Fiber Reinforced Polymer Strips Research Articles

Improving out-of-plane behavior of aerated concrete walls with carbon fiber-reinforced polymer strips

Aerated concrete walls, mostly used in reinforced concrete structures, can be damaged under dynamic forces such as earthquake and can create danger for the life of the occupants. Many experimental studies have proved that infill walls have high strength against lateral forces acting in plane, but strength against same forces is quite low out-of-plane. Therefore, an experimental study is conducted for increasing out-of-plane strength of the aerated concrete walls and obtaining improvements at other behaviors. Out of the 11 specimens manufactured, one is considered as reference specimen, and the rest are strengthened with carbon fiber-reinforced polymer strips with different layouts. Test specimens are tested at four points by bending setup for simulating out-of-plane lateral force. The variables that are examined in this study are carbon fiber-reinforced polymer strip layout and carbon fiber-reinforced polymer anchorage usage. The performance of the proposed method of strengthening is determined by examining stiffness, ductility, energy dissipation capacity, and failure modes. ACI 440 Committee report is used for calculating the capacity of the specimens. This technique, strengthening aerated concrete walls by using carbon fiber-reinforced polymer strips, has enabled the improvement in strength by 12.5 times, ductility ratio 6.1 times and energy capacity 102.4 times.

NLFEA sulfate-damage reinforced concrete beams strengthened with FRP composites

A nonlinear finite element analysis (NLFEA) model is proposed, validated then expanded to predict the contribution of fiber reinforced polymer (FRP) composites in regaining structural performance and controlling failure of shear-deficient and sulfate-damaged RC beams. The model was validated using experimental data, before expanded to consider the effect of FRP form (sheet against strip), and its fiber orientation (45° against 90°), and bond area (web against U-wrap) in addition to sulfate-damage level. The performance was evaluated using ultimate shear strength, stiffness, toughness, and crack-opening toughness. Furthermore, shear stress versus slip for the FRP composites, strain-depth distribution in the high shear region, and cracking modes at failure for repaired beams were generated and compared.The NLFEA results showed that repairing shear-deficient and sulfate damaged RC beams with various configurations of (carbon and glass) FRP strips and sheets helped enhancing their loading carrying capacity by (10–39)% and (4–32)%, respectively. External 90° FRP U-wrap sheet provided larger increase in ultimate shear capacity than that with 90° FRP web sheets. As expected, carbon and glass FRP web strips showed better structural performance when oriented at 45° rather at 90°. Except for that with U-wrap FRP sheet, repair techniques with glass fibers imparted better overall structural performance to repaired beams than that of the FRP composites with carbon fibers.

A mechanical model for FRP-strengthened beams in bending

We analyse the problem of a simply supported beam, strengthened with a fibre-reinforced polymer (FRP) strip bonded to its intrados and subjected to bending couples applied to its end sections. A mechanical model is proposed, whereby the beam and FRP strip are modelled according to classical beam theory, while the adhesive and its neighbouring layers are modelled as an interface having a piecewise linear constitutive law defined over three intervals (elastic response – softening response – debonding). The model is described by a set of differential equations with appropriate boundary conditions. An analytical solution to the problem is determined, including explicit expressions for the internal forces, displacements and interfacial stresses. The model predicts an overall non-linear mechanical response for the strengthened beam, ranging over several stages: from linearly elastic behaviour to damage, until the complete detachment of the FRP reinforcement.

Retrofitting Earthquake-Damaged RC Structural Walls with Openings by Externally Bonded FRP Strips and Sheets

The majority of research studies on the behavior of reinforced concrete members with externally bonded fiber reinforced polymer (FRP) sheets have been focused on beams, columns, and beam-column joints. However, limited experimental studies have been conducted to investigate the performance of structural walls retrofitted by wrapping FRP strips or sheets, especially on structural walls with openings. The validated retrofitting schemes for strengthening damaged walls without openings may not be suitable for walls with openings. Therefore, a series of experimental studies were carried out at Nanyang Technological University, Singapore, to study the effectiveness of the proposed repair and strengthening schemes in recovering the seismic performance of the damaged walls with irregular or regularly distributed openings. The strut-and-tie approach was utilized to design the repair schemes. The repaired walls managed to recover their strength, dissipated energy, and stiffness reasonably, indicating that the strut-and-tie approach can be a good design tool for FRP-strengthening of structural walls with openings. Moreover, the shear and sliding capacities of repaired walls were enhanced by using fiber anchors. The repaired walls failed primarily because of debonding of the fiber reinforced polymer at the base of the walls.

Finite element analysis of masonry panels strengthened with FRPs

The nonlinear structural analysis represents a powerful tool for the study of the seismic response of new and existing constructions. In particular, nonlinear static analysis, also denoted as pushover analysis, is widely used for RC and steel structures and, recently, new seismic codes extended the use of this type of analysis to the case of masonry structures. The reliability of the results deduced from nonlinear analyses is not always ensured since the numerical results can depend on the modeling strategy and on the adopted solution techniques. Moreover, in the case of masonry structures strengthened with fiber reinforced polymers (FRP), particular regards has to be addressed not only to the masonry material model but also to the modeling of FRP-system, taking into account the complex phenomena characterizing the interaction between the FRP and the masonry supports. Aim of the present paper is to discuss some strategies, in the framework of the nonlinear finite element analysis, regarding the elements choice and the models to adopt for reliable nonlinear analyses of masonry structures reinforced with FRP strips. Specific structural problems are described and solved, discussing the role and the reliability of the adopted strategies. Finally, the seismic analysis of a masonry façade and the design of suitable FRP strengthening systems are performed.

Repair and Strengthening of Reinforced Concrete Beam-Column Joints with Fiber-Reinforced Polymer Composites

Three exterior reinforced concrete beam-column joint specimens were tested under reverse cyclic loading. The joint region of these specimens suffered significant damage, whereas limited damage was observed in the beams. The damaged specimens were repaired and strengthened to prevent shear damage and strength deterioration inside the joint region and to achieve more ductile response. First, the damaged loose concrete was removed and replaced by high-strength nonshrink mortar. Then, fiber-reinforced polymer (FRP) strips were diagonally wrapped over the joint region, and longitudinal FRP strips were applied and anchored on the beams. The original strength was restored in all strengthened specimens under the application of the same loading history. Deformation capacities of the strengthened specimens were much larger than those of the original specimens.

Analytical fatigue prediction model of RC beams strengthened in flexure using prestressed FRP reinforcement

The increasing use of Fiber Reinforced Polymer (FRP) for strengthening applications initiates the need to study the behavior of the strengthened members when subjected to fatigue loading. A general Fatigue Prediction Model (FPM) of Reinforced Concrete (RC) beams strengthened using prestressed FRP reinforcement was developed and presented in this paper. The damage effect was quantified into certain variables which can be determined analytically and/or experimentally. The developed model is applicable for any type of FRP strengthening. It accounts for the degradation in the concrete stress–strain relationship and the degradation of the epoxy-concrete and the steel–concrete bonds. The model was calibrated with experimental test results of RC beams strengthened using prestressed Near-Surface-Mounted (NSM) carbon FRP strips followed by performing a parametric study concerning the different model variables. It was found that the developed model accounts properly for the non-linearity in the strain variation across the depth of the RC strengthened beams as well as the effect of changing the prestress level when subjected to fatigue loading.

Experimental analysis of timber–concrete composite beam strengthened with carbon fibres

Reconstruction of old residential buildings is most often accompanied with a tendency to preserve the original features of the building, which leads to subsequent preservation of the floor elements. The ageing of the material and the consequent lower stiffness and load bearing capacity of the floor element along with the occasional change in the purpose of the facility and the consequent increase in the load require structural reinforcement of the floor elements. A possible solution lies in replacing the classic timber planks with a concrete slab and in the corresponding strengthening of the timber beam in the tensile zone. Therefore, the paper presents an experimental study performed on composite beam test samples composed of a concrete plate connected to a timber beam with dowel-type fasteners. Since the tension failure of the timber beam is the decisive resisting criterion, the beam is strengthened at the bottom of the tensile side with a carbon fibre-reinforced polymer strip. The measured results are compared with numerical results obtained by an already developed analytical model [1] based on Mohler’s simplified formulation and on European standards for timber, concrete and steel structures. Since the experimental results demonstrate a relatively low deviation and additionally a good agreement with the numerical ones, the usage of the presented analytical model can be recommended to predict proper stiffness and load bearing capacity for these types of composite floor structures.

Structural Behavior of Arch Models Strengthened Using Fiber-Reinforced Polymer Strips of Different Lengths

The authors address the structural behavior of masonry arches strengthened through carbon-fiber-reinforced polymer (CFRP) strips at intrados or extrados surfaces and subjected to horizontal forces. The paper reports and compares results from experimental tests on 1:2-scale arches and from an analytical model that considers no-tensile strength, infinite rigid, and infinite compressive strength material. Under the assumption that the structure fails because of the formation of a four-hinge collapse mechanism, the model allows the calculation of the collapse load and the evaluation of the load-displacement diagram. The model is based on the equilibrium conditions and considers finite displacements. Results provide an evaluation of the dependence of arch structural behavior on the length of CFRP strengthening. In particular, the postpeak behavior is evaluated in the perspective of performance-based seismic design criteria, which regard the ultimate displacement as a key parameter for seismic capacity. The evaluation shows that, for the considered case study, an increase of intrados reinforcement length produces an increase of maximum load but a decrease of the ultimate displacement.

Simple Homogenized Model for the Nonlinear Analysis of FRP-Strengthened Masonry Structures. II: Structural Applications

AbstractThe homogenized masonry nonlinear stress-strain curves obtained through the simple micromechanical model developed in the first part of the paper are here used for the analysis of strengthened masonry walls under various loading conditions. In particular, a deep beam and a shear wall strengthened with fiber-reinforced polymer (FRP) strips are analyzed for masonry loaded in-plane. Additionally, single and double curvature masonry structures strengthened in various ways, namely a circular arch with buttresses and a ribbed cross vault, are considered. For all the examples presented, both the nonstrengthened and FRP–strengthened cases are discussed. Additional nonlinear finite-element analyses are performed, modeling masonry through an equivalent macroscopic material with softening to assess the present model predictions. Detailed comparisons between the experimental data, where available, and numerical results are also presented. The examples show the efficiency of the homogenized technique with resp...

Debonding resistance of FRP-to-clay brick masonry joints

Debonding at the FRP-to-masonry interface has been identified as the preferred failure mechanism in fibre-reinforced polymer (FRP) retrofitted masonry as it allows for some redistribution of forces. The results of 14 FRP-to-masonry bond tests are presented, where the FRP was near surface mounted (NSM) to stack-bonded clay brick masonry. These tests were conducted to investigate the effect that variables such as cyclic loading and FRP strip dimensions have on the debonding resistance of a NSM FRP-to-masonry joint. These results were then incorporated into a large database of FRP retrofitted masonry pull test results by various researchers over the past 10years. The database includes results for both externally bonded (EB) and NSM retrofitting techniques. From this database, local bond–slip parameters such as the maximum interface shear stress, τmax, and the maximum slip, δmax, were investigated to determine correlations between these values and masonry material properties. Further, 15 existing concrete and masonry bond strength (maximum load at the FRP-to-substrate interface) models in the literature were assessed for their use with masonry by comparing these models against the results in the pull test database. Based on the comparative statistics of the test-to-predicted bond strength it is concluded that a new FRP-to-masonry bond model is required which gives more accurate predictions. Results include a discussion on the global load–slip response and FRP-to-masonry interface behaviour.

Numerical Investigation on the Influence of FRP Retrofit Layout and Geometry on the In-Plane Behavior of Masonry Walls

Unreinforced masonry (URM) structures have shown their vulnerability to major events like as earthquakes, severe wind, blast, and impact. The present work started from experimental programs available in scientific literature related to masonry walls made of clay or natural stone units. The finite-element method (FEM) was used to describe the global behavior of tested specimens in terms of shear/displacement curves, shear capacity, and cracking pattern. FEM, and particularly detailed micromodeling, was adopted as a numerical simulation tool for masonry walls. International design codes underline that some fiber reinforced polymers (FRP) dimensions (e.g., width, thickness, and spacing of FRP strips applied as external strengthening of URM walls made of different brickworks) may influence the global behavior of strengthened masonry. The present work, starting from experimental programs by other research groups, related to walls made of solid and hollow clay units as well as natural tuff units subjected to compression/shear loading aims at identifying the actual influence of those dimensions. Diagonal and horizontal strips were investigated. Different brickwork panels having the same FRP strengthening (quantity and geometry) showed different behaviors. Present outcomes highlighted that the influence of some FRP strengthening parameters (e.g., strip spacing) is not so meaningful if compared to FRP total amount even if the code formulations predict significant differences.

Process of debonding in RC beams shear-strengthened with FRP U-strips or side strips

Reinforced concrete (RC) beams may be strengthened for shear using externally bonded fiber reinforced polymer (FRP) composites in the form of side bonding, U-jacketing or complete wrapping. The shear failure of almost all RC beams shear-strengthened with side bonded FRP and the majority of those strengthened with FRP U-jackets, is due to debonding of the FRP. The bond behavior between the externally-bonded FRP reinforcement (referred to as FRP strips for simplicity) and the concrete substrate therefore plays a crucial role in the failure process of these beams. Despite extensive research in the past decade, there is still a lack of understanding of how debonding of FRP strips in such a beam propagates and how the debonding process affects its shear behavior. This paper presents an analytical study on the progressive debonding of FRP strips in such strengthened beams. The complete debonding process is modeled and the contribution of the FRP strips to the shear capacity of the beam is quantified. The validity of the analytical solution is verified by comparing its predictions with numerical results from a finite element analysis. This analytical treatment represents a significant step forward in understanding how interaction between FRP strips, steel stirrups and concrete affects the shear resistance of RC beams shear-strengthened with FRP strips.

Bond behaviour of FRP strips glued on masonry: Experimental investigation and empirical formulation

This paper investigates the bond behaviour of carbon and glass fibre reinforced polymers (FRPs) glued on various kinds of masonry supports. It presents the results of a wide experimental campaign carried out at the Laboratory of Structures of the University of Salerno (Italy). Four different types of masonry were considered. Clay-bricks as well as some natural masonry elements (i.e., tuff or limestone, that are rather common for existing buildings in the Mediterranean Area) were employed as supports for gluing FRP sheets and perform double-shear pull-out tests. A preliminary experimental work was carried out on samples of the above mentioned masonry blocks for quantifying their key mechanical properties. Thus, a wide series of masonry specimens were tested both in compression and bending for evaluating their compressive and tensile strength and determining the key statistical parameters which describe their distribution and scatter. Then, 29 double-shear pull-out tests were carried out for observing the bond behaviour of FRP strips on the various masonry substrates. Moreover, two sets of six coupon tests were performed for evaluating the actual axial stiffness of FRP plates glued on masonry samples.The paper reports the key results of the above mentioned tests with the aim of pointing out the influence of the various relevant parameters of both masonry and composite materials on the resulting bond strength. A significant influence of masonry properties was observed and quantified, whereas no significant effect induced by the two different kinds of composite materials were actually observed. The above mentioned experimental results are employed for assessing the theoretical formula adopted by Italian Guidelines for evaluating ultimate bond strength of composite laminates glued on masonry. Finally, an alternative fully empirical relationship is calibrated on pull-out test results presented in this paper.

Three dimensional mechanical model to simulate the NSM FRP strips shear strength contribution to a RC beam: Parametric studies

This paper presents the results of a parametric study carried out by means of a mechanical model recently developed to simulate the shear strength contribution provided by a system of Near Surface Mounted (NSM) Fiber Reinforced Polymer (FRP) strips to a Reinforced Concrete (RC) beam throughout its loading process. That model, developed fulfilling equilibrium, kinematic compatibility and constitutive laws of both materials, concrete and FRP, and local bond between themselves, takes into consideration the possibility that the NSM strips may fail due to: loss of bond (debonding), concrete semi-conical tensile fracture or strip tensile rupture. It also takes into consideration: (a) interaction between progressive force transferred by bond to the surrounding concrete and its tensile fracture and (b) bi-directional interaction among adjacent strips placed on the two sides of the strengthened beam cross-section web. In the first part of the paper attention is focused on the bond-based behavior of a single NSM FRP strip mounted on a concrete prism. The influence of each geometrical–mechanical parameter on the peak force transferable through bond stresses to the surrounding concrete, excluding the possibility of either concrete fracture and strip rupture, is analyzed. In the second part of the paper attention is focused on the comprehensive behavior of a single NSM FRP strip mounted on a concrete prism. The influence of each geometrical–mechanical parameter on the peak force transferable to the surrounding concrete, also including the possibility of both concrete fracture and strip rupture, is analyzed. The third part of the paper aims at assessing the influence of each geometrical–mechanical parameter on the maximum shear strength contribution provided by a system of NSM FRP strips to a RC beam. The results of these studies are presented along with the main findings.

Long-Term Performance of RC Members Externally Strengthened by FRP Exposed to Different Environments

Environmental conditions in the Gulf region are considered to be very aggressive to most structures. The main objective of the paper is to study the performance of RC elements (beams and slabs) strengthened by externally bonded FRP using carbon fiber-reinforced polymer (CFRP) sheets and strips and exposed to different environmental conditions. Six exposure conditions were used in the investigation to represent the Gulf region various environments. The exposure conditions used are laboratory, site, high temperature and humidity, cyclic drinking water, cyclic sea water and being embedded in Sabkha soil. The results presented in this paper are a part of a 36-month study program to evaluate the performance of different strengthened RC elements. This paper includes the 18-month test results after exposing the specimens to different environmental conditions. Comparison of the test results with the virgin elements (i.e. at 28 days of age and not exposed to the different environments) is carried out to quantify the change in performance of the elements. The performance of the structural elements is evaluated by measuring various deformations such as deflections, concrete strain, reinforcing steel strain and strain in the CFRP sheets and strips. Also, the mode of failure is reported. It was found that beams and slabs strengthened with CFRP strips were greatly affected by the exposure conditions compared with those strengthened by CFRP sheets. Sabkha soil and sea water were found to be the most aggressive environments.

PREDICTIVE RESPONSE OF NOTCHED STEEL BEAMS REPAIRED WITH CFRP STRIPS INCLUDING BOND-SLIP BEHAVIOR

This paper presents the flexural behavior of notched steel beams repaired with carbon fiber-reinforced polymer (CFRP) strips. A combined experimental and computational approaches are used to examine local plasticity near the damage and the effects of CFRP-repair. A modeling approach is proposed to take into account the bond-slip behavior of CFRP-steel interface. The experimentally validated models are further used to conduct a parametric study addressing various engineering properties of CFRP composites and adhesives. The CFRP-repair is shown to restore the strength of the damaged beam. The CFRP strip relieves the stress concentration resulting from the presence of the notch, reducing the high local plasticity. The parametric study confirms the improved effectiveness of high modulus CFRP (i.e., exceeding 150 GPa) in affecting repairs of steel members. Under static loading conditions, the stiffness of the adhesive bond line influences the local behavior of the CFRP-steel interface but has little effect on the overall member behavior.

Improved hybrid bonding technique for attaching FRP to reinforced concrete beams

The use of externally bonded fibre reinforced polymer (EB-FRP) composites is becoming popular for the repair of understrengthened or damaged reinforced concrete members. However, the EB-FRP strengthened members often fail owing to fibre reinforced polymer (FRP) debonding, reducing the strength utilisation ratio of FRP and leading to catastrophic structural failure. To solve the problem, an improved hybrid bonded FRP (IHB-FRP) technique is developed in this paper. The pull-out test was conducted to select an appropriate mechanical fastener for the technique and IHB-FRP strengthened reinforced concrete beams were tested under three-point bending to examine the effectiveness of the technique. Experimental results show that the selected fastener is suitable for the IHB-FRP technique and the IHB-FRP strengthened beam fails owing to FRP rupture even for seven plies of FRP strips. Finally, the required tightening force in the fastener is estimated to ensure the effectiveness of the IHB-FRP technique.

Behavior of RC T-Beams Strengthened in Shear with CFRP under Cyclic Loading

This study investigated the shear performance of an RC beam strengthened in shear with externally bonded carbon fiber-reinforced polymer (CFRP) strips, subjected to a cyclic loading for 2 million cycles at 1 Hz. The stress level in the stirrups caused by the cyclic loading used in this study was higher than those typically used in fatigue studies, which could have caused the yielding of some stirrups from the beginning of cyclic loading. This was done to reflect the fact that many RC beams in need of shear strengthening do not meet the minimum stirrup requirement for the new and increased shear demand. The experimental results obtained in this study and a comprehensive review of the existing literature showed that RC beams strengthened in shear with externally bonded CFRP could survive 2 million cycles of cyclic loading without failure. Furthermore, the residual shear strength of the FRP-strengthened beam appeared to be greater, albeit slightly, than the static shear strength of the unstrengthened control beam. This study’s results also suggested that limiting the interfacial stress in CFRP strips to less than 1.5 MPa or 25% of its ultimate interfacial strength would increase fatigue life by avoiding debonding of CFRP strips.

Parametric study of composite T-beam

This paper provides a parametric study of a composite T-section, composed of a concrete slab and a timber beam, strengthened at the bottom tension side with a carbon fibre reinforced polymer strip. The study is innovative in the field and the design of the beam on which the study is based was carried out in accordance with European standards for timber, steel and concrete structures and guidelines for fibre reinforced polymer materials. The carbon strip contribution to the bending resistance and stiffness of composite section is presented as a function of timber grade, timber beam depth and spacing between fasteners. The results show an increase of design load-bearing resistance in the range of 26–31% as well as increase of bending stiffness in the range of 18–29%. The importance of material grade variation and the influence of time-dependent effects on load-bearing resistance are emphasised.