Journal of Composites for Construction

This paper presents the test results of a study on the bond behavior of basalt fiber-reinforced polymer (BFRP) bars to concrete. Thirty six concrete cylinders reinforced with BFRP bars and twelve cylinders reinforced with glass fiber-reinforced polymer (GFRP) bars were tested in direct pullout conditions. Test parameters included the FRP material (basalt and glass), the bar diameter, and the bar embedment length in concrete. Bond-slip curves of BFRP and GFRP bars revealed similar trends. BFRP bars developed average bond strength 75% of that of GFRP bars. All BFRP specimens failed in a pullout mode of failure along the interfacial surface between the outer layer of the bar and the subsequent core layers. The influence of various parameters on the overall bond performance of BFRP bars is analyzed and discussed. The well-known BPE and modified-BPE analytical models were calibrated to describe the bond-slip relationships of the bars. Test results demonstrate the promise of using the BFRP bars as an alternative to the GFRP bars in reinforcing concrete elements.

This paper addresses the results of accelerated hygrothermal (coupled temperature and moisture) tests on fiber-reinforced polymer (FRP) strengthened clay bricks, aimed at investigating bond degradation mechanisms. The exposures are selected to simulate different environmental conditions and the bond degradation is periodically investigated by visual inspection and by conventional single-lap shear bond tests. The changes in the properties of material constituents were also monitored; the results are presented and critically discussed. A decay model is adopted for simulating the observed degradation in the specimens. The model, once validated, is used for long-term performance prediction of FRP–masonry systems and the results are compared with the environmental reduction factors proposed by available design guidelines.

This paper presents a synthesis of the results of a broad experimental and analytical research program undertaken for evaluating the flexural behavior of unbonded posttensioned prestressed concrete (PC) members when strengthened using external fiber-reinforced polymer (FRP) composites. In the experimental part of the investigation, the findings of which were reported earlier, 24 specimens were tested. The test variables included span-to-depth ratio, area of unbonded prestressing steel, area of FRP reinforcement, and profile of the prestressing tendons. An additional 12 companion bonded PC and reinforced concrete (RC) specimens having the same design parameters as the unbonded specimens were also tested for comparison. In the analytical part, numerical nonlinear analysis was carried out for analyzing the flexural behavior of FRP strengthened unbonded PC members and for regenerating the response of the specimens tested in the experimental part of the investigation. Also, a general design oriented model was developed, vis à vis the test results, for calculating the flexural capacity of FRP-strengthened posttensioned members with an internal or external unbonded tendon system. The procedure is consistent with the approach proposed in a standard-setting report for reinforced concrete or bonded prestressed concrete members, and is applicable for both simply supported and continuous members. A flow chart was developed for illustrating the use of the proposed design procedure.

This paper discusses the effect of corrosion aging on the flexural response of reinforced concrete (RC) T-girders strengthened with near-surface-mounted (NSM) composite reinforcement. The study comprised analytical modeling and experimental testing of 15 RC T-girders. Twelve girders were strengthened in flexure with either two or four NSM carbon-fiber-reinforced polymer composite strips, and three girders were not strengthened to act as benchmarks. In some girders, the NSM composite reinforcement was supplemented by intermittent U-shaped composite wraps along the beam span. Five girders were not corroded and 10 were subjected to accelerated corrosion aging for 30 and 100 days that corresponded to average measured tensile steel mass losses of 5 and 15%, respectively. Corrosion damage and cracking reduced the load capacity of the girders strengthened by NSM composites without U-wraps. The inclusion of U-wraps in the strengthening regime prevented premature peeling failure of the concrete cover. No reduction in the load capacity was recorded for the corroded girders strengthened with NSM composites combined with U-wraps. The residual load capacities of all strengthenedcorroded girders were substantially higher than that of the control girder. An analytical model was introduced that can predict the load capacity and failure mode of corroded T-girders strengthened with NSM composites. A comparison between the analytical and experimental results demonstrated the accuracy and validity of the model.

A large amount of research has been conducted on recycled aggregate concrete (RAC) due to its social, environmental, and economic significance. However, the in situ application of RAC has so far been mainly limited to nonstructural purposes, as the performance of RAC, in both the short and long term, is inferior to its normal concrete counterpart. Existing research has shown that the performance of concrete in compression members can be significantly enhanced through external confinement using steel tubes and fiber-reinforced polymer (FRP) tubes/wraps. Some recent research has examined the behavior of steel tubes filled with RAC, but the research on the behavior of RAC confined with FRP has been rather limited. Research is therefore needed to better understand the stress-strain behavior of and develop a reliable stress-strain model for FRP-confined RAC to facilitate the design of members with FRP-confined RAC. This paper presents the results of the first systematic experimental study on the axial compressive behavior of FRP-confined RAC in which 18 FRP-confined RAC cylinders were tested. The effects of the replacement ratio of coarse aggregate (ratio between the mass of recycled coarse aggregate to the total mass of coarse aggregate) and degree of FRP confinement are investigated. Both the axial and lateral strain responses of the specimens are examined in detail. The test results show that specimens with a replacement ratio of 20% behave similarly to that of normal concrete, but specimens with a replacement ratio of 100% exhibit a lower strength and a different stress-strain response. The applicability of two existing stress-strain models for FRP-confined normal concrete to FRP-confined RAC is also examined.

An experimental study was conducted to assess the effectiveness of strengthening unreinforced-masonry (URM) shear panels with near surface-mounted (NSM) fiber-reinforced polymer (FRP) strips. A total of 23 wall panels (5 URM and 18 reinforced) were subjected to vertical precompression combined with either monotonic or increasing reversing cycles of in-plane lateral displacement under fixed-fixed boundary conditions. Two wall aspect ratios (height/length) and six different reinforcement schemes were tested. The experimental program was designed to produce diagonal cracking in the URM specimens and hence investigate the effectiveness of the various reinforcement schemes in controlling this failure mode. This was achieved for the aspect ratio 1 wall panels. The study revealed that the FRP strengthening was effective in improving the ultimate load, displacement capacity, ductility, and energy dissipation compared with the URM response. For the aspect ratio 0.5 panels, base sliding failures dominated the experimental program, making it difficult to fully assess the effectiveness of the various reinforcing schemes.

The use of fiber-reinforced polymer (FRP) composites has recently experienced a steep increase in civil engineering applications because of the high mechanical and low-density properties of such materials. Over the last few decades, concrete columns externally confined with FRP sheets have been widely investigated for their use in structural rehabilitation and the seismic strengthening of civil constructions. There is much scientific literature based on experimental results and analytical or empirical theoretical models. In fact, several numerical models and analytical procedures are able to predict the behavior of FRP-confined structural elements subjected to axial or seismic loads, and researchers worldwide have experimentally studied and analytically calibrated a wide range of significant variables. Nevertheless, there are still few results concerning the durability of FRP-confined concrete exposed to severe environmental conditions, despite being an important issue in design and safety assessment. The objective of this study is to raise awareness about the durability of FRP-confined concrete. To do so, the authors collected and analyzed the results of approximately 760 pure axial compression tests, taken from 17 different experimental studies published in the scientific literature, in order to present a critical comparison between the results of experimental studies and the theoretical models provided by design guidelines and codes. The study was conducted according to the following steps: first, experimental data available in the literature were collected; second, FRP-confined concrete cylinders subjected to axial load were classified according to the different experimental variables investigated (mainly according to the type of environmental agent to which they were exposed); third, experimental results were compared with international provisions for the design and construction of externally bonded FRP systems for strengthening concrete structures. Finally, the strength and limits of the technical codes were analyzed in terms of safety factors, and the formulation of a design equation in the short and long term was critically examined.

A finite element model denoted as FEM-C is developed in this paper for prediction of structural responses of fiber-reinforced polymer (FRP)–strengthened concrete slabs under fixed-point cyclic load. First, a finite element model named as FEM-S, which is of the same geometry as the FEM-C model, is developed and validated for analyses of structural behavior of FRP-strengthened concrete slabs under static load. Then, the finite element model is extended for analyses of structural behavior of FRP-strengthened reinforced concrete (RC) slabs under fixed-point cyclic loading in the FEM-C model, with degradations of material properties of concrete, steel reinforcement, and FRP due to cyclic loading taken into account. Geometric and material nonlinearity and bond-slip effects between FRP and concrete are included in these models. The finite element (FE) models are validated through numerical examples and demonstrated to be effective and accurate for modeling the structural behavior of FRP-strengthened RC slabs under static and cyclic loading. The FEM-C model can also model the bond-slip behavior between the FRP and concrete well. Furthermore, the effects of parameters on the structural performance of the FRP-strengthened concrete slab under fixed-point cyclic loading, including different types and widths of FRPs, are investigated using the FEM-C model. It is learned that the computed central deflection of the CFRP-strengthened slab is approximately 18% less than that of the GFRP-strengthened slab. In addition, it is found that the RC slab strengthened with 150 mm wide FRP strips performs better than that with 102 mm wide FRP strips, with approximately 8% less central deflection and 10% less slip.

Unlike carbon-fiber-reinforced polymer (CFRP) bars and the maturely developed glass FRP (GFRP) bars, the long-term performance of the newly developed basalt-fiber reinforced polymer (BFRP) bars under severe environmental conditions remains unclear. This paper evaluates the residual tensile properties of unstressed and stressed BFRP bars exposed to four types of simulated harsh environments: alkaline solution, salt solution, acid solution, and de-ionized water at 25, 40, and 55°C. Microstructural analysis using scanning electronic microscopy (SEM) was also conducted to reveal the inherent degradation mechanism of BFRP bars in alkaline environment. The residual tensile strength of unstressed BFRP bars exposed to alkaline solution was used for long-term performance prediction based on Arrhenius theory. The results showed that the effect on the durability of BFRP bars exposed to acid, salt, and deionized water was less than that for bars exposed to alkaline solution. The effects of sustained stress on the degradation of BFRP bars were not obvious when the stress level was less than 20% of the ultimate strength, but if the stress exceeded this level, the degradation processes were accelerated. The predicted exposure time required for a reduction in strength of 50% at a mean annual temperature of 5.7°C in alkaline solution, which represents an area with a northern latitude of 50°, was estimated at approximately 16.1 years for the 6-mm BFRP bar.

Two groups of cylindrical carbon fiber-reinforced polymer (CFRP) tendons were exposed in distilled water at 23°C and 60°C to study the diffusion mechanisms and the effect of moisture uptake on the tendon shear modulus. The two tendon groups had different manufacturing processes, so DMA tests and optical microscopy were used to help characterize the materials. Mass uptake readings of tendon samples were recorded and the uptake generally agreed with Fickian predictions. To study the time-dependent changes in the matrix stiffness due to exposure, torsion tests within the elastic range of loading were conducted. The tendon shear modulus was then derived from the torque versus twist plots. For both groups of tendons the measured shear modulus decreased due to exposure in water. A long-term shear modulus prediction model was developed to relate the tendon torsional shear stiffness and the moisture concentration and the results appeared to agree well with the experimental findings.

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) are an attractive form of hybrid compression members incorporating FRP. CFFTs have several advantages over traditional column forms, including their excellent corrosion resistance and ductility. Much research has been conducted on CFFTs over recent years, but no systematic experimental study has been concerned with the cyclic axial compressive behavior of CFFTs with a filament-wound FRP tube; such studies are needed for the development of a cyclic stress-strain model for the concrete in CFFTs. This paper therefore presents an experimental study on the behavior of circular CFFTs under cyclic axial compression. The experimental program included the strength of concrete as a key variable so that it also provides a much needed supplement to the very limited existing research on the cyclic compressive behavior of FRP-confined high-strength concrete (HSC). The test results are compared with a monotonic stress-strain model and a cyclic stress-strain model for FRP-confined concrete, both of which have been based on test databases that are limited to concrete confined with an FRP wrap and include only a small number of tests for HSC. The test results show that the cyclic axial stress-strain behavior of concrete in CFFTs is generally similar to that of concrete confined by an FRP wrap. The test results also show that the monotonic stress-strain model perform reasonably well for HSC in CFFTs, but revisions to the cyclic stress-strain model are needed before it can provide accurate predictions for HSC in cyclically loaded CFFTs.

Eleven decked bulb T beams were constructed, instrumented, and tested under shear loading to failure. Nine beams were reinforced and prestressed with carbon-fiber composite cable (CFCC) strands, whereas one beam was prestressed with conventional low-relaxation steel strands and one beam was reinforced with non-prestressed CFCC strands. Half the span of each beam was reinforced with CFCC stirrups, whereas the other half was reinforced with conventional steel stirrups. Both ends of each beam were tested to evaluate the performance of CFCC stirrups versus that of steel stirrups. The investigation addressed the shear performance with respect to several shear parameters, including shear-span-to-depth ratio, stirrup spacing, prestressing force, and type of longitudinal and transverse reinforcement. All test beams failed by crushing of concrete in either the web or the top flange. No rupture of CFCC stirrups was experienced in any of the test beams. The performance of CFCC stirrups was analogous to that of steel stirrups with the exception that steel stirrups demonstrated a yield plateau before concrete failure. Beam ends with CFCC stirrups attained cracking and ultimate shear capacities similar to those attained in ends with steel stirrups. Results from the experimental investigation were compared with the theoretical values predicted using some available shear design guidelines for steel and CFCC reinforcement. In addition, modifications for current AASHTO LRFD shear design equations and its possible implementation in the ACI shear design guidelines are proposed based on the experimental results.

Fiber-reinforced polymer (FRP) bars and strips are potential alternatives to conventional soil reinforcements. In this aspect, two commonly used FRP materials are glass-FRP and carbon-FRP (GFRP and CFRP). However, the differences in pullout behavior between soil-embedded GFRP and CFRP rods, and conventional steel bars, are not well understood. This paper describes the results of an experimental study that was conducted to investigate the behavior of FRP bar/strip-sand interfaces under low normal pressure. A series of pullout tests were performed on GFRP and CFRP bars and strips buried in sand. A simple model using the ideal elastoplastic interface shear stress-strain relationship is proposed to simulate the pullout behavior of the FRP reinforcements. The progressive pullout process and the evolution of interface shear stress distribution are obtained from the proposed model. The experimental and analytical results show that, in comparison with CFRP and steel reinforcements, the GFRP reinforcement has a more nonlinear and nonuniform distribution of interface shear stress, and the pullout is more progressive. The differences in Young’s modulus and interface shear coefficient lead to the different pullout behaviors of these three materials. The reduced Young’s modulus gives GFRP reinforcements a risk of excessive tensile elongation and bending deflection. The test results show that the progressive pullout behavior is also governed by the reinforcement dimensions.

Carbon fiber–reinforced polymer (CFRP) composite materials have been widely used to strengthen masonry constructions. The bond between CFRP and substrate strongly conditions the performance of reinforced masonry structures. Characterization of shear bond mechanical behavior of masonry-CFRP interface thus becomes a crucial factor. The paper presents the results of an extensive experimental investigation aimed at evaluating the bond between clay brick and CFRP sheet. Specimens with varying width and length of the bonded CFRP sheet were subjected to shear test by three different testing setups. Results allowed comparison of the effectiveness of the three types of setup and pointed out the influence of the dimensions of the bonded sheet on ultimate load and ultimate slip, load-displacement diagram, and failure modes. Measurement of local strain along the CFRP sheet also allowed an investigation on the transfer mode of the stresses from the reinforcement to the brick and the determination of the local shear stress-slip relationship.