Mark F. Green

Comparison of Confinement Models for Fiber-Reinforced Polymer-Wrapped Concrete

Fiber-reinforced polymer (FRP) wrapping can be used to retrofit existing reinforced concrete columns and is gaining acceptance as an effective rehabilitation and strengthening technique. This article reviews some of the numerous analytical models that have been used to predict the stress-strain behavior of concrete confined by FRP wraps. The authors use these analytical models in the context of a large database of test results on FRP wrapped columns. Several of the existing models are modified to provide the best fit to the experimental database. The authors conclude that, because of the variability in the test data, it appears impossible to develop simple empirical models with less than 14% mean absolute error for ultimate strength and 35% mean absolute error for ultimate strain. Examination of two current North American guidelines for the design of FRP-confined concrete demonstrated that both the ISIS Canada guidelines and CSA S806 code are conservative with at least 99% confidence (when reduction factors are used).

Fire Endurance of Fiber-Reinforced Polymer Strengthened Concrete T-Beams

Understanding the performance of fiber-reinforced polymer (FRP)-strengthened members in fire is critical to the widespread application of FRPs as repair materials for infrastructure. An investigation was undertaken to examine and document the performance of FRP-strengthened reinforced concrete T-beams under standard fire conditions. Two full-scale reinforced concrete T-beams were strengthened in flexure with FRP sheets and insulated with a patented two-component fire insulation system. The specimens were subsequently exposed to a standard fire under full sustained service load. Member deflections, strain in the steel reinforcement, and temperatures throughout the section were measured and recorded throughout the tests. A numerical heat transfer model was used to predict temperatures within the section at any time during the fire. The predicted temperatures are compared with those observed during the fire tests and are shown to agree satisfactorily. The results indicate that appropriately designed and insulated FRP-strengthened reinforced concrete T-beams can achieve fire endurances of more than 4 hours.

Innovative System for Prestressing Fiber-Reinforced Polymer Sheets

Strengthening with bonded prestressed fiber-reinforced polymer (FRP) sheets combines the benefits of excellent durability and structural improvement in terms of serviceability and ultimate conditions. The method offers the benefits of both a prestressed system that contributes to load-carrying capacity before further deformation occurs and a bonded system that sustains a major portion of load under further deformations. This work outlines a strengthening technique that involves prestressing carbon FRP (CFRP) sheets using a new external anchorage system to directly add tension to the sheets by jacking and reacting against the concrete beam itself. Results indicate that the CFRP sheets can be effectively prestressed using the developed anchorage system.

CARBON FIBER-REINFORCED POLYMER WRAPS FOR CORROSION CONTROL AND REHABILITATION OF REINFORCED CONCRETE COLUMNS

This paper discusses the use of carbon fiber-reinforced polymer (CFRP) wraps as a rehabilitation technique for corroded reinforced concrete columns. The effect of applying the wraps at early ages of the columns and its effect on corrosion propagation is addressed. Interaction between corrosion phenomena and the CFRP wrapping is studied through simple combinations of both factors. The lab work included testing of 52 reinforced concrete cylinders. The testing procedures are described. It was concluded that applying CFRP wraps significantly decreases corrosion activity when applied over the entire specimen. The application of the wraps before corrosion propagation will prevent corrosion from taking place, while the application of the wraps after corrosion occurrence will drop the rate of corrosion sharply. This effect is likely due to the epoxy saturant used to apply the CFRP sheets and not from the fibers themselves.

Fire Endurance of Fiber-Reinforced Polymer-Confined Concrete Columns

The use of fiber-reinforced polymers (FRPs) for strengthening and rehabilitating reinforced concrete structures has been the subject of numerous research projects and has seen widespread implementation in recent years. Very little information is available on the behavior of FRP materials at high temperatures, however, and this is a primary factor discouraging the widespread application of FRP wraps in buildings where fire-related issues are critical design requirements. This paper presents the results of two full-scale fire endurance tests on circular FRP-wrapped reinforced concrete columns insulated with different thicknesses of fire insulation. Test data are compared with the predictions of a numerical fire simulation model, and the model is shown to adequately predict the observed thermal and structural response. It is demonstrated that, while currently available infrastructure composites are particularly sensitive to elevated temperatures, appropriately designed FRP-wrapped reinforced concrete columns are capable of achieving the required fire endurances.

Preliminary guidance for the design of FRP-strengthened concrete members exposed to fire

An overview on the fire performance of fiber-reinforced polymer (FRP) materials and FRP-strengthened reinforced concrete (RC) members is presented. Results from an experimental and numerical resear...

RESISTANCE TO FREEZING AND THAWING OF FIBER-REINFORCED POLYMER-CONCRETE BOND

This paper presents results from an experimental and theoretical study into the effects of freeze-thaw cycling on the fiber reinforced polymer (FRP)-concrete bond. The results of flexural tests on 39 small-scale flexural beams, reinforced in tension with externally bonded FRP sheets, are presented and discussed. The beams were subjected to from 0-300 freeze-thaw cycles and were plated with 4 different FRP materials. Results indicate that little, if any, damage to the FRP-concrete bond results from freeze-thaw cycling. A simple analytical model to predict bond behavior and the potential for bond damage due to thermal cycling is discussed.

Effect of Severe Environmental Exposures on CFRP Wrapped Concrete Columns

Deterioration of concrete structures caused by corrosion of reinforcing steel, aging, and weathering is a major problem in harsh environments such as coastal areas and cold regions. In addition, a hot environment, such as in the Arabian Gulf, is recognized as one of the most severe and aggressive environments that affects concrete durability. The purpose of this study is to investigate the effectiveness of strengthening plain concrete cylinders, subjected to extreme temperature variations, by wrapping with two layers of unidirectional carbon fiber-reinforced polymer (CFRP) sheets. Thirty-six plain concrete cylinders ( 150×300 mm ) were tested. Nine specimens served as unstrengthened controls and the remaining cylinders were strengthened with two layers of CFRP sheets. Cylinders were subjected to high temperatures ( 45°C ) , to heating and cooling cycles (23 to 45°C ), and to prolonged heat exposure ( 45°C ) . Some of the cylinders that were subjected to heating and cooling, were later subjected to freezin...

Behavior of CFRP-Prestressed Concrete Beams under High-Cycle Fatigue at Low Temperature

This paper investigates the behavior of concrete beams prestressed with carbon fiber-reinforced polymer (CFRP) rods under high-cycle fatigue at low temperature. Seven precast T-beams were tested, including five beams prestressed to various levels with the CFRP rods and two beams with conventional steel strands. All beams had a history of sustained loading. Some beams were directly loaded monotonically to failure as control specimens. Other beams were subjected to three million cycles of flexural loading, either at room temperature or at -28°C , prior to being monotonically loaded to failure at the same temperature. All CFRP-prestressed beams survived the three million cycles, whereas the steel-prestressed beam did not. It was shown, however, that the bond between CFRP rods and concrete could be weakened because of cyclic loading, low temperature during loading, or high prestress level. This resulted in a premature bond failure at 70 to 90% of the full flexural strength in subsequent monotonic loading. Also, stiffness and camber gradually decreased during cyclic loading.

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.