Depiction of the spalling depth distribution of the fire-exposed concrete surface of various test specimens
Source: BAM
Concrete spalling can cause severe damage to structures in the case of fire. This causes concrete fragments to explode from the fire-exposed surface of the member, reducing the concrete cover and exposing the reinforcement. The spalling behaviour of a concrete member is influenced by a high variety of parameters such as strength class, cement type, additives, moisture content, geometry and applied load. In order to assess the tendency of concrete to spall, fire tests are carried out on full scale components. These tests are expensive. Previous research has shown that reducing the specimens size increases the influence of boundary effects and reduces spalling.
The present study investigates the spalling behaviour of intermediate scale specimens with different geometries and restraint conditions. The aim is to investigate the influence of impeded thermal expansion and thus develop a way of predicting the spalling behaviour of large scale specimens using smaller, restrained, intermediate scale specimens. Two ordinary concrete mixes containing Portland cement, fly ash as well as quartzitic and basaltic aggregates are used. The comparison of the spalling behaviour is carried out on cuboid (0.6 x 0.6 x 0.3 m³) and cylindrical (Ø 0.47 m x 0.3m) specimens without restraint and on cylindrical specimens of the same size restrained by an 8 mm thick steel ring. The thermal expansion of the concrete is inhibited by the steel ring fitted. To prevent heating and thermal expansion of the steel ring, the steel ring is insulated in the area of the fire exposed concrete surface.
The results show that restrained thermal expansion significantly increases the spalling depth of the specimens. Intermediate scale cylindrical specimens of both concrete mixes show almost no spalling without the steel ring applied. In contrast, the test specimens with steel ring show similar spalling results compared to the large scale specimens analysed in previous studies. Differences in spalling behaviour were observed between the two mixes with different aggregates. It is concluded that the experimental setup with steel ring is suitable for investigating the spalling susceptibility of concrete members using intermediate scale specimens.
Influence of thermal strain on concrete spalling
André Klimek, Ludwig Stelzner, Sascha Hothan und Jochen Zehfuß
Published in Materials and Structures, Band 57, Artikelnummer 15, Seite 1-14, 2024