The relevance of laboratory studies on delayed ettringite formation to DEFin field concretes

Many laboratory studies of delayed ettringite formation (DEF) have been con ducted on thin mortar bar specimens, heat treated, and then immersed in wat er. Under these conditions, rapid diffusion of alkali hydroxide into the su rrounding water occurs and necessarily reduces the alkali hydroxide concent ration of the mortar pore solution. Results reported recently by Famy indic ate that the DEF process is triggered as a consequence of such leaching. Wh en it is prevented by immersion into alkali hydroxide solution instead of w ater DEF expansion is delayed or prevented entirely. Results reported by Zh ang indicate that 51-mm mortar cubes behave differently than more leaching- susceptible mortar bars when exposed to the same wet environment. Mortars t hat show severe DEF as mortar bars remain almost free of DEF symptoms if th ey are stored as cubes, even after 900 days. Attention is called to the fac t that DEF in concrete is found commonly in thick concrete members where th e possibility of leaching is remote. For such concrete, the reduction in in ternal alkali hydroxide concentration that occurs with ASR can substitute f or the effect of leaching. It is postulated that without effective reductio n of alkali hydroxide concentration by one or the other process, DEF remain s latent. (C) 2001 Elsevier Science Ltd. All rights reserved.