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

摘要

Moisture diffusivity of two high-performance concretes used in concrete containment buildings of nuclear power plants is measured as a function of temperature up to 80 deg C, and the effects of high-temperature exposure up to 800 deg C and freeze/thaw cycles on its value are analyzed. The temperatures in the range of 0-80 deg C are found to have a moderate effect on the moisture diffusivities of both concretes, the typical increase being about l00-200 percent over the whole region. The high temperature exposure results 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 increase up to one order of magnitude. As the main reason for the mentioned moisture diffusivity increase, the crack formation is identified. Both thermal decomposition accompanied by release of substantial amount of gaseous substances and water freezing in the porous system can damage the intemal microstructure of the matrix due to the significant pressure increase in the porous body. The magnitude of changes of moisture diffusivity is found to be affected by the quality of aggregates rather than by the quality of cement. High quality quartzitic aggregates with very low porosity and very good mechanical properties perform much better than other more porous siliceous aggregates such as opal, and than the calcareous aggregates such as chalk.