Lean-mix cavity fill, the exact recipe?

[scottishjohn]

On 03/04/2019 at 15:25, epsilonGreedy said:

 

I was just obeying orders received in this thread. The weak concrete at 1:9 cement/ballast is for the cavity fill which is all below ground level and dpc. 

 

I mixed a strong mortar for my footing block courses as per NHBC technical advice which is where, as you say, damp/frost resistance is important.

 

Ok understand now why going for cement savings .because its below DPC anyway 

you might as well do it as a dry mix then and let ground moisture set it off-even easier  to tumble in mixer then barrow in ,

but me,i would still go for 8-1 as used in bedding for paviors -- 

[epsilonGreedy]

32 minutes ago, scottishjohn said:

... you might as well do it as a dry mix then and let ground moisture set it off-even easier  to tumble in mixer then barrow in ,

 

 

Wet/Dry? That was my next question. I think a touch of moisture in the mix will help in the shaping of the outward bevel at the top of the cavity fill.

[scottishjohn]

1 minute ago, epsilonGreedy said:

 

Wet/Dry? That was my next question. I think a touch of moisture in the mix will help in the shaping of the outward bevel at the top of the cavity fill.

don,t see a problem to being damp  mix -operators choice ?

I doubt your sand and gravel will be bone dry 

[sean1933]

I appreciate this was a whilst ago but is there any particular reason why you cannot just use blown EPS beads below DPC? 

I'm just about to start my build and the architect has simply shown blown beads above and below the cavity tray. Furthermore...why is there a need for a cavity tray in the below example if a

a normal outside fall is provided for the cavity fill mix (given EPS beads are permeable). I have to say I'm thoroughly confused as to why sometimes I see cavity trays at this level and other times I dont, just individual DPC's.

 

On 02/04/2019 at 14:46, PeterW said:

Detail from a previous submission here 

 

 

Not sure why wall ties are shown in the Denby Dale one unless they used it for SAP calcs also. 

 

[PeterW]

Firstly the EPS has a higher compressive strength which is useful below ground as you will need to compact a lot of hardcore inside the walls and not having anything in the cavity may mean the block work will move. 
 

Secondly, blowing beads into the lower

cavity below the tray may cause problems for the contractor as they won’t be able to drill properly due to the trays. You’re basically going to be making openings for water to get into as they would have to be done from the outside. 

[joe90]

We had rockwall batts above DPC and foam slabs below, reason fir DPC was to allow any damp In cavity to run down and exit via weep vents at the tray level.

[sean1933]

3 minutes ago, PeterW said:

Firstly the EPS has a higher compressive strength which is useful below ground as you will need to compact a lot of hardcore inside the walls and not having anything in the cavity may mean the block work will move. 
 

Secondly, blowing beads into the lower

cavity below the tray may cause problems for the contractor as they won’t be able to drill properly due to the trays. You’re basically going to be making openings for water to get into as they would have to be done from the outside. 

 

Thanks Peter.

Apologies I should have explained I'm going with a Jablite beam and block system rather than slab - with that in mind would you agree there isnt there isnt compressive issue to worry about? 

 

Re the cavity tray, any particular reason why they are used in this scenario? Aside from when there is an air brick to consider?

[PeterW]

Air bricks should have telescopic sleeves and DPC should be taped around them if it’s a cavity tray setup. 
 

You need to check the BBA certificate for your bead supplier too to ensure it’s certified for below DPC use. 
 

In your situation there is no real benefit of insulation below the beam level anyway so you may just be better using weak mix. 

[epsilonGreedy]

44 minutes ago, sean1933 said:

 

Thanks Peter.

Apologies I should have explained I'm going with a Jablite beam and block system rather than slab - with that in mind would you agree there isnt there isnt compressive issue to worry about? 

 

Re the cavity tray, any particular reason why they are used in this scenario? Aside from when there is an air brick to consider?

 

There is still some external ground pressure on the cavity from outside. How deep is the cavity from where the beam ends sit?

 

Be aware the cavity fill should not be brought up too high, 200mm below the beam seat height is typical.

[sean1933]

Thanks both. The jablite B&B system is the beams + EPS infill system. Architect has specified cavity fill insulation to extent at least 150mm below the beam bottom to help with thermal bridging. He has no experience with blown beads (it was my choice as we're going wide cavity on the house - 200mm) but hasnt spec'd any alternative insulation below DPC.... building control didnt seem to have an issue signing the plans off so would assume they are happy but I will check the BBA cert just to be sure.

 

Below the beams I have 2 blocks before reaching trench fill. To further help with thermal bridging I plan to use 1 course of thermalite Hi-strenth-10's (10.4n blocks spec'd below DPC) directly below the beam...so I figured using weak mix fill to the bottom of the thermalite would be fine.... any further thoughts on this setup?

 

 

[sean1933]

On 01/04/2019 at 23:53, PeterW said:

Done exactly the same as @MarkyP and @joe90 but used 120mm EPS with 20-30mm of weak mortar down the brick face side as the bricks are reclaims so getting a perfect 150mm cavity was near impossible. 

 

Just going through this issue now and our SE is now saying (despite architects plans showing cavity below ground) that he's not happy with that and wants full fill concrete to ground level. I've compromised and agreed he's happy with a full fill sheet insulation as long as it has some compression strength to avoid inward collapse. He didnt give me a specific kpa number that I need so I am wondering if the normal white EPS70 will suffice... what did you use Peter?  As our garage walls are now 2 blocks up (100mm cavity) I'm no considering doing what you did with a 70-90mm EPC and filling the outside gap with slurry cement.

 

Did anyone else go for XPS over EPS? Seems to be harder to get hold of (wickes has EPS70 in stock) and much more expensive, and if so any recommendations on product/supplier? 

 

Thanks in advance

[PeterW]

EPS70 would be fine.  Why does the SE think  there is a compressive load ..??? 

[sean1933]

Great thanks. He is talking about the same loading that you were re pressure from the ground on the outside pushing block work into the cavity. 

We're currently building the garage using the same principles as the house. He was particularly worried about vehicles driving in and that extra load on external ground pushing against a hollow cavity (well, eps beads with ltd compressive strength) below.

[PeterW]

So the 45 degree rule would indicate that the load is spread and a vehicle would have to be very close to the building to exert a load that would move a block with 12-15 more rows of blocks sat on top of it. 
 

Can see a reason for doing it at a garage doorway but not anywhere else. 

[Mike]

In 'traditional' construction the biggest risk to the cavity would probably be during construction and from inside, rather than outside - that is, from whacking down the hardcore beneath a ground-bearing floor slab, where the whacker would be working very close to the internal skin.