Die VDZ Technology gGmbH untersuchte den Einfluss der Zementart auf das Absinken des relativen dynamischen E-Moduls von Beton durch Frostbeanspruchung im CIF-Test. Im Fokus stand die Bindung und Gefrierbarkeit des Wassers. Ein Absinken des relativen dynamischen E-Moduls in der CIF-Prüfung wird üblicherweise als Indiz für eine innere Gefügeschädigung des Betons in der Prüfung gewertet. Ist das Wasser überwiegend über Kapillarkondensation und Mehrschichtadsorption im Zementstein des Betons gebunden, ist von einer Schädigung im CIF-Test auszugehen. Eine Ausnahme waren Betone mit Silicastaub: Diese wurden im CIF-Test geschädigt, auch wenn der Anteil an kapillarkondensiertem und mehrschichtadsorbiertem Wasser gering war. Die Frage, welche Bedeutung die Gefügeschädigung von Beton durch Frostbeanspruchung im CIF-Test im Hinblick auf das Verhalten des Betons im Bauwerk hat, wurde hier nicht untersucht.
Damage mechanisms of concrete under frost action: Influence of main cement constituents on internal structural damage
In order to investigate the influence of the binding and freezability of the water in the hardened cement paste on the decrease of the relative dynamic modulus of elasticity in the CIF test, laboratory cements were prepared with the other main constituents blastfurnace slag, fly ash, limestone and silica fume. Cement paste was produced with these cements. The binding of water in the hardened cement paste was characterised by means of water vapour sorption isotherms, among other things. The results show that water is bound in different pore areas of the hardened cement paste either predominantly via capillary condensation and multilayer adsorption or via physisorption and chemisorption, depending on the cement type. The type of binding correlates with the pore size distribution. Hardened cement paste with a high proportion of capillary pores bind water predominantly via capillary condensation. In the CIF test, a significant decrease in the relative dynamic modulus of elasticity can be expected if water is predominantly bound via capillary condensation and multilayer adsorption in the cement paste of the concrete at the age of 35 days. This ultimately confirms the idea of the influence of capillary pores on durability. Concrete with silica fume was an exception. The decrease in the modulus of elasticity was pronounced despite a low proportion of capillary condensed and multilayer adsorbed water. The share of cements with several main constituents in domestic shipments in Germany is about 70 %. The use of these cements contributes significantly to the reduction of CO2 emissions from cement and concrete. The results of the research project can be used equally to explain and predict the behaviour of concretes with these cements CIF test. The question of the significance of the structural damage of concrete due to frost stress in the CIF test with regard to the behaviour of the concrete in the structure was not investigated here. The transferability to practical conditions still seems at least unclear, if not questionable. With regard to e.g. the efficient use of aggregates or the use of clinker-efficient cements, i.e. resource efficiency and climate protection, it is still necessary to deal with the topic.