Concrete mix ratio?

[AndyDavis]

What is the right concrete mix ratio for a 1500 trench foundation to support a two-storey extension. I though it was 1:3:6, but got given a Wickes leaflet yesterday and it says to only use a 1:3:6 ratio for boundary walls and other small stuff. They say that you should use a 1:1.5:3 ratio for foundations. Is that right, and why?

Andy

 

[murraysnudge]

The plans that were passed by your Building Control Dept would have specified the foundation concrete. If you use ready-mix you specify a grade which would normally be C15 which is between 1:3:6 and 1:2:4(C20) but certainly not 1:1.5:3(C25), as you say Wickes recommend. Perhaps Wickes want to sell more cement? It is a good price there though.
You do need to be guided by the Building Control Dept on the grade though.
I always use the trench-fill method. I fill the trench to one block below ground level which allows for getting the level exactly right. This uses more concrete than a strip at the bottom but saves on block laying down the trench and you can often use the 16" wide bucket on the digger. If you are paying for labour then it can be cheaper. You have to fill the cavity below ground with weak-mix anyway so do consider this method.

 

[AndyDavis]

Yes my plans say a 1:3:6 mix too, for a minute there I thought maybe it was a mistake. Not sure what you mean by the weak mix though, I'm planning to mix my own concrete using Mastercrete, sharp sand and 5mm aggregates.

Andy

 

[masona]

5mm aggregates.

Are you sure this is okay for footing ? Check with your building inspector first.

5mm is normally for paths & thin section and 20mm ballast for footing.

 

[murraysnudge]

Andy, masona is absolutely right and you should order 20mm aggregate(that's 20mm down rather than single size). If you ordered Readymix you would get 20mm unless otherwise specified.
If you were using a strip foundation and two leaves of 100mm blockwork with a 50mm cavity, then you would fill the cavity below ground level with weak-mix concrete. The alternative would be to use a trench block, which is a single block with the width of a cavity wall. This detail should be specified on the plans.
I still recommend filling the trench to one block below ground level. What is the volume if you did that? It may be worth considering ready-mix. The lorries are usually 5.5m³ or 6.0 m³. You don't have to order the full load but there is a surcharge for every m³ that you don't have. I've used them for 4m³ and paid the surcharge for the 2m³ that I didn't have; I think it's worth it.

 

[AndyDavis]

Yep, I stand corrected. I'm going to need about 20 cubicM, having asked around I think readymix doesn't sound too bad.


Thanks, Andy

 

[murraysnudge]

That's quite a big extension Andy. Assuming you will use trench-fill to one block below ground level 20m³, gives about 31m length of wall. It may be best to try to work to a multiple of full lorry loads. If you had the concrete to 2 blocks below ground level i.e. 450mm, then you would only need 16m³. If you used a firm that had 5.5m³ lorries then you could order 3.
It's a good idea to have an area where any excess concrete can be used if you have got too much, such as a path or foundation for a garden wall etc.
Out of interest, is the depth of 1.5m insisted on by the Building Control office because of nearby trees or special sub-soil conditions?

 

[masona]

Out of interest, is the depth of 1.5m insisted on by the Building Control office because of nearby trees or special sub-soil conditions?

It's now very common because of the weather climate, I'm lucky I can have 1 metre footing because good clay condition. My poor mate approx 5 miles away have to have 2 metre footing including metal rods and that just for a porch !

 

[murraysnudge]

I'm semi-retired now and don't do a lot so I'm a bit out of touch then. I thought they were generally happy with 1m depth. I do remember alll the problems after the '76 summer when a lot of subsidence was attributed to shallow foundations. I think at that time the council were liable as they had approved the plans.
Soon be short bore piled foundations as standard then?

 

[masona]

I was wondering if it's a bit over the top sometime, my 1903 property have no footing, only resting on 3" slabs type footing !

 

[murraysnudge]

Perhaps there is a connection between the Council insisting on deep foundations and the income they raise from landfill charges. Is that a bit too cynical of me?

 

[AndyDavis]

Sorry for the late reply but I've been getting builder grief.

Yes, the building reg. does say 1.5m. The extension is going on a very slight slope and there used to be some big trees nearby so that might be the reason, although the trees have all gone now.