Journal of Composites for Construction

This paper aims to review and critically assess experimental research efforts on the seismic retrofit of existing reinforced concrete (RC) beam-column joints with fiber-reinforced polymer (FRP) sheets of the past 20 years. A review of the literature revealed several promising features of FRP strengthening schemes. FRP retrofits can be used to address a number of different deficiencies in nonseismically designed RC members framing into beam-column joints. A majority of studies concentrate on joint shear strengthening, and strengthening in the principal stress axis is found to be most effective. Other strategies include counteracting the weak-column/strong-beam in nonseismically designed specimens by means of column flexural strengthening, as well as plastic hinge relocation within the beams, away from the joint. Only a limited number of studies evaluate combining several of these retrofit objectives into a more complete retrofit of the joint subassemblage. In most studies, simple FRP wrapping is observed to be used for anchorage, which is not always effective. Instead, anchorage by means of FRP anchors or mechanical anchors is shown to be required to achieve adequate strengthening in most cases. Next to the detailed discussion of the literature, a database of all tested specimens is compiled and analyzed. Shear strengthening design equations from major design guidelines are assessed based on the experimental results collected in this database, highlighting the need for their further improvement. Moreover, an analysis of the database reveals a lack of tested specimens with realistic test set-ups, including scaled specimens, testing without axial load, and lack of slab and transverse beams. These parameters are found to heavily affect retrofit effectiveness and may lead to nonconservative results. Moreover, on average, the effectiveness of repairing predamaged specimens is found to be similar to that of retrofitting specimens without damage.

Serviceability requirements can often control the design of glass fiber–reinforced polymer (GFRP)-reinforced concrete flexural members due to the lower stiffness of GFRP than of steel. ACI Committee 440 crack control provisions for GFRP-reinforced concrete mimic those from ACI 318 for steel-reinforced concrete, but with the addition of a bond-dependent coefficient, $k_b$, to account for differences in bond between GFRP and concrete from steel and concrete. The recommended value for the bond-dependent coefficient in ACI 440.1R-15 is 1.4. This value is largely based on one experimental study, with one type of GFRP bar. Since that study, many more experiments have been performed to examine crack widths in GFRP-reinforced concrete. A database of these experiments that satisfy a rigorous set of criteria to ensure valid experimental results was created from the literature and used to recalibrate the bond-dependent coefficient. A value of 1.2 is recommended for all bars that include a sand coating in the surface enhancement. A value of 1.4 is recommended for all other bars.

This study introduces the use of the combined impressed current cathodic protection–structural strengthening (ICCP-SS) as a technique for repairing reinforced concrete structures that have been subjected to chloride-induced corrosion. The development of this technique is based on the combination of a carbon-fiber mesh (CF-MESH) and a polymer-modified cementitious matrix to produce a carbon-fiber-reinforced cementitious matrix (C-FRCM). First, the effects of different types and amounts of high molecular weight polymer and short chopped carbon fibers on the flexural and compressive strength, conductivity, and shear strength of a matrix with concrete substrate were investigated in order to find the optimum ingredients required for a high-performance cementitious matrix. Second, a shaped CF-MESH was bonded onto the surface of a concrete cube using the aforementioned optimized cementitious matrix. The impressed current cathodic protection (ICCP) technique was then applied to the specimens by using different current densities with the CF-MESH as the anode. During the protection period, the protection conditions of rebars subjected to ICCP were assessed by analyzing a variety of electrochemical parameters. Third, single shear tests were conducted, and the shear strengths and failure modes of the specimens were obtained and compared in order to evaluate the effects of the ICCP on shear stress transfer. The results show that the proposed technique based on the newly proposed C-FRCM composite is able to provide effective cathodic protection as well as shear stress transfer behavior to RC structures subjected to chloride-induced corrosion, leading to an improvement with respect to structural durability.

As the first study reported in the literature on the axial behavior of noncircular high-performance fiber-reinforced cementitious composite (HPFRCC) members externally jacketed by fiber-reinforced polymer (FRP) sheets, the aim of this paper is to experimentally identify the axial behavior characteristics of these members under compression. The test results of 21 specimens showed that the axial and lateral deformation characteristics of FRP-jacketed HPFRCC members are significantly different from those of FRP-confined conventional concrete members. Small voids in HPFRCCs due to the presence of fine aggregates and reduced crack widths due to the contribution of steel fibers limit the transverse deformations remarkably until the axial strain value is approximately 0.0035. The test results show that the dilation ratio increases after the axial strain reaches this value. Then, the external FRP jacket begins to noticeably affect the behavior, resulting in a significant improvement in the axial strength and deformation capacity. However, since the external FRP jacket is activated after relatively large axial strains, significant strength degradations can occur before the FRP jacket provides an effective contribution. As the stiffness of the FRP jacket increases, the strength degradation experienced before the activation of the FRP jacket decreases. Furthermore, the predictions of five available models that were developed for modeling the axial behavior of FRP-confined concrete are examined through comparisons with test results.

Flexural strengthening of steel beams using carbon fiber reinforced polymer (CFRP) laminates has gained popularity within the structural engineering community due to the many advantages it offers over traditional strengthening methods. Such advantages include ease of construction, resistance to corrosion, and minimal disruption to traffic. The interfacial shear stress transfer mechanism of the CFRP-to-steel bonded interface is critical to the performance of flexurally strengthened steel beams using externally bonded CFRP laminates. Many existing studies have investigated the behavior of CFRP-to-steel bonded joints under quasi-static monotonic loading; however, few studies have been carried out on understanding the behavior of such bonded joints under cyclic loading. This paper presents an experimental investigation aimed at understanding the behavior of CFRP-to-steel bonded joints under quasi-static cyclic loading. The experimental study presented consists of nine single shear pull-off tests of FRP-to-steel bonded joints, covering three different bond thicknesses and both quasi-static monotonic and quasi-static cyclic loading. The results are discussed in terms of their failure mode, load-displacement behavior, CFRP plate axial strain distribution, interfacial shear stress distribution, and bond-slip behavior.

This paper examines the results of material testing of hybrid carbon and glass fiber-reinforced polymer (FRP) plates for use in prestressed mechanically fastened applications. The small-scale material tests were conducted in three phases: (1) uniaxial tension without holes, (2) uniaxial tension with open holes, and (3) uniaxial tension with single- and multibolt connections. In all three phases of testing, Digital Image Correlation (DIC) was used to obtain continuous strain data, showing holistic strain field development through failure. A total of 89 tests were conducted across 17 treatment groups to develop an anchor pattern and fastener spacing which is optimized with respect to maximum capacity and behavior under anticipated service load conditions. The tests presented comprise the initial phase of a larger project that aims to employ prestressed mechanically fastened FRP (MF-FRP) plates as a retrofit repair solution for deteriorated prestressed concrete bridge superstructures. Results conclude that 2 lines of 11 bolts with diameters of 12.7 mm, with longitudinal spacing of 100 mm and transverse spacing of 38 mm, provide an end-region anchor pattern with an ultimate capacity of 206 kN which is 89% of the ultimate capacity for the examined FRP plate with holes.

This research work presents the interaction behavior of square reinforced concrete (RC) columns strengthened with different fiber-reinforced polymer (FRP) techniques, namely, near-surface mounting (NSM), external bonding (EB), and hybrid strengthening (HYB). Hybrid FRP strengthening uses a combination of near-surface mounted FRP laminates, confined laterally using externally bonded FRP composites, to achieve significant improvement in the peak strength and ductility under all combinations of axial compression (P) and bending (M) loads. Different levels of eccentricity ($e$) normalized to the depth ($h$) were considered, such as (1) axial compression ($e/h=0$), (2) low-eccentricity compression ($e/h=0.15$), (3) high-eccentricity compression ($e/h=0.63$), and (4) pure bending ($e/h=\infty$). A normalized axial compression–bending interaction diagram for the tested columns was developed based on the test results. Experimental results reveal that hybrid FRP strengthening of columns improves the peak strength considerably under all loading combinations of axial compression and bending. In hybrid specimens, bond failure of NSM-strengthened RC elements was prevented through the effective lateral confinement provided by EB carbon fiber–reinforced polymer (CFRP) fabric. The axial compression–bending interaction diagram was analytically derived using a strain compatibility procedure. Analytical predictions exhibited good correlation with the test results for all strengthening techniques.

This experimental study has shown the effectiveness of fiber-reinforced polymer (FRP) external bonding (EB) in enhancing the flexural capacity of reinforced masonry (RM) walls subjected to out-of-plane cyclic load. Twelve reinforced masonry walls were built using fully grouted concrete masonry units. The walls had three different steel reinforcement amounts, 2#3, 2#4, and 1#5, representing typical underreinforced wall sections. The strengthened walls used two FRP types, wet lay-up glass fiber sheet (GFRP) and prefabricated carbon fiber-reinforced polymer laminate (CFRP). Four RM walls without strengthening were used as reference specimens. Six walls were externally strengthened using one and two sheets of GFRP. The remaining walls were strengthened with one and two CFRP laminate. A simple model was developed to predict the FRP debonding strain. Nonlinear analysis can be conducted using the moment-curvature relation. As a result of this study, the proposed model presents an excellent prediction compared to the experimental results. Different modes of failure, including compressive concrete crushing failure, FRP rupture, shear failure, and FRP debonding from the masonry substrate, occurred in the strengthened reinforced walls.

Old concrete bridge barriers suffer from corrosion-induced degradation due to the effects of freeze–thaw cycles and the application of winter deicing salt. To replace such deteriorated bridge barriers while aiming to reduce maintenance costs and improve their life expectancy, rust-free high-modulus (HM) glass fiber–reinforced polymer (GFRP) bars with 180° hooks are utilized as primary reinforcement in the barrier wall. A proposed TL-5 bridge barrier developed in this research incorporated 15M and 13M GFRP bars as vertical (transverse) reinforcement in the barrier front and back faces at a 300-mm spacing, respectively, and 15M GFRP bars as horizontal (longitudinal) reinforcement in the barrier wall. The connection between the deck slab and the barrier wall utilized GFRP bars with 180° hooks for proper anchorage. To study the structural behavior, the crack pattern and the ultimate load–carrying capacity of the proposed barrier wall under an equivalent transverse vehicle collision force, a 40-m-long GFRP-reinforced TL-5 barrier was constructed and statically tested to failure under transverse line load at interior and end locations. While trapezoidal flexural cracks were developed at the traffic side of the barrier wall during loading, punching shear crack occurred at the loading location, leading to barrier failure. The experimental ultimate load–carrying capacities of the barriers were observed to be far greater than the load capacity required by current design codes. GFRP bar strain and concrete strain readings at the barrier deck showed very low strains at failure compared to the material’s nominal failure strain. To study the pullout capacity of the GFRP bars with 180° hooks reinforcing the lower tapered portion of the barrier wall and embedment into the deck slab, a representative concrete slab with various embedded GFRP bars was constructed. Pullout tests on GFRP bars with straight ends, hooked ends, and hooked ends with unbonded straight portions of the bar were performed. The results showed that the average pullout capacities of the tested bars are far greater than the pullout capacity of 100 kN specified in current standards for TL-5 HM-GFRP concrete barriers. The GFRP bar’s experimental pullout capacity can be used to determine the resisting moment at the barrier deck connection for compliance with the code-specified factored applied moment at this location.

The durability of adhesively bonded fiber-reinforced polymers (FRP) and concrete substrates has been the subject of recent studies. The degradation of bonded interfaces conjugated with other factors that affect the interface strength may compromise the potentialities of using FRP in externally bonded reinforced (EBR) concrete structures. However, the estimation of the effects of degradation on these bonded interfaces and the analytical methodologies to quantify them are not fully understood. The present work focuses on a local bond-slip model characterized by two parameters for which the values are obtained experimentally. Then, the determination of the local bond-slip relationship of a glass (G) FRP-to-concrete interface can be estimated. The assessment of the degradation of the bonded interface when subjected to cycles of (1) salt fog; (2) wet-dry environments with salt water; (3) temperatures between $-10°\mathrm{C}$ and $+30°\mathrm{C}$; and (4) temperatures between $+7.5°\mathrm{C}$ and $+47.5°\mathrm{C}$ is presented. The results obtained using the proposed bond-slip model led to the conclusion that after 10,000 h of exposure to temperature cycles between $-10°\mathrm{C}$ and $+30°\mathrm{C}$, there was a small change in the GFRP-to-concrete interface performance, whereas the effects on the bonded interface for the specimens subjected to temperature cycles between $+7.5°\mathrm{C}$ and $+47.5°\mathrm{C}$ were far more most severe.

This study investigated the bond strength between concrete and fiber-reinforced polymer (FRP) wet lay-up systems using resins with biobased content of various proportions as a partial replacement of epoxy, which is well known to be an unsustainable product derived from petroleum. Three different resin types were explored together with either glass or carbon fiber fabrics, namely a resin blend partially derived from vegetable and wood industry by-products (VW) with a total biocontent of 41% by weight, a cashew nut shell liquid (CN) resin blend, and an epoxidized linseed oil (ELO) resin blend. The mix ratios with epoxy were varied to obtain 20%–40% biocontent for the CN resin and 10%–40% for the ELO resin. A total of 57 small-beam bond test specimens were fabricated and tested to establish and compare bond strength in accordance with the recommendation of ACI 440.9R. It was found that the VW resin, with the highest biocontent of 41%, had similar bond strength to epoxy, whereas CN resin, at 20%–30% biocontent, resulted in a comparable bond strength to epoxy. On the other hand, a significant drop in bond strength occurred at 40% biocontent in both ELO and CN resin types. Bond strength was not only dependent on resin, but also on fiber type. In the case of glass FRP, bond strengths were generally lower those that of carbon FRPs by 12%–29% for the same resin type.

In this study, a single crack–based model is proposed for predicting the contribution of fiber-reinforced polymer (FRP) reinforcement to the shear resistance of FRP shear-strengthened reinforced concrete beams. This critical single shear crack has been found to be dominant within the shear span of many RC beams strengthened with externally bonded (EB) FRP and RC beams with near-surface mounted (NSM) FRP at large spacings. In this study, the single shear crack was assumed to occur along the principal stress trajectory (PST), along which concrete is subject to principal tensile stresses. A bilinear bond-slip model was adopted for the cohesive stresses due to FRP reinforcement and steel stirrups, and an exponential model was chosen for the cohesive stress due to aggregates. The shear crack configuration (e.g., crack width and length) was an implicit function of the external loading and cohesive stresses, which was obtained by numerical iterations. To verify the precision of the proposed model, several viable experimental studies on FRP shear-strengthened RC beams using either EB or NSM techniques were selected for comparisons. The model predicted shear crack trajectory, FRP strain distribution, stirrup strain distribution, and FRP contribution to the overall shear resistance well.

This paper presents a novel design concept for fiber reinforced polymer (FRP) composites consisting of three-dimensional (3D) printed cores and FRP helical skins as a means of ensuring adequate ductility relative to the brittle FRP systems conventionally used for internal reinforcement. The experiment demonstrated that when the FRP skins were loaded in tension, the core—which was 3D printed using acrylonitrile butadiene styrene or polylactic acid—was gradually compressed, thereby leading to plastic deformation. This behavior ensured a nonlinear load response while eliminating the unfavorable brittle failure of the FRPs. The results also indicated that the proposed FRP composite system ensured that no premature debonding/delamination occurred between the skin–skin and skin–core. The results of the parametric experimental study indicated that design parameters, such as the FRP amount, core height, core span, core shell thickness, core material, core brace, and core number (i.e., the number of inner cores used for the composite), may be optimized to realize the expected design load capacity and ductility.