High-strength and ultra low-permeability concrete (HSULPC) is a strong candidate for a radioactive waste package containing transuranic radionuclides (TRU waste) for geological disposal. The information and knowledge of the time-dependent fracturing of HSULPC and surrounding rock mass are essential to assess the long-term stability of such underground repositories. Here we measured crack velocity in andesite and HSULPC both in air and water to examine slow crack growth (subcritical crack growth) by Double-Torsion method. In air, the crack velocity in andesite increased when the temperature increased. On the other hand, the temperature had little effect on the crack velocity in HSULPC in air. In water, the crack velocity increased when the temperature was higher for both andesite and HSULPC. By using the experimental results of subcritical crack growth, the long-term strength was estimated based on the model of a single crack subjected to tension in an infinite plate. It was shown that the long-term strength of HSULPC was higher than that of andesite. When the temperature increased, the long-term strength of andesite both in air and water and that of HSULPC in water decreased. The long-term strength in water was smaller than that in air for both materials. It is concluded that water remarkably affects subcritical crack growth and the long-term strength in these materials.
