Architecture and management of a geological repository - Andra

4 – Waste disposal Packages4.1.5 The concrete used for disposal containersThe material requirements deduced from current standards in the concrete construction field [46]generally include mechanical considerations and secular durability of the structures.Within the context of container studies, Andra and the CEA have followed these standards; they havealso completed them with stricter requirements. The solutions given here are therefore based onscientific studies concerning the durability of concrete on the centennial scale [47] [48]. These studies,including laboratory tests, have been conducted since the beginning of the 1990s, both in France andabroad. Industrial suitability checks are subsequently made of the adopted solutions.The concrete used for disposal packages consists of four groups of materials:- cement, mineral additions and water;- admixtures;- granulates (sands and gravels);- reinforcements and/or metal fibres.This section is devoted to defining the characteristics of each of them. It presents the respectivecontributions to global package performance.4.1.5.1 Technical translation of expected performancesIt will be noted first of all that there is strong coherence between the durability of concrete and itsretention capacity. Indeed, to meet its objectives, concrete needs to be very compact (low porosity),with a very low diffusion coefficient and reduced permeability (no open porosity or cracking). Thesequalities promote concrete durability because they protect the concrete from penetration by water andhence prevent chemical alteration reactions engendered by the presence of water. They are alsofavourable to retention potential.The ability to use the material and obtain the best possible quality initial condition (i.e. with crackingas limited as possible in opening and in length) depends on:- low hydration heat;- limited endogenic shrinkage;- adequate relaxation of constraints generated early on by deformations blocked in the structure.Apart from the above characteristics, concrete durability also depends on preventing concretepathologies such as alkaline silica and alkaline granulate reaction, sulphate attack, hydrolysis andcarbonation [47].The consequences of a reaction between certain granulates (containing amorphous or poorlycrystallised silica) and the alkaline interstitial concrete solution are that the reactive silica dissolves.This leads to the creation of a gel, which is more or less crystallised, with a larger volume than theinitial reagents. The concrete can crack under the effect of this expansion. In the same way, theconsequences of sulphate attack can be measured in terms of mechanical deterioration of the concrete.Indeed, precipitation of expansive compounds (mainly sulphate-aluminates) leads locally tomechanical stresses which exceed the material's capacity, thus creating severely cracked zones. Thechoice of non-reactive granulates and cements with a limited concentration of tricalcium aluminates(C 3 A) helps control these pathologies.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM121/495