Pore Structure Characteristics of Admixture Shotcrete and its Quantitative Relationship with Mechanical Properties in High Geothermal Environment

Abstract

The high geothermal environment during tunnel construction leads to abnormal porosity and decreased mechanical properties of shotcrete, but the mechanism behind this degradation is not yet clear. The  strength and stress-strain curve of shotcrete with 10% ~ 30% fly ash (FA) and slag (SA)  were studied. Different from the traditional calculation of fractal dimension by using the whole pore size measured by MIP, the partition fractal dimension based on pore diameter classification is innovatively adopt to reflect the pore structure surface area, pore content, and tortuosity. Moreover, the quantitative relationship between fractal dimension of pore structure and mechanical properties was analyzed by rough set theory. The strain of shotcrete with FA or SA reaches the maximum when the amount of admixture is 20 %, but the ultimate strain decreases with the extension of curing age. The main pore structure type of shotcrete with FA and SA is transition pore (10nm ~ 100nm), accounting for 48.86% ~ 57.28% and 54.52% ~ 62.03% of the total pore volume, and the addition of 20 % FA significantly reduces the large pore (≥ 1000nm) of shotcrete.The incorporation of FA and SA reduced the tortuosity of capillary pores (100nm ~ 1000nm), but increased the tortuosity of gel pores (< 10nm). The pore volume of capillary and transition pores and the surface area of capillary pores have significant effects on the strength of shotcrete. However, the tortuosity of pores has little effect on the strength of shotcrete.

Keywords: High geothermal environment, shotcrete, mineral admixture, fractal of pore structure, mechanical properties