The easy cracking property of concrete at an early age is always an issue in engineering. One of the effective methods to control the concrete cracking at early ages is to add an expansion agent [
1,
2]. Using an expansion agent can compensate for the early autogenous shrinkage of concrete and delay the occurrence of tensile stress, thus improving the cracking resistance of concrete. At present, traditional expansion agents generally include sulfoaluminate, aluminate clinker or CaO-based expansive additives, which are widely used in Japan, the USA and China [
1]. In 1980, Mehta [
3] proposed that the expansion of MgO can be used to compensate for the shrinkage of concrete, which provided a new idea for the preparation of shrinkage-compensated concrete. Mo et al. [
1] found that, compared with traditional expansion agents, MgO expansion agents have advantages, e.g., they have a relatively lower water requirement, are chemically stable hydration products, namely Mg(OH)
2, have a controllable expansion process and their delayed micro-expansion effect matches well with the concrete cooling phase. By adjusting the calcination temperature and holding time, the delayed expansion effect can be matched with the temperature shrinkage history of mass concrete, which can be used to compensate for the shrinkage deformation of mass concrete [
4]. At present, the expansion principles of MgO expansion agents mainly include (i) crystal growth pressure caused by Mg(OH)
2 growth and (ii) expansion pressure caused by very small Mg(OH)
2 crystal water absorption. Chatterji [
5] indicated that, during the hydration of MgO, a supersaturated solution of Mg
2+ and OH
− would first form and then an Mg(OH)
2 crystal would precipitate and grow in a confined region, resulting in crystal growth pressure, which leads to the expansion of the cement pastes. After more than 30 years of theoretical and practice research on MgO expansion agents in China, MgO expansion agents have been applied to more than 50 dam projects.
Creep is an inherent property of concrete. The creep of restrained concrete members can relax the most tensile stress, which reduces the concrete’s early cracking risk [
6,
7,
8,
9,
10,
11]. Therefore, it is essential to consider creep in the evaluation of concrete cracking resistance [
12]. Research on concrete creep has been carried out continuously for decades [
7]. Hatt first discovered the creep of concrete in 1907. To date, many scholars have been devoted to this field of research. With respect to the complexity of creep, current progress is insufficient, and concrete creep is not yet fully understood. Moreover, there were a few research studies on the creep of concrete mixed with MgO expansion agents. In order to accurately evaluate the cracking resistance of the early age concrete mixed with MgO, it is necessary to study its creep. This is of great significance for the application of concrete mixed with MgO in actual dam engineering.
Creep has a great influence on the deformation and mechanical properties of concrete. Based on creep testing, many scholars suggest various creep prediction models and improved models at early stages. At present, the most widely used tensile creep model is the B3 model [
13,
14,
15]. The model dictates that the degree of hydration reaction of concrete is an important reason for its creep development. The prediction accuracy and application range of the B3 models are excellent, while the disadvantage of the B3 models is that it is difficult to apply it in actual engineering, due to its complicated calculation. De Schutter [
16] proposed a Kelvin tensile creep model with variable parameters based on the degree of hydration. Wei et al. [
17] improved the Kelvin tensile creep model by using three Kelvin units and established corresponding numerical calculation methods. Recent studies show that a single Kelvin unit in the tensile creep model based on the degree of hydration also predicts the development of the early concrete tensile creep accurately [
17,
18], and the use of multiple Kelvin units in the tensile creep model simulates the early tensile creep development further.
The benchmark concrete was called BC concrete for short. The MgO expansive agent was added to the benchmark concrete to obtain the expansion-agent concrete (EC). Under the temperature matched curing (TMC) mode and the constant temperature curing (CTC) mode, a temperature–stress testing machine (TSTM) was used to test the early stress and strain development of the two concretes in the restraint state. Based on the TSTM method of evaluating creep strain suggested by Kovler, the rules of tensile creep and specific creep were calculated and analyzed [
19]. According to the test data obtained by the temperature–stress test (TST), the tensile creep model of the 3 Kelvin units was established based on the degree of hydration. For the same material, the model accuracy was tested via the TST, under different modes. The temperature changed during the TST, which affected the creep. In this paper, we take the variable temperature factors into consideration to improve the Kelvin model [
17], and the accuracy of the model is verified. Finally, the prediction accuracy of the two models is compared and analyzed.