Study on spalling and cracking behavior of MWCNTs concrete exposed to high temperatures

This study aims to investigate the effects of multiwalled carbon nanotubes (MWCNTs) on the spalling and cracking behavior of concrete exposed to high temperatures, and also consider the mechanism of different cooling methods (including natural air cooling, spray cooling, and quench cooling) on its performance after high temperature. Meanwhile, the change rules of mechanical properties and pore structure of MWCNTs reinforced concrete during high temperature and cooling were explored by x-ray computed tomography. The results demonstrated that spalling and cracking behavior of the concrete specimens exposed to high temperatures was effectively controlled by adding MWCNTs. Consequently, the thermal resistance and residual mechanical properties of concrete exposed to high temperatures were significantly improved. Moreover, the mesoscopic and microscopic observation showed that the MWCNTs have a favorable impact on reducing the porosity of the concrete specimens below 600 degrees C. However, when the temperature exceeded 600 degrees C, MWCNTs reached the burning point and lost the bridging effect. No trace of MWCNTs was observed in the concrete specimen, but the cumulative mitigation decreased the possibility of cracking and spalling of concrete at 800 degrees C. Additionally, considering the influence of cooling methods, it is found that the spalling and cracking of the concrete specimens after high temperature caused by quench cooling were the most serious, while that of caused by spray cooling were the least.