低アルカリ性セメントを用いた吹付けコンクリートの原位置適用性試験,4; 幌延URLにおける低アルカリ性吹付けコンクリート及び岩石の物質移行特性

In-situ experiment of shotcrete using low alkaline cement in Horonobe Underground Research Laboratory, 4; Mass transport property of low alkaline shotcrete and Rock in Horonobe URL

宮原 茂禎*; 岡本 礼子*; 武田 均*; 中山 雅 ; 佐藤 治夫; 伊藤 誠二

Miyahara, Shigeyohi*; Okamoto, Reiko*; Takeda, Hitoshi*; Nakayama, Masashi; Sato, Haruo; Ito, Seiji

地層処分事業においては、コンクリートなどに由来する高アルカリ性が人工バリアの緩衝材として使用されるベントナイトの膨潤性能、及び天然バリアを構成する岩盤へ影響を及ぼすことが懸念されている。このような影響を低減するため、原子力機構では低アルカリ性セメント(HFSC)を開発しており、幌延の地下施設の140m調査坑道において、HFSCを用いた吹付けコンクリートの原位置適用性試験を実施した。原位置適用性試験においては、施工性の確認を行うとともに、施工場所から採取したコンクリート及び岩盤のコアを用いて化学的特性に関する調査を実施した。本報告では、HFSC吹付けコンクリート及び周辺岩盤の実効拡散係数や空隙構造に関する試験結果を報告する。HFSCはOPC(普通ポルトランドセメント)と比較して実効拡散係数が小さく、より緻密なコンクリートとなっていることが示された。

In Japan, any high level radioactive waste repository is to be constructed at over 300m depth below surface. Tunnel support is used for safety during the construction and operation, and shotcrete and concrete lining are used as the tunnel support. Concrete is a composite material comprised of aggregate, cement and various additives. Low alkaline cement has been developed for the long term stability of the barrier systems whose performance could be negatively affected by highly alkaline conditions arising due to cement used in a repository. Japan Atomic Energy Agency (JAEA) has developed a low alkaline cement, named as HFSC (High fly-ash silicafume cement), containing over 60wt% of silica-fume (SF) and coal ash (FA). JAEA are presently constructing an underground research laboratory (URL) at Horonobe for research and development in the geosciences and repository engineering technology. HFSC was used experimentally as the shotcrete material in construction of part of the 140m deep gallery in Horonobe URL. The objective of this experiment was to assess the performance of HFSC shotcrete in terms of mechanics, workability, durability, and so on. HFSC used in this experiment is composed of 40wt% OPC (Ordinary Portland Cement), 20wt% SF, and 40wt% FA. This composition was determined based on mechanical testing of various mixes of the above components. Because of the low OPC content, the strength of HFSC tends to be lower than that of OPC in normal concrete. The total length of tunnel using HFSC shotcrete is about 73m and about 500m of HFSC was used. This experimental construction confirmed the workability of HFSC shotcrete. Although several in-situ experiments using low alkaline cement as shotcrete have been performed at a small scale, this application of HFSC at the Horonobe URL is the first full scale application of low alkaline cement in the construction of a URL in the world.