C30 Concrete

[hotrod]

i wouldnt worry about being too precise .when the cube guys come out to site they get there concrete then we add loads of water so its all bulls**t anyway.

You will be taking test cubes anyway Chuks and getting them tested at 7,14,21 and 28 to ensure the concrete meets required spec anyway won't you?

 

[MarkSmith]

All of which is fab and everything, but to quote the man....

so whats the correct self mix quantites ???

As safe and over-engineered as it probably is, I'd still like to get as close to the specified mix as I reasonably can.

 

[Chukka63]

i wouldnt worry about being too precise .when the cube guys come out to site they get there concrete then we add loads of water so its all bulls**t anyway.

You will be taking test cubes anyway Chuks and getting them tested at 7,14,21 and 28 to ensure the concrete meets required spec anyway won't you?

ive been on top notch jobs and the most cubes ive ever been asked to make were for 7 and 28 day tests.

 

[Chukka63]

i was on a job once overseen by the BCO and the piling guys had left tons of cement on site so the main contractor ordered weaker concrete than spec'd ,the lorries pulled up to the cement and X amounts of bags of cement poured into the trucks,mixed and poured.The building covered a third of an acre and its still standing.

i was an observor not a partaker.

 

[IJWS15]

You can mix it yourself but as you can see from the responses most on here have a healthy disregard for making sure thier concrete is to spec.

Adding too much water weakens the mix.

Our clients (large utilities) do not normally permit site mix because of the variables and require cubes and slump tests.

If you want to hand mix best bet is to use a C40 mix and even if you are a bit under the strength for C40 it will easily meet C30.

 

[Chukka63]

You can mix it yourself but as you can see from the responses most on here have a healthy disregard for making sure thier concrete is to spec.

Adding too much water weakens the mix.

Our clients (large utilities) do not normally permit site mix because of the variables and require cubes and slump tests.

If you want to hand mix best bet is to use a C40 mix and even if you are a bit under the strength for C40 it will easily meet C30.

i disagree we disregard any conc spec.we're just saying it as it is in real life.

no one has mentioned "too much water".
when concrete arrives on site at the wrong slump that was ordered then adding "some" water isnt too much water.

no one has actually answered this thread with an accurate hand mix solution,probably because hand mixed concrete is inaccurate anyway.

if you watch 50 people hand mix concrete/mortar they will all end up with a different mix , due to different sized heaps on the shovel.9 times out of 10 the cement heap on the shovel is half the size of the sand/stone.
shovel heap sizes also vary with the sand/cement.

i have been on sites where the labourers have had to use buckets to gauge the mix so all mixes are technically the same.but thats rare.

 

[MarkSmith]

i have been on sites where the labourers have had to use buckets to gauge the mix so all mixes are technically the same.but thats rare.

That's exactly what I was planning to do, hence trying to work out the proportions by volume.

If you want to hand mix best bet is to use a C40 mix and even if you are a bit under the strength for C40 it will easily meet C30.

Thanks!

 

[hotrod]

i was on a job once overseen by the BCO and the piling guys had left tons of cement on site so the main contractor ordered weaker concrete than spec'd ,the lorries pulled up to the cement and X amounts of bags of cement poured into the trucks,mixed and poured.The building covered a third of an acre and its still standing.

i was an observor not a partaker.

If that is genuinely the case, and not just site bravado/hearsay, the site agent wants shooting.

 

[Chukka63]

was in 1983 and i think/hope things have been tightened up since then.

but then again,the engineer had designed the reinforcement mesh/fabric in such a way the holding down bolts couldnt be positioned where they needed to be.out comes the disc cutter, out comes a small area of steel and hey presto,the bolts now fitted.

so its not just the on site guys that change things and get things wrong.

 

[MarkSmith]

OK, I now have half a ton of ballast, sand and cement in my garage - and I think I may have ordered the wrong sand. I have building sand, and the recipes for concrete on the back of the cement packets say sharp sand.

How important is this?

 

[Chukka63]

easier to use sharp sand.
could have got ballast,where the stone and sand is already mixed.

 

[MarkSmith]

I've got ballast as well. Pieced together all the info I could find, and decided that my best bet for "not worse than C30" was for a 5:1 mix, or more accurately 3 ballast : 2 sand : 1 cement. So I have all that, but building sand instead of sharp sand.

How do you mean it will be easier?

Thanks!

 

[MarkSmith]

Looking back on this, I think there is confusion about what "aggregate" is. I think some sources refer to aggregate as being stone, and some people refer to it as being stones+sand. ("Ballast" seems to mean, unambiguously, stones+sand.)

What I have is 20 bags of ballast, 10 bags of sand, and 10 bags of cement. (Bags of cement are same weight but smaller in size, so this is right for a 3:2:1 mix by volume.)

If someone could possibly confirm whether I want ballast+sand+cement or just ballast+cement, that would be great. Also whether building sand is suitable, or whether I need to change for sharp sand.

Thanks,
M.

 

[hotrod]

Hi mark, right I'll try and clarify things you. The problem is that the terminology used sometimes confuses matters and differs geographically.

1. "Building sand" aka "soft sand" is no good for concrete - normally used for mortar.

2. "Sharp sand" aka "grit sand" aka "concreting sand" is what you use along with "aggregate" (normally 10mm or 20mm limestone chippings and OPC) for concrete.

3. Alternatively, you can use an all in ballast to which you just add OPC for concrete.

I prefer to use sharp sand and limestone when mixing concrete by hand as it gives you more control over the mix and it's the way I was always taught.

hth

 

[Deluks]

Aggregate is the stone only*, therefore you shouldn't have ballast, you should just have bagged stone. Adding your ballast to the sand will create a mix with too little aggregate in it. Even if you did have just the stone, you would also have the wrong sand. You need either ballast, or stone and sharp sand.

Go and get more ballast. Keep the sand for building with.

Mixing by volume is good but I've read several articles that suggest that going by weight is the most accurate way to gauge. I don't know how any moisture that may be in the sand will affect that, it's also not very practical, but level buckets will do fine.


*Technically the sand is also aggregate, and in concrete does the same job as the stone, it's just there to 'fill the gaps'

*edit* pipped by hotrod, can't believe I spent 6 minutes typing that. Must have been distracted by this sandwich.