Prestressed Carbon Fiber-reinforced Polymer Research Articles

Use of 3D laser scanning on evaluating reduction of initial geometric imperfection of steel column with pre-stressed CFRP

A novel method for obtaining initial geometric imperfections by three-dimensional (3D) laser scanning is developed to study a discovery of reducing initial geometric imperfections using pre-stressed (PS) carbon fiber reinforced polymer (CFRP) strips. New methodologies for establishing digital geometric models of specimens based on point cloud data are presented. The initial deflections along the weak axes of two series of specimens with I-shaped section, 10 specimens in total, are obtained by these methods. It is found that the initial deflections along the weak axes of the steel columns are reduced by the PS CFRPs. Then, the force method is implemented to establish a theoretical model for steel columns strengthened with PS CFRPs. The model illustrates the mechanism for decreasing geometric imperfections in a self-balanced process. Corresponding equations based on the theoretical model are proposed to explain the reduction of the initial deflections. Finally, the model is verified by comparing its predictions to the measurement results.

Mixed mode I/II fatigue crack arrest in steel members using prestressed CFRP reinforcement

In the present study, a strengthening design approach is proposed for the mixed mode I/II fatigue crack arrest in existing structural steel members using prestressed unbonded carbon fiber reinforced polymer (CFRP) composites. Through the analytical formulation of mode I and II stress intensity factor ranges, a design model is proposed to determine the strengthening solution, including the required prestressing level and/or the cross-sectional area of the reinforcement, which would ensure the complete arrest of an existing mixed mode I/II fatigue crack in a steel member. In parallel, sets of stepwise high-cycle fatigue tests were carried out on reference unstrengthened and prestressed CFRP-strengthened precracked steel plates of grade S355J2+N under various mode mixities. The experimental results revealed that the maximum tangential stress (MTS) criterion fairly predicts the state of the mixed mode I/II fatigue cracks (i.e., crack arrest or growth) in unstrengthened specimens, while the proposed design model provides a conservative estimation of the mixed mode I/II fatigue threshold in prestressed CFRP-strengthened specimens. Furthermore, the crack propagation characteristics of grade S355J2+N steel, i.e., Paris’ law parameters (C and m) and the crack closure parameter (U), were determined and demonstrated to be independent of the material rolling direction. Based on the analytical and experimental results of the current study, it can be concluded that the proposed model can be used for the safe design of strengthening solutions, which is an increasing need to extend the service life of existing fatigue-damaged steel structures; certain recommendations are provided in this regard for practical strengthening applications.

Vertical seismic response of a box bridge strengthened with posttensioned, prestressed CFRP sheets

ABSTRACTIn this paper, a finite element model is developed to analyse the responses of a box bridge to different seismic waves before and after strengthening using posttensioned, prestressed carbon fibre reinforced polymer (CFRP) sheets. The box beam and pier of a four-span continuous box bridge in Tianshui City were strengthened, and the seismic performance was studied. Based on the vertical seismic loads, the dynamic time-history of the entire bridge was calculated, and the effect of the posttensioned prestressed CFRP sheet on the seismic performance of the bridge was examined. The dynamic responses of the box bridge to three waves show that the vertical displacement and acceleration of the sections under the Tianjin wave decreased after their peaks; however, the vertical displacements and accelerations for the El Centro and Lanzhou waves slowly stabilised. Consequently, the Tianjin wave caused more damage to the bridge structure than did the other two waves. The seismic responses of the bridge under the actions of the three waves can be effectively reduced after reinforcement with a posttensioned prestressed CFRP sheet.

Steel columns strengthened/reinforced by prestressed CFRP strips: Concepts and behaviors under axial compressive loads

A novel strengthening method using prestressed (PS) carbon fiber reinforced polymer (CFRP) strips was proposed. This method can be used for repairing existing steel columns or constructing new columns for high-rise buildings, power transmission towers and other buildings. Specifically, a prestressing system (PS system) can be used to prestress CFRP strips without using a hydraulic jack. A steel column, PS CFRP strips and the PS system comprise a self-balanced system that offers much greater stiffness against overall buckling than a pure steel column. To verify this strengthening method, axial compression tests are conducted. The results show that the buckling load of the PS-strengthened specimen is 2.5 times greater than that of an unstrengthened specimen, and the failure mode ranges between the first order symmetric and the second order asymmetric overall buckling mode. In addition, finite element analysis (FEA) was adopted to obtain the optimal prestress value, which can also be used as a guide in design. Finally, a simple calculation method of axial bearing capacity was proposed.

Experimental study on prestressed concrete hollow slabs in service strengthened with prestressed CFRP plates

PurposeMost existing studies are confined to model beam tests, which cannot reflect the actual strengthening effects provided by prestressed carbon-fiber-reinforced polymer (CFRP) plates to existing bridges. Hence, the actual capacity for strengthening existing bridges with prestressed CFRP plates is becoming an important concern for researchers. The paper aims to discuss these issues.Design/methodology/approachStatic load tests of in-service prestressed concrete hollow slabs before and after strengthening are conducted. Based on the results of the tests, the failure characteristics, failure mechanism and bending performance of the slabs are compared and analyzed. Nonlinear finite element method is also used to calculate the flexural strength of the strengthened beams prestressed with CFRP plates.FindingsTest results show that prestressed CFRP plate strengthening technology changes the failure mode of hollow slabs, delays the development of deflection and cracks, raises cracking and ultimate load-carrying capacity and remarkably improves mechanical behavior of the slab. In addition, the nonlinear finite element analyses are in good agreement with the test results.Originality/valueStrengthening with prestressed CFRP plates has greater advantages compared to traditional CFRP plate strengthening technology and improves active material utilization. The presented finite element method can be applied in the flexural response calculations of strengthened beams prestressed with CFRP plates. The research results provide technical basis for maintenance and reinforcement design of existing bridges.

Prestressed CFRP-strengthening and long-term wireless monitoring of an old roadway metallic bridge

This paper presents an application of prestressed carbon fiber-reinforced polymer (CFRP) plates for the strengthening of metallic girders of a roadway bridge in Melbourne, Australia. The study also describes the application of a wireless sensor network (WSN) system for long-term structural monitoring of the retrofitted bridge girders. A flat prestressed unbonded retrofit (FPUR) system was developed to apply prestressed CFRP plates to the steel girders of the Diamond Creek Bridge (122 years old), which is subjected to daily passenger and heavy vehicles. The first section explains the results of sets of static and fatigue tests performed in the laboratory to examine the efficiency of the proposed FPUR system prior to its installation on the bridge. The second section presents details of different aspects of the CFRP strengthening of the bridge girders, fatigue design criteria, and layouts for short- and long-term monitoring. For the short-term measurements, the bridge was loaded with a 42.5-ton semi-trailer before and after strengthening. The CFRP plates were prestressed up to approximately 980 MPa (≈38% of the CFRP ultimate tensile strength), which resulted in about 50% reduction in the maximum tensile stress in the bridge girders. The third section discusses the development, installation, and preliminary results of the WSN system used to monitor the pre-stress level in the CFRP plates. The results of the short- and long-term measurements in this study show that the FPUR system is very effective for flexural and fatigue strengthening of bridge girders. Finally, a set of recommendations for long-term structural monitoring is provided.

The Gradient Anchorage Method for Prestressed CFRP Strips: from the Development to the Strengthening of an 18 M Long Bridge Girder

Abstract The external bonding of carbon fiber-reinforced polymer (CFRP) strips by two-component epoxy adhesive on the concrete surfaces of buildings and bridges is a retrofitting method accepted worldwide. The gradient anchorage (GA) is an anchoring method especially developed to anchor prestressed CFRP strips to concrete elements without a need for mechanical clamping after the installation phase. This method takes advantage of the adhesives property to undergo accelerated curing when heated. The results of more than fifteen years of research on the development of the gradient anchorage at the Swiss Federal Laboratories for Materials Science and Technology (Empa) are presented in this paper. The basic principles and application steps are explained, and the main results starting from the development of the technique up to the testing of real scale girders are described, and the new challenges posed by this innovative system are highlighted. The gradient anchorage is a valid alternative to a mechanically anchored system for prestressed FRP (P-FRP).

Experimental Study on Fatigue Behavior of Cracked Rectangular Hollow-Section Steel Beams Repaired with Prestressed CFRP Plates

AbstractThe bending fatigue test results on cracked rectangular hollow section (RHS) steel beams repaired with prestressed carbon fiber-reinforced polymer (CFRP) plates are presented. The steel bea...

Behaviour of Prestressed CFRP Anchorages during and after Freeze-Thaw Cycle Exposure.

The long-term performance of externally-bonded reinforcements (EBR) on reinforced concrete (RC) structures highly depends on the behavior of constituent materials and their interfaces to various environmental loads, such as temperature and humidity exposure. Although significant efforts have been devoted to understanding the effect of such conditions on the anchorage resistance of unstressed EBR, with or without sustained loading, the effect of a released prestressing has not been thoroughly investigated. For this purpose, a series of experiments has been carried out herein, with concrete blocks strengthened with carbon fiber-reinforced polymer (CFRP) strips, both unstressed, as well as prestressed using the gradient anchorage. The gradient anchorage is a non-mechanical technique to anchor prestressed CFRP by exploiting the accelerated curing property of epoxy under higher temperatures and segment-wise prestress-force releasing. Subsequently, strengthened blocks are transferred into a chamber for exposure in dry freeze-thaw cycles (FTC). Following FTC exposure, the blocks are tested in a conventional lap-shear test setup to determine their residual anchorage resistance and then compared with reference specimens. Blocks were monitored during FTC by conventional and Fabry–Pérot-based fiber optic strain (FOS) sensors and a 3D-digital image correlation (3D-DIC) system during gradient application and lap-shear testing. Results indicate a reduction of residual anchorage resistance, stiffness and deformation capacity of the system after FTC and a change in the failure mode from concrete substrate to epoxy-concrete interface failure. It was further observed that all of these properties experienced a more significant reduction for prestressed specimens. These findings are presented with a complementary finite element model to shed more light onto the durability of such systems.

Structural analyses of extruded CFRP tendon-anchor system and applications in thin-walled aluminum/CFRP bridge

Pre-stressed carbon fiber reinforced polymer (CFRP) tendon system is developed. Extrusion method guarantees the strength of the CFRP tendon could be fully used in strengthening. The pre-stressed CFRP tendons are applied to strengthen aluminum beams and portable aluminum bridge to improve the flexural rigidity and load carrying capacity of these structures and the improvements are revealed by the experiments. Applying extruded CFRP tendon-anchor system, the load carrying ability of the pre-stressed aluminum bridge has an improvement of 25%. Structural models are proposed to investigate the pre-stressing effects, including the deflection and the load capacity under different design criteria. Finite element method (FEM) is applied to analyze the nonlinear behavior of the pre-stressed beam and reveal the plastic torsional buckling failure style. All these analyses agree well with the experiments and suggest accurate methods to design pre-stressed aluminum/CFRP structures.

Mode I fatigue crack arrest in tensile steel members using prestressed CFRP plates

Numerous studies in the literature have shown that the strengthening of steel members using carbon fiber reinforced polymer (CFRP) composites can significantly extend the fatigue life of these structures. However, not enough attention has been focused on the potential of prestressed CFRP reinforcements for fatigue crack arrest in such members. In the current study, a simple analytical model is proposed to calculate the required prestressing level in the CFRP reinforcements in order to arrest the propagation of an existing fatigue crack in tensile steel members. Furthermore, a novel mechanical unbonded system is developed to anchor the high prestressing forces in CFRP reinforcements to the steel substrate using friction. A set of fatigue tests are performed on unstrengthened and strengthened precracked steel plates to verify the proposed model. The experimental results of the current study showed that the application of nonprestressed ultra-high modulus CFRP plates as externally bonded reinforcements can increase the fatigue life of precracked steel plates by a factor of 4.3. However, fatigue crack arrest is only possible when prestressed CFRPs of a certain prestressing level are used. Based on the analytical, numerical, and experimental results of the current study, it can be concluded that existing fatigue cracks in tensile steel members can be arrested using the proposed prestressed unbonded reinforcement system with the initial prestressing level calculated using the proposed model. In addition, some design recommendations are provided for fatigue crack arrest in practical cases.

Fatigue Assessment Model of Corroded RC Beams Strengthened with Prestressed CFRP Sheets

This paper presents a fatigue assessment model that was developed for corroded reinforced concrete (RC) beams strengthened using prestressed carbon fiber-reinforced polymer (CFRP) sheets. The proposed model considers the fatigue properties of the constituent materials as well as the section equilibrium. The model provides a rational approach that can be used to explicitly assess the failure mode, fatigue life, fatigue strength, stiffness, and post-fatigue ultimate capacity of corroded beams strengthened with prestressed CFRP. A parametric analysis demonstrated that the controlling factor for the fatigue behavior of the beams is the fatigue behavior of the corroded steel bars. Strengthening with one layer of non-prestressed CFRP sheets restored the fatigue behavior of beams with rebar at a low corrosion degree to the level of the uncorroded beams, while strengthening with 20- and 30%-prestressed CFRP sheets restored the fatigue behavior of the beams with medium and high corrosion degrees, respectively, to the values of the uncorroded beams. Under cyclic fatigue loading, the factors for the strengthening design of corroded RC beams fall in the order of stiffness, fatigue life, fatigue strength, and ultimate capacity.

Fatigue Behavior of Reinforced Concrete Beams Strengthened with Externally Bonded Prestressed CFRP Sheets

AbstractAn experimental study was conducted to investigate the fatigue behavior of RC beams strengthened with posttensioned prestressed carbon fiber-reinforced polymer (CFRP) sheets. The experiment...

Flexural strengthening of reinforced low strength concrete slabs using prestressed NSM CFRP laminates

The effectiveness of the Near Surface Mounted (NSM) technique with prestressed CFRP (carbon fiber reinforced polymer) laminates for the flexural strengthening of reinforced concrete (RC) slabs of low strength concrete was assessed. Four RC slabs were tested, a reference slab (without CFRP), and three slabs flexurally strengthened using NSM CFRP laminates with different prestress level of the ultimate tensile strength of the CFRP: 0%, 20% and 40%. The experimental program is described and the main results are presented and analyzed in terms of the structural behavior of the RC slabs, failure modes and performance of the NSM technique with prestressed CFRP laminates. The results show that prestressing CFRP laminates with NSM technique is an effective technology to increase cracking, service, yielding and maximum loads of RC slabs made by low strength concrete. By applying NSM CFRP laminates prestressed at 20%, the cracking, service and maximum loads have increased, respectively, 258%, 123% and 125%, when the corresponding values of the reference slab are taken for comparison purposes, while 400%, 190% and 134% were the increase when applying laminates prestressed at 40%. Using available experimental results obtained with the same test setup, but using RC slabs of higher strength concrete, it can be concluded that as minimum is the concrete strength as more effective is the NSM technique with prestressed CFRP laminates in terms of serviceability limit states. A numerical strategy was used to evaluate the load-deflection of the tested RC slabs and to highlight the influence of the percentage of CFRP, the percentage of tensile longitudinal bars and the elasticity modulus of the CFRP on the effectiveness of the NSM technique with prestressed CFRP laminates for the flexural strengthening of RC slabs, by performing a parametric study.

Analytical and numerical solution of the interfacial stress in reinforced-concrete beams reinforced with bonded prestressed composite plate

The objective of this present research is to propose a simplified analytical and numerical approach to predict the interfacial stress of simply supported reinforced-concrete beams reinforced with prestressed carbon fiber-reinforced polymer composite plate. Analytical design methodology is based on equilibrium and deformations compatibility requirements in and all parts of the strengthened beam, i.e. the concrete beam, the prestressed carbon fiber-reinforced polymer plate, and the adhesive layer. The results show that there exists a high concentration of both shear and normal stress at the ends of the laminate, which might result in premature failure of the strengthening scheme at these locations. In numerical analysis, the mesh sensitivity test shows that the finite element results for interfacial stresses are not sensitive to the finite element mesh. The finite element analysis then is used to calculate the interfacial stress distribution and evaluate the effect of the structural parameters on the interfacial behavior. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution. This research is helpful for the understanding on mechanical behavior of the interface and design of the interfacial stress in reinforced-concrete beams reinforced with bonded prestressed carbon fiber-reinforced polymer plate.

Performance of steel beams strengthened with prestressed CFRP laminate

Prestressed Carbon Fibre Reinforced Polymer (CFRP) system is generally used for strengthening reinforced concrete beams where CFRP laminate can improve both the strength and serviceability behaviour of reinforced concrete beams via increasing the overall member stiffness. However, the applicability of this technique to strengthening steel structures is still lagging behind its application to concrete structures. In this study, the flexural behaviour of steel I-beams strengthened with prestressed CFRP laminate using a mechanical anchorage system is experimentally investigated. A total of nine steel beams subjected to flexural loading are tested in various conditions to evaluate the effectiveness of the proposed strengthening system. The experimental investigation confirmed that CFRP prestressing increases the ultimate load of the strengthened steel beams and moderately delays the premature debonding failure of the CFRP laminate. Even with low level of CFRP prestressing, significant enhancement in the ultimate load of the strengthened beam was recorded. Beams strengthened using non-prestressed CFRP laminate mainly failed due to premature debonding of the laminate with a slight increase in the failure load. Mechanical end anchorages maintain the CFRP laminate prestress after releasing the jacking force without encountering any debonding of the CFRP laminate till final failure of the strengthened steel beam. The results of the experimental programme and its outcomes are presented and discussed.

Mechanical Analysis and Economic Research of RPC Beam Bridge

RPC(Reactive Powder Concrete) is a kind of high performance concrete which has many excellent mechanical properties, e.g. ultra-high-strength, high tenacity and good durability, and it has been applied in engineering practice. However, at present the economic research about RPC bridge is few at home and abroad, which has hindered the development of RPC bridge to some extent. By means of the mechanical analysis and economic research about NC(normal concrete ) beam bridge of pre-stressed steel strand (the span is 40m), RPC beam bridge of pre-stressed steel strand and RPC beam bridge of pre-stressed CFRP(Carbon Fiber Reinforced Polymer) tendon after the sectional optimization has been done, this paper reach the conclusion that the RPC beam bridge can greatly reduce the sectional size of beam bridge and reduce the weight by 23.7% , besides, the total cost of RPC beam bridge of pre-stressed steel strand is almost same with the total cost of NC beam bridge during the entire service period of bridge. Therefore, considering the long-term perspective, the integrated economy of RPC beam bridge of pre-stressed steel strand is good.

Determination of minimum CFRP pre-stress levels for fatigue crack prevention in retrofitted metallic beams

The majority of fatigue strengthening studies focus on reducing propagation rates of existing cracks, ignoring the crack initiation stage. Many existing metallic bridge members however do not contain existing cracks, but rather are nearing their design fatigue life. Limited research exists on the prevention of crack initiation using carbon fiber reinforced polymer (CFRP) materials. In this paper, constant life diagrams (CLDs) are used to determine the minimum level of CFRP pre-stress required to indefinitely extend the fatigue life of existing metallic beams. It is shown that by applying a compressive force to an existing fatigue-susceptible detail using pre-stressed CFRP plates, the mean stress level can be reduced such that the detail is shifted from the ‘finite life’ regime to the ‘infinite life’ regime. The proposed fatigue strengthening approach is advantageous particularly when the stress history from the prior traffic loadings is not known. To validate the proposed method, a pre-stressed un-bonded CFRP reinforcement system is introduced and tested on four metallic beams. The proposed un-bonded CFRP system is advantageous over traditional bonded CFRP systems as it can be applied to rough or obstructed surfaces (surfaces containing rivet heads or corrosion pitting for example). Additionally, the new un-bonded CFRP system offers a fast on-site installation (no glue and surface preparation are required) and an adaptive pre-stress level. Experimental results show that strengthening using pre-stressed CFRP plates are capable of shifting the working stresses from a finite fatigue-life zone to an infinite fatigue-life zone preventing crack initiation. Although according to many structural standards, the stress range is the main parameter that affects the fatigue life of a metallic detail, the results of this study clearly show that the mean stress level also plays a significant rule in the detail fatigue life. Based on the proposed CLD approach in this paper, the combined effects of the stress range and mean stress level can be taken into account for prediction of fatigue life of metallic members.

Effect of punching shear on load–deformation behavior of flat slabs

Punching shear resistance usually constitutes the decisive design criterion for reinforced concrete flat slabs supported by columns. Modeling approaches based on Kinnunen and Nylander’s rotation-symmetric sector model, such as the Quadrilinear Sector Model (QSM), allow the prediction of the slab’s load–rotation behavior. To take into account an orthogonal reinforcement layout, the steel Young’s Modulus is usually reduced. A mechanically better justified flexural stiffness reduction factor is derived here which depends on the longitudinal reinforcement and modular ratio. The QSM also neglects shear deformations, which may reduce the flexural capacity. A Modified Sector Model (MSM) is derived which considers the influence of shear on the flexural behavior. A strength reduction factor for the shear crack-crossing longitudinal reinforcement is introduced which depends on the mechanical longitudinal reinforcement ratio. The factor takes into account that the longitudinal reinforcement also has to transmit forces resulting from shear. The MSM can also be applied to predict the load–rotation responses and punching shear resistances of flat slabs strengthened with prestressed carbon fiber-reinforced polymer (CFRP) straps.

A new hybrid methodology according to near surface mounted carbon fiber reinforced polymer technique for the flexural strengthening of reinforced concrete beams

The objective of this paper is to propose a new hybrid methodology according to near surface mounted (NSM) technique, using carbon fiber reinforced polymer (CFRP) reinforcement for the flexural strengthening of reinforced concrete (RC) beams. This NSM hybrid flexural strengthening technique combines non-prestressed and prestressed CFRP laminates in the same application in order to provide a good balance in terms of load-carrying and ultimate displacement capacity to the strengthened elements. An experimental program composed of six RC beams was carried out to assess the benefits of this NSM hybrid technique when compared to the use of non-prestressed or prestressed NSM CFRP laminates (NSM prestressing technique). For this purpose, the performance of both techniques in terms of crack width, prevailing failure mode, ultimate displacement capacity, energy absorption, and load-carrying capacity of the strengthened beams was assessed. The experimental tests were also simulated by executing advanced 3D nonlinear finite element analysis. Moreover, the potentialities of other configurations for the NSM hybrid technique by adopting different non-prestressed CFRP reinforcement ratios were numerically assessed executing a parametric study, and the relevant results are presented and discussed.