  Damage caused by freezing wet, porous materials is a widespread problem, but
is hard to predict or control. Here, we show that polycrystallinity makes a
great difference to the stress build-up process that underpins this damage.
Unfrozen water in grain-boundary grooves feeds ice growth at temperatures below
the freezing temperature, leading to the fast build-up of localized stresses.
The process is very variable, which we ascribe to local differences in
ice-grain orientation, and to the surprising mobility of many grooves -- which
further accelerates stress build-up. Our work will help understand how freezing
damage occurs, and in developing accurate models and effective
damage-mitigation strategies.
