Anyone created soil cement "concrete"?

Mercenary Keyboard Warrior · 22 Mar 2022 at 12:59

Ive seen a few vids from the US where people actually use soil to make a concrete floor.
Now its not as good as using pure sand etc but I just wondered if anyone had used this technique?
(I get its kind of equivalent to a cob type thing)

Ive been sort of planning to make some raised beds out of concrete at the allotment, but the issue is going to be its a pain to move all the aggregate. With proper cement they would be roughly 2 inches think, I would probably go to 3 inches for this method.

Which got me wondering. I have a surplus of "soil" up there. *

Actually making raised beds out of soil and cement would not only save me a load of £, but it would also help me get over my excess issue. Plus the massive amount of transport to shift large bags of sand/shingle up there.

* Now the actual bed"ground" is a very very hard compacted sand/flint mix. There is a gravel and sand pit entrance around 1/2 mile from my house and I am assuming its the same bed they are digging and sorting.
I am pretty convinced this would be fine for mixing with cement. Its got no organic matter in it that I can see. Its untouched from I suspect the ice age. It used to be farmland, but the farmer removed the top foot before the area was built upon, and its clearly not been ploughed. (Its broken rotivators when people initially tried to dig it. Many resorted to buying top soil and raising beds as it was basically back breaking work to pick and dig by hand.)

So my assumption is this would be somewhere between the soil that people use and properly sorted aggregated mix sold for the purpose.

Gammawolf · 23 Mar 2022 at 14:35

I'm no builder but I guess the only thing to consider is your ratios, as long you can get close to a standard concrete mix in terms of sand/aggregates (flint) then it should be fine for relatively none load bearing structures such as raised beds (with appropriate use of rebar). Hard to tell without seeing quality of the material

Mercenary Keyboard Warrior · 23 Mar 2022 at 16:11

Gammawolf said:
I'm no builder but I guess the only thing to consider is your ratios, as long you can get close to a standard concrete mix in terms of sand/aggregates (flint) then it should be fine for relatively none load bearing structures such as raised beds (with appropriate use of rebar). Hard to tell without seeing quality of the material

Cheers

I think it makes sense. From what I have seen people use around 1:4 which is fairly normal. They are using stuff with organic matter in it, which reduces its effectiveness since it will still decay over time.
My base looks like its mainly sand, in fact i tested a bit at lunchtime and its like sand pit sand. I may be able to bag it and flog it

Im going to dig a better hole at the weekend, take a clean sample out since the top soil keeps falling in right now so it gets a bit contaminated.
Im going to "dissolve" that into some water and try to kind of stratify it, but also to check its not got organic matter in it. If its got some organic I would be tempted to up the mix a bit more, maybe to 3:1

Looking at floors people do its not much different to normal concrete to be honest, but i think it softens over time as the organic matter breaks down and the cement is in effect not adding to the strength.
Of course if it weakens over time I can soon pound it back to fineish matter and re do the exercise.

I will probably grab a bag of cement when i got o wickes and give it a go

Gammawolf · 23 Mar 2022 at 16:36

For the small projects i've done, i tend to default to the 1:2:3 ratio of cement:sand:stone, so as long as your soil is around 2:3 sand vs flint then that 1:4 should be on par for a normal concrete mix (ignoring organics as i really can't picture how much you have). Too much sand will make it weaker and more like a mortar mix, too much flint and it'll not bind properly. If you document, i'd be curious to see the outcome of any tests

Mercenary Keyboard Warrior · 23 Mar 2022 at 20:31

Oh no its nothing like that content on flint
I would guess its much more like 5% flint and that varies in size from small pieces to housebrick size pieces. Well housebrick is the largest I have found so far.

I'm not that far from grimes graves https://www.english-heritage.org.uk/visit/places/grimes-graves-prehistoric-flint-mine/

Im kind of working on the assumption for a sand cement only mix and if the odd smaller piece of flint creeps in then so be it

a1ex2001 · 24 Mar 2022 at 07:16

Check your allotment rules an awful lot ban building permanent structures and the use of concrete.

Mercenary Keyboard Warrior · 24 Mar 2022 at 11:41

a1ex2001 said:
Check your allotment rules an awful lot ban building permanent structures and the use of concrete.

Thats a good point actually its a long time since I read the rules.
I know people have used concrete and our rules on buildings etc have relaxed a lot over the years so I dont really envision any issue, but its worth a look, thanks!

TBH for this year I would probably only make up one raised bed anyway, which I would also stain brown to match my others. More than anything I would want to see its overwinter survival before I went all in producing a lot.
Would also allow me to play with the design a bit over winter and make a few more moulds at the same time.