Experimental study on residual axial strength capacity of CFRP wrapped concrete cylinder (full and partial Confinement) after exposure to elevated temperature

Carbon Fiber Reinforced Polymer (CFRP) has been extensively used in retrofitting structural members of buildings and different infrastructures in both normal and severe environments like fire and elevated temperatures. CFRP with its high strength and lightweight properties has performed well in normal environments. However, the performance of CFRP as retrofitting material for axially loaded members when subjected or exposed to temperature above the glass transition temperature of the CFRP composite still needs further investigation. Because of the lack of understanding of CFRP under elevated temperature, the American Concrete Institute (2017) suggests that the strengthened capacity of structural members should be neglected in case the glass transition temperature of the CFRP composite material is reached given any circumstances. This paper investigates further the performance of axially loaded compression members retrofitted with CFRP through experimental tests of concrete cylinders wrapped with full and partial CFRP under elevated temperature. Forty eight (48) concrete cylinder samples were subjected to three levels of elevated temperature (100°C, 160°C, and 240°C). The CFRP confined concrete cylinders were exposed to elevated temperature for 3 hours and were cooled down to room temperature for 24 hours prior to compressive testing. For partial CFRP confined specimens, the clear spacing ratio of CFRP is varied to investigate the effect of clear spacing ratio and elevated temperature on concrete cylinder when subjected to axial compressive load. To fully understand the crack behavior of the concrete cylinders with full and partial CFRP confinement under the three levels of temperature, Digital Image Correlation (DIC) using OpteCAL software is used. DIC has become a useful and effective tool in observing the crack and failure behavior of concrete specimens under testing. Experimental data shows that the strengthened capacity of the CFRP confined samples remains significant even after being exposed to temperature above glass transition state of the CFRP composite. Moreover, it was observed that fully confined concrete samples, developed strains and cracks on a larger spread of area during the compressive testing in comparison to the strains and cracks propagation for partially confined samples, which tends to concentrate on unconfined regions of the cylinder. In relation, in terms of types of failure, there was an observation that samples with clear spacing ratio of 0.4 tend to fail due to concrete localized cracks located at unconfined concrete region; on the other hand, samples cylinders with clear spacing ratio of 0.25 tend to fail due to rupture of the CFRP. Through this experimental study, a clearer understanding of the compressive strength performance and crack development of CFRP-confined concrete cylinders under elevated temperature using DIC tools is achieved