Luke Bisby

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.

A contemporary review of large-scale non-standard structural fire testing

In recent years, large-scale structural fire testing has experienced something of a renaissance. After about a century with the standard fire resistance test being the predominant means to characterize the response of structural elements in fires, both research and regulatory communities are confronting the many inherent problems associated with using simplified single element tests, on isolated structural members subjected to unrealistic temperature-time curves, to demonstrate adequate structural performance in fires. As a consequence, a shift in testing philosophy to large-scale non-standard fire testing, using real rather than standard fires, is growing in momentum. A number of custom made, non-standard testing facilities have recently been constructed or are nearing completion. Non-standard fire tests performed around the world during the past three decades have identified numerous shortcomings in our understanding of real building behavior during real fires; in most cases these shortcomings could not have been observed through standard furnace tests. Supported by a grant from the Fire Protection Research Foundation, this paper presents a review of relevant non-standard structural fire engineering research done at the large-scale around the world during the past few decades. It identifies gaps and research needs based both on the conclusions of previous researchers and also on the authors’ own assessment of the information presented. A review of similar research needs assessments carried out or presented during the past ten years is included. The overarching objective is to highlight gaps in knowledge and to help steer future research in structural fire engineering, particularly experimental research at the large-scale.

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.

Thermal Behavior of Fire-Exposed Concrete Slabs Reinforced with Fiber-Reinforced Polymer Bars

This paper describes the results of experiments carried out to investigate the performance in fire of concrete slabs reinforced with carbon or glass fiber fiber-reinforced polymer (FRP) bars. Several variables were examined to determine which were critical to the fire resistance of these slabs. The parameters examined included the reinforcement type, slab thickness, concrete cover thickness to the reinforcement, the aggregate type, and the effectiveness of other fire insulation. The thickness of the concrete cover to the reinforcement and the type of reinforcement were found to be the key parameters in determining the fire resistance of FRP-reinforced slabs. Overall, the results indicated that the qualitative and heat transfer behavior of FRP-reinforced slabs was similar to that of steel-reinforced slabs.

High Temperature Residual Properties of Externally-Bonded FRP Systems

Synopsis: The use of externally-bonded FRP plates and sheets to strengthen existing reinforced concrete structures is now widely recognized. However, a primary concern that still discourages the use of FRPs in some cases is their assumed susceptibility to fire. While recent studies have demonstrated that the overall performance of appropriately designed and insulated FRP-strengthened reinforced concrete members is satisfactory, the specific behavior of FRP materials at high temperature and after exposure to high temperature remains largely unknown, particularly for externallybonded FRP strengthening systems. As a first step in an effort to learn more about the high temperature properties of these systems, an initial series of tests is presented to study the high temperature residual properties of externally-bonded carbon and glass FRP systems for concrete. Axial tension tests, single-lap bond tests, thermogravimetric analysis, and differential scanning calorimetry are all used to elucidate high temperature residual performance. The potential consequences of these initial results for the fire-safe design of FRP-strengthened reinforced concrete members are discussed.

Temperature Effects in Adhesively Bonded FRP Strengthening Applied to Steel Beams: Experimental Observations

FRP plates can be used to strengthen a steel beam in flexure, but this method relies critically upon the adhesive used to bond the FRP plate to the existing steel member. When the temperature of the strengthened beam is increased, differential thermal expansion occurs between the steel and FRP. In addition, the glass transition temperature of a typical two-part ambient-cure epoxy adhesive is typically between about 50°C and 65°C, and the stiffness and strength of the adhesive will decrease at temperatures somewhat below the glass transition temperature. This paper reports tests conducted on steel beams strengthened with CFRP plates and ambient-cure epoxy adhesive. Load was applied to the beams in four-point bending, and the temperature of the strengthening was then increased until failure occurred. Slip deformations were directly observed across the adhesive joint, giving an indication of the performance of the strengthening at elevated temperatures. The consequences of this preliminary study upon the design of externally-bonded FRP strengthening for steel structures are discussed.

Effect of Warm Temperatures on Externally Bonded FRP Strengthening

AbstractFiber-reinforced polymer (FRP) plate strengthening relies critically upon the adhesive that is used to bond it to the existing structure. A typical two-part ambient-cure epoxy adhesive for structural strengthening has a glass transition temperature of approximately 40°C–70°C, but the stiffness and strength of the adhesive typically decrease at temperatures somewhat below this characteristic temperature. This paper investigates the implications of the changes in adhesive properties at warm temperatures (<100°C) for FRP-strengthened beams, through short-term experimental and analytical work. Tests were conducted on FRP-strengthened steel beams subjected to sustained load and increasing temperature; the results, however, are also relevant to strengthened concrete beams. Digital image correlation was used to measure the slip between the strengthening plate and beam, and hence to observe the behavior of the adhesive joint. A bond analysis was also developed to predict the slip across the adhesive joint...