Tome Architecture and management of a geological repository - Andra

5 – B waste repository zoneThe quantity of alkaline elements (K and Na) in the cement, adjuvants and other constituents is as lowas possible, to limit the risk of silica alkaline reaction (level of alkaline Na 2 O not exceeding 0.6%).The class of cement resistance to obtain a high performance concrete (HPC) class of concrete must beat least 42.5 Mpa.• Water dosage and ratio of total water to total cementitious materialThe weight ratio of total water (W) to total cementitious material (C = cements plus mineral additives)must be less than 0.40. To ensure that this is the case, apart from using a reasonably maximized doseof cementitious material, water (W) must also be minimized. A reasonable quantity of cementitiousmaterial, taking into account an aggressive environment and current normalization, is within the rangeof 300 to 550 kg/m3.This choice makes a significant contribution to obtaining highly compact concrete, i.e. minimizingporosity. This provides better protection from chemical attack and improves mechanical andconfinement performances.The low dose of water also helps limiting the hydrogen source term by radiolysis of the envelope,particularly for disposal containers holding primary irradiating packages type B1 and B5 in particular.5.3.3.3 AdjuvantsFor implementation, adjuvants are necessary. They compensate for the low water content, providingthe concrete with the fluidity required while pouring. However, since adjuvants are mostly organicmaterials, the dosage must be adjusted to limit the quantity of complexing species likely to influenceradionuclide water transfer mechanisms (solubility).5.3.3.4 GranulatesThe choice of granulates (sands and gravels) results from the objective of preventing alterationphenomena due to alkali silica reaction.Considering the time scales considered, particularly for the variant with reinforced retention capacity,granulate must be a physico-chemically stable limestone. Dolomitic limestone (containingmagnesium), or limestone with a high alkaline content, must be avoided. The limestone granulatesenvisaged must contain at least 95% calcium carbonate (CaCO 3 ). The impurities constituting theremaining 5% must also be free of any alkali silica reaction.The diameter of granulates must be limited (dimensional classification less than 12.5 millimeters) toimprove the concrete's mechanical strength and density. The granular section can also be refined toimprove the concrete's density therefore enhancing performance and facilitate concrete casting(injection, possibility of pumping). Finally, granulates must be non-reactive.5.3.3.5 Possible fibres and reinforcementAny fibres and reinforcement contribute to the need of mechanical reinforcement and durability of thecontainer.The use of stainless steel reinforcement rods and/or fibres avoids the risk of cracking linked to theexpansion of non-alloyed steel corrosion products.The addition of fibres to the concrete improves the quality while reducing the risk of microcrackingand provides characteristics of ductility and tensile strength. A tensile strength of 5 MPa has thereforebeen targeted.5.3.3.6 Formulation, industrial implementation tests and performance obtainedSeveral concrete formulations have been developed in the laboratory, then tested using industrialsuitability tests. During these tests, particular attention was paid to the "pumpability" of the concrete,Dossier 2005 Granite - ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL REPOSITORY125/228