate flexural limit state, failure occurs by either concrete crushing or CFRP rupture. The consequence is an increase in strength and decrease in ductility. In this study, a parametric analysis is presented where strength and energy ratios are investigated as a function of existing steel and strengthening CFRP reinforcements. Both normal-strength and high-strength concretes are considered. A graphical representation showing the controlling strength limit state is developed. Strength and energy ratios for various service, yield, and ultimate limit state combinations show that strength increase and ductility loss are significantly affected by the amounts of steel and CFRP reinforcements. The ultimate-to-yield strength ratio increases rapidly with decreasing steel and CFRP reinforcements; consequently, the service strength approaches the yield strength. Ductility loss is nonlinear and inversely related to the amounts of steel and CFRP reinforcements. For the parameters considered, ductility relative to the unstrengthened condition was reduced between 35 and 93%.

The mechanical performance of the bond between fiber-reinforced polymer (FRP) plates and cast-in-place concrete (referred to as the wet-bond interface) is investigated. The wet-bond interface is named with respect to the dry-bond interface. The dry-bond interface is a chemical bond that is developed to adhere FRP sheets or plates to cured concrete, whereas the wet-bond interface is a chemical-bond interface that is developed to connect stay-in-place FRP formwork and cast-in-place concrete. The double-lap shear test method is adopted to evaluate the effect of the bond. The mechanical properties of the wet-bond interface are compared with that of the conventional dry-bond interface using the same epoxy adhesive, including the load-slip curves, strain distributions, load capacity, and interfacial fracture energy. The results show that the failure modes of the wet-bond and dry-bond interfaces are different. In the dry-bond specimen, debonding occurs in the thin concrete layer and is accompanied by an inclined crack near the loading end; however, in the wet-bond specimen, debonding occurs in the shallow mortar layer adjacent to the concrete-adhesive interface, and no inclined crack appears. The shear stress and slip between the FRP and concrete are obtained by differential and integration operations on the measured strain values. A bond-slip constitutive model for the wet-bond interface is proposed by a regression analysis of the test data. Numerical simulations are performed using the ANSYS finite-element software, incorporating a spring unit to simulate the overall mechanical performance of the wet-bond interface. The results show that the average wet-bond strength value is approximately 0.54–0.68 times that of the dry-bond strength value. Finally, the fracture energy and the load capacity value of the wet-bond are compared. The results show that the proposed constitutive model in this paper sufficiently simulates the overall mechanical performance of the wet-bond interface.

The results of out-of-plane strength tests on unreinforced and reinforced concrete masonry wallettes that were constructed using $190\times 190\times 390\text{-}\mathrm{mm}$ hollow concrete masonry units are reported in this paper. The reinforcement was external and was accomplished by spraying one side of the wallettes with layers of glass-fiber-reinforced polymer (GFRP). Eight unreinforced and 28 reinforced wallettes were constructed and tested in four-point bending spanning either vertically or horizontally. Twelve additional reinforced wallettes were constructed but tested cantilevered from either the bottom or one of the sides and loaded either along the top or along the nonsupported side. Overall, the results indicate that the strength of unreinforced masonry can be greatly improved by external reinforcement. The behavior of the wallettes is also discussed, with the most worrisome observed behavior being the failure of the webs of the masonry units in the direction parallel to the direction of the applied loads. As part of the research, simplified models to predict the capacity of hollow concrete masonry externally reinforced with sprayed GFRP were also developed and presented in this paper.

Because concrete bridge deck slabs are much longer in the traffic direction, they experience transverse early-age cracks due to volumetric instability and restraint. In the last decade, the use of the noncorrodible fiber-reinforced polymer (FRP) bars in concrete bridges has significantly increased. This paper presents the experimental results of four full-scale, cast-in-place glass FRP–reinforced concrete (GFRP-RC) deck slabs, subjected to early-age shrinkage. The slabs measured $\mathrm{2,500}\mathrm{mm}\mathrm{long}\times 765\mathrm{mm}\mathrm{wide}\times 180\mathrm{mm}\mathrm{thick}$. The longitudinal reinforcement ratio (0.30, 0.50, 0.70, and 1.1%) was the main parameter investigated in this study. The test results were presented in terms of cracking pattern, width and spacing, strains in both GFRP bars, and concrete over a period of 90 days after casting. The experimental results were compared with provisions and predictions from a published analytical model. It was concluded that the published numbers can reliably provide a minimum reinforcement ratio for bridge deck slabs reinforced with GFRP bars. Also, the measured width of shrinkage cracks agreed with the results of the analytical model.

This study proposes the use of artificial neural networks (ANNs) to calculate the compressive strength and strain of fiber reinforced polymer (FRP)–confined square/rectangular columns. Modeling results have shown that the two proposed ANN models fit the testing data very well. Specifically, the average absolute errors of the two proposed models are less than 5%. The ANNs were trained, validated, and tested on two databases. The first database contains the experimental compressive strength results of 104 FRP confined rectangular concrete columns. The second database consists of the experimental compressive strain of 69 FRP confined square concrete columns. Furthermore, this study proposes a new potential approach to generate a user-friendly equation from a trained ANN model. The proposed equations estimate the compressive strength/strain with small error. As such, the equations could be easily used in engineering design instead of the invisible processes inside the ANN.

Fiber-reinforced polymer (FRP) plate debonding is commonly caused by the fracturing of concrete, but few studies of fracture debonding models exist, from which the failure load of the concrete cover layer can be evaluated. This paper presents a parametric study for plate end debonding using the global energy balance approach (GEBA), which has been proposed recently for determining the structure-debonding load. GEBA determines the debonding load using moment-curvature ($\mathrm{M}\text{-}\kappa$) models, and can thus be used to determine how debonding is affected by the beam’s flexural design. This paper presents parametric results using debonding contours on plots of moment capacity against the plate curtailment locations, and shows that beams with the same depth-to-fracture energy ratio give virtually the same debonding contour. This helps to generalize debonding determination for beams with different depths, and can be conveniently used for design. The parametric study lays a foundation for the application of fracture mechanics in FRP-plate retrofitting design using conventional $\mathrm{M}\text{-}\kappa$ models to cover a wide range of flexural retrofitting situations.

Fiber reinforced polymer (FRP) bars have been increasingly used in bridge deck slabs over the last decades to overcome corrosion issues. However, there is still a critical need for a rational method to analyze the load capacity of such bridge deck slabs. Because FRP bars possess a linear stress-strain relationship, the moment-curvature response of FRP concrete section, either reinforced or prestressed, does not exhibit a distinct yield-plateau, thus it is unclear if conventional yield line theory is applicable. Therefore, the concept of an equivalent plastic moment capacity for FRP concrete sections is introduced and applied in yield line theory for bridge deck slabs. A full-scale bridge deck slab with precast panels reinforced and prestressed with aramid fiber reinforced polymer (AFRP) bars is experimentally tested under different wheel and axle load configurations on either interior spans or overhangs. The results confirm the high accuracy of the failure analysis using the yield line theory, where the load capacity predictions are within 3% accuracy of the experimentally observed results.