Fire exposed surface of three different concrete specimens (photography and distribution of the spalling depth); Source: BAM, Division Fire Engineering
Source: BAM, division Fire Engineering
Due to its high compressive strength, good durability and formability, concrete is a versatile building material used in civil engineering. Concrete has an adequate fire resistance as a result of its non-combustibility and low thermal conductivity. However, in case of fire, thermally induced damage processes can lead to explosive spalling, which reduce the load-bearing cross-section and decrease the fire resistance of concrete members. Investigating the spalling behaviour of a concrete mixture using large scale members is complex and expensive. Therefore, it is the aim to reduce the testing effort. The influence of the specimen size on the spalling intensity under comparable boundary conditions has not been sufficiently investigated, yet.
The presented study investigates the spalling behaviour of large scale slabs (1.8 m x 1.2 m x 0.3 m), intermediate scale, cuboid shaped specimens (0.6 m x 0.6 m x 0.3 m) and small scale, cylindrical shaped specimens (Ø = 0.15 m; h = 0.3 m) for six different concrete mixtures. Except for the small scale specimens, the intermediate and large scale specimens were reinforced. Two of the six concrete mixtures represent concretes for tunnel linings with and without polypropylene fibres. The other four mixtures represent concretes for building constructions. These mixtures include two high performance and two standard concrete mixtures. All specimens were tested unrestrained and without mechanical load. To increase the comparability a special set-up was built to test all specimens under the same test conditions. Additionally, the set-up enables the fire test of one large scale slab as well as simultaneous fire tests of six intermediate scale specimens or 12 small scale specimens.
The results show a significant reduction of the spalling depth with a decreasing size of the fire exposed surface due to an increasing influence of boundary effects. The free expansion of the specimens leads to a reduction of compressive and tensile stresses in form of cracking on the fire exposed surface and the unexposed sides. Additionally, water escapes through the cracks from larger specimen depths and thus, thermohydraulic induced stresses are relieved. Based on the results, a test set-up for the intermediate scale specimens under restrained conditions is developed. It is the aim to reproduce the spalling behaviour of large scale specimens more closely.
Fire induced concrete spalling in combination with size effects
André Klimek, Ludwig Stelzner, Sascha Hothan, Andreas Rogge
published in Materials and Structures, Vol. 55, Issue 8, Article number 216, page 1-14, 2022
BAM Safety of Structures department
BAM Fire Engineering division