Tensile creep is an important factor affecting the early cracking resistance of concrete. The tensile creep model can effectively predict the development of tensile creep. In order to establish an appropriate tensile creep model, a temperature–stress testing machine (TSTM) was employed to test the development of temperature, deformation and restraint stress of benchmark concrete and concrete mixed with the MgO under different temperature curing modes. The development law of early age stress, strain and creep was analyzed via the test data of the TSTM. The early age tensile creep of concrete was predicted with the existing Kelvin creep model. The effect of variable temperature on creep was considered in this study, and an improved Kelvin creep model was proposed. The prediction accuracy of the two models was compared and analyzed. The results indicate that MgO has little influence on the creep and specific creep of concrete. The early age cracking resistance of MgO concrete is better than benchmark concrete. The improved Kelvin model based on the microprestress-solidification (MPS) theory predicts the early tensile creep of concrete more accurately in variable temperature conditions. These are significantly helpful for the application of the MgO expansion agent in dam engineering.

The creep of face-slab concrete in a rockfill dam is critical for determining the restrained stress and cracking resistance of the concrete at an early age. In this article, the mix proportion of the face-slab concrete for a rockfill dam under construction without any cracking resistance additive was taken as the reference concrete (called JC) mix proportion. The carbon nanotubes (CNTs) and magnesium oxide expansion agent (MEA) were incorporated into JC to prepare face-slab concretes called NC and PC, respectively. The temperature–stress tests under temperature matching curing (TMC) and constant temperature curing (CTC) modes were conducted on these three kinds of concretes to investigate the effects of CNTs and MEA on the early-age creep properties of the face-slab concrete under variable stress conditions. The results showed that the creep performance of NC concrete under CTC mode was lower than that under TMC mode. Combined with mercury intrusion porosimetry test results, the mechanism of the effect of CNTs and MEA on creep was analyzed. The results showed that the temperature change may lead to the CNTs debonding from the cementitious matrix or matrix cracking for the NC concrete. The incorporation of CNTs can increase the early-age creep and improve the cracking resistance of concrete.