Online concrete calculators?? - Gnaaarrrggh!

[David WE Roberts]

Gnaaarrrggh! Confused - you will be. I know I am.

Just talked to my friendly local builder (about finding a labourer to help with barrowing ready mixed concrete) and he asked why I wasn't mixing it myself.

Told him the figures from the online calculator priced up to be more than ready mixed.

He was not convinced (to put it politely, which he didn't). He said (as others here have) that he would mix it himself on site.

On his recommendation I talked to a local concrete firm and asked for a quote for 3.5 cubic metres of a 5:1 mix. They said "That's just under 5 (25 kg) bags of cement per ton - shall I quote you 5 bags?" and "That comes up at just over 5 tons - shall I quote you for 5 tons (tonnes?)?"

O.K.

5 tons loose ballast £147:20 25 bags cement £91.20 (625 kilos)

Total £246.40 including VAT.

Weekend hire for a mixer is about £50.

Grand total of £300.

me I need 7.77 tonnes of ballast

me I need 3990kg of gravel + 2660kg of sand = 6650kg ballast

735 kg cement [By the way, 'practicaldiy' tells me that I need (3.855 tonnes sand + 5.11 tonnes gravel) = 8.965 tonnes OR 7.77 tonnes ballast. Now I can see that if they were calculating by volume then you would need a larger VOLUME of sand and gravel to get the same WEIGHT of ballast (because the sand fits between the gaps in the gravel) but a larger WEIGHT??]

But moving on. The calculators say that I need 7.77 tonnes or 6650 tonnes of ballast (however see rant above) but my local firm says I need 5. At 5:1 this comes out as 38.85 bags cement for the 7.77 tonnes.

The two online calculators don't seem to agree on the weight of ballast but the higher is more than 50% higher than my local firm. If they are correct then the materials (scaling up the price) should cost about £383 which is remarkably close to the 'mix at the door' price (not including barrow running).

So I am now heading towards mixing my own.

My first calculations were based on yet another online calculator which I can't find at the moment but which (working back from the spreadsheet) told me I needed about 5.5 tonnes (equating to 7 * 0.8tonne bags) and 25 * 50kg bags of cement. Looking at these figures again surely that must have been 25

• 25kg bags of cement. (And don't call me Shirley.) Whatever, using bagged prices from Jewsons this came to over £500.

O.K. I know that I didn't want to put loose ballast on my block paved drive but at £75 saving over bagged I can afford to lift and relay a few blocks.

So I am now back to the drawing board.

Today (!) I think I will order 5.5 tonnes of loose ballast to be sure I have enough and get 25 bags of cement because I can easily get more cement from the sheds or merchants and I can hide any extra ballast round the back of the shed.

The clincher is the need to hire someone to barrow the ready mixed. Haven't priced a local labourer yet (last time I checked they were about £80 per day) but mix-at-kerb charge £30 per cubic metre + VAT to do the barrowing which comes out at an extra £123.38. For that price I can hire a mixer, a ball and socket truss and a masseuse then do it all myself.

So - finally - what does the team think about the volumes?

Trust the local firm recommended by my builder (as I would have before t'Internet raised its ugly head) or go with the online calculators?

Cheers

Dave R

[Phil L]

big snip>

If I were mixing it I would order the materials loose (getting bagged for these volumes is futile, as a lot of it will end up on the drive anyway) and I would pay the following:

4t limestone @ £20 per tonne = £80 2t grit sand @ £20 per tonne = £40 40 OPC @ £4 per bag = £160

Total £280

5 tonnes of loose ballast isn't going to be enough, and 5 cement per tonne of ballast isn't strong enough - if you say that its 2 parts stone to 1 part sand, this means that a third of each tonne will be sand IE 333kg - putting 125kg of cement with this isn't strong enough - it would have to be over 150kg to get near to a C20 mix.

but it's all immaterial anyway as you can't weigh the aggregates if you mix it yourself :-p you can guestimate 50kg of sand and 100kg of stone and add a bag of cement to each mix and that's the best you can hope for, but there's an art to mixing concrete - adding the cement at the wrong time will cause almost all of it to stick to the inside of the mixer, resulting in a weak mix, then the next mix may be double strength if the cement from the first mix mixes into it!

I always mix it like this: Water, half the stone, half the sand, more water, all the cement, remainder of stone, then sand....adding water periodically if required

[John Rumm]

Part of the complexity here, is the there are local variations as to what constitutes ballast. That tends to mean that were you to specify a traditional mix of 4:2:1 you could say that by volume that would be around 4.5:1 mixed aggregate to cement, allowing for most of the the sand and cement to vanish in filling the voids between the gaps in the gravel. However when using ballast you may need different proportions depending on what your local ballast is like. In these parts, the ballast tends to be sharp sand and small pebble/shingle style aggregate that does not pack together in quite the same way as your traditional gravel and sand and works out a bit heavier per cube.

I usually estimate based the "standard" ratios, and then cross check the weight on an assumed finished density of around 2400kg/cube

If you use McCormack's C20 estimator, it gives

1120kg cement, 2205kg sand, 4095kg gravel, and 1050kg water. A total mass of 8470kg. To check 3.5 x 2400 = 8400, so allowing for some water evaporation that sounds about right.

Not 8.4 tonnes there, even with another 600kg of water.

Might be a bit much - although not far off its like the stuff we get. I would probably have ordered a tonne less.

I was using

usually seems to work for me when estimating.

Yup, again sounds like too much based on the final target weight.

That I can understand...

That sounds like a bit too much variation though!

Personally I would not dream of doing that much with a mixer...

You will probably find the Jewson price comes down with a bit of suggestion!

I doubt bulk bags will move the paving if it was laid right in the first place. The pressure exerted by a bag will be far less than a car wheel for example.

I think you will need more ballast unless you local stuff is very light per cube...

[David WE Roberts]

Interesting that you use limestone instead of gravel.

What are your estimates for a 'shovel' of each? If I was mixing by weight I would obviously do 2 of limestone to one of sand but I have no idea of the relative densities and therefore the relative volumes.

I am a little confused by your calculations on cement.

25 bags * 25kg of cement is 625 kg in total. Divided by 3.5 is 178kg per ton of ballast which is over your 150kg (tying in with the 'under 5 bags per tonne' from my local firm).

My (somewhat rusty) mixing method is much the same as yours, apart from being used to using ballast instead of stone and sand.

How many tonnes of ballast would you estimate it would take to make 3.5 cubic metres of concrete?

Cheers

Dave R

[David WE Roberts]

There (as they say) lies the rub!

My concundrum is roughly " Do I trust the local firm which knows the local agregate or an online calculator which must generalise?".

I will take cross sightings from other firms etc. but as you have said the online calculators seem a bit high (and don't agree).

As to mixing it all myself I am reasonably sure that I am physically up to it and it is a lot less stressful to contemplate than having to shift 3.5 cubic metres of concrete the length of the garden within 45 minutes.

Having moved over 6.6 cubic metres of MOT "due to a terrible miscalculation of scale" when I was expecting to move under 3 cubic metres I reckon I can move 3.5 cubic metres of concrete.

Indeed, having had the final bag of MOT freeze into something akin to a mega ice cube I think I can more or less tackle anything!

Thanks, as always to all contributors.

Cheers

Dave R

[Phil L]

It's more widely available here than any other type of stone, I assume it's geographical - some places use what looks like beach gravel, some use limestone some use granite

no one does, which is why it goes on weight - no one can say how much free space is in a tonne of gravel / stone, therefore they have no idea how much volume is stone and how much is air, likewise with wet and dry sand, but this is easier to measure by the volume rather than the weight, IE a barrow of sand is a barrow of sand, regardless of the water content.

Yes but you don't measure the cement to ballast ratio, you measure the sand / cement ratio, which should be 2:1 in a C20 mix

I don't know of anyone who buys all-in ballast, that is, sand and stone mixed - it's more expensive and harder to estimate WRT mixing (as you are finding) than purchasing the 2 ingredients seperately, especially for large projects such as this. I suppose it could be useful to buy a tonne bag if you were putting up a concrete fence or patching up a path or something similar, where a failry large amount is needed and buying 25kg bags of everything would be prohibitive

[Tim Lamb]

In message , David WE Roberts writes

Right. So you don't need a labourer other than for tamping:-) After Phil... draw off mixed concrete and park barrow. Load mixer but leave final water. Run barrow to site, empty and return to mixer which should now be ready for the last bit of water... etc.

Personally I think it is a two man job as 3.5 cube is a days work and the stuff you mix in the morning will be too hard to tamp by lunch.

regards

[John Rumm]

All in one is about all you can buy easily here... otherwise the nearest you can get to gravel is Type 1 MoT, but that's a bit coarse for concrete.

[Tim Lamb]

20mm ballast is normal for Herts. I suspect they mix it from sharp sand and separated pebbles. *As dug* used to be available from local pits. Nowadays, ballast delivered from St. Albans is ordered from Leicestershire!

regards

>

[David WE Roberts]

Ballast is the norm around here as well - AFAIK it still comes from local pits. The MOT I have been using looks like ground up concrete with added chunks (yum).

[David WE Roberts]

Sigh. I shall be joining Tim in remedial maths.

I should have worked forward and not backwards (especially not using a factor of 3.5 instead of 5).

150kg cement per ton (working on your assumption that 1/3 of the ballast is sand) comes in at 6 bags per ton - one more bag per ton that suggested by the local firm.

I am assuming that 5:1 is a volume measure of 5 shovels (or other more accurate measure) ballast to one shovel cement. Which I again assume means that (as suggested elsewhere) the weight to provide this volume may differ depending on the density of the ballast. Will investigate further and confirm they were estimating for C20 and not a leaner mix..

[David WE Roberts]

Alternatively, put in the initial water and a few shovels of ballast to give the drum a quick clean and stop any build up of cement on the paddles, and leave running. Carry on mixing after running the barrow.

I shall have two 'light duty' labourers for some of the tamping and final spreading.

I will also be checking to see how much a local labourer will cost for a day's work.

I keep chasing my own tail on this one - if I am going to hire a labourer for a day then it is going to be a lot easier to use him to barrow ready mixed concrete.

The cost of ready mixed + a labourer is much higher than me running as mixer but if I add in a labourer to help with the mixing and barrowing then the cost difference comes down again.

As a backup strategy is there a reasonably safe way to lay this slab in 2 days? I am assuming that I should be looking at laying one full side then the other full side, not filling the whole site half full then putting the top on the following day (some sources recommend having jointing steel between the two slabs). My problem with this strategy is getting half level and tamped without a board of some sort to support the temporary end of the pour - won't the edge of the slab slump? Putting a board in isn't going to be easy with membrane down and reinforcing steel on top. Any tips and tricks?

Cheers

Dave R

[Bruce]

Quite the opposite, actually. MOT Type 1 has far too many fines to make a good concrete. There are insufficient voids for the cement paste and the high proportion of fines means that a very large amount of cement paste would be needed to coat them all. A non-starter.

The grading is closer to that of an unbound macadam.

[David WE Roberts]

The MOT we have just put down and whacked does look quite like tarmac without the tar - almost solid enough to skim and ignore the concrete layer :-)

[dennis]

If its going to take a long time you can get stuff to retard the setting.

[David WE Roberts]

** Calculating for 3.5 cu m concrete **

My dodgy arithmetic makes this 6300 kg of sand/gravel and so presumably equivalent to just over six tons of ballast.

1120kg cement equals 45 bags.

This is close to Phil's "4t limestone @ £20 per tonne = £80

2t grit sand @ £20 per tonne = £40 40 OPC @ £4 per bag = £160 "

Either way it is more than 5 tons + 25 bags.

I then made the mistake of reading the Building Construction Handbook by Chudley & Greeno which tells me that C15 and C20 are suitable for unreinforced plain concrete or reinforced using lightweight aggregate (such as clinker, perlite etc.). For reinforced (and I have been persuaded to add some reinforcement) I should apparently be using grade C25.

Cross purposes, I think.

I am happy with bulk bags craned onto the drive - that is what I did with the MOT. I was less happy with a loose load being tipped onto the drive because the tipper will probably have to put the rear wheels on the drive which might cause damage. The ballast itself is not a problem. However I am now considering the risk of minor damage to save money by having a bulk delivery of loose ballast.

[John Rumm]

Most of our MoT in pinkish, and definitely limestone... occasionally we get the grey stuff, but it still looks like stone rather than concrete lumps.

[Tim Lamb]

In message , David WE Roberts writes

Then you will be standing around while it mixes.

Hiring the dumper truck avoids the 2 day issue. Something else you need to consider is weather. Rain is not good on wet concrete. Hiring a dumper and ordering readimix needs to be last minute.

On the half and half method.... you could attach a couple of temporary supports to your side shuttering, fit your board above the reinforcing and use a some timbers to brace from the unfilled end. If you use a reasonably stiff mix it should not slump under the board too badly and you can always top up a bit.

regards

[David WE Roberts]

Tell me about it :-( The weather is either lows coming in from the west bringing warmth but heavy rain, or highs sitting over the UK and bringing clear skies but freezing winds down from the north and snow and frost to the east coast. I need to catch a slot when the weather is changing over between wet and dry, but before the frost sets in. The ready mix brigade normally need a couple of days notice and the weather forecast isn't that consistent at the moment - they keep changing their minds. One thing I don't fancy is trying to keep cement bags dry whilst hand mixing in the rain.

I think that is feasible. It would be a lot easier to put down the bottom half of the slab and put the top on the next day but that gives a join across the whole surface of the slab instead of one small one across the slab.

On the gripping hand I could go to the Canaries until the weather improves. Hang on - they've just had floods as well.

[Tim Lamb]

In message , David WE Roberts writes

We had two days of steady rain on Madeira last month! Locals said it was the wettest Christmas for years.

regards