Concrete Shrinkage Taken Into Account as Crack Width Assessment

In very large buildings with connected walls, such as large Nuclear Power Plants, concrete shrinkage strains have to be considered because of elements differential strains, as required in the new European construction code (EC 2).. The common engineering method consists in considering shrinkage as an equivalent thermal strain, which is in fact computed as internal forces. In EPR, for Flamanville 3 conditions, this method could lead to unusual in the lower part of the buildings, resulting in very high reinforcement ratio hard to set in place, so EDF with its partners have proposed a new methodology based on crack width assessment. At first, we compute the shrinkage differential strains, “ε_s”, depending on moisture conditions and elements thickness. Then we fix a reinforcement section as a calculation hypothesis in order to estimate the distance “S_s” between cracks, which is independent from the loads. Consequently, we can assume that a certain crack width value “W_s” is consumed by the shrinkage itself, with W_s = ε_s S_s R_ax (R_ax restriction factor). So the available crack width for the other loads is the remaining crack width.. From cracking theory and according to EC2; we can deduce steel stress σ_d. So the structure design (reinforcement mainly) can be undertaken with this allowable limit value σ_d., in the load combinations where shrinkage as to be considered. This new methodology is more realistic than the equivalent thermal load based method, so safety requirements are satisfied by focusing on crack width assessment: this is a performance approach. It has allowed EDF to size a reinforcement ratio, which is compatible with concrete technical rules, especially in the areas near the raft. Of course this method remains in accordance with durability hypotheses and other requirements connected to nuclear specificities.