Reassessment of the very long term basic creep of concrete

The prediction of delayed deformations of concrete at very long term is particularly important for prestressed structures such as nuclear power plant containments. The basic creep, which corresponds to the deformation of the concrete under load, without exchange of water with the environment, is a major component of the delayed strains. The analysis of several experimental results indicates that this component can be expressed in a logarithmic form. However, Brooks very long term tests (more than 30 years) seem to indicate a deviation from this trend and an acceleration of creep after 3 years of testing. This could be due to a non-linear behavior but these deviations can also be explained by experimental artefacts. In the case of Brooks tests, because the sample were stored in renewed water, calcium leaching could explain the evolution of the behavior on the very long term. Assuming a simple model for leaching and that the leached part of the sample does not sustain stress, it is possible to estimate the evolution of the stress within the core of the sample. Using the creep model fitted on the first part of the tests (before 1000 days) and in the framework of the superposition principle, it is possible to show that leaching indeed could explain the very long term behavior.