Power floated slab. Now rethinking micro-cement

[flanagaj]

My original plan was to save time and simply install a 125mm slab with UFH attached to the rebar mesh and then get a company to pour and power float the concrete.   That would be the floor finish across the whole ground floor of the house.

But now I am wondering whether

a)  Will I find a company who are good enough to create a power floated finish that will be acceptable as a finished floor, without having to have it polished?
b)  How do you protect the floor as the brickies will be throwing down block and brick remnants as they build up the internal walls? 

 

So I was wondering whether it might be easier and work out similar in cost, to pour the slab without power floating and then apply a 10mm (not sure if that is the depth) micro cement finish.

Keen to hear if anyone has done this.

[nod]

I borrowed a sit on power float for our garage floor Quite easy the following morning 

[Oz07]

You'd have to osb the lot I reckon for decent protection. Probably could source some second hand boards from an old hoarding job. I think the finish would be ok but you'd still need some kind of sealer on it surely?

 

@nod what time of year was your floor done? We've troweled up a garage floor before the next day morning but it was like Jan/Feb, this was after pouring at around 9am too. I don't think you'd get away with it in summer

[-rick-]

I've been looking at this a bit though no practical experience. The conclusions I've reached so far is that if I went with this type of floor I'd want some sort of dry shake on top and to get a decent power floated finish you might want to look at commercial concrete installers rather than your normal crew as these guys are used to dry shakes and power floated finishes. But these guys might not want to touch a small floor or charge a hefty premium for it.

 

Check out https://concria.com/ they seem to have a more developed/modern solution than others that might have benefits and do have certified installers in the UK. I especially like their solution to polishing (if you wanted to do that) as it just uses the powerfloat to do it.

 

If I get the chance to do this and the costs added up I would always be doing it on the basis of it being a trial run with the option of microcementing (or similar) over the top if it didn't work out well. (Need to plan floor levels around doors, etc, very carefully for that to be an option).

 

Unless you DIY the microcement it looks like it's a pretty expensive option for flooring. Tiling might be cheaper.

Edited Sunday at 09:15 by -rick-

[nod]

1 hour ago, Oz07 said:

You'd have to osb the lot I reckon for decent protection. Probably could source some second hand boards from an old hoarding job. I think the finish would be ok but you'd still need some kind of sealer on it surely?

 

@nod what time of year was your floor done? We've troweled up a garage floor before the next day morning but it was like Jan/Feb, this was after pouring at around 9am too. I don't think you'd get away with it in summer

August to September from memory 

Troweled it Then hit it the following morning with the ride on float 

[flanagaj]

4 hours ago, nod said:

I borrowed a sit on power float for our garage floor Quite easy the following morning 

Unfortunately, I just would not feel comfortable doing that job myself.

[BadgerBodger]

So I’ve overseen a reasonable amount of power floated floor finishes and @-rick- has it figured. We use dry shake and a curing agent/sealer. We don’t protect the floors after and build entire power plants atop. There is some damage but not a huge amount considering we drive 120t cranes and 185ft cherry picker on it for 18 months. 
 

If I were doing it in my house (which i likely wouldn’t because of my partners taste but also it’s an f*ing hard surface with no texture underfoot) I’d do as above and use some proplex 3mm. OSB at a stretch. But it is repairable and there are a plethora of products to do so. 
 

Commercial concrete floor layers are definately your best bet. Probably think weekends. 

[-rick-]

15 minutes ago, BadgerBodger said:

So I’ve overseen a reasonable amount of power floated floor finishes and @-rick- has it figured. We use dry shake and a curing agent/sealer.

 

I'm always impressed at no matter the subject this forum always seems to have someone who has been there and done that! Thanks for speaking up.

 

Any views on which products to use/avoid?

 

15 minutes ago, BadgerBodger said:

it’s an f*ing hard surface with no texture underfoot

 

I view it as a base layer to be augmented with rugs, etc, to soften it and hope thats sufficient to offset the hardness.

[BadgerBodger]

Fosroc for the dry shake I’ll dig into the spec for the actual product and I usually see the Adocure products for the curing agent/sealer. 
 

I know the dry shake we had last time was on special order from France. So maybe not that one! 

[-rick-]

6 hours ago, flanagaj said:

So I was wondering whether it might be easier and work out similar in cost, to pour the slab without power floating and then apply a 10mm (not sure if that is the depth) micro cement finish.

 

I thought microcement was expensive enough (ex labour) that you would really want to have a good level surface before even laying it (so would want the powerfloating in any case). The product itself isn't too expensive but the labour is and all the pricing I could find was >£100/m2 for someone to install it (on top of a decent surface). Like plastering and rendering it looks like a job you don't want to DIY as mistakes are very visible.

 

Someone on here ( @Iceverge ??) used a paint/polymer/epoxy product as floor covering that they were very happy with and IIRC was much cheaper than microcement - much less labour to lay.

[nod]

1 hour ago, flanagaj said:

Unfortunately, I just would not feel comfortable doing that job myself.

If you can hire a sit on Its a real easy job My mate drove out to Scandinavia to collect his So I’m guessing there’s not lots of these about 

[Iceverge]

40 minutes ago, -rick- said:

 

I thought microcement was expensive enough (ex labour) that you would really want to have a good level surface before even laying it (so would want the powerfloating in any case). The product itself isn't too expensive but the labour is and all the pricing I could find was >£100/m2 for someone to install it (on top of a decent surface). Like plastering and rendering it looks like a job you don't want to DIY as mistakes are very visible.

 

Someone on here ( @Iceverge ??) used a paint/polymer/epoxy product as floor covering that they were very happy with and IIRC was much cheaper than microcement - much less labour to lay.

 

Not me. 

 

Maybe @IanR

[-rick-]

13 minutes ago, Iceverge said:

Not me. 

 

Damn, sorry for tagging you then. I was ~90% sure it was you and couldn't really think of what search terms to use to verify quickly.

[IanR]

1 hour ago, Iceverge said:

Not me. 

 

Maybe @IanR

Not me either. I did look at it, but went with poured resin directly on the power-floated RC slab.
 

 

[-rick-]

7 minutes ago, IanR said:

Not me either. I did look at it, but went with poured resin directly on the power-floated RC slab.

 

Poured resin is what I had in mind. Thanks.

[markharro]

6 hours ago, BadgerBodger said:

But it is repairable and there are a plethora of products to do so. 

MBC power floated our slab as we told them we were going to use it as the finished floor. Its largely good but some rough areas but it fits in with our aesthetic in the house so its ok....

On a separate point there are some minor cracks here and there - how best to repair these?

[BadgerBodger]

Depends on the size of the crack and potentially, what its cause is. When did it crack? During curing? After? What it when it was loaded. When did you load it after pouring? What was loaded? What were the weather conditions when you poured… yadayadayada the list goes on but each cause may require a different action for “satisfactory” rectification. 
 

Different structures actually have different permissible crack widths, it’s expected to some extent and designing out cracking can result in some hefty tonnages of steel (structure dependent). The problem is, in my experience, most repairs for cracks are fairly destructive involving injection, raking out and in some cases simply recasting. 

 

If they really are minor, you already have a decorative finish and it’s not a water retaining structure, I’d leave it. 
 

. 

[Gus Potter]

13 hours ago, flanagaj said:

My original plan was to save time and simply install a 125mm slab with UFH attached to the rebar mesh and then get a company to pour and power float the concrete.   That would be the floor finish across the whole ground floor of the house.

But now I am wondering whether

a)  Will I find a company who are good enough to create a power floated finish that will be acceptable as a finished floor, without having to have it polished?
b)  How do you protect the floor as the brickies will be throwing down block and brick remnants as they build up the internal walls? 

 

So I was wondering whether it might be easier and work out similar in cost, to pour the slab without power floating and then apply a 10mm (not sure if that is the depth) micro cement finish.

Keen to hear if anyone has done this.

Yes you will find folk that promise you this. 

 

But then if you presented them with a specification and said Gus is going to check their work and hold them to the specification..  they will either run a mile or infate the price. 

 

Do your slab and then put a 50mm screed on top. 

 

I've done hundreds of raft slab designs, well probably (by defualt)  thousands as I was a development SE that wrote the calculations for one of the biggest suppliers in the UK of raft slabs, long before MBC/ Advance Foundation Technology etc came on the scene, and the economy comes from keeping it simple stupid, buldable and allowing you to use local contractors as opposed to specialists.   Part of the stuff I wrote for the software included practical tolerance levels for flatness and level of the slabs. 

 

There is no free lunch here, trying to save on your structrual slab by introducing tighter tolerances is false economy when a self builder. 

 

In summary it might sound counter intuitive but the extra thickness of concrete is small beer. 

[saveasteading]

19 hours ago, -rick- said:

been there and done that! 

Concrete floors always have cracks. If perfectly made these are microcracks, but thousands of them.

Mineral shake makes it harder, perhaps prettier, but it is still industrial.

Any spills will show the cracks and stain the concrete.

You'll gather I am in favour for forklift use but not domestic.

 

I suggest think of 2 stages.

1.Structure and heavy work.

2. Internals and Finishes .

 

It's all been done before and it is very brave to consider innovation.

 

[jack]

We have an MBC slab topped by a ~65 mm polished concrete screed that was installed by a specialist polished concrete company.

 

A few thoughts in no particular order:

 

1. You can't predict the outcome of a pour

The quality of surface finish of a structural slab can't reliably be predicted in advance, and it isn't necessarily to do with the installer. In our case, the slab was poured in January. I believe (without any real evidence) that the concrete we were supplied with wasn't ideal in terms of slump. I think, in particular, that it had too much water in it. In any event, we started pouring first thing in the morning, but even by 11 o'clock that night it hadn't gone off enough to power float. It got very cold that night and we ended up with really bad spalling on large areas of the slab.

 

To their credit, MBC offered to fix at their cost by raising the frame and covering the slab with a self-levelling screed. We went with polished concrete instead so that wasn't necessary.

 

The main point I'm making here is that if something goes wrong with the pour or finishing and you haven't allowed for the additional buildup of a screed to fix, you'll need to think about microscreed or other thin finish to rectify.

 

2. Polished concrete is usually not just "concrete"

As others have said, ideally you should add the right additives to the mix and use a surface hardener (I think our guys sprayed something on right before the power floated, or maybe before they started polishing). I don't know how this all works when you're talking about doing everything based on the structural slab, but if you're getting a specialist in to do the polishing, you should probably be getting their advice in advance, and possibly even onsite input during and immediately after the pour.

 

3. Polishing is best done before the frame goes up

We made the mistake of leaving the decision on flooring until the frame was already up. Large polishing machines can't get right up to edges, and corners are a real challenge. These areas are polished with smaller hand machines which definitely don't do as good or consistent a job as the large machines. We have lots of waviness and visible artefacts along edges and in corners. We've convinced ourselves they add character.

 

We also have a couple of large cracks. We did have crack-relief cuts made in the appropriate places, but because the frame was in the way, the cuts couldn't be made all the way to the edge. This wouldn't have happened if it had all be done before the frame went up.

 

4. Polishing isn't perfect

Along with the edge and corner inconsistencies, we have several spots in our floor where you can see blade marks from power float being stopped. They look like big '+' signs.

 

The underlying aggregate is inconsistently exposed. This is something you might be able to improve by being present during the polishing (and particularly the early part where the coarser grinding is done).

 

1 hour ago, saveasteading said:

Concrete floors always have cracks. If perfectly made these are microcracks, but thousands of them.

 

Yes, and yes! Our floors are completely covered in fine surface microcracks. You can't see them in most places unless you look closely, but they're more obvious where there's wear or staining.

 

5. Concrete doesn't (in my personal experience) age well

This is just my experience, but we've found the concrete to be an absolute bear to keep clean, especially in the kitchen. Every spill, particularly containing oil, risks a permanent stain on the surface unless it's wiped up immediately. Some months after moving in, my wife took the rubbish bag out of the bin in the kitchen and put it on the floor. She forgot to take it out and it leaked overnight. The resultant large and obvious stain is still there over 9 years later. There are smaller stains underneath the bin, in front of the fridge, along the bottom edge of the dishwasher, and at various random points throughout the kitchen/dining area.

 

It's just my personal impression, but it also just feels like it's become a bit grubby over time. It's partly due to the stains mentioned above, but also the polished surface has dulled in areas of high traffic. 

 

To me, there's a difference between a rustic, "honest" finish that shows the construction method (e.g., edge/corner inconsistency and float marks) and the history of the house (general wear, such as more matte areas where the polished surface has worn down over time), and stains that just make the place look dirty. 
 

We're not super tidy people but we are quite clean, so the staining really annoys me. I don't personally see how you can avoid staining. Perhaps regularly applying a sealant would reduce the problem (we've only done that once or twice after the original sealant was applied), but the big stain mentioned above happened only a few months after the first industrial sealant was applied.

 

6. You might be stuck with it

If you really hated a tiled or wooden floor after a while, it would be costly to replace, but it wouldn't necessarily be that difficult to break up a tiled floor or remove a wooden floor. It's potentially a real challenge to do the same with concrete.

 

In my case, I don't know what the process for taking up the whole screed would be. I suppose we could cut down through it along walls and around kitchen units, then chop it into sections for removal. A breaker probably wouldn't be ideal, as there's anti-crack mesh, and I wouldn't want to damage the underlying structural slab and its UFH pipework.

 

Where the structural slab is polished, it might be even worse. You're unlikely to have much space to add a different floor on top of the existing slab. In our case, there's only a few millimetres' clearance underneath a couple of the external doors, so even adding a topping layer such as a microcement screed would be challenging (quite aside from the fact we couldn't afford it anyway!).

 

Probably the best option would be to apply some sort of industrial epoxy topper, but that really limits the end look. 

 

Other thoughts
It's not all bad. In general, it looks pretty good other than in the kitchen where all the stains are.

 

It's very easy to run a broom, vacuum cleaner, or mop over - far less resistance than even a wooden surface. This is great if (like us) you have a dog that sheds hair and drops dirt/dust around the place.

 

It's brilliant for underfloor heating and cooling - it feels nice and cool in summer and warm in winter.

 

Some people think the surface will feel unpleasantly hard, but I don't find that bothers me at all. We have rugs in some places to soften it out, but even without them I don't have any issues.

 

On balance, my wife and I agree that we definitely wouldn't go for concrete again. She'd prefer wooden floors throughout. I'd prefer tiles (either large format, or possibly those wood-effect tiles you can get, although I'd want a closer look at them before committing).

[-rick-]

12 hours ago, Gus Potter said:

Do your slab and then put a 50mm screed on top. 

12 hours ago, Gus Potter said:

economy comes from keeping it simple stupid, buldable and allowing you to use local contractors as opposed to specialists.

 

You make very good points. Doing it this way does indeed make many detail points of setting out the slab easier but AFAIK the surface of a cemfloor type screed is still less 'finished' than a good powerfloated finish. If you want the visual change from a dry shake then you would still have to do this + powerfloat on top of the screed and doing so at time of screed means there are walls in the way and it's a tricker job. So it seems to me you may end up spending more to get this than just doing a slab + level and resin type finish or even tiles if self lay them.

 

12 hours ago, Gus Potter said:

There is no free lunch here, trying to save on your structrual slab by introducing tighter tolerances is false economy when a self builder. 

 

Everything is indeed a trade off. I don't have a site or final design yet so my thinking has all been based on a couple of conceptual ideas but my focus has been trying to think through how to make each stage involving a contract job as simple and straightforward as possible. Basically, leave the fiddly stuff to me either pre-arrival of trades or after the trades have done their bit. So finding a contractor who's bread and butter is commercial power floated floors and presenting them with a slab to pour that has a big flat area with minimal/no obstructions for services, etc, at least at this stage of my research still seems to win for cost/effort/effect.

 

I've said before, but at least for me, the idea of a powerfloated finish is not necessarily the total end goal. It's something that I hope could be done for a relatively small uplift over a normal slab that is sufficient to live with while focusing on all the other details and then when everything else is done is something that could be overlaid if it turns out too much of a compromise and funds are available.

[-rick-]

Thanks for posting @jack lots of useful info here. Apologies in advance for all the questions below! Hope all this is relevant to @flanagaj's original point and I'm not diverting things too far away.

 

38 minutes ago, jack said:

We have an MBC slab topped by a ~65 mm polished concrete screed that was installed by a specialist polished concrete company.

 

Sounds like you actively chose polished concrete over other options for it's look rather than cost. Is that right? If so what guided your decision?

 

34 minutes ago, jack said:

2. Polished concrete is usually not just "concrete"

 

Indeed. Do you remember any more details about what you got? Was the screed a normal cemfloor type? Did the mix have fibres? Did they top it with stones/minerals for the look? Or a dry shake?

 

34 minutes ago, jack said:

5. Concrete doesn't (in my personal experience) age well

This is just my experience, but we've found the concrete to be an absolute bear to keep clean, especially in the kitchen. Every spill, particularly containing oil, risks a permanent stain on the surface unless it's wiped up immediately. Some months after moving in, my wife took the rubbish bag out of the bin in the kitchen and put it on the floor. She forgot to take it out and it leaked overnight. The resultant large and obvious stain is still there over 9 years later. There are smaller stains underneath the bin, in front of the fridge, along the bottom edge of the dishwasher, and at various random points throughout the kitchen/dining area.

 

I've seen in torches used non-domestic settings to burn off the stains from the concrete. Is that a possibility that's been mentioned to you? I have no idea if its something thats even an option on a polished/domestic floor and I assume it would introduce the possibility of cracking. I assume the stain seems to have penetrated deeply so repolishing the area is not an option?

 

34 minutes ago, jack said:

We're not super tidy people but we are quite clean, so the staining really annoys me. I don't personally see how you can avoid staining. Perhaps regularly applying a sealant would reduce the problem (we've only done that once or twice after the original sealant was applied), but the big stain mentioned above happened only a few months after the first industrial sealant was applied.

 

One of the attractions to me was that concrete is a hard wearing surface and it's disappointing to hear you've found staining to be such a problem. Some of the dry-shakes marketing is around making the surface more stain resistant so something for me to look into further.

 

34 minutes ago, jack said:

6. You might be stuck with it

 

I'm definitely of the view that you need to plan up front for the possibility of wanting to cover it in the future and set levels to allow it. Meeting the level entry requirements at the same time is tricky but I think doable.

[saveasteading]

7 hours ago, -rick- said:

surface of a cemfloor type screed is still less 'finished' than a good powerfloated finish.

Indeed. It will be ripped and dimpled and isn't especially strong, so it needs covering. 

But a power floated floor isn't pretty either and still has 3mm dips.

OR you get the surface ground off it and expose the stone.

Don't ever spill red wine.

 

Why, really, do you want exposed concrete? To impress? To be different? A brutalism  lover?

You MUST see a few finished examples before proceeding any further along this line. Then I'd love to read your comments.

[-rick-]

13 minutes ago, saveasteading said:

Why, really, do you want exposed concrete? To impress? To be different? A brutalism  lover?

 

I mentioned some of my thoughts above, but firstly personally, I'm more looking at a lightly ground surface. Not the full exposed chunky aggregate of some polished concrete but also not raw powerfloated. Enough grinding to even the surface a bit and get rid of the powerfloat marks. @flanagaj started the thread asking about a more pure powerfloated finish.

 

I don't have a site yet so everything I'm saying is based on early research not a fully formed plan but when I look at budgeting I think I can afford to build a relatively small property conventionally but I would prefer something a bit larger* and one way I can see of doing that is to strip requirements down to get a building finished and livable and then have a plan to build from there to make it nice as future funds become available. I tend to like modernist verging on minimalist architecture and design so if a concrete finished floor makes financial sense then it likely would fit quite well with what I want to do - again with the proviso that after living there for a few years I may want to add a layer on top. Karndean, resin, even tile if I can preserve that option.

 

* all dependent on finding a site an how much I can get for my current place which due to cladding issues has lost a lot of value (which I hope will at least partially recover once works are completed)

 

For reference, my current thought is to use the concria system:

https://concria.com/decorative-concrete/

https://concria.com/polishing/

 

Somewhere between their 3 step and 7 step finish, not overly keen on any of their colours, other brands have better options from what I've seen so nothing finalised.

 

Quote

You MUST see a few finished examples before proceeding any further along this line. Then I'd love to read your comments.

 

Of course, I'm still at the research phase, once I get a practical project then that is on the list to do. @flanagaj is further along with his project so keen to hear his thoughts.

[saveasteading]

Thanks for the link. It looks good in the pictures of course.

I'd expect that the ground surface needs to be sealed.

Also for it to be hideously expensive. Have you asked?

 

I once had a project that required grinding, albeit for super flatness, not appearance. The grinding cost about as much as the concrete.

 

Power floats are big, with a circular sweep. How do they reach into corners?