Journal of Composites for Construction

This article presents an experimental study investigating the long-term shear performance of basalt fiber–reinforced polymer (BFRP) bars wrapped in seawater–sea sand concrete (SSC) immersed in different solution environments. Horizontal shear tests and transverse shear tests of immersed bare and SSC-wrapped BFRP bars were conducted. The various tested parameters included the following: (1) SSC wrapping thickness (0, 10, and 20 mm); (2) solution environment (alkaline solution, seawater, and tap water); (3) immersion time (15, 30, 90, 180, 360, and 540 days); and (4) conditioning temperature (∼26°C, 40°C, and 60°C). In addition, the test findings of the durability of BFRP bars wrapped with SSC were compared with those wrapped with conventional concrete. The results showed that alkalinity is the main factor controlling the performance degradation of BFRP, and the degradation of BFRP is strongly correlated with the concrete wrapping size (the greater the wrapping thickness is, the greater the degradation is). SSC leads to a higher pH of the immersion solutions and combines the reaction of the Na⁺ and K⁺ contained in the sea sand and silica of the basalt fibers, resulting in the shear strength retention of BFRP bars wrapped with SSC being lower than that of bars wrapped with conventional concrete under the same conditions.

Ten large-scale high-strength concrete (HSC) circular columns were constructed and tested to failure. Nine columns were internally reinforced with glass fiber-reinforced polymer (GFRP) bars and spirals, whereas one was reinforced with steel bars and spirals to serve as a reference. All columns had a diameter of 350 mm. The variables tested were reinforcement type, spiral pitch, slenderness ratio, eccentricity-to-diameter ratio (e/D), and type of loading (axial or four-point bending). Experimental results showed that both reinforcement types (steel or GFRP) and the spiral pitch did not have a significant effect on the behavior of GFRP-reinforced HSC columns up to the peak load. In addition, a decrease in the axial capacity of the columns as the e/D ratio increased was observed. This was consistent for specimens of both slenderness ratios of 14 and 20. Columns with a higher slenderness ratio showed a lower axial capacity for all specimens tested under the same e/D ratio. Furthermore, slender columns with higher e/D ratio underwent much larger deformations; both axially and laterally. For columns of both slenderness ratios, axial load–bending moment interaction diagrams were produced using the experimental results and were compared to the predictions of available codes and guidelines.

This study first presents an experimental investigation on the compressive behavior of carbon fiber–reinforced polymer (CFRP)-confined square seawater–sea sand concrete (SSC) columns with embedded epoxy-coated reinforcement (ECR). Thirty-three columns were tested to explore the influence of several parameters, including clear spacing ratios, FRP wrapping schemes, FRP volumetric ratios, and internal steel reinforcement ratios. Test results reveal that the location of CFRP strips with respect to the steel hoops has a significant impact on the mechanical behavior of specimens, especially for the deformation capacity. Furthermore, more obvious improvements of strength and ductility are detected for the reinforced columns with smaller clear spacing ratios and higher volumetric ratios of FRP wraps. In specific, the maximum enhancements of load capacity and ductility index for the reinforced SSC specimens with two-layer CFRP strips are 11.8% and 92.9%, respectively, while the improvements increase to 15.6% and 189.2% when the thickness of CFRP jackets doubles. A regression-based stress–strain model is developed to evaluate the ultimate conditions of FRP partially wrapped concrete columns, and a better agreement can be achieved by the developed model compared with other existing models. Finally, a simple expression is also proposed based on the experimental observations, which can accurately predict the load capacity of plain and reinforced concrete columns wrapped with FRP strips.

Steel reinforcing bars, helices, and fibers in concrete columns are susceptible to corrosion, particularly in aggressive environments. This study experimentally investigated the use of corrosion-free glass fiber-reinforced polymer (GFRP) bars and GFRP helices in hollow-core circular glass fiber concrete (GFC) columns. The influence of the addition of the glass fibers, loading conditions, and pitch of the helices was investigated. The experimental program consisted of 12 circular specimens with an outer diameter of 214 mm and an inner circular hole diameter of 56 mm. Nine specimens (850 mm high) were tested under concentric and eccentric axial loading and three specimens (1,500 mm long) were tested under four-point bending. The experimental results showed that, for a similar amount of reinforcement, the GFRP bar-reinforced hollow-core glass fiber concrete (GFRP-HC-GFC) specimens achieved 12%–18% lower maximum axial load than the GFRP bar-reinforced hollow-core nonfibrous concrete (GFRP-HC-NFC) specimens under concentric and eccentric axial loadings, as evident in the P–M interaction diagrams of GFRP-HC-NFC and GFRP-HC-GFC. However, GFRP-HC-GFC specimens achieved 5%–20% higher ductility than the GFRP-HC-NFC specimens under different loading conditions. The experimental results also showed that the GFRP-HC-GFC specimens experienced a lower number of cracks and smaller crack widths than the GFRP-HC-NFC specimens under eccentric axial loading and four-point bending. In addition, the GFRP-HC-GFC specimens with 30-mm pitch of the GFRP helices achieved 2%–34% higher maximum load and 7%–55% higher ductility than the GFRP-HC-GFC specimens with 60-mm pitch of the GFRP helices under different loading conditions.

The axial load capacity and stiffness of carbon fiber–reinforced polymer (CFRP) and glass fiber–reinforced polymer (GFRP) rods glued in timber is investigated under cyclic loading as the main design consideration for structures that experience load reversal (e.g., due to wind loading). Load cycles at 20%, 40%, 60%, and 80% of the ultimate load and three repetitions per load cycle were considered. The main parameters examined are the effect of FRP rod, anchorage length, and construction scenario. The construction scenarios represent full contact between timber faces, gaps in joints due to long-term effects (e.g., viscoelastic creep) and manufacturing tolerances, and contact with other materials. The GFRP rods exhibit 23% higher axial load capacity and 20% lower axial tensile stiffness than CFRP rods for an embedment length of 5D, where D = diameter of the rod. The axial load capacity of the GFRP rods tends to plateau with increasing bonded length at anchorage lengths greater than 10D. Small gaps significantly decrease the axial compressive stiffness of the glued-in FRP rods at the first load cycles and the axial stiffness varies along the bonded length. An analytical methodology is presented to describe the bond stress transfer mechanism and the progressive bond degradation. The analytical tensile slip values agree fairly well with the experimental results when debonding takes place at 80% of the ultimate load.

This research experimentally investigated the applicability of glass fiber–reinforced polymer (GFRP) bars as postinstalled reinforcement in structural connections. The test specimens included two identical vertical elements, which were anchored into a reinforced concrete base using postinstalled GFRP bars replicating a beam–column or beam–wall connection. The variables investigated included the GFRP bar embedment depth (150 and 290 mm), concrete compressive strength (20.7 and 41.4 MPa), and postinstalled GFRP bar size (Nos. 13, 19, or 25). A total of 16 specimens were subjected to combined tension and shear loading. Concrete breakout in tension was the failure mode in all specimens. The concrete breakout strengths improved by increasing the embedment depths and concrete strengths and worsened as the GFRP bar diameter increased while keeping the embedment depth and concrete strength constant. The authors recommend using anchor provisions from current standards to estimate the concrete breakout strength of post-installed GFRP bars (Nos. 13–19) tested with the parameters described herein.

Highway bridge piers could be underdesigned for a multihazardous condition involving vehicle collision and air blast based on the current design code. The piers are often positioned in such a fashion that it is neither possible nor economically feasible to place protective devices around them. This paper numerically investigated two potential strengthening techniques, the placement of a fiber-reinforced polymer (FRP) wrap and polyurea coating onto the column surface, to improve column resistance against the extreme demands from combined collision and blast loads. Finite-element models of isolated reinforced concrete (RC) columns with an FRP wrap and polyurea coating were developed using a commercial software, LS-DYNA. The modeling techniques were validated separately against impact and blast experimental results. Collisions were supplied from a medium-size F800 single-unit truck at various velocities, and air blasts of varying magnitude were represented using the arbitrary Lagrangian–Eulerian approach. Bare and strengthened column response to various collision and blast demands was compared to assess the effectiveness of strengthening. The results indicated that using the FRP wrap and polyurea coating on columns could effectively mitigate the effects of combined vehicle collision and air blast. The effectiveness of each scheme differed as a function of geometric and material properties and column demands. Parametric studies were then conducted to evaluate the influences of various design parameters on column performance and identify optimal strengthening schemes.

A new concrete recycling method is to crush demolished concrete into distinctly large recycled concrete lumps (RCLs), which are in a direct mix with fresh concrete, leading to the so-called compound concrete. Not only can this method decrease the recycling cost by simplifying the recycling process, it can also increase the recycling ratio. However, existing studies have demonstrated that such compound concrete is inferior to normal concrete. To improve the performance of compound concrete, an effective technique is to confine the compound concrete using fiber-reinforced polymer (FRP)–confining tubes, as demonstrated by a limited number of studies through tests on circular compound concrete columns. However, no studies have been done on FRP-confined rectangular compound concrete columns. Moreover, the possible column size effect in such columns has never been investigated; indeed, existing studies have revealed that FRP-confined rectangular normal concrete columns of different-sized specimens may exhibit obvious behavioral difference. Against this background, this paper presents the results of the first-ever experimental program on glass FRP (GFRP)–confined square compound concrete columns of three different sizes. The columns of different sizes had the same effective FRP confinement stiffness, and, therefore, the possible column size effect could be revealed. It was observed that the column size effect was obvious in the test columns in terms of compressive strength. Finally, three existing compressive strength models originally developed for FRP-confined normal concrete were evaluated using the present test results.

Geopolymers have been employed as a replacement material for cement in construction because they are better for the environment than are other replacement material options. In recent decades, externally bonded carbon fiber–reinforced polymer (CFRP) laminates have been used to strengthen or repair structures and improve their durability. The efficiency and reliability of a fiber-reinforced polymer (FRP) application depends significantly on the effectiveness of the bond between the FRP and the concrete substrate. This study examines the effect of the size of the coarse aggregate (5–10, 10–16, and 16–20 mm) on the bond between CFRP and metakaolin-based geopolymer (MKGP) concrete. The failure mode, failure load, distribution of strain along the bond length, maximum interfacial shear bond stress, effective bond length, local slip at the maximum interfacial shear bond stress, and bond–slip relationship between the CFRP sheet and the concrete substrate are investigated. Results show that the failure loads and maximum interfacial shear bond stress of MKGP specimens decreased with an increase in the size of the coarse aggregate, which can be attributed to the decreased splitting tensile strength of MKGP concrete with increasing size of the coarse aggregate. It was shown that this size affected the bond capacity between the CFRP and the concrete substrate and increased the effective bond length of ordinary Portland cement (OPC) and MKGP concrete by up to 24.5% and 46.8%, respectively. The average local slip between CFRP and MKGP concrete at the maximum interfacial shear bond stress increased with the size of the coarse aggregate, due to the decreased splitting tensile strength of MKGP concrete. Experimental results were used to recalibrate prevalent models used to calculate the effective bond length. Models to calculate the effective bond length are proposed that consider the influence of the coarse aggregate size. Popovics’s equations were employed to develop a relationship between the local bond stress and local slip by incorporating the effect of the size of the coarse aggregate. The findings of this study may contribute to the tailoring of bond behavior between CFRP and geopolymer concrete for engineering applications.

A novel composite-wedge anchorage is proposed here for prestressing fiber-reinforced polymer (FRP) tendons. The composite wedge was designed and optimized in terms of the length and elastic modulus of the wedge segments via finite-element analysis. The manufacturing technologies for the composite wedge involving a molding process, are presented in detail. Static, creep, and fatigue tests on basalt FRP (BFRP) tendons with the proposed anchorages were conducted. The tensile capacity, long-term deformation, and fatigue life of the tendon–anchor assemblies were analyzed. The experimental results show that the proposed anchorage exhibits an anchor efficiency factor of up to 91%; the corresponding value for a conventional steel–wedge anchorage is only 80%. The tendon–anchor assembly displays a creep behavior similar to that of BFRP tendons. Furthermore, the system survives 200 million cycles of fatigue load at a maximum fatigue load of 0.5 F_u (F_u is the tensile capacity of the tendon–anchor assembly) and a load range of 0.05 F_u. Wedge–sleeve and tendon–wedge sliding under a sustained load or cyclic load cannot cause considerable prestress loss in practical engineering. The results validate the effectiveness of the proposed anchorage under service loads and demonstrate the application prospects of this anchorage.

The bond between fiber-reinforced polymer (FRP) bars and concrete is the weakest link in the chain of parameters affecting the behavior of FRP-reinforced structures. In this study, eight glass fiber–reinforced polymer (GFRP)-reinforced concrete beams were tested in flexure. There were two control beams and six beams with lap splices. The first control had continuous rebars, whereas the second control had lap splices. A lap length of 40 times rebar diameter was used in tension in the constant moment region. The test variables included the spacing of confining stirrups (no stirrups, 100, and 50 mm) and a gap between rebars in the lap splice zone (0 and 18 mm). The test results revealed that the bond strength of GFRP rebars increases with the enhancement in the gap between rebars and the decrease in the spacing of confining stirrups. The available development length and bond-strength equations were modified to incorporate the effect of the GFRP rebar gap and spacing of confining stirrups in the lap zone. Besides the experimental program, nonlinear finite-element models were prepared to model the behavior of test beams and validate the proposed bond strength equations.

The seismic behavior of fiber-reinforced polymer-reinforced concrete (FRP-RC) columns is far from being fully explored. Therefore, numerical and analytical studies were performed to address the effects of different parameters and evaluate the current design provisions for confinement reinforcement under seismic loading. Using a commercially available software package, a three-dimensional nonlinear finite-element model (FEM) was constructed and validated against the experimental results of full-scale glass FRP (GFRP)-RC circular columns previously tested by the authors. The validated FEM was, then, used to conduct an extensive parametric study investigating the effect of concrete compressive strength, spiral pitch, axial load level, and column aspect ratio (i.e., shear span-to-depth ratio). It was found that increasing the concrete strength caused an increase in lateral load capacity and initial stiffness, whereas the drift capacity decreased. The latter property was also significantly affected by the variation of spiral pitch and axial load level. On the other hand, the aspect ratio had a marginal effect on moment or drift capacities. Using the results of the FEMs, two new design models were proposed. The proposed models showed remarkably better predictions than the equation adopted by the Canadian standard for the design of FRP-RC structures.

In this study, the compressive behavior of a load transfer component (LTC) for a fiber-reinforced polymer (FRP) cable anchor system (CAS) was improved using three types of microfibers, including 8-μm-diameter and 13-μm-diameter glass microfibers and 7-μm-diameter carbon microfibers, and implemented in a full-scale 37-tendon basalt FRP (BFRP) cable. The results showed that the corresponding optimal fiber lengths for the LTCSFs were 150, 900, and 75 μm, respectively. The compressive failure of the LTC modified by single-length microfibers (LTCSFs) was caused by microfiber debonding, pullout, and fracture. The compressive strengths and elastic moduli of the LTCSFs were positively related to the microfiber content. The compressive properties of the LTCSFs could be well predicted by a cubic polynomial fitting formula with R² > 0.99. When it was modified by multiple-length 8-μm-diameter glass microfibers (LTCMGF-8), an LTC with a hybrid ratio of 3:2 showed the optimal compressive strength and ductility. Then, to further release the radial stress of the FRP cable, a CAS with a variable-stiffness LTC could be optimized by decreasing the loading-end elastic modulus and increasing the free-end elastic modulus of the LTC. Finally, the effectiveness of the optimized variable-stiffness LTC was verified in a 37-tendon BFRP cable with a tendon diameter of 4 mm. Compared with the nominal breaking load of 761 kN, the experimental ultimate load of the BFRP cable was 766 kN and the corresponding anchor efficiency was 101%.

Laminated bamboo lumber (LBL) is an engineered bamboo with consistent and reliable mechanical properties that can be utilized in structural applications. Experimental studies showed that LBL columns perform well under compression, but the addition of eccentricity could induce excessive tensile stress, which may cause premature failure to bamboo fibers. This paper investigates the use of Carbon Fiber-Reinforced Polymer (CFRP) to suppress cracking of LBL in the tensile zone of eccentrically loaded columns. The mechanical properties of CFRP strengthened chamfered LBL columns with four eccentricity values of 30, 60, 90, and 120 mm were tested. Obtained results were analyzed to propose two analytical models to predict the ultimate bearing capacity of LBL columns under the considered loading case. Test results showed that most of the specimens failed due to delamination at the contact surface between CFRP and bamboo, resulting in fracture of CFRP at the midheight of the column. Consequently, mechanical connections in LBL also failed, and columns suffered a complete failure. Strain distributions within the column cross section showed linear variation showing compatibility with elastic bending theory. LBL columns strengthened using CFRP showed higher resistance under eccentric loading when compared against typical LBL columns; as the eccentricity was increased from 30 mm to 120 mm in 30 mm intervals, the ultimate load increased by 10.9%, 6.3%, 13.7%, and 17.9%, respectively. The results predicted using the two proposed models showed good agreement with test results.

The application of fiber-reinforced polymer (FRP) reinforcement in coral aggregate concrete (CAC) structures is a new solution to the shortage of construction materials on islands and reefs far from the mainland and to the insufficient durability of steel-reinforced concrete members. However, the understanding of the durability of FRP bar-reinforced CAC members is not yet clear, which greatly limits the evaluation and design of their long-term performance. In this paper, accelerated degradation tests were carried out in a high-temperature and high-humidity marine environment to study the durability of CAC beams reinforced with basalt FRP (BFRP) bars and stirrups. Four-point flexural tests were performed on beams that have been subjected to environmental conditions to study the effects of environmental time, temperature, and concrete type on the failure mode, cracking and ultimate moment, midspan deflection, crack width and pattern, and ductility. In addition, an engineered cementitious composite (ECC)–CAC composite beam reinforced with BFRP bars was proposed to improve the mechanical properties and durability of the CAC beam reinforced with BFRP bars. The results showed that, in the early stage of environmental conditions (1–2 months), the effect of the environment on the mechanical properties of the CAC beam reinforced with BFRP bars was beneficial. With increasing environmental condition time, the effect of environmental conditions on the mechanical properties of the CAC beam reinforced with BFRP bars gradually weakened. The failure mode, deflection, crack width, and crack pattern of the ECC–CAC composite beam reinforced with BFRP bars were greatly improved compared with those of the CAC beam reinforced with BFRP bars.

A new kind of structural system, ultrahigh-performance concrete (UHPC) shear walls reinforced with carbon fiber–reinforced polymer (CFRP) and steel bars, is proposed in this study. CFRP bars were used in the web region of the shear walls to obtain favorable self-centering capacity, and steel bars in the boundary element were used to achieve high energy-dissipation capacity. Quasi-static tests of four shear-wall specimens were conducted. The self-centering capacity, energy-dissipation capacity, and strength and stiffness degradation of the specimens were analyzed. The tested UHPC shear walls exhibited favorable self-centering and good energy-dissipation capacity. The experimental results showed that the stress in the CFRP bars is the key factor to determine the effectiveness of CFRP bars. The authors proposed a formula for calculating the proper location of CFRP bars and a design axial-load ratio limit for UHPC shear walls with hybrid CFRP/steel reinforcement bars.

In this study, the bond behavior of sand-coated glass fiber-reinforced polymer (GFRP) bars embedded in cementitious composite matrix reinforced with the recently developed basalt fiber (BF) pellets and steel fibers (SF) was evaluated. Several studies have attempted to introduce empirical models to estimate the bond strength of FRP-reinforced concrete (RC) specimens. In this study, the applicability of these models to predict the bond stress of straight and headed-end FRP bars embedded in normal concrete and fiber-reinforced concrete (FRC) was assessed. Test results of 413 pullout specimens, which were divided into four groups based on the type of the FRP bar or the cementitious matrix, were used to evaluate the available models. In addition, a proposed model to predict the bond performance of headed-end FRP bars with FRC pullout specimens was introduced. The model was calibrated against experiments conducted previously by the authors on FRP–FRC specimens. The model was capable of predicting the maximum bond strength with a mean experimental-to-predicted value of 1.09 and a standard deviation of 0.19.

The diffusion and adoption of coal gangue sintered bricks can effectively relieve the negative impact of coal gangue on the ecological environment. In China, this type of brick has been widely used for the construction of masonry structures. Existing masonry structures that use these types of bricks can experience severe problems related to safety, and strengthening by textile-reinforced mortar (TRM) is a valid measure for improving their performance. In this paper, the compressive performance of TRM-confined coal gangue sintered brick masonry piers was experimentally investigated. The influences of the number of textile layers, textile types (carbon and basalt), and additional FRP confinement at the upper and lower ends of the specimens on the compressive performance were mainly considered. The failure mode and increased effectiveness of the compressive strength, ultimate strain, and energy dissipation after TRM confinement were discussed. An existing model was also used to analyze the compressive strength. The results indicated that the compressive behavior of gangue sintered brick masonry piers could be significantly improved after TRM confinement, and the existing equation demonstrated relatively good prediction ability.

Fiber-reinforced polymer (FRP) ties can be used to provide tensile capacity to seismic-deficient concrete diaphragms, but there are no existing research or design provisions and guidance on how to design such ties is limited. Three commonly used design guidelines, ACI 440.2R, CNR-DT 200, and fib bulletin 90, have various provisions for calculating the axial tension capacity of FRP ties. These provisions have been summarized and their predictive ability compared with two databases, one for unanchored FRP ties and another one for FRP-anchored FRP ties. The unanchored tie data, compiled previously by others, consist of 855 test results from 135 documents; however, the range of parameters is limited, reporting primarily short, thin, and narrow FRP ties that are not representative of real practice. The second database was compiled by the authors and consists of 213 test results from eight papers; although it too is limited to short, thin, and narrow FRP ties anchored with small and shallowly anchored FRP anchors. Nevertheless, the behavior of anchored FRP ties from the database is described and guidance is provided to shape future research, necessary to confirm the suitability of FRP for diaphragm retrofits and develop design guidance.