Sustainable application of processed TBM excavated rock material as green structural concrete aggregate

Abstract

In this study, we investigate the potential use of the material excavated by tunnel boring machines (TBMs) as structural concrete aggregate. Accordingly, several experimental and statistical investigations on the mechanically and thermally processed material were carried out. The raw TBM excavated material contained a considerable amount of macro–micro quartz sand. Additionally, the poor-grain size distribution of the TBM material and its weak interface bonding demonstrated the impracticality of using it as a sand filler. Microstructural investigations of the crushed limestone material revealed that around 20% of it was composed of calcium carbonate, whereas no pozzolanic compounds were observed. The results indicate that the TBM excavated material can be used as a structural concrete aggregate. The properties of the fresh TBM-aggregate-based concrete were well within the acceptable range. The porosity and chloride diffusivity of the TBM-aggregate-based concrete mixtures were slightly higher than those of the control mixture. The aggregate processing method had a negligible influence on the strength and durability of the concrete. Furthermore, the concrete porosity and permeability exhibited a decreasing trend with the increase in the TBM aggregate content. The compressive strength of the TBM aggregate concrete was less than that of the control mixture. However, thermal treatment of the TBM aggregate can reduce this loss in strength. In addition, reasonably reliable isoresponse models were developed to predict the strength and durability of the TBM-aggregate-based concrete.