The effects of thermal load and frost cycles on the water transport in twohigh-performance concretes

Moisture diffusivity of two high-performance concretes used in concrete con tainment buildings of nuclear power plants is measured as a function of tem perature up to 80 degreesC, and the effects of high-temperature exposure up to 800 degreesC and freeze/thaw cycles on its value are analyzed. The temp eratures in the range of 0-80 degreesC are found to have a moderate effect on the moisture diffusivities of both concretes, the typical increase being about 100-200% over the whole region. The high temperature exposure result s in a much more significant increase of moisture diffusivity, up to three orders of magnitude, and the freeze/thaw exposure of 400 cycles to an incre ase up to one order of magnitude. As the main reason for the mentioned mois ture diffusivity increase, the crack formation is identified. Both thermal decomposition accompanied by release of substantial amount of gaseous subst ances and water freezing in the porous system can damage the internal micro structure of the matrix due to the significant pressure increase in the por ous body. The magnitude of changes of moisture diffusivity is found to be a ffected by the quality of aggregates rather than by the quality of cement. High quality quartzitic aggregates with very low porosity and very good mec hanical properties perform much better than other more porous siliceous agg regates such as opal, and than the calcareous aggregates such as chalk. (C) 2001 Elsevier Science Ltd. All rights reserved.