Journal of Composites for Construction

In the last decade, extensive research has been done in the field of the seismic upgrading of reinforced concrete (RC) columns using fiber-reinforced polymer (FRP). The enhancement in the column’s ultimate strength and significant improvement in the column’s displacement ductility combined with the ease of FRP installation resulted in an increasing use of FRP as an effective retrofit solution. Recently, similar enhancements to the RC columns have been reported for reinforced masonry columns (RMCs) upgraded using FRP. However, due to the limited available studies, a gap still exists in understanding the lateral response of the upgraded RMCs using different types of FRP. This study presents an experimental investigation of the effect of changing the FRP type (carbon and glass) on the lateral response of RMCs tested under quasistatic cyclic loading. Based on the experimental data, both FRP materials showed higher strength and ductility compared with control specimens not FRP-wrapped. However, the carbon FRP (CFRP) upgraded RMCs showed slightly higher strength and higher ductility levels than that of glass FRP (GFRP). Also, the difference between carbon and glass FRP RMCs performance was more noticeable as the confinement ratio increased. Moreover, with the current migration of the design codes from forced-based to displacement-based design, there is a need to have simple analytical tools capable of predicting the full load-displacement relationship. Therefore, 12 masonry prisms having various numbers of FRP layers and configurations were tested under concentric compression loading to calibrate the stress-strain material model. Consequently, this stress-strain model was implemented in a simple backbone model capable of predicting the lateral load-displacement backbone relationship of the upgraded RMCs. The model was capable of calculating the ultimate strength, displacement at ultimate strength, and initial stiffness of the tested RMCs with average errors of 6, 22, and 30%, respectively.

Glass fiber-reinforced polymer (GFRP) composites offer several advantages over loose-plate steel connections when used in vertical joints between concrete wall panels as a connector. Advantages include resistance to corrosion, higher tensile strength, and the ability to conform to uneven surfaces. The performance of continuous and intermittent GFRP composite connections between precast concrete wall panels under simulated seismic loads is investigated using six specimens. Each specimen was comprised of two precast concrete wall panels connected with a GFRP composite system. The laminate in the system consisted of bidirectional GFRP sheets applied in a wet layup configuration over a layer of epoxy–putty adhesive. The connection was applied on one side of the wall panels to meet architectural and construction demands. Cyclic shear was introduced to the GFRP composite connection through in-plane horizontal quasi-static cyclic loads, while horizontal and vertical constraints at the bottom and top corners of each specimen were engaged. The variables in the research are the presence of epoxy–putty adhesive in the seam between the wall panels, the percent of seam height covered with bidirectional GFRP composite, and the use of GFRP spike anchors. The presence of epoxy–putty adhesive in the seam significantly increased the horizontal load capacity of the GFRP composite connection. The use of GFRP spike anchors increased the horizontal load, horizontal displacement, shear transfer, and energy dissipation capacity of the GFRP composite connection. The GFRP composite connections performed very well and could be used in the seismic retrofit of existing connections between precast concrete wall panels.

An experimental program that investigated the seismic performance of concrete-filled square steel tube with fiber-reinforced polymer (FRP)–confined concrete core (FCCC) column (SCFC) was conducted. Fourteen SCFC specimens were fabricated with four parameters of axial compression ratio, strength grade of concrete, thickness of FRP tube, and thickness of steel tube and tested under constant axial compression and reversed-cyclic lateral loading. Examinations of the test data resulted a number of significant conclusions with regard to the influence of the investigated column parameters on the performance of SCFC columns. Of primary importance, SCFC columns exhibit favorable energy dissipation and ductility, even when the columns were subjected to high axial loads. The results also indicate that SCFC columns delivered satisfactory lateral moment-bearing capacity and displacement ductility under high axial compression ratio. In addition, a good level of average stiffness and equivalent viscous damping ratio were examined. Furthermore, the variations of hoop strains along column height, the relationship of axial displacement–lateral drift ratio, and the plastic hinge length were examined to evaluate the seismic performance of SCFC columns.

This paper reports on load tests conducted on seven full-scale RC beam specimens. The objective of this study was to determine the feasibility of using an emerging and greener material, basalt fiber–reinforced polymer (BFRP) composite, as a candidate material for flexural strengthening and rehabilitation of RC beams. The test parameters chosen in this study are the flexural reinforcement ratio, corrosion level, number of layers of BFRP composite, and cross-strapping scheme. The study finds that BFRP composite is a suitable material for flexural strengthening and rehabilitation of RC beams because a large gain in the strength can be achieved by using externally bonded BFRP composite, especially if premature debonding-induced failure of a RC beam can be avoided through implementation of a proper cross-strapping scheme. The study also finds that BFRP flexural strengthening and rehabilitation can reduce the ductility of RC beams; however, the reduction is limited to 30%.

The seismic performance of slotted reinforced concrete walls strengthened with fiber-reinforced polymer (FRP) was investigated. This reinforcement method aims to increase the ultimate deformation capacity and avoid compressive damage to the concrete. To compare the performance of the slotted RC walls with and without FRP reinforcement, pseudostatic tests were conducted on two conventional slotted RC walls and two FRP-reinforced slotted RC walls. The slotted RC walls reinforced with FRP exhibited significantly improved deformation capacity and were able to sustain a $1:30$ drift ratio without obvious damage, while the ultimate deformation of the slotted RC wall without FRP reinforcement could only sustain a drift ratio of about $1:50$. Furthermore, a partial FRP retrofitting method can fully recover the load-bearing capacity of the wall. The results confirm the effectiveness of FRP reinforcement in a slotted RC wall. However, the initial stiffness and energy dissipation capacity were not regained completely through the FRP reinforcement.

The performance of simply supported concrete-filled steel tube (CFST) beams strengthened with unidirectional carbon fiber–reinforced polymer (CFRP) sheets was investigated. Twelve circular and rectangular beams were tested under flexure. Different CFRP strengthening configurations were investigated. In addition, a nonlinear finite-element (FE) analysis was performed to investigate the influence of different design parameters. The results show that the CFRP sheets significantly improved the moment capacity, flexural stiffness, and energy absorption (EA) capacity of the CFST beams. For example, the moment capacity and the EA capacity of circular beams increased by up to 38 and 18%, respectively, depending on the strengthening configuration. The strengthening scheme consisting of CFRP sheets applied on two sides for 75% of the beam length resulted in a similar strength enhancement compared with CFRP sheets applied along the entire beam length. Based on the results of the FE analysis, an empirical method was developed to predict the moment capacity of the CFST beams that were strengthened with CFRP sheets.

A bonded sleeve connection was developed for joining tubular glass fiber–reinforced polymer (GFRP) beams and columns. It utilizes a sleeve connector made by welding a steel tube to a steel endplate. The steel tube is then inserted into and adhesively bonded with the GFRP beam end and the endplate is connected to the GFRP column using through-bolts. Four beam-to-column specimens with different bond lengths between the GFRP beam and steel tube and different numbers of bolts were tested. The moment-rotation responses, failure modes, and local strain responses were experimentally received and comparatively studied. The measured initial rotational stiffness of the proposed bonded sleeve connections was evaluated against a standard classification and compared to existing connection systems. Numerical analysis was also performed through finite element (FE) modeling. Paired contact elements and pretension elements were used to model the bolted connections, taking into account effects of friction, pretension force, and bolt hole clearance. Relevant failure criteria were also used to indicate yielding of the steel endplate and cohesive failure. Modeling and experimental results compared well in terms of moment-rotation behavior and failure modes. Further parametric study evidenced dominant effects of endplate thickness on the initial rotational stiffness of the connections.

Sandwich beams made of bio-based fiber-reinforced polymer (FRP) composite facings and foam cores were studied. The FRP facings consisted of a plant-based bidirectional flax fiber fabric ($400\mathrm{g}/{\mathrm{m}}^2$) and a bio-based epoxy resin (30% bio content), and the foam cores were made of 75-mm-thick closed cell polyisocyanurate. A total of nine sandwich beam specimens (1,200 mm long and 150 mm wide) were prepared and tested under three-point bending. The parameters of the study were core density (32, 64, and $96\mathrm{kg}/{\mathrm{m}}^3$) and facing thickness (one, two, and three layers of flax fabric). Three failure mechanisms were observed during testing, including top face wrinkling/crushing, core shear, and tensile rupture of the bottom face. It was shown that the foam with the density of $96\mathrm{kg}/{\mathrm{m}}^3$ was stiff and strong enough to achieve the tensile rupture of the flax FRP (FFRP) facing. Also, a nonlinear behavior was observed for the sandwich beams. A bilinear stress-strain model for FFRP facing was proposed and, based on that, closed-form moment-curvature and load-deflection equations of the sandwich beams were derived for design applications. The proposed design-oriented model was verified against the test data of this study and an independent study capturing the stiffness, strength, and nonlinearity of the test specimens.

Structural applications of recycled aggregate concrete (RAC) have been rare in practice owing to its inferior strength and durability compared with natural aggregate concrete (NAC). Among the various methods attempted to facilitate the wider structural applications of RAC, the newly proposed method of fiber reinforced polymer (FRP) jacketing is particularly attractive due to the dual benefits of confinement and corrosion insulation. The few relevant studies on FRP-confined RAC have all been conducted on relatively small-scale specimens with no attention being paid to the possible size effect. This paper presents a series of axial compression tests on carbon FRP (CFRP)-confined RAC/NAC cylinders at three different scales with diameters ranging from 150 to 300 mm. In particular, some different-sized specimens were designed to have the same FRP confinement stiffness to allow for the first-ever examination of the possible size effect in FRP-confined RAC. The test results indicate that no appreciable size effect appears to exist in FRP-confined RAC or in FRP-confined NAC in the size range considered. The test results also show that FRP confinement effectiveness is marginally affected by the use of recycled aggregate. The compressive behavior of the two types of concrete can be almost equally well predicted by using stress-strain models originally proposed for FRP-confined NAC.

Four full-scale specimens were constructed and tested under reversal cyclic loading to study the seismic behavior of exterior glass fiber-reinforced polymer (GFRP)-reinforced concrete (RC) beam-column joints with lateral beams. The test variables were joint shear stress (0.85, 1.0, and 1.1 times the square root of the concrete compressive strength) and type of reinforcement (steel or GFRP). Test results indicated that well-designed exterior GFRP-RC beam-column joints confined with lateral beams exhibit linear behavior with minimum residual damage up to 5% drift ratio. A specialized software was used to construct a finite elements model, capable of simulating hysteresis behavior of reinforced concrete beam-column joints under seismic loading by taking into account the nonlinear behavior of concrete, confinement provided by the internal reinforcement, and the effect of cracks and bond-slip relationship between reinforcing bars and surrounding concrete. The model was used to investigate the effect of joint shear stress, size of lateral beams, reinforcement material, column axial load, and geometrical configuration on the joints’ behavior.

The majority of experimental and analytical investigations on the stress-strain behavior of rectangular concrete columns externally confined with carbon fiber–reinforced polymer (CFRP) composites have concentrated largely on unreinforced concrete columns of small size. However, research investigating the influence of larger cross-sectional size and internal steel reinforcement on the stress-strain behavior of CFRP-confined rectangular RC columns under axial compression loading has been limited. Therefore, the first part of this paper aims to present the results of experimental tests on 28 larger-sized rectangular plain and reinforced concrete (RC) columns confined with CFRP wraps. The primary test parameters were (1) cross-sectional size and aspect ratio; (2) number of CFRP composite layers; (3) volumetric ratio of hoop steel reinforcement; and (4) nature of loading (i.e., monotonic and cyclic). The experiments showed that the stress-strain curves of the monotonically loaded CFRP-confined columns exhibited a softening second branch followed by an ascending or almost flat third branch. All the confined columns experienced a significant enhancement in axial strain but only a slight enhancement in axial strength. An important finding of the study was that the internal longitudinal and hoop steel reinforcement influenced the shapes of the axial stress-strain envelope curves, the unloading and reloading paths, and the plastic strain values. A model was finally proposed for predicting the ultimate strength of CFRP-confined rectangular RC columns. The performance of the proposed model was validated against the results of current and existing experiments.

The contribution of near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars to the shear strength of unreinforced masonry (URM) walls built with concrete masonry units (CMU) was evaluated, and results are presented in this paper. This evaluation was achieved by reviewing test results from three types of research programs that were previously reported in the literature. Experiments were grouped as follows: (1) pull-out tests of NSM FRP bars embedded in CMUs, (2) pull-out tests of NSM FRP strips in clay brick prisms, and (3) in-plane diagonal compression tests of URM walls strengthened with NSM FRP bars/strips. Experimental results clearly indicate a significant difference in the failure mode observed between pull-out and in-plane diagonal tests, and statistically the maximum strains registered in NSM FRP bars/strips were significantly higher in pull-out type tests. Results from the pull-out tests were subsequently used to develop a bond stress–slip model for NSM FRP bars embedded in concrete masonry. Finally, the contribution of NSM FRP bars to the shear strength of concrete URM walls derived from different approaches is presented in this paper.

Carbon fiber–reinforced polymer (CFRP) laminates are widely used in strengthening reinforced concrete slabs, beams, and columns. However, the application of this advanced composite material to strengthen steel structures is still evolving, and more research is needed to facilitate deployment. This paper presents the results of an experimental program aimed at investigating the behavior of steel columns subject to eccentric loading and strengthened using bonded high-modulus CFRP laminates. Thirty hollow steel section (HSS) column specimens ($48\times 48\times 2\mathrm{mm}$) were eccentrically loaded to failure using load eccentricity values of 25, 50, and 100 mm. The efficiency of bonding the CFRP laminates to the HSS column in enhancing the column’s capacity was evaluated for different column slenderness ratios ($kL/r$) ranging between 68 and 131. Depending on the slenderness ratio and the load eccentricity, it was found that the increase in axial load capacity of the CFRP-strengthened columns ranged between 40 and 107%, and the increase in the ultimate moment ranged between 34 and 135%; the maximum increase in a column’s capacity corresponded to the medium column slenderness ratio. Two successive failure modes for the strengthened HSS columns were encountered: CFRP laminates debonding and flexural buckling of the column. Flexural buckling was the dominant mode of failure for all the unstrengthened columns (control specimens). On the other hand, debonding of CFRP laminates was observed either before or after flexural buckling for the strengthened columns depending on the load eccentricity value and the slenderness ratio.

This paper presents an original study aimed at predicting the ultimate flexural and shear strengths of glass fiber–reinforced polymer (GFRP) RC squat walls subjected to quasi-static reversed cyclic lateral loading. Predicting the strength of GFRP-reinforced squat walls is critical for their safe design and acceptance of GFRP bar as reinforcement in such lateral-resisting structural elements. The test results of squat walls reinforced solely with GFRP bars have demonstrated their feasibility in resisting lateral loads and pointed to the necessity of adequate prediction of ultimate strength and failure modes. The predictions based on fiber-reinforced polymer (FRP) design codes and guidelines were investigated and compared with the test results of five large-scale GFRP-RC squat walls with different failure modes. The results demonstrated that in the one code, the concrete contribution to the shear resistance should be considered, and in other design guidelines, the 45° shear crack–angle assumption should be modified for squat walls with different properties and should consider the increase in concrete shear contribution after initiation of the first shear crack. The confinement at boundary elements has been shown to improve significantly the ultimate flexural strength that should be integrated in the design.