Filling trench foundations with just concrete or hardcore and concrete

21 Oct 2010
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United Kingdom
Hi, I am building a 7x5m shed in my garden. I have dug trenches of 350mm width x 500mm depth for the perimeter, and also a central trench for floor joists support. I will be filling the trench with Concrete 1:5 ratio (Ballast/Cement) and will also use Formwork to ensure the concrete form is 100mm off the ground. So, in total my filling space is 600mm deep. Question: Can I chuck loose concrete blocks/broken bricks/hardcore in the bottom 400mm of the trench, leaving concrete only for the top 200mm or must I fill the entire 600mm depth with concrete? It seems quite overkill to fill all the space with concrete for an all timber structure on top, used as a playroom.

Thank you for your advice.

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What does your BCO advise? (For that floor space you'll need building regs I believe).

What you can usually do is just drop 200mm depth of concrete into the bottom of the trench and then build up with blockwork but doing blocks in a trench is a bit of a pain to get square and level (if you're doing it yourself- trench gets in the way of your diagonal setting out strings). 200mm for 31m of trench 350mm wide is going to take 2.5 cube of concrete- hope you're not planning on doing it by hand, it'll really hurt :)
I would say that if your hardcore is clean then starting off with a wet mix and making sure that your added hardcore is well and truly dunked in to the mix, it will be OK. This based on cutting the finished foundation in half, if you find an old bit of concrete in the mass of new concrete, it would be more or less as strong as its surrounding new concrete. What you don't want are voids where the new concrete has not flowed around/into the hardcore, so leaving unconsolidated areas with holes.
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Thanks to all for your replies. If I lay dry concrete blocks at the bottom of the trench, and pile them up to a certain height, say 3 courses, loose, with no mortar between them but pea shingle on their sides, and then I pour about 300mm deep x 350mm wide of concrete on top of the loose blocks- what is the risk here? The blocks have a relatively large footprint (440x215) and should not sink into the ground, and the concrete above it will not have any voids at all and will be relatively thick and wide. Isn't it the case that the concrete will be heavy enough to stop any rocking of the loose blocks, making it stable? What am I missing?
[QUOTE="hope you're not planning on doing it by hand, it'll really hurt :)[/QUOTE] - I will be renting a Concrete Mixer and then screeding it at the top of my formwork by hand - is that what you meant would be painful? or did you mean mixing it by hand? (If so, no way...) Sorry, as I have never done this before (i.e. work with such volume of concrete) not sure what you meant...
Have you got a surplus of concrete blocks? Only asking because if you're paying for them they aren't a lot cheaper than concrete. If your trench bottom is flat (or malleable clay so the blocks will prod well down) then your scheme could work but why risk it? If you are determined to do it by hand then you could pour say 50mm concrete, puddle in all your halfbricks etc thoroughly (so you don't have any air voids in the foundations) then pour more concrete then repeat but it is going to be very time consuming and labour intensive.

And doing 2 cubic metres with a mixer will be painful- each cube will need about 2 tons of ballast and more cement than you'd imagine. You'll never get the whole trench done in the 2 hour life of the mix so you won't form a contiguous structure (which is the whole point of concrete)- for a timber superstructure it won't be the end of the world but it seems a shame to bodge the job to save at most a few hundred quid.

Have you actually costed out any of your plan (including how long it will take). Pouring those footings with Readymix (and no messing about chucking rocks in the trench) to whatever depth you fancy will take a morning, the only sweating involved will be if you (or the readymix lot) have to barrow from the street.

If you go for shallow fill and block up then (after leaving the footings for a week to go off properly) a decent brickie will then take an hour or so to set out the rectangles and half a day to bring up your 2 courses.
Job jobbed and happy building inspector (assuming you're bothering, which I suspect you're not).
oldbutnotdead, thanks for your time. Yes, I have a surplus of blocks otherwise I would not be considering them. For my precise dimensions, I calculated I would need 6 bulk bags of ballast and 41 cement bags (to make 3 cubic meters of concrete). This is costing me about £450 including mixer hire. Concrete pumping for 3 cubic meters and a pump to pour comes at about £910, but my main worry is that I cannot carefully start/stop as needed to ensure a perfect level at the top of my formwork as I would be under pressure from the chap operating the truck. I thought that by doing it myself I can work better without rushing, but I haven't considered the effect of taking too much time and the concrete setting as I go around filling the trench. Is this a showstopper then (for the manual mix)? Is there no way to bind fresh concrete with not completely set concrete?
No, the only way is to embed bits of rebar in the first bit that stick through your formwork. When the first bit has gone off hard (3days+?). Remove the formwork, wire brush the new concrete face to expose the aggregate, give it a spray of water and then pour next mix. You hope that your second mix will adhere to the rebar and face of the older concrete.
What is going into this hut ?, you seem to be building the foundations for a two storey brick built house with a slate roof. The shed I built in my last house was on a sloping site, about 12" drop along its 12' length. I dug through the top soil into the clay subsoil, 6"?, layed a 6" wide strip foundation. On this I built a single skin brick wall , four bricks high at one end and one at the other. Put the dirt into the big hole. Put a screed on top of the earth and just overlapped the brick tops. I had a 1000kg milling machine on this along with a 5 cwt lathe and loads of metal.
Your foundations are meant to become part of a stable earth layer, so tossing blocks into a trench will not do it, unless you put down a layer of bedding mortar first. So are you down to a stable layer of soil? , even colour, even texture when whacking it with a hammer equal load carrying ability?
Is the locality flat (its not going to slide downhill?).
If the answer to all of these questions are yes then I would use the mixer to make up some mortar, shovel in to the trench smooth it off with a spade, drop your blocks on, and thump them down with a big hammer. Repeat next course overlapping the first, so you are crudely building a wall in the bottom of the trench. Don't get to neurotic about the squareness, because when you have come up to just below ground level, you can then move to concrete and your formwork. Because you trench is so wide and deep you will have to back fill and thump down earth on the inside of the blockwork if your cement has not filled the cavity.
It is very unlikely that the top surface will be flat enough to carry floor joists without some more fiddling around with mortar and a DPC.
How much of that readymix cost is the pump? Your costs for the hardcore & cement look similar to up here, readymix barrowed from the road to the job by the driver comes in at about £160 per cube inc VAT for 3 cubic metres (that was this time last year, price per cube gets lower the more you have). If it is a long way (>50m) then you'll need 2 or 3 barrows to get it done in time but that can be either supplied by concrete lot or cost you a few slabs of beer for your mates to come and get stuck in. Either way, the flow rate is not as relentless as with a pump so you'll have time to tamp down and float to the top of your formwork- you've got 31 linear metres to do so 4 minute per metre gets the job done within the 2 hours.

And I like Franks' foundation scheme, it'll also give you somewhere to get rid of some of the spoil from the trench. He's probably right about the level as well, but you can sort that with some slate shims (though a course of blocks with DPC on top will be more elegant, not to mention conventional)
Guys - thanks for your help. To answer Frank's comment about the structure, I will be building an all-timber-frame building, 120x50mm floor joists doubled up at the perimeter, 100x50mm for the wall frame, 195x50 roof joists, felt on warm roof + ply/osb as decking/sheathing + plasterboard inside and solid insulation throughout. I guess I went a little overboard with the trench, but I wanted to make sure my foundations were deep to avoid any soft soil shifting. The building will be for table tennis so there will be some movement inside. The ground is now levelled and the soil I reached is pale brown in colour with some chalk - it feels very solid. The cost of the pump is 350+VAT so a big chunk of the total 900-1000 cost of buying readymix it is.

So...if I understood correctly...if I was to use my hardcore/blocks rather than go full concrete fill, I should put some mortar at the bottom of the trench (on the soil), then properly and fully embedded hardcore/blocks, then mortar again, and so on, until I am ready to begin with concrete all the way up to the top of my formwork above ground? What mortar mix would work for such crude sandwiching? I know it may be more elegant to finish the concrete at ground level (or thereabouts) and then build a course of blockwork, but I cannot see how my being inexperienced with blockwork would yield me a better level result than using formwork to go above ground with concrete. I intend to put a DPC on the concrete before my floor joists. I thought that since the width of my form is only 350mm, then with the formwork well levelled it would be relatively easy to ensure a proper level at the top of the form by removing excess concrete using a piece of timber - why are you suggesting it will be difficult? Because of the coarseness of the ballast in the concrete? It doesn't need to be smooth, or perfectly flat - just perfectly levelled.

Frank - I am confused about "Because you trench is so wide and deep you will have to back fill and thump down earth on the inside of the blockwork if your cement has not filled the cavity." In my scheme there is no blockwork, only concrete - what did you mean? Cheers
Your trench is 300mm wide the blocks are 215mm wide, so when you pour your cement, you have to make sure that it gets right down to the sides of the blocks, which is a narrow, 85/2 ~ 40 mm wide. Not to easy to do, or at least very slow so as not to get a lot of soil mixed in with concrete.
Makes sense. Thanks for the clarification, Frank. What mortar mix do you recommended for such crude bottom-of-trench blockwork?
I would use 1:4 cement/sharp sand, its very strong, other would say 1:5 soft sand with plasticiser, fairly soft, the plasticiser makes it easy to level the blocks.
I think you've been advised to do a nonsense foundation that is bound to fail.

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