Freeze-thaw resistance of sprayed concrete in tunnel linings

摘要

Freeze-thaw exposure of wet-mix sprayed concrete for permanent tunnel linings is characterized by thermal and moisture gradients between wet temperate rock mass/concrete and cold dry concrete/air surface. Hardened sprayed concrete with water/binder ratios in the range of 0.45-0.47 has been studied. The standard freeze/thaw tests like e.g. salt frost scaling according to CEN/TS 12390-9 and rapid freeze/thaw cycling according to ASTM C666 represent much more severe exposure conditions than the worst possible cases in tunnel linings. The sprayed concrete did not pass the salt frost scaling test with 3 % NaCl and rapid freeze/thaw cycling in water but showed no deleterious freezing dilation during sealed freezing following 4-6 months submersion in water. The porosity characteristics, air void content, spacing factor and protective pore factor also indicate a certain freeze-thaw resistance. A new functional performance freezing test method simulating the moisture and freezing gradient exposure conditions in a tunnel lining in a more realistic manner is presented. Reproducible cyclic freeze-thaw exposure during the different test series was measured. Frost damage was measured by resonant frequency on disc shaped specimens prepared from the core sample after gradient frost exposure. The new gradient frost method also indicates frost durability at full capillary saturation on concrete that deteriorated in salt frost scaling-and rapid freeze/thaw tests. The findings of this study suggest that the sprayed concrete in a sprayed concrete lining structure waterproofed with membrane based on ethyl-vinyl-acetate copolymer, is not prone to freeze-thaw damage under the tested moisture and thermal conditions.