Experimental Study on Compressive Behavior of FRP-Confined Expansive Rubberized Concrete

This paper presents the results of an experimental study into the behavior of rubberized concrete-filled fiber-reinforced polymer tube (CFFT) columns, where the fiber-reinforced polymer (FRP) confining layer is prestressed by using an expansive agent (EA). The study focuses on utilizing the incompressibility property of rubber in improving the strength properties of crumb rubber concrete (CRC). A total of 27 CFFT columns were tested under axial compression and the effect of rubber content, prestress level, confinement amount and curing condition was studied. Different EAs with cement replacement ratios of 0%, 7.5%, and 15% were used to examine the influence of different levels of hoop prestress on the axial compressive behavior. The influence of amount of confinement was examined with specimens prepared with either one or two layers of CFRP. The influence of the curing condition was also examined by preparing half of the one-layer specimens with steel plates confining the specimens in the axial direction during curing. Finally, both flexible and stiff molds were used to examine the influence of mold stiffness on prestress development during curing. The lateral prestress provided by the expansive agent and FRP confinement was used to mitigate the typical strength reduction associated with CRC. The positive effect of this technique is two-fold. First, the pressure produced by the expansive agent compresses the cement paste and rubber particles together, reducing porosity and increasing interaction and interface friction between the rubber particles and cement paste. Second, rubber is generally considered nearly incompressible, with a Poisson's ratio of approximately 0.5, hence as the rubber is compressed in one direction, it expands significantly in the other directions. Using rubber in unconfined and non-prestressed concrete results in strength reduction; however, the combined effects of FRP-confinement and lateral prestress on CRC can lead to significant increases in the columns' axial stiffness and strength. The stresses and strains developed during curing for the expansive mixes were found to be considerably higher in CRC compared with conventional concrete (CC), indicating that an EA is more effective in developing prestress in rubberized concrete. Moreover, the mold stiffness was found to have a noticeable influence on the compressive strength of concrete. Using stiff molds resulted in unrealistic and unsafe strength evaluation and, hence, should be avoided if expansive concrete is used, or the results should be modified to account for the effect of confinement provided by the stiff molds. It was also observed that the confinement effect in CRC was higher compared with that of CC, which is due to the incompressibility of rubber. Finally, applying confinement plates to the ends of the concrete during curing had insignificant impact on the compressive behavior.