Some questions about concrete (i'm confused)

[STEVEROG]

I was considering mixing my own concrete on a future project. Some questions:

1. What's the correct mix ratio for "C20" concrete? (using ballast) Depending where I look this varies from 5:1 up to 6:1. Any ideas?

2. On my paricular job it will be much more practical to buy 25kg bags of ballast than a big giant bag. I was thinking of using the Wickes stuff (pretty cheap and easy for me to get hold of). However, I'm confused as to whether the Wickes stuff is acceptable or if I need to use something different, specifically I keep seeing "20mm ballast" being called for, not sure if the Wickes stuff is 20mm.

3. To mix the stuff up I would like to bung one bag of ballast (25kg) into the mixer then the appropriate amount of cement (5kg if using 5:1 ratio). However, I guess this would mean I need to find a container to guage 5kg of cement. Perhaps a better option would be to guage both the ballast and the cement by volume (5 buckets of ballast, 1 bucket of cement).

Any tips/suggestions much appreciated

 

[jeds]

Mixing by hand is a very imperfect process and, within reason, small differences in aggregate size and ratio will not make that much difference. Generally concretes are a mix of aggregate, sand and cement.

Aggregate should be between 15-20mm. Sand means stuff graded from 0 to 5mm - don't confuse with fine pit sand which is really for mortar. A C20 mix would be in the ratio 4 aggregate, 2 sand and 1 cement - mixed by volume. Ballast is premixed aggregate and sand. So if you use ballast you should use 6 to 1.

By far the most important factor in concrete strength is the water content and curing process. Too wet and it will be weak. You could get the mix fairly wrong and still make a reasonable concrete as good as one of the right mix but the water wrong. Concrete should be cured slowly by keeping it as wet as possible for as long as possible - up to 100 years or so. I worked in Aussie years ago and they used to flood the filled trenches with water throughout the day.

 

[STEVEROG]

Thanks for the input.

Looking like, 6:1 then.

Still not sure if the Wickes ballast is correct. I will see if they have any more information on their web-site

Thanks for the input regarding the water. I guess perhaps I could bang some plastic over the stuff once it was poured to keep the moisture from getting away too fast.

Steve

 

[^woody^]

There is not some special quarry supplying crap stuff to Wickes. Its the same stuff that other local merchants sell.

You only need to cover concrete if it will be subject to rapid drying or evaporation - ie hot summer sunshine and hot drying wind

Mixing by volume, ie six shovels of ballast and one of cement will be fine for any domestic work

Don't over analyse it. Its concrete, you are not building a motorway bridge ... are you?

 

[MarkSmith]

Having done this myself recently, I found a good easy way to get the ratios right: got an old paint container, and determined that a bag of aggregate was (say) 5 containers, and therefore for my 5:1 mix I wanted one bag of aggregate to one container of cement. (The numbers are from memory, but you get the idea.)

I too had to mix to a certain specification - think it was C30 - and in my case it did need to be strong enough as it was structural. As others have said, an accurate mix is very hard to achieve (depends on all sorts of things you can't easily measure, like chemical composition of the cement IIRC, something like that anyway) so I mixed to a C40 recipe (stronger), the idea being that even allowing for innacuracies it would definitely be better than C30.

To get the water content right you need to measure the "slump" of the concrete: essentially fill a cone with it, lift the cone off and measure how far it slumps down. I realise that's not enough detail to do it, but should get you started on Google. How much water you add naturally depends on whether the aggregate has been sitting in the rain for a week.

 

[noseall]

When you build a dam or a bridge - agreed, the recipe needs to be spot on.

However, as woody has said there is a time and place for such accuracy.

Make sure the muck is mixed well.

Make sure that there is enough cement.

Makes sure that you add enough water.

Job done.

 

[hammerswinger]

The Paving Expert site has loads of information on concrete and all other aspects of ground works, I've always found it to be a very informative site.

http://www.pavingexpert.com/mortars.htm#mixos

I've used wickes ballast before for concrete, 6 buckets of ballast to one of cement should give you a C20 mix.