# Laboratory Evaluation of Tensile Creep Behavior of Concrete at Early Ages
## 1 Introduction  
It is well recognized that under sustained loading conditions, concrete exhibits time-dependent viscoelastic behavior, such as creep or stress relaxation [1; 2; 3]. Concrete pavement is a special type of structure undergoing sustained environmental loads, causing thermal or drying shrinkage deformation from the ambient temperature and moisture conditions. In a newly placed concrete pavement, restrained shrinkage stress will gradually develop with time because the deformation becomes limited in concrete due to internal restraints from aggregate as well as hardening cement paste matrix [4; 5]. However, early-age concrete subjected to restrained shrinkage will experience significant stress relaxation, which is beneficial with respect to concrete cracking. Cracking is expected to occur when the stress after relaxation exceeds the material's resistance to cracking. But the cracking time might be delayed with the help of creep and relaxation, reducing the magnitude of tensile stresses, which consequently allows for a wider saw-cut-timing window [6]. It should be noted that saw cut timing is affected by various factors such as concrete mixtures, curing temperatures, and relative humidity, etc. In hot weather conditions, saw cutting may start as soon as a couple of hours after the concrete has been finished. In cooler weather, sawing may not start for 12 h after pouring [7]. The factors that can influence the creep of the concrete structures include concrete mix proportioning, ambient humidity and temperature, and concrete member shape and size, etc. [8; 9]. Influential factors for the concrete mix that induce creep are the type and content of cementitious materials, slump, air content, fine aggregate percentage, aggregate type, and fibers [8; 9; 10; 11]. The early-age deformation of high-performance concrete (HPC) is known to be higher than normal concrete, as HPC is mixed with high cement and supplementary cementitious materials [12]. In addition, lower ambient relative humidity increases the-creep coefficient [8]. And the volume-to-surface ratio or minimum thickness of the concrete specimen affects the creep coefficient too because of resistance to moisture transport from the interior to the atmosphere [5]. To date, existing studies on concrete creep have been focused on the compressive creep of concrete [13; 14; 15], while few efforts have been carried out on issues concerning the tensile creep of concrete. The tensile creep of concrete is creep deformation under constant tensile loading conditions. Reviewing various research results on the compressive and tensile creep of concrete, [PERSON] et al. [16] reported that the development of creep strain is different between compressive and tensile creep in most studies, while the ratio of compressive creep to tensile creep decreases with time. Under similar testing conditions, such as specimen geometry, curing conditions, and loading conditions, similar creep results in compression and tension were demonstrated by [PERSON] et al. [17]. Performing creep tests in tension is known to be more difficult than conducting those in compression, especially at early ages [10]. A number of aging creep models have been proposed, e.g., ACI equation [18], the triple power law [19], the microbrestress-solidification theory (MPS) [20; 21; 22], and the Dischinger model [23]. However, the empirical constants and functional parameters in these models have been derived from long-term compressive creep test data for aged concrete. Other studies modified these models to fit the creep of early-age concrete [24; 25]. These modified models provide a good basis for the description of aging effects on the creep behavior of early-age concrete. These models are generally formulated via production or summation of the power laws, each of which represents age-dependent and age-independent behavior. Due to this functional complexity, further approximation is required for the representation of the relaxation behavior and structural stress analysis.