Investigation of moisture migration of MWCNTs concrete after different heating-cooling process by LF-NMR

Abstract

Moisture migration and distribution have a significant influence on the durability of concrete exposed to high temperature after cooling process. And adding multi walled carbon nanotubes (MWCNTs) into concrete could greatly improve its mechanical properties at room temperature. In order to study the moisture content, distribution characteristics and migration law of MWCNTs concrete after different heating-cooling process, a non-destruction method, low field nuclear magnetic resonance (LF-NMR) detection technology was applied in this study. The MWCNTs concrete specimen was heated at 200, 400, 600 and 800℃ followed by two cooling modes, including quenching in pure water and spraying by high pressure water gun. For comparison, plain concrete (without MWCNTs) specimen under the same experimental conditions was prepared as a reference. And then the transverse relaxation time (T2) was measured by Low field nuclear magnetic resonance (LF-NMR) to distinguish the moisture condition of the concrete specimen and the influence of MWCNTs on the moisture migration of concrete after heating-cooling process. Meanwhile, the weighing method also conducted to measure the moisture content to compare with the results obtained from LF-NMR. In addition, mechanical test and micro test (TG-DTA and SEM) were performed to confirm the influence of MWCNTs on the residual properties of concrete. Results from this study showed that the transverse relaxation time T2 of concrete presents a satisfactory correlation with moisture content, residual compressive strength and porosity of concrete after heating-cooling process. And the results of weighing are consistent with those of LF-NMR. Both heating-cooling process affected the moisture migration in concrete, leading to different mechanical performance and pore structure. Furthermore, the MWCNTs have significant effects on moisture migration and pore size distribution of concrete. Based on the LF-NMR test results, the relationships between the residual compressive strength, porosity and moisture content were established.