Fiber Reinforced Polymer Grid Research Articles

Shear behavior of circular concrete short columns reinforced with GFRP longitudinal bars and CFRP grid stirrups

Although fiber-reinforced polymer (FRP) reinforcements have become popular, the post-pultrusion bending leads to significant deficiency in bent portions of FRP hoops and spirals. This study adopts flexible carbon FRP (CFRP) grids as stirrups, and investigates the shear behavior of circular short columns reinforced with GFRP bars and CFRP grid stirrups. Six specimens with different shear reinforcement ratios, axial load ratios and types of stirrups (CFRP grids and steel hoops) were tested to shear failures. The failure modes, crack and strain developments, and shear load-horizontal displacement responses were presented for discussion. All specimens experienced shear-compression failures, and the critical diagonal crack widths increased approximately linearly with shear loads. The typical shear load-horizontal displacement curve consisted of three branches: a linear elastic branch, a cracking branch with progressive slope deteriorations, and an almost flat failure branch. Increasing CFRP grid stirrup ratio could not only enhance strength and ductility performance, but also alleviate the slope degradations, while increasing axial load ratios could enhance the initial cracking loads, restrain crack propagations, and reduce the lateral displacements. In terms of strength performance, steel hoops could be replaced by CFRP grid stirrups even with smaller stirrup ratio. Additionally, analytical investigations suggested that the effective stresses and strains of CFRP grid stirrups at failure may be lower than code limits.

Tensile behavior of ultra-high-performance concrete (UHPC) strengthened with fiber reinforced polymer (FRP) grid

In order to enhance the tensile strength and toughness of Ultra-High Performance Concrete (UHPC) at the macro level, this study developed Fiber-Reinforced Polymer (FRP)-reinforced UHPC composite materials. The steel fibers and FRP grids in UHPC play a reinforcing and toughening effect on UHPC at two different levels. Therefore, this paper examined, evaluated 48 FRP grid-UHPC specimens. The study includes independent parameters such as grid type (Carbon-FRP, Basalt-FRP, stainless steel wire mesh), grid size (5×5 mm, 10×10 mm, 20×20 mm), steel fiber content (0%, 0.5%, 1.0%, 1.5%, 2.0%), grid layers (1 layer, 2 layers, 3 layers, 4 layers), and the fiber bundle impregnation status. Based on the test results, a simplified tensile stress-strain relationship for FRP grid-UHPC was proposed. The test results show that when the steel fiber content is less than 1%, the FRP grid does not exert its reinforcing effect. The FRP grid-UHPC composite material exhibits strain-softening behavior. However, when the steel fiber content is greater than 1%, the FRP grid-UHPC specimens all exhibit strain-hardening phenomena. Compared to other grids type, the enhancement and toughening effects on UHPC are more pronounced when using CFRP grids. Specifically, when using a 20×20 mm CFRP grid, the tensile stress can be increased by 30.36%. Regardless of whether dry grids or impregnated grids are used, increasing the grid layers greatly improves the peak strain and stress of UHPC. Compared to dry grids, impregnation with epoxy resin further improves the stress state between fiber filaments in FRP grids, resulting in a more significant improvement in the tensile performance of FRP grid-UHPC composite materials. Finally, based on experimental data, a tensile stress-strain model for FRP grid-UHPC considering different mesh ratios was proposed and validated against experimental results. These conclusions and findings are of great significance for understanding and optimizing the performance of FRP grid-UHPC composite materials, providing valuable references for the future research and application of FRP grid-UHPC composite materials.

Study on the uniaxial tensile behavior of an FRP grid-ECC composite layer

In this paper, an experimental and theoretical study on the uniaxial tensile behavior of a fiber-reinforced polymer (FRP) grid-engineered cementitious composite (ECC) composite layer is described. Based on the test parameters, including the ECC material properties, the ECC cross-sectional area and the FRP grid cross-sectional area, three groups of ECC specimens and six groups of composite layer specimens were tested to reveal their joint action mechanism and tensile behavior. The experimental results showed two failure modes: fibers were pulled off or pulled out in the ECC specimens, and in the composite layer specimens, part of the fiber bars in the FRP grid was pulled off. The stress–strain curves show three stages: the elastic stage, softening stage and hardening stage. Due to the reinforcement of the FRP grid, the ultimate stress of the composite layer increased significantly compared with that of the ECC specimens. The composite layer softening strain was primarily related to the ECC softening strain, and the ultimate strain was primarily related to the FRP grid fracture strain. Moreover, a three-stage model was proposed to predict the stressstrain relationship, exhibiting a higher prediction accuracy and smaller dispersion compared with the three existing models.

Experimental study to evaluate the flexural behavior of concrete beams reinforced with CFRP grid

Fiber reinforced polymer (FRP) is evaluated as an excellent material to replace rebar in reinforced concrete due to its advantages such as high tensile strength-to-weight ratio and high corrosion resistance. Many studies have been conducted to use FRP as substitutes for steel rebar. However, most of these studies were conducted using FRP bars, and research on using FRP grids in the construction field is focused on the field of repair and strengthening. Therefore, this study reports the results of an experimental study to evaluate the flexural behavior of concrete beam with Carbon FRP (CFRP) grid tensile reinforcements as a basic study to evaluate the feasibility of using FRP grid as a as substitutes for steel reinforcement. Thirteen test specimens were prepared for the experiments. The CFRP grids were applied to the specimens as tensile reinforcement. The concrete cover thickness, layers of the CFRP grid and the spacing between the CFRP grids were considered as test variables. The experiment results showed that the specimens without spacing between the CFRP grids were destroyed by side CFRP grid fracture. Also, as the CFRP grid layer increased, the ultimate load increased by 53% to 94%, and when the cover thickness decreased by 10 mm, the ultimate load decreased about 10%. The ratios of the nominal flexural strengths calculated according to the CSA S806–12 and ACI 440.1R-15 standards were 1.004 and 1.09, respectively. Thus, the use of the CSA S806–12 standard enables a relatively accurate prediction of the nominal flexural strength of concrete beams reinforced with CFRP grids.

Punching Shear Behavior of FRP Grid-Reinforced Ultra-High Performance Concrete Slabs

Fiber-reinforced polymer (FRP) grid-reinforced ultra-high performance concrete (UHPC) slabs are new structural solutions that take advantage of the mechanical properties of FRP and UHPC. However, the punching shear behavior of this new slab type has not been characterized. Therefore, punching shear tests were conducted to eight carbon fiber-reinforced polymer (CFRP) bars or grid-reinforced UHPC square slabs (600 mm side width × 40 mm thick). Several influential factors [e.g., use of CFRP bars or grids as flexural reinforcements, type of strengthening short fiber, steel fiber (SF) content, and presence of shear reinforcements] were investigated. The test results showed that FRP grids and short fibers helped to distribute the applied loads and dissipate the input energy; therefore, more cracks were observed, and higher punching shear capacities were achieved. Furthermore, increasing the reinforcement ratio in the FRP grids led to a more significant postcrack ductility response, which increased the punching shear capacity by 17%. In addition, SF addition could enhance the initial cracking load of the slab (Vcr), and polyethylene (PE) fiber addition could intensify the postcrack ductility response, both enhanced the punching shear capacity. The installation of shear reinforcements (eight pieces of 80 mm long CFRP grid strips) appeared to be more cost-effective than increasing SF content. Finally, compared with the current design provisions for conventional reinforced concrete slabs, a more accurate (average error of 8%) punching shear model was proposed for FRP grid-reinforced UHPC slabs with or without SF additions. However, the robustness of the proposed model will be assessed with more test data in the future.

Lattice discrete particle modeling of the cycling behavior of strain-hardening cementitious composites with and without fiber reinforced polymer grid reinforcement

Strain-hardening cementitious composites (SHCC) has become increasingly prevalent in structural design. Compared to ordinary concrete, SHCC exhibits ultra-high tensile ductility, significant strain-hardening behavior, very fine multi-point cracking, high energy dissipation, and good durability. One robust model for simulating reinforced concrete behavior is the so-called Lattice Discrete Particle Model (LDPM), and specifically LDPM-F which includes the effect of fiber reinforcement. The present cycling constitutive law implemented in LDPM and the cycling fiber-bridging law in LDPM-F though cannot accurately capture the residual tensile plastic strain, loading–unloading path, and energy dissipation of SHCC during cyclic tension. To solve these concerns, the cycling tension–compression constitutive law and the nonlinear cycling fiber-bridging law were reformulated. Further, the new model was used to simulate the cycling tensile behavior of plain concrete, SHCC, and Fiber-Reinforced Polymer (FRP) grid reinforced SHCC (FRP-SHCC). Simulation results show that the multi-point cracking, crack widths, and ultra-high ductility properties are correctly captured by LDPM-F. In addition, LDPM-F with the modified cycling constitutive model can effectively simulate the cycling tension–compression behavior, accurately reproducing the stress–strain relationship, residual plastic strain, and overall dissipated energy. Finally, the simulated failure modes agree well with the actual fracture planes of these materials under cyclic tension.

Flexural strengthening of RC beams using GFRP grid bonded with sprayed polyurea

Flexural strengthening of reinforced concrete (RC) beams with fiber reinforced polymer (FRP) was investigated in this research. The bonding agents were an epoxy paste and a sprayed polyurea. Four RC beams were strengthened with FRP composites according to the following techniques: (a) sprayed polyurea with and without glass FRP (GFRP) grid reinforcement; (b) manual layup with one GFRP grid; and (c) manual layup with one GFRP sheet. Experimental results clearly shows that flexural strengthening with polyurea technique is an effective scheme. Other than fast setting, the major advantages of using polyurea over other organic or inorganic matrices are as follows: (a) no slippage of the FRP grid from the polyurea or cracking of the polyurea occurred during the tensile test; (b) no debonding of the polyurea system from strengthened RC beams was observed during the bending test. Application of the polyurea system, key experimental results, and comparison with findings from other research programs reported in literature are presented in this paper.

Behavior of FRP grid-reinforced ultra-high performance concrete (UHPC) pipes under lateral compression

In this paper, ultra-high performance concrete (UHPC) pipes internally reinforced with fiber-reinforced polymer (FRP) grid (abbreviated as “FRP grid-UHPC pipe”) are proposed. The lateral load carrying performance of FRP grid-UHPC pipes was explored by testing 15 pipe specimens. The test variables included the presence and amount of FRP grid reinforcements, pipe thickness, as well as the presence of carbon FRP (CFRP) jacketing. Results showed that separation failure and flexural fracture were the primary failure modes for the pipes with and without FRP grid reinforcements, respectively. The typical load-vertical displacement curve of a FRP grid-UHPC pipe exhibited a two-stage ascending behavior. Without CFRP jacketing, the two ascending segments were linked by a load reduction, while the transition was smooth with CFRP jacketing. For the variables, it is found that the presence of FRP grid reinforcements could enhance the pipe initial cracking load by approximately 30%, while increasing the FRP grid layers could lead to a much steeper rising in the second ascending segment. Increasing pipe thickness and applying CFRP jacketing could increase the slopes of both initial and second ascending segments. Therefore, increasing pipe thickness (from 30 mm to 45 mm) and applying 1-ply CFRP jacketing could enhance the pipe ring stiffness by 422% and 115% respectively, which was more efficient that increasing the FRP grid layers.

Influence of displacement rate in the tensile testing of dry yarns for composite materials used in masonry strengthening

One of the most recent strengthening techniques of masonry structures involves fibre-reinforced polymer (FRP) grids as internal reinforcement of mortar plasters to produce what is normally referred to as Composite Reinforced Mortar (CRM) systems. The behaviour of such systems relies on the performance of the FRP grids which in turn depends on the properties of their constituents (fibres and resin).By comparing results of tensile tests at different displacement rates on glass fibre yarns, the effects of the strain history on the strengths exhibited by such products is here investigated. Further tensile tests were then performed on glass FRP grids made of the same previously analysed yarns in their warp direction and the influence of the displacement rates again studied.The analysis of the results of the performed experimental activity allows studying the influence of strain rates on the mechanical properties and failure behaviour of both yarns and their relative composites.

Flexural behavior of pre-damaged and repaired reinforced concrete beams with carbon fiber reinforced polymer grid and engineered cementitious composite

In this study, the flexural behavior of pre-damaged reinforced concrete (RC) beams repaired with carbon fiber reinforced polymer (CFRP) grids and engineered cementitious composite (ECC) was investigated. Fifteen beams were designed and subjected to four-point bending tests. Three types of specimens were tested: undamaged, pre-damaged with removed load (i.e., loaded to the target level and then unloaded), and pre-damaged under sustained load. The main testing parameters were strengthening material (ECC alone, or CFRP grid plus ECC), CFRP grid thickness, main reinforcement ratio, preloaded level, and repairing process. The experimental results showed that most of the repaired beams exhibited flexural failure caused by concrete crushing, and a few others failed by debonding. The proposed repairing technique was effective in enhancing the flexural stiffness and bearing capacity of pre-damaged RC beams. For the repaired beams, the flexural bearing capacities did not significantly decrease as the preloaded level increased from 0 to 0.45 and slightly decreased when the preloaded level exceeded 0.65. A mathematical model based on the plane section assumption was proposed to predict the flexural capacities of the repaired beams, considering whether the load causing the damage was removed or sustained. The predicted capacities were in good agreement with the experimental results.

Experimental investigation on fire endurance and residual strength of reinforced concrete beams strengthened with CFRP grids

Fiber-reinforced polymer (FRP) reinforcement is known to be extremely susceptible to deterioration of mechanical properties when exposed to fire. FRP grids encased with polymer mortar have received much attention lately as an efficient and fire-resistant technology for strengthening concrete structures. An experimental investigation was undertaken to examine and document the performance of carbon-fiber-reinforced polymer (CFRP) grids strengthened reinforced concrete beams under the sustained service load and the standard fire. Tests results on fire indicated that, even without any insulation, the CFRP grids strengthened beam can achieve a satisfactory fire endurance of more than 2 hours. The polymer mortar layer allows the strengthened beam to have longer fire endurance time than the un-strengthened beam, while under larger service loads. Moreover, this paper also presents a residual strength study of fire-tested beams. It has demonstrated that, if the FRP grids are able to maintain below the decomposition temperature when exposed to fire, the FRP grids strengthening system will be able to retain almost full effectiveness after cooling down.

Compression properties of carbon fiber reinforced polymer grid sandwich structure

Carbon fiber reinforced polymer (CFRP) grid sandwich structure with four different grid configurations was prepared using the interlocking assembly process in the present work. All test specimens were subjected to quasi-static out-of-plane uniform compression. The equivalent density, height, geometric configuration, and other factors of the structural core layer were analyzed, and their effects on the failure mode, damage mechanism, and energy absorption characteristics of CFRP grid sandwich structure were discussed. The results showed that there are four distinct stages in the load versus displacement curve of CFRP grid sandwich structure specimens subjected to uniform compression, namely, preloading stage, elastic stage, load drop stage, and platform stage. The main failure modes of CFRP grid sandwich structure are the local buckling and compression fracture of the core rib. The failure load, compression strength, breaking absorbed energy, and specific absorbed energy of CFRP grid sandwich structure are positively correlated with the equivalent density of the structure core layer, and negatively correlated with the height of the structure core layer. When the equivalent density of the core layer of the Mixed CFRP grid sandwich structure was increased by 10 kg/m3, the compressive strength and specific energy absorption of the structure increased by about 5.08 MPa and 186.79 mJ, respectively, and when the core layer height was increased by 10 mm, the compressive strength and specific energy absorption decreased by about 12.04 MPa and 67.39 mJ, respectively; The strength and stability of the Mixed CFRP grid sandwich structure are significantly higher than those of the Triangular, Diamond, and Square CFRP grid sandwich structures.

Flexural behavior of FRP grid reinforced ultra-high-performance concrete composite plates with different types of fibers

Ultra-high performance concrete (UHPC), which has an excellent compressive strength and an extraordinary durability, has become increasingly popular in structural engineering. However, UHPC has an inferior tensile property to its compressive property, and thus appropriate reinforcing material is necessary in UHPC structures. To this end, fiber-reinforced polymer (FRP) grids have been proposed for UHPC plates as reinforcements (the composite plates are referred to as FRP grid-UHPC composite plates). As the steel fibers in UHPC would still suffer from corrosion problems in aggressive environments, non-metallic fibers can be in UHPC so that steel-free FRP grid-UHPC composite plates are developed. A total of 26 plates (600 mm in length × 120 mm in width × 40 mm in thickness) are prepared for three-point bending tests in this study. The effects of the fiber type, fiber content and the fiber length on the flexural behavior of FRP grid-UHPC composite plates are investigated. Additionally, an analysis into the microstructure based on scanning electron microscope (SEM) of the FRP-UHPC composite plates is conducted. Moreover, a simple evaluation between the costs and the flexural capacities of the FRP grid-UHPC composite plates is conducted. It is economical to adopt 12-mm-length polyethylene (PE) fibers with a 1 % volume fraction to manufacture composite plates. Finally, the ultimate flexural capacities of FRP grid-UHPC plates were predicted using the section analysis approach.

INVESTIGATION OF THE PERFORMANCE OF RC BEAMS REINFORCED WITH FRP AND ECC MATERIALS

This paper investigates the structural working behavior of reinforced concrete beams bonded with fiber reinforced polymer and engineered cementitious composite materials subjected to bending using structural stressing state theory. First, six reinforced concrete beams externally bonded with composite reinforcement layer and one control beam are tested to investigate the effects of the bond length, fiber reinforced polymer grid thickness and fiber content on the flexural behavior. Then, the finite strain data of RC beams are interpolated by the numerical shape function method. The generalized strain energy density model is established to characterize the stressing state of the structure. Through the MannKendall criterion, the characteristics load P and Q of the beams are detected, and the whole loading process is divided into three stage. Finally, the analysis of the strain and deformation on the beams reveals the effect of different parameters on different stage. The characteristic load P increases as the bond length increases, and the characteristic load Q increases as the thickness of the FRP and the fiber content increase. The vertical deformation of the strengthened beam for the characteristic load Q and ultimate load is significantly smaller than that of the unreinforced beam.

Research on Impact Resistance of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer Grid and Engineered Cementitious Composites.

When reinforced concrete structures are subjected to impact loads, they may suddenly yield or fail, or even collapse as a whole. In this paper, the impact resistance of reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP) grid and engineered cementitious composites (ECC) was studied. Drop hammer impact tests were conducted on eight beams, then the finite element model was used to simulate the impact test, finally a simplified two-degree-of-freedom (TDOF) model was proposed for CFRP grid reinforced ECC layer strengthened RC beams under impact loading. The results showed that CFRP grid reinforced ECC layer significantly improved the impact resistance of RC beams. When the ECC and CFRP grid were used, the crack development was inhibited after the concrete cracked in the tensile area, avoiding the brittle damage of concrete beams with one crack to the end. Compared with the control beam, the reaction force of RC beams strengthened with CFRP grid and ECC under impact load increased by 16.2%~34.5%, the maximum mid-span displacement decreased by 16.3%~31.6% and the mid-span residual displacement decreased by 36.02%~49.53%. The finite element model and the proposed TDOF mode were demonstrated to effectively simulate the strengthened beam under impact loading.

Strain Measuring of Composite Grid Using Digital Image Correlation

Digital Image Correlation (DIC) is a new advanced technique for measuring the deformation of a specimen using high-resolution images. It has been used by numerous researchers since it can measure the deformation of specimens without interference because it is contactless. Moreover, the DIC technique can be applied to any materials to which normal measuring equipment is difficult to attach such as Fiber-Reinforced Polymer (FRP) grid in this paper, undulated and small surfaces. The DIC technique is easy to set up and provides reliable results compared to conventional equipment like a strain gauge. Although it is good, its associated equipment is too expensive to be readily affordable. Hence, this paper uses an open-source DIC program called Ncorr that works in MATLAB to analyze deformations of six FRP grids from direct tensile tests by comparing their results to the results from a strain gauge. Young’s modulus—calculated from ASTM 3039, ACI 440-3R-04, and regression analysis—of each specimen will be used for comparison. The results show that the difference between Young’s modulus from DIC and strain gauge is <5% if the stress–strain graph of data from the strain gauge is perfectly linear without straying.

Fatigue performance of corrosion-damaged beams strengthened with FRP grid-reinforced ECC matrix composites

An investigation on the flexural fatigue behavior of corrosion-damaged reinforced concrete (RC) beams that were strengthened with a fiber-reinforced polymer (FRP) grid-reinforced engineered cementitious composite (ECC) matrix is presented in this paper. The concrete cover, which was damaged by the corrosion of the tensile steel bars, was chipped away and reinstated with an ECC layer that was embedded with an FRP grid. A total of 12 beams were constructed, and 11 of them were subjected to cyclic fatigue loading. The test results indicated that the final failure mode of the strengthened beams was still dominated by the brittle fracture of the tensile steel reinforcements. The fatigue life of the beams increased significantly with increasing strengthening amounts of the FRP grid but decreased with the corrosion ratio and the upper limit fatigue load level. At a sufficient strengthening amount, the fatigue life of strengthened beams could be restored to that of the original uncorroded beams. By regression analysis of the stress range–fatigue life (S–N) curve, a prediction formula for fatigue life was derived with a satisfactory correlation coefficient. In addition, the tested midspan deflections and the results that were calculated using available empirical models or theoretical calculation approaches were compared and evaluated.

Shear performance of 3D printed concrete reinforced with flexible or rigid materials based on direct shear test

Flexible and rigid materials, such as stainless steel cable (SSC), stainless steel cable with blades (SSCB), stainless steel cable mesh (SSCM), and carbon fibre reinforced polymer grid (CFG), were used to reinforce printed concrete. Based on the directed double-sided shear test, the shear responses of printed concrete, such as the failure mode, shear strength, and shear load–displacement curve, were obtained, and the enhancement efficiencies of different reinforcing methods were discussed by considering the variation in the ratio of reinforcement. Furthermore, the mechanical mechanism was proposed, and prediction models were established based on the different contributions of the flexible and rigid reinforcing materials to the shear behaviour of printed concrete. The results indicated that the stiffness of the reinforcing materials evidently affects the shear performance of printed concrete, and lateral reinforcement is beneficial for improving the shear strength and toughness of printed concrete.

Axial-flexural behavior of FRP grid-reinforced geopolymer concrete sandwich wall panels enabled with FRP connectors

This paper presents experimental and theoretical investigations conducted on a novel precast concrete sandwich wall panel (SWP) subjected to eccentric axial loading. The proposed SWP is composed of geopolymer concrete wythes which were embedded with basalt fiber reinforced polymer (FRP) bars/grids. Hollow Glass FRP (GFRP) tubes were used as connectors. Nine such SWPs were prefabricated and tested. The key design test parameters considered were slenderness ratio of the SWP, load eccentricity, and type of longitudinal reinforcement (FRP bar or grid) in wythes. The failure modes, load-deformation behavior, and load-strain behavior were studied and reported. It was found that the axial load capacities of the eccentrically loaded SWPs subject to load eccentricity-to-sectional thickness ratios of 0.15, 0.45, and 0.63 were reduced by 35%, 75%, and 85% (average of the load capacity values for the slender and squat SWPs), respectively, compared to their counterpart concentrically loaded SWPs. A second-order theoretical analysis was also conducted to obtain the axial load-moment interaction curves of equivalently assumed sections, and the outcomes were compared with the experimental ones to validate the modeling.

Experimental investigation of corrosion-damaged RC beams strengthened in flexure with FRP grid-reinforced ECC matrix composites

In this study, composites fabricated using a fiber-reinforced polymer (FRP) grid-reinforced engineered cementitious composite (ECC) matrix were used for the flexural strengthening of corrosion-damaged reinforced concrete (RC) beams in combination with a patch repair technique. After the longitudinal reinforcements were corroded to the expected ratio by the electrochemically accelerated method, the corrosion-damaged concrete cover was removed, and then repaired with FRP grid-reinforced ECC matrix (FGREM) composites. The investigated test variables were mainly FRP grid types (carbon and basalt), strengthening amounts, corrosion level, and longitudinal reinforcement ratio. The experimental results indicated that the bearing capacities of the corroded beams were sufficiently improved using this strengthening technique. The intermediate debonding at the concrete substrate-to-ECC interface caused by flexural cracks could be completely suppressed. The use of a carbon fiber reinforced polymer (CFRP) grid with higher stiffness could produce a weaker ductile behavior and prematurely trigger critical diagonal crack-induced debonding. Furthermore, the flexural bearing capacities of the basalt-FGREM strengthened beams could be reasonably predicted according to the ACI-549.4R-13 provisions. However, the predicted results were relatively dangerous for carbon-FGREM strengthened beams, which failed under the control of the critical diagonal crack-induced debonding mode.