Journal of Composites for Construction

This paper deals with the punching shear resistance of flat slabs reinforced with glass fiber-reinforced polymer (GFRP) reinforcing bars. A total of six large-scale interior GFRP and steel reinforced slab-column connections with the varying longitudinal tensile reinforcement ratio were tested. A new fracture-mechanics-based empirical formula for estimation of the punching shear resistance of interior GFRP reinforced slab-column connections is also presented in the paper. The formula takes into account effects, span to the effective slab-depth ratio, depth of the compression zone, dowel action, and the size effect on the punching shear resistance of flat slabs reinforced with GFRP bars. The accuracy of the proposed formula was verified against test results obtained by the authors and also by other researchers. In comparison with already known formulas, the proposed formula is in good agreement with the test results and provides predictions of the punching shear resistance with the smallest scatter. A physically reasonable relation between experimental punching resistances and reinforcement ratios of both the GFRP slabs and the steel reinforced concrete (SRC) slabs was obtained when equivalent GFRP reinforcement areas were used for SRC slabs. Predicted punching resistances of SRC slabs, calculated based on the equivalent GFRP reinforcement area, were found to be approximately equal to the resistances of GFRP slabs when the behavior (load-displacement diagram) of SRC slabs resembled the one of GFRP reinforced slabs.

The anchorage of fiber-reinforced polymer (FRP) composites when applied to reinforced concrete (RC) structures as externally bonded reinforcement is an effective means to achieve higher levels of fiber utilization prior to premature debonding failure. Commonly documented anchorage methods for FRP-to-concrete applications demonstrating encouraging results include FRP U-jackets, FRP anchors (also known as spike anchors, among other names), patch anchors (utilizing unidirectional and bidirectional fabrics), nailed metal plates (also known as hybrid bonding), near-surface mounted rods, mechanical fastening, concrete embedment, and mechanical substrate strengthening. Anchorages applied to FRP systems have been verified through experimental testing and numerical modeling to increase the ductility, deformability, and strength of the member and also prevent, delay, or shift the critical mode of FRP debonding failure. Although the benefits of anchorage solutions have now been widely acknowledged by researchers, further studies are required in order to establish reliable design formulations to negate the requirement for ongoing laboratory verification by industry. The present paper is a state-of-the-art review of experimental studies conducted in the area of FRP anchorage systems applied to FRP-strengthened RC flexural members. Available experimental data are compiled and catalogued and an anchorage efficiency factor for each anchorage type under investigation is assigned in order to quantify the anchor’s efficiency. Finally, current shortcomings in knowledge are identified, in addition to areas needing further investigation.

Despite extensive research on externally bonded fiber-reinforced polymer (EB-FRP) for rehabilitation of concrete structures in the past two decades, the industry has been slow to embrace the technology owing to some difficult problems that are yet to be resolved fully. Among these, premature debonding is one of the critical problems. Since the tensile strength of concrete is inherently low and unreliable (in the long term), EB-FRP by surface adhesion might not be the final solution; augmentation of the interfacial bond by mechanical fastening may be unavoidable. To this end, the first author proposed a bond augmentation system, hybrid-bonded FRP (HB-FRP), where a special type of mechanical fasteners are used to enhance the interfacial bond. Although the HB-FRP system has been experimentally shown to increase bond strength several fold, effectively overcoming the debonding problem, there has been no systematic theoretical study of the system. In this paper, HB-FRP joints are characterized through experimental testing and analytical modeling, leading to a theory for design and construction of the HB-FRP system.

RC beams shear strengthened with externally bonded fiber-reinforced polymer (FRP) U strips or side strips usually fail owing to debonding of the bonded FRP shear reinforcement. Because such debonding usually occurs in a brittle manner at relatively small shear crack widths, some of the internal steel stirrups intersected by the critical shear crack may not have reached yielding at beam shear failure. Consequently, the yield stress of internal steel stirrups in such a strengthened RC beam cannot be fully utilized. This adverse shear interaction between the internal steel shear reinforcement and the external FRP shear reinforcement may significantly reduce the benefit of the shear-strengthening FRP but has not been considered explicitly by any of the shear strength models in the existing design guidelines. This paper presents a new shear strength model considering this adverse shear interaction through the introduction of a shear interaction factor. A comprehensive evaluation of the proposed model, as well as three other shear strength models, is conducted using a large test database. It is shown that the proposed shear strength model performs the best among the models compared, and the performance of the other shear strength models can be significantly improved by including the proposed shear interaction factor. Finally, a design recommendation is presented.

Fiber reinforced polymer (FRP)-wrapped circular concrete columns under axial compression usually fail as a result of FRP rupture in the hoop direction. The average measured hoop strain in the FRP jacket at failure of such columns, i.e., the apparent FRP rupture strain, is typically considerably lower than the ultimate tensile strain obtained from tensile tests of flat coupons. A large scatter also exists in the measured rupture strains over the surface of an FRP-wrapped column at ultimate failure. No rational model based on sound theories is currently available to explicitly consider the reduction of the apparent FRP rupture strain in design. This paper discusses the possible failure modes of FRP-wrapped concrete columns and the factors that may play a role in the initiation of failure. Further research needs and the possible consequences for the optimized design of FRP-wrapped concrete columns are discussed.

This paper investigates the behavior of two 8-story cantilevered RC shear walls rehabilitated using carbon fiber-reinforced polymer (CFRP) composite sheets when subjected to base excitations from a shake table. The two original reduced size walls (1:0.429) were tested on the shake table of École Polytechnique de Montréal to investigate the effect of higher modes of vibration on the behavior of multistory RC walls. The walls were subjected to several levels of ground motion excitation that matches the design spectrum of Montréal city in Québec, Canada. The original walls showed significant inelastic deformations at the 6th-story level in addition to those at the base plastic hinge. After the shake table tests on each of the two original walls, the damaged walls were rehabilitated and resubjected to the same levels of the ground motion excitations. This paper focuses on the FRP-rehabilitation of the original walls. The rehabilitation scheme for the two walls aimed to increase the flexural and shear capacities of the wall at the 6th-story panel because of the observed increase in demand at that level, whereas the base panel was confined using CFRP sheets to increase the ductility capacity without increasing strength. The rehabilitated walls showed satisfactory performance with improved flexural strength at the 6th-story panel. The rehabilitation scheme resulted in a reduced wall rotation and lower strain values of the flexural steel rebars at the 6th-story panel. The shear demands and bending moments on the FRP-rehabilitated walls were higher than those of the original ones.

This paper presents the results from an investigation into the reliability of reinforced concrete bridge girders strengthened in shear using fiber-reinforced polymers (FRP). Two expressions for the shear design of FRP-strengthened girders were developed as part of a National Cooperative Highway Research Program project. The expressions were developed for two distinct cases, namely, bonded and anchored FRP reinforcement. Uncertainties inherent in the new design models were first assessed using an extensive database of hundreds of experimentally tested specimens. Variabilities in material, fabrication tolerances, dead and live loads, and distribution factors obtained from the literature were then included in a limit state function for the shear strength mode of failure. The reliability of a representative design space comprising 18 bridge girders that cover different span lengths, shear deficiency levels, and girder location (exterior versus interior) was calculated for both bonded and anchored strengthening techniques. The use of approximate expressions for reliability index calculations was deemed unacceptable because of the high scatter in the results. This paper discusses the difference between two reliability methods [Monte Carlo simulations and first-order reliability method (FORM)] that were first used to ensure the validity of FORM in analyzing the developed limit state function. The results showed that the new design expressions result in reliability index values in the range targeted by most design codes (3.00–3.50), albeit lower than the target value used in the calibration of AASHTO load and resistance factor design (LRFD) ($\beta =3.50$) in some cases.

This paper presents an experimental investigation on the effectiveness of various types of spike anchors in combination with U-shaped fiber-reinforced polymer (FRP) jackets for shear strengthening of reinforced concrete T-beams. The parameters examined include the orientation, the number and spacing of anchors, and the role of carbon versus glass fibers in the anchors. It is concluded that anchors placed inside the slab are many times more effective than those placed horizontally inside the web, and anchors of similar geometrical characteristics (e.g., embedment length) display similar effectiveness despite the difference in fiber type.

Recent research has been conducted to evaluate the benefits provided by externally-applied discrete fiber-reinforced polyurea (DFRP) coating systems. This coating was proposed to allow for ease of construction in repair-retrofit situations and to provide multihazard benefits, ranging from blast or impact fragmentation mitigation to seismic reinforcement and general strengthening. This phase of research investigated the flexural and shear reinforcement capabilities of the systems. Testing parameters included type of structural failure, polyurea, and fiber volume fraction. Additionally, the effects of the thickness of the composite system were considered. Concrete beam specimens were fabricated and coated per the designed test matrix and subjected to four-point bending. Analysis was based on ultimate capacity, deflection, overall ductility, and the qualitative observations of coating adhesion and fragmentation confinement. In addition, a theoretical model was developed and validated to describe the flexural behavior of the polyurea-coated beams and to normalize the test data for comparison. Results presented in this paper suggest measurable strengthening for both flexural and shear capacity provided by the coating system and substantial gains in ductility.

The rehabilitation and strengthening technique using near-surface mounted (NSM) fiber-reinforced polymer (FRP) has proved to be a reliable alternative to the existing methods for deficient concrete structures. To effectively apply this method, bond characteristics at the interface need to be fully understood. This paper presents the results of the bond performance of a wide range of embedded FRP NSM reinforcements with various surface configurations, cross-sectional shapes, material types, adhesive types, and groove sizes. A total of 109 concrete specimens (with a dimension of $350\times 300\times 150\mathrm{mm}$) were tested under direct pull-out loading configuration. It was found that both specimen sets using NSM bars treated with spirally wound and sand coating with indentation and groove displayed the best performance. Strip shape was more effective than round and square cross-sectional shapes. Higher bond strength was obtained by using carbon FRP than glass and adhesives with higher material bond strength. Increasing the groove sizes was fairly effective in improving the bond capacity. The existing bond strength models were also assessed in comparison with the test data in this paper.

A space truss is formed of interconnected structural components that transfer loads axially. Pultruded glass-fiber reinforced polymer (GFRP) composites exhibit their best material strength in the axial (pultrusion) direction, and their inherent lack of material stiffness can be compensated by the structural stiffness achieved by space configuration. A novel connector made of pultruded GFRP profiles is proposed in this paper to join diagonal and chord members and to form all-composite free-form space trusses. The proposed truss unit was assembled and tested statically, and the load-displacement responses were recorded. The final collapse of the structural unit evidenced a pull-out shear failure mode in the bolt connections, and the ultimate load can therefore be well predicted by this mechanism. A detailed finite element (FE) analysis considering the interaction between the bolts and FRP profiles was conducted for the structural unit to investigate the effects of pretension force of bolted connections and the stress concentration at the joint region. A simplified FE approach was used to demonstrate one- and two-dimensional space trusses with satisfactory structural stiffnesses.

A concept for an ultra-lightweight buckling-restrained brace was conceived, and a prototype was designed that utilized an aluminum core and bundled glass fiber-reinforced polymer pultruded tubes for the buckling restraint. Prediction of global stability in compression was made using analytical methods based on single-degree-of-freedom (SDOF) and previously established Euler buckling models. Detailed finite-element simulations of the proposed prototypes utilized a constitutive model calibrated from experimentally obtained reversed cyclic coupon testing of 6061-T6511 aluminum alloy at 2–4% total strain amplitude. Analytical formulations were compared with monotonic and cyclic numerical results from a parametric study varying restrainer stiffness, end moments induced by frame drift, and core reduced section length. The study concluded that SDOF and Euler formulations may underestimate the required restrainer stiffness by a factor of two or greater. The resulting ultra-lightweight brace prototypes satisfying global buckling restraint were calculated to weigh 27 and 41% of traditional mortar-filled tube and all-steel buckling-restrained brace (BRB) configurations, respectively.

This paper presents results of an experimental study on the behavior of square and rectangular high-strength concrete (HSC)-filled fiber-reinforced polymer (FRP) tubes (HSCFFT) under concentric compression. The effects of the tube thickness, sectional aspect ratio, and corner radius on the axial compressive behavior of concrete-filled FRP tubes (CFFT) were investigated experimentally through the tests of 24 CFFTs that were manufactured using unidirectional carbon fiber sheets and high-strength concrete with 78 MPa average compressive strength. As the first experimental investigation on the axial compressive behavior of square and rectangular HSCFFTs, the results of the study reported in this paper allow a number of significant conclusions to be drawn. First and foremost, test results indicate that sufficiently confined square and rectangular HSCFFTs can exhibit highly ductile behavior. The results also indicate that confinement effectiveness of FRP tubes increases with an increase in corner radius and decreases with an increase in sectional aspect ratio. It is also observed and discussed that HSCFFTs having tubes of low confinement effectiveness may experience a significant strength loss at the point of transition on their stress-strain curves. Furthermore, it is found that the behavior of HSCFFTs at this region differ from that of normal-strength CFFTs and that it is more sensitive to the effectiveness of a confining tube. Examination of the test results have also lead to a number of important observations on the influence of the key confinement parameters on the development and distribution of the hoop strains on the tubes of CFFTs, which are presented and discussed in the paper.