Assessment of Post-Fire Performance of FRP-Confined RC Columns
This study focuses on assessing the post-fire performance of reinforced concrete (RC) columns confined with fiber-reinforced polymer (FRP) composites. The main goal of this research is to reduce uncertainty regarding the post-fire performance of FRP-confined RC columns. A comprehensive parametric experimental program was implemented to provide new insights into the post-thermal exposure behavior of said columns. The specimens were tested after exposure of moderately high temperatures (100-400 ℃) and extreme fire environments (+1000℃). The critical temperature for the FRP composites concrete was found to be in the 250-300℃ range depending on the type of fiber and polymer matrix. FRP-confined concrete specimens were suseptable to exploding during the thermal exposure protocol. The phenomenon was attributed expansion of the concrete being resisted by the shrinking confinement which has a negative thermal expansion coefficient. The onset of ignition of the polymer matrix was determined to be 300 ℃. The performance of different innovative fire protection schemes was assessed. The use of proper insulation was found to be beneficial in reducing the degradation of specimens. The extent of the reduction in degradation varies depending on the thermal properties and thickness of the fire protection system as well as the temperature and duration thermal exposure. Additionally, for the first time, the post-thermal exposure behavior of hybrid carbon and glass FRP-confined concrete was investigated. The hybrid composite-confined specimens were found to be extremely susceptible to ignition and explosion at a temperature of 300℃ and above. This can be attributed to glass fiber fabric having a higher absorbtion of the epoxy matrix which acts as addiitonal fuel to the fire. Large-scale FRP-confined columns were tested under axial compression. The results of the experimental program were used to develop simplified procedures to predict the residual behavior of FRP-confined columns post-thermal exposure. Nonlinear heat transfer simulations were conducted using ANSYS® Mechanical Transient-Thermal module to extract the thermal profile of the specimens exposed to temperatures and insulated with varying thicknesses of fire protection systems. Numerical simulations of post-fire seismic behavior of RC columns confined with FRP composites were also conducted via Opensees® to calibrate the design equations for FRP confined columns. The Opensees® FRPConfinedConcrete02 material model was found to be accurate in predicting residual axial behavior of FRP-confined columns after exposure to elevated temperatures, provided that the material properties are known. Additionally, lateral cyclic simulation was performed on confined and unconfined RC-columns before and after high-temperature exposure. Both the confined and unconfined columns experience a reduction in their ductility and energy dissipation capacities after exposure to elevated temperature. The outcome of this study will increase the confidence level structural engineer via provided a wealth of information that will assist in predicting the residual strength of RC columns confined with FRP composites. Results also provide a confirmation on the necessity of using proper fire protection systems to increase reliability of composites in repair and strengthening applications. The protocol used in building the Opensees® model can be included in this public-domain software to assist structural engineers in assessing the effect of fire and the resulting degraded mechanical properties associated with such events on the overall performance in any reinforced concrete structure. Results obtained from the experimental program revealed the necessity of changing the geometry and details of RC members such as columns when tested for fire rating. It was found that the bulk majority of published experimental work utilized specimens with unprotected end surfaces in contrary to the actual case of a typical RC column in a building or a bridge where the roof (or bridge deck) and the floor (or foundation such as pile cap) act as protective and insulative media. For this reason, and based on experimental program experience, it is strongly recommended to ad thick portions of concrete at the column ends to realistically simulate the fire damage and delay the premature local failure of specimen ends. Finally, recommendations were made for future research work. This includes experimental testing of larger-scale columns, slabs, beams and beam-column joints strengthened with different types of FRP composites with and without thermal insulation. The other important topic is performing residual tests simultaneously or after a relatively short time of the end of the exposure time. This will provide a more realistic simulation of the behavior of such columns while specimens are still hot and degradation rate is high.