Journal of Composites for Construction

In this study, the bond behavior of sand-coated glass fiber-reinforced polymer (GFRP) bars embedded in a novel type of cementitious composite, containing nanosilica, slag, and reinforced with recently developed basalt fiber pellets (BPs), was evaluated. Steel fibers (SFs) were incorporated in control mixes for comparison purposes. Ninety-five cubic pullout specimens were prepared and tested according to Canadian standards. The test parameters were the type of cementitious composite, the bar diameter, the type of fibers, the dosage of BP, and the type of GFRP bar end. The bond performance was elucidated through the compressive strength of the composite and load–slip response of GFRP bars. In addition, the fiber bridging mechanism was determined by evaluating the residual bond strength and toughness indices. The provisions for bond strength in the Canadian and American codes were also assessed. The test results showed the effectiveness of the BP in enhancing the failure pullout load compared to counterpart specimens comprising SF, which makes them a good candidate for infrastructural applications including rehabilitation.

Shear is the predominant failure mode in most reinforced concrete (RC) short columns under seismic loads. The overall structural performance of short columns can be improved by shear strengthening the columns accompanied by confinement. Nevertheless, as the aspect ratio of short columns is lower than that of other columns within the same story, the formation of plastic hinges at low drift ratios can be of high risk during an earthquake. Therefore, in addition to providing adequate shear capacity and confinement for short columns, it may be necessary to strengthen them in flexure to transfer the plastic hinges to the beams. In this study, five RC short columns, one unstrengthened and four strengthened in shear/flexure by fiber-reinforced polymer (FRP), were tested under constant axial load and reversed lateral cyclic loadings. The test parameters included the number of longitudinal carbon FRP (CFRP) layers (0–2), the installation method as externally bonded reinforcement on grooves (EBROG) and externally bonded reinforcement (EBR), and the partial use of longitudinal FRP sheets. The results indicated that shear–flexural retrofitting of short columns using longitudinal CFRP sheets applied through the EBROG method plus an adequate number of confining CFRP wraps improved the load-carrying capacity, ductility, and dissipated energy by up to 83%, 168%, and 1,200%, respectively, compared to those obtained for the unstrengthened specimen. These enhancements were respectively 29%, 26%, and 137% compared to the specimen strengthened in shear using confining CFRP wraps. When the EBR method was used to install the longitudinal FRP sheets, however, no positive effect on the overall behavior of the retrofitted short column was observed.

The application of recycled aggregate concrete (RAC) has so far mainly been limited to nonstructural components because of its inferior short- and long-term performance compared with normal concrete. Encasement of RAC with a fiber-reinforced polymer (FRP) tube has been demonstrated to be an effective technique to improve both the mechanical properties and durability of RAC. A number of studies have been carried out on FRP-confined RAC in circular columns. However, only a limited number of studies have examined the behavior of FRP-confined RAC in rectangular columns, and the existing test specimens were all small-scale. Existing studies have shown that there may exist a significant behavioral difference between small- and large-scale FRP-confined rectangular concrete columns (i.e., column size effect). Against this background, a first-ever experimental study on glass FRP (GFRP)-confined square RAC columns of three different sectional widths ranging from 200 to 400 mm was conducted and the results are presented in this paper. The same FRP confinement stiffness was adopted for the columns of different sizes so that the possible column size effect could be examined. The test results showed that the size effect indeed existed in the test FRP-confined square RAC columns and the presence of recycled aggregate amplified such effect. The size effect should be considered in the design of such columns especially when the coarse aggregate replacement ratio is relatively large. The accuracies of three existing strength models for FRP-confined normal concrete in square columns were also evaluated using the test results.

Confinement reinforcement, such as tubes, stirrups, and wraps, can significantly improve the strength and, more importantly, ductility of eccentrically loaded concrete prisms. However, the confinement effect in flexural analyses is sometimes not considered in design codes because there is no simple approach for quantifying it. Although the finite-element method has been proposed to analyze laterally confined concrete prisms under eccentric loads, the complicated computational program may hinder its wide applications. In this paper, based on fundamental partial-interaction shear-friction and bond–slip material properties, a passive stress–strain model of concrete with the confinement effect is introduced and reduced to a rectangular stress block. A closed-form solution is then derived for the ductility of fiber-reinforced polymer (FRP)-wrapped concrete prisms under eccentric loads. The main advantage of the solution is that the size effect is considered and no structural tests are required. After the validity of the solution is verified with experimental results, a parametric study is conducted to quantify the effects of various key factors on the ductility of FRP-wrapped concrete prisms under eccentric loads.

Confinement with external bonding (EB) of fiber-reinforced polymer (FRP) composites is effective for improving the performance of reinforced concrete (RC) columns under axial compression. However, EB confinement is less effective for sections with a side aspect (width/depth) ratio of more than 2. Thus, a hybrid FRP strengthening technique that combines the advantages of EB and near-surface mounting (NSM) of carbon FRP (CFRP) laminates was explored. Twenty-four rectangular RC columns of different aspect ratios (b/d = 1.0, 2.0, and 3.0) were strengthened and tested under axial compression to investigate the effect of shape on the efficiency of hybrid FRP strengthening. Two specimens each for control, EB confinement, NSM, and hybrid FRP strengthening were tested at each aspect ratio to decouple the interaction effects in hybrid FRP strengthening. A three-dimensional nonlinear finite-element (FE) analysis was conducted using commercially available software. Three-dimensional FE models of different RC columns of varying aspect ratios were created with EB confinement, NSM, and hybrid FRP strengthening and were analyzed. FE models were validated with test results and subsequently used for parametric study. A detailed parametric investigation on the effect of the hybrid combination of a varying NSM ratio (0.4%–1.6%) and a constant EB confinement ratio was studied. Both experimental and FE results indicated that the efficiency of confinement reduces with an increase in the aspect ratio. The hybrid FRP technique was effective in improving both the axial strength and ductility of columns with large aspect ratios.

For interfaces between externally bonded fiber-reinforced polymer (FRP) laminates and concrete, the incremental fatigue analysis for each load cycle using a finite-element (FE) model is exceptionally time-consuming. This study introduces an accurate and efficient numerical approach that accounts for the bond–slip effect under fatigue loading. A preload cycle was adopted to model the damage caused by the previous load cycles. The formulae for the upper limit of the preload and the unloading stiffness in the local bond–slip model were established. The reloading and unloading stiffnesses in the bond–slip model for an arbitrary load cycle were defined. Moreover, different bond–slip models were adopted for a numerical analysis to compare the effect of the bond–slip relations on the fatigue response of a carbon FRP (CFRP)-strengthened concrete structure. The comparisons between the numerical results and the experimental data show that the proposed approach is capable of accurately predicting the fatigue response of CFRP-strengthened structures. The debonding process of the CFRP plate from the concrete surface under fatigue loading can be predicted by the proposed model. Compared with the existing bond–slip models, the proposed model in this study is more effective in predicting the mechanical response of the CFRP-to-concrete interfaces subjected to fatigue loading.

This paper proposes a confining combination of steel and innovative rectangular basalt fiber-reinforced polymer (BFRP) ties at the plastic hinge region on the seismic response of square reinforced concrete (RC) bridge columns. Six concrete specimens were constructed to test the bend parts' strength for comparison to the tested straight parts' strength. Seven square columns were constructed, considering the spacing between ties and axial load level as the parameters for experimental study. A lateral load versus drift ratio with ductility parameters was applied to assess the column performance. BFRP ties test results showed that BFRP ties had overcome the flaws of conventional pultruded FRP rod ties. The column test results showed enhancement in the columns' deformation ductility and deterioration stiffness even with increasing axial load level from 0.03 to 0.2 P_o, indicating that the BFRP ties provided effective confinement, resulting in a delay of rebar buckling and an increased displacement ductility by more than 28%. BFRP ties with ρ_fw = 1.8% provide the same displacement ductility as a BFRP jacket with 0.666 mm thickness.

The aim of this study is to define the cyclic axial behavior of hybrid FRP (fiber reinforced polymer)-confined concrete based on the results of an experimental study presented here. Two different types of fiber sheets with different ultimate tensile strain capacities were used together in a suitable epoxy resin matrix to confine concrete. The inner and outer jackets of the concrete confinement were constituted with carbon (or glass) sheets with a relatively low tensile strain capacity and polyethylene terephthalate (PET) sheets with a high tensile strain capacity. PET fibers, which are a relatively new type of fiber, are made from recycled plastics. By varying the number of layers of the outer fiber sheet, different combinations were formed for the hybrid jackets. To characterize the cyclic axial behavior of hybrid FRP-confined concrete, experimental data were utilized to obtain the axial stress–strain relationship and dilation behavior. Based on the results, a stress–strain model for the envelope curve of the cyclic response of hybrid FRP-confined concrete is proposed.

Concrete-filled glass fiber-reinforced polymer (GFRP) tube (CFFT) columns benefit from significant concrete confinement effects that enhance its structural performance compared to conventional internally reinforced concrete members. CFFTs are currently utilized mostly for exterior applications, as the uncertainty of their performance in fire situations has slowed their adaptation in buildings. To address this uncertainty, a series of ambient temperature and fire tests were conducted on CFFT columns with slenderness ratios (KL/r) ranging from 6.9 to 62.4, using GFRP tubes of 113 mm outer diameter and 3.8 mm wall thickness. The strength increase provided by the GFRP tubes as compared to plain concrete columns was reduced from the initial 214% increase at a KL/r of 6.9 to 104% at a KL/r of 62.4. The fire tests revealed that unprotected CFFT columns with KL/r = 57.4 were able to sustain the applied design service loads (44% of ultimate capacity) for up to 108 min in a standard fire. However, it was determined that the unprotected GFRP tubes lost their structural integrity after approximately 10 min of fire exposure, when the temperature on the inside of the tube reached 350°C, thereby only providing thermal protection to the concrete core after the 10-min mark. The unique outcomes of this new study indicate that CFFT columns can be a viable design option in fire situations.

Sandwich insulation wall panels (SIWPs) are used extensively as thermal resistance components in buildings. Connectors in SIWPs are subjected to axial and shear forces to resist the load of the outer layer and transfer it to the inner layer and structure. In some areas, the insulation thickness needs to increase to achieve a satisfactory heat preservation effect, which leads to an increase in connector length. Most existing connectors do not satisfy the mechanical requirements for a thick insulation layer, e.g., 300 mm. In this work, a series of pull-out and shear tests were conducted for a novel long glass fiber-reinforced polymer I-shaped connector. The ply design and anchorage design of the connectors are key factors that affect the connector’s mechanical properties. Through comparative tests, the influence of these two factors on the axial and shear stiffness, capacity, and failure mode were explored. The addition of ±45° and 0°/90° multiaxial fiber cloth had little effect on axial performance, but the vertical shear stiffness and horizontal shear capacity improved. The pull-out and vertical shear capacity can be improved by optimizing the anchorage design and adding a row of anchorage rebar in the flange.

When reinforced concrete (RC) slabs are supported directly on columns, a combination of shear forces and unbalanced bending moments is transferred between the slab and the columns at slab–column connections. The unbalanced moment transferred between a slab and a column can be significantly increased by the application of horizontal loads, such as wind and seismic loads. According to current design codes and standards for steel–RC structures in North America, a portion of the unbalanced moment is assumed to be transferred by the eccentricity of shear, while the remaining portion is assumed to be transferred by flexure. Previous research shows that the slab width that contributes to transferring the flexure portion is bounded by lines located at a distance of 1.5 times the slab thickness on each side of the column. This study investigates whether the same slab width can be used to transfer the unbalanced moments in slab–column connections reinforced with glass fiber–reinforced polymer (GFRP) reinforcement. A similar approach to that used for steel–RC connections is followed to analyze data from 27 GFRP-RC connections without shear reinforcement reported in the literature. Furthermore, a previously validated finite-element model is used to investigate the strain distribution in the longitudinal slab reinforcement along the transverse direction of the slab. The results show that a smaller slab width bounded by lines located at a distance of 0.5 times the slab thickness on each side of the column is recommended for GFRP-RC slabs.