Prestressed Carbon Fiber-reinforced Polymer Research Articles

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.

Effects of Gradually Anchored Prestressed CFRP Strips Bonded on Prestressed Concrete Beams

Four prestressed concrete beams were constructed and tested to investigate the effectiveness of flexural post-strengthening with prestressed carbon fiber-reinforced polymer (CFRP) strips. One of the beams served as a reference beam, another was bonded with an unstressed CFRP strip, and the remaining two specimens were strengthened with prestressed CFRP strips at two prestressing levels. The gradient method was used for the anchorage of the prestressed CFRP strips. Experimental and analytical calculations are compared with the test results. Further, different failure modes are explained. On the basis of this investigation, recommendations for the use of prestressed CFRP strips anchored with the gradient method are given.

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.

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.

Flexural behaviour of concrete beams strengthened with prestressed carbon fibre reinforced polymer sheets subjected to sustained loading and low temperature

This paper presents results of an experimental investigation that examined concrete beams post-tensioned with bonded carbon fibre reinforced polymer (CFRP) sheets and then exposed to either room (+22 °C) or low (-28 °C) temperatures. The primary objective of this investigation was to assess the flexural behaviour of the strengthened beams after sustained loadings at both room and low temperatures. The strengthened beams showed significant increases in flexural stiffness and ultimate capacity as compared with the control-unstrengthened beams. Failure of the strengthened beams was by tensile rupture of the prestressed CFRP sheets. Test results showed that the long-term and low temperature effects did not adversely affect the strength of the beams. Long-term loading and low temperature exposure caused prestress losses in the CFRP sheets. The load–deflection behaviour of the long-term beams was predicted accurately by an analytical model. These results suggest that bonded prestressed CFRP sheets could be used to increase the strength of damaged prestressed concrete girders under extreme environmental conditions.Key words: strengthening, anchorage, low temperature, long-term, prestress losses, sustained load, carbon fibre reinforced polymer sheet.

External Prestressed Carbon Fiber-Reinforced Polymer Straps for Shear Enhancement of Concrete

The use of fiber-reinforced polymers (FRPs) for the strengthening and repair of existing concrete structures is a field with tremendous potential. The materials are very durable and, hence, ideally suited for use as external reinforcement. Although extensive work has been carried out investigating the use of FRPs for flexural strengthening, a fairly recent development is the use of these materials for the shear strength enhancement of concrete. The current system investigates the use of posttensioned, nonlaminated, carbon fiber-reinforced polymer (CFRP) straps as external shear reinforcement for concrete. Experiments were carried out on an unstrengthened control beam and beams strengthened with external CFRP straps. It was found that the ultimate load capacity of the strengthened beams was significantly higher than that of the control specimen. Existing design codes and analysis methods were found to underestimate the ultimate resistance of the control specimen and the strengthened beams. Nevertheless, the modified compression field theory provided insight into possible failure mechanisms and the influence of the strap prestress level on the structural behavior. It is concluded that the use of these novel stressed elements could represent a viable and durable means of strengthening existing concrete infrastructure.