Cracking by restrained shrinkage in reinforced concrete structures

Shrinkage-induced cracking is responsible for around 20% of disorders observed in reinforced concrete structures. These disorders have a number of aesthetic and structural consequences. From a structural point of view, this cracking very rarely leads to partial structural failure (for example, in the case of facade elements, it may lead to the detachment of parts of the structure). On the other hand, the consequences are more significant for the durability of the structure, or the serviceability of structures where concrete plays a waterproofing role. With regard to durability (all structures are concerned), cracks, for example, encourage the penetration of aggressive elements such as carbon dioxide (permeation, diffusion mechanism) or chloride ions (diffusion, advection mechanism), leading to premature corrosion of reinforcement. With regard to sealing, the structures involved are :

• DAMS;

• concrete tunnel linings ;

• tanks ;

• containment systems for nuclear power plant reactor buildings, etc.

In terms of leakage rate, it can be shown that this is proportional to the cube of the crack opening!

If the estimated cracking is too compromising, it should be remedied either by injecting the cracks (which increases the cost of the work by necessitating additional operations, and can also lead to delays in the works), or by a thin-layer repair. Care must also be taken to ensure dimensional compatibility (shrinkage) between the initial structure and the material used for injection or repair, otherwise the latter may crack again!

Although structural design is now taught at all engineering schools, cracking induced by restrained shrinkage is often only partially addressed. Similarly, in the regulations currently used in reinforced concrete structures (Eurocode 2), while it is clearly stated that it must be taken into account, no clear methodology is proposed.

After defining shrinkage deformations (mechanisms, influential parameters, amplitude of deformations), it is indicated which shrinkage deformation should be considered, depending on the structure and composition of the materials used. Elements for predicting the risk of cracking are then presented. As a result, the calculation is very complex in reality (it is currently the subject of worldwide research). For example, the risk of shrinkage cracking does not depend solely on shrinkage deformation! A more precise calculation requires finite-element numerical simulations, which are not presented in this document. Finally, constructive and material-related provisions are given to limit the risk of shrinkage cracking....