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]. Han 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 °C to 150 °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 and properties of components. The improvement in compressive strength due to drying must be related to changes in microstructure and the interaction of individual components [
15]. Wittmann [
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. Maruyama 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.
4. Discussion
The composition and properties of concrete at each scale play a role in its strength and affects it at the upper scale due to the multiscale nature of concrete. In this section, the drying effects on compressive strength will be discussed form four scales―C–S–H, cement paste, mortar, and concrete.
In C–S–H scale, C–S–H gel is made up of the aggregation of C–S–H granules. The interaction between water and C–S–H gel determines the mechanical properties of cementitious materials [
15]. Water removal from the interlayer space and intraglobular pores (IGP) causes the C–S–H globule collapse and results in the changes of C–S–H sheets in shape and orientation, relative to their nearest neighbors [
28]. As shown in Figure 13 in Reference [
30] by Zhang et al., the removal of interlayer water and gel water caused by heating leads to the C–S–H grain shrinkage, and further, to shrinkage of concrete at three scales. Furthermore, the impact of water on the performance of C–S–H gel was interpreted from the perspective of molecular dynamics. Water molecules can change the connections in C–S–H gel by screening the Ca–O connections and replacing the ionic–covalent bond with unstable H-bond connections, which can be shown from Figures 8 and 11 in Reference [
31]. The hydrolytic weakening of water molecules reduces the stiffness and cohesive force of C–S–H gel, as a result of the reduction in compressive strength. The molecular simulation of Hou et al. [
31] had indicated that compressive strength of dry C–S–H was 33.3% higher than that of saturated C–S–H. As the dominant cement hydrate, the reduction of compressive strength in C–S–H scale provides an explanation for the strength changes caused by drying in higher scales of concrete.
Cement paste scale is composed of hydration products (C–S–H, CH, AFt, ...), dehydrated clinkers, and capillary pores. According to the Kelvin–Laplace equation, water confined in capillary pores is under compression. When paste is exposed to drying conditions, capillary water is gradually lost and smaller and smaller pores are emptied. The water loss from large capillary pores has less contribution on shrinkage than that from small capillary pores, so in the early stage of the drying process more water loss just results in relatively small shrinkage. The drying shrinkage of paste is only about 1/4 of the total shrinkage when about 70% of the water is lost, as shown in
Figure 6a. On the other hand, the dry and saturated paste behaves differently under compression loading. For the dry paste, which has a higher stiffness than saturated paste, more energy is required when microcracks are driven to extend through, and microcracks in other weak regions may propagate or new microcracks may initiate to reduce local stress intensity. This will result in a higher compressive strength a columnar failure mode, and the development of more microcracks (
Figure 5b). For the saturated paste, however, the growth of microcracks is easier because water reduces the surface free energy of solid phase [
5]. Thus, the saturated paste fails in a lower compressive strength and presents a failure mode with a main crack on surface (
Figure 5a).
The mortar scale is composed of cement paste, sand, and interface transition zone (ITZ) between paste and sand, of which cement paste is the origin of strength and ITZ is the weakest phase [
15]. Due to the introduction of sand, mortar has higher stiffness and lower water content than cement paste with the same water/cement ratio, and thus presents the smaller shrinkage with the loss of water. When subjected to compression loading, microcracks in mortar grow mainly along the ITZ as shown in
Figure 9. They propagate and coalesce into macro-cracks. On one hand, drying makes paste stiffer and stronger, on the other hand, it causes the bonding between sand and paste to get weaker, as well as increasing the porosity within 100~1000 nm. Therefore, the strength increase of mortar (13.27%~15.45%) caused by drying is not as pronounced as that of cement paste (23.91%~25.73%), as shown in
Figure 4.
Concrete scale is composed of mortar, coarse aggregate, and ITZ between mortar and coarse aggregate. The introduction of coarse aggregate with higher stiffness not only results in the smaller deformation of concrete than mortar and cement paste in drying process (
Figure 6c), but also results in the more complicated ITZ with a higher volume content. Similarly, when subjected to compression loading, failure of concrete results from the propagation, and coalescence of microcracks, which depends to a large extent on the performance of ITZ. For dry concrete, the harmful effect to strength due to the weakened bonding between coarse aggregate and mortar is greater than mortar. Therefore, the saturation effect on compressive strength of concrete is less pronounced (4.23%~5.26%) than that of mortar (13.27%~15.45%), as shown in
Figure 4.
From above analysis, it can be seen that the drying effect on compressive strength is different at three scales. On one hand, the denser microstructure and further cement hydration caused by drying promote the strength increase. On the other hand, the porosity increase and the coarsened pore structure, as well as the weakened bonding between paste and aggregate, would cause strength reduction. The drying effect on compressive strength of concrete is the result of competition between the strengthening effect and the weakening effect. From cement paste to mortar and concrete, the drying-strengthening effect on strength decreases with the decrease of paste content. The authors analyzed some experimental results from other researchers and found that the difference in compressive strength between dry and saturated concrete at different scales was closely related to the paste content. Results from the literature [
4,
9,
12,
24,
31,
32,
33], as well as our test data, are plotted in
Figure 10. It is evident that the drying effect on compressive strength decreases significantly with the decrease of paste content.
5. Conclusions
The isothermal drying process of cement paste, mortar, and concrete, as well as drying effect on the compressive strength were investigated in this paper, and the main conclusions are as follows:
(1) The isothermal drying process of concrete at three scales is very similar and can be divided into three stages: rising-rate drying, falling-rate drying, and slow drying stage. Water-losing rate, final water loss, and the time required to completely dry state are all related to the paste content.
(2) After isothermal drying, cement paste, mortar, and concrete all exhibit apparent shrinkage; The porosity and average pore diameter of cement paste and mortar increase. This porosity increase in cement paste is mainly attributed to pores less than 100 nm, while in mortar it is attributed to the pores within 100~1000 nm.
(3) Drying has a strengthening effect on the compressive strength of cement paste, mortar, and concrete, which increases with the increase of paste content; the increase in compressive strength of concrete mainly comes from the increase in paste strength and water/cement ratio has little influence on this strengthening effect.
(4) The drying effect on compressive strength of concrete is the result of competition between the strengthening effect and the weakening effect. The former results from the densified microstructure and further hydration due to drying, and the latter results from the propagation of microcracks and the increase of porosity caused by drying.