Journal of Composites for Construction

Reinforced concrete bridge wall piers constructed according to older codes experience damage during earthquakes, which can lead to bridge collapse. Deficiencies include inadequate design and detailing of longitudinal and transverse steel reinforcement and deficient lap splices. Three wall piers were constructed using as-built reinforcement details conforming to pre-1973 requirements to evaluate flexural strengthening strategies. The first wall pier was used as the control specimen. The second wall pier was retrofitted using carbon fiber-reinforced polymer (CFRP) jackets and transverse CFRP anchors placed through the section for the confinement of the lap-spliced region and vertical CFRP anchors to improve flexural capacity. The third wall pier was retrofitted similarly to the second pier by using vertical near-surface mounted (NSM) CFRP rods. The piers were tested under cyclic lateral load until failure. Both retrofitted wall piers were able to achieve significantly higher flexural capacity, stiffness, and hysteretic energy dissipation when compared with the control wall pier specimen. The as-built wall specimen was not able to develop the theoretical flexural capacity due to lap splice clamping failure; both retrofitted specimens exceeded the theoretical flexural capacity of the as-built wall specimen.

Basalt fiber–reinforced polymer (BFRP) has been the most recent fiber–reinforced polymer (FRP) composite material introduced as reinforcement and prestressing tendons for concrete. Measuring the mechanical properties of BFRP rod/tendon is challenging using the existing mechanical testing methods due to their relatively lower transverse direction strength. Hence, gripping of the BFRP rods using the conventional wedge-type anchors becomes unsuitable because it causes stress concentration and exerts localized high transverse pressure on the specimen, which results in premature failure at the grips, instead of the desired rupture of the gauge section. This study is focused on the development of a new anchor for the prestressing of the BFRP rods based on using expansive cement as a grout material. The goal is to ensure a uniform distribution of pressure around the BFRP rods to avoid their failure within the wedge during prestressing. Pull-out tests were conducted on several samples with different anchor dimensions to study the interaction between the expansive cement and the BFRP rod. The parameters affecting the gripping behavior were investigated, and the design of the anchor was optimized based on finite-element simulations using commercially available software.

To investigate the size and slenderness effect on the axial-compressive behavior of basalt fiber–reinforced polymer (BFRP)-confined predamaged concrete, five groups of concrete cylinders with different sizes and slenderness ratios were designed and tested. The cylinders were axially preloaded to three predamage levels, then repaired using BFRP, and reloaded. The results showed that the concrete predamage had an adverse effect on the ultimate strength and initial elastic modulus of BFRP-confined concrete. Except for the smallest specimens affected by the wall effect, the initial analysis found that the ultimate strength of BFRP-confined concrete decreased with an increase in size and slenderness ratio, and the size and slenderness effect decreased with an increase in BFRP confining pressure, while these increased with the severity of concrete predamage. However, there was no obvious size or slenderness effect on the ultimate strain of BFRP-confined concrete. Through multifactorial analysis, it was confirmed that the ultimate strength of BFRP-confined undamaged and predamaged concrete was influenced by the slenderness. Considering the effect of size, slenderness, and predamage, monotonic and cyclic models were developed for BFRP-confined concrete. Finally, a uniaxial material object was added into OpenSees to provide an effective numerical material model for theoretical analyses and engineering applications.

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been studied extensively under almost all loading conditions, except torsion. In this pioneering study, six 168-mm-diameter samples were tested in pure torsion, consisting of two hollow glass-FRP tubes and four CFFTs. Two tubes with distinctly different laminate structures were tested and compared, namely, near-cross-ply and angle-ply tubes. A specially designed Z-shape setup with unique features intended to avoid second-order effects and facilitate gripping of the circular section was developed and used to apply pure torsion. The results showed that the postcracking response was heavily dependent on the FRP tube laminate structure. The tube confined the concrete core diagonal struts, as evident by a state of combined stresses in the tube, which ultimately fractured. The concrete core prevented buckling of the tubes and resulted in a 250% increase in ultimate torques over the hollow tubes. It also increased twist angles by 14% and 45% for CFFTs with near-cross-ply and angle-ply tubes, respectively. The angle-ply CFFT achieved a 118% increase in ultimate torque but was 48% lower in ultimate twist, compared to the near-cross-ply CFFT, with the latter showing a significant nonlinear response. A simplified analytical model based on the superposition of the truss action and membrane shear action of the tube was developed and verified. It was then used in a parametric study to examine the effect of the laminate structure and tube thickness on torsional strength, ranging from the lower bound based on the [0°/90°]_s cross-ply tube to the upper bound based on the [±45°]_s angle-ply tube.

This paper assesses the seismic response of large circular concrete columns internally confined with glass fiber–reinforced polymer (GFRP) spirals. The experimental program included testing of eight 508-mm-diameter columns reinforced with GFRP spirals and steel longitudinal bars under simulated earthquake loading. Shear–deflection, moment–curvature responses, and various ductility parameters were used to evaluate the performance of the columns. The results showed that the GFRP spirals provided effective confinement to the concrete core until the rupture of spirals at least two locations, corresponding to a strain several times larger than the steel yield strain. GFRP spirals were found to provide increasing confining pressure to the large column core with increased deformations, even more efficiently than comparably smaller 356-mm-diameter columns from a prior study. All the columns satisfied the ductility requirements of the North American codes.

In this study, a mechanical anchorage for prestressed carbon fiber–reinforced polymer (CFRP) rods was proposed. The proposed anchorage consisted of a steel barrel with a conical hole and three separate aluminum wedges that are in direct contact with the CFRP rod. The anchorage system relied only upon friction, without any adhesives required. The static and fatigue behaviors of the anchor were experimentally investigated according to the European guidelines for testing post-tensioning kits and fulfilled their requirements. The average tensile strength of the anchorage system for the 8-mm CFRP rods was 2,371.4 MPa, being greater than the guaranteed value of 2,047 MPa. The fatigue tests revealed that the wedge seating distance (prior to pulling the rod) significantly affected the occurrence of slippage between the anchor components. Therefore, a new displacement-controlled presetting system was developed to ensure adequate wedge seating distance. It eliminated the need for hydraulic jacks and demonstrated the capability of applying presetting forces greater than 110 kN. Furthermore, the anchors were tested under loading frequencies of 5, 17, and 23 Hz. The proposed anchorage was observed to be insensitive to the loading frequency because no slippage or temperature rise occurred under these loading frequencies.

The American Concrete Institute recently updated their one-way shear provisions for steel–reinforced concrete (RC), and it is time to consider updating provisions for one-way shear in fiber-reinforced polymer (FRP)-reinforced concrete (FRP–RC). The shear strength of FRP–RC is lower than steel–RC mainly because FRP, particularly glass–FRP, has a much lower modulus of elasticity than steel. This study evaluates the performance of current design provisions at predicting the one-way shear strength of RC members without stirrups that are internally reinforced with longitudinal FRP bars, based on 147 tests available in the literature. Further, shear parameter effects (concrete strength, size effect, span to effective depth ratio, modulus of elasticity, and longitudinal reinforcement ratio) were also investigated. One-way ANOVA analysis was used to classify each provision, and results show that the codes, standards, and guidelines are identified as more consistent groups compared with the proposed models. The method proposed in the CSA S806-12 standard is the most consistent of the studied approaches at predicting the shear strength of the tested members with a mean predicted-to-experimental strength closest to one and the minimum average absolute error.

In this study, in-plane shear tests were performed on 20 masonry wall specimens. The research factors considered included the length, height, thickness, fiber material, and number of textile-reinforced concrete (TRC) layers of the specimens. The failure modes, shear strength, pseudoductility, and energy dissipation of masonry walls under the influence of different factors studied were discussed and analyzed. The results showed that the unstrengthened masonry walls exhibited typical brittle failure characteristics and low shear strength at failure, which could be significantly improved by TRC strengthening. Under the test conditions of this study, the greater the length and height of the unstrengthened specimens with the same thickness, the lower the shear strength, and the higher the percentage increase in the shear strength after strengthening. For unstrengthened specimens with the same length and height, the greater the thickness, the greater the shear strength, and the lower the percentage increase in the shear strength after strengthening. After applying the same TRC layers on double-leaf walls, the shear strength and pseudoductility increased by 107.9% and 9.4 times for carbon-TRC strengthened specimens, and the two parameters of specimens strengthened by basalt-TRC increased by 50.3% and 7.0 times, respectively. In addition, the strength and ductility of the specimens increased as the number of TRC layers increased. Finally, the calculated and design values of shear capacity of each specimen were calculated according to current design standards and compared with the obtained test results. The conservativeness of the related design methods was also discussed.

This paper presents an experimental investigation on the application of textile-reinforced mortar (TRM) layers as a means of increasing the flexural capacity of two-way reinforced concrete (RC) slabs with cut openings. The investigated parameters included the number of TRM layers (one versus two layers), the textile type (two carbon fiber textiles with different weight and geometry), the strengthening configuration (full coverage versus partial coverage via the use of diagonal strips), and the role of the matrix material (mortar versus epoxy resin). For this purpose, six large-scale two-way RC slabs with a central opening were constructed and tested to failure under monotonic loading distributed at four points. It was mainly concluded that TRM substantially enhanced the flexural capacity of the two-way RC slabs with a central cut opening and restored the original capacity of the slab without the opening. The effect of the various investigated parameters is discussed in terms of flexural capacity, post-yielding stiffness, and load resistance at the serviceability limit state. Finally, an already available simple design tool was refined based on the new results of this study. It was found to provide a good estimation of the flexural moment of resistance of the TRM-retrofitted slabs.

Although prestressing carbon fiber–reinforced polymer (CFRP) strands outperform steel strands on different levels, such as strength and durability, their performance under elevated temperatures remains a susceptible design issue that requires careful evaluation. Moderate increase in the temperature of prestressing CFRP strands takes place during construction due to concrete curing. CFRP strands can also experience increase in temperature if the CFRP-prestressed structural element is subjected to fire during service. This paper addresses the effect of increasing the temperature on the strength of prestressing CFRP strands as well as the level of prestressing force. Two sets of CFRP strand specimens with two different diameters were prepared and evaluated for strength degradation triggered by the increase in temperature to 350°C (662°F). Two more sets of prestressed CFRP strands were evaluated for prestress loss due to increase in temperature to 204°C (400°F). The prestress loss due to temperature increase was verified by constructing and monitoring half-scale decked bulb T-beams prestressed with CFRP strands. Test results showed that tensile strength of CFRP specimens decreased with the increase in temperature. In addition, first heating cycle of prestressed CFRP strands led to a slight permanent strand relaxation and a corresponding prestress loss. Subsequent cycles of heating and cooling did not seem to generate additional relaxation of the strands as long as the temperature of the first cycle was not exceeded. Furthermore, CFRP specimens subjected to heating and cooling cycles showed no reduction in the strength when tested at ambient conditions afterwards.

A novel technique to improve the effectiveness of fiber-reinforced polymer (FRP) confinement for strengthening rectangular reinforced concrete (RC) columns is to implement section curvilinearization (SC) before FRP wrapping. In this SC technique, the four flat sides of a rectangular column are transformed into slightly curved sides, which increases the sectional size only slightly but can significantly enhance the FRP confinement effectiveness. However, existing research on curvilinearized square/rectangular columns (CSCs or CRCs) (particularly on CRCs) is very limited and has been conducted using small-scale specimens. To this end, the first-ever experimental program of axial compression tests on large-scale FRP-confined CRCs was recently carried out, and the results are presented in this paper. The experimental program covered the rise-to-span ratio of the curved sides, the corner radius ratio, and the sectional aspect ratio as the key test variables. The experimental results show that the slope of the linear second segment of the stress–strain curve of FRP-confined concrete in a CRC is much larger than that of the corresponding column without SC, demonstrating the effectiveness of the SC technique. On the basis of the present test results, an evaluation of the accuracy of the only stress–strain model for FRP-confined concrete in CRCs available at the time of conducting the present study is reported.

Externally bonded carbon-fiber-reinforced polymer (CFRP) composites are increasingly being used for repair of concrete infrastructure. Both the mechanical performance and the long-term durability of such a repair have resulted in a significant body of research. However, polyurethane-based composites for concrete repair have seen only limited treatment to date. This paper investigates the effect of seven different environments on the durability and failure modes of two different wet lay-up CFRP systems applied to flexural reinforcement of concrete: a two-part epoxy and a preimpregnated, water catalyzed polyurethane with aromatic chemistry as a matrix. Durability of concrete, CFRP laminates, and small-scale CFRP-strengthened concrete flexural beams was investigated for each duration (125, 250, and 365 days) and accelerated conditioning environment. Inverse analysis with a numerical model was used to develop conditioned bond–slip models for each composite system. Results and failure modes of control and conditioned specimens showed that degradation of CFRP-strengthened beams was controlled by the conditioned concrete tensile strength and bond cohesive energy in the epoxy and polyurethane systems, respectively.