Prediction of Long-Term Creep for Nuclear Concrete Structures

Abstract

Creep is a long-term deformation that can cause redistribution of stresses, large deformations and prestress or post-tensioning losses in prestressed or post-tensioned structures, respectively. A major challenge in quantifying the effect of creep in concrete structures is that creep of concrete is known to continue for decades. Additionally, concrete creep research in the past, has focused primarily on uniaxial response. However, in biaxially prestressed concrete structures, multiaxial stress state can complicate the behavior of the viscoelastic material. Significant creep strains may be induced in directions transverse to each principle stress due to Poisson’s effect. In this dissertation, a unique, miniature version of the standardized concrete creep frame is designed that is amenable to placing in climate chambers and temperature ovens. This enables the use of the time-temperature superposition principle to predict long-term basic creep of mature portland cement mortar from short-term creep experiments conducted at multiple elevated temperatures. The 3D basic creep response of mature cement mortar is examined using the miniaturized confined compression creep test that allows direct determination of the full stress and infinitesimal strain tensors in a single test, which enables the determination of Viscoelastic Poisson’s Ratio (VPR). The VPR and the basic creep compliance of cement mortar can be used to upscale to concrete creep using advanced computational composite models. To validate the model, few concrete creep tests are conducted at room and elevated temperatures. The primary finding from this dissertation is that a basic creep compliance master curve of cement mortar can be developed for nearly 60 years at 20°C using experimental creep data obtained at higher temperatures for test durations of 600 days. From the confined compression experiment, the VPR of sealed, mature cement mortar is found to be nearly constant and equal to the elastic at room temperature, while the VPR gradually increases with time when measured at 60°C. Excellent agreement between the upscaled simulated concrete creep and experimental data was found for a period of 25 years.