The Brew Cabin

hi all great site.
i am about to start laying bradstone carpet stones on my driveway.in the bradstone brochure it states you have to have a concrete base of between 100-150mm,then a bed of sharp sand/cement mix 40mm deep ,under the carpet stones.im going to cover an area of 60m2.
i was wondering if i could get away with laying 100mm of type1,vibrate it down,then the sand/cement mix 40mm on top of that.i dont know why they want a concrete base ,when i dont think you need it for monoblock.can anyone advice

The reason why the folk at Bradstone recommend laying the Carpet Stones on a concrete base is the old "stiff on stiff" rule of thumb, ie, if you're using a stiff bedding, which would be the case with a sand/cement bed and mortared joints, then the base should be stiff, also. The thinking is that, if you use a rigid bed/jointing on a flexible sub-base, then any slight movement in the sub-base will result in cracking of the bedding/jointing.

Monoblock, or block paving as it's called south of Hadrian's Wall, is a flexinble paving. The bedding and the jointing are unbound, that is, no cement, and so the blocks can be laid on a standard sub-base.

The problem with the Carpet Stones is the joint width - they're too wide to rely on an unbound jointing material, such as sand, so you're more or less obliged to use a mortar (or a polymeric sand), which, because of the small unit size of the individual "stones" means that a bound bedding is required, which, in turn, requires a bound base. However, you could get around this on a patio with foot traffic only be laying the Carpet Stones onto a 75-100mm thick bed of concrete laid over a sub-base. That would be strong enough to take pedestrians in low numbers, but would not be suitable for a driveway, such as you have, where the only sensible option is to use at least 100mm of concrete as a bed/base, or, as it says in the brochure, lay a concrete base and rely on a 20-30mm layer of mortar to bed the damned things.

thanks for the advice.
the bedding mixture for these carpet stones is 6 sharp sand to 1 cement,does this mixture still thoroughly harden.when the stones are laid,i am suppsed to brush in a mix of 1 part cement with 3 parts sharp sand.i was under the impression that the mix wouldnt harden thoroughly and would be slightly flexible.
i know laying concrete is the correct way,but the last thing i want is the expense and labour of laying it.is there no mix i could use on these stones that would give me the flexibity required.

You've lost me there! There are no cement-bound products that are flexible - it's just not possible. Cement is a hard, non-plastic material when cured, and so it can never be flexible, at least, not at the sort of joint width we're considering!

A 6:1 bedding layer will be reasonably hard, somewhere around 15-20 Newtons, I'd estimate, and a 3:1 jointing mix would be even harder/more brittle, at around 30-40 Newtons, so your plan to use a flexible jointing material is a non-starter with any cement-based material. What made you think that a cement-based material would be flexible?

I can't understand why you would want a flexible jointing material, anyway, when you're laying on a rigid bed. Remember what I said in my previous post - "Stiff on Stiff" - if you use a stiff bed, then you should have a stiff joint. If you really want a flexible joint, then you need to look at a bitumen-based sealant, or some other elastomeric compound, such as Nitoseal, but the cost would be astronomical. A rough calculation tells me it would cost more for the Nitoseal to fill the joints of a single mat of Carpet Stones than it would for the mat itself!

Why not contact Bradstone and ask what they suggest? I bet they have only one option, the one in the brochure!

contacted bradstone,they said i probably could get away with type 1 as a base,so i tried it and hey presto, it worked,no movement at all.i managed to get a hold of a flexible mortar substance called polymartic.managed to get it from a local builders merchants.its a new product from canada.its like a mortar like susbstance with a pernament plasticiser bound in it.you can dye it the colour of the carpet stones.it great as in its gives lots of movement of the stones,a bit like chewin gum.

Why would you want your bradstones to move around?

I missed something here anyway, what I want to know is reading this post gave me an idea, I have a concrete drive a bit uneven in places with a join here and there and the odd crack, I assumed I would have to have it all dug up and start again for any improvment to the driveway, but after reading the posts on this topic I wondered, do you think I could lay carpetstones or something similar on the top of my concrete drive?. what you think.

Cheers Nev

In response to fras - you may think there's no movement of your carpet stones on a DTp1 bed, but there is! It may be minimal, but they will be moving - that's the natuire of the best with an unbound bed.

I'm not sure about this Polymartic stuff. It can't be a flexible mortar, and my guess is that it's just another polymeric sand. You say it contains a "permanent plasticiser", but that doesn't make any sense, as a plasticiser keeps mortar plastic, ie, workable, unset. I'll have to see if I can find out anything about it.

To Nev - have a read of the Paving over an existing base page to see if your idea is feasible.