Avoidance of Alkali Silica Reaction by Fly Ash due to External Alkali Supply

Project Number 323

Due to the damage in concrete pavement currently observed which, based on initial investigations, 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 this project, the future use of fly ash in pavement concrete shall be enabled by extensive proof of the durability of pavement concrete made with fly ash and, in excess thereof, the proof of the avoidance of ASR by using fly ash.

The proposed investigations aim at testing the conditions under which damaging alkali silica reactions due to the external supply of additional alkalis can be avoided for aggregate of the sensitivity class EIII-S (critical) if fly ash is used in combination with German Portland cements (CEM I) having a regular Na 2 O equivalent.
Investigations on damaging ASR are planned for concretes made with different fly ashes and cements of different alkali content available in Germany together with aggregate of the sensitivity class EI (harmless) and EIII-S (critical) according to the DAfStb Alkali Guideline.

Tests will also be carried out using fine concrete made with duran glass in order to determine the effect of fly ash on external alkali attack. In addition, investigations are planned with conventional pavement concretes and concretes for marine structures made with alkali sensitive aggregate. Changes in length and ultra sonic pulse velocity are to be measured. In selected cases, crack formation and, possibly, gel deposits will be investigated using SEM and optical microscopy on thin sections. The effect of different levels of fly ash content on ASR damage due to external alkali supply is to be determined by comparing changes in length and assessing the degree of damage to concrete prisms and cubes.

Concrete compositions will be selected for the chemical analysis of pore solutions. The hardened paste microstructure and the formation of phases will be characterized and the penetration of Na + and Cl - ions determined.

In order to simulate the exposure of structural components to alkalis in the spray region of roads, 300 mm concrete cubes will be stored in natural outdoor conditions and sprayed with a 3 % by mass NaCl solution in winter.

The project is assigned to the VGB research programme “Waste Management of Coal-fired Power Plants and Waste Incineration Plants” (ERKOM). Starting in December 2008 and finishing in May 2011, the project is carried out by the Institute for Building Materials and Construction at the Technical University of Munich under the direction of Prof. Detlef Heinz. It is technically supervised by the VGB Technical Committee Power Plant By-products . The project is funded by the Federal Ministry of Economics and Technology via the German Federation of Industrial Research Associations "Otto von Guericke" ( AiF Project No. 15866 N).