Abstract: To investigate the quantitative relationship between pore distribution characteristics and the compressive strength of medium-to-high strength concrete, we conducted comprehensive compressive strength tests by using a low-field nuclear magnetic resonance (NMR) imaging analysis system in conjunction with a fully automatic pressure testing machine. By validating and modifying the Ryshkewitch semi-empirical equation model, we established the quantitative relationship model between pore distribution characteristics and compressive strength. Test results demonstrate several key findings. First, an increase in sand rate and initial entrapped air content of concrete leads to a gradual increase in porosity. Increasing the initial air content proves to be more effective in generating a more variable porosity compared to increasing the sand rate. Specifically, by increasing the sand rate, the percentage of non-capillary pores increases by 17.2% while the percentage of capillary pores decreases by 13.0%. On the other hand, increasing the initial air content results in a 12.7% increase in capillary pores and a 12.6% decrease in gel pores. By employing the model based on modified Ryshkewitch semi-empirical equation, we achieved accurate results with an R² value close to 1 and a P-value less than 0.05, effectively describing the quantitative relationship between pore distribution characteristics and compressive strength.

Key words: medium-to-high strength concrete, compressive strength, pore distribution, low frequency NMR, sand rate, initial air content, porosity, Ryshkewitch semi-empirical equation