Tensile and compressive creep deformations of hardening concrete containing mineral additives

In hardening concrete the creep behaviour is one of several key properties which determine the stress build-up and the related crack sensitivity of restrained structural members. Therefore good knowledge about creep of young concrete is necessary to achieve reliable crack assessment methods. Because the typical stress histories in early age concrete contain a compressive and a tensile phase, creep tests under both conditions have been performed. The paper gives an overview of the test program, including description of the experimentally investigated concretes, which are variants over one typical infrastructure concrete where parts of the cement are replaced by fly ash or granulated blast furnace slag. The experimental results, and description of the determination of creep deformations from raw data, are presented. One important point is that the uncertainty of the creep strains in tension is considerably larger than in compression simply due to the small size of the total strains in the tensile test, and the related problems occurring when the creep strains are determined as the difference between the total strains and the autogenous deformation. However, the main observation is that the mineral additives both increase early age creep and reveal different creep behaviour in both compression and tension. Especially for the lowest ages at loading it is clearly seen that the instantaneous deformations per stress unit are smaller in tension than in compression. Afterwards the creep strain rate is largest in tension, and consequently, the final result is larger stress dependent deformations in tension than in compression.