i have ladder spikes fitted to my ladders, after reading on this forum about them a couple of years ago.these dig in on the smallest crack and are great.
be wary of buying Anything made of rubber as a safety feature,including ladder feet,because if you use rubber outside if the ground is damp the rubber can slip as if its on ice.
i knowthis because i had a set of laddermats (rubber mats) and one October morn i put them down on a slightly damp path and once i got at the top of my ladder,WHOOSH out it slid at the bottom and i was very lucky i didnt break both legs, as i fell the full drop . At the time i didnt know rubber slides on damp ,iv since read various scary tales of rubber slipping .
Laddermatrix ,which has tiny metal spikes might be a better safety idea though i havnt got one.