Moisture Conditions in High Performance Concrete. Mathematical Modelling and Measurements

Doktorsavhandling, 1997

The work presented in this thesis has been focused on developing a model to calculate the moisture conditions in concrete of low water/binder ratio (W/B) which is normally used in high performance concrete. To do this, the moisture conditions in high performance concrete have been studied, from a very early age. The separate processes involved have been clarified and combined into a complete model. The main processes are the chemical and physical binding of moisture in the concrete and the moisture flow through the concrete. Since self-desiccation is of great importance for concrete of low W/B, this has been studied separately and a model for self-desiccation has been developed which represents an important part of the model for moisture conditions in high performance concrete. In order to describe the moisture conditions in concrete a number of material properties were required. These have been determined from literature studies and from laboratory measurements. For example, the influence of curing conditions on the rate of reaction has been studied by measuring the degree of reaction of binders in concrete stored at different levels of RH at low ages. The desorption isotherms for a few different concrete compositions at different age of maturity have been measured and also moisture transport properties of high performance concrete have been measured at very early age. A calculation model for estimating the moisture conditions in concrete structures subjected to certain curing and climatic conditions has been developed. This model takes both self-desiccation and moisture transport into consideration. The model has been used to simulate the moisture conditions in real concrete structures subjected to different climatic conditions and the results from the model agree reasonably well with measured data, except for concrete submerged under water where the measured data are much lower than the calculated. The reason is probably that there are other phenomena involved, that are not included in the calculation model, causing the dry moisture conditions inside the concrete structure.