concrete mix ratio for foundations

Hi, I know this doesn't answer your question but from my experience you are better getting concrete supplied for something that size. The mix is consistent, correct, you only pay for what you need and it's done in no time.

 

C25 Mix

800MM for a 2 story extension???? Has it been passed by BCO.

 

C25. ☺

 

5.1 will do, make sure it a measured ratio using something like a bucket to measure.

I must say I'm concerned about your trench size, only 300m wide this is the exact width of the brick, cavity, block, a standard foundation is 450mm wide and 1000mm deep.

 

C25 wasted in a normal found. I always specify ST 1 which is about 9:1. Do take the trouble to get the top level and matching existing courses: this will save you a lot of effort.

Just over 2 cube so not a major deal to mix and lay: just keep the stuff reasonably stiff which will mean you keep a tight face and good levels and also be easier to barrow (not slopping everywhere, weight on wheel not your arm) an of course the concrete will be stronger.

 

Wasted or not its the specified mix for foundation.

http://www.easymixconcrete.com/news/types-of-concrete-and-their-strengths/


9.1 that's a mix so weak I don't think I've ever used, even as a lean mix.

 

If you actuallyread the stuff you've posted they also suggest Gen 0 or Gen 1 for domestic founds. These re weaker than ST1.

What does concrete do in the ground? It bridges the gap between the ground and your bricks so it doesn't really need to be any stronger than the ground which is what is supporting the wall at the end of the day.

Lean mix can be as low as 18 or even 20 to 1. Believe it or not we often have problems in road construction because it is too strong!! It is now referred to as CBGM (cement bound granular material)

 

I did read it and used that and the fact that I deal with BCOs every day, most demand C30 for foundations.

Of course it has to be stronger than the ground, otherwise we would just build off the ground, you have to take into account many factors re: the ground, type of soil, moisture content, surrounding area etc.

They are always changing the name of what effectively is the same thing, years ago when I was doing reinstatement work for the Gasboard, they started using foam concrete as back fill for us to tarmac on to, well that didn't last long before it started compacting and the trenches sinking, costing a small fortune to repair.

As for being to strong, thats not really the problem, the problem is that the concrete isn't laid thick enough to handle its own strength, hence it cracking.

 

Not quite - the role of a foundation is to spread the load; that's why softer ground will require wider foundations (and very soft ground will need a raft foundation or even piling). Then you also need to take into account shear forces, bending moments, etc. The stronger the concrete, the more resistance to those forces; there are many reasons why an SE will specify concrete strength for a particular foundation.

 

With respect this is a case of "a little knowledge..". The question related to a deep strip foundation. Back in the day we'd have dug a hole, put in 6-8 inches of concrete and bricked up from that: how much strength does the brickwork have relative to a lump of concrete?

It's very rare to have anything other than 450 width unless you are at the site boundary and putting the wall at the edge of the foundation when you'll be at 600. Raft? Piles? Now you're talking proper engineering design and reinforced concrete.

I repeat my view that for normal deep strips ST 1 is fine in 99% of cases: I have specified it for 14 years over at least 10 different LAs and only had queries on about 2 cases where the ground water conditions required sulphate resistance. BC are never going to tell you your job is over specified are they?