Prestressed Carbon Fiber Reinforced Polymer Laminates Research Articles

Flexural Strengthening of Concrete Structures Using Externally Bonded and Unbonded Prestressed CFRP Laminates: A Literature Review

Carbon fiber-reinforced polymer (CFRP) materials have been widely used in prestressed and non-prestressed systems for the flexural strengthening of concrete structures in many construction projects worldwide. Strengthening with non-prestressed CFRP materials, which is known as passive repair, could lead to an increase in the ultimate capacity of the concrete member. However, it has little effect on its serviceability performance (e.g., cracking, yielding, and deflection properties). Flexural strengthening with prestressed CFRPs provides an active repair technique that positively effects the strength and serviceability of structures. In addition, strengthening with prestressed fiber-reinforced composites efficiently uses the material’s high tensile strength. Although different fiber-reinforced polymer (FRP) materials and forms have been used in prestressing applications, this paper focused on the use of CFRP laminates (e.g., sheets, strips, or plates). These laminates have been widely used due to their excellent mechanical properties, ease of application in various geometrical shapes, good conforming surface area of contact with structures, and low installation costs. A comprehensive literature review was presented on strengthening using prestressed CFRP laminates in conventionally and internally prestressed concrete structures (PCSs). The externally bonded and unbonded prestressed CFRP laminate applications that used different anchorage systems were presented. The flexural behavior, failure modes, and serviceability performance of the strengthened concrete members were discussed. The effects of the prestressing level on ductility and energy absorption and a summary of the recommended optimum prestressing level discussions were presented in this paper. Furthermore, a review of the theoretical studies that were conducted for prestressed CFRP applications was provided. The knowledge gaps in the research area and future research recommendations were presented.

Bifurcation and propagation of mixed mode fatigue crack in steel plates repaired with prestressed CFRP laminates

The mixed-mode crack propagation model was proposed to predict crack growth trajectory of a central cracked tensile steel plate with prestressed carbon fiber-reinforced polymer (CFRP) reinforcement through verification of previous experiments. A sensitivity analysis of different parameters including CFRP fiber orientation, initial crack geometry, and prestress level was conducted to investigate CFRP repairing efficiency by using a combination of the programs FRANC3D and ABAQUS. The results show that the applied loading direction primarily governs the bifurcation angle of crack propagation. Increasing prestress level of CFRP along the applied loading direction can achieve ten-fold repairing efficiency of damaged steel structures.

Flexural behavior of light steel purlins reinforced by prestressed CFRP laminates

A reinforcing technology using prestressed carbon fiber-reinforced polymer (CFRP) laminates was proposed for reinforcing steel purlins under bending. The reinforcing system can prestress CFRPs without using a hydraulic jack, which enables convenient construction and high reinforcing efficiency. The flexural behavior of the reinforced steel purlins was studied experimentally and theoretically. First, the flexural failure caused by the local buckling or yielding of the upper flange was successfully delayed by the prestressed CFRPs; the flexural capacity and stiffness were improved by 14% to 18% and 28% to 31% in tests, respectively. Then, a calculation method for determining the flexural capacity and corresponding deflection was proposed, which gave accurate results compared with the test. This study provides useful references for reinforcing cold-formed thin-walled steel beams with prestressed CFRPs.

Innovative prestressing method for externally bonded CFRP laminates without mechanical anchorage

Strengthening of reinforced concrete (RC) structures by externally bonded carbon fiber reinforced polymer (CFRP) laminates has been widely accepted as an effective and cost-efficient method. It is well known that advantages offered by the bonded CFRP laminates can be further increased by prestressing the laminates prior to bonding. Mechanical anchors are essential, in this case, to prevent debonding since interfacial stress at areas close to the ends of the strengthening laminate is several times higher than the strength of the concrete substrate. Common anchorage solutions often consist of bolted metallic plates to clamp the prestressed CFRP laminate. Besides labor-intensive installation operation, the anchor plates are vulnerable to galvanic corrosion, which further complicates the inspection and increases the maintenance costs. There are also doubts about the long-term performance of such anchorage systems as it highly depends on the quality of the adhesive layer between the plate and laminate, and the level of pre-tension in clamping bolts.This paper presents the work conducted at Chalmers University of Technology on the development of an innovative prestressing method and a tool which allow for the application of prestressed CFRP laminates without mechanical anchors. The principles of the novel method and the prestressing system are explained. Experimental and numerical work carried out on an RC beam strengthened with this method is presented. Results indicate that CFRP laminates with high prestressing forces (approximately 30% of CFRP tensile strength) can be safely anchored without the need for mechanical anchors. Numerical results based on finite element analyses show that the proposed prestressing method can reduce the interfacial shear stresses in the CFRP-concrete adhesive joint below the bond strength with a reasonable safety margin.

Flexural behavior of preloaded reinforced concrete beams strengthened by prestressed CFRP laminates

This study focuses on evaluating flexural behavior of preloaded reinforced concrete beams (RC) strengthened with pre-tensioned (prestressed) carbon fiber reinforced polymer (CFRP) laminates. A technique for tensioning and application of pre-tensioned CFRP laminates strengthening system were developed. In order to verify the effectiveness, a total of twelve RC beam specimens were strengthened and tested under monotonic three-point loading up to failure. For comparison purposes, one unstrengthened control (as-built) RC beam and another specimen strengthened with conventional non-tensioned CFRP laminates were tested. The other ten RC beam specimens were strengthened with pre-tensioned CFRP laminates and the performance was compared to the control specimens. Several influential experimental parameters were considered including: (i) pre-tensioned strain level, (ii) CFRP reinforcement ratio, and (iii) sustained loading level. Experimental results indicated that the first two parameters of pre-tensioning strain and CFRP reinforcement ratio had significant influence on both flexural strength and stiffness of the strengthened beams. However, experimental results indicated that the level sustained load had less influence, especially when failure mode is in the form of CFRP laminates rupture. Also, the behavior of retrofitted beams was simulated using nonlinear finite element analyses. Good correlation between experimental and numerical results was achieved. Results indicated that in cases where sustained loads were larger than 40% of the control beam ultimate load, the capacity of strengthened beams was notably reduced. However, this negative effect could be avoided by maintaining the pre-strain level to 3000.0∼4000.0με.

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.

Advanced Active Prestressed CFRP in RCC Structures

. The need for rehabilitation of reinforced concrete structures is rapidly increasing. Fibre reinforced polymer (FRP) composite materials for concrete structures have high strength-to-weight ratios that can provide high prestressing forces while adding minimal additional weight to a structure. They also have good fatigue properties and exhibit low relaxation losses, both of which can increase the service lives and the load carrying capacities of reinforced concrete structures. Carbon fiber reinforced polymer (CFRP) composite system is integrated system based on carbon fibres and epoxy resins. By prestressing the CFRP laminates, the material is used more efficiently as a part of its tensile capacity is utilised and it contributes to the load bearing capacity under both service and ultimate load condition. This is an ideal technique as it combines the advantage of using noncorrosive and lightweight advanced composite material in the form of FRP laminates with high efficiency offered by external prestressing. An innovative mechanical anchorage system was developed to prestress the FRP laminates directly by jacking and reacting against the RCC structure.This paper describes the use of Prestressed CFRP laminates for strengthening of RCC structures including practical applications on slabs and bridges. Also it elucidates the post strengthening testing carried out for the validation of this technique.

A Probabilistic Model for the Flexural Capacity of Reinforced Concrete Structures Strengthened with Prestressed CFRP Plates

Strengthening concrete structures with prestressed carbon fiber reinforced polymer (CFRP) materials has gained popularity in recent years due to its ability to efficiently utilize the material's strength. However, the reliability of structures strengthened with prestressed CFRP laminates is a key issue. In this paper the statistical characteristics of design variables of reinforced concrete beams strengthened with prestressed CFRP plates are assessed. Accounting for the gradient effects, the Weibull distribution is used to describe the probability distribution function of the size and stress. The loss of prestressing force is analyzed considering the prestressing method. The probabilistic models of the flexural capacity are established for the three failure models, and the influence of variables on the flexural capacity is also studied. The probabilistic model derived in this paper can be used as the theoretical basis of reliability analysis and safety assessment of structures strengthened with CFRP materials.

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.

CFRP-Strengthening and Long-Term Performance of Fatigue Critical Welds of a Steel Box Girder

Empa’s research efforts in the 1990s provided evidence that a considerable increase of the fatigue strength of welded aluminum beams can be achieved by externally bonding pultruded carbon fiber reinforced polymer (CFRP) laminates using rubber-toughened epoxies over the fatigue-weak welding zone on their tensile flange. The reinforcing effect obtained is determined by the stiffness of the unidirectional CFRP laminate which has twice the elastic modulus of aluminum. One can therefore easily follow that an unstressed CFRP laminate reinforcement of welded beams made of steel will not lead to a substantial increase in fatigue strength of the steel structure. This consideration led to the idea of prestressing an external reinforcement of the welded zone. The present investigation describes experimental studies to identify the adhesive system suitable for achieving high creep and fatigue strength of the prestressed CFRP patch. Experimental results (Wöhler-fields) of shear-lap-specimens and welded steel beams reinforced with prestressed CFRP laminates are presented. The paper concludes by presenting a field application, the reinforcement of a steel pendulum by adhesively bonded prestressed CFRP laminates to the tensile flanges of the welded box girder. Inspections carried out periodically on this structure revealed neither prestress losses nor crack initiation after nine years of service.

Effectiveness of prestressed NSM CFRP laminates for the flexural strengthening of RC slabs

An experimental program was carried out to investigate the effect of the prestressed Near Surface Mounted (NSM) Carbon Fiber Reinforced Polymer (CFRP) laminates on the behavior of reinforced concrete (RC) slabs. 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 solution to increase cracking, service, yielding and maximum loads of RC slabs failing in bending. By applying 20% of prestressed NSM CFRP laminates, the service and ultimate loads have increased, respectively, 55% and 136%, when the corresponding values of the reference slab are taken for comparison purposes, while 119% and 152% were the increase when applying 40% of prestressed laminates. A numerical strategy was used to evaluate the load–deflection of the tested RC slabs, and a good agreement between experimental and numerical results was obtained.

RC Slabs Strengthened with Prestressed and Gradually Anchored CFRP Strips under Monotonic and Cyclic Loading

This paper presents the test results of reinforced concrete slabs strengthened with prestressed and gradually anchored carbon fiber–reinforced polymer (CFRP) strips under monotonic and cyclic loading. To take full advantage of the externally bonded CFRP technique, it is beneficial to apply the laminates in a prestressed state, which relieves the stress in the steel reinforcement and reduces crack widths and deflection. The aim of the monotonic tests was to determine the strengthening efficiency of the new prestressing technique and to investigate serviceability and ultimate states. The cyclic tests were performed to identify the fatigue behavior of the strengthened slabs and to investigate the influence of long-term cyclic loading and elevated temperature on the bond properties of the prestressed CFRP laminates and the ductility and flexural strength of the strengthened slabs. A nonlinear analytical model of reinforced concrete members strengthened with passive and prestressed CFRP strips under static loading is proposed in the paper. A comparison of the experimental and predicted results reveals an excellent agreement in the full range of loading.

Effect of prestress levels in prestressed CFRP laminates for strengthening prestressed concrete beams: A numerical parametric study

This paper presents the effect of prestress levels in prestressed, carbon-fiber-reinforced-polymer (CFRP) laminates for strengthening prestressed concrete beams based on a validated, nonlinear, three-dimensional finite-element-analysis model. A total of nine medium-scale prestressed concrete beams, six of which were externally strengthened with prestressed CFRP laminates, were studied to examine the contribution of various prestressing levels in the CFRP laminates to flexural behavior of the strengthened beams. The focus of this study was on the load-deflection responses, strain development in the internal and external reinforcement, elastic and plastic energy characteristics, and failure predictions. For a practical use of this type of strengthening method, the recommended level of prestress is between 20% and 30% of the ultimate design strain of the CFRP laminates. More research, however, may be warranted to draw a general design guideline for the prestress level, including various size effects and reinforcement ratios of prestressed concrete beams.

Fatigue Performance of Tension Steel Plates Strengthened with Prestressed CFRP Laminates

An experimental and analytical study was conducted to investigate the fatigue behavior of tension steel plates strengthened with prestressed carbon-fiber-reinforced polymer (CFRP) laminates. A simple fracture mechanics model was proposed to predict the fatigue life of reinforced specimens. Double-edge-notched specimens were precracked by fatigue loading and then strengthened by CFRP laminates at different prestressing levels. The effects of the applied stress range, CFRP stiffness, and prestressing level on the crack growth were investigated. Experimental results show that the increase of the prestressing level extends the fatigue life of a damaged steel plate to a large amount. The CFRP with the highest prestressing level performed best, prolonging fatigue life by as much as four times under 25% higher fatigue loading. Theoretically, predicted results were in a reasonable agreement with the experimental results. A parametric analysis was also performed to investigate the effects of the applied stress ran...

Experimental studies of concrete beams strengthened with prestressed CFRP laminates

Externally bonding prestressed carbon-fiber reinforced polymer (CFRP) laminates to reinforce concrete beams is an effective and convenient technique to improve beams 'flexural strength. This paper describes a modified anchorage system for prestressing high-strength CFRP laminates in which the maximum prestress level was about 60% of the CFRP laminate's ultimate tensile strength of 2500 Mpa (362 ksi). Experimental studies on nine 2.7-m-long (9ft) reinforced concrete beams investigated the influence of influence of prestress level, width of the high-strength CFRP laminates, and the tensile reinforcement ratio on the flexural behavior of the strengthened beams. The results indicated that prestressed CFRP laminates provide the best crack control, mitigate laminate debonding failure, and increase the flexural capacity of the concrete beams.

Preservation of Hopkins Street Bridge via externally bonded prestressed CFRP laminates

Several fascia beams of an existing prestressed concrete box-girder bridge, located in the City of Defiance, Ohio, USA were severely deteriorated due to the influence of de-icing salt applied during wintertime. An in-depth visual inspection and subsequent full-scale load tests were conducted to identify the extent of deterioration. A strengthening strategy was devised involving advanced carbon fiber reinforced polymer (CFRP) composite strips, post-tensioned via the Stress Head system that was developed and patented in Switzerland. The technique, the first of its type in the USA, offers numerous advantages over conventional repair and rehabilitation methods. The inherent attributes of this innovative approach include lighter weight for easy handling, lower loss of prestress due to higher initial tensile strain, corrosion resistance laminates, optimal use of high tensile strength of CFRP plates, optimal cost/performance ratio, and increased durability with reduction of crack width and tensile steel strain. ...