Glass Fiber-reinforced Polymer Layers Research Articles

Strengthening of Laminated Veneer Lumber Slabs with Fiber-Reinforced Polymer Sheets—Preliminary Study

Analyzing the feasibility of reinforcing new and existing wooden structures is a valid problem, being the subject of numerous scientific papers. The paper presents the preliminary results of a study on reinforcing Laminated Veneer Lumber (LVL) panels with composite materials bonded to exterior surfaces using epoxy resin. Glass-Fiber-Reinforced Polymer (GFRP) sheets, Carbon-Fiber-Reinforced Polymer (CFRP) sheets, and Ultra-High-Modulus (UHM) CFRP sheets were used as reinforcement. The variables in the analysis were the type of reinforcement and the number of reinforcement layers. The tests were carried out on small samples (45 × 45 × 900 mm) subjected to the so-called four-point bending test. Reinforcement positively affected the mechanical properties of composite section. The highest increases in load bearing were 37 and 48% for two layers of GFRP and CFRP, respectively. The bending stiffness increased up to 53 and 62% for two layers of CFRP and UHM CFRP, respectively. There was a change in failure mode from cracking in the tension zone for unreinforced beams to veneer shear in the support zone (for CFRP and GFRP sheets) and sheet rupture (UHM CFRP). Good agreement was obtained for estimating bending stiffness with the presented numerical and mathematical model; the relative error was up to 6% for CFRP and GFRP and up to 20% for UHM CFRP. This preliminary study proved the effectiveness of combining LVL with FRP sheets and indicated their weak spots, which should be further analyzed to improve their competitiveness against the traditional structures. The key limitation was the shear strength of LVL.

Experimental study and finite element modelling of the torsional behavior of self-compacting reinforced concrete (SCRC) beams strengthened by GFRP

The present study experimentally and numerically investigates the behavior of ten self-compacting reinforced concrete (SCRC) beams strengthened by various means of externally bonded glass fiber reinforced polymer (GFRP) strips and subjected to pure torsion. The experimental matrix is designed to explore the impact of the number of GFRP layers, the width of the strengthening layers, and the spacing between those layers on the ultimate torsional capacity, the ultimate angle of twist, ductility, and failure patterns with respect to an un-strengthened specimen. Tests revealed that the specimen that was strengthened along its whole length using three layers of GFRP sheets yielded the highest gain in the ultimate torsional moment of 33.3% compared to the un-strengthened specimen. Additionally, the highest gain in the energy dissipation capacity among all strengthened beams (209.4% higher than the control beam) was obtained when four layers of GFRP sheets spaced every 300 mm were installed. Moreover, a three-dimensional (3D) non-linear finite element analysis is carried out via ABAQUS software to predict the torsional characteristics of SCRC beams either strengthened by GFRP strips or kept bare. The numerical model well captured the failure patterns, torque versus angle of twist responses, stress distribution, and the ultimate load of SCRC beams strengthened by GFRP strips with respect to the experimental findings.

Improvement of mechanical properties of oak-wood by bi-directional laminations – Efficacy of standard and pre-stressed glass fibre implants

This paper brings results of the bending testing of oak-wood laminated elements for structural use. The bi-directional laminations were reinforced with six types of implants made of glass fibre reinforced polymer (GFRP), some of which were pre-stressed before embedding in epoxy resin. Previously we reported on improving the mechanical properties of oak-wood laminated elements using carbon fibre implants (CFRP). Although it proved as an efficient improvement of the mechanical properties, it also raised the question of the cost effect to the final product. Therefore, within the larger study on investigating effects of different reinforcement systems, we tested glass fibres reinforced polymer (GFRP) as an alternative. Results showed that the introduction of implants improved the effective stiffness between 4 and 51 % and the ultimate load to failure up to 55 %. Furthermore, not only that insertion of the GFRP layers significantly contributed to the strength and stiffness improvement, but the ductility of the elements increased up to 32 %. Pre-stressing of the glass fibres reinforced polymer implants in tension before their installation contributed to the strength increase to a much greater extent than the increase in the number of layers. The best reinforcement effect is obtained if two layers of pre-stressed GFRP are used.

Axial compressive behavior of concrete-filled steel tubes with GFRP-confined UHPC cores

The steel–concrete-GFRP-UHPC (SCGU) composite column is a new type of column consisting of steel, concrete, ultra-high performance concrete (UHPC) and glass fiber reinforced polymer (GFRP). The sectional form of the SCGU column is a circular steel tube as the outer layer, a GFRP-wrapped UHPC column as the core, and concrete between the core column and the steel tube. In this paper, axial compression tests were conducted on twenty-four SCGU composite columns, and their damage morphology, load–strain curves and influencing factors were analyzed. The results show that the damage modes of the specimens present multiple convex curves of the external steel tube, corresponding to the fragmentation of its plain internal concrete, the fracture of GFRP and the large axial crack of the UHPC core column; that the load–displacement curves of the composite columns can be roughly simplified to elastic, elastoplastic and plastic flow stages; that the composite columns still have high residual bearing capacity after compression damage; and that effect of tube thickness, number of GFRP layers and UHPC core column diameter of the specimen on the load bearing capacity and ductility of composite columns. Based on the mechanical analysis of each component of the SCGU composite column, a formula of axial compression bearing capacity of the SCGU composite column was presented, and agreement between the formula predicted results and the experimental results was obtained. In addition, finite element (FE) models that can simulate the SCGU composite column are developed.

The Effect of GFRP Wrapping on Lateral Performance of Double Shear Lap Joints in Cross-Laminated Timber as a Part of Timber Bridges

Timber elements, such as timber bridges, are exposed to heavy loads. Therefore, reinforcement might be useful. Due to a lack of wood supplies, poplar, a fast-growing tree, could be used to construct CLT (cross-laminated timber). The low density of fast-growing wood species directly impacts the mechanical properties of CLT. Therefore, in this study, a CLT panel was reinforced with GFRP (glass-fiber-reinforced polymer), and the lateral resistance of double shear lap joints in reinforced CLTs with 0-90-0° arrangements in two strength directions was investigated. Lag screws (Ø = 8 mm) at the end distances of 1 and 3 cm were employed for making the lateral test specimens. First, the effect of the number of GFRP layers on lateral resistance of the joints was investigated. The results revealed that, as the number of GFRP layers changed from one to three, the lateral resistance increased by 45.47%, and then, by four layers, it decreased by 1.3%. Since the joints with three layers of FRP had the highest strength, the effects of the end distance and the CLT panel strength directions on the lateral performance of the reinforced and non-reinforced specimens were investigated. The results indicated that the lateral resistance of reinforced CLTs with GFRP was about 26.5% more than the unreinforced ones. Moreover, CLTs in the major strength direction showed 4.2% more lateral resistance than those in the minor strength direction. Moreover, lag screws at the end distance of 3 cm had 60% more lateral resistance than those at the end distance of 1 cm. In terms of failure modes, bearing, shear, and net-tension modes were observed in the CLTs, while Is, IIIs, and IV modes were observed in the lag screws.

Axial compressive behavior of GFRP-confined seawater coral aggregate concrete incorporating slag-based alkali-activated materials

• A new type of GFRP-confined alkali-activated seawater coral aggregate concrete (AACAC) was developed. • Axial compression characteristics of GFRP-confined AACAC were investigated. • The FRP confinement was an effective method in enhancing the loading capacity and deformation of the concrete. • The ultimate strength and strain models for GFRP-confined AACAC were proposed. This paper investigated the experimental study on the axial compressive behavior of glass fiber-reinforced polymer (GFRP)-confined concrete, including cement-based seawater coral aggregate concrete (CAC) and slag-based alkali-activated seawater coral aggregate concrete (AACAC). The axial compression characteristics of the CAC and AACAC cylinders wrapped with different GFRP layers (i.e., 0, 2, 4, and 6 layers) were tested and analyzed. Experimental results showed that both CAC and AACAC specimens had a fairly similar failure pattern. The ultimate strain increment ratios are significantly higher than the ultimate strength ones, and AACAC exhibited a higher strain enhancement ratio than CAC. Moreover, the ultimate strength and axial strain gradually improved as the thickness of FRP increased. Approximately 2.88- and 5.79-times improvements in the ultimate axial strain were observed for the 6-ply CAC and AACAC specimens, respectively, compared with their un-confined companions, demonstrating that FRP jackets can effectively improve the axial deformation capacity of concrete cylinders. Additionally, experimental results were compared with some representative empirical models and then preliminary ultimate strength and strain models were proposed for GFRP-confined CAC and AACAC.

Structural Behaviour of Polystyrene Foam Lightweight Concrete Beams Strengthened with FRP Laminates

Lightweight concrete (LWC) is one of the most important building materials nowadays. Many research studies were focused on LWC produced using lightweight aggregates. However, limited work was cited for LWC produced using polystyrene beads. In this study, LWC beams strengthened with carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) were experimentally tested to investigate the improvement in their flexural and shear behaviours. LWC in this investigation was achieved by partial replacement of normal aggregate by polystyrene beads and resulted in approximately 30% less weight compared to Normal weight concrete. Fourteen Reinforced Concrete (RC) LWC beams of 100 mm by 300 mm cross section having an overall length of 3250 mm were tested under four-point bending. These beams were designed, detailed, and tested to obtain flexural and shear mode of failure. These beams were divided into two groups based on the intended failure mode. In each group, six beams were strengthened using CFRP and GFRP laminates, while the remaining one beam was used as control. The tested parameters were the type of FRP, the width of the laminates used in shear strengthening, and the number of layers used in flexural strengthening. It was found that strengthening of LWC beams using CFRP and GFRP layers resulted in increasing the loading capacity and decreasing deflection as compared to control. The strengthening with CFRP and GFRP is also suitable in reducing the crack width and crack propagation which is more significant in LWC beams. The experimental results were also compared with the expressions in codes for forecasting the strength of LWC beams and it was that these expressions are compatible with the experimental results.

Compressive Behavior of Concrete Containing Glass Fibers and Confined with Glass FRP Composites

In this paper, numerous experimental tests were carried out to study the behavior of concrete containing glass fibers and confined with glass fiber-reinforced polymer (GFRP). Concrete specimens containing different fiber percentages ( 0.3 wt.%, 0.6 wt.%, 0.9 wt.% or 1.2 wt.%) and with different strengths of concrete (8.5 MPa, 16 MPa and 25 MPa) and different confinement levels (two, four and six layers of GFRP) were used as research parameters. The samples were tested to failure under pure axial compression. The results imply that the confinement effect with GFRP is relatively higher for concrete samples containing glass fiber (GFCC) with a percentage equal to 0.6 wt.%. The theoretical of stress ratios (fcc/fco) estimated by using existing ultimate strength models are found to be close to the experimental results for high strength of GFCC, but not close to the experimental results for low strength of GFCC.

A novel technique to improve the compressive strength and ductility of glass fiber reinforced polymer (GFRP) composite bars

Glass fiber reinforced polymer (GFRP) composite bars have excellent durability compared to traditional steel reinforcements but their strength contribution in the design of compression members is generally neglected. The non-homogeneous and anisotropic nature of GFRP bars including their complex behavior in compression and the lack of standard testing procedure have hindered their use as compressive reinforcements. To improve their behavior and strength in compression, a novel technique has been developed by winding additional GFRP layers around the longitudinal fibers. One, two and three winding layers with a thickness of 1.5 mm for each layer and a winding angle of ± 83.3° have been used. Compressive strength tests have then been conducted on 20 mm and 30 mm high GFRP bars with a core diameter of 20 mm to determine the improvement in compressive strength, ductility, and failure behavior. Test results showed that the 20 mm core diameter GFRP bars have shown promising improvement in the compressive strength and ductility with the increased number of winding layers wherein the compressive strength of the 20 mm and 30 mm high bars increased by 74% and 63%, respectively for bars with three winding layers. Comparison of the mechanical properties in compression and tension showed that the optimum strength and ductility improvement can be achieved with two winding layers. A simple theoretical strength model has been developed which can reliably predict the compressive strength of the winded GFRP bars.

Experimental and analytical investigation of using externally bonded, hybrid, fiber-reinforced polymers to repair and strengthen heated, damaged RC beams in flexure

PurposeThis study aims to conduct an experimental study and finite element model (FEM) to investigate the flexural behavior of heat-damaged beams strengthened/repaired by hybrid fiber-reinforced polymers (HFRP).Design/methodology/approachTwo groups of beams of (150 × 250 × 1,200) mm were cast, strengthened and repaired using different configurations of HFRP and tested under four-point loadings. The first group was kept at room temperature, while the second group was exposed to a temperature of 400°C.FindingsIt was found that using multiple layers of carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) enhanced the strength more than a single layer. Also, the order of two layers of FRP showed no effect on flexural behavior of beams. Using a three-layer scheme (attaching the GFRP first and followed by two layers of CFRP) exhibited increase in ultimate load more than the scheme attached by CFRP first. Furthermore, the scheme HGC (heated beam repaired with glass and carbon, in sequence) allowed to achieve residual flexural capacity of specimen exposed to 400°C. Typical flexural failure was observed in control and heat-damaged beams, whereas the strengthened/repaired beams failed by cover separation and FRP debonding, however, specimen repaired with two layers of GFRP failed by FRP rupture. The FEM results showed good agreement with experimental results.Originality/valueFew researchers have studied the effects of HFRP on strengthening and repair of heated, damaged reinforced concrete (RC) beams. This paper investigates, both experimentally and analytically, the performance of externally strengthened and repaired RC beams, in flexure, with different FRP configurations of CFRP and GFRP.

Predicting interlaminar damage behaviour of fibre-metal laminates containing adhesive joints under bending loads

This study includes experimental and numerical investigations on fibre-metal laminate structures containing adhesive joints under static bending loads. Experimental tests were carried out on Glare® 4B specimens manufactured in-house and containing doubler joint features. Numerical analyses were performed using Abaqus software including damage in the glass fibre reinforced polymer layers, ductile damage in the resin pockets (FM94 epoxy) and plasticity in the metal layers. A new cohesive zone model coupling friction and interfacial shear under through-thickness compressive stress has been developed to simulate delamination initiation and growth at the metal/fibre interfaces with the adhesive joint under flexural loading. This model is implemented through a user-defined VUMAT subroutine in the Abaqus/Explicit software and includes two main approaches, firstly, combining friction and interfacial shear stresses created in the interlaminar layers of the fibre-metal laminate as a result of through-thickness stresses and secondly, considering elastic-plastic damage behaviour using a new cohesive zone model based on the trapezoidal law (which provides more accurate results for the simulation of toughened epoxy matrices than the commonly used bilinear cohesive zone model). Numerical results have been validated against experimental data from 4-point bending tests and a good correlation observed with respect to both crack initiation and evolution. Delamination and shear failure were noted to be the predominant failure modes under bending stresses as expected. This is due to the higher mode-II stresses introduced during bending which cause different damage evolution behaviour to that seen for axial stresses. Finite element results revealed that both friction and shear strength parameters generated from through-thickness compression stresses have a significant effect in predicting damage in fibre-metal laminate structures under this type of loading.

Repair of Fire Damaged Axially Loaded Short RC Columns Using GFRP Wrap

Fire damaged columns can be repaired with glass fibre reinforced polymer (GFRP) wrap to regain its full strength or even more. In the present work, short reinforced concrete (RC) rectangular columns which are exposed to elevated temperature at 4000℃ for 2 hours duration are repaired using GFRP wrap glued with epoxy resin. Strengthening of existing columns using GFRP wrap is a simple, easy and economical solution. GFRP wrap enhances the stiffness of the column in lateral direction, which improves the load carrying capacity of the column. The behaviour of GFRP wrapped column is different from the behaviour of column without GFRP wrap in response to the applied load. The behavior of GFRP strengthened column depends on various factors such as column cross sectional area aspect ratio, radius of rounded edges of the column, number of GFRP layers wrapped, width of GFRP strip, wrap pattern, epoxy properties etc. Various wrap patterns are tried to obtain effective one which greatly enhances the load taking capacity of short axially loaded columns. GFRP wrapping using strips showed good results than using continuous layer. The load carrying capacity of fire damaged GFRP repaired columns is increased by 42.3% compared with undamaged column without GFRP wrap and increased by 112.6% compared with fire damaged column without GFRP. Designer should fairly estimate the capacity of the strengthened column. Mathematical models are developed to predict the strength of repaired columns.

Axial Compressive Performance of a Composite Concrete-Filled GFRP Tube Square Column

A composite concrete-filled glass fiber reinforced polymer (GFRP) tube square column is a new type of composite column, where GFRP is externally wrapped over several GFRP square tubes to form a multicavity GFRP tube, and then concrete is poured inside. External GFRP wrapping methods can be divided into two types: entirely wrapped and strip-type wrapped methods. The former is superior to the latter in terms of performance under stress. However, difficulties are introduced in the construction process of the former, and substantial materials are required to wrap the entire structure. To examine the axial compressive performance for this new type of composite column and the impact of the wrapping method, we designed and fabricated one type of entirely wrapped composite column and two types of strip-type wrapped composite columns with clear spacings of 85 mm and 40 mm, respectively, and performed static axial compression tests. Through tests and numerical simulations, we obtained the failure mode, load–displacement curve, and load–strain curve of the specimen, and analyzed the impact of the externally wrapped GFRP on the mechanical behavior of the composite column. The results show that the composite column reached the peak load before the fracture of the GFRP tube fiber occurred, and the bearing capacity declined sharply to approximately 75% of the peak load after the fiber fractured, then entered a platform section, thereby displaying ductile failure. As the wrapped layers of GFRP strips increased, the load capacity of the specimen exhibited a linear growth tendency. Compared with the performance of the entirely wrapped method, the load capacity of the specimens in the W5040 group declined 9.8% on average, and the peak efficiency of the GFRP strips increased by 50%, thereby indicating that the use of appropriate GFRP layers and strip distance intervals can ensure the appropriate bearing capacity of composite columns and full utilization of GFRP material properties.

Effects of Thermal Residual Stresses on Tensile and Interlaminar Shear Behaviors of GLARE Laminates

The residual stresses caused by curing, forming and machining process significantly affect the mechanical behaviors of composite materials, especially for super hybrid structure like GLARE (glass fiber reinforced aluminum laminates). In this work, the curing thermal residual stresses characteristics of GLARE and corresponding effects on tensile and interlaminar shear behaviors were investigated. Firstly, the characteristics of residual stresses were described by a finite element (FE) model and verified by layer removal method. Then, thermal residual stresses were incorporated into FE mechanical behavior prediction models in the form of predefined field to analyze the effects on tensile and interlaminar shear behaviors. The corresponding simulation conclusions were consistent with the experimental observations. The results indicated that there were about 40 MPa tensile residual stress in aluminum layers and 70 MPa compressive residual stress in GFRP (glass fiber reinforced polymer) layers of GLARE. Residual stresses had obvious reduction effect on the yield strength of GLARE, and the prediction accuracy was hence improved by 9%. Moreover, the failure mode of tensile tests was hardly affected by the residual stresses, but the tensile strength and the initial damage loads were reduced slightly. In interlaminar shear tests, the peak load and initial damage load were reduced by the residual stresses in the adhesive layers of GLARE, and the interlaminar shear strength was slightly influenced. An edge failure mode caused by edge effect was observed during the propagation of the delamination cracks, which was verified by interlaminar shear finite element models and SEM (scanning electron microscope) images.

Compression behavior and modeling of FRP-confined high strength geopolymer concrete

Fiber-reinforced polymer (FRP) has proven its efficiency as a strengthening material of ordinary concrete. However, the research on the effect of these materials on geopolymer concrete is still less understood. Due to the excellent mechanical and environmental properties of geopolymer concrete, the compression behavior of confined high strength geopolymer concrete (HSGC) is experimentally and analytically investigated and compared with high strength cement concrete (HSC). The confined cylindrical test specimens of HSGC and HSC were tested in compression. The cylinders were confined using three schemes of FRP, namely one and two layers of Glass FRP (GFRP) and one layer of carbon FRP (CFRP). The stress–strain behavior of the HSGC is compared with HSC for different levels and types of confinement. For low confining pressure, there was a definite peak followed by descending portion of the stress–strain curve, whereas for high confining pressure, initially, the stress increases nonlinearly with strain, and after reaching a certain level of stress, it becomes asymptotic to a positive slope straight line. A single equation is proposed for predicting all curves of HSGC and HSC for low as well as high confining pressures. The validation of the model is also successfully shown with the data taken from the literature for a wide range of confinement levels.

Strength Capacity Cracks Propagations Deflection and Tensile Enhancement of Reinforced Concrete Beams Warped by Glass Fiber Reinforced Polymer Strips

Different approaches were adapted to strength the structural elements to increase the load capacity and reduce the deformation such as deflection. The easiest and light external strengthening of reinforced concrete members are Fiber Reinforced Polymer (FRP) family such as Armed, Carbon, Glass and Basalt, respectively. This paper presents the theoretical approach to check out the experimental tests of reinforced concrete beams strengthened by glass fiber reinforced polymer (GFRP) using finite elements method by ANSYS software in which all models are simulate the tested beams. All models have the same geometry and mechanical properties but differ in GFRP layers and width. The main objectives of present work are evaluating the strength capacity, cracks propagations, deflection and tensile enhancement of reinforced concrete beams warped by GFRP strips subject to four points static load. Analysisof results indicate that the presences of GFRP sheets enhance the capacity and ductility of reinforced concrete beams in additional to delay the post crack concrete. The delay in the formation of first crack, increase in the number of cracks and ultimate loads of the models compared with the control model. There are improvements in flexural strength based on the modulus of rupture. Also, the cracks propogation become less in case of presence of GFRP and there is improvements in tensile resistance due to flexural. Analysis results inicated that the presence of GFRP at the bottom face of reinforced concrete beam in case of two layers gave increase in ultimate load 104.3% as compared with the control model. The reduction of the deflection for same models is 10.84%. Factor of the modulus of rupture range between (0.76-1.36) that is more than with ACI code suggested as 0.6. All model results were close to the experimental tests.

Flexural Behavior of Slab-Rib Integrated Bridge Decks with GFRP Skin and Polyurethane Foam Core

This paper presents experimental and analytical studies on flexural behavior of slab-rib integrated Sandwich composite decks. The influences of layers of glass fiber-reinforced polymer (GFRP) facesheets, foam densities, and the existence of webs and cross beams are discussed herein. The test results showed that the existence of vertical webs remarkably improved the debonding of the facesheets from the foam core, thus increasing the ultimate load by 59% compared with the specimens without webs. However, the existence of horizontal webs has insignificant effect on the failure mode and ultimate load. Increasing the number of layers of GFRP facesheets from 2 to 4 and 6 results in 100% and 214% increments in ultimate loads, respectively, while the specimen with lower density of foam had a higher ultimate load than the specimen with higher density of foam due to deformation compatibility between GFRP skins and foam core with low density. The analysis software Abaqus Explicit was used to simulate the flexural behavior of test specimens, and the numerical results agreed well with the test data. The verified finite element model was extended to analyze the influences of the number of GFRP layers on the top of decks and the height of vertical webs. Based on equivalent method and compatibility of shear deformation, the flexural and shear rigidities were estimated. Then, analytical solution for displacement of the slab-rib integrated Sandwich composite decks subjected to four-point load was derived out. Comparison of analytical and experimental results shows that the displacements can be precisely predicted by the present theoretical model.

Design-Oriented Stress-Strain Model for FRP-Confined Lightweight Aggregate Concrete

Despite the advantages of lightweight aggregate concrete (LWAC), its brittleness in compression has limited its vast application. Jacketing by fiber-reinforced polymer (FRP) tends to be one method to overcome this negative point. The current study attempted to recognize the FRP confinement impact on the LWAC compressive monotonic behavior. Both fine and coarse aggregates were Scoria aggregate. The variables include the type and number of FRP layers; 150 × 300 mm cylindrical specimens were wrapped with 1, 2, 3, and 4 layers of glass FRP (GFRP) and carbon FRP (CFRP) strips. An investigation was performed on the monotonic stress-strain curve belonging to LWAC confined with FRP, and key points, including ultimate stress and strain on the curve, were explored. The results revealed that the key points would significantly improve by increasing FRP layers. Although the influence of GFRP confinement on enhancing the ultimate strain appears to be more considerable than the CFRP confinement, CFRP jacketing of LWAC upgrades the ultimate strength to the higher level compared to wrapping with GFRP. Based on the interpretation of experimental results, a design-oriented stress-strain model was established to predict the monotonic compressive response of Scoria-LWAC circular specimens confined with CFRP and GFRP. The validity of the model was examined. The predicted output was appropriately in line with the experimental findings.

The axial and lateral behavior of low strength concrete confined by GFRP wraps: An experimental investigation

The purpose of this investigation is to investigate the axial and lateral behavior of low strength concrete confined by glass fiber reinforced polymer (GFRP) wraps. A total of 48 cylindrical samples having 150 mm diameter and 300 mm height were strengthened and examined. The mix designs were provided with four different target strength (5, 10, 15, and 20 MPa). The axial and lateral strain were studied to evaluate the axial strain relation and the lateral strain for axial compression. The results of this investigation present a comparison between the nine models previously reported for normal strength concrete and the lesser-known context of low strength concrete confined by fiber reinforced polymer (FRP) composites. The results obtained from this study show that the majority of models reported for normal strength concrete, can accurately predict the stress improvement ratio for low strength concrete (LSC), however, one of the models exactly predicts the strain improvement ratio for all ranges of LSC. The results also show that an increase in compressive strength of concrete causes a decrease in both stress improvement ratio and strain improvement ratio. The analysis of variance (ANOVA) was applied to study the percentage effect of the key criteria to the stress improvement ratio, strain improvement ratio, and the strain factor of LSC, where the results reveal that the compressive strength is a more effective criterion on the stress and strain improvement ratios and the strain factor than the confinement level (number of GFRP layers). Finally, lateral strain-axial strain behavior of GFRP and actively confined concrete is compared and it is understood that lateral strain- axial strain behavior of confined specimens is affected by lateral pressure and unconfined concrete strength.

Experimental and analytical behaviour of sandwich composites with glass fiber-reinforced polymer facings and layered fiber mat cores

This paper presents the results of experimental and analytical studies on the behaviour of sandwich beams fabricated with layered cores and glass fiber-reinforced polymer (GFRP) composite facings. The GFRP facings were fabricated using a unidirectional fiberglass fabric and epoxy resin, and the cores were fabricated using a thin non-woven continuous-strand polyester fiber mat with a thickness of 4.1 mm. A total of 30 sandwich beams with the width of 50 mm were prepared tested with five varying core configurations including cores made with one, two, or three layers of the fiber mat core and with or without the inclusion of intermediate GFRP layers. The specimens were tested up to failure under four-point bending at two different spans to characterize flexural and shear properties of the specimens. Two types of failure were observed, namely crushing of the compression facesheet and core shear. The load-deflection, load-strain, and moment-curvature behaviour were analyzed and using the results the flexural stiffness, shear stiffness, and core shear modulus were calculated. An analytical model was also developed to predict load-deflection behaviour and failure loading of sandwich specimens with varying core layouts. After verification, the analytical model was used for a parametric study of cases not considered in the experimental study, including additional GFRP and fiber mat core layers. It was shown that additional fiber mat core layers and the inclusion of intermediate GFRP layers can increase the strength and overall stiffness of a sandwich beam, while additional GFRP layers can only increase the overall stiffness of the system. The analytical model can be used to optimize the configuration of layered sandwich composites for cost effective rehabilitation techniques of culverts, pipelines, and other curved-shape structures where a thin, flexible core is needed to accommodate the curvature of the existing structure.