ballast to concrete ratio please?

[stevie888]

I was taught that the correct mix is 1 cement, 2 sand, 3 (20mm) stone. If you use ready mixed ballast then for best you need to mix large quantities. If wish a weaker mix (i.e. a little cement then an extra 1/2 measure of both sand and stone but NO weaker)
As for the right amount of water I go by sound - I aim to hear a 'flumf' sound. Start with half a measure of water in the mixer before adding the dry materials, add a little more as the mix progresses to ensure the mix is workable.

Your teacher was a fool or incompetant.

"Correct" for what? Concrete comes in myriad forms for myriad applications.

We can vary the strength, how quickly it gains that strength. how easy it is to place, its density. We vary the mix to suit the size of the element we're casting. A mass concrete dam will have different size aggregate to a lintel.

What we need always to do is use a mix appropriate to the job and use a bit of care.

 

[Diver Fred]

Your teacher was a fool or incompetant.

He most definitely wasn't either. Not one house he laid footings for has fallen down; not one concrete path he laid has failed. And that is from 55 or so of being in the building trade.

Would you like me to say about my mixing ratio's above 'for general building purposes'?

 

[muckington]

I was taught that the correct mix is 1 cement, 2 sand, 3 (20mm) stone. If you use ready mixed ballast then for best you need to mix large quantities. If wish a weaker mix (i.e. a little cement then an extra 1/2 measure of both sand and stone but NO weaker)
As for the right amount of water I go by sound - I aim to hear a 'flumf' sound. Start with half a measure of water in the mixer before adding the dry materials, add a little more as the mix progresses to ensure the mix is workable.

I do not know what you mean by a fluff sound. But I was putting 10 shovels of bales in the barrow and two shovels sprinkeled over the top. Then I would turn over with the shovel till mixed. Then I would make a well in the middle and add about a pint and a half, then drag the outers into the inner and mix (same as making bread dow)

 

[stevie888]

He most definitely wasn't either. Not one house he laid footings for has fallen down; not one concrete path he laid has failed. And that is from 55 or so of being in the building trade.

Would you like me to say about my mixing ratio's above 'for general building purposes'?

I'd have to stick to fool even if with good intentions because he's used much more cement than he need have over the years. He could have used half the cement and those houses would still be standing: founds need very little strength.

Even with groundbeams when we are designing elements to carry quantifiable loads it generally makes negligable difference to the amount of steel if you use 25N or 40N concrete.

Usually the extra strength is of no consequence except to budget but ocassionally there are consequences. One such is road construction using lean concrete (CBGM in modern construction speak). We're talking about a semi dry mix as weak as 20:1 carried in tippers and spread like aggregate. Historically there are many miles of such roads with transverse cracks every 10m or so which then reflect up through subsequent layers.

I spent a fair chunk of my early career dealing with them and they are due to the concrete being to strong toO flex as intended.

I do like your flumf though: shows you're not using excess water. Well done.

 

[stuart45]

A 1/3/6 OPC/sand/gravel used to be a standard mix for a lot of residential foundations. 1/2/3 is really strong. 1/2/4 was normal for insitu lintels.

 

[muckington]

Well i have now run out of ballast!
I was told that an 800kg bag would do me 2X45 gallon/205 litres drums and it has only done 1 and 2/3rd of the drums. I am still 1/3 of a drum short!