Journal of Composites for Construction

An experimental study was conducted to investigate the interfacial behavior between glass-fiber-reinforced polymer (GFRP) plate and cast-in-place (CIP) concrete placed against this plate—a method known as wet-bonding. The experimental program consisted of 33 single-shear specimens with various initial placement times of concrete, durations of concrete curing, and interface preparations. Control specimens using a conventional dry-bonded technique were also tested. A model was proposed that was able to predict the interfacial ultimate pullout load and local shear stress-slip relationship of a wet-bonded interface with reasonable accuracy. It is shown that (1) wet-bonded interfaces exhibit lower capacity and interfacial fracture energy than dry-bonded interfaces although the load to cause initiation of debonding is not significantly different; (2) the time of initial placement of the CIP concrete onto the wet, set, or cured resin significantly influences the ultimate pullout load and interfacial fracture energy. When initial placement of concrete follows initial set of the resin, at about 30 min, the ultimate pullout load and interfacial fracture energy are greatest; and (3) the nature of the interface also strongly affects the ultimate pullout load and interfacial fracture energy. Specimens in which fine aggregate is spead onto the resin immediately after its placement exhibit improved interface performance. Nonetheless, wet-bonded interfaces do not approach the capacity of conventional dry-bonded interfaces.

Although the practice of repairing or strengthening concrete infrastructure with externally bonded carbon-fiber-reinforced polymer (CFRP) has become common, the development of robust guidelines and techniques for its quality assurance, inspection, and monitoring is lagging significantly. Whereas common inspection methods (e.g., visual inspection or acoustic sounding) are sufficient to broadly identify deboned regions, it is difficult to reliably and accurately define the boundaries of bond defects, as is often required by acceptance criteria. Infrared thermography is a more sophisticated alternative, but the results of its application are influenced by ambient environmental conditions and operator interpretation of the data. Further, for applications in which multiple layers of CFRP are required, none of these techniques are capable of fully characterizing existing bond defects, including the depth through the thickness. To this end, pulse phase thermography (PPT) has the potential to more reliably and accurately detect the location and size of bond defects. By observing images in the frequency domain, one is able to identify a specific “blind frequency” at which a certain defect is no longer visible, and thereby also predict the defect’s depth through the thickness. This paper presents a procedure using PPT to develop a calibrated equation based on blind frequency, specific to the strengthening system, which allows for one to predict the interface at which a bond defect is present for externally bonded CFRP-to-concrete systems. With the development of more robust techniques enhancing quality assurance and reliability, and the facilitation of regular monitoring and inspection, infrastructure owners will gain more confidence in this repair or strengthening technology, ultimately leading to its widespread acceptance and adoption.

This paper presents an experimental investigation on the influence of prestressing fiber reinforced polymer (FRP) confining shell on the axial compressive behavior of high-strength concrete (HSC)-filled FRP tube columns. A total of 24 aramid FRP (AFRP)-confined concrete specimens with circular cross-sections were manufactured in the form of concrete-filled FRP tubes (CFFTs). The influence of the amount of prestress was examined by preparing the specimens with three different levels of lateral prestress ranging from 4.1 to 7.3 MPa. In addition to the prestressed specimens, companion specimens with no applied prestress were manufactured and tested to establish reference values. Results of the experimental study indicate that the influence of prestress on compressive strength is significant, with an increase in ultimate strength observed in all prestressed specimens compared to that of nonprestressed specimens. On the other hand, the influence of prestress on axial strain was found to be dependent on the amount of confinement, with lightly-confined and well-confined prestressed specimens displaying a decrease and increase in ultimate strain respectively, compared to their nonprestressed counterparts. The results also indicate that prestressing the AFRP shell prevents the sudden drop in strength, typically observed in FRP-confined HSC specimens, that initiates at the transition point that connects the first and second branches of the stress-strain curves. Finally, it was observed that prestressing the AFRP tube results in a significant increase in the specimen toughness as well as in the hoop strain efficiency of the AFRP shell.

Experimental results of the axial stress-strain response of eighteen carbon-fiber-reinforced polymer (CFRP) confined circular, square, and rectangular column specimens when subjected to cyclic axial compression are presented and discussed. Guided by these test results and other test data reported in the technical literature, a constitutive axial stress-strain material model of CFRP-confined concrete under generalized loading is developed. The proposed model, which is composed of a monotonic envelope response and a cyclic response, accounts for a wide range of test parameters and assumes a more simplified approach than existing models available in the literature. The model covers all important parameters in a unified manner, and predicts both ascending and descending postpeak responses. In addition to its simplicity in application, despite little discrepancy, the model was able to reproduce the test results generated in the experimental part of this investigation and other test data reported in the technical literature with good accuracy. The model can be incorporated in numerical analysis schemes for predicting the cyclic response of CFRP-confined concrete structural members.

Structural sandwich panels with fiber-reinforced polymer (FRP) skins are becoming an increasingly popular system because of their remarkable light weight, ease and speed of installation, and high thermal insulation capabilities. This paper looks into the potential for replacing conventional glass fiber–reinforced polymer (GFRP) skins with bio-based skins made of unidirectional flax fibers and a resin blend consisting of epoxidized pine oil. A comprehensive material testing program was first carried out on 70 standard tension and compression coupons. Then, 36 sandwich panels of $\mathrm{1,000}\times 150\times 75\mathrm{mm}$ were tested under four-point loading. The number of flax fiber–reinforced polymer layers of skin was varied from one to five [in comparison to one layer of GFRP, an epoxidized pine oil resin blend to epoxy, and the wet layup (WL) fabrication method to vacuum bag molding]. Sandwich panels with three layers of flax fibers provided equivalent strength and stiffness, but better deformability, compared to panels with one layer of glass fibers. Epoxidized pine oil-based skins decreased strength by up to 23% compared to epoxy-based skins. Vacuum-bagged panels decreased strength by up to 27% compared to WL as a result of thinner skins. As the number of flax layers increased from one to five, failure modes transitioned from skin wrinkling in the constant moment region to core shear failure or compression skin crushing in the shear span.

An innovative retrofit system to prestress carbon fiber reinforced polymer (CFRP) plates and attach them to existing metallic beams was developed (patent number CH 706 630 B1) and tested. The system does not require any glue between the CFRP plates and the beams; therefore, surface preparation is not necessary, which reduces the time and cost of retrofitting. The proposed prestressed unbonded reinforcement (PUR) system includes a pair of mechanical clamps that function based on friction. Each clamp can simultaneously hold and attach three CFRP plates to the beam. The design considerations of the clamps, which are the most important elements of the PUR system, were explained. The system has a trapezoidal configuration that offers an easy on-site installation and uninstallation procedures without residual damage on the metallic beam. Three 5-m-long steel beams were statically tested until failure, including one reference unstrengthened beam and two beams that were strengthened with 15% and 31% CFRP prestress levels. A considerable increase in the yielding and ultimate load capacities of the retrofitted beams was achieved. The ultimate load-carrying capacity of the strengthened beams with 15% and 31% CFRP prestress levels increased by more than 23% and 31%, respectively, compared to that of the reference specimen. A finite-element (FE) model was created to simulate the behavior of the retrofitted beams. Next, the FE results were compared with those from the experiments.

The use of carbon-fiber-reinforced polymer (CFRP) is an increasingly common solution for the strengthening of structures. However, the majority of research and applications have focused on the retrofitting of concrete structures. The applications of adhesively bonded CFRP to enhance the load-carrying capacity of metallic elements has been widely studied in the aeronautical industry, but it is also a promising technique for the area of civil engineering. This paper presents an experimental study that was designed to verify the effectiveness of the use of CFRP for the strengthening of the net section of steel elements under tensile loading. A series of tensile tests were conducted with different bond lengths, number of layers, and surface preparations of the steel. The specimens consisted of double lap joints and steel plates with various hole configurations. The ultimate load, the failure mode, and the effective bond length for CFRP-strengthened specimens were determined. The results showed that using CFRP sheets for the strengthening to prevent net area failure provides no gain on the ultimate strength, a small gain at the elastic limit, and a larger gain if the designer accepts extending the limit state from the elastic limit to the debonding limit.

This paper presents an experimental study that investigated the physical, mechanical, and durability characteristics of three different types of fiber-reinforced polymer (FRP) bars made of basalt and glass fibers with vinylester and epoxy resins, namely basalt/vinylester (B/V) FRP bars, glass/vinylester (G/V) FRP bars, and basalt/epoxy (B/E) FRP bars. First, their physical and mechanical properties were assessed. Then, a comparative durability study was performed on the three types of FRP bars under alkaline exposure simulating a concrete environment. The alkaline exposure was achieved by immersing the bars in an alkaline solution for up to 5,000 h at 60°C. Thereafter, the properties were assessed and compared with the unconditioned (reference) values. The test parameters were (1) fiber type (basalt or glass); (2) resin type (vinylester or epoxy); and (3) conditioning time (1,000, 3,000, and 5,000 h). The test results reveal that the G/V composite had the best physical and mechanical properties and lowest degradation rate after conditioning in alkaline solution. The B/E composite ranked second, while the B/V composite evidenced the lowest physical and mechanical properties and exhibited significant degradation of its physical and mechanical properties after conditioning.

Buckling-restraint braces (BRBs) are used extensively in seismic-resistant structures. They consist of a core energy dissipation and external restraining component. A novel glass fiber–reinforced polymer (GFRP) steel BRB is proposed. In the GFRP steel BRB, four GFRP pultruded tubes, which are tied together by GFRP wrapping layers, are used to restrict core steel component buckling instead of conventional steel tube and infilled concrete or mortar. This GFRP steel BRB is extremely lightweight. Techniques for manufacturing GFRP steel BRB were developed for convenient large-scale industrialized production. Quasi-static tests were carried out to validate the performance of the GFRP steel BRB under cyclic loading. The tests focused mainly on energy dissipation capacity and ultimate failure mode of the proposed GFRP steel BRB. The effect of applying additional reinforcements to the GFRP external restraining component on the performance of GFRP steel BRB was also investigated. The performance of the appropriately designed GFRP steel BRB was satisfactory; however, it is prone to damage from local failure of external restraining GFRP components and additional reinforcements may enhance its performance.

This paper investigates the manufacturing technology and anchoring effectiveness of a proposed novel wedge anchor for multitendon fiber-reinforced polymer (FRP) cables. Manufacturing of the FRP cable used 19 FRP tendons with a diameter of 4 mm, and its anchor system was first studied by addressing the reliable bonding between each tendon and the realization of the variable elastic modulus of the load-transfer component (LTC) in the anchor zone. The experimental demonstration included evaluating the tensile properties of the FRP cable, the strain distribution in the anchor zone, and the stress-strain relationship of the cable. The anchor effect was evaluated through a comparison of strain distributions among the finite-element simulation and experimental results. Furthermore, the anchor efficiency of the tested cables was evaluated. The results indicate that the winding of fiber roving around each tendon at the anchor zone benefits the integration of the tendons, and the variable elastic modulus of the LTC can be realized through the winding fiber roving with variable angle inclinations and the assistance of compression molding technology. The strain distribution in the anchor zone demonstrates the realization of the variable modulus of the LTC, which benefits smoother and longer shear-stress transfer along the longitudinal direction of the anchor zone. The high anchor efficiency of the tested FRP cable demonstrates that the proposed anchor obtains the potential tensile strength of each tendon without any reduction, thus resulting in a high-strength cable.

This paper presents the results of experimental testing a novel dual-tube self-centering buckling-restrained brace (SC-BRB) with pretensioned basalt fiber-reinforced polymer (BFRP) composite tendons. Cyclic tensile experiments are first conducted on two sets of BFRP tendons with different diameters. The results confirm that the BFRP tendons have a stable cyclic elastic modulus and sufficient elongation capacity, making them suitable for use as the self-centering tendons in SC-BRBs. Quasi-static experiments are performed using two dual-tube SC-BRB specimens with different core plate areas. Before the failure of the self-centering system, both specimens stably exhibit the expected flag-shaped hysteresis response, with a relatively small residual deformation. The internal forces in the BFRP tendons and the gaps between the tubes and the end plates are measured to validate the performance of the self-centering system. Both specimens meet the requirements for ultimate ductility and cumulative ductility in braces. For a similar initial pretension, a greater core plate area results in greater residual deformation; however, the cumulative energy dissipation and the equivalent viscous damping are also greater.

Reinforced concrete structures strengthened by fiber-reinforced polymer (FRP) always suffer dynamic loadings. Furthermore, the dynamic loadings are always added at the base of static loadings. The success of this strengthening method relies on the effectiveness of the bond of the FRP sheet to the concrete. Although numerous experimental studies have investigated this bond, experimental data concerning dynamic tests on basalt FRP (BFRP) sheets applied on concrete specimens under different initial static loadings are still lacking. This paper presents an experimental investigation on the dynamic bond behavior between the BFRP sheet and concrete under different initial static loadings (0, 30, 50, 80, and 100%) and displacement rate of $70\mathrm{mm}/\mathrm{s}$. Double-lap shear specimens were used for the tests. The results of the dynamic tests are reported and discussed to evaluate and compare the influence of initial static loading on the dynamic bond behavior between BFRP sheets and concrete. A nonlinear bond stress-slip relationship of the BFRP-concrete interface under different initial static loadings is determined based on an analysis of displacement data, which comprise four empirical parameters, namely, dynamic maximum bond stress ${\tau }_{\max i}^d$ under different initial static loadings, corresponding slip $s_0$, curve characteristic constant $n$, and local slip $s$. The test results show that (1) the dynamic bond capacity of the BFRP-concrete interface decreases with increasing initial static loading; (2) the failure mode of all specimens is debonding in the concrete layer; (3) the dynamic effective bond length of the BFRP-concrete interface increases with increasing initial static loading; and (4) the dynamic maximum bond stress decreases with increasing initial static loading. The calculation models of dynamic bond capacity and dynamic effective bond length considering the influence of initial static loading are also presented.

Fire safety is a significant concern for fiber-reinforced-polymer (FRP)–strengthened RC structures, particularly for indoor applications. To satisfy fire resistance requirements, fire insulation layers may be provided to protect FRP-strengthened RC members. This paper presents a simple, design-oriented method for predicting temperatures in insulated FRP-strengthened RC members under standard fire exposure. The proposed method consists of two sets of formulas: one set for predicting temperatures in unprotected FRP-strengthened RC members exposed to a standard fire; and another set to convert a fire insulation layer into an equivalent concrete layer. As a result, an insulated FRP-strengthened RC member can be analyzed as an unprotected RC member with an enlarged section for which a similar simple method has previously been established by these authors. In the present study, a finite element (FE) approach for the temperature analysis of insulated FRP-strengthened RC members was first developed and then verified using existing test data. Then the verified FE approach was employed in a parametric study to generate extensive numerical data, on which the second set of formulas were established. The proposed temperature prediction method is shown to provide accurate predictions of both FE results and test data of insulated FRP-strengthened RC members.

The axial compressive behavior of concrete confined with fiber reinforced polymer (FRP) has received much attention over the past two and a half decades, with over 90 empirical and semiempirical models developed to predict the compressive stress strain behavior. While there is no doubt that in general these models show a good correlation to the dataset from which they were derived, when applied to a global dataset, accuracy is reduced. In response to the largely empirical analysis approaches, which should only be applied within the bounds from which they were developed, a new, mechanics-based approach for predicting the axial and lateral stress–strain relationships of concentrically loaded FRP-confined cylinders is presented. The approach uses shear-friction theory to simulate the formation and displacement of sliding planes as concrete softens. It is shown that cylinders can fail through two shear-friction mechanisms, namely, through either the formation of a circumferential wedge, or, a single sliding plane. Importantly, from this is shown that although each mechanism is defined by the same shear-friction material properties different stress–strain relationships result and this may explain some of the scatter of test results. In this paper, the mechanism of a single sliding plane is derived and compared with that of a circumferential wedge.

This study investigates the behavior and failure modes of fiber-reinforced polymer (FRP) confined concrete wrapped with different FRP schemes, including fully wrapped, partially wrapped, and nonuniformly-wrapped concrete cylinders. By using the same amount of FRP, this study proposes a new wrapping scheme that provides a higher compressive strength and strain for FRP-confined concrete, in comparison with conventional fully wrapping schemes. A total of 33 specimens were cast and tested, with three of these specimens acting as reference specimens and the remaining specimens wrapped with different types of FRP (CFRP and GFRP) by different wrapping schemes. For specimens that belong to the descending branch type, the partially-wrapped specimens had a lower compressive strength but a higher axial strain as compared to the corresponding fully-wrapped specimens. In addition, the nonuniformly-wrapped specimens achieved both a higher compressive strength and axial strain in comparison with the fully-wrapped specimens. Furthermore, the partially-wrapping scheme changes the failure modes of the specimens and the angle of the failure surface. A new equation that can be used to predict the axial strain of concrete cylinders wrapped partially with FRP is proposed.

Reinforced concrete (RC) frame structures are widely used in earthquake regions, and damage to these structures has been reported after many earthquakes. Small parts of seriously damaged RC frames should be demolished and reconstructed, and a large number of earthquake-damaged RC frames can be retrofitted and used again. In this paper, the seismic behavior of predamaged reinforced concrete columns repaired using early-strength cement mortar and basalt fiber–reinforced polymer (BFRP) has been investigated through lateral reversed cyclic tests on seven large-sized RC columns. Two predamage levels [(1) moderate damage, and (2) severe damage] were assigned to the RC columns through cyclic loading. The axial load and residual displacement were kept constant during the retrofitting process, and the retrofitted columns were retested after 6 days of mortar and epoxy curing at room temperature. The ductility and energy dissipation capacities of the fiber-reinforced polymer (FRP)-retrofitted predamaged RC columns were greatly improved after early-strength cement mortar perfusion and BFRP wrapping compared with the original intact columns. The flexural capacity could be fully restored for the moderately predamaged RC column and partially restored for the severely predamaged column, whereas the initial elastic stiffness of the BFRP-retrofitted predamaged RC columns was lower than that of the original intact columns and deceased with increased predamage level. The influences of axial load, residual displacement, and predamage level on the repair effect of FRP-retrofitted predamaged RC columns were analyzed, and the predamage level was the main influencing factor.