Using rubble in foundations

11 Apr 2006
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United Kingdom
I'll be breaking up an 8 inch thick concrete slab to clear the oversite for my new porch.

The new porch will have 1M deep, 600mm wide foundations that I'm going to trench-fill with concrete, to just below ground level.

The breaking process will yield a lot of small bits of rubble that I'll use as hardcore for under the new floor, and a number of larger chunks. How small do these larger chunks need to be for it to be acceptable for me to throw them in the middle of my trench fill?


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I'd "wait" for a reply from Noseall on this one, he'll advise you, but any rubble or chunks of concrete that are too big, and thrown into a foundation, will be unstable, and not compact the same as smaller material.
The new porch will have 1M deep, 600mm wide foundations that I'm going to trench-fill with concrete, to just below ground level.

How many storeys will this porch have? ...and does it include a loft conversion?
Does seem like overkill to me. Putting rubble in foundation concrete is normally a no-no. The rubble breaks the continuity of the concrete and can cause it to crack. In this case though - unless it is a three storey porch - with a 1m deep 600 wide trench fill, it won't be going anywhere. Rubble or not.
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It's a single storey 2M x 4M front extension, thermalite block inner leaf, brick outer, so as you say, it's not going to be going anywhere!

It is not something we would normally do as the savings are peanuts against the peace of mind of a solid foundation.

That said you will incur little structural weakness if you were to throw a barrow full of brick sized pieces into the muck as you were laying. But why bother for a barrow of concrete?

Try and make use of the concrete spoil elsewhere.
My main motivator isn't to reduce concrete production costs especially, but to avoid having to hire a skip. As I'm reusing the existing storm shelter roof tiles there'll be very little other waste generated from the site (that can't be put in a car and taken to the tip) so I thought it'd be a nice win-win situation if it was okay to use the slab in that way.

I'll do some volume calcs when I get home and see what percentage 'rubble' the trench fill would be.


My foundations requirement is around 3.5m3, and the slab I'm taking up is 1.4m3, of which I'd hope to use 33% has hardcore.

So I'd be talking about 1m3 of rubble in a 3.5m3 trench. That does sound like a lot when I put it like that. I think I'll do a couple of trips to the tip in the car then!

one lump in the wheely bin every week for the next two years... Finally, a use for Council tax!
Yes, the wheelie bin does come in handy for minor disposal of trade waste :LOL:

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