Disaster, advise needed ASAP!

[Jamieharnett]

Hi all,

I’ve employed a builder to install a Limecrete floor supplied by mike wye, on return today his screeders have mixed as to his instructions on 9 bags per tonne.

We was supplied with 7 tonnes of sharp sand with 66 bags of NHL5 on my return to see the new screed laid I noticed 24 bags of lime had not been mixed within!

On calling the builder he said it will be fine.

Please can someone explain the consequences of the mix used? So I can get this sorted or will it be okay?

Many thanks in advance

 

[plasticpigeon]

I suppose you could pay him just over half the agreed price and when he complains tell him it will be fine.

 

[Jamieharnett]

Very true but as I’m currently living off site costing me by the week I need a resolution or plan b. No doubt someone will fall short but at the present I need to know if this needs ripping out and starting again.

 

[plasticpigeon]

Mike Wye is the person to talk to, and your builder probably needs to talk to him too. THey need to rip it out really and the earlier they do it, the easier it will be.

 

[Jamieharnett]

I think your right, it’s such a fundamental part of the house which if not right will truely come back to bite with floorfinishes.

 

[malcolm]

A cement mix for a floor might be 1 in 5 by volume, but a lime mix is more likely to be 1 in 3. That might be where the mistake is. As PP says, a call to Mike Wye first thing has to be the way forward.

 

[paulc]

Looking at www.stastier.co.uk/nhl/guides/pdfs/Exposed_Concrete_Floors.pdf

7 tonnes of dry sharp sand should equate to approximately 4.375m³. According to the above PDF, you'd need around 1750Kg of NHL5 lime. So just 42 bags (1050Kg) is quite a bit short. Even 66 bags (1650Kg) is a bit on the weak side. Mike Wye should be able to confirm the mix ratio....

I'd suggest the builder needs to remove the floor and put a new one down at his expense, and quickly.

 

[robgil]

Where I live its been very hot and dry, not the best weather for using lime, especially a large slab like a floor, my guess is if its dry and hot where you are the builder didn't want it going off too quickly and cracking, so really it shouldn't have been attempted if that's the case, I would dream of doing lime work during a heat wave.

 

[MatthewC]

On calling the builder he said it will be fine.

"Well, he would say that, wouldn't he?"

Presumably this was NHL5 (or 3.5)? I agree with PP and Malcolm - speak to Mike Wye, and check who gave precisely what instruction to the builder.

Best of luck

Matthew

 

[88v8]

Yes, best ask Mike Wye.

With two-thirds the lime, it is at the low end of permissible ratios, and will be under-strength especially in the short term, and potentially a bit crumbly.
What is the span, how thick is it, and on what sub-base?
And what do you plan to lay on top?

Ivor

 

[Minkeycat]

Purely to add weight to the above suggestions, I'd speak to Mike Wye and go solely on what they say. I've always found them very knowledgeable and helpful and I'd take their word over the builder unless the builder can take you to another floor he's done the same way and physically demonstrate that it's on the same base as yours, the mix he used and how it's cured. Without proof that it'll work, I'd follow Mike Wye's suggestion. My limited experience is that lime goes off quite slowly compared to other things (don't know about in this heat though) so that will save the builder time and money as it will be easier the sooner he gets on with it. It sounds like it was his deliberate decision rather than an error so an awkward conversation ensues. Sorry this has happened but good on you for catching it now. I hope you get it resolved without too much aggro.

 

[Jamieharnett]

Many thanks for all the replies, I spoke with mike wye today and they supplied materials on the basis of a 2:1 mix, hence the left over bags meaning the slab has been installed with a 3:1 mix. Mike wye previously recommended a mix of 3:1 however recently they changed it to 2:1. They’ve reassured me the mix will be fine as screed was laid 100mm thick with 200mm Geocell. Panic over time to have a beer and relax up! Many thanks for everyone’s input.

 

[plasticpigeon]

Really good news, as good as it could have been!!! It would have been confidence inspiring if your builder could have explained that but at least they have not done anything awful. To be fair, I thought 3:1 was standard anyway.

 

[paulc]

To be fair, I thought 3:1 was standard anyway.

I did some empirical measurements recently with some very dry sharp sand. Took a 250ml beaker of sand, weighed it then transfered to a small bowl. Filled the beaker with water and then poured the sand back in. After leaving it to settle of a few minutes, tipped the excess water out and checked that the sand was still at the 250ml level. weighed it again, subtracted the original weight (all in grammes) - The final figure gave an indication of how much lime is required for an "ideal" mix using this particular bag of sand.

It came out as a 2.5:1 mix of sand & lime - For plaster and render, I'd probably go for a 2:1 mix, and for pointing mortar, 3:1.

 

[MatthewC]

I spoke with mike wye today and they supplied materials on the basis of a 2:1 mix, hence the left over bags meaning the slab has been installed with a 3:1 mix. Mike wye previously recommended a mix of 3:1 however recently they changed it to 2:1.

I would love to know why Mike Wye have changed their minds to a 2:1 mix. That sounds quite a dramatic shift - my limecrete floor is rock hard anyway at 3:1.

Being still not sure that I understand the figures, I just weighed 10 litres of both sand and NHL: the sand weighed 13.5 kg and the NHL 6.4 kg. so the sand is just over double the weight of the NHL, say it's 2.1. Hence a tonne of sand at 3:1 needs approx 1000/(3x2.1) kg of NHL = 159kg, which is 6.3 bags (at 25kg/bag), so that's pretty well correct as they used 6 bags per tonne. (Obviously sand weight can vary with its water content)

Matthew

 

[88v8]

Hmm, this is all a bit confusing, given that most of us mix lime/sand on the basis of volume.
Or at least, I do.

Ivor

 

[MatthewC]

Please read what I wrote, Ivor. When you buy by weight and measure by volume, how do you work out how much you need to buy to get the right mix?

 

[Flyfisher]

I spoke with mike wye today and they supplied materials on the basis of a 2:1 mix, hence the left over bags meaning the slab has been installed with a 3:1 mix. Mike wye previously recommended a mix of 3:1 however recently they changed it to 2:1.

I would love to know why Mike Wye have changed their minds to a 2:1 mix. That sounds quite a dramatic shift - my limecrete floor is rock hard anyway at 3:1.

Yes good question. Quite a significant change.

Also (and I've asked this before with no answer) I've never quite understood the need for 'rock hard' floor slabs and the standard use of NHL5 compared with NHL3.5 or even NHL2. A correct mix of any NHL would surely create a strong enough floor slab (providing the sub-base is properly prepared) - after all, the only force to be resisted is compression, something that mortars are very good at. I suppose it could be argued that a 'softer' slab might be less resistant to scuffing or point-load damage, but how many properties have bare slabs? I'd bet they are 99.99% covered with something that would protect from scuffing and other wear-and-tear anyway.

I only mention all this because the whole point of a limecrete slab seems to be breathability yet the 'harder' the NHL grade the less breathable the slab.

 

[malcolm]

I should imagine failures caused the recommendation to change from 3:1 to 2:1. The folk supplying the lime don't have much control over the sand used in the job. A good concreting sand/aggregate mix ought to have everything from big chunks all the way down to fine sand and fines and a 3:1 would be OK for that. I notice some suppliers have taken to washing even concreting sand which takes out the fines and I guess that is what caused the advice to change.

I had a bit of a render failure with plastering sand which turned out to have been washed. After some testing I found it needed a 1:1.25 mix to work. Bad practice - better to mess with the sand than increase the mix. I've still got a load of the stuff and am getting rid of it by mixing 50:50 with a soft sand that has all the fines and am using a 2.5:1 mix for exterior rendering and interior base coat plastering.

 

[Flyfisher]

According to this http://roundtowerlime.com/technical-information/sand-selection.html, soft sands and fines should be avoided and the best sand is washed (no fines) sharp (angular grains for bonding strength) sand.