Mortar sand:cement ratio for below DPC concrete blockwork

[unleashtheboys]

Evening all,

Hopefully a relatively simple question for the builders out there.

Should it warm up a bit this weekend I'm planning on building my single skin solid concrete blockwork shed up to the DPC level. The strip foundations are in and I'm ready to go. However I've been reading around the net and found varying advice on the sand:cement mix for the subterranean blockwork - anywhere between 3:1 to 5:1.

What do people recommend?

Thanks for your time.

 

[tony1851]

1:3 might be a bit too strong, 1:5 with some plasticiser would be OK.

 

[unleashtheboys]

Thanks for your response Tony.

I was planning on 1:5 for the rest of the blockwork (i.e. above DPC) so is there no need to worry about making the mix any stronger for those blocks below ground or will that 1:5 ratio above DPC ratio be too strong?

Also in light of the recent cold weather (which is supposed to be warming up slightly this weekend) I was thinking of using frostproofer instead of plasticiser in the mix - is this a wise move?

 

[tony1851]

If the shed isn't too large, 1:5 throughout would be fine.

Frost proofer is OK, but better to use normal plasticiser and cover the work if frost threatens.

 

[unleashtheboys]

I'll forget the frostproofer then and just go with the normal plasticiser.

I've ordered some hessian roll to cover the work overnight. Is it all likely to go off as it should providing I lay at >3 degrees and the temps don't drop below 1 degrees at night (with the work covered) or is this still too cold at night even with the covers on?

Thanks again for your response.

 

[stuart45]

Recommended NHBC mortar mix for below DPC is 3.5/1 sand/OPC +plaz, or 4.5/1/0.5 sand/OPC/lime.

 

[noseall]

All sub-terranean masonry mortar - 3:1.
Anything below ground or below DPC should be dense 7n concrete blocks or FL rated bricks, i.e. semi-engineering or similar.

 

[unleashtheboys]

Thanks for all the replies.

The blocks are dense 7N concrete ones so they are the required spec.

The replies received mirror my findings on the net - opinions ranging between 3:1 to 5:1. I guess it comes down to individual experience and recommendations from relevant authorities (e.g. NHBC).

With respect to night time temps with work covered with hessian roll - what's the minimum temp it can drop to overnight without compromising the mortar used that day?