Fiber Reinforced Polymer Strengthening System Research Articles

Static and multi-impact performances of RC beams strengthened with large rupture-strain FRP composites

To solve the problem of inadequate plastic deformation capacity and energy dissipation of traditional fiber-reinforced polymer (FRP) strengthened structures under impact loading, this study proposes the use of novel large-rupture-strain fiber-reinforced polymer (LRS-FRP) materials combined with different strengthening schemes to improve the impact resistance of existing structures. This study conducted three-point bending and drop hammer tests to investigate the static and dynamic behaviors of flexure-dominated LRS-FRP-strengthened beams. Experiment results showed that the end-anchored FRP strengthening system at the beam soffit significantly improved the load capacity of beams under static loading and reduced the maximum mid-span deflection under impact loading, and the ductility factor of LRS-FRP strengthened beams under static loading was 2.7 times higher than that of the carbon-FRP (CFRP) counterpart. Specimens strengthened with a mid-span LRS-FRP wrapping scheme presented improved ductility and multi-impact performance. Specimens strengthened with a combined scheme enhanced the static load capacity and ductility of by 71.9 % and 159.7 %, respectively, compared to the unstrengthened beam, and exhibited the best energy dissipation capacity (23.8 kJ) under multiple impact loads. These findings from the test investigations can facilitate design guidance and evaluation methods for strengthening schemes for RC structures under impact loading.

A Review of Fibre Reinforced Polymer Structures

This paper reviews Fibre Reinforced Polymer (FRP) composites in Civil Engineering applications. Three FRP types are used in Structural Engineering: FRP profiles for new construction, FRP rebars and FRP strengthening systems. Basic materials (fibres and resins), manufacturing processes and material properties are discussed. The focus of the paper is on all-FRP new-build structures and their joints. All-FRP structures use pultruded FRP profiles. Their connections and joints use bolting, bonding or a combination of both. For plate-to-pate connections, effects of geometry, fibre direction, type and rate of loading, bolt torque and bolt hole clearance, and washers on failure modes and strength are reviewed. FRP beam-columns joints are also reviewed. The joints are divided into five categories: web cleated, web and flange cleated, high strength, plate bolted and box profile joints. The effect of both static and cyclic loading on joints is studied. The joints’ failure modes are also discussed.

Flexural behavior of RC beams externally bonded with polyethylene terephthalate (PET) fiber reinforced polymer (FRP) laminates

This paper presents an experimental study on the flexural behavior of reinforced concrete (RC) beams externally strengthened with a unique and sustainable type of fiber reinforced polymer (FRP), named polyethylene terephthalate (PET). Compared to conventional FRPs, PET-FRP is characterized by its large rupture strain and low elastic modulus. The flexural behavior of RC beams strengthened with one and two layers of PET-FRP was compared to that strengthened with one carbon FRP (CFRP) layer and an unstrengthened beam specimen. Test results showed that the specimens strengthened with two PET-FRP layers and one CFRP layer displayed equal and significant strength enhancement of 47% compared to the unstrengthened beam. However, the former exhibited higher ductility than the latter by 33%. Lower strength enhancement was depicted in the specimen strengthened with one PET-FRP layer. Despite that, the ductility of the PET-strengthened beam was substantial, which even outweighed that of the unstrengthened beam by 9%. The results of this study showed that PET-FRP is a promising strengthening material that improves structural performance and provides a sustainable alternative to conventional FRP strengthening systems.

Analytical study on the effectiveness of hybrid FRP strengthening on behaviour of slender reinforced concrete square columns

Strengthening of reinforced concrete (RC) columns using external bonded (EB) fibre-reinforced polymer (FRP) composites has become popular since last three decades. The effectiveness of EB confinement reduces for slender columns due to the secondary moment arising from applied axial loads. The effectiveness of the hybrid FRP strengthening system for enhancing the performance of RC slender square columns is analytically investigated. Hybrid FRP strengthening uses a combination of EB FRP wrapping for confinement and longitudinal near-surface mounted (NSM) FRP laminates for bending. Hybrid FRP strengthening aims to reduce the second-order lateral displacement of the slender RC column and increase its axial capacity and flexural stiffness. A numerical modelling approach is presented to capture second-order effects in hybrid FRP strengthened slender RC square column based on Newmark's central difference scheme. Both material and geometric nonlinearities are accounted for in the calculations. The predictions are validated with test results available in the literature. After validation, a parametric study is carried out to evaluate the influence of slenderness ratio, the number of EB FRP layers, NSM laminate ratio, modulus of NSM laminate, and buckling of FRP composites. Results show that hybrid FRP strengthening is effective in improving the performance of slender RC columns.

Modeling Strategies of Finite Element Simulation of Reinforced Concrete Beams Strengthened with FRP: A Review

Fiber-reinforced polymer (FRP) composites do not only possess superior mechanical properties, but can also be easy to tailor, install, and maintain. As such, FRPs offer novel and attractive solutions to facilitate strengthening and/or retrofitting of aging, weakened, and upgraded structures. Despite the availability of general code provisions, the design and analysis of FRP-strengthened concrete structures is both tedious and complex—especially in scenarios associated with unique loading conditions. As such, designers often leverage advanced finite element (FE) simulation as a mean to understand and predict the performance of FRP-strengthened structures. In order to narrow this knowledge gap, this paper details suitable strategy for developing and carrying out advanced FE simulations on FRP-strengthened concrete structures. The paper also covers techniques related to simulating adhesives (bonding agents), material constitutive properties and plasticity (cracking/crushing of concrete, yielding of steel reinforcement, and delamination of FRP laminates), as well as different material types of FRP (CFRP, GFRP, and their hybrid combinations), and FRP strengthening systems (sheets, plates, NSM, and rods) under various loading conditions including ambient, earthquake, and fire. The principles, thumb rules, and findings of this work can be of interest to researchers, practitioners, and students.

Modified Epoxy for Fibre Reinforced Polymer Strengthening of Concrete Structures

Fibre-Reinforced Polymer (FRP) is a preferable material for repairing concrete structure due to excellent material properties and effective installation cost over the long-term maintenance of structures. The successful application of FRP strengthening system very much depends on the bond between the concrete substrate and the FRP material using epoxy adhesive. Epoxy acts as a bridge to transfer stress from the concrete to the FRP material. The use of wet lay-up technique to apply FRP onto concrete structure requires epoxy to undergo a curing process normally referred to as cold curing. This paper intends to give a review of the problems with cold-cured epoxy and its effect on structural performance. Cured epoxy is characterised as brittle; therefore, modifications of epoxy are required to toughen the epoxy to suit the purpose of repairing a concrete structure. The methodological approaches from previous studies on modified epoxy were collected and reviewed in this paper. This review also offers some important insights regarding the use of sustainable materials, as well as recommendations for new epoxy in the future.

DEVELOPMENT OF BONDING AGENT USED IN FRP STRENGTHENING SYSTEM

Fiber Reinforced Polymer (FRP) composite is an attractive material for strengthening various civil infrastructures. The strengthening system that based on FRP composites consists of many elements. These are: strengthening material (FRP), strengthened element (concrete, steel, wood, etc) and bonding agent. The latter is the most important elements which affect the performance of the whole strengthening system. Different types of bonding agent are used based on service conditions. Epoxy-based adhesive (EBA) is the most commonly used to strengthen concrete or steel structures. Due to the limitations of the application of EBA, cement-based adhesive (CBA) is utilized as an alternative bonding agent. The CBA have been developed by modifying its properties. The modification was conducted using different powder-state materials or by magnetic water treatment. The development of bonding agent that used in FRP strengthening system is addressed in this paper in order to highlight the most modern bonding agent which consider as a promising material to employ in the actual outdoor structures.

Environmentally-induced loss of performance in FRP strengthening systems bonded to full-scale masonry structures

The issue of building materials deterioration due to environmental actions, which are expected to worsen in the future according to climate change predictions, is recognized as high-priority by the scientific community. However, the impact of material deterioration on the structural and seismic performances of masonry and historical structures strengthened with composites has not been investigated in detail, even though potentially significant. This paper aims to evaluate the environmentally-induced loss of performance in fiber reinforced polymer (FRP) strengthening systems bonded to masonry structures. An efficient damaging block-based modelling strategy, recently developed by the authors, is used to model masonry. FRP strips are originally introduced in the model and bonded to masonry blocks through a contact-based formulation with cohesion. This novel numerical approach is validated through FRP-strengthened masonry wall benchmarks. Numerical analyses are performed on FRP-strengthened in- and out-of-plane loaded masonry full-scale walls and houses with non-degraded material properties to predict the structural response at design conditions. Then, degraded material properties are deduced from accelerated ageing laboratory tests which investigated environmental degradation in FRP-strengthened masonry and are adopted in the same numerical analysis framework to predict the structural response at degraded conditions. Therefore, the environmentally-induced loss of performance in FRP strengthening systems bonded to masonry structures is deduced by comparing the structural response at degraded conditions with the one at design conditions.

Bond performance of NSM FRP bars in concrete with an innovative additional ribs anchorage system: An experimental study

Abstract Bond failure due to a lack of anchorage capacity is often observed when using FRP reinforcement as a strengthening method in RC structures, thus restricting the efficiency in upgrading their structural performance. To address this issue, our research group has recently developed a simple and innovative additional ribs anchorage system for FRP bars, which has been verified to have the high efficiency of improving the anchorage performance of FRP bars reinforced in concrete. As a promising strengthening method in RC structures, the near-surface mounted (NSM) fiber-reinforced polymer (FRP) technique has become popular. However, bond failure also exists in the NSM FRP strengthening system. It is also important to initiate a study on the bond performance of NSM FRP bars which are mounted in concrete using the proposed additional ribs. In this study, a total of 36 direct pull-out specimens and 12 beam pull-out specimens were employed. In particular, the influences of additional ribs on the bond behavior of FRP bars mounted in concrete in terms of the failure modes, local bond stress distribution, and load-slip curves were discussed. Several important parameters expected to affect the bond performance were investigated, namely, the type and the bonded length of the FRP bars, groove size and section configuration, and adhesive species. The test results and corresponding analysis in this paper demonstrated the feasibility and significant efficiency of using the proposed additional ribs anchorage system to enhance the bond performance of NSM FRP bars in concrete.

An approach of steel plate hybrid bonding technique to externally bonded fibre-reinforced polymer strengthening system

The strengthening efficiency of externally bonded fibre-reinforced polymer to concrete structure is usually limited owing to the unexpected debonding of fibre-reinforced polymer laminates. In this study, a new steel plate hybrid bonding technique was developed to supply additional anchorage for traditional externally bonded fibre-reinforced polymer strengthening system. With this approach, the fibre-reinforced polymer debonding can be effectively prevented. Moreover, the stress concentration, which probably results in a premature fracture of fibre-reinforced polymer laminates as that performed for available hybrid bonding anchorage techniques, can be eliminated by introducing a steel plate between the mechanical fasteners and fibre-reinforced polymer strips. To verify the effect of this new method, 21 carbon fibre–reinforced polymer–strengthened beams were studied on the flexural behaviours. Test results showed that, compared to available hybrid bonding anchorage techniques, steel plate hybrid bonding is more capable of making the full use of fibre-reinforced polymer laminates and further enhance the ultimate capacity and ductility of externally bonded fibre-reinforced polymer–strengthened beams. Based on the experimental results, the effect of interfacial treatment, ply of carbon fibre–reinforced polymer and mechanical fastener spacing on the failure mode and ultimate load ratio were discussed. Eventually, a simplified analytical procedure was proposed and verified to estimate the flexural resistance of steel plate hybrid bonding – fibre-reinforced polymer–strengthened beam.

Assessing the Contribution of the CFRP Strip of Bearing the Applied Load Using Near-Surface Mounted Strengthening Technique with Innovative High-Strength Self-Compacting Cementitious Adhesive (IHSSC-CA).

Efficient transfer of load between concrete substrate and fibre reinforced polymer (FRP) by the bonding agent is the key factor in any FRP strengthening system. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) was recently developed by the authors and has been used in a number of studies. Graphene oxide and cementitious materials are used to synthesise the new adhesive. The successful implementation of IHSSC-CA significantly increases carbon FRP (CFRP) strip utilization and the load-bearing capacity of the near-surface mounted (NSM) CFRP strengthening system. A number of tests were used to inspect the interfacial zone in the bonding area of NSM CFRP strips, including physical examination, pore structure analysis, and three-dimensional laser profilometery analysis. It was deduced from the physical inspection of NSM CFRP specimens made with IHSSC-CA that a smooth surface for load transfer was found in the CFRP strip without stress concentrations in some local regions. A smooth surface of the adhesive layer is very important for preventing localized brittle failure in the concrete. The pore structure analysis also confirmed that IHSSC-CA has better composite action between NSM CFRP strips and concrete substrate than other adhesives, resulting in the NSM CFRP specimens made with IHSSC-CA sustaining a greater load. Finally, the results of three-dimensional laser profilometery revealed a greater degree of roughness and less deformation on the surface of the CFRP strip when IHSSC-CA was used compared to other adhesives.

Assessment of bond strength of NSM CFRP strips embedded in concrete using cementitious adhesive made with graphene oxide

Efficient transfer of load between concrete substrate and fibre reinforced polymer (FRP) by the bonding agent is the key factor in any FRP strengthening system. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) was recently developed by the authors and has been used in a number of studies. Graphene oxide and cementitious materials are used to synthesise the new adhesive. The successful implementation of IHSSC-CA significantly increases carbon FRP (CFRP) strip utilization and the load-bearing capacity of the near-surface mounted (NSM) CFRP strengthening system. A number of tests were used to inspect the interfacial zone in the bonding area of NSM CFRP strips, including physical examination, pore structure analysis, and three-dimensional laser profilometery analysis. It was deduced from the physical inspection of NSM CFRP specimens made with IHSSC-CA that a smooth surface for load transfer was found in the CFRP strip without stress concentrations in some local regions. A smooth surface of the adhesive layer is very important for preventing localized brittle failure in the concrete. The pore structure analysis also confirmed that IHSSC-CA has better composite action between NSM CFRP strips and concrete substrate than the other adhesives, resulting in the NSM CFRP specimens made with IHSSC-CA sustaining a greater load. Finally, the results of three-dimensional laser profilometery revealed a greater degree of roughness and less deformation on the surface of the CFRP strip when IHSSC-CA was used, compared to other adhesives.

Destructive <i>In Situ</i> Tests on Masonry Arches Strengthened with FRCM Composite Materials

In the last few decades, fiber reinforced polymer (FRP) composites have been widely employed in several strengthening and rehabilitation applications of existing masonry buildings. Fiber reinforced cementitious matrix (FRCM) composites are a newly-developed strengthening technique comprised of high strength fibers embedded in a cementitious matrix. FRCMs usually offer several advantages such as the high resistance to fire and high temperatures or vapor permeability with masonry substrate, therefore they appear to be a promising alternative to traditional FRP strengthening systems. In this experimental work, the results of destructive in situ tests performed on existing masonry arches strengthened with FRCM composites are reported. FRCM strips consist of a balanced bi-axial mesh made of basalt fibers embedded in a cementitious grout. Three different configurations of the strengthening system have been considered. Load responses and failure modes are presented.

Fibre-reinforced polymer strengthening and fibre Bragg grating–based monitoring of reinforced concrete cantilever slabs with insufficient anchorage length of steel bars

Reinforced concrete cantilever slabs are among structures that are most likely to develop structural integrity problems, as they are statically determinate and often exposed to the outdoor environment. This article presents an experimental study on the strengthening of reinforced concrete cantilever slabs where the anchorage of the top steel reinforcing bars into the adjacent wall was insufficient. The experimental study involved the use of a fibre-reinforced polymer strengthening system and fibre Bragg grating sensors for strain monitoring. The fibre-reinforced polymer strengthening system consisted of glass fibre–reinforced polymer sheets and glass fibre–reinforced polymer spike anchors which connected the glass fibre–reinforced polymer sheets to the adjacent concrete wall. The test results showed that the fibre-reinforced polymer strengthening system was effective in improving the load-carrying capacity of reinforced concrete cantilever slabs and the fibre Bragg grating sensors worked efficiently and reliably for strain monitoring. The debonding in glass fibre–reinforced polymer sheet/glass fibre–reinforced polymer anchor-to-concrete bonded joints was found to be a progressive process associated with an increasing load. The fibre-reinforced polymer strengthening system examined in this study is thus a potential ductile solution for deficient cantilever slabs.

Bond behaviour between NSM CFRP strips and concrete at high temperature using innovative high-strength self-compacting cementitious adhesive (IHSSC-CA) made with graphene oxide

Strengthening of reinforced concrete (RC) structures using the near-surface mounted (NSM) fibre reinforced polymer (FRP) method is becoming an attractive technique for upgrading the existing structural elements. The exposure of the NSM FRP strengthening system to high temperatures greatly affects the bond between FRP, adhesive material and the concrete substrate. Organic adhesives are widely used in the NSM strengthening technique. However, their low fire resistance is a major drawback. This study investigates the performance of NSM carbon FRP strengthening technique using single-lap shear tests (pull-out tests) with innovative high-strength self-compacting cementitious adhesive (IHSSC-CA) under high temperatures. Graphene oxide and cementitious materials were used to synthesise the IHSSC-CA. Single-lab shear tests were conducted on NSM strengthened specimens which were exposed to high temperatures of 800°C. The results obtained from the pull-out tests signifies that the samples made with IHSSC-CA were less affected by high temperature, and maintain considerable residual bond strength. Theoretical equations are proposed to simulate the effect of high temperature on bond strength.

Influence of plate length and anchor position on FRP-to-concrete joints anchored with FRP anchors

This paper presents the details of an experimental and analytical study that addresses the influence of fibre-reinforced polymer (FRP) anchors upon the strength and behaviour of FRP-to-concrete bonded interfaces. Single-shear joints are utilised to represent the bonded interface. The primary variables under consideration are (i) the influence of anchor position in relation to the loaded end of the joint, lanc, (ii) the length of plate between the anchor and the unloaded plate end, lend, and (iii) total length of plate, lfrp. These variables, which have received limited direct attention to date by the research community, are important parameters for understanding the influence of concrete cracking upon the behaviour of the FRP strengthening systems containing anchorage devices. The results of forty-one anchored joints and two unanchored control joints are presented and it is shown that lend is more influential than lanc. Details of a partial interaction model are also presented and the model is shown to adequately replicate the load, slip and strain distribution responses of a selected test joint.

Comparison between TRM and FRP strengthening systems at preventing buckling failure of brick masonry walls

Fibre Reinforced Polymer (FRP) and Textile Reinforced Mortar (TRM) have been studied, compared and applied to strengthen brick masonry walls. The comparison of their performance against second order bending effects is addressed in this paper for the first time. Experimental and analytical data from previous researches and new analytical data for TRM cases are summarised, ordered and systematically compared to analyse the structural response of strengthened brick masonry walls. The results show a similar performance for both systems in terms of load bearing capacity and in-plane response. However, TRM strengthened cases showed greater lateral deformation than FRP ones.

Fire resistance design of un-protected FRP-strengthened RC beams

In the strengthening of reinforced concrete (RC) structures with fiber-reinforced polymer (FRP) composites, fire resistance design is an important issue. If fire insulation is not provided, the fire resistance of an FRP-strengthened RC beam can be approximated as that of an equivalent bare RC beam with a much higher load ratio as the mechanical resistance of the unprotected FRP strengthening system disappears within minutes of fire initiation. Due to this difference in load ratio, existing fire resistance design methods for conventional RC beams cannot be used in the fire resistance design of these equivalent bare RC beams. This paper aims to address this deficiency in existing research by presenting a reliable design method for the fire resistance of bare RC beams that provides close predictions for a wide range of load ratios, particularly those of high load ratios. Numerical results obtained using an advanced finite element approach developed by the authors are presented to examine the effects of various parameters on the fire resistance of bare RC beams, including the load ratio, the concrete cover depth, the reinforcement ratio of tension steel rebars, the distribution ratio of tension steel rebars, the cross-sectional dimensions, and the aggregate type of concrete. A design equation for predicting the fire resistance period of bare RC beams under standard fire exposure, formulated on the basis of the numerical results, is proposed. It is demonstrated that the proposed equation provides reasonably close fire resistance predictions for bare RC beams with a wide range of load ratios.

The effect of shear strength on load capacity of FRP strengthened beams with recycled concrete aggregate

Shear failure of reinforced concrete members is brittle and catastrophic, and should be avoided. This work presents a comparative study of concrete beams with natural crushed stone and those incorporating recycled concrete aggregate (RCA). The study indicates that the equation presented in ACI 318-14 Code requirements for predicting the shear strength of concrete is less conservative for RCA concrete beams. The presented experimental investigations show that strengthening with fiber reinforced polymer (FRP) fabrics can be designed so that the shear capacity of the beams with weaker (RCA) concrete is higher than that of the control beams with natural aggregate concrete. The design of the FRP strengthening system and the reasons for the effectiveness of the design are explained.

Fire behaviour of FRP-strengthened reinforced concrete structural elements: A state-of-the-art review

The last two decades have seen increasing applications of fibre reinforced polymer (FRP) materials in civil engineering structures due to their many advantages over traditional strengthening and reinforcing materials. Among the most common applications is bonding or wrapping FRP products (strips or sheets) to the exterior of reinforced concrete (RC) members to increase their strength or deformability. However, widespread application of FRP strengthening systems in buildings, where structural fire ratings are required, is hindered due to unknowns surrounding the reduction in their mechanical and bond properties at elevated temperatures. This paper presents a state-of-the-art review on the fire performance of FRP-strengthened RC structural elements. The review addresses first the mechanical behaviour at high temperature of the constituent materials of FRPs and how their bond to concrete is affected when heated. The paper then discusses available experimental and numerical studies on the fire behaviour of FRP-strengthened RC beams, slabs, and columns. Available design guidance is also discussed. Finally, recommendations for future research are given.