THEORY OF SPALLING IN HSC COLUMNS SUBJECTED TO FIRE

Abstract

Concrete is generally considered to perform well when exposed to fire since it is inorganic, non-combustible and does not give off noxious fumes. Moreover, concrete is a poor conductor of heat and has a relatively high specific heat capacity, which facilitates its use as a protection material to other elements and even as a heat insulating medium. With development in concrete the construction industry has shown significant interest in the use of high strength concrete (HSC). This is due to improvements in structural performance, such as high strength and durability, that it can provide as compared to traditional normal strength concrete (NSC). High strength concrete column when exposed to fire tends to loose strength much earlier and is more prone to explosive spalling due to low permeability and high brittleness than the normal strength concrete. This tendency to spall is influenced by the moisture content of the concrete, the permeability of the concrete, the rate of heating, the nature of the aggregate and the load applied to the concrete. Although these separate contributing mechanisms have been identified, their relative contribution and their interaction are less well understood. On the basics of strong column and weak beam concept during failure if column fails first there is high risk of failure of entire structure, so it becomes necessary to undertake a detailed study of high strength concrete column under fire. In present study, a 3-d finite element modelling of high strength reinforced concrete column has been done to study the spalling phenomenon due to fire and axial compressive loads. Although experiments have played a significant role in the research of concrete structures, but it is very difficult to study spalling experimentally due to its unpredictable nature. The study is carried out for reinforced columns with M60 concrete grade and Fe-415 steel materials. The column considered for the study has cross-sectional dimensions of 400 X 40( mm and of 1000 mm in length. The column is reinforced with four twenty-five diameter rod and eight mm ties with spacing equal to one fifty mm c/c. The concrete is modelled as solid model and hence the effect of mass transfer is not accounted in this study. Tr reinforcement has been modelled as a solid instead of modelling it using link element whit gives better understanding of the model and the behaviour of the concrete in column.