Confused about mortar mixing by weight or volume...

[Army]

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

[Pilgarlick]

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

[John Schmitt]

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

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

[Rick]

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

[dg]

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

[Chris Bacon]

I agree with this post.