Effets combinés de différents facteurs sur l'expansion des bétons causée par la formation différée de l'ettringite

Abstract

In order to better understand, explain, and predict expansion caused by delayed ettringite formation, an experimental programme was designed. The influence of certain parameters regarded as significant on the behaviour of over 1000 samples of mortar and concrete, was examined. An elevated temperature during setting and hardening, as well as the use of a cement sensitive to DEF are the necessary factors for this phenomenon to occur. However, DEF does not necessarily entail expansion. Expansion due to DEF can be reduced, and in certain cases prevented, by using mixtures containing entrained air. Other sources of void creation, such as microcracking caused by thermal shock, drying and wetting cycles or freezing and thawing, or simply the volume of entrapped air, can significantly decrease expansion.In cases where the potential for expansion is low, expansion can be hidden by the creation of microcracks.In many methods of evaluating the risk of expansion caused by DEF, acceleration of this expansion is based on the formation of microcracks. However, microcracks change the behavior of concrete. We have developed a new accelerated method to evaluate the sensitivity of a given mortar or concrete. The advantage of applying this method, in which the samples are kept in water continuously, is that they do not crack due to the application of the method itself. A statistical analysis of the effect of various factors such as maximum concrete temperature during setting and hardening, volume of entrained air, choice of cement, existence of microcracks, thermal history, given service conditions, as well as the interaction of these factors on the potential for DEF, was undertaken. It is possible to determine certain predominant factors in DEF and consequent expansion. However, the number of factors having a significant effect on expansion is important, as well as the number of interactions.In cases where a cement"sensitive" to DEF must be used and where temperature during setting and hardening will be unavoidably high, a study of the specific mixture is required. An efficient tool in diminishing expansion due to DEF is entrained air. The determination of air void characteristics required to avoid expansion (L[overline] and volume), as a function of the mixture's sensitivity (including materials and mixture characteristics), could be the focus of further study.