FRP Strips Research Articles

Parameters affecting the behaviour of RC beams strengthened in shear and flexure with various FRP systems

An experimental investigation was carried out on rectangular RC beams strengthened with FRP in different strengthening forms in shear and flexure. The parameters examined in this study are as follows: The shear span to effective depth ratio (a/d), the number of FRP layers in shear, strengthening form (completely wrapping, U-wrapping, and side-bonding), FRP types (Glass or Carbon), and FRP strips width to spacing ratios (wf/sf). The experimental results showed that strengthening with FRP enhanced the load-carrying capacity, deflection capacity, initial stiffness, and ductility capacity with respect to the reference beams. Increasing the number of FRP layers in shear did not provide a proportional increase in the contribution of FRP to shear strength. The FRP types, FRP strips width to spacing ratios (wf/sf), the number of FRP layers in shear, a/d, and strengthening form affect the cracking patterns and loads of tested beams compared to reference beams. In addition, strengthening with FRP could change the failure modes of RC beams from brittle shear failure to more ductile flexural failure. It was also found that the equations used to calculate FRP contribution to shear strength (ACI 440.2R (2017), Fib-TG 9.3 (2001), Khalifa (2002), Triantafillou (1998), Bukhari (2010)) overestimated the FRP contribution and gave unconservative results, especially for U-wrapped and side bonded beams compared to the experimental results.

The behavior of concrete beams strengthened with NSM CFRP strips: a numerical parametric study

A non-linear finite element (NLFE) model was developed to predict the flexural response of concrete beams, strengthened with near surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. The model was employed for predicting the influence of a wider spectrum of key parameters, concrete compressive strength (25–55 MPa), NSM FRP strips number (2–4) or spacing (20–35 mm), bond length (100–450 mm), FRP type (carbon, glass, and basalt) and loading pattern (four-point, three-point, and distributed load), upon the performance of these beams. The results showed a clear improvement in the performance of the CFRP-strengthened concrete beams when made at a higher-strength class of concrete. Using a higher number of NSM FRP strips of the same cross-sectional area contributed to enhancing the mechanical performance noticeably. Moreover, the model developed was capable of capturing the negative impact of using a lower spacing for a constant number of NSM CFRP strips. The impact of the FRP strips' type varied upon the mechanical property predicted and the strength grade of concrete used in the casting of the beams. The load resistance of NSM FRP-strengthened specimens was the highest under three-point load configuration, followed, in sequence, by distributed and four-point loads, respectively.

Low-Cycle Fatigue of FRP Strips Glued to a Quasi-Brittle Material

This paper aims at further advancing the knowledge about the cyclic behavior of FRP strips glued to quasi-brittle materials, such as concrete. The results presented herein derive from a numerical model based on concepts of based on fracture mechanics and already presented and validated by the authors in previous works. Particularly, it assumes that fracture processes leading to debonding develop in pure mode II, as is widely accepted in the literature. Starting from this assumption (and having clear both its advantages acnd shortcomings), the results of a parametric analysis are presented with the aim of investigating the role of both the mechanical properties of the interface bond–slip law and a relevant geometric quantity such as the bond length. The obtained results show the influence of the interface bond–slip law and FRP bond length on the resulting cyclic response of the FRP-to-concrete joint, the latter characterized in terms of S-N curves generally adopted in the theory of fatigue. Far from deriving a fully defined correlation among those parameters, the results indicate general trends that can be helpful to drive further investigation, both experimental and numerical in nature.

Strengthening of timber beams with pretensioned CFRP strips

The article presents the results of research on the strengthening of timber beams with the use of pretensioned CFRP strips. In the beginning, the review of literature regarding both passive and active strengthening methods of timber beams with steel bars, steel mats and FRP strips is presented. The review is followed by the description of a test program concerning the strengthening of solid timber beams with pre-tensioned composite strips. In four series of beams various levels of prestress were applied. The article discusses all stages of beams production, including the prestressing process. The behavior of beams over 28 days after prestressing is reported. The results of four-point bending of strengthened beams are described. The ultimate forces, mechanisms of failure as well as the “load–deflection” relationships in individual beam series were analyzed in comparison with the reference beams. The possibility of prestressing of solid timber beams and the reduction of their deflection due to prestress was proved.

Compression Tests on Large Angle Columns in High Strength Steel

AbstractThe paper discusses the stability behaviour of large angle columns in high strength steel subjected to compression loads. It presents a number of compression tests on such columns that have been performed at University of Liege with different eccentricities and slenderness and, as a result, different buckling failure modes observed. The experimental campaign is intended to widen the knowledge about the behaviour of high strength steel columns with large angle sections in compression and bending and so to complement previous experimental studies. The tests have been accompanied by numerical analyses and calculations of the load carrying capacities through present Eurocode prescribed methods. The numerical simulations have been carried out by application of the FEM software FINELG using beam elements and considering of relevant imperfections as well as geometrical and material non – linearities. The numerical and analytical results are compared and validated with the corresponding experimental ones and summarized in the present paper. The experimental campaign and the numerical simulations are part of the European‐funded RFCS project ANGELHY “Innovative solutions for design and strengthening of telecommunications and transmission lattice towers using large angles from high strength steel and hybrid techniques of angles with FRP strips”.

Shear strengthening of reinforced concrete beams using NSM/EBR techniques

This paper presents the experimental results of research into the behavior of shear-enhanced reinforced concrete R.C beams using steel stirrups, Fiber Reinforced Polymers FRP rods, and Fiber Reinforced Polymers FRP strips. This enhancement was accomplished by the Near Surface Mounted technique NSM. The NSM technique contains a groove on the outside surface of the concrete member to adjust the depth to be less than the cover of the member. After cleaning, the epoxy paste was used to fill half of the groove's depth. In the groove, the particular FRP element is then installed. Finally, the groove is filled with epoxy and the outside surface of the concrete is levelled with so much epoxy. This method enables the fiber reinforcement polymer FRP materials is covered completely by epoxy. The objective of this research is to study the effect of NSM technique on shear resistance for stressed beam. 13 experimental studies of half-scale R.C beams were involved in this paper. The experimental program included two specimens strengthened with steel stirrups, eight specimens strengthened with stirrups of Glass Fiber Reinforced Polymer GFRP rods with the shape of deferent end anchorage and angle, two specimens strengthened with externally bonded GFRP strips. The remaining un-strengthened specimen was allocated for comparison as a control one. The test results included ultimate load of capacity, deflection, cracking, and failure mode. All beams enhanced with GFRP rods showed a capacity improvement ranging from 14% to 85% compared to the reference beam, and compared to the reference beam, beams enhanced with GFRP strips showed a capacity improvement ranging from 7% to 22%.

Behavior of RC columns and those strengthened with FRP composite under an innovative reversing cyclic eccentric axial loading

The fundamental role of columns in structural stability calls for an in-depth understanding of their behavior under different loadings. In this study, an innovative test was designed and the required apparatus was manufactured to apply axial eccentric cyclic loading in two opposite directions as an effort to create more realistic conditions for eccentric axial loading of columns in the structures under cyclic loads. For this purpose, six reinforced concrete (RC) columns were tested. The columns included non-strengthened specimens, those strengthened with longitudinal FRP strips installed using the externally bonded reinforcement on groove (EBROG) technique, and those strengthened with full FRP wraps. All the specimens subjected to axial loading with an eccentricity of 90 mm in both monotonic and reversing cyclic method (the innovative method developed herein) to compare the effects of both cyclic and monotonic loadings. The specimens exhibited reduced bearing capacity and ductility under cyclic loading when compared with those subjected to monotonic. Moreover, the trend of the load-deformation relationship of RC columns and those strengthened with FRP wraps were almost the same under monotonic and cyclic loads, while strengthened specimens with longitudinal FRP exhibited different load-deformation trends. Comparison of the specimens strengthened via the two methods used in this study revealed that using longitudinal FRP sheets via the EBROG technique caused a significant (about 60%) increase in bearing capacity under high eccentric loads through a reduction in ductility. Application of FRP wraps, however, exhibited almost no effect on bearing capacity under the same loads but led to the columns’ enhanced ductility.

Introduction and comprehensive theoretical framework of analysis of Smart Plate retrofitted reinforced concrete members (SPRRCs)

Several reasons result in the need for retrofitting of existing reinforced concrete structures, and extensive effort is devoted to invent and implement different methods for strengthening RC members. One of the leading rehabilitation techniques is attaching FRP strips, but FRP materials’ low ductility has caused several issues such as FRP-structure debonding, boundary layer cracking, and brittle behaviour of the strengthened RC members. To overcome these drawbacks, the Smart Plate (SP) which is a ductile retrofitting cover, is proposed for the first time. This cover is formed by embedding Shape Memory Alloy (SMA) wires in an elastomeric substrate. The SMAs exhibit certain properties, including superelasticity, shape memory effect, and temperature-dependent behaviour, while the elastomer can withstand severe stretches. To investigate SP-retrofitted RC beams (SPRRC) characteristics, at first, a computer program has been developed based on the fiber analytical method to analyse the strengthened concrete members. The program is verified against experimental and finite element method (FEM) data for ordinary, FRPRC, SMARC and SPRRCs members. Following verification, the program and FEM are implemented to conduct a comprehensive analysis of the influential parameters affecting the behaviour of SPRRCs including SMA percentage and working temperature. As a result, the SPRRCs show improved stiffness and strength compared to ordinary beams. They also behave in a ductile way contrary to FRP-strengthened beams. The FMA program and the FEM simulations both indicate that the SMA phase transition temperature significantly alters the performance of SPRRCs. Moreover, FMA and FEM outcomes approve that the smart plate is less likely to debond from the strengthened structural member, which is a remarkable advantage. Additionally, a parametric study is conducted, and a characteristic moment–curvature diagram is proposed, which approves the merits of using SP as an alternative retrofitting cover. These advantages are the significant enhancement in moment capacity, yielding moment, ultimate curvature, and ductility.

Seismic performance of a masonry house prototype retrofitted using frp strip: a numerical study

This study presents a series of finite element analyses on the prototype of masonry house retrofitted using FRP strips . The model of the masonry house refers to the prototype scaled to 1:3 tested in an experimental campaign. The non-linear behavior of masonry is modeled through the Concrete Damage Plasticity (CDP) model, while the FRP and adhesive are modeled as two separated isotropic solid materials with elastic-plastic behavior. A good agreement between experimental and numerical result is obtained, indicating the increase of bearing capacity of the reinforced masonry prototype.

A new and robust hybrid artificial bee colony algorithm – ANN model for FRP-concrete bond strength evaluation

The present study describes the bond strength estimation between the FRP strip and the substrate concrete using machine learning approaches. To provide a precise model, a database including 656 single and double-lap direct shear test results was compiled from the literature. Artificial neural networks (ANN) and the combination of ANN with the artificial bee colony optimization algorithm (ABC-ANN) were implemented for the model development. The concrete compressive strength, the elasticity modulus of FRP strip, FRP thickness, the width of the FRP strip, and the width of concrete block were considered as input parameters of the models. The results of the ANN and the hybrid ABC-ANN were compared with those of existing models and international procedures. It was indicated that the accuracy of the proposed models outperforms the existing models. The correlation coefficients of the ABC-ANN and ANN approaches are 0.97 and 0.93, respectively. The robustness of the models was investigated using the distribution of absolute error values. It was demonstrated that the proposed ABC-ANN model is robust and resulted in the least error values, distributed in small ranges (less than 10%), compared to ANN and other methods. Using the hybrid ABC-ANN, a straightforward formulation for bond strength evaluation was proposed.

Experimental study on shear strengthening of RC beams with basalt FRP strips using different wrapping methods

This paper presents experimental and analytical studies on the shear strengthening of reinforced concrete (RC) beams by basalt fiber reinforced polymer (BFRP) composites. The effects of BFRP composite and wrapping methods are investigated on shear strengthening of RC beams. In addition, the calculation methods suitable for these strengthening methods are investigated by comparing the results of the experimental study with the existing calculation methods. The beams with rectangular cross section were produced with steel reinforcement and low strength concrete and designed to have a sufficient flexural moment capacity but insufficient shear capacity. However, that beams with two shear regions are designed to be one side shear strength sufficient and the other side insufficient. The shear insufficient regions of the RC beams were strengthened with BFRP composites; full wrapping, side bonding and strips methods. Specimens were tested by four-point flexural test under monotonic loading. The change in shear strength, deflections, shear crack opening and shear deformations were investigated from the results of the experiment. According to the experimental results, the shear capacity of RC beams strengthened with BFRP composites increased between 43% and 100%. The best results were gained by fully wrapped sheets and fully wrapped strips configurations among strengthening configurations. In addition, an appropriate prediction model has been determined to the shear strengthening configurations.

Experimental and Numerical Study of Basalt FRP Strip Strengthened RC Slabs under Impact Loads

Basalt fiber-reinforced polymer (BFRP) has been applied for strengthening concrete structures. However, studies on reinforced concrete (RC) slabs strengthened by BFRP strips under impact loads are limited in open literature. This study investigates the efficiency of using BFRP strips with various strengthening layouts and anchoring schemes on the impact resistance of RC slabs. A total of 11 two-way square slabs were prepared and tested, including one reference specimen without strengthening and ten slabs strengthened with BFRP strips and/or anchors. The RC slabs were impacted by a drop weight with increasing height until slab failure. The observed failure modes include punching shear failure, BFRP sheet debonding and reinforcement fracture. The failure modes and the effects of using various strengthening schemes on the impact resistant capacity of RC slabs were examined. The quantitative measurements, such as impact velocity, indentation depth and diameter, were compared and discussed. In addition, numerical studies were carried out by using LS-DYNA to simulate the impact tests of RC slabs with and without BFRP strengthening. With the calibrated numerical model, the impact behavior of slabs with various dimensions and strengthening layouts under different impact intensities can be predicted with good accuracy.

Heterogeneous FE model for single lap shear tests on FRP reinforced masonry curved pillars with spike anchors

A 3D-FE heterogeneous approach is proposed to reproduce experimental single lap shear tests performed on curved masonry prisms reinforced either at the intrados or extrados with FRP strips fixed to the masonry surface by both epoxy resin and an anchor device placed in the middle of the strip. Bricks and mortar are modeled with a concrete damage plasticity material exhibiting softening, whilst for both the FRP and spike a linear-elastic behavior is assumed. An elastic uncoupled cohesive interface between FRP/substrate and anchorage/substrate is also introduced in the model to properly reproduce experimental stiffness and damage diffusion inside masonry. The model is benchmarked against four sets of single lap shear tests performed by the authors at the University of Florence, Italy. A good match between experimental evidences and numerical predictions is obtained. Sensitivity analyses are finally performed in order to furnish a prediction of the delamination strength also in cases not studied experimentally.

Numerical Analysis of Compressed Masonry Columns

This paper presents a three-demensional micro-model for an unreinforced masonry (URM) column and masonry columns reinforced by FRP wrapping (RM). The column with dimensions of 0.3 x 0.3 x 1 m was constituted by twelve rows of solid burnt bricks bonded by mortar joints. The RM column was wrapped in four external FRP sheets. A heterogeneous model, in which masonry units, mortar joints and reinforcement are materially and geometrically accurately described, was chosen. Both bricks and mortar are modelled with independent behavior in compression and tension. The FRP reinforcement is assumed to behave linear-elastically. The reinforcement was modelled under the assumption of perfect adhesion between the elastic FRP strips and the damageable masonry support. Numerical results are compared with experimental ones in order to confirm conclusions about the behavior of URM and RM columns under concentric compressive load determined based on the experimental part of the research program. The comparison between experimental and numerical findings shows good agreement and the adopted numerical model is suitable to predict the ultimate load and allows a better understanding of the behavior of brick masonry columns under concentric compression before and after reaching the ultimate load. For all simulations, the commercial software package ABAQUS was used.

Using grooving and corner strip‐batten techniques for seismic strengthening of square reinforced concrete columns with fiber‐reinforced polymer composites

AbstractThis study represents the results of an empirical program investigating the behavior of square reinforced concrete (RC) columns retrofitted using longitudinal carbon fiber‐reinforced polymer (CFRP) composites under simulated seismic loading. The main challenges to tackle are the premature buckling and the debonding of the longitudinal FRP composites on the tension and compression sides of the columns, respectively. To postpone both, the two versions of the grooving methods (GM), that is, the externally bonded reinforcement on grooves (EBROG) and the externally bonded reinforcement in grooves (EBRIG), were employed and the results were compared with those obtained under the traditional externally bonded reinforcement (EBR) procedure. Another aspect of the study involves investigating the influence of the recently developed corner strip‐batten (CSB) method on confining column specimens. For these purposes, seven half‐scale RC columns strengthened using FRP sheets were subjected to reversal cyclic loading. Experimental results exhibited that using the EBRIG and EBROG methods simultaneously with CSB technique could prevent altogether the debonding and buckling of longitudinal CFRP sheets. It was also found that the application of FRP strips utilizing the GM and CSB methods could produce an important increment in the performance of the RC columns in different terms such as ductility and lateral strength.

Axial compressive behavior of partially CFRP confined seawater sea-sand concrete in circular columns – Part II: A new analysis-oriented model

For the comprehensive analysis of the mechanical performance of concrete structures wrapped with partial FRP strips, the development of an accurate and versatile axial stress-strain model is of significant importance. No research, however, has been conducted to present a satisfactory analysis-oriented stress-strain model for partially FRP confined concrete. To this end, this paper firstly investigates the performance of several existing analysis-oriented models in assessing the FRP partially wrapped columns. The results show that all selected models cannot provide reasonable predictions for the test specimens, especially for the specimens possessing strain-softening stress-strain relationships. Therefore, the dilation properties and the peak axial strength expressions of active-confinement base models were both revised based on the test observations and a new analysis-oriented model was proposed for partially FRP confined concrete. Finally, a large database of CFRP partially wrapped columns was assembled through an extensive literature review to evaluate the accuracy of models. By comparing the test results with the predicted calculations, the proposed model can provide accurate and reasonable predictions both for the ultimate conditions and the complete axial stress-strain relationships, which were obviously superior to the other existing models.

Punching shear strengthening of flat slabs with EBROG and EBRIG – FRP strips

The present study investigates the strengthening of flat slabs with Externally Bonded Reinforcement “On Grooves” (EBROG) and “In Grooves” (EBRIG) FRP strips and they are compared with Externally Bonded Reinforcement (EBR) FRP strengthening. 28 slabs consisting of one control and 27 strengthened were defined and analyzed nonlinearly. The EBRIG and EBROG strengthening layout consisted of 1- or 2-layer FRP strips under slabs on 2 grooves of 4- or 8-mm width and 8, 10 or 12 mm depth. First, the results of the finite elements (FE) analysis were verified by experimental results available in the literature and it has confirmed that the FE nonlinear analysis can be used to estimate the strengthened slabs behavior. The analysis results of EBR, EBRIG and EBROG strengthened samples showed that the FRP strengthening of slabs in EBRIG and EBROG format, increased the punching capacity of the slab up to 60%, while EBR strengthening could increase this capacity about 28%. The cracking load was increased about 50% compared to EBR strengthened samples. Also, the EBRIG and EBROG strengthening in some of the samples could change the slab failure from shear to shear flexure and the FRP rupture was resulted in them. The grooves width and depth directly affected the cracking load, punching shear capacity and failure type of slabs.

Dynamic and Structural Study of a RC Building Braced by FRP Composite Materials

Dynamic problems analysis in the field of construction is necessary to guarantee the reliability of the structures in different applications, especially in case of structures subjected to seismic solicitations. In that context, the interest of this work is to study the effects of the composite materials used in the bracing concept for a building by using displacement dimensioning methods and establishing the “Pushover method” fragility curves. For this reason, a set of reinforced concrete buildings designed accordingly to the usual seismic practices, is used to study the influence of the bracing based on the innovative plates (FRP) compared to the one of the reinforced concrete sail which is based on the stability of the work during an earthquake. From one hand, the assumption of adherence is considered perfect between the FRP strips and the concrete, and on the other hand, between the reinforcements and the concrete, which makes it possible to analyze efficiently the behavior of the mechanical response of the proposed variants in this study. The aim is to improve the understanding of bracing in case of reinforced concrete construction in quasi-static (Pushover) and dynamic regimes. Several seismic numerical simulations based on the tool of massive finite elements have been conducted in order to identify the interest of different variants through the indicators’ evaluation of partial and total degradation of the superstructure. The study of the influence of each design variant on the stability of the construction has ultimately clarified the role of a new bracing proposal developed based on FRP materials.

An experimental approach for shear strengthening of RC beams using a proposed technique by embedded through-section FRP sheets

This article presents a new technique for enhancing the shear capacity of RC beams with FRP materials, which instead of bonding of FRP strips on the external face of element, involves perforating holes along the beam web, inserting FRP reinforcement inside these holes, and filling the holes with grout. Although according to technical literature, externally bonded and near-surface mounted FRP reinforcements are well known as effective solutions for increasing the shear capacity of RC elements, they are ill-suited for the cases where the concrete cover lacks sufficient bonding strength, and a debonding may allow the beam to fail before maximum capacity utilization. Furthermore, in the structures where the beam is connected to a floor, the FRP wrapping involves drilling holes in the floor to install the reinforcement, which creates shear vulnerability around the floor-beam contact area. Experimental results show that the beams reinforced with the proposed technique perform similar to or better than those with externally bonded FRP reinforcements. Also, the comparisons of the Applied Strengthening Material Index (ASMI) for specimens shows that the ratio of FRP shear contribution to the amount of FRP consumed is greater in the proposed technique than in the external bonding methods.

Recent Advances in the Straps/Strips Technique for Out-of-plane Strengthening of Load-bearing Masonry Walls

As is well known, the main contribution of the FRP strips to the strength of load-bearing walls is an improvement in the in-plane strength. This paper deals with the possibility of applying the FRP strips in way to modify the strengthening mechanism of the FRP reinforcing system, from an in-plane to an out-of-plane strengthening mechanism. In order to achieve this goal, a second reinforcement system – derived from the CAM system (Active Confinement of Masonry) – provides connections between the FRP strips placed on the opposite sides of the wall. This new strengthening technique – called the straps/strips technique – establishes a stiffness constraint that forces the opposing FRP strips to behave like two flanges of an FRP I-beam embedded in the wall. Consequently, the use of FRP strips also improves the flexural strength of the wall. The present paper briefly summarizes the results obtained in previous works with the straps/strips technique and proposes an improvement of this strengthening technique, based on some weak-points emerged in the early experimentations. The paper also shows the results of a further experimental test, performed with the improved straps/strips technique. Finally, the similarity between FRP strips with transversal connection and concrete wythes of a sandwich panel with flexible connectors leads to interpret the behavior of the ideal I-beam in terms of composite action established between the FRP strips. This paves the way for analytical modeling of the straps/strips technique.