The sulphate internal reaction due to Delayed Ettringite Formation (DEF) is a cause of disorders likely to damage concrete structures severely. If the first cases of DEF appeared abroad in some precast concrete elements subjected to a heat treatment unsuited to the composition and the environment of the concrete (like, for example, railway sleepers), this pathology was observed in France, for the first time in 1997, on bridges whose concrete had been cast on site. The bridge parts damaged by DEF were primarily massive structural elements (piers, crossbeams on piers or abutments, etc.) in contact with water or subjected to high moisture. Differing from the more traditional sulphate reactions where the sulphates attack the concrete from outside and create a progressive degradation from the surface towards the heart of the element, DEF affects the whole concrete without calling upon an external source of sulphates. It leads to a concrete swelling and the cracking of the structure. The paper describes the lessons learned from the expertises conducted on several cases with the objective to write a guide in order to prevent disorders in new structures that have to be built. Detailed expertises were carried out on eight bridges in order to reveal the parameters that were simultaneously present and necessary for the development of DEF. These parameters were classified in four groups: parameters related to the temperature, the cement; the concrete and the environment. From this analysis, a strategy was built and based on a crossing between, on the one hand the category in which the structure is classified according to the level of risk of occurrence of disorders that can be accepted, and on the other hand the environmental conditions to which the structure is exposed during its service life. This strategy defines four level of prevention with related precautions that have to be applied. The precautions are mainly based on the limitation of the maximum temperature reached within the heart of the structure parts during hardening of the concrete, and on the choice of an adequate composition of the concrete. The prevention of disorders due to DEF is presented in the technical recommendations released by LCPC in august 2007.
Lessons learned from structures damaged by delayed ettringite formation and the French prevention strategy