Alkali Silica Reaction (ASR) - Variation of Prestorage Time - Further investigations (Continuation 349)
Project Number 402
In the VGB research 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 VGB project 329, the effect of prestorage duration (7, 14, 21d at 20°C in moist air) has been 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.
Investigations are necessary to understand the processes occurring and to transfer the results to other concrete compositions with fly ash and other testing methods.
The prestorage aims at an approximation of the pozzolanic reaction of the fly ash to real conditions, 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.
Within the VGB projects 300, 329 and 349 essential questions have already been answered. These results explain the comparable behaviour of fly ash concrete with dense aggregate in the 40°C fog chamber, the 60°C performance test and the outdoor storage. Based on this it is now possible to compare the test procedures and predict the resistance of fly ash concrete with dense aggregate against ASR.
The reasons for the discrepancy in the cracking development of fly ash concretes with EIII-OF material between the 40°C fog chamber and the outdoor storage are not yet known and shall be investigated within this project by continuation of the storage accompanied by measurement of the damage and microscopic and macroscopic characterization of the concrete structure.
In case of success, the results will support the modification of the German alkali-guideline with the aim of an enlarged application area of fly ash concrete.
The project is assigned to the VGB focal point research programme "Waste Management of Coal-fired Power Plants and Waste Incineration Plants" (ERKOM) and is supervised by the VGB Technical Group Power Plant By-products.