Hardcore for Patio

[Wurzel83]

Hi

I am building a patio 4.05m by 2.25m on a well compacted lawn with clay soil. It will consist of both 18" and 24" square slabs, most of which I have got free of charge. I would therefore like to minimise the cost of the base layers.

I have looked at a few guides online and most of which say compact 2-3" of hardcore then apply 1-2" of mortar under the slabs and avoid the 5 dab meathod. One or 2 websites have said that the hardcore sub-base is not essentail if the patio is for light use / on solid level ground etc.

So, 3 questions:

What purpose does the hardcore sub-base actually serve and what are the long term effects of not using it?
Under what circumstances can you do away with it?
Does hardcore mean rubble or 40mm to dust aggregate?

Many thanks in advance

 

[r896neo]

If the flags are pressed concrete you could get away with laying them on sand. But in that case it would need to have a hardcore sub-base. This is usually made up of 'mot type 1' which is a mix of 20mm-dust and compacts well.

For the price of a few tons of mot why would you risk not putting it in? probably under 20 quid a ton plus delivery.

 

[Wurzel83]

Thanks for the advice.

I have been quoted £18 per tonne plus vat by a local gravel yard. Im going to put a 1 tonne 5mm sub base down as the price is cheaper than I thought and they will allow me to take half a tonne at a time in my trailer.

I have around 1/3 of a tonne of old pea gravel to get rid of - is it acceptable to bulk out the aggregate by mixing this in before compacting or will that mean it doesnt hold together as well?

 

[r896neo]

IT'd be ok if you were putting down 4 inches of sub base but for only 50mm depth it will as you say prevent it from binding well. If you really want to use it you could mix it up in a mixer as a very weak concrete like 8 stone 2 sand 1 cement but then your into buying sand and cement etc too but it may be cheaper than a skip if that was your only option.

 

[J_N30]

I'm paying £35 per tonne for scalpings round my way!

 

[r896neo]

come to belfast! I get it for 12.50 loose.

 

[Wurzel83]

Some places around here delivery dumpy bags for £35 and I know Wickes charge £42. I have heard though that some price differences are due to some places selling "Scalpings" where mud is a small percentage of the content as opposed to MOT Type 1 which tends to be dearer.

Not sure whether there is actually any difference in performance?

 

[Wurzel83]

I got a loose half tonne load of type 1 today with a view to gettin another half tonne tomorrow. I went to a well known Sand and Gravel supplier. I have wheel barrowed it from my trailer to my patio excavation and given it an innitial rake. I must say I am really disapointed as there is barely any dust in it and it looks like it will be difficult to compact. I would like some opinions before I go back to them about it. Here is a link (hopefully) to the Picture I have uploaded:

//www.diynot.com/network/Wurzel83/albums/9312/35602

 

[r896neo]

doesn't look great, your exactly right in that when it is mainly stone like that it is hard to compact.

 

[Wurzel83]

Thanks. Any other opinions before I get back to the people I bought it off? Can I do anything with it without moving it eg. is dust available on its own that I can add to it?

 

[r896neo]

yes you can buy just dust.

 

[RedHerring2]

It might be called "Quarry Dust".

 

[Wurzel83]

Thanks for the replies. I bought another half tone of finer aggregate. See additional photos in the album. I have raked it on top of the type 1 and have about 3 inches of cover.

Do i need to mix it in or Will the dust work its way down when compacted? Shall i use the roller or hire a wacker plate? Is it best to go over it with a hose first?

By the way the 2nd half tonne is a third stone dust and two thirds 14mm to dust.

Thanks

 

[r896neo]

I personally would try and mix it up a little. A whacker plate is needed and probably better than a vib roller. Wetting it well will help it mix but don't plate it then wet it, wet it while it's still roughed up or else you get a crust of wet dust on the top.