# Isothermal Drying Process and its Effect on Compressive Strength of Concrete in Multiscale
## 1 Introduction  
Concrete, a most widely used building material at present, is a typical heterogeneous and porous material. The existence and migration of pore water would affect its mechanical properties and durability [1; 2; 3; 4]. For concrete structures exposed to water environments, or those in an environment where humidity varies considerably, the effect of moisture content is significant and should be considered in structural design. In recent decades, investigators have achieved many satisfactory results in which strength, elastic modulus, creep, and shrinkage of concrete are generally recognized as functions of moisture content [5; 6; 7]. As a basic and most important engineering indicator of concrete, compressive strength under different moisture content has been extensively studied. Isothermal drying is generally used as an effective method to control water content of concrete in the laboratory [1; 8; 9; 10]. When the drying temperature is not too high, a positive effect on strength termed as \"drying-strengthening\" occurs [11]. Studies have shown that the compressive strength of dry concrete is higher than that of saturated concrete [8; 9; 10]. [PERSON] et al. [12] also found that the drying effect on compressive strength of concrete with different strength grades was influenced by temperature, which decreased first and then increased as temperature increased from 60 \({}^{\circ}\)C to 150 \({}^{\circ}\)C. Moreover, the same conclusion can be drawn from the study of mortars [9; 13; 14]. What happens during the drying process of concrete that affects the strength of concrete, and is this effect the same in different scales? These questions have not yet been clarified.  
As a multiphase material composed of paste, aggregates, an interface transition zone, and pores with different sizes, the macro-mechanical properties of concrete depend on its microstructure andproperties of components. The improvement in compressive strength due to drying must be related to changes in microstructure and the interaction of individual components [15]. [PERSON] [16] pointed out that the decrease of humidity caused by drying would cause the increase of surface tension of hardened cement paste, consequently resulting in drying shrinkage and strength increase. [PERSON] et al. [17] explained the variation in compressive strength of concrete caused by drying as the joint interaction of the change in paste strength and damage accumulation due to different volume changes between aggregate and mortar. He further indicated that the variation in strength of cement paste caused by drying is attributed to its microstructural change as well as C-S-H (Calcium-Silicate-Hydrate) globule densification [18]. Many researchers focused mainly on the change of compressive strength caused by drying, however, few detailed descriptions about the drying process of concrete, as well as generally accepted explanations about the relationship of changes occurring in drying process and the compressive strength, have yet been reached. To understand the nature of the drying effect on compressive strength of concrete, it is necessary to experimentally study its drying process. The isothermal drying process of concrete differs at different scales due to the multiscale nature and complex microstructure. In this paper, experimental studies were carried out to investigate the drying process and the drying effect on the compressive strength at three scales--cement paste, mortar, and concrete, and some explanations for strength changes were given.