On the explosive spalling behavior of ultra-high performance concrete with and without coarse aggregate exposed to high temperature

Abstract

Experimental investigations are conducted on the explosive spalling behavior of two types of ultra-high performance concrete (UHPC), i.e., UHPC with coarse aggregate (UHPC-CA) and UHPC without coarse aggregate (UHPC-no-CA) under high temperature slow heating conditions. The influences of polypropylene (PP) fiber and moisture content on the spalling behavior of UHPC are studied. The inner vapor pressure and temperature in the UHPC specimens are measured to analyze the driving force of explosive spalling of UHPC during heating. Results show that UHPC-CA exhibits a slightly better resistance to explosive spalling than UHPC-no-CA and coarse aggregate has a beneficial effect on minimizing the explosive spalling of UHPC. Also, Moisture content of UHPC specimen remarkably affects the explosive spalling behavior of both UHPCs, i.e. the higher the moisture content, the severer the explosive spalling is. Hybrid fibers (steel fiber and PP fiber) significantly improve the resistance to explosive spalling of UHPC and can prevent some UHPC specimens from explosive spalling. These results prove that vapor pressure is the primary driving factor on the explosive spalling of UHPC under the heating conditions in this work. Additionally, layered explosive spalling occurs to both UHPCs. The temperature gradient in UHPC-no-CA is much higher than that in UHPC-CA. Many coarse aggregates without cracking damage peel off from the UHPC-CA matrix. These results indicate that thermal stress is an important factor driving the explosive spalling. However, when the UHPC specimens are fully saturated (100% moisture content), the “pressure releaser” function of PP fiber may not play the most important role in alleviating the explosive spalling in UHPCs.