Mechanism of shrinkage and cracking in high-strength concrete regulated by internal humidity under extreme drying environments

Extreme climates characterized by coupled drying, large diurnal temperature variations, and wind are widespread in regions such as northwestern China, significantly aggravating the deterioration of volumetric stability and cracking risk of high-strength concrete (HSC) during curing. Under extreme drying coupled environments, multiscale experiments were conducted to investigate the regulation of SAP-based internal curing in HSC with different water-to-binder ratios (W/B) (0.36–0.20). Drying shrinkage and restrained deformation, the evolution of internal relative humidity (RH) and temperature, and pore structure characteristics were systematically examined, and a drying shrinkage prediction model based on internal RH was established. The results show that diurnal temperature cycling induces pronounced asymmetric periodic strain responses during the shrinkage and expansion stages of HSC, which continuously accumulate under restrained conditions, with this effect becoming significantly more pronounced as the W/B decreases. Internal RH exhibits synchronous periodic fluctuations with environmental temperature, and temperature-driven RH oscillations repeatedly disturb the pore water–vapor equilibrium, triggering cyclic loading and unloading of capillary tension and thereby driving irreversible deformation accumulation; HSC with lower W/B is more sensitive to this process. SAP-based internal curing effectively reduces the deformation sensitivity of HSC to diurnal temperature cycling by mitigating RH reduction, optimizing mesopore structure, and weakening capillary-tension-driven effects. For HSC with a W/B of 0.20, the incorporation of 0.3% SAP reduced the average shrinkage peak value and strain fluctuation amplitude by approximately 34% and 35%, respectively, significantly suppressing drying shrinkage, restrained deformation accumulation, and cracking. The proposed model shows good agreement with experimental results (R 2 > 0.90), with prediction errors below 10%. This study provides a theoretical basis for mix design optimization and shrinkage–cracking risk assessment of HSC in arid regions with pronounced diurnal temperature variations. • Cyclic temperature loading causes irreversible shrinkage accumulation in HSC. • Internal RH–temperature coupling governs capillary-tension-driven shrinkage. • Low W/B increases HSC sensitivity to extreme drying environments. • SAP internal curing mitigates shrinkage and delays restrained cracking. • An RH-based model accurately predicts drying shrinkage.