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The size effect has its origin in fracture mechanics and describes the formation as well as propagation of cracks in brittle and solid materials in dependence of the specimen size. However, the size effect in concrete spalling describes the damage behaviour on a macroscopic scale for different sized specimens in case of fire. Concrete spalling is a very complex and yet not fully understood phenomenon. To reduce the effort of fire tests to analyse the spalling behaviour of concrete mixtures, this study investigates the susceptibility to spalling for six different concrete mixtures and three specimen sizes. The sizes were divided in full scale slabs (1.8 m x 1.2 m x 0.3 m), intermediate scale cuboids (0.6 m x 0.6 m x 0.3 m) and small scale cylinders (Ø0.15 m x 0.3 m). For this purpose, a novel test set-up was built to test six intermediate scale or twelve small scale specimens simultaneously to ensure a similar heating regime for every specimen. All specimens were fire exposed on one side and remained unrestrained. A size effect occurred for four of the six concrete mixtures. Compared to the full scale specimens the spalling was reduced significantly for all smaller specimen sizes. Additionally, spalling did not occur for the small scale specimens. The results show that the specimen size is an essential parameter to investigate the susceptibility to spalling of a concrete mixture. For future investigations the testing conditions must be adjusted for the intermediate scale specimens to recreate the conditions of the slabs.
Understanding the susceptibility to spalling of concrete members in case of fire is important to evaluate the residual load-bearing capacity. The investigations of the spalling phenomenon of a concrete mixture using real scale members are necessary but expensive to carry out. Reducing the specimen size leads to an increase of boundary effects that can result in a reduced spalling or absence of spalling. In this study, fire tests were carried out on unrestrained, single-sided exposed, cuboid shaped specimens (0.6 m x 0.6 m x 0.29 m) as well as unrestrained and steel ring restrained cylindrical specimens (Ø = 0.47 m, h = 0.29 m), which induce different boundary conditions. These fire tests were carried out on two ordinary concrete mixtures. The two mixtures differ only in the type of aggregates (quartz gravel and basalt grit) and were used to investigate the influence of the thermal expansion of the aggregate on the spalling behaviour of the concrete. The results show a significant increase of
the spalling depth due to the restrained thermal expansion achieved by the applied steel rings. Additionally, the type of aggregate has a direct influence on the spalling behaviour of a concrete mixture. The reduction of the boundary effects by the steel rings recreate the test conditions in the centre of a large concrete member. Thus, this type of specimen is suitable to determine the susceptibility to spalling of a material (screening-tests) as preliminary investigations to full scale fire tests.
NMR is becoming increasingly popular for the investigation of building materials as it is a non-invasive technology that does not require any sample preparation nor causes damage to the material. Depending on the specific application it can offer insights into properties like porosity and spatial saturation degree as well as pore structure. Moreover it enables the determination of moisture transport properties and the (re-)distribution of internal moisture into different reservoirs or chemical phases upon damage and curing. However, as yet most investigations were carried out using devices originally either designed for geophysical applications or the analysis of rather homogeneous small scale (< 10 mL) samples. This paper describes the capabilities of an NMR tomograph, which has been specifically optimized for the investigation of larger, heterogeneous building material samples (diameters of up to 72 mm, length of up to 700 mm) with a high flexibility due to interchangeable coils allowing for a high SNR and short echo times (50 - 80 m s).