Influence of Relative Humidity on Long-Term Drying Shrinkage of Concrete with Different Water-to-Cement Ratios at a Constant Temperature of 40 ℃

Shrinkage is one of the fundamental characteristics of cementitious materials, occurring in both the plastic and hardened stages of concrete. Volumetric shrinkage could be generated by tensile stresses due to inhomogeneous deformation within the material, which can cause harmful cracks [1]. These cracks act as preferential pathways for the penetration of aggressive agents such as chlorides and sulfates. The ingress of these agents accelerates the deterioration of concrete and adversely affects its durability and long-term service life [1-3]. Previous studies in the construction sector have shown that long-term cracks are among the most serious problems affecting concrete structures throughout their service life, owing to their considerable structural and economic impacts [4].

The repair and maintenance costs associated with shrinkage damage are also high; reports indicate that addressing cracks and related defects in residential buildings has been a significant financial burden in recent decades [4]. In addition, cracks facilitate the penetration of moisture and corrosive agents, accelerating the corrosion of reinforcing steel if proper maintenance measures are not taken. Therefore, improving the resistance of concrete to shrinkage-induced deformations is essential to enhance the durability of structures, reduce repair costs, and limit the environmental impacts associated with demolition and construction waste [4].

Shrinkage mechanisms in concrete are a complex phenomenon resulting from the interaction of several physical and chemical processes that extend from the early stages of concrete's life to the advanced stages of its service life. Depending on the dominant mechanism, shrinkage in cementitious materials is classified as plastic shrinkage, autogenous shrinkage, drying shrinkage, and carbonation shrinkage [5]. In traditional concrete with water-to-cement (w/c) ratios exceeding 0.4, drying shrinkage accounts for the largest portion of total shrinkage compared to the other types [6]. When the w/c ratio exceeds the critical value of approximately 0.42, the water content exceeds the requirements for complete hydration of cement. Consequently, the influence of autogenous shrinkage gradually decreases while the concrete's susceptibility to physical water loss increases, resulting in a corresponding increase in drying shrinkage [7]. When the w/c ratio exceeds 0.45, drying shrinkage is the main factor responsible for the physical cracks that concrete is exposed to during its service life [8]. Drying shrinkage in porous cementitious materials results from the loss of internal moisture to attain moisture equilibrium with the relative humidity (RH) of the surrounding environment [9]. The severity of drying shrinkage is closely related to several factors, most notably the curing system, pore size distribution, and thermodynamic behaviour of water within the pores. The nature and texture of hydration products within the concrete microstructure also play an important role [10, 11]. This phenomenon has been explained based on several mechanisms, including surface energy changes (Gibbs–Bangham effect), capillary forces, separation pressure, and water movement between layers of hydrated calcium silicate gel (C–S–H) [12, 13]. The Gibbs–Bangham effect is mainly effective at RH levels below 40%, while capillary tension and separation pressure work together in the RH range between 40% and 95% [14, 15]. At the critical humidity range between 40% and 50%, the controlling mechanism transitions from capillary tension forces to surface energy effects. Within this range, capillary tension forces exert their maximum effect on volumetric deformation [12, 16]. Since concrete structures are rarely exposed to RH below 40% continuously, capillary tension forces are considered the dominant mechanism responsible for drying shrinkage in cementitious materials under practical environmental conditions [17, 18].

Accordingly, numerous studies have addressed ways to reduce drying shrinkage using various methods, including mineral and chemical additives, fibres, alternative aggregates, and improvements to external and internal curing systems. In general, controlling drying shrinkage relies on three basic principles: reducing the stresses generated on capillary walls, maintaining internal moisture to promote cement hydration and densify the microstructure, and increasing the hardness and crack resistance of concrete. Together, these principles contribute to the development of high-strength and sustainable concrete materials. Despite numerous studies on drying shrinkage in concrete, the long-term behaviour of this shrinkage under high temperatures and in combination with the effects of different RH and w/c ratios remains insufficiently studied. Therefore, this research aims to study the influence of RH on the long-term drying shrinkage of concrete with different w/c ratios (0.4, 0.5, and 0.6) at a constant temperature of 40 ℃ for a period of 120 days. Previous studies have shown that drying shrinkage is highly sensitive to environmental exposure conditions, particularly temperature and RH. High temperature accelerates moisture movement and increases the drying rate, whereas RH controls the moisture gradient between concrete and the surrounding environment. However, most available studies have focused on drying shrinkage under standard laboratory conditions or on individual parameters such as w/c ratio, admixtures, or general moisture loss. Fewer studies have examined the combined effect of high temperature, controlled RH exposure, and mixture composition on long-term drying shrinkage, especially under hot-climate conditions similar to Iraq. A brief comparison between selected previous studies and the present study is shown in Table 1 to clarify the research gap addressed in this work.

The novelty of this study lies in evaluating the combined influence of RH exposure level and mixture composition on the long-term drying shrinkage of concrete under hot-climate conditions representative of Iraqi summer environments. Although drying shrinkage has been widely studied, limited attention has been given to the combined effect of monitored RH conditions and different w/c ratios under high-temperature exposure.

Therefore, this study aims to answer the following research questions: (1) How does RH level affect the long-term drying shrinkage of concrete exposed to hot conditions? (2) Does increasing the w/c ratio lead to a direct increase in drying shrinkage? (3) How does mixture composition interact with RH exposure to influence shrinkage behaviour? The practical significance of this study is to support better curing and moisture-protection practices for concrete structures exposed to hot climates.

Table 1. Summary of selected previous studies versus the present study, highlighting the research gap

3.1 Compressive strength, absorption, and porosity

The compressive strength, absorption, and porosity results were obtained from three specimens for each mixture and age. The absorption and porosity tests were conducted following the general procedure of ASTM C642:21 [25]. The reported values represent the average of three readings from three separate specimens for each mixture.

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Figure 3. Compressive strength of the concrete mixture used

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Figure 4. Water absorption (%) of the concrete mixtures used

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The error bars shown in Figures 3-5 represent the variation among the three specimens, expressed as the standard deviation (SD) of these results. The results showed a direct relationship between the increase in the w/c ratio and both porosity and absorption. In contrast, an inverse relationship was observed between the w/c ratio and compressive strength, as shown in Table 7 and Figure 3.

The mixture with a ratio of 0.4 achieved the highest mechanical efficiency (50 MPa at 90 days) and the lowest porosity (6.53). This may be attributed to the formation of a denser microscopic structure owing to reduced water content and the continuation of the hydration process over time [6, 19].

Table 7. Results of compressive strength, absorption, and porosity

Note: All values are based on three readings from three distinct specimens. SD: standard deviation.

3.2 Effect of relative humidity and water-to-cement ratio on drying shrinkage

The results of length change, shrinkage and expansion for the different mixtures are shown in Table 8 and Figures 6-8. The initial length reading taken immediately after demolding was used as the reference reading (zero-length baseline) for calculating the subsequent length changes. During the 28-day water-curing period, the specimens were periodically removed from the curing water for length-change measurements and then returned to the water. Therefore, the positive length-change values recorded before 28 days represent expansion relative to the initial reference reading. After curing the samples in water up to 28 days old, all the mixtures showed positive strain expansion ranging from 340 to 730 microstrain for all mixtures. This early-age expansion is attributed to continued cement hydration and swelling associated with the formation of C–S–H products [1, 6]. After transferring the samples to air and exposing them to different RHs, the volumetric behavior reversed between 28 and 38 days from expansion to contraction, as a result of the onset of moisture loss from the capillary pores.

Table 8. Length-change results for M(0.4), M(0.5), and M(0.6) ages counted from casting

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Figure 6. Expansion and shrinkage result for M(0.4), M(0.5), and M(0.6) at 75 ± 5% RH

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Figure 7. Expansion and shrinkage result for M(0.4), M(0.5), and M(0.6) at 55 ± 5% relative humidity (RH)

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Figure 8. Expansion and shrinkage result for M(0.4), M(0.5), and M(0.6) at 35 ± 5% relative humidity (RH)

Table 9. Shrinkage risk classification at 120 days

At 120 days, the shrinkage response varied depending on both RH condition and mixture composition. The highest shrinkage for M(0.5) was recorded under the medium RH condition, reaching −1133 microstrain, whereas M(0.4) and M(0.6) showed their highest shrinkage under the low RH condition. This behavior may be attributed to the development of sustained capillary tension within partially saturated pores under the medium RH condition, where the pores are partially filled with water. Specifically, for the M(0.5) mixture, this capillary pressure reaches its critical maximum due to the optimal balance between its pore size distribution and water-to-cement ratio. Under low RH, rapid drying may disrupt the continuity of the capillary pore-water network, thereby reducing the sustained capillary tension stresses within the pore structure [1].

In contrast, high RH (75 ± 5%) reduced the final shrinkage, with values ranging from −695 to −758 microstrain, due to the lower moisture loss and reduced vapor pressure gradient, as reported in previous studies [6, 26]. At low RH (35 ± 5%), shrinkage ranged from −856 to −1046 microstrain for M(0.4) and M(0.5), while M(0.6) reached −919 microstrain. Under this condition, rapid drying may reduce the continuity of capillary menisci over time, which may limit the persistence of capillary tension [27]. With regard to the w/c ratio, the results did not show a direct linear relationship with shrinkage, as the M(0.6) mixtures did not exhibit the highest shrinkage in all cases [1]. Therefore, the shrinkage behavior cannot be attributed to the w/c ratio alone. Because the water content was kept constant while the cement content varied, the mixtures differed not only in w/c ratio but also in cement content and paste volume; consequently, the observed shrinkage response should be interpreted as the combined effect of mixture composition rather than the isolated effect of w/c ratio alone.

The relatively high shrinkage observed for M(0.5) may be related to the combined influence of the medium w/c ratio, cement content, paste volume, and pore-structure sensitivity to moisture movement. Since the pore-size distribution, internal RH, and mass-loss measurements were not conducted in this study, the proposed mechanism should be considered as a possible interpretation rather than direct evidence.

To provide a practical interpretation of the experimental results, a simple shrinkage risk classification was developed based on the final shrinkage values at 120 days, as shown in Table 9.

This classification provides a practical indication of the RH conditions that may require greater attention during post-curing exposure of concrete in hot climates.

It should be noted that the classification presented in Table 9 is intended as a relative comparison among the investigated RH conditions rather than an absolute assessment of structural cracking risk. Although the shrinkage values at 75 ± 5% RH remained considerable, they were lower than those measured under 55 ± 5% and 35 ± 5% RH. Therefore, the classification reflects the comparatively lower shrinkage observed under the highest RH condition within the scope of this study.

Although direct drying mass-loss curves for the shrinkage prism specimens, mercury intrusion porosimetry (MIP), and internal RH measurements were not included in the present experimental program, water absorption and porosity tests were conducted on concrete specimens made from the same mixtures. These results provide indirect material evidence of differences in the pore-related properties of the mixtures. However, absorption and porosity measurements do not directly describe moisture transport during drying. Therefore, the shrinkage mechanism discussed in this study should be regarded as a possible interpretation based on the measured length-change results, absorption/porosity data, and supporting literature, rather than as direct moisture-transport evidence.