Fiber-reinforced Polymer Reinforcement Research Articles

Moment Capacity Charts for Strengthening of RC Beams With Near Surface Mounted FRP Reinforcement

Near surface- mounted fibre reinforced polymer is a new technique for strengthening
 structural members beside externally applied fibre reinforced polymer (FRP) technology.
 In NSM technique, the FRP material is applied to the tension region of the member inside
 pre-prepared grooves. In externally applied FRP the debonding of the fibres is a major
 issue thus, the full utilisation of FRP material can not be achieved, and the fibres are more
 susceptible to environmental effects. With NSM FRP, these issues can be controlled more
 effectively. ACI Committee 440 [1] has presented guidelines for design and construction
 of externally FRP method for strengthening of concrete structures. Although many
 experiments have been conducted on NSM FRP in flexure strengthening of RC beams,
 no code is introduced yet. Theoretical studies to derive equations for calculating moment
 capacity of strengthened beams have been introduced but mostly require several trial
 procedures to reach the required effective strain for FRP. In this study, a moment
 capacity chart is introduced based on several assumptions in the derivation of flexure
 capacity equation. To avoid the brittle behaviour of FRP, it is suggested that the
 utilization of FRP to be limited to 70% of its ultimate capacity for CFRP, and an
 equivalent area of reinforcement is adopted for simplicity. Experimental results of other
 researchers have been verified with the proposed charts, and a good comparison of results
 have being achieved.

Theoretical solution to fatigue bond stress distribution of NSM FRP reinforcement in concrete

Abstract This study presents an innovative theoretical model for bond stress distribution under fatigue loading of reinforced concrete structures strengthened with near-surface mounted (NSM) fiber reinforced polymer (FRP) reinforcement. The model has a unique ability in predicting the bond stress between concrete and NSM FRP reinforcement under fatigue loading by considering the bond heterogeneity due to fatigue damage. A bilinear bond-slip relation, describing the pre- and post-crack bond behavior, is introduced to model the FRP-concrete interface as a cohesive zone. Theoretical solutions on bond stress distribution are derived for regions in front of and behind the crack tip. During the fatigue loading, the propagation of crack length (i.e., debonded length) is represented by a cohesive model that is based on Paris law. One-dimensional mesh is then used to account for different levels of bond degradation along the embedded length and appropriate boundary conditions are applied to obtain the closed-form solutions. To verify the accuracy of the model, test results from an existing experiment are used, which shows satisfactory predictions. The parametric study conducted indicates that high bond strength and axial stiffness can slow down the growth of the debonded length.

Numerical investigation of parameters influencing debonding of FRP sheets in shear-strengthened beams

An effective technique for strengthening of concrete members is to provide extra reinforcements in the form of externally bonded fibre-reinforced polymer (FRP) sheets. Nonetheless, debonding often prevents strengthened beams from gaining their full load capacity and experimental studies have confirmed this. The current study proposes a non-linear three-dimensional (3D) finite element model to look into debonding failure of FRP sheets and its consequential effects in various strengthening schemes such as side-bonded and U-wrap (i.e. continuous FRP or discontinuous strips). 3D elements with linear elastic orthotropic constitutive relation was assumed for the FRP sheet and appropriate interface elements are used to simulate bond–slip behaviour between concrete, internal steel reinforcements and external FRP reinforcement. Once the model was validated against published experimental results, a parametric study was conducted to research the influence of some parameters on the shear capacity and failure mode, with a particular emphasis on the interfacial behaviour and debonding phenomena. The results show the significance of each parameter as well as FRP configuration on failure of retrofitted beams.

Shear Behavior of Circular Concrete Members Reinforced with GFRP Bars and Spirals at Shear Span-to-Depth Ratios between 1.5 and 3.0

In the last decade, the shear strength of concrete members with rectangular cross sections reinforced with fiber-reinforced polymers (FRPs) has received considerable attention. Yet no research seems to have investigated circular concrete members reinforced with FRP reinforcement under shear loads. This paper presents the results of an investigation of the shear strength and behavior of six circular concrete specimens reinforced with glass-FRP (GFRP) bars and spirals. The specimens, which measured 3,000 mm in length by 500 mm in diameter, were tested under four-point bending. The test parameters included the shear span-to-depth ratio (a/d) ranging from 1.5 to 3.0 and the GFRP spiral reinforcement ratio with different spiral spacings (100, 150, and 200 mm) and spiral diameters (13 and 15 mm). As designed, the specimens failed in shear due to GFRP spiral rupture or flexural-shear failure for the specimens with a/d>2.5 and strut crushing combined with spiral rupture for the specimens with a/d<2.5. The experimental results were compared to the current sectional models and the strut-and-tie model in codes and design guidelines as well as to the available analytical approach, which is based on the modified compression field theory. The comparison indicates that the shear capacity of FRP-reinforced concrete members with circular cross sections may be determined with the shear design provisions developed for rectangular sections within a variable degree of conservativeness.

Bond mechanism and bond strength of GFRP bars to concrete: A review

Abstract Glass fiber-reinforced polymer (GFRP) reinforcements are taken as an alternative solution for the deterioration of civil infrastructures. GFRP bars have received increasing attention due to low cost compared to carbon fiber-reinforced polymer (CFRP) bars. Bond characteristic of GFRP bars in concrete is the most critical parameter for implementation of the material to the corrosion-free concrete structures. Unlike steel reinforcement, GFRP materials behave anisotropic, non-homogeneous and linear elastic properties, which may result in different force transfer mechanism between reinforcement and concrete. With the purpose of covering the most valuable contributions regarding bond mechanism in the past work, a comprehensive review focusing on the failure mode and bond strength is carried out in this paper. A database consisted of 682 pullout-test specimens was created to observe the factors affecting bond behavior. Basic relationship between bond strength/slip and factors was analyzed accordingly. In addition, the development of bond degradation under environmental conditions, such as freezing-thawing cycling, wet-dry cycling, alkaline solutions and high temperature was presented thereafter. These environmental influences need to be further investigated.

Behavior and Design Aspects of FRP-Strengthened URM Walls under Out-of-Plane Loading

AbstractThe use of externally bonded fiber-reinforced polymer (FRP) composites for upgrading the out-of-plane flexural resistance of unreinforced masonry (URM) walls is experimentally and analytically investigated in this study. A total of six hollow concrete block walls were tested to failure using an airbag and a reaction frame to obtain a uniform load on the wall. The masonry walls were placed horizontally and tested as one-way slabs with span direction perpendicular to the bed joints. The first wall was left unstrengthened to be used as control specimen; the other five walls were strengthened using different schemes of externally attached glass-fiber-reinforced polymer (GFRP) sheets. The main parameters studied experimentally were FRP reinforcement ratio and stiffness. In addition to the experimental program, an analytical model was developed to predict the ultimate moment capacity of the URM walls. The procedure outlined in standard guidelines was also utilized to compute the flexural capacity of wal...

Analysis of the fatigue behavior of reinforced concrete beams strengthened in flexure with fiber reinforced polymer laminates

Abstract This paper presents an analytical study on the fatigue response of reinforced concrete beams strengthened with fiber reinforced polymers (FRP). The study gathers the experimental results from the international literature and from authors' experiments and investigates the critical parameters that affect fatigue performance of reinforced concrete (RC) beams. A new analytical model is proposed for predicting the fatigue life of FRP strengthened RC beams. The parameters used are the maximum stress of tensile steel (σ max ) to the yielding strength (f y ) ratio, as well as the axial rigidity of longitudinal steel (k s ) and FRP (k f ) reinforcement. The predictions of the proposed model are compared against the experimental results as well as against the predictions of fatigue models in the literature.

Behavior of Prestressed Hollow-Core Slabs Strengthened in Flexure with Near-Surface Mounted Carbon Fiber-Reinforced Polymer Reinforcement

AbstractFlexural strengthening of reinforced concrete using near-surface mounted (NSM) fiber-reinforced polymer (FRP) composites is a widely studied topic, and a promising technology. However, overlooked applications such as prestressed concrete and one-way slabs require further experimental investigation and theoretical refinement in order to develop comprehensive design guidelines. This paper investigates the behavior of prestressed hollow-core slabs strengthened in flexure with NSM carbon FRP strip reinforcement. Seven full-scale simply supported slab specimens were tested to failure under two-point monotonic load configuration. Experimental variables included two different internal prestressing reinforcement ratios, and three different NSM strengthening reinforcement ratios. This paper examines cracking behavior and modes of failure, load-strain relationships, deflections, and strengthened capacities. Also, a comparison between experimental results and theoretical predictions by relevant Canadian and ...

Shake Table Tests on FRP Retrofitted Masonry Building Models

AbstractThe current study is devised to evaluate the seismic performance of fiber-reinforced polymer (FRP) retrofitted buildings with openings at different FRP reinforcement levels. Required objectives are achieved by performing five shake table tests on 1/4-scale models of single-story boxlike masonry buildings. Out of five building models, one was an unreinforced masonry (URM) building model whereas the other four were retrofitted with different quantities and layouts of FRP reinforcement. FRP reinforcement is reduced by decreasing FRP strip widths and strip spacing and applying FRP strips on either the inside or outer faces of walls. Each building model was subjected to the same series of input ground motions with gradually increasing amplitudes. A series of diagonal compression tests was also conducted to decide the appropriate type of FRP and trial FRP reinforcement ratios for the masonry building models. Descriptions of the tested models are provided, including the structural response in terms of ba...

Seismic response of PP-band and FRP retrofitted house models under shake table testing

Current research work presents the seismic response of brick masonry houses retrofitted by different composite materials under dynamic shake table tests. Tests were performed on 1:4 scaled unreinforced masonry (URM), Polypropylene (PP-band) retrofitted, Glass Fiber Reinforced Polymer (FRP) retrofitted and FRP+PP-band retrofitted house models. A minimum reinforcement ratio of FRP was selected to use with PP-band as FRP+PP-band composite. Results show that URM is highly brittle and can hardly withstand an ordinary ground motion, whereas PP-band retrofitting has increased the ability of URM houses to withstand a severe ground motion with intense cracking and separation of wall segments. However, despite no collapses, PP-band retrofitted house model had lost its serviceability when subjected to strong input ground motion. Unlike PP-band, FRP retrofitted houses were collapsed suddenly without giving any warning with respect to ductility and energy dissipation. The sudden failure of FRP houses is attributed to very low FRP reinforcement ratio (0.0006), as it is an expensive material and its sole use in higher amounts would result in significant increase of retrofitting costs. Results show that a potential use of FRP in combination with PP-band is a good alternative as it significantly enhances the shear capacity, energy dissipation as well as the ductility of URM masonry structure at a very low retrofitting costs.

Size and shape effect in the pull-out of FRP reinforcement from concrete

Scaling phenomena in the progressive debonding between fiber reinforced polymer (FRP) strips and concrete substrate are examined on the basis of single lap shear tests, considering different bond lengths and widths. To capture a stable post-peak response of the FRP-concrete joint, the slip between the concrete support and the reinforcement strip, measured at the reinforcement end using a clip gage, was selected as feedback control signal during the tests. This allowed a quasi-static stable fracture propagation in the descending branch until complete load relaxation and debonding. Experimental tests were simulated by means of 2D and 3D finite element analyses adopting two different approaches. At first, a numerical model based on linear elastic materials with all non-linearities concentrated at the interface was assumed. It clearly revealed the inability to recognize all the aspects of the debonding process. Then, a different numerical model, based on a non-linear constitutive model for concrete and perfect adhesion between all the materials, was adopted.

Nonlinear 2D Finite-Element Modeling of RC Beams Strengthened with Prestressed NSM CFRP Reinforcement

AbstractThis paper presents a comprehensive two-dimensional (2D) nonlinear finite-element (FE) analysis of reinforced concrete (RC) beams strengthened with prestressed or non-prestressed near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) reinforcement. In this model, the bond-slip relationship between the steel reinforcement and surrounding concrete as well as that between the NSM CFRP reinforcement and concrete substrate are precisely modeled with cohesive interface elements. Based on the smeared crack approach, the tensile and shear behavior for concrete cracking are considered using appropriate models. The prestressing effect was enforced in the NSM CFRP reinforcement using the equivalent temperature method, and the anchoring effect of U-shaped steel end anchors on the NSM CFRP reinforcement is properly implemented in the model. Moreover, the radial stresses induced by the tensile steel bars on the surrounding concrete are properly considered, and the bar-end concrete cover separation fa...

Unified model for hollow columns externally confined by FRP

Nowadays, the employment of Fibre Reinforced Polymer (FRP) composites in civil engineering field has been successfully experienced. Different potential applications of solid concrete-filled FRP tubes are exploitable in marine piles, overhead sign structures, poles and posts, bridge columns and piers, girders, large pipes and tunnels (mainly circular cross-section). This technique is also used for the confinement of masonry columns, typically encountered in monuments and historical buildings (both rectangular and circular cross-section); hence the need of providing formulas for the design of an appropriate strengthening. Several analytical models are available in the scientific literature for assessing the increase of strength and ductility of concrete or stone solid elements externally confined with FRP or for hollow columns internally steal enclosed and externally FRP-confined but, there is still a lack of research about hollow columns only externally confined. The presence of an empty core implies a different stress state in the inner cylindrical surface with respect to the outer one. Inwards deformations are more significant and this behaviour is not taken into account by available models.The present study aims to illustrate a detailed summary of the existing analytical models and to provide a unified procedure for concrete and masonry hollow columns valid for both circular and square cross-sections. An iterative method that updates the geometrical parameters according to step-by-step uniaxial compressed column is shown in order to capture the real deforming behaviour of the compressed solid. This analytical approach has two important implications: the first is the ability of calculating the stress state of the column and of the external FRP reinforcement at each step; the second is that of theorizing a procedure, which is independent on the type of material used for the column. The outputs of the proposed method are then compared with experimental results currently available in the scientific literature. A good matching is obtained between the available experimental results and the analytical predictions in term of axial stress–strain curves.

Numerical and analytical modeling of concrete beams with steel, FRP and hybrid FRP-steel reinforcements

The paper presents the results obtained from a numerical and analytical analysis carried out on a set of concrete beams reinforced with steel bars, Fiber Reinforced Polymer (FRP) bars and hybrid combinations of FRP-steel bars. To this purpose a database of experimental results, available in literature, was collected.A simple and reliable two-dimensional Finite Element (FE) model was defined. In the numerical simulations, the linear and nonlinear behavior of all materials was adequately modeled by appropriate constitutive laws. To simulate the concrete post-cracking tensile behavior a specific tension stiffening model was used. In order to overcome convergence difficulties, to simulate the quasi-static response of RC beams, a dynamic approach was adopted.Furthermore, to assess the effectiveness of the current Italian guideline, on same set of RC beams, an analytical analysis was performed.The comparisons between numerical/analytical results and experimental data highlighted the reliability of both the proposed FE model and the analytical model.The results show that the tension stiffening model used in the FE analysis provides good results with low and normal reinforcement ratios, whereas the numerical predictions are not acceptable with high reinforcement ratios. The analytical results provided by the Italian guideline are satisfactory, compared to experimental data.

FRP Composites and their Using in the Construction of Bridges

Traditional material for concrete reinforcement is steel. In recent times, non-metallic fibres have been intensively investigated and some of them used for construction of bridges. FRP composites are durable materials which are different from the steel reinforcement for their resistance to the electrochemical corrosion. With respect to steel, different mechanical behavior of non-metallic reinforcement, however, involves some drawbacks - namely the lack of thermal compatibility between concrete and FRP reinforcement. FRP composites belong among anisotropic materials. Their properties depend on the type, volume and alignment of the fibres, the matrix type, form and quality of the construction. The analysis of causes of failures on German bridges shows that reinforcement corrosion initiated by chlorides makes up 2/3 of all the failures recorded in the bridge construction. Corrosion of the reinforcement initiated by chlorides is the main cause of the loss of serviceability of bridge structures. In such an environment, fibre-reinforced polymer (FRP) reinforcement can fully replace the traditional steel reinforcement. The paper presents the mechanical properties and durability of different types of the FRP rebar's and their use in construction of bridges.

Experimental Analysis of Dynamic Effects of FRP Reinforced Masonry Vaults

An increasing interest in the preservation of historic structures has produced a need for new methods for reinforcing curved masonry structures, such as arches and vaults. These structures are generally very ancient, have geometries and materials which are poorly defined and have been exposed to long-term historical movements and actions. Consequently, they are often in need of repair or reinforcement. This article presents the results of an experimental study carried out in the laboratory and during on-site testing to investigate the behaviour of brick masonry vaults under dynamic loading strengthened with FRPs (Fiber Reinforced Polymers). For the laboratory tests, the brick vaults were built with solid sanded clay bricks and weak mortar and were tested under dynamic loading. The experimental tests were designed to facilitate analysis of the dynamic behaviour of undamaged, damaged and reinforced vaulted structures. On-site tests were carried out on an earthquake-damaged thin brick vault of an 18th century aristocratic residence in the city of L’Aquila, Italy. The provision of FRP reinforcement is shown to re-establish elastic behavior previously compromised by time induced damage in the vaults.

Study on stiffness and recoverability of Fiber Reinforced Polymers-Reinforced Concrete (FRP-RC) composite T-beams with prefabricated Basalt fiber-reinforced polymers shell

This paper focuses on the stiffness and recoverability of a new type of composite beam, the Fiber Reinforced Polymers-Reinforced Concrete (FRP-RC) beam, in which steel bars are the main reinforcement. Ten pieces of beams under both static and cyclic bending load were conducted. Fiber-reinforced polymer profile made by the vacuum infusion molding process is used as reinforcement in the composite beams, in which basalt fiber-reinforced polymer sheets were axially laid at the bottom of the beams in order to improve bearing capacity, and hoop-directionally laid at the lateral side of the beams with the advantages of improving the shear resistance. A finite element model is proposed to predict nonlinear deflections and stresses. The flexural stiffness and recoverability index of this new type of composite beam are studied. The results show that the recoverability of the FRP-RC composite beam is better than that of the normal RC beam and it increases along with the fiber-reinforced polymers reinforcement ratio. Stiffness calculation formulas at various stages and residual deformation formulas are proposed. Finally, the calculation results and test results are compared, which shows that the formulas for stiffness and deformation demonstrate good precision.

Intermediate crack-induced debonding analysis for RC beams strengthened with FRP plates

This paper presents the analysis of intermediate crack-induced (IC) debonding failure loads for reinforced concrete (RC) beams strengthened with adhesively-bonded fiber-reinforced polymer (FRP) plates or sheets. The analysis consists of the energy release and simple ACI methods. In the energy release method, a fracture criterion is employed to predict the debonding loads. The interfacial fracture energy that indicates the resistance to debonding is related to the bond-slip relationships obtained from the shear test of FRP-toconcrete bonded joints. The section analysis that considers the effect of concrete‟s tension stiffening is employed to develop the moment-curvature relationships of the FRP-strengthened sections. In the ACI method, the onset of debonding is assumed when the FRP strain reaches the debonding strain limit. The tension stiffening effect is neglected in developing a moment-curvature relationship. For a comparison purpose, both methods are used to numerically investigate the effects of relevant parameters on the IC debonding failure loads. The results show that the debonding failure load generally increases as the concrete compressive strength, FRP reinforcement ratio, FRP elastic modulus and steel reinforcement ratio increase.

Implementation of Bond Durability in the Design of Flexural Members with Externally Bonded FRP

Externally bonded fiber-reinforced polymer (FRP) composites are becoming a method of choice for the repair/strengthening of civil structures, some of which are situated in environments detrimental to FRP-concrete bond durability. Based on analysis of an extensive database of durability test results from notched-beam three-point bending tests, a bond durability factor (BDF) of 0.60 was identified as an appropriate estimate of bond durability for wet-layup carbon fiber-reinforced polymer (CFRP). This factor was compared to a normalized database of durability data from the literature, and it was found that a 0.60 BDF agrees well with these data. A modified procedure for determining the ultimate design strain primarily in bond-critical applications (such as external flexural and shear FRP reinforcement), is proposed. This procedure makes a distinction between durability properties associated with the FRP rupture and debonding failure modes. When used for practically feasible strengthening ratios and in conjunction with BDF of 0.60, the proposed procedure yields overall ultimate flexural-member design strengths that are 0–15% lower than those determined using current design procedures.

Wood-based beams strengthened with FRP laminates: improved performance with pre-stressed systems

Using bonded fibre-reinforced polymer (FRP) laminates for strengthening wooden structural members has been shown to be an effective and economical method. In this paper, properties of suitable FRP materials, adhesives and two ways of strengthening beams exposed to bending moment are presented. Passive or slack reinforcement is one way of strengthening. The most effective way of such a strengthening was to place reinforcement laminates on both tension and compression side of the beam. However, the FRP material is only partially utilised. The second way is to apply pre-stressing in FRP materials prior to bonding to tension side of flexural members and this way was shown to provide the most effective utilisation of these materials. The state of the art of such a strengthening and various methods are discussed. Increasing the load-bearing capacity, introducing a pre-cambering effect and thus improving serviceability which often governs the design and reducing the amount of FRP reinforcement needed are some of the main advantages. A recent development on how to avoid the requirement for anchoring the laminates at the end of the beams to avoid premature debonding is shown, and the advantage of such a system is described.