Concrete - confusion over quantities

[ianh64]

Hi

First post.

I am building a very low oval retaining walled enclosure (< 500mm height at max, 150mm min height) cut into a slight slope and have excavated the foundations. I have used Landscape Detailing - Enclosures by Michael Littlewood and An Introduction To Landscape Design & Construction by James Blake to verify my construction plans.

I calculated my quantity of concrete for the foundations to be approximately 1.4m3. I intended to use a 4:2:1 and over estimating slightly I intend to order 1.5m3 quantity.

Using the calculator for C20 mix here at pavingexpert.com, it suggested 525Kg cement, 1050Kg sand, 1650 aggregate.

Our local builders merchants supply ballast in 850Kg (min weight) bulk bags, enough according to them for approximately 0.6m3 - approx. Some sources that I have seen said that a jumbo bag was 1 cu.yard, approx 0.7m3. From varous sources, a jumbo bag seems to correspond to between 34 and 40 x 25Kg bags of ballast.

Based on approx 0.6m3 per bag, I need 2 1/2 bags ie 2 jumbo + approx. 20x25Kg bags. Total weight = 2200Kg.

Using the calculator, I would need 1050+1650 = 2700Kg assuming that a bag of ballast is approx 2:1 mix.


Can someone please explain the descrepancy? I would guess its all down to approximations and that maybe a bag of ballast is not approximately a 2:1 agg/sand mix.


For the present, I have ordered 2 bags of ballast due for imminent delivery and already have 20 bags of cement. I intend to make up any shortfall with loose bags bought when needed.

Would it be best to make up the shortfall with more ballast or would it be better to use loose aggregate mixed in where a stronger mix is required - possibly difficult to judge?

[Tony McC]

I would NOT use ballast full stop. Ballast is an unregulated mix of sands and gravels (nad the odd bit of crap that is neither!) - you have absolutely no control over the ratio. You could end up with a big bag of very sandy gravel that would give a weak, practically useless concrete, or you could end up with a big bag of gravelly sand that would give a weak, practically useless concrete.

That's why professionals and concrete supply companies don't use ballast - it's too unpredictable.

[IanO]

Tony

Please would you advise what professionals use if ballast is not suitable? Apart from ReadyMix for large quantities, I have used ballast in the past - and was about to order more.

The more specific the better! If it helps with your reply, I am about to lay a small (2.8m x 1.4m) utility room floor.

BTW - I gather you are not a great fan of Belle gear, but I recently picked up a Minimix 150 with a honda engine for £227 from B&Q. Some on the list might be interested at this price for a petrol mixer?

Thank you for your help.

Ian.

[ABILITY]

Ian.

Tony is stating that ballest is non stanard mix of gravel to sand.
To gain a certain concrete mix you should be adding measured quanities of gravel, grit sand and cement to give correct mix strength. Not possible with pre mixed ballast.

Thats a very good price for Belle petrol mixer are you sure its a Belle! with Honda. Thats the usual price for a electric one! Nothing wrong with Belle make - think you are refering Tonys comments on their ' Wackers ' which are a bit light weight for many jobs.

[IanO]

Ability

Thank you for the explantion. You'll be pleased to know I have now read the material here on concrete mix - but, frankly it is still a bit confusing to the uninitiated. ie. I am not sure how I go about ordering the aggregate. The pages here describe varying sizes, which is self-explanatory, but what do you actually specify to the BM? A jumbo bag of graded aggregate please?

I am very sure about the mixer - it's in my shed. I know the price is good which is why I mentioned it for the good folks on here. I took Tony's advice some time ago about a Wacker and bought a second hand Benford(?).

Actually, I also bought a new Birchwood block splitter which is the dogs biscuits (wife taught me that polite description!).

Thanks again for your help.

Ian.

[Tony McC]

When have I ever criticised Belle mixers? They're a handy enough bit of kit for small quantities, but for anything more than a cubic metre of concrete, that size of mixer is not a tool I'd choose. Belle are a fine brand - my only concern would be the capacity.

Anyway - aggragtes. You go into the BM and tell them you want so many tonnes of such-and-such a size of aggregate, be it 10mm, 14mm or (rarely) 20mm. When the clueless rain-dodger behind the counter has finished bullshitting you and trying to sell you what's most convenient for them, you ask to speak to manager and repeat the order. If s/he is none the wiser (and often, theyre not!) tell them not to bother and you'll call in at a real BM and see if they understand basic English.

It really isn't that difficult!

[IanO]

Tony, I apologise if I mis-represented your views. I had wrongly attributed comments to a Belle block splitter to all Belle gear. Pleased to hear your endorsement as I now have the mixer in my shed!

It is intimidating going into a BM (and I am not a shrinking violet) because none of us likes to make a prat of ourselves by asking a 'dumb' question. Usually, reading up helps, especially on your site here, but some 'techie' details are not always clear - ie. how do you know whether to specify 10mm, 14mm or 20mm?

Finally, I saw some references to your book - has it been published yet?

Thank you again for this excellent resource.

Ian.

[Tony McC]

The size of aggregate is usually determined by tyhe end-strngth and finishing required of the concrete. When a fine finish is needed, a 10mm agg is the one to choose: when it's bedding for kerbs or haunching for edge courses, then a 20mm agg would be fine. For fine moulding, such as fence panels, some manufacturers will use a 6mm agg or even smaller.

For your floor, I reckon a 14 or 20mm would be adequate. 20mm is cheaper than 14mm (not by much, though), but it may be that your nearset BM only has ¾" (18mm), so you could use that.

The book? Fecking publishers! I've no idea what they're playing at. I've heard nowt since before Christmas, but as soon as the kids are back at school and I can get near the phone again, I'll be chasing them up. At this rate, the damned thing will be out-of-date by the time it hits the shops.

[IanO]

Tony, thank you for this explanation. I now have some appreciation of the significance and application of different aggregate sizes. I'll test the BM and ask them what they can supply!

Sorry about the book, I'll keep my eyes peeled for news.

Regards

Ian.

[dig dug dan]

Tony

I am now confused by what you said "do not use ballast"

I use a concrete company who mix as you go, you only pay for what you need. They use ballast
My other readymix company fill their trucks with ballast, from the quarry that also supplies my BM with ballast.
Are they doing it wrong or am i missing the point?

A assumed ballast made concrete, but i bow down to your better judegemtn if you think otherwise

[Tony McC]

I'm not familiar with your suppliers, Dan, but it may be that the ballast they use is a regulated blend of fine and coarse aggregate (sand and gravel in layman's terms) rather than any-pld ratio , as is often the case with ballast.

One problem is that many 'mix-as-you-go' concrete suppliers are not members of QSRMC (Quality Assurance Scheme for Ready Mixed Concrete), and so the concrete they supply may not meet the requirements. If you ask them for, say, a C25, you might actually get a C20 and have no comeback.

It might be worth asking them how they determine the end strength of their products.

For other readers, you should be aware that "ballast", as used in this discussion, is more or less unique to the (geologically) impoverished south-east of Eng-er-land. In terms of availability, it's like that god-awful hoggin shite that they use down there to daub their drivews and garden paths: it might be shite, but it's the best they have, because they don't have any decent rock like we have in the rest of these islands. They have clay, they have sand-and-gravel, and they have chalk, so they do the best they can with a very limited palette. None of us might use the sticky claggy mess that is hoggin, but then we are blessed with real aggregates and for that, we should be eternally grateful.

[dig dug dan]

Thanks for that tony

I take on board about the c25 etc. and the quality assurance scheme.
I will enquire next time i use them.
It may be colder up there, but you have good aggregates!

[Tony McC]

...and good beer!