Estimating concrete - the results (long)

[David WE Roberts]

Well, the slab is laid (75% done Saturday and the last 25% on Sunday).

Hired a Belle Premier 100XT diesel mixer.

6 tons of loose ballast, 40 bags of cement (4 bags spare).

After measuring a 5:1 mix using buckets and estimating shovel loads we settled on 22 shovels of ballast and one 25kg bag of cement per load. This worked out at six mixer loads per ton (nominally a 110 litre mixer but this works out at 167 litres; we could have pushed it and mixed a little more but it was easiest working at one bag of cement per mix). Barrow rated at 88 litres and two almost full loads so we were somewhere near. However I can't help thinking that we were nearer 140 - 150 litres and the ballast was slightly under six tons. Whatever.

The six tons of loose ballast used 36 bags of cement so (all things being equal) we were working at the suggested six bags per ton.

There wasn't enough ballast.

Went to B&Q Warehouse on Sunday with the trailer and got a 'ton' bag of ballast and another couple of bags of cement.

With this extra we just scraped home - ended up about a pint short and scraped all the residue out of the mixer :-) We didn't get six full mixes out of the bag so reckon there was less than a ton in there - but nobody knew more than that the bag was rated at 1 ton. Getting anything out of the builders yard end of B&Q warehouse is a farce. It took three people over half an hour to organise a fork lift and get the bag onto the trailer. Five minute job at Jewson. However, B&Q is open on Sundays.

So:

assuming my estimate of volume (3.5 cubic metres) was approximately right the quarry companies who work on 1.5 tons per cubic metre are under estimating. 2 tons per cubic metre would be closer to the mark.

The contenders were (checking back)

Ballast comapnies - 5.25 tons Paving Expert - 6.65 tons or 6.3 tons using McCormack's C20 estimator 'practicaldiy' - 7.77 tons

Estimated actual about 6.9 tons. Could be less if the 'six tons' loose was slightly under and the B&Q bag was slightly under. In which case Paving Expert was about spot on.

seems to be pretty good - certainly within 5% - I am assuming that my slight overshoot was due to the 8"/200mm area round the edge being slightly larger than estimated in parts.

Thanks to all for the help.

Pictures to be updated on photo bucket shortly.

Slab is now covered in hessian (to give it a string vest) and a layer of plastic.

Saturday was glorious and sunny. Sunday was damp and miserable. Monday is cold, temperature is below 2C, and it is snowing a little.

I am still not at all sure how much I have laid; thinking back one sure way to know would be to fill the site up with water (inside the damp proof liner) measuring the water on the way in then pumping it out again. Life is, however, too short.

Now to start planning the construction of the shed, and waiting for the weather to warm up enough for block laying.

Cheers

Dave R

Oh, and the valve for the tyre on the builder's barrow packed up over night Saturday. Fortunately I had a second barrow (not quite the same shape as a builder's barrow but it was adequate).

[JimK]

nice one! NB if you had had enough ballast would you have finished in 1 day? how many pairs of hands all told? how do you feel?!

Seriously I hope you are well pleased, not least because you *know* it's done right with proper materials, no unnecessary rushing, no unavoidable bodges, or typical hassles with readymix or truckmix etc..

Cheers JimK

[Huge]

You were lucky. The B&Q in Wellingborough refused to get their fork lift out a couple of weeks ago on the grounds that Elfin Safety forbids it's use during opening hours!

[Huge]

The solid tyres sold by

(and, coincidentally, by B&Q) are very good.

[David WE Roberts]

We didn't run out of ballast until half way through Sunday.

There were two of us full time (myself & lady wife) and two more for about three hours (friend and his partner). Two people sharing the mixing and barrowing did speed things up. We had decided not to push for doing the whole slab in one day, which was wise. We were all pretty tired at the end of the first day.

It was obvious that trying to barrow a load of ready mixed stuff in under an hour would have more or less killed me, even with another barrow runner and without the added complication of trying to get it all spread and levelled before the mixer went off site.

3.5 cubic metres is doable in one (summer) day if you plan to work from about 09:00 through to 19:00. We had to stop and clear up when it started getting dark about 16:30.

Tired, but not as tired as after Saturday. Sunday was only a quarter of the slab, with a lot of pratting about at B&Q.

After the rain overnight I know that I have a couple of low spots - only a few millimetres but enough to form a very shallow puddle. So no playing billiards on the concrete base :-(

The good news is we managed to get the slab laid in what looks like the only suitable two day slot this month. By Thursday we expect heavy snow.

Cheers

Dave R

[Phil L]

Did you put in any mesh or did you not bother with it?

I wouldn't worry about a few low spots here and there - these are unavoidable even for a professional, which is why god invented tiles and/or carpets.

If it's smooth, you can paint it more easily when the time comes, and even now it should be 'trowelable' if you still want to give it another blast over should you need to, but this is personal choice and for a shed floor I wouldn't worry too much - a bit of 'roughness' isn't a bad thing WRT grip, especially for older people (!).

Don't worry about the weather - as long as it stays damp, the concrete will set harder and with fewer cracks, and frost will do nothing unless it's a deep heavy frost and even then it would only cause a light spalling on the face, easily repaired in warmer weather with a strong mix of sand/cement.

If you want your floor to remain 'pristine', you'll have to put something down prior to building the walls. I suggest a sheet of membrane about 3ft wide, and place it under the DPC and build on it, then when you've finished, slice it off with a sharp blade - the remainder that stays in acts as a double DPC What have you chosen for the walls? - there's a lightweight block which is fairly cheap and easy to use - we built a garage from them about 8 years ago, they are made with what looks like small balls stuck together, but the balls are air filled, it's like building with aero bars!! - but they have no weatherproofing qualities so they'll need to be rendered externally, which is easily offset by the price, I think they were about 30% cheaper than cocnrete blocks and about a third of the weight.

[Phil L]

Search google UK for pumice blocks

[David WE Roberts]

We put down one layer of mesh, and incorporated some old (but clean) concrete fence posts with built in reinforcing into the deep bits.

Had a look at some - they are like rice crispies. Lightness is a plus, having to render is not. We were planning not to render, just to paint (cost is less important than my back on this one) which led us to concrete blocks. Heavy, not especially good thermal insulation, but solid.

If you know of a structurally strong lightweight block that is also weatherproof then please let me know :-)

I assume the concrete blocks are a bit hard to drill into, as well, for fixing insulation, shelves etc.

Chosing the blocks is the next stage.

Cheers

Dave R

[John Rumm]

How did you handle the transition from the first days pour to the second?

[Tim Lamb]

In message , David WE Roberts writes

Big bags of building sand from my merchant are 850kg. Don't know about bagged gravel.

Now you are in practice, I have 12 cu/m to lay in the next week or so...

regards

[Steve Walker]

I don't know about B&Q, but Wickes do similar bags and quote them as minumum 850kg.

SteveW

[David WE Roberts]

As shown in the photos we put some wood across to form a vertical barrier, and had the reinforcing under it. The concrete was still 'green' the following morning and so seemed fine to join to. Only time will tell.

I would have been more cautious in the summer where the concrete could have gone off quite a lot more but an overnight pause at this time of year seemed O.K.

There is an inherent problem in stopping a pour of reinforced concrete - you want the sheets of reinforcing to overlap and you can't do that and get a clean vertical joint.

Professionals have special shuttering with holes in (mmm......) and put reinforcing bars through the holes to bind the slabs together.

Couldn't face the added complication and so decided to leave a sloping joint with the top 50mm vertical from the wood acrosss the slab. The top bit of wood enabled us to level up to it.

Cheers

Dave R

[David WE Roberts]

Jewson also quote a minimum of about 850kg - which means you could get anything from there upwards but most likely near the minimum. It would be interesting some time to get a few bags and dry them then put them on a weighbridge. No doubt the results would be much like drying a pound of bacon - some suppliers would have quite a bit of water in there.

The B&Q bag was a lot drier than the loose ballast - we could tell when we had to add more water to each mix.

[David WE Roberts]

£100 per cubic metre sound about right (plus materials) ?

[Tim Lamb]

In message , David WE Roberts writes

Fortunately I can either back the readimix wagon up to the site or move mixed concrete in the farm grain bucket. Tamping may be an issue as the building is 6.5m wide and 13.5 long. Two drops clearly but splitting it into three may make the job easier.

Major concession on my last delivery as they waived the *empty haul* charge.

regards

[The Medway Handyman]

Couple of years ago I did a job in a local factory owned by the Tarmac Group. They packaged sand, ballast, type one, gravel, slate chippings etc into 25kg sacks & 850k big bags.

They had stocks of bags printed up for Wickes, B&Q, Homebase, Travis Perkins, Jewson, Buildbase, Selco etc. Everything came from the same great big heap :-)

[Rick Hughes]

In case others are interested there are full calculators on line to do the quantities for you (pavingexpert.com)

[David WE Roberts]

If you read back a little we are discussing how accurate these estimators are.