Influence of pore size distribution on concrete cracking with different AEA content and curing age using acoustic emission and low-field NMR

Concrete pore sizes are in the range of a few nanometers to a few millimeters, and these pores directly affect the cracking characteristics of concrete. In this study, the influence of the pore size distribution on concrete cracking damage was evaluated. Low-field nuclear magnetic resonance (LF-NMR) was used to test the effects of curing age and air-entraining agent (AEA) content on the pore structure of concrete. Three sets of concrete samples with different AEA dosages (0%, 0.7%, and 1.4% of the cementitious material) were prepared, and standard curing was carried out for 3, 7, 28, and 90 days before the tests. During the uniaxial compression test, the cracking signal of the concrete specimen was collected with an acoustic emission (AE) system. The results show that as the curing age increased, the proportion of macropores decreased and the combined proportion of gel pores and transition pores increased. The proportions of macropores and capillary pores increased with AEA content. During the compression process, the relative ringing count at low strain levels was positively correlated with concrete cracking damage. Based on the AE and LF-NMR data, a mathematical model between concrete cracking and the pore size distribution was established, and the effect of the pore size distribution on concrete cracking was quantitatively described by multiple regression equations.