C40 concrete recipe - Kitchen worktops

[mnlad743]

I know that this request is not strictly a paving question - however it is a concrete question. I'm looking at creating concrete worktops for a kitchen. Having done a load of research I'm looking to use a C40 concrete with a 10mm aggregate (standard 20mm is too big given the thichness of the worktops). Could anyone give me the complete breakdown on the ratio of 'ingredients' as follows? To make it simple (for simple minds like mine) would appreciate numbers per cubic metre.

Water
Fine Aggregate
Course Aggregate
Cement

Thanks, Bruce

[Brucieboy]

Hi - the figures I'm quoting below are based on typical batch weights used by the ready mixed concrete (rmc) industry for a C40 based on good quality 10mm aggregate (gravel, limestone or granite) and sharp sand. The other factor to take into consideration is the cement type. Most bagged cements are now General Purpose Cements containing limestone filler and are much weaker than those used by the rmc industry. The General Purpose Cements are mostly (not all) classified as 32.5 CEM II/L or LL (the 32.5 refers to the nominal strength of the cement) whereas the cements used by the rmc industry will generally be 42.5 for CEM II/B or CEM III/A (Ordinary Portland Cement blended with pfa or ggbs) or 52.5 for CEM I (100% Ordinary Portland Cement).

Bagged 52.5 cements are available from builders merchants (most probably by special order) such as "Procem" from Lafarge or "High Strength" from Hanson. White cement such as "Snowcrete" from Lafarge is aso 52.5.

Typical batch weights for a medium workability concrete are as follows (dry):

Cement 52.5 - 400 kg/m3
10mm aggregate - 1000 kg/m3
Sharp sand - 800 kg/m3
Water (mains) - 200 litres/m3 (same as kg/m3)

Ideally, to minimise shrinkage/plastic cracking/surface dusting, it's best to incorporate a water reducing admixture such Sika Plastiment 180 or Sikament 160 or even one of their mid-range water reducers. If used, these may also permit a slight reduction in cement content, say 5-10%. The water content will be reduced by about 10% (follow the admixture dosage guidance given by Sika). Do not use a mortar plasticiser as they contain an air entraining agent as well as a plasticiser. The air will result in a lower concrete strength.

The above aggregate and sand weights will need to be increased slightly to alow for as delivered moisture content - say 2% for gravel (limestone and granite are generally dry) and 6% for sand. Deduct the corresponding amounts from the water content. Note that the water content is a guide and depends on the type/shape of aggregate, grading of sand, actual workability required etc. The aggregate and sand weights may also need to be adjusted slightly to achieve the finish required. If you deduct 50 kg/m3 from the aggregate add the same amount onto the sand to keep the yield (volume) the same - personally I wouldn't adjust it by more than about 50 kg/m3.

I must stress that the only way to be absolutely certain of the concrete strength is to make test cubes and have them tested in compression (crushing) after 28 days. For obvious reasons the strength cannot be specifically guaranteed.

Other factors that must be taken into consideration are adequate compaction - essential to remove the air voids; curing - keep the concrete moist after placing / demoulding using polythene sheeting or similar; reinforcement - this could be mesh, fibres etc but seek advice as they are dependent on unit thickness and overall dimensions. Some bespoke manufacturers even used core materials such as mdf to reduce the weight.

Good luck - I hope it goes well.

[cemcraft]

Brucieboy - (That's my nickname as well!)

Thanks very much for all your help and advice. The ready-mix companies in my area weren't all that helpful as the quantity I was looking at wasn't really going to be worth their while, and I wanted white cement to boot! The latter is something most don't want to deal with or they charge you a hefty clean-out fee. We're going to be looking at the addition of pigments (hence the white cement) so quite a bit of testing will be required in order to get the colour right - with your numbers we will be able to run as many test batches as it takes.

Again, many thanks.

Bruce

P.S. Don't know of a good online calculator for render by any chance, do you?

[lutonlagerlout]

an interesting book i own is concrete counter tops by fu tung chen
not that technical but some great ideas
sadly in this country most readymix companies are not interested in unusual requests
I know from experience that crushed 10 mm aggregate is stronger than 10 mm pebbles,more interlock
other than that i defer to my erstwhile colleague brucieboy who is the resident concrete expert
i looked in to making them but TBH with granite getting cheaper it made sense for me to use granite
let us know how you get on
cheers LLL

[mnlad743]

lutonlagerlout,

Thanks - have a copy of that book myself. It is a little short on substance but you're right about it being a useful 'Ideas' resource. I've considered going across for one of his workshops, either in California or Rhode Island. The only problem with his web site, and I suppose it's not surprising, is that it doesn't give too much away - primarily designed to sell his pre-packaged mixes, pigments etc. His books and the training DVD are all available from Amazon here in the UK.

Bruce

[Brucieboy]

LLL - with regard to high strength concrete (C50 and above), you are quite correct that crushed rock such as limestone and granite is better than gravel due to enhanced interlock and rougher surface texture. Flint/quartz type gravels, although generally harder than most crushed rocks, tend be more rounded/irregular in shape and have a smoother surface texture therefore the bond with the cement matrix isn't as good. Most ready mix concrete suppliers will generally limit gravel to a C50 although in saying that we have achieved cube strengths as high as 75 N/mm2 using conventional admixtures - even higher with the additon some of the special materials, particular the combination of microsilica and a superplasticiser.

[Brucieboy]

Bruce - ps. The inclusion of microsilica will significantly improve the moulded surface finish - like polished glass.

[DNgroundworks]

Jeez Bruce you sure know your concrete my friend

[Mikey_C]

I don't imagine that there are many people in this world that would enjoy this thread, but I did immensely, brucieboy you know and love your concrete.

As I was reading your initial response, I was wondering if the OP was going to complain, as he had used the phrases "ingredients" and "To make it simple (for simple minds like mine)" but all's well that ends well.

thats interesting to note about the snowcrete strength, as I big fan of strong (i.e. over engineered mix).

[GB_Groundworks]

thoroughly enjoyable read, mikey_c remember stronger isn't always better some degree of flexibility can be better than completely rigid

[Brucieboy]

Many thanks for the comments fellas - hope I can be of help on this excellent forum. I took early retirement about 18 months ago after 45 years in technical management in the concrete/mortar/screed industry but still do a bit a consultancy work for my last employer to keep my hand in. I love reading all the topics - some excellent advice given (particlularly useful when I laid my first sandstone patio/path last summer).

[Mikey_C]

I hear ya Giles.

It's the tree that bends in the wind that survives the storm.

[mnlad743]

"with the additon some of the special materials, particular the combination of microsilica and a superplasticiser".

Do you know of a source for microsilica (preferrably not in ten tonne loads)!

Bruce

[Carberry]

Brucieboy, I think you have found the only people in Britain who would be happy to listen to you talk about concrete :laugh:
Interesting read.

[lutonlagerlout]

i agree fascinating stuff
LLL