Influence of pre-demolding early-age loading on post-demolding microstructure and compressive strength evolution of self-compacting concrete under different curing regimes: Experiments and multi-scale model

During tunnel lining concrete construction and curing, the evolution of self-compacting concrete's (SCC) microstructure and mechanical properties is critically influenced by both early-age loading and the post-demolding curing regime, causing performance development to deviate from design specifications. To accurately predict the performance development of SCC in actual tunnels, this study employed a custom reaction frame to simulate early-age loading from surrounding rock deformation and sealed steel molds to simulate formwork trolley restraint. Compressive strength tests, Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscopy (SEM), and X-Ray Diffraction (XRD) were conducted on SCC subjected to different early-age loading under varying demolding times and curing regimes. Results show that under 2.0 MPa early-age loading and standard curing, SCC demolded at 1, 3, 5, and 7 days exhibited 7–28 day compressive strength growth rates of 14.29 %, 18.94 %, 19.32 %, and 18.61 %, respectively, consistently exceeding unloaded specimens (13.73 %, 12.97 %, 14.70 %, 14.11 %). Furthermore, compared to SCC without early-age load, specimens under 2.0 MPa early-age loading for 7 days before demolding showed an 8.06 % reduction in total porosity and a 19.32 % reduction in 10–1000 nm porosity at 28 days, concurrently with enhanced microstructural density and higher diffraction peaks for hydration products. Delayed demolding consistently increased compressive strength, reduced porosity, and improved microstructural density regardless of loading. Standard curing post-demolding proved optimal, yielding lower porosity, denser morphology, and more hydration products versus low-temperature/low-humidity curing. Finally, a multi-scale compressive strength model was refined by incorporating early-age loading effects on cement particle spacing (reduction/dilation). The simulation results can effectively predict the development of compressive strength of SCC under different demolding times and curing regimes. Moreover, it can accurately predict the phenomenon that the development rate of compressive strength of SCC with demolding times of 1–7 days is 0.56 %, 5.97 %, 4.62 %, and 4.50 % higher than that without early-age load when the coupled effects of spacing reduction and dilation are considered. • Early-age loading causes cement particle spacing reduction-dilation effects in SCC. • Reduction-dilation effects show multi-scale coupling with load level and demoulding times. • Early-age loading and curing regimes jointly govern the performance evolution of SCC. • Multi-scale model for strength was developed based on hydration products formation kinetics.