Journal of Composites for Construction

The effective strain in composites, along with their potential rupture and debonding, plays a crucial role in predicting the strength of retrofitted reinforced concrete (RC) beams. However, only limited experimental data on these phenomena are available, mainly because of the inadequacy of traditional deformation and strain measurement techniques. This paper presents a comparative analysis of instrumentation for monitoring retrofitted RC elements. In particular, the paper addresses beams retrofitted with composite materials, FRPs (fiber-reinforced polymers), and FRCMs (fiber-reinforced cementitious mortars). It also considers strain gauges, fiber Bragg grating (FBG) sensors, LVDTs, digital image correlation (DIC), and acoustic emission (AE) sensors for monitoring strain, displacement, cracking, and debonding. Experiments on six beams are carried out, and the measured data from the monitoring devices are compared. The accuracy of DIC for strain and displacement monitoring is shown to match the performance of traditional methods, with the added benefit of providing full-field monitoring. The use of AE for detecting cracks and debonding, which is not readily possible using traditional methods, is also demonstrated. This is of particular interest for composite-strengthened RC elements, where accurate measurements of effective strain and debonding of the composite material can lead to the development of more precise design formulas.

This paper presents an analytical investigation of the inelastic local buckling of steel I-section beams strengthened with bonded fiber-reinforced polymer (FRP) sheets. Attention is focused on the ability of the FRP sheets to increase the beam rotations before onset of local buckling takes place. An inelastic stability analysis technique is presented that models the beam as a group of connected plates. The behavior of the steel beyond the elastic limit was modeled using the deformation theory of plasticity. The model results were verified through comparison with the results of a finite-element model and some reported experimental data. The developed model was used to conduct a parametric study to investigate the effect of each of the parameters of the problem focusing on FRP type and strengthening configurations. The study results showed that FRP materials with high longitudinal elastic modulus do not necessarily result in higher increases in the inelastic local buckling capacity. The FRP in-plane shear modulus is found to be more influential when buckling is controlled by the flange, while the transverse modulus becomes more effective when buckling is controlled by the web.

Composite timber beams strengthened by $U$-shape steel sections and carbon fiber–reinforced plastic (CFRP) are studied experimentally in this paper. Specimens with various strengthening, i.e., steel only, CFRP only, and a combination of the two, were considered under three-point flexural tests. Failure, displacement and strain response, ductility, bending capacity, and structural efficiency were evaluated for the present tested models. Dramatic enhancement of the capacity in addition to improved deflection and ductility were gained for the strengthened beams relative to the plain specimens, indicating the effectiveness of the reinforcement on the flexural strength of such composite beams.

Precast, prestressed hollow-core (PHC) slabs are widely used in floor and roof decks because of their economic benefits. Because no shear reinforcement can be provided because of fabrication restraints, designers have to opt for deeper profile or filling PHC slab cores with concrete when the load demand exceeds the limited concrete shear capacity of PHC slabs. This research project is exploring an innovative technique for strengthening the web-shear capacity of PHC slabs by bonding external fiber-reinforced polymer (FRP) sheets along the internal perimeter of the slab voids. Experimental testing and numerical simulation on full-scale single web, I-shaped beam specimens cut out longitudinally from full-width hollow-core slabs have been performed during the first phase of this research to investigate the feasibility and effectiveness of this new technique. The influence of several design parameters, including the length and the width of the strengthened zone, the thickness of applied FRP sheets, and the prestressing level of PHC, have been investigated. Results show that once the minimum required strengthened zone length and FRP sheet thickness are satisfied, the proposed novel shear strengthening technique would provide a considerable enhancement on the shear capacity and ductility to the strengthened specimens.

Textile-reinforced concrete (TRC) is currently being used as an effective inorganic reinforcement material due to its high bearing capacity, anticracking capacity, and corrosion resistance. However, the compressive performance of TRC-strengthened concrete columns under chloride corrosion is unclear. Therefore, this paper first studied mechanical properties of RC columns strengthened with TRC with a variable number of textile layers and overlap lengths in a conventional environment. Then, compressive performances of RC columns strengthened with TRC under chloride wet-dry cycling and coupling effects of chloride wet-dry cycling and sustained axial compressive load were studied. The experimental results indicated that the TRC’s constraint ability could make better use of the compressive performance of concrete and improve the failure mode of the compression column. The bearing capacity and ductility of RC columns was improved with an increasing number of textile layers; the increase in the lap length of the textile had only a slight impact on bearing capacities of columns, which slightly improved the ductility of the TRC-strengthened RC columns. The bonding properties of the interface between the TRC and concrete decreased slightly due to the corrosion effect of chloride ions. Thus, the ultimate bearing capacity and deformation capacity of RC columns strengthened with TRC declined after chloride wet-dry cycling; the coupling effects of chloride wet-dry cycles and the sustained axial compressive load negatively affected the bearing capacity and ductility of TRC-strengthened RC columns. The ultimate load and ductility of strengthened columns decreased with the increase in the sustained load.

A large number of substandard reinforced concrete (RC) frames exist in potential earthquake regions across the world. These frames are at high risk of collapse during possible strong earthquakes in the future. In order to mitigate the risk, pre-earthquake seismic strengthening of these frames is urgently needed. This paper presents an experimental study on the seismic performance of substandard RC frames strengthened with externally bonded carbon-fiber-reinforced polymers (CFRP) composites using shake table tests. Two 1/2-scale 4-story and 2-bay RC frames were fabricated and tested on a shake table. One specimen was tested first as the control specimen, while the other specimen was retrofitted with CFRP before test. The retrofitting scheme was designed according to the failure mode of the control specimen, and involved CFRP wrapping at and close to the beam-column joints at the lower two stories of the frame. The test results showed that the seismic performance of the substandard RC frame could be significantly improved by the CFRP strengthening technique. The maximum applied peak ground acceleration (PGA) that could be resisted by the control frame specimen was approximately $0.6g$, while the retrofitted frame specimen could endure seismic loading with a PGA value of $1.0g$ without significant damage. The horizontal displacements, interstory drift ratios, and torsional displacements of the retrofitted frame specimens were also found to be much smaller than those of the control frame at the same earthquake levels.

The results of an experimental study of the cyclic axial stress-strain response of rectangular concrete columns confined with a combination carbon-fiber-reinforced polymer (CFRP) jacket and CFRP anchors are presented and discussed. The test variables include aspect ratio of the column section, area of CFRP wraps, and area and configuration of the CFRP anchors. It was found that adding CFRP anchors improves the confinement effectiveness of the CFRP jackets, leading to substantially enhanced envelope stress-strain response and ductility of axial failure. Furthermore, changing the anchor configuration while maintaining the total anchors’ cross-sectional area does not produce a noticeable effect on the envelope stress-strain curve. The cyclic part of the stress-strain response of the CFRP-anchored specimens is similar to that of the companion specimens confined with CFRP sheets without anchors. Based on the results of this study, analytical expressions are derived for predicting the stress-strain curve, taking into account the effect of CFRP anchors. The proposed model is able to predict the envelope and cyclic responses of concrete columns confined with CFRP sheets only and columns confined with a combination of CFRP sheets and anchors.

This research investigates the feasibility of using prestressed glass fiber reinforced polymer (GFRP) bars for enhancing the in-plane shear strength of unreinforced masonry (URM) walls. In this application, URM walls were strengthened with prestressed GFRP bars by using the near-surface mounted reinforcement (NSM) technique. Prestressed GFRP bars would be used for repointing/closing wide open cracks in existing masonry structures. In total, eight URM walls were tested under in-plane diagonal compression. Experimental results validated the higher shear capacity of the strengthened URM walls and this increase was more pronounced for walls strengthened with prestressed GFRP bars. Furthermore, the calculated drift ratio in strengthened walls was considerably higher when failure was along stair-stepped diagonal cracks. Experimental results were used to modify and calibrate a theoretical shear friction model for predicting the in-plane shear capacity of strengthened URM walls with NSM prestressed GFRP bars. The model was capable of predicting reasonably well the shear capacity of strengthened URM walls.

The application of textile-reinforced mortar (TRM) as a means of increasing the flexural capacity of two-way reinforced concrete (RC) slabs is experimentally investigated in this study. The parameters examined include the number of TRM layers, the strengthening configuration, the textile fibers material (carbon versus glass), and the role of initial cracking in the slab. For this purpose six large-scale RC slabs were built and tested to failure under monotonic loading distributed at four points. It is concluded that TRM increases substantially the precracking stiffness, the cracking load, the postcracking stiffness, and eventually the flexural capacity of two-way RC slabs, whereas the strengthening configuration plays an important role in the effectiveness of the technique. Simple design equations that provide good estimation of the experimental flexural moment of resistance are proposed.

Because of the premature debonding of fiber-reinforced polymer (FRP) materials that results in a reduction in ductility, the problem of how to exploit moment redistribution (MR) in FRP-strengthened continuous reinforced concrete (RC) structures is unresolved. To date, limited research has been conducted into MR in such structures; a reliable and rigorous solution for quantifying MR throughout the loading cycle remains elusive. This paper aims to quantify MR and predict the capacity at reasonable accuracy, to encourage the use of FRP for the strengthening of existing continuous RC structures. Experiments conducted on 12 continuous T-beams are reported, and the findings are discussed. Strengthening configuration and anchorage scheme are the main variables. A new analytical strategy is described for quantifying MR, and the analytical results are then validated against the experimental results. Both experimental and analytical results confirm that there is no reason to restrict MR into strengthened zones. More importantly, MR, out of FRP-strengthened zones, can indeed occur, provided that the FRP is sufficiently anchored; reliable exploitation of this is now possible.

A half-scale, three-story, two-bay, and three-span reinforced concrete model frame retrofitted with carbon fiber–reinforced polymer (CFRP) strip cables was tested under sudden removal of two side middle columns in the long direction on the ground floor to study dynamic responses, resisting mechanism, and CFRP retrofit effects. The CFRP strip cables were anchored to concrete slabs by specially designed anchorage. The experimental results are compared with previous tests of the same frame without CFRP retrofit. The comparison shows that the CFRP strip cables were effective in preventing the total collapse of the frame after the loss of columns. With additional CFRP strip cables, the frame structure sustained the applied gravity load as moment-resisting frames without a transition to catenary action. To further demonstrate the effects, the CFRP strip cables were released, causing the frame structure to deflect downward and eventually collapse. A simplified method was proposed to calculate the average strain on CFRP strip cables and a combining resisting mechanism was established to compute the ultimate capacity provided by slabs, beams, and CFRP strip cables.

Fiber-reinforced polymer (FRP) sandwich systems as primary load-bearing elements are relatively new concepts in lightweight civil infrastructure. These systems offer a combination of light weight, high strength, thermal insulation for some types, and service-life benefits. Recent developments and applications have demonstrated that these composite systems have emerged as a cost-effective alternative, especially when each material component is appropriately designed. Still, some issues and challenges need to be addressed if FRP systems are to gain widespread use in civil infrastructure. This paper provides an overview of the state-of-the-art research, development, and applications of FRP sandwich systems. It also identifies the challenges and future opportunities for the broad use of these advanced systems in civil engineering and construction.

The main objective of this research was to evaluate the behavior of fiber-reinforced polymer (FRP) bar-reinforced ballastless track slab subjected to a fatigue loading. Five ballastless track slabs were experimentally tested, which were reinforced with steel fiber–reinforced polymer composite bars (SFCBs), basalt fiber–reinforced polymer (BFRP) bars, and traditional steel bars, respectively. All slabs were loaded in three-point loading and designed to have the same initial stiffness of cross section. To simulate the wheel cyclic load, different maximum loads were selected, but the minimum load was the same for all slabs. At the same maximum load of 1.76 static wheel loads, the stiffness degradation and crack width in the SFCB slab were similar to those of the traditional RC slab and survived 3,000,000 fatigue cycles, showing good fatigue performance and a residual strength reduction of only approximately 3%. At the maximum load level of 60% of the ultimate load static capacity, both the SFCB slab and RC slab could satisfy the requirements of 2,000,000 fatigue cycles, whereas the stress in SFCB was only approximately 66% of the steel bar, and the crack width in the SFCB slab was 1.15 times greater than that in the RC slab. After 2,000,000 fatigue cycles, the maximum load level was increased to approximately 65% of the ultimate load static capacity. The stiffness of the RC slab decreased by 21% and was close to failure, whereas the SFCB slab only decreased by 6%. Finally, the BFRP slab had the shortest fatigue life (only 1,600,000 cycles) with a maximum load of only 52% of the ultimate load static capacity owing to the possibly reduced bond strength between concrete and BFRPs.

Fiber-reinforced polymer (FRP) strengthening systems are widely accepted in engineering practice as means to enhance the flexural strength of RC members, but resulting in decreased ductility. Brittle failure, i.e., FRP rupture, FRP delamination, or concrete cover separation, impairs the reliability of FRP strengthening system. A near surface–mounted (NSM) FRP strengthening system has similar drawbacks. As a possible solution, a design philosophy is proposed to enhance the ductility of the NSM-FRP system by triggering a global slip at the FRP–adhesive interface. The corresponding design method is developed to induce the ductile failure and predict the ultimate strength of NSM FRP–strengthened RC member in flexure. A series of experimental tests were conducted to verify the effectiveness of the proposed method. The designed RC beams strengthened with NSM–carbon FRP (CFRP) outperformed the externally bonded CFRP systems both in ductility and flexural strength. Additionally, the proposed design method was experimentally validated by precisely predicting the ultimate strength of tested specimens.

This paper aims to study the nonlinear flexural response of continuous RC slab strips strengthened with near surface–mounted (NSM) composite reinforcement in either the sagging or the hogging region. The study comprised experimental testing and three-dimensional finite-element (FE) modeling of eight two-span RC slab strips, $400\times 125\times \mathrm{3,800}\mathrm{mm}$ each. The amount of internal steel in the strengthened regions was approximately 30% of that of the unstrengthened regions. Flexural strengthening with NSM composites improved the stiffness and load capacity of continuous RC slab strips with no or insignificant reduction in the deflection at ultimate load. The gain in the load capacity due to strengthening was more pronounced for the slab strips strengthened in the sagging regions. Flexural strengthening reduced the moment redistribution ratio of the slab strips. The ratio further decreased by increasing the axial stiffness of the NSM composite reinforcement in the strengthened regions. The developed FE models provided more than reasonable prediction of the ultimate load by simulating the nonlinear structural behavior of the tested slab strips with good accuracy and predicting strain data not captured experimentally in some specimens. The validated FE models are considered a valuable and reliable alternative to the laboratory testing, which is costly, labor-intensive, and time-consuming.

Many studies have been carried out on enhancing the positive moment capacity of the midspan areas of RC beams with fiber-reinforced polymer (FRP) reinforcement under monotonic increasing loads. The present study focuses on retrofitting of beam support regions under reversed cyclic loading conditions representing seismic conditions. The tests were conducted on seven full-scale RC beam-column-slab subassemblages with a part of transverse beams perpendicular to the main beam. The specimens were divided into three series that represented different geometrical characteristics of beams and columns in frames such as wide beam–narrow column, narrow beam–wide column, and beam and column of equal widths. One reference specimen in each series was tested as is, while the remaining four specimens were tested after being retrofitted with near surface–mounted (NSM) carbon fiber–reinforced polymer (CFRP) ropes. Two different retrofitting details in terms of anchorage of CFRP ropes at the support region were applied [NSM and embedded through-section (ETS)] and their effectiveness is investigated. Furthermore, the validity of design code formulations currently available for near surface–mounting method is investigated and a modification is proposed to consider the effects of cyclic loading conditions. The results show that the retrofitted beams achieved an enhancement in the moment capacity but it could not be sustained at large deformations due to weakening of bond between concrete and adhesive under cyclic actions and localization of cracks at the maximum moment zone.

A set of three old suspended timber floors were flexurally strengthened with carbon fiber–reinforced polymer (CFRP) strips in order to investigate the effectiveness of externally bonding FRP to their soffits. The specimens were from an old building and 740-mm-wide bands were transferred to the laboratory in order to be tested in a four-point bending test. One specimen was tested with no strengthening system and the results obtained were used as reference values for comparison with the specimens that were externally bonded and reinforced (EBR) with CFRP strips. Two similar EBR systems were studied: (1) keeping both ends of the CFRP strips free of any restriction (traditional technique), and (2) embedding both ends of the CFRP strips into the timber, thus providing a bonding anchorage of the strips (new technique). The installation of the new strengthening system comprises the opening of holes in the timber and the creation of a transition curve between the holes and the timber surface. This transition curve allows a smooth transition of the CFRP laminate between the hole and the timber surface, thus avoiding stress concentrations in this area. After the opening of the holes, the resin is applied inside the hole and on the beam surface, and then the CFRP laminate is mounted. The load-carrying capacity of the specimens, the rupture modes, and the strains and bond stress distributions within the CFRP-to-timber interface are presented. A nonlinear numerical simulation of the specimens based on the midspan cross-sectional equilibrium is also presented. The results showed that the use of the new strengthening system enhances the performance of the specimens when compared with the traditional strengthening system.

Composite sandwich panels with reinforcement ribs or webs are being increasingly considered for structural applications in civil engineering. Such panels are prone to creep when subjected to significant permanent loads and therefore the effects of this phenomenon must be duly accounted for in their design. Data regarding the viscoelasticity of sandwich panels and their constituent materials is still scarce and often cannot be directly used for creep predictions in design practice. To address this issue, this paper presents the experimental assessment and the analytical modeling of the viscoelastic response of two types of sandwich panels, with and without reinforcement ribs. Panels comprising glass fiber–reinforced polymer (GFRP) faces, cores of polyurethane (PU) foam, and longitudinal GFRP ribs are considered. The ribs increased the flexural strength and stiffness of the panels by a factor of two, while providing a threefold reduction in their creep compliance. A composite creep modeling (CCM) approach is assessed, inputting independently determined viscoelastic properties of the individual materials into Timoshenko beam theory to estimate the creep response of the full-scale panels. The predicted and experimental creep curves presented good agreement for different load levels, attesting the CCM’s adequacy for the analysis of creep in sandwich panel design.

The need for repair and rehabilitation is a crucial issue today in order to maintain a safe and efficient transportation network. One of the most recent innovative composite materials that has been proposed to the market is a fabric reinforced cementitious matrix (FRCM) strengthening system. This system consists of two components: a structural reinforcement mesh and a cementitious matrix. The structural reinforcement mesh consists of a polyparaphenylene benzobisoxazole (or PBO) fiber composite, while the cementitious matrix is a nontoxic grout system based on portland cement with a low dosage of dry polymers. The first aim of this study was to evaluate the fatigue resistance of reinforced concrete (RC) beams strengthened with FRCM under fatigue loading and postfatigue flexural strength to failure. The second aim was to investigate the durability performance of the FRCM after exposure to environmental conditioning including temperature, moisture, and sustained stress. All of the beam specimens examined in this study were subjected to fatigue loading for 2 million cycles before flexure testing. The stiffness reduction of both unstrengthened and strengthened RC beams was evaluated. The effects of environmental exposure were also examined. Results indicated that the FRCM strengthening system enhanced the fatigue and flexure performances of RC beams. Higher stiffness degradation was observed in beam specimens that were exposed to environmental conditioning. The exposed beam specimens exhibited improved performance under the postfatigue flexure test.

This paper presents analytical and experimental studies on the axial load-bending moment behavior of glass fiber–reinforced polymer (GFRP) bars and helices RC columns. The nominal axial load and bending moment of the columns were analyzed based on the stress-strain behavior of the cross-sectional components. A numerical integration method was used to determine the compressive force of concrete in the compression region. The analytical results were verified with experimental results of 12 circular specimens reinforced with GFRP bars and GFRP helices. Out of these 12 specimens, eight specimens were taken from available literature and four specimens were tested in this study. The influences of different parameters such as loading conditions, spacing of the GFRP helices, and wrapping the specimens with carbon fiber–reinforced polymer (CFRP) sheets on the behavior of GFRP-RC specimens were investigated. A parametric study was also carried out to investigate the effects of longitudinal and transverse GFRP reinforcement ratio and slenderness ratio on the axial load-bending moment diagrams of GFRP-RC columns. It was found that the slenderness effect is more pronounced on the confined cross sections under eccentric loads at the ultimate state condition.

Parking garages are among the concrete structures that suffer from corrosion and deterioration due to exposure to deicing salts. The 40-year-old La Chancelière parking garage in Québec (Canada) showed severe corrosion-related deterioration and was in need of costly rehabilitation. As its structural system consisted of two-way flat slabs and the steel reinforcement was severely corroded in most of the slabs, the City of Québec (structure’s owner) decided to replace the structure’s flat slabs (a total area of about $\mathrm{3,160}{\mathrm{m}}^2$) with new ones, while maintaining the main supporting elements (columns and retaining walls). The consulting firm produced two designs with steel reinforcing bars and glass-fiber-reinforced polymer (GFRP) reinforcing bars. Based on the comparative cost analysis of the steel-reinforced and GFRP-reinforced designs, the city opted for GFRP bars. The flat-slab system was designed according to CAN/CSA S806-12 with GFRP bars as main reinforcement, the world’s first application of its type. The slabs were instrumented at critical locations to measure strain with fiber-optic sensors (FOSs) attached to the surface of the GFRP bars or embedded in concrete. This study provides details on the design, construction, and performance of GFRP-reinforced-concrete (GFRP-RC) flat slabs under real service loads and conditions over 3.5 years. In addition, it provides a comparative cost analysis of the steel-RC and GFRP-RC designs. The cost comparison confirms that the initial higher cost of GFRP compared to steel does not necessarily lead to a higher total cost and that a cost-effective design could be achieved.

This paper presents a novel beam-testing method to assess the bond performance of fiber-reinforced polymer (FRP) bars in reinforced-concrete resisting systems subjected to tension–compression reversed cyclic loading. Special attention was paid to ensuring the stability of the proposed test setup, limiting all unwanted forces generated in the system and allowing for the application of tension and compression forces to the FRP bar without premature bar buckling. Preliminary test results are presented specifically for the cyclic bond stress-slip relationship under different loading conditions—tension–tension and tension–compression cyclic loading—in addition to the monotonic-tension loading to assess the suitability of the novel testing method.