Hi,

Been looking at http://www.source4me.co.uk/calculate_concrete_mix.php
and similar for screed.

I notice that the quantities/ratios are mass rather than volume.

This is of course fine if it involves lobbing 25kg bags of cement whole or
half into a mixer. And SBR and water can be assumed to be 1kg/l

But with stuff like sand and aggregate, when I'm going to be shovelling them
out of a 1 ton crane bag, how do you normally gauge the mass?

Would you just assume a standard density and go by the bucket full, given
density may vary according to dampness of materials (mine have been well
rained upon and are thus damp) - or would you mark up a bucket line
according to "10kg" for both, say, using scales to set that up. This seems
a bit pedantic, so I wondered how much it actually mattered...

Cheers

Tim

Tim S wrote:

Been looking at http://www.source4me.co.uk/calculate_concrete_mix.php
and similar for screed.

I notice that the quantities/ratios are mass rather than volume.

If you look at Cormack's site they do it by volume which can make it
simpler:

http://www.pavingexpert.com/mortars.htm

This is of course fine if it involves lobbing 25kg bags of cement whole or
half into a mixer. And SBR and water can be assumed to be 1kg/l

But with stuff like sand and aggregate, when I'm going to be shovelling them
out of a 1 ton crane bag, how do you normally gauge the mass?

Most builders etc will simply do it approximately by volume. X shovels
of sand to 1 of cement. This works well enough if you don't need exact
control over mix or colour. Just try to keep the shovels consistent -
especially when doing the cement since it can be harder to get a nice
rounded shovel full of cement out of a small bag.

Would you just assume a standard density and go by the bucket full, given
density may vary according to dampness of materials (mine have been well
rained upon and are thus damp) - or would you mark up a bucket line
according to "10kg" for both, say, using scales to set that up. This seems
a bit pedantic, so I wondered how much it actually mattered...

The only time I would make an attempt at converting to mass is for
working out approx ordering quantities. If you need a very precise mix
then use a receptacle to gauge the mix - say a bucket or similar.

One area to watch is when converting between concrete mixes that are
specified in gravel, sand, and cement, to use "all in one ballast" and
cement. the volume translations are not always obvious - depending on
what "all in one ballast" is actually like from your local suppliers.

A unit of gravel + a unit of sand is equal in volume to something less
than 2 units, since the sand will occupy spaces not available to the
gravel alone. So, if your ballast is a mix of gravel and sand - you may
need more cement than first thought to allow for the rise in density of
the ballast. If however you ballast is similar to what we get round
here, a mixture of sharp sand and rounded pebble type stones, you can do
a rough translation of say 4:2:1 - 6:1 (although its probably closer
to 5.5:1)

Final comments would be - if doing any quantity at a time (say more than
a cube or so), its probably cheaper and easier to get it mixed for you
(either delivered mixed or mixed on site). If you go for a mix on site,
try and pick a supplier that has a decent accuracy mixer and not one
with a big rotating drum on the back!

--
Cheers,

John.

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Hi John,

Thanks for this - most useful

John Rumm coughed up some electrons that declared:

Tim S wrote:

Been looking at http://www.source4me.co.uk/calculate_concrete_mix.php
and similar for screed.

I notice that the quantities/ratios are mass rather than volume.

If you look at Cormack's site they do it by volume which can make it
simpler:

http://www.pavingexpert.com/mortars.htm

Ah - yes. That's most useful.

I'm reckoning on using a medium C20 (1:2:4) concrete for everything, unless
the BCO calls for something better. He's coming on Wednesday anyway.

This is of course fine if it involves lobbing 25kg bags of cement whole
or half into a mixer. And SBR and water can be assumed to be 1kg/l

But with stuff like sand and aggregate, when I'm going to be shovelling
them out of a 1 ton crane bag, how do you normally gauge the mass?

Most builders etc will simply do it approximately by volume. X shovels
of sand to 1 of cement. This works well enough if you don't need exact
control over mix or colour. Just try to keep the shovels consistent -
especially when doing the cement since it can be harder to get a nice
rounded shovel full of cement out of a small bag.

OK - I will probably do it by the bucket which is fairly practical - don't
trust my shovel-fulls(!)

Would you just assume a standard density and go by the bucket full, given
density may vary according to dampness of materials (mine have been well
rained upon and are thus damp) - or would you mark up a bucket line
according to "10kg" for both, say, using scales to set that up. This
seems a bit pedantic, so I wondered how much it actually mattered...

The only time I would make an attempt at converting to mass is for
working out approx ordering quantities. If you need a very precise mix
then use a receptacle to gauge the mix - say a bucket or similar.

One area to watch is when converting between concrete mixes that are
specified in gravel, sand, and cement, to use "all in one ballast" and
cement. the volume translations are not always obvious - depending on
what "all in one ballast" is actually like from your local suppliers.
A unit of gravel + a unit of sand is equal in volume to something less
than 2 units, since the sand will occupy spaces not available to the
gravel alone. So, if your ballast is a mix of gravel and sand - you may
need more cement than first thought to allow for the rise in density of
the ballast. If however you ballast is similar to what we get round
here, a mixture of sharp sand and rounded pebble type stones, you can do
a rough translation of say 4:2:1 - 6:1 (although its probably closer
to 5.5:1)


Indeed.

Final comments would be - if doing any quantity at a time (say more than
a cube or so), its probably cheaper and easier to get it mixed for you
(either delivered mixed or mixed on site). If you go for a mix on site,
try and pick a supplier that has a decent accuracy mixer and not one
with a big rotating drum on the back!


I've run up a spreadsheet and it seems the biggest single session lay will
be about 0.3 m3 - I have a high volume 2nd hand builder's mixer - should
manage with that.

Tell me more about high accurracy mixers. My mate used Mixamate once for a
driveway slab and they did indeed have a drum mixer.

BTW - as two sets of floor screed call for an SBR screed (25mm screed) and
SBR is 40 quid/5l in the builder's yards, I since discovered:

http://www.permagaurd.com/index.php?...products_id=77

Which is a far more sensible price (I need about 25l including the
cement/SBR slurry primer layer).

Thanks again!

Cheers

Tim

Tim S wrote:
Hi,

Been looking at http://www.source4me.co.uk/calculate_concrete_mix.php
and similar for screed.

I notice that the quantities/ratios are mass rather than volume.

This is of course fine if it involves lobbing 25kg bags of cement whole or
half into a mixer. And SBR and water can be assumed to be 1kg/l

But with stuff like sand and aggregate, when I'm going to be shovelling them
out of a 1 ton crane bag, how do you normally gauge the mass?

Would you just assume a standard density and go by the bucket full, given
density may vary according to dampness of materials (mine have been well
rained upon and are thus damp) - or would you mark up a bucket line
according to "10kg" for both, say, using scales to set that up. This seems
a bit pedantic, so I wondered how much it actually mattered...

Cheers

Tim
X shovels of aggregate to Y shovels of cement.

Its not that critical for most purposes.

3-5 shovels of sandy stuff plus one of cement..

The Natural Philosopher coughed up some electrons that declared:

Tim S wrote:
Hi,

Been looking at http://www.source4me.co.uk/calculate_concrete_mix.php
and similar for screed.

I notice that the quantities/ratios are mass rather than volume.

This is of course fine if it involves lobbing 25kg bags of cement whole
or half into a mixer. And SBR and water can be assumed to be 1kg/l

But with stuff like sand and aggregate, when I'm going to be shovelling
them out of a 1 ton crane bag, how do you normally gauge the mass?

Would you just assume a standard density and go by the bucket full, given
density may vary according to dampness of materials (mine have been well
rained upon and are thus damp) - or would you mark up a bucket line
according to "10kg" for both, say, using scales to set that up. This
seems a bit pedantic, so I wondered how much it actually mattered...

Cheers

Tim
X shovels of aggregate to Y shovels of cement.

Its not that critical for most purposes.

3-5 shovels of sandy stuff plus one of cement..

Ta.

Think I'm ready now... Order to builder's merchant time

Cheers

Tim

Tim S wrote:
The Natural Philosopher coughed up some electrons that declared:

Tim S wrote:
Hi,

Been looking at http://www.source4me.co.uk/calculate_concrete_mix.php
and similar for screed.

I notice that the quantities/ratios are mass rather than volume.

This is of course fine if it involves lobbing 25kg bags of cement whole
or half into a mixer. And SBR and water can be assumed to be 1kg/l

But with stuff like sand and aggregate, when I'm going to be shovelling
them out of a 1 ton crane bag, how do you normally gauge the mass?

Would you just assume a standard density and go by the bucket full, given
density may vary according to dampness of materials (mine have been well
rained upon and are thus damp) - or would you mark up a bucket line
according to "10kg" for both, say, using scales to set that up. This
seems a bit pedantic, so I wondered how much it actually mattered...

Cheers

Tim
X shovels of aggregate to Y shovels of cement.

Its not that critical for most purposes.

3-5 shovels of sandy stuff plus one of cement..

Ta.

Think I'm ready now... Order to builder's merchant time


Unless you are doing big volumes fast, get aggregate in, but buy cement
as needed. Nothing goes nasty faster than a bag of opened cement. Buying
one at a time to put in the car boot is less expensive than throwing
away half a dozen spoilt half bags..


Cheers

Tim

The Natural Philosopher coughed up some electrons that declared:



Unless you are doing big volumes fast, get aggregate in, but buy cement
as needed. Nothing goes nasty faster than a bag of opened cement. Buying
one at a time to put in the car boot is less expensive than throwing
away half a dozen spoilt half bags..


True - that's how I've been buying plaster.

I cheated though recently and hired a skilled labourer for a few weeks to
help me through the heavy work (wise move in retrospect, should have done
that a month ago - the man is gifted at serious hard graft, reasonably
priced and I still control everything).

So I reckon we'll deal with all the concreting and screeding work in one
hit.

Cheers

Tim

On 24 May, 15:55, Tim S wrote:
The Natural Philosopher coughed up some electrons that declared:



Unless you are doing big volumes fast, get aggregate in, but buy cement
as needed. Nothing goes nasty faster than a bag of opened cement. Buying
one at a time to put in the car boot is less expensive than throwing
away half a dozen spoilt half bags..

True - that's how I've been buying plaster.

I cheated though recently and hired a skilled labourer for a few weeks to
help me through the heavy work (wise move in retrospect, should have done
that a month ago - the man is gifted at serious hard graft, reasonably
priced and I still control everything).

So I reckon we'll deal with all the concreting and screeding work in one
hit.

Cheers

Tim

Just to pick up on John R's comment about concrete quantities larger
than a cube, I was in the situation of requiring near enough exactly a
cube for a shed floor..

I went to my next door neighbour who is in the drainage and
landscaping business and asked if he had contacts for getting the tail
end of a concrete lorry load.. " no bother,just tell me when".

I told him "when" a week later, and got about 40 minutes notice of the
delivery. This was just enough time to phone a friend for assistance
and get the dpm down. The lorry parked on the curb and shot the
concrete down its chute through the garden hedge. So a cube of
concrete arrived all at once at one end of the site, with a certain
amount not making over the shuttering and having to be rescued. Then
having got it into place and started to level it we found that it was
about 0.1 cube short; fortunately I had cement and ballast at hand to
make that up, but that wasn't really part of the equation.

So take what warnings you want out of that !!

Rob