Avoidance of Alkali Silica Reaction (ASR) by Fly Ash due to External Alkali Supply - long term behaviour

Project Number 372

Due to the damage in concrete pavement which was attributed to the alkali silica reaction (ASR), the German Federal Ministry of Transport, Building and Housing (BMVBW) published the general circular letter no. 15/2005 entitled “Avoidance of Damage to Concrete Pavement due to ASR”. The letter specifies a limit of 0.8 % by mass on alkali content of the cement and the use of binder compositions containing fly ash is not considered.

In the previous AiF funded project (VGB Project 323) it was shown under which conditions a damaging alkali silica reaction due to the external supply of additional alkalis can be reduced or avoided by using fly ash as concrete addition. The effect of different levels of fly ash content on ASR damage due to external alkali supply has been determined by comparing changes in length and assessing the degree of damage to concrete prisms and cubes under laboratory conditions.

In order to simulate the exposure of structural components to alkalis in the spray region of roads, 300 mm concrete cubes have been stored in natural outdoor conditions and sprayed with a 3 % by mass NaCl solution in winter. Till the end of the AiF project in most of the cases (concretes containing fly ash and reference mixtures) no damage caused by ASR occurred.

Despite a storage time of over 3 years in natural outdoor conditions an evaluation of the different concrete mixtures was not possible. After the end of the AiF-project these concrete cubes have not been removed and still have been treated like described before, which makes it possible to continue the investigations. Within the next two years it is likely to see differences between the different concrete mixtures. It is estimated that the same positive effect of concretes containing 30 % fly ash, which was observed under laboratory conditions, will also be seen in outdoor conditions.

With these results a modification of the German alkali-guideline with the aim of an enlarged application area for fly ash concrete could be possible.

The research project is supervised by the VGB-Technical Committee “Power Plant By-products”.