This study focused on bug-holes on sidewalls of tunnel lining concrete. The bug-holes of lining concrete may negatively affect aesthetics and durability of NATM (New Austrian Tunneling Method) tunnels. Most bug-holes appeared on concrete surface are generated from entrapped air during consolidating concrete. In particular, the sidewall of tunnel lining is constructed with negative angles, so bug-holes are often observed on the sidewall. Seven concrete specimens were prepared to simulate conventional tunnel lining. In the experimental investigation, breathable-waterproof material sheets and permeable sheets were used. The primary experimental parameters are (a) covering-sheet materials and (b) form surfaces such as ceramic-coated steel with grooves. The study examined bug-hole distributions in the concrete specimens using the various sheets. The bug-hole distributions were quantified by using an image analysis developed in this study. The test results show that quantity of bug-holes of concrete using the sheets is lower than the quantity of concrete without sheets. It was noteworthy that the bug-holes were hardly observed in the test using the form covered with the permeable sheets.
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