Alkali-Silica Reaction (ASR) - Effect of Prestorage Time on Fly Ash Concrete in 40 and 60°C Concrete Performance Test

Project Number 349

In the VGB project 300 “Avoidance of Alkali Reactions by Fly Ash - Examinations of Concretes During Long-term Storage” concretes made with fly ash and reactive aggregate were stored up to seven years in outdoor conditions and compared to specimens stored in the 40°C fog chamber.  Differences were observed for some fly ash concretes made with EIII-OF aggregate. The assessment of the performance of these concretes based on the fog chamber 40°C storage was incorrect for certain replacement levels of cement by fly ash and certain types of fly ash. It is not clear why these concretes exhibit no signs of damage in the fog chamber, although the specimens stored outdoors were damaged, and how a correct assessment should be achieved.

In another current VGB project 329, the effect of prestorage duration (7, 14, 21d at 20°C in moist air) is being investigated using fly ash concretes in the 60°C performance test. By changing the testing procedure, the assessment of the different concretes was the same as for the fog chamber. It is expected that this will also apply to concretes made with EIII-OF aggregate.

Investigations are necessary to understand the processes occurring and to be able to apply this to other concrete compositions with fly ash and other testing methods.

The prestorage aims at a more real occurrence of the pozzolanic reaction of the fly ash, i.e. without the acceleration produced by immediate storage in the fog chamber. This also avoids disturbance of the solubility equilibrium of the pore solution. These effects are most likely decisive for the correct assessment of fly ash concretes with EIII-OF material. Thus, storage in the fog chamber will be supplemented by prestorage (28, 91d) of the specimens at 20°C in moist air.

In order to draw conclusions on the development of microstructure and pore solution of the concretes, PE bottles are filled with fresh concrete or cement paste and prestored like the fog chamber specimens. The microstructure is investigated using mercury intrusion porosimetry and the extracted pore solution analyzed chemically using ICP-OES. These investigations are planned for concretes with EIII-OF material as well as concretes with greywacke used in the 60°C test as reactive aggregate.

In case of successful tests, the results will support the modification of the German alkali-guideline (in preparation) with the aim of an enlarged application area for fly ash concrete.

The project is supervised by the VGB-Technical Committee "Power Plant By-products".