Spalling is considered as one of the most complex and poorly understood phenomenon occurred at high temperatures. (Khoury 2000; Tenchev and Purnell 2005) Mostly it is assumed that this phenomenon occur at high temperature but it can be observed at early ages of fire (Canisius et al. 2003) and at a temperature of around 200 °C.
If the spalling is severe, it might have destructive consequences on the strength of reinforced concrete structures. Spalling can expose the steel reinforcement to fire by reducing or even eliminating the layer of concrete cover over it, resulting strength of reinforcement is reduced and overall deterioration of whole structures takes place.
Spalling also reduces the physical strength of reinforced structures by reducing the cross-section of the member. Reduced cross section results in increased stress on the other parts of the member due to imposed loading.
Spalling is an important phenomenon, as it takes place at comparatively small temperatures; even any other deleterious effects of heating takes place on the strength of concrete.(Fletcher et al. 2007) Spalling occur due to the involvement of high thermal stresses in the member/structure either due to rapid temperature change or buildup of high pressure resulting from moisture evaporation and concrete is unable to release these high thermal stresses.
It results in the fractures development and later on the removal of material from surface in the form of hunks. So in general it can be concluded that moisture content of at least 2% and sudden change in temperature are prerequisite for spalling within the material.
(Schneider 1988) In the present age, the world is shifting towards the use of “high-strength” or “high performance” concrete with the advent of technology. Compressive strength of “high strength” or “high-performance” concrete is significantly higher than normal-strength concrete. Despite of higher compressive strength it is noticeably moisture absorbent and less porous. It is harder for water molecules to leave the concrete surface as it has generally less water content and hence the water-cement ratio. It is occasionally claimed that lower porosity of high-strength concrete makes it more prone to spalling as lesser porosity helps in achieving high pressure inside the member. (Hertz and Sørensen 2005) However, the research in the recent years shows contraction to this by performing tests that indicate “high-strength” or “high-performance” concrete possess higher spalling resistance, (Ali et al. 2004) which means that the improved tensile properties efficiently counter the growth in activities that encourage the affinity of spalling