Study on applicability of low alkaline shotcrete in Horonobe URL project

Cementitious tunnel supporting system is the most expectable candidate in order to keep stability of host rock in deep geological tunnel construction. Hyper alkaline solution such as 12.5 in hardened OPC may effect alteration of rock and bentonite for a long period, although OPC has been used in actual construction in civil engineering field. Highly flyash contained silicafume cement which is called HFSC, has been developed in JAEA, and its in-situ applicability tests are planning in Horonobe URL since 2007. In this study, mix proportion of HFSC shotcrete was investigated and workability was assessed. Since 36 N/mm2 is designed as 28 days strength in supporting system in Horonobe URL, suitable mix proportion to fulfill this design strength should be proposed. 0.4 in W/B was needed in case of OPC as base cement of HFSC, and 0.45 in W/B is needed in case of HPC in order to fulfill the design strength. The actual scale shotcreting test was carried out with hardening accelerator. Workability of both 0.4 and 0.45 in W/B shows good performance. Although porous zone was observed in the surface, unit weight of inner part was satisfied with design value. The mean core strength of both types of shotcreting is more than 48 N/mm2 at 28 days, they could fulfill the require design strength in Horonobe URL. pH of pore water from HFSC shotcreting was 12.4 at 28 days, since pozzoranic reaction has not been enough progressed yet. Mix proportion will be fixed by actual using materials in Horonobe URL construction, and assessed of pH changing in long term period will be carried out in future work. (author)