Fire spalling of concrete is a complex phenomenon with many influencing factors. The concrete mix, geometry and cross section, stress state, fire exposure, age, pre-conditioning, moisture state and the amount of spalling reducing polypropylene fibres added all influences in different ways if a fire exposed concrete structure is prone to spalling or not. During fire testing of concrete structures for tunnels it is in practice difficult to do fire test on real designs with real boundary conditions. Therefore, more or less representative test methods are used. If small test specimens are used, around 600 x 500 x 200 mm3, these tests are in general only indicative and shall mainly be used as “screening tests”, i.e. to optimise the mix before scaling up to more realistic sizes. Further on, the small test specimen size is not suitable for evaluating the fire resistance including heat penetration for longer duration of fire exposure as the geometrical and boundary effects are not representative for larger specimens.