Hi all,

I began to lay the first of my foundation blocks for my conservatory
yesterday and am not sure whether I made the morter correctly.

I needed to know the weight of cement which I was adding to the sand
in order to be able to add the correct amount of FebMix+ plasticiser
which is specified at about 2.8 mls per 1kg of cement.

I understand the correct ratio to be 1:4 cement to builders sand so I
measured 4 small buckets of sand and put them in my large mixing
bucket. I then poured 1 small bucket of cement and weighed it. It
came in at 6kg. I then thought, hang on, 6kg of cement means I should
have used 24kg of sand yet I've used less than half the 25kg bag. So,
I weighed what was left of the sand in the bag (SWMBO not pleased with
non-standard use of bathroom scales) and determined that I actually
had 12kg of sand in my big mixing bucket, so I tipped some of my
measure of cement back into it's bag so that I had 3kg of cement and
then added that to my big bucket, mixed the mortar and sucessfully
laid (or so I hope) 9 blocks with it. I'm now a little worried that I
should have mixed by volume and that my morter is never going to set
or will set with little strength.

Help!


Mike

I've always used volume to produce the mix. I've seldom seen professionals
use other than a practised eye so it doesn't seem to be too critical.

I've always measured, as I don't do the work often enough to rely on sight
and if you don't get the mixes of similar proportions you end up with
multi-coloured joints.

Regards

Pilgarlick


"Army" wrote in message
...
Hi all,

I began to lay the first of my foundation blocks for my conservatory
yesterday and am not sure whether I made the morter correctly.

I needed to know the weight of cement which I was adding to the sand
in order to be able to add the correct amount of FebMix+ plasticiser
which is specified at about 2.8 mls per 1kg of cement.

I understand the correct ratio to be 1:4 cement to builders sand so I
measured 4 small buckets of sand and put them in my large mixing
bucket. I then poured 1 small bucket of cement and weighed it. It
came in at 6kg. I then thought, hang on, 6kg of cement means I should
have used 24kg of sand yet I've used less than half the 25kg bag. So,
I weighed what was left of the sand in the bag (SWMBO not pleased with
non-standard use of bathroom scales) and determined that I actually
had 12kg of sand in my big mixing bucket, so I tipped some of my
measure of cement back into it's bag so that I had 3kg of cement and
then added that to my big bucket, mixed the mortar and sucessfully
laid (or so I hope) 9 blocks with it. I'm now a little worried that I
should have mixed by volume and that my morter is never going to set
or will set with little strength.

Help!


Mike

On Mon, 24 Oct 2005 13:25:35 +0100, Army wrote:

I needed to know the weight of cement which I was adding to the sand
in order to be able to add the correct amount of FebMix+ plasticiser
which is specified at about 2.8 mls per 1kg of cement.

I understand the correct ratio to be 1:4 cement to builders sand so I
measured 4 small buckets of sand and put them in my large mixing
bucket. I then poured 1 small bucket of cement and weighed it. It
came in at 6kg. I then thought, hang on, 6kg of cement means I should
have used 24kg of sand yet I've used less than half the 25kg bag.

If the sand was damp, it will have gone bulky. For most jobs in DIY the
strength is ample for the loading imposed. For civil engineering projects,
the concrete strength is rather more critical as the normal design
parameter is a factor of safety of 2:1, i.e. the structure is designed to
take twice the maximum likely load. For a bridge, for example, one would
probably assume nose-to-tail 38 ton trucks in every lane and a 100 mph
wind, notwithstanding that the trucks would not be allowed onto the bridge
in such conditions. On top of this, a concrete mix is specified with a
minimum crushing strength after 28 days, commonly either 30 or 50 N/mm^2,
although over 80 is possible. Typically, 30 N concrete runs about 40N
after 28 days and ultimately makes about 50, jacking the factor of safety
closer to 3. The companies which make bulk concrete use large weighbins to
get an exact mix and small inter-batch variation. Another thing common in
the building trade is to use shovelfuls rather than buckets. Not only does
cement not pile onto a shovel as generously as sand, it is rather quicker
to chuck eight shovels of sand and two of cement into the mixer. In this
case, if you want 4:1 by weight, it appears that you should use a bucket
per bag. However if you are using a 5 litre bucket, you should find that
the weight of cement is more like 7.5-8 kg. Mechanical bathroom scales are
notoriously inaccurate, particularly near the lower end of their range.

John Schmitt

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On Mon, 24 Oct 2005 12:25:35 GMT, Army wrote:

Hi all,

I began to lay the first of my foundation blocks for my conservatory
yesterday and am not sure whether I made the morter correctly.

I needed to know the weight of cement which I was adding to the sand
in order to be able to add the correct amount of FebMix+ plasticiser
which is specified at about 2.8 mls per 1kg of cement.

I understand the correct ratio to be 1:4 cement to builders sand so I
measured 4 small buckets of sand and put them in my large mixing
bucket. I then poured 1 small bucket of cement and weighed it. It
came in at 6kg. I then thought, hang on, 6kg of cement means I should
have used 24kg of sand yet I've used less than half the 25kg bag. So,
I weighed what was left of the sand in the bag (SWMBO not pleased with
non-standard use of bathroom scales) and determined that I actually
had 12kg of sand in my big mixing bucket, so I tipped some of my
measure of cement back into it's bag so that I had 3kg of cement and
then added that to my big bucket, mixed the mortar and sucessfully
laid (or so I hope) 9 blocks with it. I'm now a little worried that I
should have mixed by volume and that my morter is never going to set
or will set with little strength.

Help!


Mike

My brickie trainer taught me to do it by the shovel, 8 rounded shovels
of sand, half a bag of cement = 4:1, or 12 shovels to half a bag of
cement for 6:1. The easiest way to get half a bag of cement is to cut
the full bag in half with a saw.

For the plasciciser, it was one good glug to the bucket of water, I
now use mastercrete cement, which has it added anyway. I can't see it
makes a whole bunch of difference. The biggest way to make the mix
different is to add too much water. If the bricks are wet you need a
dryer mix, but its so much easier to keep the bricks dry.

I have been using
building sand 4:1 for 7N blocks
buliding sand 6:1 for 4N thermalite blocks
washed sand 4:1 for stonework.

Rick

You are wasting your time trying the guage by volume or weight for
domestic work. Firstly it is simply not required, and secondly your
materials will have different weights and volumes and you have no
accurate batching plant.

Just use a shovel as the guage, and make sure that the quantity picked
up is roughly the same

dg

dg wrote:
Just use a shovel as the guage, and make sure that the quantity picked
up is roughly the same

I agree with this post.