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Posted: Thu May 24, 2007 6:35 pm
by jaseandwave
I'm just starting to lay Bradstone sandstone flags.
Your site recommends 10-1 for bedding layer, Bradstone's web site reckons on 3-4. That's a bit of a difference!
Why would Bradstone recommend such a strong mix if it aint necessary.
Posted: Thu May 24, 2007 7:54 pm
by lutonlagerlout
the strength of the mix is also affected by the coarseness of the aggregate,so bradstone must cover them selves
personally i use 6:1 this works well for me and i have never had a patio fail (yet)
but the grit sand where i live is quite soft (south east) up north and Scotland its a lot coarser and stronger for it
my general sandstone paving mix equates to 12.5 kg (half a bag) of cement in the mixer and 15 shovels of grit sand plus water and PVA
hope this helps
Regards LLL
Posted: Sat May 26, 2007 1:39 pm
by Tony McC
The BS mix for flag bedding is 3-4:1, but this is, in my experience, way over the top for your average patio, and because of the relatively high cement content, gives reduced working time, which makes an already awkward job that much more difficult for DIYers.
Over 30-odd years of laying flags, I found that a 10:1 mix gives more than adequate strength, with a much extended working time, and lower costs. Ifd you yhink about it, a 10:1 mix of grit sand with cement is not much different from the old 1:3:6 mix for medium strength concrete (C7.5) and is roughly the same strength as the bedding we use for road kerbs.
LLL is right that many manufacturers feel obliged to stick to BS recommendations to cover their own arses, but it's ridiculously over-strength and doesn't take into account the problem of shrinkage when a cement-rich mortar (or concrete) is used as a wet mix.
Posted: Sat May 26, 2007 5:00 pm
by lutonlagerlout
i have a feeling the aggregate is a lot coarser oop north tony,which makes the mix inherently stronger
us southern softies only get relatively soft sand,which i have seen used on floors screeded with a weaker mix and fail.
cheers LLL 