Polished concrete with UFH

[Rishard]

I’ve been ‘floating’ the idea of a polished concrete ground floor slab. The build up would be hardcore/insulation/ufh within the slab. Thickness yet to be decided. 
 

Has anyone had any success with this? When I speak with my local concrete/screed pump suppliers they said they have stopped offering it as a service due to the concrete cracking due to expansion/contraction. He said it is possible but carries with it the cracking risk, even outside of the usual control joints. 
 

Would love to know if anyone has successfully achieved a polished finish with UFH as it seems like a good option for the slab/finished floor. 
 

 

[saveasteading]

All concrete shrinks and cracks. The skill is to have millions of tiny cracks that are invisible. That is done by concrete mix and reinforcement design.

 

Power floating is a skill. The machine is big and powerful and is difficult to control, especially in tight areas. In a warehouse there is a big central area, and the odd edge not quite polished doesn't matter. In a house there is a lot of edge and little middle.

 

There is no secret to successful slab construction, but there is a lack of knowledge. 

Mix design, mix control, precise preparation, and skilled workers.

DIY and the local builder, and it is likely to crack. And polished concrete shows up cracks beautifully.

 

Yes, UFH pipes will complicate the issue slightly, as regards crack control, but the main worry is damage from careless workers or impact from a tool or concrete pump nozzle. That may be a good reason for contractor's giving the sensible advice to be wary.

 

I suggest looking closely at the concrete floor of any local retail park shop. Almost certainly there will be cracks, often roundish shapes, coin sized.

 

If I had to provide a polished floor, I would first check that the client was prepared to pay the high price, and then suggest a specialist laid screed, which is done after the floor has done its shrinking.

But don't let me talk you out of it...it is my default to aim for good value and no gimmicks. 

 

 

[Rishard]

Thanks, the supplier even advised against a screed in this situation as he said it doesn’t hold up particularly well as a finished floor. Seems lengthy to lay a slab, screed and then tiles ect. I gather it may work out more cost effective that way. 
 

Small cracks could look fine with an industrial looking polished slab. I’ve never seen any on the slabs I’ve poured in the past which have been no bigger than 50m2 but Sod’s law says it’ll crack if you want it to be seen as a finished floor! 

[Dudda]

I've put polished concrete floors into a lot of projects. Huge polished floor to the atrium of a Performance Theatre, Several Universities Projects, lots of houses and even my own house. Many of the projects won prestigious RIAI Awards. I've also seen polished concrete floors by others go horribly wrong. Some put in the wrong reinforcement, some use the wrong mix, etc.

 

 

I wrote the below for someone at work but is relevant for you too. If you've any questions on any parts let me know.

Concrete
The concrete is 35N10 which is reinforced with plastic fibres at 900g per cubic meter of concrete. As a polished floor goes in later it might need to also be a pump mix as you’ll have doors, windows and roof on. Whoever you get to do your floor will help input on this.

Placing of concrete
It’s not just the grinding and polishing of the concrete that’s important. The placing and power floating of the concrete is critical. Unlike a regular concrete floor, a floor poured which will be polished has to be super flat and power floated for hours after. Very few people in Ireland can do this correctly. Formwork for steps and other edges has to be perfect and have 45 degree edges to allow trowels into the corners. Little things like dragging a vibrator, shovel or rake through a floor when placing the concrete the wrong way will result in the drag mark being visible when the floor is polished. These things may not be visible to the untrained eye but are flaws in a lot of cheap or poorly installed polished floors that can never be fixed. Spend time and money on this part as a poorly poured floor can’t be fixed by grinding and could make the errors even more visible.

Crack Joint
These have to be around every 6 meters max and are a saw cut made the day after in the concrete about 30-50mm deep which is later filled with a flexible mastic. Don’t have any areas too small as they could rock like paving slabs. Don’t have them long and thin either as they can crack in the middle like a seesaw. If you have a few pipes in the floor, eg heating pipes coming from a manifold it’s a good idea to have a joint here. It’s also good to line these up with any columns you have. Hide them under lightweight internal stud walls and have them at all doors, etc. You have to think about underfloor heating pipes and ensure they’ve sufficient coverage before cutting the crack joints. That’s where the thickness of the floor is important.

Expansion joint
Not to be confused with a crack joint these are flexible day pour joints and are also positioned above expansion joints in the structural floor below. As a result they’re slightly wider than a crack joint but again are filled with flexible mastic. For a house you probably won’t have one however we had one on a larger project I worked on a major project.

Coloured Stone
This is optional. I’ve done it in some projects but not my own floor. Make sure you get the contractors to quote for 4-5 samples if you plan on using coloured stone as you don’t want to get a claim later. You can pick whatever stone you want and also the size of stone. Usually they’re sprinkled over the top during powerfloating. They can be mixed through at the concrete plant but you’ll use a lot more coloured stone and it will be a lot more expensive. Glass which can also be used has to be lead glass as ordinary glass can shatter when grinded.

Slip Resistance
The slip resistance of polished concrete usually complies with all regulations. Like any regular floor it can be more slippery when wet. Additional grinding and polishing doesn’t necessarily impact the slip resistance of the floor so thinking a shiny polished floor will be a lot more slippery isn’t right. As it’s a domestic situation I wouldn’t worry about this however if it’s a public building you’ll need to include in your specification a slip resistance pendlium test to be carried out at the end of the project to prove the floor is safe. This is a useful document to have in the safety file should someone slip in the future. If you want to get it carried out for peace of mind you can.

Sealing
Make sure you ask in your quotes for sealing the floor after it’s grinded and polished as the concrete is porous and will absorb tea/coffee spills and heavy traffic if not sealed. The sealing will make the floor a tiny bit darker but in my opinion is worth it.

Protection
Include in the spec for the contractor to protect the floor after its poured for the duration of the works. We had two layers of cardboard over the floor for three months. It dired out more in areas where it wasn’t protected and at joints but once exposed the variation disappeared. Just ensure it’s well protected everywhere particularly if its not yet sealed as any paint spills etc, will destroy it.

Grinding and polishing
You can very lightly grind the floor and then polish it as normal if you don’t want to expose aggerate or alternatively grind it to expose the aggerate and then polish it. Grinding a floor is time consuming and therefore expensive so include for grinding the floor to expose a lot of aggerate and for a lot of polishing. You can then later decide with samples not to grind as much or go for a reduced polish. If you don’t specify the amount of grinding required they’ll assume a very light grinding with no aggerate revealed. The floor will be grinded in stages starting at 10 grit working through 15 grit, 25 grit, 50 grit, 100 grit, 200 grit, etc. 800 grit is matt finish and 1500 grit would be a medium shine with 3000 grit a high shine. If you want lots of stone aggerate exposed they’ll spend longer at the 10 and 15 grit before moving up.

Reinforcement
We used a plastic fibres reinforcement. You can get thin ones and heavier ones but the heavier ones can appear if you look closely in the finish. It’s 900g per meter cubed that use used which is standard. Don’t use steel fibres and don’t use steel mesh. I researched a lot of failed concrete floors and one major issue with steel mesh is when they cut the crack joints they don’t cut through the steel mesh so the floor can’t crack at the crack joints. The joints are still reinforced and therefore crack elsewhere.

Perimeter
You’ll need flexible insulation 12-15mm thick around all perimeters and at columns, service popups and penetrations, etc. Don’t use rigid insulation as this won’t allow the slab to move and therefore risk cracking. Another issue is with stone walls which are very uneven require thicker flexible insulation. If you intend to dryline do this after the floor is poured and you’ll hide the perimeter insulation easily.

Thickness
Our floor is 100mm thick and that’s the optimum. Don’t go below 75mm as below this is too thin and you’ll risk cracking. If the floor changes thickness anywhere you’ll need additional crack joints. For example if you’ve steps, ramps or around all recessed matwells as you have a change in thickness in the concrete.

Services
All electrical trunking and mechanical services should be cut out of the insulation below the concrete floor. If a 50x150mm electrical trunking was placed on top of the insulation the concrete would crack over the trunking. Where we had several pipes close to each other in the floor we had to include a steel plate to rest over to ensure the concrete remained 100mm think and didn’t flow down between the pipes increasing the depth but also preventing the slab from moving.

Underfloor heating
This works great with polished concrete floors due to the thermal mass of the floor and the slow release of heat. It’s important to have the floor well insulated so put in as much insulation as you can afford otherwise you’ll be paying to heat the ground under the building. Another key point is to ensure the underfloor heating pipes are firmly clipped down to the insulation as any pipes which become loose or if the plastic staples become loose they can be exposed or damaged in the polishing of the floor. Also you’ll be cutting into the slab for the crack joint and if the pipes rise a bit you risk cutting them.

Edited March 10, 2022 by Dudda

[Ajn]

Our polished structured concrete floor with UFH has been down about 18 months and no damage that we have noticed, heating has been on for about four.

Issues we had were with the builder we used to get us to DPC and the subfloor work suffered from slope in his spirits and bent string.

The outside walls were woodcrete ICF or glass.

All partitions were done after the floor.

Glass frames were done after the floor.

The finished floor level was set to the threshold of the sliding doors (bedroom 4.5m living room 7m)  and the main enterance door which was part of a 9m glass wall.

To enable the floor to be finished at this level I started with 150x50 timber which was reduced in thickness by a Aldi bench plane machine (took a while) to the correct thickness and then screwed to the outer blockwork in the correct place. The concrete floating could then be done over the top of these levels.

Next problem was the shower drains. Two were required. I couldn't find anything abvailable in the uk. So I modified two very expensive slot type drains from screwfix and set levels using the same process as for the doors.

The main man from the floor supplier was more than happy as his power floaters had unrestricted access.

The floor is 150mm with two layers of steel mesh and spacers. The UFH is between the mesh, I think.

There are a number of expansion slots cut in the floor but most of these are under the intended partitions and edge of the mesh.

The man incharge of the men that came to float the floor said we cant put a slope in the floor for the showers as concrete is flat. I pointed out that the other main man said you could.

The two nice men doing the floating said they would come back later and do those bits by hand.

They spent a long time power floating, well past midnight. Then a few weeks later the polishers came.

 

Polished concrete is NOT a quick process. Our is a darker grey and is a fine finish with just the odd bit of aggregate showing.

 

Edited March 10, 2022 by Ajn

[Rishard]

@Dudda- thanks for your informative response. There is a lot of very helpful information in there. Am I right in thinking you didn’t use any reinforcement mesh in your floor? Is this also your main slab or is this a 2nd pour over a structural ground bearing slab? Next step is to try find someone comfortable in placing and grinding the slab to this standard. 
 

@Ajn- are any of your partitions structural walls or just timber partitions? I think I follow your timber support. Was that for the concrete to be screeded off to give the correct heigh of your ffl? Was your slab running over the top of your internal blockwork? In your shower room, how did you achieve a fall to your drains ? I assume the whole ground floor slab is flat and you had your slot drains set slightly lower? I’ve seen Americans sink a tub into their wet rooms to allow for a trap to be fitted below the ffl height. Was the bathroom shuttered off and poured separately to be worked by hand? Do you have any in-progress or completed pictures? 

[Ajn]

2 hours ago, Rishard said:

re any of your partitions structural walls or just timber partitions? I think I follow your timber support. Was that for the concrete to be screeded off to give the correct heigh of your ffl? Was your slab running over the top of your internal blockwork? In your shower room, how did you achieve a fall to your drains ? I assume the whole ground floor slab is flat and you had your slot drains set slightly lower? I’ve seen Americans sink a tub into their wet rooms to allow for a trap to be fitted below the ffl height. Was the bathroom shuttered off and poured separately to be worked by hand? Do you have any in-progress or completed pictures? 

No structural timber partitions. Timber was glued to the finished floor. Yes Timber in place of frames gave the ffl. No internal blockwork. The nice man with a small power plane and hand trowel did the slopes. Whole floor is one level and drains were set lower allowing for a slope to the drain. It was all done in one pour, just under 100sqm. No pictures on this device but I will see what was taken.

[Dudda]

I

3 hours ago, Rishard said:

@Dudda- thanks for your informative response. There is a lot of very helpful information in there. Am I right in thinking you didn’t use any reinforcement mesh in your floor? Is this also your main slab or is this a 2nd pour over a structural ground bearing slab? Next step is to try find someone comfortable in placing and grinding the slab to this standard. 

Correct I didn't use reinforcement mesh as it was non structural or 2nd pour. Buildup I've used is sub floor, insulation and then polished concrete. I can't think of a project off hand where I've had it as structural. The guy who sits behind me at work did do it as a main slab polished structural floor in his own house last year. I'd be a bit worried about it getting damaged. Otherwise no reason you can't do it. Put down a few sheets of ply for fear a block or lump or steel would take a chunk out of it. Also don't let the water in the underfloor heating pipes freeze and crack the slab. 

[Rishard]

My main thinking was to reduce the need for double layers of concrete. Is it best to pour and polish it while the building is at dpc level to make it easier to power float the whole area? 

[7dayworker]

On 10/03/2022 at 18:25, saveasteading said:

All concrete shrinks and cracks. The skill is to have millions of tiny cracks that are invisible. That is done by concrete mix and reinforcement design.

 

Power floating is a skill. The machine is big and powerful and is difficult to control, especially in tight areas. In a warehouse there is a big central area, and the odd edge not quite polished doesn't matter. In a house there is a lot of edge and little middle.

 

There is no secret to successful slab construction, but there is a lack of knowledge. 

Mix design, mix control, precise preparation, and skilled workers.

DIY and the local builder, and it is likely to crack. And polished concrete shows up cracks beautifully.

 

Yes, UFH pipes will complicate the issue slightly, as regards crack control, but the main worry is damage from careless workers or impact from a tool or concrete pump nozzle. That may be a good reason for contractor's giving the sensible advice to be wary.

 

I suggest looking closely at the concrete floor of any local retail park shop. Almost certainly there will be cracks, often roundish shapes, coin sized.

 

If I had to provide a polished floor, I would first check that the client was prepared to pay the high price, and then suggest a specialist laid screed, which is done after the floor has done its shrinking.

But don't let me talk you out of it...it is my default to aim for good value and no gimmicks. 

 

 

Thanks for that info. It has pushed me to a different flooring . It all looks easy on the tv shows…

[saveasteading]

1 hour ago, 7dayworker said:

has pushed me to a different flooring

Are we Philistines, discouraging challenge and aesthetics? 

No I don't think so. Letting people hear the pros and cons.

[Conor]

On 13/03/2022 at 20:03, Rishard said:

My main thinking was to reduce the need for double layers of concrete. Is it best to pour and polish it while the building is at dpc level to make it easier to power float the whole area? 

I did the sums and there is little or no saving. You're talking £100m² + for the finished polished floor. Think insulation plus screed was about £35m², then £50m² for your tiles and it's still under the cost of the polishing. 

[Rishard]

I’ve just listened to this podcast which touches on their experience with polished concrete in a passive build. The end results look great. It isn’t ufheated but I think if your willing to except some defects then it could be done as a cost effective use of the structural slab as finnished floor. He did spend 90 hours grinding it which would have cost a fortune for a specialist company to do but in the spirit of self build I would say don’t rule it out, just lower your expectations… If it’s that bad, allow for a thin engineered flooring to cover it up. 
https://www.houseplanninghelp.com/hph310-important-takeaways-from-an-exemplar-straw-panel-passivhaus/

[saveasteading]

1 hour ago, Rishard said:

the structural slab as finished floor.

Don.t ever drop red wine/jam on it. OR apply lots of sealer and top it up regularly.

 

Also, depending on location, some concrete always has lignite in it (fossilised wood in the gravel) and it floats to the surface.

Not good in an industrial slab, but awful in domestic.

[Rishard]

I would say the same for a wood floor or a carpet… Don’t drop wine on it, it’ll stain. I imagine it probably does stain the concrete over years of family abuse but that’s an aesthetic choice surely? Personally if I had an industrial looking floor like the one in the video, if it had a few dings and scratches in it, all the better. If I wanted a mirror flat finish, I would either have to pay a specialist or choose not to do it.
 

 

[markharro]

On 10/03/2022 at 20:06, Dudda said:

Crack Joint
These have to be around every 6 meters max and are a saw cut made the day after in the concrete about 30-50mm deep

Hi @Dudda we are planning to have our 150mm insulated passive slab as our finished floor surface in our new passive house. There will be UFH pipes laid 50mm below the surface. The company laying the slab will power float it to finish but have cautioned that they cant guarantee the finish. They have also raised the issue of cracking and recommended "expansion joints" but said that they wouldn't install these. From some reading I think I would probably need these crack or contraction joints you mention but my concern would be hitting a UFH pipe. Our slab will be on 2 levels and a total of about 150m2. Is cracking inevitable without joints? How bad could it be? Can cracks be filled if they occur? thanks

[Sharonj472]

On 12/04/2022 at 19:12, Rishard said:

I would say the same for a wood floor or a carpet… Don’t drop wine on it, it’ll stain. I imagine it probably does stain the concrete over years of family abuse but that’s an aesthetic choice surely? Personally if I had an industrial looking floor like the one in the video, if it had a few dings and scratches in it, all the better. If I wanted a mirror flat finish, I would either have to pay a specialist or choose not to do it.
 

 

 

On 13/03/2022 at 19:15, Ajn said:

No structural timber partitions. Timber was glued to the finished floor. Yes Timber in place of frames gave the ffl. No internal blockwork. The nice man with a small power plane and hand trowel did the slopes. Whole floor is one level and drains were set lower allowing for a slope to the drain. It was all done in one pour, just under 100sqm. No pictures on this device but I will see what was taken.

Hello @Ajn just reading your post and interested how you waterproofed your walls in the wet room areas where you installed a linear drain? We are looking to do the same but I’m concerned water will get under stud walls.

[Ajn]

Sorry not been on for a while. Elswhere in the build we used a couple of special tile trays (they look good but) from Topps tiles. These were new stock to them and while ok they did also provide a waterproof paint substance that is intended to be used under these shower tiles when installing on a wooden floor. So while the timber frame was glued to the concrete we put libral coats of this on the timber frame. We also used tile board on the frame.

 

[Gus Potter]

On 10/03/2022 at 18:25, saveasteading said:

All concrete shrinks and cracks.

Correct.

 

What about sitting on the fence. Plan your UF slab on the basis that you are going to polish it. Design it, cure it and turn the heating on. If you have made an error.. not achieved the correct quality control etc, lay a timber floor / carpet over to hide the cracks.

 

If not cracking what about using a polished concrete overlay, yes I know it's not the real deal but who is really going to know? I was at a job where they used an overlay on a normal reinforced concrete wall, you could not tell the difference and in fact looked great cf some (a lot of) the rubbish concrete jobs you see. It's one of the few times I've see a concrete finish good enough that I would have in my own house.

 

Below is a random link to overlays.

 

https://nichepolishedconcrete.co.uk/overlays/

 

 

 

 

[SarahHutch]

On 19/11/2022 at 14:08, markharro said:

Hi @Dudda we are planning to have our 150mm insulated passive slab as our finished floor surface in our new passive house. There will be UFH pipes laid 50mm below the surface. The company laying the slab will power float it to finish but have cautioned that they cant guarantee the finish. They have also raised the issue of cracking and recommended "expansion joints" but said that they wouldn't install these. From some reading I think I would probably need these crack or contraction joints you mention but my concern would be hitting a UFH pipe. Our slab will be on 2 levels and a total of about 150m2. Is cracking inevitable without joints? How bad could it be? Can cracks be filled if they occur? thanks

Hi @markharro I was just wondering if you have installed this yet. And if so how it is looking/performing. I am also interested in using this approach if at all possible.

Many Thanks

[cheekmonkey]

We've recently done something similar.

 

150mm powerfloated finish (Not polished - i'm not sure why you would?)

 

mesh in bottom 40-50mm with UFH pipes tied to the mesh. so circa 100mm on top but some places a touch less where levels werent perfect.

 

we have saw cuts made throughout the slab the following day circa 30mm deep, crisscorssing the whole building. we have chosen not to mastic them as quite like them, but have mastic sealed other places as we have the old concrete barn protuding through the floor. most builds probably wouldnt have as many cuts but with the fixed structure with 8 posts we felt safer to cut to each to allow it to crack in all directions

 

we have only got one major crack not down the cut line which is right down our hallway but i think probably due to the hot water supply to the other end of the house running under that bit. although that's even under the mesh at the bottom it must have had an impact

 

Edited June 11 by cheekmonkey
added pics

[AppleDown]

1 hour ago, cheekmonkey said:

We've recently done something similar.

 

150mm powerfloated finish (Not polished - i'm not sure why you would?)

 

mesh in bottom 40-50mm with UFH pipes tied to the mesh. so circa 100mm on top but some places a touch less where levels werent perfect.

 

we have saw cuts made throughout the slab the following day circa 30mm deep, crisscorssing the whole building. we have chosen not to mastic them as quite like them, but have mastic sealed other places as we have the old concrete barn protuding through the floor. most builds probably wouldnt have as many cuts but with the fixed structure with 8 posts we felt safer to cut to each to allow it to crack in all directions

 

we have only got one major crack not down the cut line which is right down our hallway but i think probably due to the hot water supply to the other end of the house running under that bit. although that's even under the mesh at the bottom it must have had an impact

 

 

 

Did you seal it with anything?

[saveasteading]

2 hours ago, cheekmonkey said:

(Not polished - i'm not sure why you would?)

Harder, smoother.

But yours looks polished to me. It is done with the same machines, just after the surface is hard enough to walk on, with a different angle on the blades.

 

2 hours ago, cheekmonkey said:

we have chosen not to mastic them as quite like them

These are very neat straight cuts.

the reason for filling with mastic is to keep biscuits and other detritus out of them, and to restrain the edges which will otherwise get hipped away over time (with chairs dragging etc).

Theoretically it keeps the crack free of obstructions , should the slab ever want to expand again...but they don't expand again.

[markharro]

21 hours ago, SarahHutch said:

Hi @markharro I was just wondering if you have installed this yet. And if so how it is looking/performing. I am also interested in using this approach if at all possible.

Many Thanks

Yes it was done last year. It worked well. We cut no line in the end and so far I think have been lucky as no evidence of cracking. I will look out some photos

[cheekmonkey]

15 hours ago, AppleDown said:

Did you seal it with anything?

No not yet. Osmo do an oil but its quite a faff to do now we've moved in. It's marked a bit in kitchen area from drops of grease but it doesn't really show due to the nature of the natural imperfections in the finish