Lean-mix cavity fill, the exact recipe?

[epsilonGreedy]

Could someone confirm the ingredients of lean mix concrete for below ground wall cavity fill?

 

I understand the basic ratio is 1:3:6 (Cement/sand/lumpy aggregate).

 

Is that soft or sharp sand? And what about the lumpy portion, 10mm or ballast, round/sharp... so many options.

 

Here is what I can order from my local quarry:

• 0/2mm softsand
• 0/4mm sharp sand
• 0/4mm plastering sand
• 0/4mm Dust granite
• 1/4mm horticultural grit
• Screened fill sand
• 0/10mm Washed ballast
• 0/20mm Washed ballast
• Type-1 granite
• 1" down limestone
• 1" clean limestone

 

All of the above come in at under £30 in VAT per ton loose.

Edited April 1, 2019 by epsilonGreedy

[Mr Punter]

1 part cement, 9 parts all in ballast.

[ToughButterCup]

I would use the 10mm ballast - if you can trust it.

There be sum shoit  ballist roun' yur  : ' happen 's  wuy it cost  £25 bagged and delivered. Loads a' glass innit. 

[epsilonGreedy]

1 hour ago, recoveringacademic said:

I would use the 10mm ballast - if you can trust it.

 

 

Ok 10mm washed ballast it is. The quarry has a samples shed where customers can have a touchly feely experience with the 50 or so aggregates they sell.

 

1 hour ago, recoveringacademic said:

There be sum shoit  ballist roun' yur  : ' happen 's  wuy it cost  £25 bagged and delivered. Loads a' glass innit. 

 

 

Was that £25 for a 1 ton bag delivered?

[Construction Channel]

Have you poured the oversite yet? If not order a bit more, then you won't even have to mix it. 

 

If you have. Just get the all in ballast. (10/20mm washed, which ever is cheapest) 

Half a bag of cement and fill the mixer up. This time it really is just a gap filler

[MarkyP]

Or chat your BCO and see if they will agree to dispense  with the lean fill altogether. In my case ours was fine and I used 150mm eps to full fill the cavity below the tray formed by the dpm down to top of the footing. If I recall the leanfill detail is to prevent high loads from vehicles or high earth build up from pushing the block/brickwork and collapsing into the cavity. I think it's a bit of a belt and braces old fashioned detail unless there really is a build up of earth or risk of crushing. In our case the soil was 150mm above footing, sometimes less. Insulating to top of footings will improve the thermal performance of the wall by extending  the heat loss path of the inner leaf.    

 

 

Edited April 1, 2019 by MarkyP

[joe90]

I too had insulation right down to the strip foundation, it was part of the insulation buildup to alleviate a cold bridge. I forget the name of the house that I copied this from on the internet but here is a drawing I supplied the BCO and they passed it with no question.

Edited April 1, 2019 by joe90

[MarkyP]

I think I got the idea from the Denby Dale passive houses. Mine looks exactly like your picture, albeit with a lower external ground level. 

[Cambs]

4 minutes ago, joe90 said:

I too had insulation right down to the strip foundation, it was part of the insulation buildup to alleviate a cold bridge. I forget the name of the house that I copied this from on the internet but here is a drawing I supplied the BCO and they passed it with no question.

 

 

Golcar passivhaus by Green Building Store, methinks....  they have some good case studies and information on their site under the ”Free Resources” tab

 

https://www.greenbuildingstore.co.uk/golcar-passivhaus-ground-floor-foundations/

 

[PeterW]

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. 

[epsilonGreedy]

13 hours ago, Construction Channel said:

Have you poured the oversite yet?

 

 

I will have beam & block floors so the floor screed pour could be much later when the roof is on.

 

The pour of the ground bearing garage floor will happen sooner but I had assumed that this grade of concrete would be too good and too expensive for cavity fill particularly if I follow other advice received here ( @JSHarris and @PeterW ) to purchase fibre reinforced concrete in place of rebar.

 

Time for me to price out a cubic meter of home mixed lean cavity fill concrete v. upping the garage floor concrete order quantity.

 

10mm washed ballast is £21 per ton or (21 * 1.6 * 0.9) for 1 m3 = £30.24

7 x 25kg bags of cement @ £4 = £28.

 

Then there is bulk delivery at £45 for the small 11 ton lorry.

 

How much do I need for my footings...

 

House 55 linear meters of 100mm cavity blockwork two blocks high but I need an extra min distance margin below the base of a floor beam and the top of the cavity fill minus the outward chamfer to direct cavity dribbles away.

 

Cubic meters of house cavity fill = 55 x 0.1 x 0.4 = 2.2 m3

Cubic meters of garage cavity fill = 38 x 0.1 x 0.2 (one course of trenchblocks) = 0.76 m3

 

Which means I need 3.0 m3 of cavity fill which equates to 3 x 1.6 x 0.9 = 5 tons of ballast = £150 delivered loose with some left over for the lean mix base of ground bearing garage floor slab + 21 bags of cement = £84. So at the end of the day 3 tons of lean concrete mixed on site = £78 per ton = not a lot less than regular concrete.

 

However I will be mentally and physically maxed out coping with my first garage slab pour hence no time to rush around filling the cavity with the extra premixed concrete.

 

I cannot find an online price for fibre reinforced concrete?

 

 

[epsilonGreedy]

13 hours ago, joe90 said:

I too had insulation right down to the strip foundation, it was part of the insulation buildup to alleviate a cold bridge.

 

 

Due to the flash flood risk I need an extra 2 courses of bricks up to damp and I would also like something to stop blown beads dropping down to the top of the cavity fill. I was considering solid insulation between top of the concrete cavity fill to just below damp. However if water does enter the cavity would it run down the beads and then pool on top of the hard insulation which I assume is stuffed forcibly in the cavity.

 

One solution would be a cavity tray like yours however I am adverse to this idea in fear of creating a mortar moisture bridge. I your case with a 200mm cavity the risk is substantially lower.

[PeterW]

17 minutes ago, epsilonGreedy said:

cannot find an online price for fibre reinforced concrete?

 

I pay £6+vat per tonne extra for glass fibres

[joe90]

We had two damp courses!!!, one as per the above diagram but on the internal wall at floor level (which meant it was below ground level on the outside wall), the second was just in the outside wall level with the internal floor level. The reason fir this is my builder, who has done as per the above diagram, found the internal plaster cracked at the damp course level internally which was above the skirting board, by lowering it and providing a second DPC this problem was sorted. 

[epsilonGreedy]

16 hours ago, joe90 said:

here is a drawing I supplied the BCO and they passed it with no question.

 

 

Did you use basalt wall ties as per the diagram?

[PeterW]

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]

And the insulation under the slab was changed to 150mm PIR on 25mm EPS. 

[Simplysimon]

i'm surprised at these well insulated floor/wall sections, yet they all show no thermal break in the inner leaf.

3 hours ago, PeterW said:

 

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

are you saying they are now building two single leafs and not using ties or is it just the basalt ones?

[joe90]

3 hours ago, epsilonGreedy said:

 

Did you use basalt wall ties as per the diagram?

 

No, someone did some sums for me and the thermal bridge was very small and in my opinion not worth the expensive price of basalt ties. I spent the money on more insulation in the roof ?

[Construction Channel]

8 hours ago, epsilonGreedy said:

 

I will have beam & block floors so the floor screed pour could be much later when the roof is on.

 

The pour of the ground bearing garage floor will happen sooner but I had assumed that this grade of concrete would be too good and too expensive for cavity fill particularly if I follow other advice received here ( @JSHarris and @PeterW ) to purchase fibre reinforced concrete in place of rebar.

 

Time for me to price out a cubic meter of home mixed lean cavity fill concrete v. upping the garage floor concrete order quantity.

 

10mm washed ballast is £21 per ton or (21 * 1.6 * 0.9) for 1 m3 = £30.24

7 x 25kg bags of cement @ £4 = £28.

 

Then there is bulk delivery at £45 for the small 11 ton lorry.

 

How much do I need for my footings...

 

House 55 linear meters of 100mm cavity blockwork two blocks high but I need an extra min distance margin below the base of a floor beam and the top of the cavity fill minus the outward chamfer to direct cavity dribbles away.

 

Cubic meters of house cavity fill = 55 x 0.1 x 0.4 = 2.2 m3

Cubic meters of garage cavity fill = 38 x 0.1 x 0.2 (one course of trenchblocks) = 0.76 m3

 

Which means I need 3.0 m3 of cavity fill which equates to 3 x 1.6 x 0.9 = 5 tons of ballast = £150 delivered loose with some left over for the lean mix base of ground bearing garage floor slab + 21 bags of cement = £84. So at the end of the day 3 tons of lean concrete mixed on site = £78 per ton = not a lot less than regular concrete.

 

However I will be mentally and physically maxed out coping with my first garage slab pour hence no time to rush around filling the cavity with the extra premixed concrete.

 

I cannot find an online price for fibre reinforced concrete?

 

 

That's why we fill the cavities first ?

 

How are you getting the concrete in for the garage floor? (pump, barrows, straight out of the lorry) 

 

If you can do it with insulation then that sounds a great idea to me. 

Mixing 3m3 of concrete by hand is a good mornings work at least......

 

Maybe wait until after the garage floor. Any excess goes in the cavity then calculate and mix the rest by hand

[PeterW]

14 hours ago, Simplysimon said:

.

are you saying they are now building two single leafs and not using ties or is it just the basalt ones?

 

Sorry wasn’t clear - I meant it’s unusual to see them in section drawings, they are normally in the notes section. 

[epsilonGreedy]

15 hours ago, Construction Channel said:

How are you getting the concrete in for the garage floor? (pump, barrows, straight out of the lorry) 

 

 

The lorry should be able to back up to within 1m of the garage on the 150mm of 2" hardcore laid last autumn. The cavity fill at the garage door threshold will consist of a 3m run of strong concrete, not seen that documented anywhere but just seems like a good idea.

 

My recollection from the foundation pour is the delivery chute can reach out around 2.5m.

 

15 hours ago, Construction Channel said:

Maybe wait until after the garage floor. Any excess goes in the cavity then calculate and mix the rest by hand

 

 

Good tip, I will leave a few meters of cavity unfilled incase my concrete floor maths is wrong.

[scottishjohn]

Is this very stingy use of cement just to save money .

just thinking back  to a wall I had pointed by a local --who mixed it very weak ,and it always holds the dampness more than the next bit ,that  i did my self , after seeing how he charged for a rough job ,

but i mixed good and strong and the water from rain does not stay on it  or in it like the weak mixed section .

 so my point being if any dampness is about  it could track through the weak mix and hold moisture ?

where as strong it won,t go through it or hold it .

comments please

 

Edited April 3, 2019 by scottishjohn

[epsilonGreedy]

1 hour ago, scottishjohn said:

Is this very stingy use of cement just to save money .

just thinking back  to a wall I had pointed by a local --who mixed it very weak ,and it always holds the dampness more than the next bit ,that  i did my self , after seeing how he charged for a rough job ,

but i mixed good and strong and the water from rain does not stay on it  or in it like the weak mixed section .

 

 

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.

 

Edited April 3, 2019 by epsilonGreedy

[joe90]

24 minutes ago, epsilonGreedy said:

who mixed it very weak ,and it always holds the dampness

23 minutes ago, epsilonGreedy said:

Is this very stingy use of cement just to save money .

 

Anything below DPC on my plot is lawyer saturated anyway ?