HAZARDOUS-WASTE CONTAINERS IN CEMENTITIOUS MATERIALS : LEAKAGE TESTS ON SMALL-SCALE SPECIMENS AND ON FULL-SCALE PROTOTYPES

Leaching is the selective transport of particles occurring in a material once seepage of water is established, which in turn may be driven by the long-term weathering of the material. Hence concrete repositories should be designed and produced with the objective of minimizing the risk of leaching, and consequently the risk of releasing and dispersing dangerous agents in the soil or in the atmosphere. To this end, cementitious composites exhibiting low permeability and favourable durability are ideally suited to the construction of waste barriers. In this project, the characterisation of suitable concrete mixes has been attained through leakage tests, carried out on small concrete slabs (300x300x120 mm). Then, four full-scale box-like containers ("caissons", 305x209x170 cm, wall thickness 15 cm) have been cast and submitted to waterproofing tests. Testing the caissons brought in evidence on two critical points. One, of general character, is related to the nearly unavoidable presence of "construction joints", even those that are hardly visible. These joints are likely to impair liquid tightness, that is of primary concern for the environment. The second critical point is related to the lack of homogeneity of the concrete, that is unavoidable in the peculiar conditions of the caissons, where limited wall thickness and reinforcement congestion required the abandonment of concrete vibration and the use of superplasticizers. Sonic surveys helped tracing in detail the flaws in the concrete, due to the lack of homogeneity in the walls and in the bottom slab. While concrete mechanical resistance is only marginally affected by the lack of homogeneity (especially in box-like statically-redundant structures), water tightness is crucial, because "weak" sections cannot be compensated by "strong" sections in adjacent areas.