On Fri, 20 Aug 2010 03:37:13 -0700, Jim K wrote:
oh concrete will set, just keeping it where you want it whilst it does
if raining/water flowing at time - Bin liner - i.e. effectively make "a
bag of concrete"? will mould to contours around pipe and sides of ditch
and not get washed out whilst setting....

I think you only need it to be rain-free for 24 hours or so; I just did
some concrete work around our wood chute and it was solid when I took the
forms off in less time than that. Of course it'll take much longer to
fully cure, but I think it'll do so eventually - the critical bit's just
not having it completely saturated by water during that first phase.

cheers

Jules