White cobbles - Sourcing white cobbles/other questions

[Tom Carey]

Hi. Great site, really informative!
I have a small back yard (c. 13 sqm) which is currently covered in gravel and decking, underneath which there is concrete. I would like to replace the gravel and decking with small white cobbles (suitable for walking on but no vehicles). I have three questions:
1. The concrete seems to be sound and it doesn't puddle - water runs off to a drain by the house. To lay cobbles, will it be okay to put the bedding down directly onto the concrete which is already there, and if so, would this be to the normal 50-75mm depth? Or do I have to knock through to the sub-grade beneath the concrete?
2. Do you know of any suppliers (and/or contractors - I haven't decided whether or not to undertake the job myself!) near London who might do small white cobbles?
3. I was thinking of using a traditional bedding as the house is Victorian and it might look more in keeping. Do you have any experience of what sort of bedding might look best with white cobbles? I have seen a thread which recommended using black ash for traditional cobbling Also, if I do end up using a mortar based bedding, I understand that there are ways of colouring the mortar?
Thanks again for the excellent site.

[Tony McC]

1 - you can lay over the existing concrete as long as there's no problem with DPC levels. You need to maintain a level diff of at least 150mmm between paved surface and any DPC.

2 - You'll struggle to get a contractor interested in only 13m² but you should be able to source the cobbles via CED in West Thurrock or Rock-Unique near Sevenoaks. Contact details on Links page

3 - If I could get hold of boiler ash or crushed cinders, I'd be tempted to use a lime cinder-crete as this would give me a grey bedding that should maintain it's colour over time and not be too difficult to clean. The alternative is to use a coloured mortar, but as the predominant sands used in london Village are those orangey-yellow types, getting a grey colour is damn near impossible - it always ends up being brown. You might be able to buy a pre-coloured mortar from a local BM (I think Brendan Moran in Kilburn stocks them), but failing that, buy a powder or liquid cement dye from a local BM and do a few trial mixes to see how much dye is needed to give you a colour you can live with.

[Tom Carey]

Thanks Tony, just a couple of follow up questions. Firstly, where do you think I could get sufficient quantities of ash from and does it have to be coal ash? Secondly, what would be the proportions of ash, cement, lime and water to make a lime cinder-crete? Sorry, amateur here!

Also, on the existing concrete of the back yard, will I be okay if I only put down say 50 mm depth of the bedding? Would like to get away with as shallow as possible.

Thanks again, Tom

[Tony McC]

Getting hold of cinders depends on where you are. They used to be commonplace with all the steam engines, boilers and coal-fired powers stations, but changes in technology means stocks of new cinders are rare and you often have to rely on dumps. These can be relatively easy to find in coal-mining areas, or those locations with older power stations, but elsewhere, they may not even exist any longer.

Occasionally, an aggregate supplier will be able to locate a source, but it really is luck of the draw.

Coal ash/cinders isn't essential but it is the best. Pulverised fly ash (PFA) is a common by-product (a waste product, really) of modern power stations and is ok-ish. I find it to be difficult to work with, but it does almost guarantee a charcoal grey colour when mixed with lime. Ground granulated blast slag (GGBS) is more readily available in some steel-working areas, and while it is better than nowt, it's usually very heavy and a bugger to work with when mixed with cement or lime.

A typical mix for ash-crete would be 6 parts ash/cinders to one part lime/OPC. Using cement (OPC) gives a harder, more brittle concrete, whereas the lime is softer and more amenable to movement., It's also far more authentic for use with cobbles.