# Concrete mix for Foundations

#### bars03

Hi

I am putting together plans for building a small extension at the rear of my property (roughly 4*3 mtrs).

I was just wondering what the correct mix of concrete:Sand:Aggregate & water for the foundations is? I will be hiring a small mixer to do the job.

This will be a trench fill job, (depth of concrete- 1.2m, width - 0.6m)
Total required will be approx 7.2m3. This will bring the foundation up to 15cm below ground level, where I will start the brickwork. (depth of foundation has been dictated by invert of sewer pipes under build).

Also, is there a recommended gap between laying the foundation and starting the brickwork?

I know building regulations would advise when I submit my plans but I wanted to get an idea of the cost/times before hand.

Hi

I am putting together plans for building a small extension at the rear of my property (roughly 4*3 mtrs).

I was just wondering what the correct mix of concrete:Sand:Aggregate & water for the foundations is? I will be hiring a small mixer to do the job.

The strength of a concrete mix is measured in grades. There are different concrete grades for example C15, C20, C25. The grade of the concrete means the concrete compression resistance after 28 days. It is done in Newtons per square millimetre. A concrete of Grade C15 means that the concrete will have a compresion resistance (withstanding a compression) of 15 N/mm2 per square millimetre in 28 days.

Cement : Sand : Gravel
1 : 2 : 5 for grade C15(general purpose concrete)
1 : 2 : 4 for grade C25(strong)
1 : 2 : 3 for grade C30(very strong)

Remember the proportion is done in terms of volume. This means the above proportions are correct if using the same measure.

For example: 1 bucket cement + 2 buckets sand + 5 buckets gravel

bars03, do you intend to mix the 7.3m3 yourself in 1 day?

Cheers alastairreid

Wavetrain, I was looking at mixing it over a weekend, with a little help from some friends.

Ideally your strip founds should be poured in one day.

Cheers alastairreid

Wavetrain, I was looking at mixing it over a weekend, with a little help from some friends.

Recenlty I read on this forum from an experienced forum member that two good lads could mix 2m3 in one day and that would be a hard day's work for both.

Therefore what you intend doing is not possible.

If you look into the price of the materials for the concrete and compare it to the price of ready mix there won't be much in it. As Wavetrain said it's not really practical to mix up that much on site.

A bit more thought needed, me thinks. As I see it you have a few options:

a) 3 mixers, 6 guys, 3 barrows and mix it all on site in one day. You'll need all the materials close to hand.

b) IIRC, 6 M³ is the maximum load in ReadyMix, so a bit expensive for 1 and a bit loads. (IIRC 8 M³ is available in France). So alternative, 1 full load and mix the remianing 1.2 M³ on site, same day, 1 mixer, 2 guys.

c) Mixamate, I've not used this method, but might be worth investigating. Apparently they mix as much as you need on site.

d) Reduce the concrete required and top-up with trench blocks.

c) Mixamate, I've not used this method, but might be worth investigating. Apparently they mix as much as you need on site.

I've used them before and would recommend them over standard readymix. I just got another price from them for a garage floor. 3 cubes £324ex. They also have a small tracked dumper equiv to about .25 cube which saves your back if barrowing over distance or up inclines. thats £40ex. They give you 20mins waiting/laying time per cube and don't hassle you for timekeeping. If you go over the time allowed they charge you extra rather than just driving off or dumping the load in a pile for you to panic. The operator adjusts the mix there and then via a control panel, stronger /wetter etc.

c) Mixamate, I've not used this method, but might be worth investigating. Apparently they mix as much as you need on site.

I've used them before and would recommend them over standard readymix. I just got another price from them for a garage floor. 3 cubes £324ex. They also have a small tracked dumper equiv to about .25 cube which saves your back if barrowing over distance or up inclines. thats £40ex. They give you 20mins waiting/laying time per cube and don't hassle you for timekeeping. If you go over the time allowed they charge you extra rather than just driving off or dumping the load in a pile for you to panic. The operator adjusts the mix there and then via a control panel, stronger /wetter etc.

Totally agree with you on that, i had them for the garage base of about 2 cube with the mini dumper and they were spot on. The base price is £128 all in for the first cube and about 90 quid for each one after and saves A LOT of mixing!

b) IIRC, 6 M³ is the maximum load in ReadyMix, so a bit expensive for 1 and a bit loads. (IIRC 8 M³ is available in France). So alternative, 1 full load and mix the remianing 1.2 M³ on site, same day, 1 mixer, 2 guys.
.
You can get 8 metre trucks here in Somerset.

The 'Volumetric' trucks will carry 8cu metres. (I suspect the dry materials weigh less than mixed material.) These, as said, will also mix exact quantity required on site so worth having a look at a local supplier who operates these trucks.

You can get 8 cubic Metre trucks in wet mix as well.

You can get 8 cubic Metre trucks in wet mix as well.

never heard of or come across them... so who is doing that....

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