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Synopsis On the basis of experimental results reported more fully elsewhere, a stress-strain relationship is proposed for plain concrete that is applicable to all combinations of biaxial tension and compression. The proposed equation accounts for both non-linear and orthotropic behaviour of concrete. Also based on experimental results, a biaxial ultimate strength criterion is proposed for concrete in the form of a failure stress envelope. The proposed equations are given in a form such that they may be incorporated readily in plane-stress analysis of reinforced concrete using the finite-element approach.

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Synopsis Experiments designed to investigate the effects of moisture content, condition of the cement gel, time of loading and time of unloading upon the creep and creep recovery of sealed and unsealed concrete specimens are described. Comparisons are made between the creep and creep recovery responses of specimens sealed immediately after curing and of unsealed specimens. The results of the experiments carried out on the drying (unsealed) specimens were easily predictable by existing theory. However, the experiments performed on the sealed specimens produced data not so easily explained. For example, under certain conditions, the creep of unsealed specimens was less than that of sealed specimens, and a period of time during which the creep potential of sealed specimens increased with age at time of loading was observed. These results are discussed in terms of the major variables affecting the creep of concrete: the degree of hydration at the time of loading, the amount of water present and not chemically combined at the start of loading and while the specimen is under load, and the amount of m icro cracking developed in the system before and during the time under load.