FRP Sheets Research Articles

Effect of the FRP sheet's arrays and NSM FRP bars on in-plane behavior of URM walls

This paper studies the results of compression diagonal tests conducted on a series of retrofitted and non-retrofitted small-scale masonry walls (which are also known as wallettes). Wallettes with the same characteristics and mechanical properties of large masonry walls were retrofitted by using two arrays of glass fiber reinforcement polymer (GFRP) sheets (grid and diagonal), two arrays of carbon fiber reinforcement polymer (CFRP) sheets (grid and diagonal), and near surface mounted bars (steel and GFRP). FRP sheets were applied to both surfaces of the wallettes, and rebar was mounted onto one of the surfaces in two horizontal and two vertical arrangements. All of the methods significantly increased loading capability and ductility of wallettes. Wallettes retrofitted with near surface GFRP bars had the highest value of final load, and specimens retrofitted with CFRP sheets in a diagonal array demonstrated the highest ductility among all tested specimens. Wallettes retrofitted with near surface mounted steel bars showed poor performance in terms of both loading capability and ductility. This study also includes discussion of failure modes, elastic characteristics, shear stresses and strains, and other variables that impact retrofitting.

Simulating Intermediate Crack Debonding on RC Beams Strengthened with Hybrid Methods

Abstract The externally bonded (EB) and the near-surface mounted (NSM) are two well-known methods for strengthening reinforced concrete (RC) beams. Both methods are unfortunately prone to fail prematurely through debonding when the amount of strengthening reinforcement provided is high. In response to this, a hybrid method that combines the EB and NSM method was introduced. The method allows the amount of reinforcement needed for EB and NSM methods to be reduced; this, in theory, should lower the interfacial stresses, thus reducing the possibility of debonding failures. While debonding failure can be prevented, certain amounts of debonding would still occur through the interfacial crack (IC) debonding mechanism which can affect the strength and stiffness of hybrid strengthened beams even if it does not directly cause failure. This paper presents a method to simulate IC debonding of hybrid strengthened beams using the moment-rotation approach. The proposed method allows a better prediction of maximum load and stiffness of the beams. The method is also less dependent on empirical formulations compared to the commonly used moment-curvature approach; this allows the method to be applicable to all material and shape of hybrid strengthening reinforcement, assuming correct material models are used. The proposed method was then used to perform parametric studies; among the important findings is the length of IC debonding tend to increase when FRP sheet with higher elastic modulus is used, thus negating most of the benefit from the higher modulus.

Experimental investigation on the debonding failure mode of basalt-based FRP sheets from concrete

In this paper, the debonding failure mode of basalt-based FRP (BFRP) reinforcements from concrete supports is experimentally investigated. In detail, more than 40 push-pull double shear tests on BFRP-concrete specimens with different geometrical configurations were carried out. Results in terms of debonding load and strain patterns arising at the BFRP layer are presented and discussed, furnishing indications about the influence of: composite thickness, width ratio between the BFRP width and the concrete one, anchor length. On the basis of the proposed experienced evidence, the design indications provided by the Italian Technical Document CNR DT200/R1 and addressing the assessment of the debonding load have been critically analysed. Accordingly and as a further result, a novel effective calibration of the corresponding empirical correction parameter kg (introduced to improve the previsional effectiveness of the fracture-mechanics-based theoretical estimate of the debonding force), has been provided. In detail, in order to ensure the best agreement between proposed experimental data and theoretical predictions associated to concrete elements externally strengthened by the in-situ application of BFRP sheets, the value kg=0.117 mm (5% fractiles 0.079) is specifically proposed, resulting very different from the indication by the Italian technical document (kg=0.077 mm, 5% fractiles 0.037 mm).

Effect of FRP Strengthening on the SHS brace collapse mechanism

During an earthquake diagonal braces are designed to dissipate energy by yielding in tension and buckling in compression. However, local buckling occurring in the middle of the brace leads to immediate fracture. With the aim of strengthening braces against local buckling, wrapping FRP sheets in transverse direction is proposed in this study. Hitherto, the effect of FRP strengthening on the post-buckling behaviour of Square Hollow Section (SHS) tubes has not been investigated. A numerical model was generated and verified by previous research. Then, a comprehensive parametric study was conducted and the effect of slenderness ratio, number of FRP layers and FRP coverage percentage on post-buckling response of strengthening brace was explored within the study. Results indicate that utilising FRP is certainly successful at mitigating local buckling mode of long SHS braces Moreover, for short braces, applying enough numbers of FRP layers can change the mode of buckling from local to overall. Finally, an optimized length of FRP was suggested for strengthening of braces in accordance with their slenderness ratio.

Modeling of Shear Behavior of Reinforced Concrete Beams with Corroded Stirrups Strengthened with FRP Sheets

AbstractBecause stirrups corrode earlier and faster than longitudinal steel bars, reinforced concrete (RC) structures may fail in shear instead of bending. Since shear failure is brittle and catast...

STRUCTURAL BEHAVIOR OF THE HYBRID COMBINATION OF CFRP-BFRP SHEETS IN RETROFITTING OF RC BEAMS BASED ON NONLINEAR FINITE ELEMENT ANALYSIS

The use of composite materials such as fiber-reinforced polymers in strengthening and repairing of reinforced concrete elements is widely spreading. However, for successful and costeffective applications, engineers must improve their knowledge with respect to the actual behavior of strengthened structures. The CFRP sheets used in strengthening applications have high strength; however, they are brittle materials with low ductility. Basalt fiber reinforced polymer (BFRP) sheets on the other hand have relatively lower strength compared to CFRP, however they have higher ductility and cheaper than carbon fibers. As a result, there is growing interest among researchers and practitioners in combining different types of FRP sheets to produce an enhanced strengthening system in terms of strength and ductility. This hybrid system is designed to enhance the properties of composites, where it combines the high strength of CFRP and high ductility of BFRP sheets, respectively. This paper presents the nonlinear finite element analysis (FEA) that has been carried out to simulate the behavior of failure modes of reinforced concrete (RC) beams strengthened in flexure using CFRP sheets, BFRP sheets, and their hybrid combination (CFRP-BFRP). Besides, new technical approach is presented numerically to show that concrete cover replacement with high strength concrete one provided better load capacity and failure mode, indicating utmost utilize of the hybrid combination of CFRP-BFRP in strengthening of RC beams. The commercial and general finite element analysis; ABAQUS software is used for modeling and nonlinear analysis. Load deflection relationships, failure mode, ductility, ultimate load and ultimate deflection were obtained and compared with the recent experimental results available in literature. From the analysis, it is found that FEA can predict accurately the load-displacement relation and good agreements were obtained when compared to the experimental data. In addition, the proposed FE analysis can be reliably used as a cost-effective tool to predict the inelastic behavior of strengthened beams.

Using Response Surface Methodology and providing a modified model using whale algorithm for estimating the compressive strength of columns confined with FRP sheets

This study presented a modified model for determining the compressive strength of square and rectangular concrete columns using whale algorithm and a widespread database. In this model, the effective strain coefficient of FRP and cross section coefficient considered as an individual coefficient. Furthermore, the coefficients of the rising strength for specimens with unconfined compressive strength in upper and lower of 35 MPa are differently considered. In addition, Response Surface Methodology used for estimating the compressive strength of the confined specimens by FRP. Finally, results compared with the existing models. The predictions of the modified model and RSM show satisfactory estimations. So that RSM and the modified model have increased R2 approximately 34% and 26% rather other models, respectively.

Pressure-sensitive bond fatigue model with damage evolution driven by cumulative slip: Thermodynamic formulation and applications to steel- and FRP-concrete bond

In this paper we introduce a thermodynamically consistent bond-interface pressure-sensitive damage model with cumulative sliding strain measure as a fundamental source of fatigue damage. The modeling approach provides a clear physical interpretation of the dissipative mechanisms governing the propagation of fatigue damage within the concrete-steel/FRP interface so that it is possible to reproduce both the monotonic and the cyclic behavior of the bond with a consistent set of material parameters. The model has been applied for simulation of the degradation process in the bond between concrete and reinforcement bars and between concrete and FRP sheets under pullout fatigue loading. The paper presents numerical studies of the fatigue pullout behavior with detailed analysis of the damage propagation during loading and unloading stages, and at different levels of imposed lateral pressure. To validate the ability of the model, simulations of pullout tests published in the literature have been conducted.

Performance of rubberised reinforced concrete members under cyclic loading

This paper presents an experimental investigation into the cyclic behaviour of reinforced concrete members incorporating a significant proportion of recycled rubber particles as a replacement for mineral aggregates. Tests were carried out on thirteen large scale members of circular cross-section, with and without external confinement, and with different proportions of rubber content and axial loads. The specimens were subjected to inelastic lateral cyclic displacements and predefined levels of co-existing axial loading. After describing the testing arrangement and specimen details, the main results and observations are provided and discussed. The test results enable a direct comparative assessment of the key response characteristics of the specimens, with focus on stiffness properties and strength interaction, as well as ductility and energy dissipation. It is shown that rubberised reinforced concrete members can offer a good balance between bending capacity and ductility in comparison with conventional reinforced concrete members, particularly for low levels of axial loads. In the presence of relatively high axial loading and when a significant proportion of rubber content is used, external confinement such as through FRP sheets as employed in this study, can be adopted to recover the required capacity and to provide highly stable hysteretic response. The implications of the findings on the use of rubberised reinforced concrete members in practice, and procedures that can be used to determine the main design parameters, are also highlighted within the discussions.

Behavior of FRP-strengthened RC elements subjected to pure shear

The shear behavior of fiber reinforced polymer strengthened reinforced concrete (FRP-strengthened RC) has been the focus of extensive research studies. However, the mechanism of this complex phenomenon has not been fully clarified. Recent analytical models which were developed for predicting the shear capacity of FRP strengthened RC girders were based on test results of simply supported beam specimens with various shear span-to-depth (a/d) ratios. In such tests no region of the specimen is subjected to uniform stress conditions, Therefore, the results of such tests cannot predict the true pure shear behavior due to non-uniformity of stresses, the presence of flexural and other non-shear related effects such as a/d ratio that cannot be filtered out. Therefore, proper design of shear strengthening using FRP requires testing of elements that are subjected to pure shear case primary before adding other governing effects. This allows a careful investigation and full understanding of the behavior at the element level. In order to accomplish this task, panel testing of representative RC specimens strengthened with FRP sheets were needed. This paper reports the testing of 10 FRP strengthened RC panels subjected to pure shear stress field. The tests were carried out to evaluate the effects of three variables: FRP stiffness, FRP wrapping scheme, and transverse steel reinforcement ratio. The test results showed that these three variables greatly affected the shear behavior due to various types of failure modes associated with FRP strengthening. In addition, it was observed that the magnitude of increased shear capacity associated with the application of FRP sheets depends not only upon the stiffness of FRP, but also on the stiffness of internal shear reinforcement. With the increase of internal steel shear reinforcement, the effectiveness of shear gain due to externally bonded FRP decreases.

Improvement of the FRP sheet bonded to concrete substrate by silane coupling agent

We experimentally studied the fine lightweight aggregate with the particle size range of 3.15-4.75 mm used as functional bridge between FRP sheet and concrete substrate. However, problems would appear and how to deal with the interfacial transition zone (ITZ) and make it stronger is the key point for this concept. Considering that silane coupling agent (SCA) can provide a better bond on a silicon-containing material surface, it was introduced as a modifying material to further improve the bond quality of the ITZ between lightweight aggregate and cement paste. Results indicated that the water absorptivity of lightweight aggregate can be controlled by SCA solutions, and the pull-off bond strength, mechanical strength, and microhardness were increased, which was attributed to the optimized microstructure under the condition of an appropriate concentration of SCA.

Characterization of Shear Strength of FRP Anchors

A critical performance aspect of FRP retrofitted concrete elements is the bonding of the FRP sheet to the concrete surface. In general, the performance is limited by the debonding of the loaded FRP sheets from the concrete surface. One method to delay debonding and enhance the capacity is the use of FRP anchors which interlock the FRP sheet to the concrete body. FRP anchors are made of rolled FRP fibres epoxied into in predrilled boreholes. There are a considerable number of studies on FRP strengthening methods available, and also FRP anchors attract more attention of the research community recently. However, to date FRP anchors were tested in a system together with the FRP sheet attached to the concrete, inhibiting the development of general design models. Moreover, the anchor behaviour was never tested for cyclic loads, though most applications are for seismic retrofitting schemes and cyclic shear loading generally results in reduced load capacity due to fatigue failure. To overcome the deficit in knowledge, shear tests on various FRP anchors were carried out. For these tests, FRP anchors were installed in concrete specimens on a separating steel section. The FRP anchor was then directly loaded to determine the capacity of the isolated component. This paper describes the testing approach and procedure. Details on the experimental results for static tests are presented and an outlook on seismic tests is given.

A practical model for simulating nonlinear behaviour of FRP strengthened RC beam-column joints

Generally, beam-column joints are taken into account as rigid in assessment of seismic performance of reinforced concrete (RC) structures. Experimental and numerical studies have proved that ignoring nonlinearities in the joint core might crucially affect seismic performance of RC structures. On the other hand, to improve seismic behaviour of such structures, several strengthening techniques of beam–column joints have been studied and adopted in practical applications. Among these strengthening techniques, the application of FRP materials has extensively increased, especially in case of exterior RC beam-column joints. In current paper, to simulate the inelastic response in the core of RC beam–column joints strengthened by FRP sheets, a practical joint model has been proposed so that the effect of FRP sheets on characteristics of an RC joint were considered in principal tensile stress-joint rotation relations. To determine these relations, a combination of experimental results and a mechanically-based model has been developed. To verify the proposed model, it was applied to experimental specimens available in the literature. Results revealed that the model could predict inelastic response of as-built and FRP strengthened joints with reasonable precision. The simple analytic procedure and the use of experimentally computed parameters would make the model sufficiently suitable for practical applications.

Effects of tensile steel bars arrangement on concrete cover separation of RC beams strengthened by CFRP sheets

This study was conducted to investigate the effect of reinforcement arrangement in strengthened RC beams on the concrete cover separation. For this purpose, 12 beam specimens with the dimensions of 250 mm × 300 mm × 2200 mm and different reinforcement ratios were prepared and tested under four-point bending. From the standpoint of the strengthening technique, the specimens were categorized into two groups: six specimens were strengthened by Externally Bonded Reinforcement (EBR) method and the other six specimens were strengthened using Externally Bonded Reinforcement on Grooves (EBROG). On the condition that the reinforcement ratio is constant, reducing the diameter and increasing the number of bars increased the probability of concrete cover separation. The specimens with low and medium ratios of reinforcement, in which small-diameter bars are used, demonstrated a higher load carrying capacity sustaining more deflections at mid-span before debonding of FRP sheets.

Axial-strength model for FRP-confined concrete based on the improved twin shear strength theory

External confinement by wrapping with FRP sheets is a very effective method for the retrofit of a concrete structure subject to either static or seismic loads. For the reliable and underlying analysis of the confinement, an accurate and theoretical strength model is required for FRP-confined concrete. In this paper, the math-deductive method is introduced based on a review of the existing models. The theoretical method uses the failure criterion as the foundation to establish the axial-strength model. To consider the stress condition of FRP-confined concrete, an improved failure criterion is developed based on twin shear strength theory. Based on a proposed failure criterion, an axial-strength model is proposed for FRP-confined concrete columns. The coefficients in the proposed model are computed with respect to the range of the unconfined concrete strength, and the ranges of the coefficients show obvious trends. The predictions of the proposed model agree well with the comprehensive database.

Finite element model for predicting the shear behavior of FRP-strengthened RC members

The shear behavior of FRP strengthened reinforced concrete (FRP strengthened RC) membrane elements can be predicted by developing logical models that satisfy the principles of mechanics of materials namely stress equilibrium, strain compatibility, and constitutive relationships of concrete, steel and, FRP reinforcements. The Softened Membrane Model (SMM), which was developed for predicting the shear behavior of reinforced concrete (RC) membrane elements, is extended to FRP strengthened RC members subjected to shear. This new analytical model, referred to as the Softened Membrane Model for FRP strengthened RC members (SMM-FRP), considers new constitutive laws for each material component of the member. Similar to the case of the SMM model for RC, this new SMM-FRP model can predict the entire stress-strain curve, including pre- and post-cracking, and the ascending and descending branches. The SMM-FRP is implemented into an OpenSees-based finite element program for a membrane 2-D element that will allow structural engineers to predict the monotonic responses of FRP strengthened RC members subjected to shear. The developed program is validated in this paper by the prediction of the monotonic responses of 10 FRP strengthened RC panels subjected to pure shear stresses. The good agreement between the experimental and analytical results confirms the validity of the analytical model in predicting the shear behavior of RC members strengthened with FRP sheets.

하이브리드 FRP시트로 보강된 철근콘크리트 원형 기둥의 휨 거동

철근콘크리트 원형기둥의 보강재료로써 하이브리드 FRP시트의 적용성을 평가하기 위해 중심축하중과 반복횡하중을 동시에 받는 기둥의 휨 거동 실험을 실시하였다. 주요변수는 유리섬유보강, 탄소섬유보강, 하이브리드섬유보강 및 무보강 기둥이다. 실험결과 섬유로 보강된 기둥의 항복내력과 최대내력은 무보강 실험체보다 각각 1.1 배 및 1.06 배 증가하였다. 특히 하이브리드 섬유로 보강된 실험체는 유리섬유보강 실험체에 비해 매우 연성적으로 거동하였으며, 에너지 연성도는 유리섬유보강 실험체보다 약 174% 이상 증가하였다. 탄소섬유와 유리섬유를 혼합한 하이브리드 섬유는 철근콘크리트 원형기둥의 연성능력을 높이는데 매우 효과적임을 확인하였다.

08.31: Analysis of the load transfer between materials in composite concrete encased steel columns loaded axially

ABSTRACTThe analysis of the load distribution between steel and concrete parts of the composite concrete encased I‐shaped steel columns subjected to short‐term axial load is presented. The variable parameters were: two loading conditions − load scheme I (specimens loaded through the entire cross‐section simultaneously), load scheme II (specimens loaded only through the steel profiles protruded from the top end) and concrete class (C25/30 and C40/50). Obtained in the experiment longitudinal strains of both steel and concrete were used to compute the force transferred through the steel and concrete parts of the columns. To provide a reference level for the discussion on the results the analytical solution assuming the perfect bond between steel and concrete, Hooke's law for steel and the Rüsch formula for unconfined concrete were used. The force for which the stress in steel and concrete computed using the strain readings obtained from the experiment starts to deviate from the full bond analytical model predictions can be interpreted as the force for which the loss of adhesive bond occurs. Furthermore on the basis of the equilibrium of the infinitesimal cross section of a column, method for estimating the shear stress at which the debonding occurs is shown. The analysis showed that for columns under the load scheme I composite action of steel and concrete is in effect for the entire load range regardless of the concrete class. For elements under the load scheme II the composite action of both materials is in effect only up to a specific value of the force that changes along the height of the column. Presented method can be applied to composite steel‐concrete columns under any configuration of boundary conditions subjected to axial compression. Moreover it can be used to specify the area for which the shear connectors should be provided, to validate constitutive models of shear stress – slip behaviour that can be used is FEM analysis and to analyse adhesive bond properties also in columns made of another materials, for example reinforced concrete members strengthened with FRP sheets.

Experimental Study on Performance of Short-Bonding Interface between CFRP and Concrete

Based on double shear test, to study the performance of FRP and concrete bonding interface as the FRP sheet bond length is relatively short (hereinafter referred to as short bond). This paper mainly focuses on the research of interface failure modes, ultimate bearing capacity, interface fracture energy, the development and distribution regularity of strain and stress. Therefore, the interface bond slip relationship curve is obtained through differential operation and data regression analysis on FRP strain values which was gained from test. Results show that: (1) under the condition of short-bonding, damage form of FRP and concrete bonding interface is shear failure which caused sudden destruction to the inner of concrete block; (2) The ultimate bearing capacity, interface fracture energy and maximum shear stress increase with the improvement of concrete strength; (3) the interface shear stress - slip curve is obviously featured as nonlinear, composed of rising and falling, basically presents as a parabolic shape.

Lateral load-displacement response of low strength CFRP-confined capsule-shaped columns

This paper describes the behavior of Carbon Fiber Reinforced Polymer (CFRP)-confined capsule-shaped columns with large section aspect ratio subjected to pushover and simulated seismic load. To enhance the FRP confinement effect in case of rectangular columns, two shape modification technique are proposed to form capsule-shaped sections. The columns studied herein are representing building columns in Singapore which were built around 50years ago with low strength concrete and minimum transverse steel reinforcement. Existing concrete columns and piers are often deficient in shear strength and displacement ductility due to inadequate steel reinforcements. This inadequacy in reinforcements may lead to the brittle shear failure during severe earthquakes. To prevent the shear failure in a deficient column, the column has been confined by transversely bonded FRP reinforcements around the entire height of the column. Additionally, to improve the ductility, the plastic hinge of column has been confined by external FRP systems. The effectiveness of the proposed shape modification technique has been examined by comparing the shear capacity and ductility of FRP-confined columns with the unconfined columns. The CFRP confinement with one ply of FRP sheet in full height of the column prevented the shear failure of these columns; and the additional ply in the plastic hinge zones significantly enhanced the ductility of columns for all three shape parameters. It is evident from the present study that the proposed shape modification technique is effective to provide better confinement effect to rectangular columns and thus, can improve the deformation characteristics. The proposed retrofitting method can be used for low strength concrete buildings with shear-deficient rectangular columns to enhance their seismic performance.