6:1

[Stuart]

Is there any other explanation other than I have been done?

Took delivery of 1 tonne aggregate to mix into concrete, as per normal I shovelled it into the mixer at the ratio of 6 shovels of aggregate to one 1 of cement. Curiously I only used 5 bags (125kg) of cement. Now this only means two things I had done a 1:8 mix or i had only received ¾ tonne of aggregate. So on the second 1 tonne delivery of aggregate I measured it into the mixer By the bucketful (has to be as accurate as reasonably practical) Same results, just over 5bags cement done the whole tonne. The supplier of the aggregate assures me his tonnes are accurate, So before I take the matter further and embarres myself, is there any other reason why I am getting the above results?...

[Colin Wilson]

weight of the aggregate vs the weight of the cement maybe ?

[Peter Taylor]

Stuart wrote

Stuart - by counting shovelfuls you are actually gauging the mix 1:6 by volume, not by weight. It doesn't follow that the weight of the cement and the aggregate would also be 1:6 unless the densities of the two materials are identical. Gauging by volume is the correct way to do it.

Peter

[Set Square]

A Google search on "specific gravity"+cement comes up with a number of sites holding tables of density or SG for various materials - including cement powder and aggregates. I'm not referencing any of them because they don't all agree with each other - but some do seem to indicate that the SG of cement is anything up to 1.5 times that of sand/gravel aggregate.

If we use a figure of 1.5, a mixture of 6:1 by volume would actually be 9:1 by weight.

[Set Square]

Correction: If we use a figure of 1.5, a mixture of 6:1 by volume would actually be 4:1 (not 9:1 as previously posted) by weight - because cement is *denser* than aggregate.

[wanderer]

I don't *really* think that comes into the calculation when it's one shovel of cement to five of aggregate.....

[Ian Stirling]

I suspect that the density might come into how high you tend to pile the shovel.

[Andy Wade]

I'm sure those are wrong. I got quite confused by this a few years back, when I saw similar information in a book. So I did some test weighings. The SG of cement (OPC) straight out of the bag is about 1.4, sharp sand is about 1.7 and 20mm gravel weighed-in at about 1.6. I wasn't using all-in ballast, so can't quote a mesured figure for that, but it will be denser than the stone or sand, as the sand fills in the voids between the stone chips. I'd guess an SG in the 2 - 2.2 region.

So, for the OP, a tonne of ballast is going to be about 450 - 500 litres. At 1:6 he'd need 75 - 80 litres of cement, i.e. something like 110 kg. It sounds to me as if his gauging was pretty accurate.

[Set Square]

You're probably right!

I guess that the only undisputed fact in all this is that if you're mixing substances of different densities, the volume ratio will be different from the weight ratio.

[Stuart]

And it sounds like the bloke that I buy my aggregate off is indeed supplying me with the correct weights......I just incorrectly assumed that if you bought 6 tun of aggregate you would need a tun of cement, where in fact you would only need about ¾of a tun

thanks all

[Harry Bloomfield]

It happens that Stuart formulated :

You are measuring the aggregate by volume as you put it into the mixer, your supplier measured it by weight.

[John Schmitt]

In Civil Engineering, specific gravity refers to the density of the particles, and bulk density to the "bucket" density. Cement and aggregate are about the same at 1.6 relative to water. Wet and dry concrete sit around 2 bulk density.

what a pain the experiment is to determine SG. In almost all cases it is safe to assume SG ~2.65 for gravels, clays and sands.

The original poster can compare the volume of the gauging bucket times the number of bucketsful times 1.6 to see if he was given significant short measure.

John Schmitt