The Effect of Elevated Temperatures on the Moisture Migration and Spalling Behavior of High-Strength and Normal Concretes

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Title: The Effect of Elevated Temperatures on the Moisture Migration and Spalling Behavior of High-Strength and Normal Concretes

Author(s): Naysan Khoylou and George England

Publication: Special Publication

Volume: 167

Issue:

Appears on pages(s): 263-290

Keywords: Fire tests; heat transfer; high-strength concretes; moisture; permeability; pore water pressure; spalling; temperature; thermal expansion.

Date: 3/1/1997

Abstract:
Moisture migration in non-uniformly heated concrete is a complex phenomenon. It depends upon many factors, both intrinsic to the concrete mix and its local environment. At temperatures above 100°C pore vapour pressures dominate the mass transfer behaviour and lead to creation of dry zones containing superheated steam and zones of excessive wetness and physical saturation where condensation has occurred. Spalling of concrete, in fire, is strongly related to the water content of concrete at the time of heating and its moisture flow properties. During heating, as the temperature rises, the free water, contained in the porous structure of concrete, will expand whilst sustaining an increasing saturated vapour pressure. The continuous expansion of water together with the moisture flow frequently leads to physical saturation of the pores. Further heating will then generate additional strains in the solid envelope surrounding the pores and can lead to cracking and hydraulic fracture of the solid skeleton. High strength concrete is particularly vulnerable to this behaviour because of its inherent, low porosity, low permeability to water flow and high percentage of initial pore saturation. This paper describes numerical/theoretical modelling procedures, for the prediction of temperature-dependent moisture flow in non-uniformly heated concrete. The flow is considered to be governed dominantly by the pore pressures. A mathematical description is also provided to help understand the spalling process caused by the hydraulic fracture of the solid skeleton during heating of the water in saturated pores.