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Concrete In Australia : September 2014
Concrete in Australia Vol 40 No 3 51 specific materials, environmental and test factors. • Upon heating, the hydrated cement paste intially expands but subsequently undergoes dehydration and shrinks rapidly. Aggregates, in contrast, expand. The thermal incompatibility arising from expanding aggregates and contracting cement pastes causes internal stresses and possible cracking. • Aggregates, occupying between 60% and 80% by volume of concrete, influence the strength and thermal stresses within the heated concrete, as well as the thermal deformation and stability. The selection of aggregates is thus of major importance for concrete fire behaviour – as far as strength degradation of concrete upon heating is concerned: – Desirable features of aggregates include (1) high thermal stability; (2) low thermal expansion to minimise thermal incompatibility with hydrated cement paste; and (3) rough angular surface to improve the physical bond with the hydrated paste. – The lower the silica content of an aggregate, the lower its coefficient of thermal expansion. It is thus beneficial to use aggregates of less silica content. – Carbonate aggregates (such as limestone and dolomite) absorb heat during their decomposition to drive off CO2, retarding the rate of temperature rise. Concrete with carbonate aggregates thus has better fire endurance than that with siliceous aggregates. Figure 1: (a) Typical physicochemical changes in concrete during heating; (b) Forces acting in heated concrete, together with gradients of temperature, pore pressure and moisture (Khoury, 2008). (a) (b) 50-53 - Vinh.indd 51 50-53 - Vinh.indd 51 26/08/14 10:23 AM 26/08/14 10:23 AM