Prestressed Carbon Fiber-reinforced Polymer Sheets Research Articles

Study on axial compressive properties of RC square columns strengthened with prestressed CFRP sheets

Carbon-fiber reinforced polymer (CFRP) has the advantages of high strength, high elasticity, light weight, corrosion resistance, high fatigue resistance, and ease of construction and has become the main seismic strengthening strategy for civil engineering structures. This paper presents a new technique to quickly strengthen reinforced concrete square columns using prestressed CFRP. The axial compression performance of three groups of 10 prestressed CFRP reinforced concrete square columns was tested. By observing the effects of different prestressed CFRP wrapping methods and the range and degree of prestressing on the axial compression, it was possible to analyze the mechanism by which CFRP actively restrains the reinforced columns. Changes in the ductility of the reinforced components were examined and their axial bearing capacity was studied theoretically. The test results and analytical studies show that, the larger the degree of prestressing and range of wrapping, the stronger the lateral deformation constraint placed on the core concrete, and the greater the bearing capacity and ductility. The increase in bearing capacity can reach 65.91%. Increasing the degree of prestressing of the CFRP is also more effective than increasing its reinforcement range. The axial bearing capacity of the reinforced columns was also calculated theoretically by establishing strength calculation models for the confined concrete for different methods and ranges of reinforcement. The predicted results were in good agreement with the test results, so, the models can provide a firm theoretical basis for practical applications.

Design method for concrete columns strengthened with prestressed CFRP sheets

To improve upon the bearing capacities of short circular concrete columns strengthened with carbon fiber reinforced polymer (CFRP) sheets, this paper conducted experimental and numerical investigations to study the influencing factors on the compressive strength of CFRP-strengthened concrete columns, including the preloading of columns, the prestressing applied to CFRP sheets and the number of layers of CFRP sheets. The test columns consisted of an un-strengthened control column in addition to 10 preloaded and non-preloaded columns strengthened in compression with one or two CFRP layers at prestress levels of either 300MPa or 600MPa. These columns were tested under an axial load. The failure modes, stress changes, bearing capacities and load–longitudinal displacement behaviors of the tested specimens were recorded and discussed. Furthermore, the finite element models (FEM) of 10 strengthened columns were developed and validated by comparing the load–longitudinal displacement curves obtained through finite element analysis (FEA) with those retrieved experimentally. A parametric study was subsequently performed using the validated FEM to investigate the effects of preloading, prestressing and the number of CFRP layers. The parametric study demonstrated that the strain lag caused by preloading was negatively related to the compressive strength of the CFRP-strengthened columns. In addition, prestressing could reduce the negative effects of preloading and could improve on the utilization of CFRP tensile strength. Furthermore, the yield load of CFRP-strengthened concrete columns was improved correlative with an increment of the number of layers and the prestress levels of the CFRP sheets. Conversely, whereas the ultimate load only improved with an increment of the number of layers of CFRP sheets, it had no obvious relationship with the prestress levels of the CFRP sheets and was instead related to their ultimate tensile strength. Based on the test data, the bearing capacity formulas for preloaded and non-preloaded short circular concrete columns strengthened with prestressed CFRP sheets were proposed. Subsequently, the presented formulas were confirmed to be reasonably accurate by comparing the bearing capacities calculated numerically with those measured experimentally.

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...

Repair of flange damage steel-concrete composite girders using CFRP sheets

Damaged steel-concrete composite girders can be repaired and retrofitted by epoxy-bonded carbon fiber-reinforced polymer (CFRP) sheets to the critical areas of tension flanges. This paper presents the results of a study on the behavior of damaged steel-concrete composite girders repaired with CFRP sheets under static loading. A total of seven composite girders made of I20A steel sections and 80mm-thick by 900mm-wide concrete slabs were prepared and tested. CFRP sheets and prestressed CFRP sheets were used to repair the specimens. The specimens lost the cross-sectional area of their tension flanges with 30%, 50% and 100%. The results showed that CFRP sheets had no significant effect on the yield loads of strengthened composite girders, but had significant effect on the ultimate loads. The yield loads, elastic stiffness, and ultimate bearing capacities of strengthened composite girders had been changed as a result of prestressed CFRP sheets, the utilization ratio of CFRP sheets could be effectively improved by applying prestress to CFRP sheets. Both the yield loads and ultimate bearing capacities had been changed as a result of steel beam's flange damage level and CFRP sheets could cover the girders' shortage of bearing capacity with 30% and 50% flange damage, respectively.

Mechanical Prestressing System for Strengthening Reinforced Concrete Members with Prestressed Carbon-Fiber-Reinforced Polymer Sheets

AbstractA new, practical mechanical prestressing system was developed to prestress carbon-fiber-reinforced polymer (CFRP) sheets. The system is used for strengthening reinforced concrete members. The main features of the system are as follows: (1) the prestressing forces are applied manually without using hydraulic jacks, (2) the prestressing level of the CFRP sheets can be controlled by using an electronic load cell, (3) the system is light weight and can be installed without using heavy lifting equipment, (4) the CFRP sheets are extended to the ends of the member where high shear stresses occur, and (5) releasing of the prestressing CFRP sheets is accomplished under a slow strain rate. An experimental program was conducted to check the validity and efficiency of the prestressing system by testing three reinforced concrete beams. One of these beams was used as a control beam, whereas the others were strengthened with prestressed CFRP sheets. Results show that the system is efficient and practical for pre...

Bond and Short-term Prestress Losses of Prestressed Composites for Strengthening PC Beams with Integrated Anchorage

This article presents modeling of bond performance and short-term prestress losses of prestressed carbon fiber-reinforced polymer (CFRP) composite sheets for strengthening prestressed concrete beams, using an integrated anchor system that consists of steel plates bonded with CFRP sheets. A simple fracture mechanics model, validated with the experiment, is developed to examine the bond performance of the CFRP sheets for the plate-type anchor system. To predict the short-term prestress losses of the prestressed CFRP sheets used in the integrated anchor system, a closed-form solution is developed. Seven prestressed concrete beams strengthened with prestressed CFRP sheets are used to validate the proposed model. Fracture energy and tensile modulus of the CFRP sheets are the most critical factors affecting debonding failure of the plate-type anchor system. For design purposes, it is recommended that the short-term prestress loss be 10% of the applied prestress using the proposed anchor system.

Flexure of Two-Way Slabs Strengthened with Prestressed or Nonprestressed CFRP Sheets

This paper presents a study on the flexural behavior of two-way reinforced concrete slabs externally strengthened with prestressed or nonprestressed carbon fiber-reinforced polymer (CFRP) sheets. Four large-scale flat plate slabs (3,000 mm×3,000 mm×90 mm) are tested and a nonlinear three-dimensional finite-element analysis is conducted to predict the flexural behaviors of the tested slabs, including the load-deflection response, strain distribution, crack propagation, and crack mouth opening displacement. An increase in the load-carrying capacity of 25 and 72% is achieved for the slabs strengthened with nonprestressed and prestressed CFRP sheets, respectively, in comparison to the unstrengthened slab. A reduction of the deflections up to 32% in service is noted for the strengthened slabs. The unstrengthened slab shows very ductile behavior, whereas, progressive failure is observed for the strengthened slabs, exhibiting pseudoductility in postpeak behavior. Stress redistribution between the internal and ex...

Ductility and Cracking Behavior of Prestressed Concrete Beams Strengthened with Prestressed CFRP Sheets

This paper investigates the flexure of prestressed concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets, focusing on ductility and cracking behavior. Structural ductility of a beam strengthened with CFRP sheets is critical, considering the abrupt and brittle failure of CFRP sheets themselves. Cracking may also affect serviceability of a strengthened beam, and may be especially important for durability. Midscale prestressed concrete beams (L=3.6m) are constructed and a significant loss of prestress is simulated by reducing the reinforcement ratio to observe the strengthening effects. A nonlinear iterative analytical model, including tension of concrete, is developed and a nonlinear finite-element analysis is conducted to predict the flexural behavior of tested beams. The prestressed CFRP sheets result in less localized damage in the strengthened beam and the level of the prestress in the sheets significantly contributes to the ductility and cracking behavior of the strengthened beams. Consequently, the recommended level of prestress to the CFRP sheets is 20% of the ultimate design strain with adequate anchorages.

Live Load Distributions on an Impact-Damaged Prestressed Concrete Girder Bridge Repaired Using Prestressed CFRP Sheets

This paper describes detailed flexural behavior, including live load distributions, of a four-span prestressed concrete girder bridge supported by 14 m long C-shape girders (4 @ 14.05=56.2 m). The bridge has been damaged by frequent impact from heavy trucks, and repaired using prestressed carbon fiber-reinforced polymer sheets. A calibrated finite element analysis is conducted to investigate the flexural behavior (i.e., stress redistribution, deflection, live load distribution, and applied load effects) of the bridge in three different phases (i.e., undamaged, damaged, and repaired states) under various loading configurations. Strain localizations are noted at the damaged and repaired locations. Assessment of existing bridge codes such as the Association of State Highway and Transportation Officials Load Resistance Factor Design and Canadian Highway Bridge Design Code is conducted. The bridge codes predict well the nominal live load effect on the exterior girder, but underestimate the effect on the interior girders. A refined analysis may be recommended for this type of bridge.

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Using Nonmetallic Anchor Systems

This paper presents the flexural behavior of reinforced concrete beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) sheets using nonmetallic anchor systems. The developed nonmetallic anchor systems replace the permanent steel anchorage. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The flexural behavior of the tested beams, including detailed failure modes of each nonmetallic anchor system, is investigated. The study shows that the developed nonmetallic anchors are more effective in resisting peeling-off cracks compared to the permanent steel anchors and the beams strengthened with the nonmetallic anchors provide comparable load-carrying capacity with respect to the steel anchored control beam.

Flexural Strengthening of RC Beams with Prestressed CFRP Sheets: Development of Nonmetallic Anchor Systems

This paper presents a novel anchoring technique for strengthening reinforced concrete beams with prestressed carbon fiber- reinforced polymer (CFRP) sheets. Permanent steel anchors are commonly used for the application of prestressed CFRP sheets. The steel anchors are, however, susceptible to corrosion and may not blend into the aesthetics of the original structure. As a result, it may be preferable to remove the steel anchors after transferring the required prestress to the structure with minimal losses of sustained prestress. A technique for replacing the steel anchors with nonmetallic anchors is investigated and reported herein. Nine doubly reinforced concrete beams are tested with various types of nonmetallic anchor systems such as nonanchored U-wraps, mechanically anchored U-wraps, and CFRP sheet-anchored U-wraps. The developed nonmetallic anchorages successfully transfer the sustained prestress in the CFRP sheets with insignificant prestress losses. A closed-form solution for the transfer of prestress is developed and compared to the experimental results.

Description of a Mechanical Device for Prestressing of Carbon Fiber-Reinforced Polymer Sheets - Part I

A mechanical device for prestressing carbon fiber-reinforced polymer (CFRP) sheets was developed, and its application for the flexural strengthening of reinforced concrete (RC) beams was investigated in this research program. Significant features of this mechanical device are 1) the CFRP sheets are directly anchored to the device itself; 2) prestressing is easily achieved while the device is not yet attached to the RC beam; 3) the prestressing is applied with a manual torque wrench without the need for power-operated hydraulic jacks; 4) fiber-reinforced polymer (FRP) U-wraps can be easily attached to the end of the prestressed CFRP sheets; and 5) the prestressing transfer is accomplished under slow strain rates. This paper describes in detail the main features and the application procedures for this mechanical device. To investigate the feasibility of using this device for flexural strengthening, the experimental program consisted of testing seven RC beams under a four-point bending test setup. Test results indicate that the device was efficient in applying prestressing to the CFRP sheets and, as predicted, in comparison with the control beams, the flexural capacity and serviceability of the beams were increased after strengthening with the prestressed CFRP sheets. In a second accompanying paper, the experimental and analytical results obtained from the flexural program are discussed in further detail.

Repair of Bridge Girder Damaged by Impact Loads with Prestressed CFRP Sheets

A sound repair on a 40 year old four-span prestressed concrete girder bridge is performed with an innovative strengthening method using prestressed carbon fiber reinforced polymer (CFRP) sheets. In fact, this application is the first North American field application of its type. An adequate repair design is conducted based on the American Association of State Highway and Transportation Officials Load Resistance Factor Design (AASHTO LRFD) and the Canadian Highway Bridge Design Code. To ensure the feasibility of the site application using prestressed CFRP sheets, tests are conducted and closed-form solutions are developed to investigate the behavior of the anchor system that is necessary for prestressing the CFRP sheets. A full-scale finite-element analysis (FEA) is performed to investigate the flexural behavior of the bridge in the undamaged, damaged, and repaired states. The AASHTO LRFD exhibits conservative design properties as compared to the FEA results. The repaired bridge indicates that the flexural...

Flexural Strength of Reinforced Concrete Beams Strengthened with Prestressed Carbon Fiber-Reinforced Polymer Sheets - Part II

In an accompanying paper in the same issue of this journal, the development of a prototype mechanical device for prestressing carbon fiber-reinforced polymer (CFRP) sheets was described. To investigate the feasibility of using this device, seven reinforced concrete (RC) beams were strengthened with prestressed CFRP sheets. This paper describes theoretical and experimental results related to prestressing levels that can lead to debonding of the sheets immediately after transfer and during flexural testing. An investigation regarding the increase in the flexural capacity that results from prestressing and anchoring the CFRP sheets at the beam ends with U-wraps is also discussed. Analytical and experimental results suggest that values of the tensile strength of concrete are likely to be higher than suggested by ACI. The predominant premature failure mode in RC beams prestressed with CFRP sheets is by concrete cover separation unless the CFRP sheets are effectively anchored with U-wraps. For the prestressed sheets anchored with U-wraps, concrete cover separation was averted and the predicted theoretical capacity was achieved. Results also showed that the addition of prestressed and non-prestressed sheets increased the cracking load by nearly 3 to 6 times, respectively compared with the control beam. The yield and ultimate loads were increased by nearly 25%, but displacement ductility capacity of the system was reduced by nearly 5 times.

Prestressed FRP Sheets for Poststrengthening Reinforced Concrete Beams

Four large-scale reinforced concrete beams were constructed and tested to investigate the effectiveness of external poststrengthening with prestressed fiber reinforced polymer (FRP) sheets. One of the beams served as a control specimen, another was strengthened with nonprestressed carbon FRP sheets, and the remaining two were strengthened with prestressed carbon FRP sheets. Presented is a method of prestressing multiple layers of the carbon fiber sheets during the application process and the experimental and analytical behavior of the beams under quasi-static loading. Comparisons are made between the control beam, the beam reinforced with nonprestressed carbon FRP sheets, and the beams strengthened with prestressed sheets. Serviceability and ultimate conditions are considered in the theoretical prediction of beam behavior, including the effects of multiple layer prestressing and external loading. The bonding of prestressed FRP sheets to the tensile face of concrete beams improved both the serviceability a...