Foundations and Slump

[bobloes]

Now that I have got my walls set out I just need a bit advice/general thoughts on the foundations. Two points really

1. How do you know when you have dug to a suitably stable piece of ground? I know - how long is a piece of string. Any thoughts would be useful.

2. Surprisingly, considering my first question I have built a couple of walls before and considering the way I did them I am surprised they are still standing. I thought in both cases that I probably had the concrete too 'solid'. I am told that a 50% slump would be ideal. Given that I will be using a 3:1 mix how do I determine how much water to add other than a suck it and see approach?

On a separate subject I previously asked a question about hardcore as the first layer of the footings and was shot down about using hardcore. I have just re-read the DIY book and it specifically says to put hardcore down first. I know that you guys are very good so can you help me out of this conundrum?

As ever thanks.

 

[garyo]

If you mean foundations for building control, then at least 1 mete, and they have a handy table on page 35 which helps you identify the various ground conditions on the basis of things like 'you can easily push your finger in 10mm'

http://www.planningportal.gov.uk/uploads/br/BR_PDF_AD_A_2004.pdf

Gary

 

[jeds]

If you mixed concrete in proportions to create the strongest possible mix (ignoring other factors such as curing conditions etc) the concrete would actually be too stiff to work. So, as a matter of practicality, concrete is always mixed too wet. The aim therefore is to make the concrete just wet enough to work, and no wetter. There are additives that will aid workability but that shouldn't be necessary in a simple foundation.

Trench bottom depends on what sort of soil you are on? Generally minimum 900mm to a metre deep. If it's clay it should be quite difficult to drive a 50x50 square stake in.

 

[noseall]

.......and don't mix concrete 3:1.

It should be 6 parts ballast to 1 cement + water.

Or breaking the ballast down to its constituent parts; 4 parts gravel, 2 parts sharp sand to 1 cement + water.

 

[Chukka63]

i was alwasy told that 4-2-1 equates to 5-1 when using pre mix ballast.

A foundation isnt there to strengthen the ground,its go give you a solid and stable "foundation"

 

[r896neo]

Don't put hardcore into the base of foundations. Imagine what you are doing?

Foundations as said are to give a solid stable base for walls which will not settle. You dig out the ground to a solid base then pour concrete which forms a single piece which then does not sink relative to other parts of itself.

Hardcore, not matter how well compacted will always settle to some degree which will leave voids at best or large cracks at worst under your foundation.

In very slimy clay there is a theory of adding a scattering of clean stone in order to act as a grip to stop the concrete sliding as one piece but the situation where that happens i would imagine is pretty rare.

 

[bobloes]

I knew you guys would come up with answers which give the meaning behind the data. I feel more confident now that I will not be wasting my time with the extra digging. Re-read the book re the mix - agreed. I am sure I will be back. Cheers all

 

[stuart45]

i was alwasy told that 4-2-1 equates to 5-1 when using pre mix ballast.

A foundation isnt there to strengthen the ground,its go give you a solid and stable "foundation"

We were taught at Tech that 1/3/6 cement/sand/gravel was the same as 6/1 all in ballast/cement as roughly half the amount of sand to gravel will fill all the voids in the gravel, depending on the stone sizes.
I've never sieved out the sand to test the theory, but when you look at No fines concrete you can see a lot of voids.