Determining hydraulic properties of concrete and mortar by inverse modelling
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Determining hydraulic properties of concrete and mortar by inverse modelling Sébastien Schneider1, Dirk Mallants1, Diederik Jacques1 1 Performance Assessments Unit, Belgian Nuclear Research Centre SCKCEN, 2400 Mol, Belgium. ABSTRACT This paper presents a methodology and results on estimating hydraulic properties of the concrete and mortar considered for the near surface disposal facility in Dessel, Belgium, currently in development by ONDRAF/NIRAS. In a first part, we estimated the van parameters for the water retention curve for concrete and mortar obtained by calibration (i.e. inverse modelling) of the van Genuchten model [1] to experimental water retention data [2]. Data consisted of the degree of saturation measured at different values of relative humidity. In the second part, water retention data and data from a capillary suction experiment on concrete and mortar cores was used jointly to successfully determine the van Genuchten retention parameters and the Mualem hydraulic conductivity parameters (including saturated hydraulic conductivity) by inverse modelling. WATER RETENTION CURVES OF CONCRETE AND MORTAR Concrete constitutes one of the main materials used in engineered barriers limiting radionuclide leaching to the environment, especially in case of near surface disposal of low-level radioactive waste. It is then crucial to accurately determine the (unsaturated) flow and transport properties of the envisaged concrete components, as such properties have an effect on the longterm performance of engineered barriers, including limiting water flow and providing for retardation of contaminant migration. Also, the coupling between flow and transport properties is important to develop defensible models of physical and chemical degradation of concrete. In this paper methodology and results on estimating hydraulic properties of the concrete and mortar considered for the near surface disposal facility in Dessel, Belgium, are presented. Experimental water retention data Water retention curves have been estimated for the concrete and mortar samples referenced, respectively, as C-15-A and M1 [2]. The concrete C-15-A is a mix of CEM I, calcium carbonate, calcareous aggregates and superplasticizer, whereas the mortar M1 is a mix of CEM III, silica fume, limestone and superplasticizer. Absorption and desorption isotherms have been determined by letting 50-mm diameter and 5-mm thick concrete samples equilibrate in a closed chamber until constant weight (different humidity levels were controlled by different saturated aqueous solutions). In total 11 different controlled atmospheres have been imposed by using saturated aqueous solutions covering a relative humidity range from 11.3% to 97.6%. All equilibria were reached in rooms having a controlled temperature fixed to 21°C. In order to determine the water retention curves, relative humidity conditions were changed into matric potential Pc [Pa] in a capillary tube using the Kelvin-Laplace relationship:
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Pc
(1)
RT U w ln HR M
where M is the atomic mass of water (0.018
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