Analysis on enhanced pullout resistance of steel fibers in ultra-high performance concrete under cryogenic condition

This study aims to investigate the implication of cryogenic condition on the pullout properties of straight and deformed steel fibers embedded in ultra-high-performance concrete (UHPC). For this, two environmental conditions, i.e., ambient and cryogenic, approximately −170 °C, were considered along with three steel fiber types, i.e., straight, half-hooked, and twisted, and two inclination angles of 0° and 45°. In order to rationally explain the test results obtained, optical micrograph and scanning electron microscope images were captured and analyzed. The test results indicated that the bond strength of all steel fibers generally increased at cryogenic temperatures, and the effectiveness was higher when they were aligned than inclined. The straight fiber led to a much higher enhancement of the pullout resistance in terms of strength and energy when compared to the hooked and twisted fibers. The matrix damage became more severe as the fiber was geometrically deformed, inclined, and tested under cryogenic conditions. The fiber deformation improved the initial pullout properties but deteriorated the surrounding matrix and later pullout properties due to the fiber rupture or severe matrix spalling. Consequently, a lower efficiency of the deformed fibers as compared to that of the straight fiber was obtained under inclined or cryogenic conditions.