The evolution of microstructure and compressive strength of self-compacting concrete affected by early-age loading: Experiments and multi-scale coupling model

This study investigated the effects of early-age loading on the compressive strength , porosity, and microstructural evolution of self-compacting concrete (SCC) before formwork removal. A specialized steel mold and reaction frame were designed to simulate early-age loading conditions. The results of compressive strength tests, mercury intrusion porosimetry (MIP) tests, and scanning electron microscopy (SEM) tests revealed that increasing early-age loading induced a compaction effect , reducing porosity and enhancing the densification of mortar around aggregates, thereby increasing compressive strength. However, excessive early-age stress during hydration led to a damage effect, characterized by an increase in pore (>1000 nm) volume, aggregate and mortar crushing, and a subsequent decline in compressive strength. This demonstrated that early-age loading within the formwork exerted a multi-scale coupling influence on SCC's microstructural evolution and compressive strength, dependent on stress level and time scale. Based on these findings, a multi-scale coupling model incorporating capillary porosity evolution was developed for compressive strength analysis. The model was validated using experimental data from varying early-age loading paths, demonstrating its capability to accurately predict porosity and compressive strength evolution of SCC under different early-age loading conditions within the formwork. • Early-age loading causes compaction and damage effects in SCC. • Compaction and damage effects show multi-scale coupling with load level and time. • The effect of early-age loading on SCC 's mechanical properties is linked to microstructure. • Multi-scale model for strength was developed based on capillary pore evolution.