  In this study, we propose a laboratory-scale methodology, based on X-ray
micro-tomography and caesium (Cs) as a contrast agent, to advance the
understanding of cracking due to alkali-silica reaction (ASR) in concrete. The
methodology allows achieving a completely non-destructive and time-lapse
characterization of the spatial-temporal patterns of both the cracks and the
ASR products. While Cs addition slightly accelerated the ASR kinetics, the
crack pat-terns, with and without Cs addition, were statistically equivalent.
Cracks with ASR products appeared first in the aggregates, close to the
interface with the cement paste. They propagated afterwards towards the
aggregates interior. Some products were then extruded for several mm into air
voids and cracks in the cement paste. This process suggests that, in the early
stage, the ASR products may be a low-viscosity gel that can flow away from the
source aggregate and may settle later elsewhere as a rigid phase, upon calcium
uptake.
