Why weak mix? cavity fill

[bandekk]

Anyone know why you need to use a weak mix concrete 1:6 to fill between cavity walls on footings? only ask as I may need to use 3 aggregate blocks to get the width I need for my first few footings courses, thanks

 

[hotrod]

No pressure on the cavity from external ground forces above ground is there?

 

[noseall]

You could almost fill the cavities with ballast, but this would not be quite good enough.

8:1 is about right.

 

[bandekk]

No, theres nothing above ground, however my concrete foundation is approx 600mm below ground and I'm building off that.

I also now know that my foundation block work needs to be at least 355mm to accomodate a step in the facing brickwork and the inside course. Now that I've taken delivery of my aggregate blocks, it seems the only way to make the blockwork wide enough is to place 1 skin face down, and 1 on its side creating an overall width of 315mm, plus cavity.

Does this make sense? in hindsight I should have just ordered 355mm trench blocks, buts thats a wonderful thing.....

 

[hotrod]

Does this make sense? in hindsight I should have just ordered 355mm trench blocks, buts thats a wonderful thing.....

355mm trench blocks aren't available as far as I'm aware - the widest trench block readily available is 300mm.

I also now know that my foundation block work needs to be at least 355mm to accomodate a step in the facing brickwork and the inside course. Now that I've taken delivery of my aggregate blocks, it seems the only way to make the blockwork wide enough is to place 1 skin face down, and 1 on its side creating an overall width of 315mm, plus cavity.

This 'step', does it extend around the perimeter of the building? Does it need to be built off the foundations? Think I've got it straight in my head, that you are going to bring the two 100mm skins off the 315mm 'homemade' trenchblock to give you a 115mm cavity. Is that correct?