1992

Kukko, Heikki

High strength concrete is expected to be highly durable, and its use is often based on this assumption. However, knowledge of material properties affecting frost resistance, especially at microstructural level, is still relatively poor. The aims of this study are to identify and analyse the applicability of experimental methods and methodologies through studies of the freeze thaw durability of high strength concretes with different binder compositions, and to elucidate the microstructural changes that occur during freeze thaw degradation. As background to the experimental study, a survey is done of the main features of concrete microstructure, existing analysis methods and main theories of concrete frost resistance. An automatic image analysis method for thin sections was developed for direct measurement of pore and crack properties of concrete. Five high strength concrete mixes with various binder compositions and one medium strength concrete were prepared for the experimental study. They were subjected to as many as 1000 freeze thaw cycles and the defects were studied with several methods. Strength loss was used as the basic measure of degradation. Ultrasonic pulse velocity was not a reliable indicator of frost defects. The image analysis method was used to collect information on frost defects along with mercury porosimetry, ultrasonic pulse velocity measurements, water immersion tests, scanning electron microscopy and visual optical analysis of thin sections. Image analysis is an asset to automatic thin section analysis of the porosity and cracking of concrete. The results can be used in mathematical modelling of strength changes of concrete, and correlate well with visual results. Visual optical analysis of thin sections was proved to be a reliable method, but laborious and not capable of quantitative analysis of cracking. Mercury porosimetry results did not give a reliable basis for the estimation of frost crack increase due to ettringite formations in the cracks. Scanning electron microscopy is a valuable tool for detecting the causes of changes found in porosimetric analysis. The principle of critical degree of saturation applies to high strength concrete. The mathematical modelling of strength loss during frost tests can be based on a combined model including changes in total porosity and maximum crack length.