Autoradiographic methods for identifying alkali-silica reaction gel

The current method for identifying ASR gel uses ultraviolet fluorescence to image the distribution of a uranyl acetate stain that has a high affinity for the silica gel. However, UV fluorescence can also be produced by other minerals. As an alternative, methods based on imaging natural or activated radiation have been investigated. Potassium in the, gel can be imaged by both the beta and gamma ray radiation of K-40. The storage photostimulable phosphor imager (SPP) system can have spatial resolution as fine as 50 mu m and a dynamic range of >10(5). Digital images of K-40 distribution in actual concrete cores with K2O % <1% have been acquired with counting times on the order of 24 h. Similarly, using the conventional uranyl acetate stain approach, the distribution of uranium can be imaged by the natural radioactivity of its daughter products, using the SPP plates with counting times on the order of 7 days. Alternatively, the uranium can be imaged by inducing fission and using fission track detectors. Autoradiography can thus be used to survey concrete structures for areas of possible ASR. Further research is necessary to standardize the methods and to evaluate their performance relative to the conventional stain methods.