Journal of Composites for Construction

Rubberized concrete possesses viscous damping that is higher than that of conventional concrete, making it a promising candidate for construction in high seismic regions. Confining rubberized concrete with glass fiber–reinforced polymers (GFRP) may result in superior performance. This paper investigates the behavior of rubberized-concrete-filled fiber-reinforced polymer tubes (RCFFT) under different strain rates. The rubberized concrete had 0, 10, and 20% volume replacement of fine aggregate with shredded rubber. Each test specimen’s behavior was compared to that of conventional concrete-filled fiber-reinforced polymer tubes (CFFT). The GFRP tubes were fabricated using wet-layup with different numbers of GFRP layers. Three different strain rates (representing static, earthquake, and severe earthquakes) were used to test these specimens under cyclic axial loading. The RCFFT behaved in a manner that was similar to conventional CFFTs. The rubberized concrete’s confinement limited the reduction in the confined compressive strength more than it did in the unconfined compressive strength. The confinement of both conventional and rubberized concrete resulted in an increase in both the compressive strength and the ductility. The increase in the strain rate by two and three orders of magnitude resulted in increases in the compressive strength and ductility of both the CFFT and RCFFT. High increases in the modulus of elasticity of both CFFT and RCFFT were observed with the increase of strain rates. The strain rate changed the behavior of the concrete that was confined with a lower confinement ratio. This effect was reduced when the confinement ratio was increased. The plastic dilation was also investigated. The addition of rubber resulted in a smooth transition in the dilation of concrete.

Hybrid fiber-reinforced polymer (FRP)–concrete–steel double-skin tubular members (DSTMs) are a new form of hybrid members that consist of an outer tube made of FRP and an inner tube made of steel, with the space between them filled with concrete. The existing studies on hybrid DSTMs have been mainly focused on their use as compression members, with only a very limited number of studies on their use as flexural members [i.e., hybrid double-skin tubular beams (DSTBs)]. This paper presents the first ever experimental study on large-scale hybrid DSTBs with headed shear studs; the effect of an integrated deck is also examined. The main parameter examined in the experimental program was the section configuration. The test results show that both the DSTBs and the DSTB-deck unit possessed a very ductile response, and that the headed shear studs effectively reduced or eliminated slips between the steel tube and the concrete. This paper also presents a theoretical model based on conventional section analysis. The predictions from the theoretical model are in reasonably close agreement with the test results.

The adhesive bonding of fiber-reinforced polymers (FRP) to tensile faces of reinforced concrete elements is now widely considered as a simple and effective means for enhancing load-carrying capacity and decreasing deflection. There is, however, uncertainty surrounding the long-term durability of the FRP-to-concrete bonded system. Hence, a comprehensive database of available shear bond tests is collated, categorized, and analyzed to quantitatively assess the impact of environmental exposure on debonding failure modes, bond properties, and residual strength. Trends reveal that the durability of joints is generally not attributable to the type of fiber, strengthening scheme, surface preparation method, or differences in conditioning regimes, but rather the strong influence is by the durability of the adhesive layer. Significantly, deterioration of the adhesive layer alters the mechanical behavior of the joint, often to an extent where tests can fail to capture the full strain development along the bonded length, and therefore not fulfill the intended purpose of simulating crack-induced debonding failures in flexural elements. These findings allow for the development of regression models that predict the degradation of bond strength in FRP-to-concrete joints under a number of environmental conditions.

A load-strain model for a steel-concrete-FRP-concrete (SCFC) hybrid column section in compression is proposed. The section layout has a square steel tube as the outer layer and a circular fiber-reinforced polymer (FRP) tube as the inner layer, and concrete is filled between these two layers and inside the FRP tube. Thus the section can be regarded as a concrete-filled steel tube (CFST) with a FRP-confined concrete core (FCCC), in which the FCCC is essentially a concrete-filled FRP tube (CFFT) in sectional configuration. However, the mechanical behavior of a SCFC is superior to the simple superposition of CFST and CFFT without consideration of the interaction mechanisms among the different materials. The load-strain behavior of a SCFC differs from that of a CFST or CFFT in that it includes an initial parabola portion, a second linear portion, and a postpeak portion. The model is established by superposing four load-strain models of the constituent layers and attempting to reveal the mechanical responses of the SCFC sections under axial compression. In the modeling, several mechanical characteristics, namely yielding point, peak strain, peak load, and postpeak residual bearing portion, are investigated and the effects of three parameters, FRP thickness, steel thickness, and concrete strength, are examined. Comparisons between the modeling results and experimental results show good agreement in terms of yielding strain, yielding load, peak strain, and peak load. Furthermore, a set of predictions for peak load covering a greater range of parameters than the experiments is developed according to this load-strain model.

In this paper, a fiber beam model previously developed by the authors for the nonlinear analysis of strengthened elements, including the effects of shear, is used to predict the response of reinforced concrete (RC) beams strengthened in shear with fiber reinforced polymers (FRP) sheets. In the previous version of the model, debonding failure of FRP was not included; hence, its application was limited to the simulation of wrapped configurations. The model is now extended to account for debonding failure in order to allow for its application to beams strengthened with U-shaped and side-bonded configurations. Existing experimental tests on RC beams strengthened in shear by FRP sheets in both wrapped and U-shaped configurations were numerically simulated. The model reproduces, with reasonable accuracy, the experimental failure loads, the load-deflection behavior, and the strains in FRP and stirrups with increasing load. The advantages of this proposal are related with the simplicity and straightforwardness of the beam models to be applied in practical engineering problems.

This experimental study was aimed to investigate the in-plane shear performance of externally strengthened masonry walls using two types of fiber-reinforced polymer (FRP) sheets, namely, carbon FRP (CFRP) and polyethylene terephthalate-FRP (PET-FRP) sheets. Among these two, PET-FRP has a low tensile strength but possess a higher fracturing strain than CFRP. Twelve masonry walls made from clay brick were tested for static lateral loading under constant compression, after bonding CFRP and PET-FRP sheets onto their surfaces in three different configurations. The ultimate shear strength and deformation at peak load were the two important observations. The mechanisms by which load was carried were observed, varying from the initial uncracked state to the final, fully cracked state. The results demonstrate that a significant increase in the in-plane shear capacity of masonry can be achieved by bonding these two FRPs to the surface of the walls but ductility is compromised when CFRP is used. Walls retrofitting with PET-FRP in a crossdiagonal fashion show a good ductile behavior in both prepeak and postpeak regimes. The experimental data were used to assess the effectiveness of the strengthening of one FRP over the other. PET-FRP in diagonal configuration was found to be the most effective way of strengthening without compromising the two most essential aspects of masonry, that is, strength and ductility.

This paper presents the results of an experimental study on the axial and flexural behavior of square concrete members reinforced with glass fiber-reinforced polymer (GFRP) bars and embedded with pultruded GFRP structural sections under different loading conditions. The main parameters investigated in this study were the influence of the type of internal reinforcement (steel bars, GFRP bars and pultruded GFRP structural I-sections and C-sections) and magnitude of load eccentricity on the flexural and compressive behavior of square concrete members. To fulfil the objectives of this study, 16 reinforced concrete specimens were tested, of which 12 were tested as columns under compression loading and 4 were tested as beams under flexural loading. The concrete specimens were square in cross section with a side dimension of 210 mm and a height of 800 mm. The experimental results have shown that the steel-reinforced specimens have a higher load-carrying capacity than specimens reinforced with GFRP bars for all loading conditions. In addition, for concentrically loaded specimens, steel-reinforced specimens have a better ductile performance than specimens reinforced with GFRP bars. In terms of eccentric loading, specimens reinforced with GFRP bars experienced similar ductility as compared to the corresponding steel-reinforced specimens. However, the eventual failure mode of specimens reinforced with GFRP bars was sudden and brittle in nature. However, specimens encased with GFRP structural sections have a higher load-carrying capacity but considerably lower ductility than the steel-reinforced and GFRP bar–reinforced specimens.

Current design manuals for fiber-reinforced polymer (FRP) structures only allow limited application of FRP in high-temperature environments. However, the residual mechanical properties of FRP composites at high service temperatures should be considered. This paper presents the results of bending tests on FRP coupon specimens at temperatures from 25 to 120°C and of compression tests on FRP components at temperatures from $-40$ to 90°C. Although the mechanical properties decrease with increasing temperature, they retain residual strength and stiffness at high service temperatures. The influence of subzero temperatures on the mechanical properties is negligible. Additionally, the flexural properties of the coupon specimens and the compressive properties of the components were experimentally investigated after high-normal temperature cycles, which alternated between 45 and 135°C every 12 h. The results show that repeated high-normal temperature cycles have little effect on the mechanical properties. The simultaneous effects of loading and high-temperature environments on FRP structures should be considered during design. Thus, this paper proposes a design method for calculating the loading capacity of FRP members at different temperatures. Experimental data from literature and this study were normalized and compared with the results predicted by this method. As expected, the proposed method provides a lower envelope of experimental data for both strength and modulus. Thus, the design method can conservatively estimate various mechanical properties of FRP structural members under different loading conditions at high service temperatures. Additionally, the method can be conveniently established and applied in design.

Carbon fiber-reinforced-polymer (CFRP) plates have significant properties for applications in structural engineering, automotive, aviation, space, and shipbuilding. CFRP plates have the advantage over conventional materials owing to their resistance to corrosion, low weight, and high tensile strength. The coefficient of friction between CFRP plate and copper plate is an important parameter required for designing anchors for CFRP plates. The friction behavior of CFRP plates in contact with copper plates of different hardness (as-received and annealed) under high contact pressure (50–175 MPa) was investigated. The shear stress versus contact pressure relationship was linear for the as-received copper plates with a constant coefficient of friction of 0.31. However, for the annealed copper plates, the coefficient of friction decreased with contact pressure from 0.39 to 0.30. For all tests, the sliding distance before rupture of CFRP plate decreased with the increase of contact pressure. Ploughing, wear, debris accumulation, and stick-slip behavior were observed and illustrated in this article.

The information on long-term durability of the carbon fiber–reinforced polymer (CFRP)-concrete bond interfaces in various environmental conditions is necessary to predict the service life of the structures. The assessment of the bond interfaces under moisture conditions were evaluated by shear and tension bond tests using six popular commercial CFRP and epoxy resin systems for the maximum immersion period of 18 months. The bond tests were also accompanied by the test in the mechanical properties of the resins and concrete. Two of systems showed 25 and 16% reductions in average shear bond strengths, while the remaining systems showed either improvement or a small reduction. Observation of the failure modes suggested that the durability against water-related deterioration is worst when the adhesion bonds between the concrete and resin interface are weaker than the cohesive bonds of the adjacent layers. Similarly, the average tensile bond strength reduction was found to vary from 19 to 41%, indicating that the durability of the bond is highly dependent on the CFRP composite systems.

Bridge deck construction using stay-in-place (SIP) fiber-reinforced polymer (FRP) structural forms has shown promise as an efficient, rapid, and low-waste alternative to conventional methods. Several studies have shown that the load capacity of this system is typically governed by punching shear strength, although no design-oriented equations have yet been developed to predict failure load or deflection at service. In this paper, two simple design equations are proposed for the ultimate load and stiffness of concrete deck slabs with SIP FRP forms, derived from the results of a comprehensive parametric study using a rigorous finite-difference computer model that is readily available. The equations were then validated by using experimental results from a database of 52 tests reported in the literature including FRP SIP forms of several shapes, sizes, surface treatments, spliced connections, boundary conditions, environmental exposures, and loading protocols. The equations showed an average predicted-to-experimental nominal strength of 87% with a standard deviation of 19%. When applying the design code member resistance factor of 0.75 consistent with shear failure of FRP-reinforced decks, a conservative strength was predicted for the entire database. Stiffness, on the other hand, was less accurately predicted; however, the model provided reliable and accurate indication of whether deflection limit is satisfied or not.

Bond behavior is an important subject in the design and performance of reinforced concrete structures. In this research, the bond property between sand-coated glass fiber–reinforced polymer (GFRP) bars, a corrosion-resistant substitute to steel bars, and fly ash–based geopolymer cement (GPC) concrete, a more environmental friendly alternative to ordinary portland cement (OPC) concrete, is investigated. Pullout test specimens containing GFRP bars embedded in GPC and OPC concrete cylinders with 100-mm diameter and 170-mm height were prepared. Three different embedment lengths were tested: three, six, and nine times the bar diameter. Average concrete compressive strengths of approximately 25 and 45 MPa and GFRP bar diameters of 12.7 and 15.9 mm were the other test parameters. For each specimen, the test results include the bond failure mode, the average bond strength, the slip at the loaded and free end, and the bond-slip relationship curves. The test results showed that GFRP-reinforced GPC concrete has similar bond strength as that of GFRP-reinforced OPC concrete. The increase in embedment length resulted in the decrease of the bond strength as well as a change in the failure mode of the specimens. Furthermore, the experimental results were used to generate a constitutive bond-slip law. Finally, finite-element modeling is performed by using the constitutive bond-slip law to investigate strain and bond distribution along the embedment length of the bar.

Carbon fiber–reinforced polymer (CFRP) sheets/plates are widely used to strengthen deficient RC structures. Existing studies show that the effectiveness of externally bonded CFRP materials generally depends on the bond between the CFRP element and concrete. Most of the research studies developed so far have focused on the bond behavior of the CFRP sheet-concrete interface under static loading. In this work, the bond behavior was experimentally investigated from the dynamic standpoint, through the drop-mass impact test method, with the aim of highlighting the effect of the loading rate on the bond strength. The test results show that the strain distribution gradient of the CFRP sheets under impact loading was larger than under static loading, and that the loading rate significantly influences the bond strength, while only moderately affecting the effective bond length. A practical bond-slip model is proposed to simulate the CFRP-to-concrete interface bond behavior under dynamic conditions, which considers the strain-rate effect based on the recommendations for the strength of concrete under impact loading. Furthermore, starting from the equations given in some existing guidelines, a design proposal is developed to accurately predict the effective bond length and the bond strength of the CFRP-to-concrete interface under impact loading.

This paper presents the axial compression behavior of standard $Ø150\times 300\mathrm{mm}$ concrete cylinders after exposure to elevated temperatures of 300, 500, and 700°C for heating periods of 2 h (for all temperatures) and 3 h (only for 700°C). A total of 192 specimens were tested in static compression to investigate the stress-strain relationships and failure modes of fire-damaged concrete before and after strengthening with carbon fiber–reinforced polymer (CFRP) wraps. The test results showed that the exposure temperature and duration, unconfined concrete strength, and cooling method (air or water cooling) influence the postfire compressive strength, strain at compressive strength, and modulus of elasticity of concrete. Low-strength concrete is more susceptible to the loss in residual properties caused by fire than high-strength concrete. The CFRP wrapping can significantly enhance both strength and ductility of concrete after exposure to elevated temperatures. The level of strength enhancement by CFRP confinement for fire-damaged concrete is higher than undamaged concrete. The confinement effectiveness increases with an exposure temperature, especially for the lowest-strength (20-MPa) water-cooled concrete. In contrast, the level of ductility enhancement on fire-damaged concrete is lower than undamaged concrete. The application of an equation to predict the compressive strength of CFRP-confined fire-damaged concrete is also discussed. It was found that the equation conservatively predicts the ultimate strength of CFRP-confined fire-damaged concrete. However, the prediction becomes less accurate as exposure temperature increases.

Due to the complexity and difficulty involved, the interfacial bond-slip relationship of externally bonded (EB) fiber-reinforced polymer (FRP)-to-concrete joints has been investigated, mainly through experimental testing. This work investigates the interfacial behavior using the finite-element method (FEM). Damage plasticity theory is employed to model concrete material and the pull-off test of EB-FRP-to-concrete joints is simulated by FEM to study the interfacial behavior. Debonding process, failure mechanism, local bond strength distribution, and characteristics of the local bond-slip relationship are examined. It is found that the local bond-slip relationship has a shape different from all existing bond-slip models. However, such change in shape does not make a significant difference to the global interfacial load-slip response.

The ductility performance of reinforced concrete (RC) members strengthened with carbon-fiber-reinforced polymer (CFRP) reinforcement using near-surface-mounted (NSM) technique has a tendency to decrease with the increase of the prestress level applied to the CFRPs. Hence, in this study, first to assure a sufficient degree of ductility for these prestressed members, a methodology was proposed to determine the maximum prestress level that can be applied to the NSM CFRP reinforcement. Using this methodology, a simplified analytical formulation was developed to determine this maximum allowable prestress level. In the second part, after demonstrating the good predictive performance of a developed three-dimensional finite-element model, first the effect of partial unbonding of prestressed NSM CFRP reinforcement at midspan to increase the ductility performance of strengthened slabs was numerically assessed. Then, a new hybrid-partially bonded system, combining the fully bonded non-prestressed and partially bonded prestressed CFRP reinforcements in the same application according to the NSM technique, was proposed for the flexural strengthening of RC slabs. This hybrid-partially bonded system was capable of providing a better balance in terms of load-carrying and ultimate deflection capacity of prestressed strengthened slabs. At the end, a comparison between the potentialities of fully bonded, partially bonded, and hybrid-partially bonded systems for the flexural strengthening of RC structures is made, and the relevant results are presented and discussed.

Segmental bridges are widely used for accelerated construction and lower costs and environmental impact. The major concern with such bridges is in the corrosion protection of posttensioning steel strands. Noncorrosive carbon fiber strands can serve as viable alternatives to help reduce maintenance costs. A novel carbon fiber strand was tested for posttensioning of a $3.5:1$ scaled model of the Long Key segmental box girder bridge, and its performance was compared with prior test data for steel strands and another type of carbon fiber. The most important distinction between the two types of carbon fibers is their elastic moduli, which are 77 and 93% of that of steel strands. Numerical simulation was also conducted and the effect of different elastic moduli of the tendons on the structural performance was investigated with a parametric study. While the study confirms the feasibility of both types of carbon fiber strands for segmental bridge applications and their similar serviceability behavior, strands with higher elastic modulus are shown to improve structural performance and minimize displacements after joint opening.

The current study is devised to evaluate the seismic performance of fiber-reinforced polymer (FRP) retrofitted buildings with openings at different FRP reinforcement levels. Required objectives are achieved by performing five shake table tests on $1/4$-scale models of single-story boxlike masonry buildings. Out of five building models, one was an unreinforced masonry (URM) building model whereas the other four were retrofitted with different quantities and layouts of FRP reinforcement. FRP reinforcement is reduced by decreasing FRP strip widths and strip spacing and applying FRP strips on either the inside or outer faces of walls. Each building model was subjected to the same series of input ground motions with gradually increasing amplitudes. A series of diagonal compression tests was also conducted to decide the appropriate type of FRP and trial FRP reinforcement ratios for the masonry building models. Descriptions of the tested models are provided, including the structural response in terms of base shear, displacement, and damage patterns. The results of diagonal compression tests and building model shake table tests show that FRP can significantly enhance the seismic resistance of new and existing URM buildings.

To study the stress-strain behavior of axially loaded fiber-reinforced polymer (FRP)–confined concrete columns in the event of a localized failure, 32 circular concrete cylinders were tested with heavy instrumentations. The test specimens had a diameter of 150 mm and a length varying from 300 to 700 mm. Another test variable was confinement, of which 6 different levels were used. To identify the localized failure zone and its effect, a large number of electrical strain gauges and digital image correlation (DIC) techniques were used to capture strain variation along the full height of the cylinders. Measured strain fields and local load-deformation curves were used to identify the length of the zone and the compression fracture energy required for stress-strain modeling of columns with failure localization. The test results show that both the length of the failure zone and the compression fracture energy increase when confinement increases. The experimental results from this work provide adequate data and information for development of a stress-strain model of FRP-confined concrete columns involving failure localization.

In this study, a total of 16 reinforced concrete beams (dimensions of $203\times 305\times \mathrm{3,861}\mathrm{mm}$) strengthened with near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) rods and strips were tested under four-point bending fatigue loading. Monotonic loading was applied to their corresponding control specimens to obtain the static monotonic flexural strength. Fatigue loading with a range of 10% of the corresponding static strength to various upper limits was applied to determine the empirical relationship for each type of specimen. It was found that the dominating failure mode under fatigue was rebar rupture. Mixed failure modes of rebar rupture and unstable cracks in concrete were also observed in specimens with longer fatigue life. As the specimens underwent more fatigue cycles, an increase was observed in their tensile strains of CFRP and deflections due to accumulated damages and the losses of stiffness. A fatigue bond model was developed, which simulated the process of fatigue loading as fatigue crack growth at the NSM-epoxy interface. Based on the comparison with experimental data, the model predicts well the bond behavior and progression of the debonded length for most of the fatigue life.

The behavior of fiber-reinforced polymer (FRP) tube reinforced concrete (FTRC) columns under different loading conditions was investigated in this study. Four groups of 16 specimens were cast and tested. Specimens in the first group (the reference group) were reinforced with longitudinal steel bars and steel helices (Group REF). Specimens in the second group were reinforced with intact glass FRP tubes (Group IT). Specimens in the third group were also reinforced with intact glass FRP tubes. In addition, polymer grid was embedded into the concrete cover to reduce the cover spalling (Group ITG). Specimens in the fourth group were reinforced with perforated glass FRP tubes (Group PT). One specimen from each group was tested under concentric loading, one under 25-mm eccentric loading, one under 50-mm eccentric loading, and one under four-point loading. Results from the experimental study show that FRP tubes significantly increase the load-carrying capacity and ductility of FTRC specimens. Group ITG specimens performed better than the other groups of specimens. Experimental and analytical interaction ($P-M$) diagrams also show the enhanced performance of FTRC specimens.

The aim of the present research is the experimental investigation of the response of carbon-fiber-reinforced polymer (CFRP) strengthened reinforced concrete (RC) beams of large scale under fatigue loading. Reinforced concrete beams were cast for this purpose, both with rectangular and T cross-sections, with web dimensions of $200\times 500\mathrm{mm}$ and 3,050 mm in length. Beams were strengthened in flexure with externally bonded CFRP laminate [externally bonded reinforcement (EBR)] or with CFRP laminates bonded into slits on the concrete cover surface [near-surface-mounted (NSM)]. Transverse CFRP sheets were placed on the shear spans of the beams to avoid fragile shear failure. All specimens were tested under fatigue loading, with load-unload cycles of 2 Hz frequency. Two different amplitudes of cycles were investigated. The different fatigue life and modes of failures are discussed. Beams subjected to the low loading range, simulating the service loads of a bridge beam, could sustain more than 2 million cycles. Beams subjected to the high loading range, according to the maximum limit of industry standards, presented fatigue failure of the reinforcing steel in cycles less than $1.5\times {10}^6$. The ratio of the axial rigidity of the tensile steel bar $k_s$ to the axial rigidity of fiber-reinforced polymer (FRP) strengthening $k_f$ plays a significant role in extending the fatigue life of steel and thus of RC FRP strengthened beams. The lower the $k_s/k_f$ ratio, the higher the fatigue life of the retrofitted member. NSM strengthening may maintain a full bond with the concrete substrate even after the fatigue fracture of steel bars. Therefore, it may provide residual capacity to the retrofitted member to survive detrimental collapse.