| Abstrakt: |
In this contribution, selected fundamental properties of concrete are presented and discussed. Basic mechanisms of shrinkage are first described in some detail. With increasing age of concrete different mechanisms of shrinkage become activated. During the first few hours after mixing, dissolution of cement is the main shrinkage mechanism. Depending on the humidity of the surrounding air, capillary pressure in the pore water may cause serious damage in the first few hours. After this initial loss of water, the remaining aqueous solution is retreated in the pores between fine aggregates and first hydration products. At this scale, the role of disjoining pressure is pointed out in detail and it becomes clear that the influence of capillary pressure on shrinkage of hardened concrete can be neglected. Additionally, at relative humidity lower than 50%, the dominating mechanism is the increasing surface energy of the drying gel particles. Shrinkage of high-strength concrete has a totally different time evolution. The moisture distribution naging high-strength concrete is completely different. If high-strength concrete is exposed to an environment with RH higher than 80%, there will be no shrinkage at all but swelling. These fundamental aspects of shrinkage have to be taken into consideration when long-term shrinkage is to be predicted. Creep of concrete can be described in a realistic way by means of rate theory. Elements of rate theory are briefly described. The influence of temperature and applied stress is outlined. Most important, however, is the influence of humidity content. As the humidity content of high-strength concrete is comparatively low, creep is significantly reduced as compared to normal concrete. Prediction of creep can be significantly improved if these fundamentals are taken into consideration. |
