On the Very Long-Term Delayed Behavior of Biaxially Prestressed Structures: The Case of the Containments of Nuclear Power Plants

摘要

The typical French reactor building consists of two concentric containments. The outer containment is made with reinforced concrete. The inner containment is biaxially prestressed, from 80 to 120 cm thick, and designed to withstand an internal pressure of 0.5 MPa (absolute) in case of an accident. In order to avoid tensile stresses in concrete, a prestress is applied, corresponding to compressive stresses in concrete of about 8.5 MPa and 12 MPa along vertical and tangential directions, respectively. In order to maintain enough prestressing in the structure, the delayed behavior of concrete on the very long term should be correctly assessed. The measurements of the very long term deformations of larges bridges and of nuclear containments seem to indicate that, on the very long term, creep strains are function of the logarithm of time. In particular, the measurements of delayed strains in the containment wall show two distinct kinetics: - the difference between tangential and vertical strains, which exhibits a fast kinetics and becomes almost constant after 500 days, - the average of tangential and vertical strains undergoes a slow kinetics. If vertical and tangential strains rates are equal (despite different values of the prestressing), this means that the long term biaxial creep deformations are only sensible to the mean stress and independent of deviatoric ones. In the recent Model Code 2010 creep is expressed as a combination of an asymptotic and a logarithmic function. The logarithmic function corresponds to basic creep while drying creep is asymptotic. Using this model for delayed strains, and adding the possibility to adjust the parameters of the relations proposed by MC2010 on experimental results, the evolution of the strains is calculated and compared to in situ measurements. We finally discuss the set of adjusted possible parameters.