Explosive spalling of concrete in fire: Test report

A sufficient resistance of concrete structures to fire was always taken for granted. The concrete’s low thermal conductivity ensures an overall good thermal protection of the concrete’s reinforcement, even during longer fire impact. In contrast to ordinary performance concrete used in most structures, the behavior of high (HPC) and ultra-high performance concrete (UHPC) at high temperatures exhibits an increased tendency to explosive spalling and must be considered as critical. Explosive spalling might occur at high temperatures, leading to a direct fire exposure onto the reinforcement steel and a reduction of the cross section. The risk of explosive spalling of HPC and UHPC can be reduced with a modified concrete mixture including polypropylene fibers (PP-fibers); however, temperature-related damage cannot be excluded in general. Another possibility is the use of fire protective concrete linings, acting as a thermal barrier or narrow spaced stirrups. However, their application is still difficult due to the lack of existing design criteria. Within the framework of a research project at the Institute of Structural Engineering (IBK) at ETH Zurich, several tests on the analysis of the risk of temperature-related explosive spalling of HPC and UHPC was carried out. In this report, the explosive spalling of unprotected concrete cylinders (ø = 150 mm, h = 300 mm) was investigated using linear heating. Chapter 2 describes these tests. In chapter 3, the behavior of HPC and UHPC containing PP-fibers was analyzed. Different amounts of PP-fibers with varying fiber geometry were tested with the same base concrete mixture. The influence of the PP-fiber’s geometry on the intensity of explosive spalling was clearly indicated. Tests on the porosity and permeability of concrete after cooling from high temperatures are described in chapters 4 and 5. Small concrete fragments (l = 4 mm) were used to determine the cumulative pore volume using mercury intrusion porosimetry. Concrete before and after explosive spalling was analyzed, as ...