Polished Concrete Slab crack management.

[MikeSharp01]

Hi All.

 

Just working through the design of our slab with the architect, SE and insulation formwork provider! It will be a polished concrete floor with the slab forming the finished floor. One question that came up yesterday was how we would prevent surface cracking in the slab while remembering that the slab integrity is key to the structure. My thinking is that we just use surface slots but I thought I might ask what the experience is. Anybody done it?

 

Mike

[Oz07]

Why are 2 professionals asking the customer how to achieve the customers goal?!

[Dudda]

The structural engineer may not want to advise as it's a finishing screed not structural. That happened me on a few jobs and the contractor won't make any decisions in case something goes wrong and has to dig it out and redo it. They'll give advice but you can be sure they won't take any responsibility for its design. It's left to the architect but they may not have experience with them.

 

What's the thickness of the polished concrete and what are you using to reinforce it. Don't use steel mesh unless the whole project is very carefully planned and I've seen the metal fibres show in the finish a bit. If using the plastic fibres then cut the slab at every 6 meters and under all doors the day after pouring about 30mm deep into the slab with an angle grinder. They can be filled later with mastic. Keep the cut areas square and large. Long thin ones can crack in the middle and small squares can rock like paving slabs. Do you have any columns or other structural walls protruding into the space. You may need additional crack joints near or inline with these. Also are the walls smooth? We had one project with rough stone walls so the edge perimeter insulation has to be thicker to allow for the undulating surface and avoid cracking. Make sure the perimeter insulation isn't rigid too as it needs to be able to compact and move if required.

Edited October 14, 2016 by Dudda

[Temp]

Our screed wasn't polished but we did get cracks that could be avoided...The occurred where the screed in two rooms met at a doorway. Looks like each area shrank towards its own center. So I would definitely put expansion joints at doorways.

[Dudda]

2 hours ago, Temp said:

So I would definitely put expansion joints at doorways.

An expansion joint is slightly different. A crack induced joint where you cut the slab the day after where you want it to crack is usually sufficient.

[MikeSharp01]

Thanks Guys. Think the professionals lack confidence with a passive slab - monolythic, all on and encased around in EPS, - no traditional footings so the loads are on the edge of the slab and in a couple of places in the center supporting the ridge beam and to some extent the first floor which also sits on the walls and hence is trasfered to the edge. They all seem to know what they don't want to happen but are not clear on how to stop it so the above if very helpful. They would prefer me to screed it but I want to polish it once laid, protect it and build on it because it will be easier to polish it when its just a flat sheet without the walls, also sticking the wall plates down and sealing them will be much simpler if the surface is polished. Reinforcement design, which will be steel rebar is with the slab supplier but they cannot start until the SE provides final loadings and so we are in a bit of loop at the moment and I am trying to break the log jamb. Essentially I have taken away that we don't need sliding joints just crack propagation prevention although I think that does mean that the slab will be broken through, down to the EPS, in those places.

 

 

[Alexphd1]

We are planning exactly the same but on a diy basis. Maybe I am wrong but I can't see how grind/polish etc will crack a structural slab, or am I missing something? Due problems with the original engineer who specd 150mm concrete for main slab (material ordered to this spec) and the new engineer spec 100mm and is happy with 150mm "over spec" . So we are happy with grinding away a few mm to show the aggregate. 

Edited October 15, 2016 by Alexphd1

[Declan52]

By my reckoning it won't matter that it's getting polished or not. Just follow normal procedure and have the screeders put expansion joints at all the door ways when they are laying the floor. Same goes with the edge insulation have all the walls done so that the floor can move. If you are building stud walls on top I would put an expansion joint under each of these as well so in effect you are dividing the floor up into the rooms that form your house layout.

[Alexphd1]

Declan, I think mikesharp01 and myself are speaking about a single pour structural raft foundation above eps insulation. the house is then build off this inc external walls (icf)/internal wall (timber). 

[MikeSharp01]

Yep, thats it - single pour structural raft, I am DIYing the polish but not down to aggrigate as SWMA (She who must agee) feels its not the look she wants - pheew. Am doing a smaller slab in the garden room first to test out the processes and develop my skill set. I won't have a problem putting the slits in below the internal walls. Not sure if we have concensus on the process yet though.

 

Mike

Edited October 16, 2016 by MikeSharp01

[Declan52]

Could you use something like a brick expansion joint buried in the concrete where your internal walls are. Have it set in position and haunched up with some motar/concrete so when you come to cut it out it's only a 10-15mm slice.

 

https://www.travisperkins.co.uk/Fillcrete-Brickfill-Expansion-Joint-Roll-10mm-x-100mm-x-10m/p/837980

[gavztheouch]

Did you end up using control joints in the slab?

[markharro]

Yes @MikeSharp01 I would be interested to know how this went too. We will have a Passive insulated slab installed in March with steel reinforcement and UFH pipes 50mm under the finished surface which will be the powerfloated slab. Our installer says they won't fit any expansion gaps etc I think because they would be paranoid they might hit one of the UFH pipes. 

That leaves me in a difficult position because how do I minimise any cracking to the floor when this is to be used as the final finish? A quick look at this - https://www.hanson.co.uk/en/ready-mixed-concrete/technical-information/concrete-expansion-joints-explained guidance suggests that @Dudda 's advice about cutting slots is spot on. However with our 150mm slab that would mean ideally 37mm deep slots which is painfully close to the UFH pipes.

 

Actually this reminds me - technically I think I need contraction joints not expansion - https://www.cement.org/learn/concrete-technology/concrete-construction/contraction-control-joints-in-concrete-flatwork

 

Do I ask my contractor to tool these in during the instal to thus avoid completely the risk of cutting pipes? I am thinking that if the joints were put under where our internal walls will be they would then be hidden. Would it be best however to continue across door openings too? thanks

[MikeSharp01]

We finally laid ours in 2018 on the hottest day of the year. It went sort of OK build-up exactly as described but the polishing went wrong on about 20% of surface so we had to have it ground off. We had no expansion cracks and we still have no cracks. There are two layers of mesh with the UFH in between and given there is nothing to stop the concrete moving, it is only constrained by the polystyrene former, I cannot see it will ever crack. 

[gavztheouch]

On 05/02/2023 at 10:46, markharro said:

Yes @MikeSharp01 I would be interested to know how this went too. We will have a Passive insulated slab installed in March with steel reinforcement and UFH pipes 50mm under the finished surface which will be the powerfloated slab. Our installer says they won't fit any expansion gaps etc I think because they would be paranoid they might hit one of the UFH pipes. 

That leaves me in a difficult position because how do I minimise any cracking to the floor when this is to be used as the final finish? A quick look at this - https://www.hanson.co.uk/en/ready-mixed-concrete/technical-information/concrete-expansion-joints-explained guidance suggests that @Dudda 's advice about cutting slots is spot on. However with our 150mm slab that would mean ideally 37mm deep slots which is painfully close to the UFH pipes.

 

Actually this reminds me - technically I think I need contraction joints not expansion - https://www.cement.org/learn/concrete-technology/concrete-construction/contraction-control-joints-in-concrete-flatwork

 

Do I ask my contractor to tool these in during the instal to thus avoid completely the risk of cutting pipes? I am thinking that if the joints were put under where our internal walls will be they would then be hidden. Would it be best however to continue across door openings too? thanks

Hi Mark did you end up using control joints ie cuts in the slab. I’m going to do my slab soon. It’s 250mm thick with 4 layers or A393 mesh. Underfloor heating tied to mesh. 

[gavztheouch]

On 05/02/2023 at 21:34, MikeSharp01 said:

We finally laid ours in 2018 on the hottest day of the year. It went sort of OK build-up exactly as described but the polishing went wrong on about 20% of surface so we had to have it ground off. We had no expansion cracks and we still have no cracks. There are two layers of mesh with the UFH in between and given there is nothing to stop the concrete moving, it is only constrained by the polystyrene former, I cannot see it will ever crack. 

Good to know. Our architects own house is a polished slab like this they have done a few others. I will ask them what they have done themselves for the collective knowledge bank. I would hope having a lot of mesh in the slab should help matters

[markharro]

Hi no we didnt in the end - just too much hassle trying to organise. Luckily we have no issues on the 150mm slab. It may have helped that we covered it with sheeting for a few weeks after the pour.