Research on the capillary absorption capacity of concrete of different particle sizes after high temperature based on nuclear magnetic resonance technology

Abstract

Concrete is subjected to thermal stress and thermal loading in a high temperature environment, resulting in the expansion and contraction of internal pores, which adversely affects the stability and durability of concrete structures. To explore the effects of temperature and aggregate particle size on the capillary absorption capacity of concrete, the capillary absorption tests were carried out on the specimens with aggregate particle sizes of 1.25–2.5 mm, 2.5–5.0 mm and 5.0–10 mm after high temperature treatment at 300 °C, 400 °C and 500 °C.In this paper, The capillary absorption coefficient was measured by mass change method, and the migration change law and spatial distribution of water during capillary absorption were monitored by NMR technology, the changes in water content in different pores during water absorption were calculated by nuclear magnetic resonance T2 spectroscopy, and a method for calculating the capillary absorption coefficient based on T2 spectra was proposed. The results showed that the capillary absorption coefficient of the samples without high-temperature treatment decreased with increasing of particle size. The capillary absorption coefficient increases at 300 °C and decreases slightly at 400 °C. At 500 °C, the capillary absorption coefficient increases dramatically. This shows that at 500 °C, the durability of concrete is drastically affected. The capillary absorption coefficient calculated based on the T2 spectrum is linearly related to the results of the traditional weighing method. In this paper, the fractal geometry theory and regarding methods are used to deduce the relationship between the average capillary rise height and t12DT, and the rise rate is related to the pore structure. And the correctness of the model was verified by MRI. This study can provide theoretical guidance for the safety assessment of concrete structures after high temperature exposure.