Screed deflection and microcement curing time

[Adsibob]

Our screed is about 75mm thick. It was poured in early October. Apart from one 3.5m long, but very thin crack which developed on day 125 or so, it seemed to cure very well. Around day 70 we started putting gentle heat through it. Well, I say gentle but my manifold can only do 35C as a minimum. Maybe that was a bit early and what caused the hairline crack, I don’t know. 
Around day 150 the company that was going to install microcement over the screed came and “stitched” the crack and also filled a couple of small holes that I’m not sure how I missed because they were almost 2cm in diameter. We also did about 2mm of self levelling compound to raise a very small area near a threshold. About 2 weeks later they installed the microcement, which consists of a thin mesh, topped with various layers of microcement which adds about 3.5mm to 4mm to the Screed. The microcement was laid during 3.5 days and sealed on the last afternoon (first coat of sealer) and on the 4th day (second coat of sealer). The installer said I could walk on 2 hours later without shoes on, and with shoes on the day after. He told me to leave it uncovered for a week so that it can breathe.

I then had a dispute with the company because I realised the installer had ruined various walls that had been painted a month ago by not using low tack masking tape. So there hasn’t been much by way of aftercare in terms of customer service.

 The dispute caused me to read the small print of their website Ts and Cs and I spotted that it is recommended that during the first 72h after installation one maintains the ambient temperature in the room above 18C. It also says not to do this with UFH or with any heating that will dry the air. Not exactly sure how one is meant to do that.

 The house isn’t fully air tight yet, so even with UFH heating on in the two areas adjacent to the room where we have had microcement installed, it doesn’t get Enough heat into the room with underfloor heating. At night, the microcement is around 11C and during the day it gets to 15.5C. 
I found another website which talks about curing microcement in general and it also says about keeping it above 18C for the first few days, but also says that if you can’t do this all that will happen is that the curing will happen more gradually.

The company who installed the microcement say that after a well one can turn the UFH on gradually. Should I delay that by a few days to account for the slower curing time at 11C to 15.5C ?

 What about floor deflection? I’m due to get a stove installed next week which will sit on a heavy steel plate that will be supported by three concrete plinths cast onto the floor. Total weight of the steel top and stove is about 150kg, though it is spread out onto the three plinths which are about 70cm long each, so shouldn’t really cause too much deflection I hope. Would you delay the stove installation to allow the microcement to fully cure? 

[nod]

No

i wouldn’t delay it 

As a tiling contractor I break All curing 

RECOMMENDATIONS on a daily basis 

With little or no consequence

We are currently tiling the floors of two 1.3 million pound homes Screeds have been down three weeks 

No gas to fire up the UFH 

Both have to be handed over in April 

It currently snowing outside 

[Gus Potter]

Interesting topic. before you go for these types of floor finishes please consider this. I hope this is helps a bit.

 

To start from the top, these types of polished floors are expensive and you don't want them to fail.

 

If you have a beam and block floor say the following is not applicable.

 

If you have a ground bearing slab, maybe the UFH is in this slab. Under this could be insulation, or it could be on top but all generally rests on a bit of hard core on virgin ground. Now if the ground is a sand then this tends not to shrink settle so  much, but if clay it will shrink and settle by different amounts, often more than a sand type soil. Clays shrink when they lose moisture. Thus when you put a roof / building over clay the moisture content will drop. Sand is less effected. But there is more so you are not off the hook yet.

 

You builder or you may put down a layer of hard core and compact it well but the soil underneath will still move about, it is not your builders fault.

 

For the avoidance of doubt you often see specified for a ground bearing slab that is not carrying other loads "anti crack" reinforcement.. could be say an A142 or A193, sometimes A252 mesh.

 

This description "anti crack" is acually a misnomer for the unwary. All concrete cracks and during the curing and hardening process the forces generated in the concrete are quite large. Steel is quite stretchy! Actually in the modern design codes we talk about "crack limitation" , "crack control" They recognise that your structural slab is going to crack and that you design to limit the crack size to what is acceptable. Now if you are designing a nuclear waste storage facility you want to make the cracks as small as possible, and if you are putting a polished concrete type screed over the top you need to take the same approach. Thus you often need to spend some extra money early on and design all the stuff underneath for your fancy / expensive floor finish.

 

Next when you pour a screed or slab you get an effect what is called curling in the concrete @saveasteading has some stuff on this. I experimented on my own house to see if this was just applicable to big stuff or not.. turns out that even for my DIY  (even on a 3.5 x 3.0m slab) job the theory is correct. What happens is that during the curing / drying process the top of the slab shrinks more than the bottom and this causes it to bend up, particularly the corners. I went round with a bit of wood tapping the floor and it was solid in the middle and sounded hollow at the corners. Now I know if I add a bit too much load to the floor at the corners it will crack until the mesh takes up the load. I'm not fussed as I have a wooden floating floor so if the concrete does crack we won't see it.

 

So the key things for me are that if you are intending to have polished concrete floors / fancy finishes then you need to spend the money on designing from the virgin soil up. The companies that provide these floor finishes know this and I have touched on the ins and outs.. you have little chance to if any to make any claim against them unless you have a grasp on the design from the virgin soil up.

 

In summary, you can't polish a jobby. You can take the risk of having a cheep standard slab and steel mesh say A142 under and have confidence that you can gold plate it with polished concrete.. but much will depend on how much shine you can apply.. all shine wares off eventually.

 

 

 

 

 

 

[saveasteading]

1 hour ago, Gus Potter said:

@saveasteading has some stuff on this

The above advice is all good, and I feel that I am expected to add something.

 

Two more thoughts.

1. People often have too much expectation about beautiful finishes. Our houses are made of ground and plants, in all sorts of weather and in the middle of a field:. Very little can be perfect.

The floor is built and it is not going to settle or float away. It is safe to walk on , jump on and support a stove. If it cracks a bit more it won't go anywhere, but you may have some cracks which you can cover..

 

This only really presents a problem if the industrial finish of the concrete is intended to remain uncovered. Look at the floor in a retail outlet shed and there are normally millions of visible cracks.

 

2. You can fit floor finishes when it suits after just a month or so.  The times stated for concrete to cure and dry are generally extremely conservative. This gives all the contractors an excuse if there are problems, and also sells sealing products (which trap any moisture in for ever).

 

The exception to all this if the concrete slab is laid with too much added water (which makes it easier for the contractor to handle) This evaporates and leaves the equivalent as voids in the concrete, which will crack more and may be soft on the surface.

 

3 hours ago, Adsibob said:

maintains the ambient temperature in the room above 18C.

That would be nice. This is Britain.

Not turning the heating pipes on too early is essential as it will upset the chemistry of curing, but much lower temperatures (above 5?) will be fine. I would leave the UFH off for at least a week.

[Adsibob]

This is helpful thanks. Our build up of the floor in question is:

 

3.5mm of microcement

0.5mm mesh specified by microcement company

70-80mm of sand and cement screed containing UFH pipes

100mm tacker insulation

DPM

sand blinding

150mm of concrete slab containing a A142 mesh to BS4483

machine compacted hardcore

London clay

 

I take comfort from the fact that due to various delays, the concrete slab was laid a year before the microcement and the screed was laid about midway through that year, so hopefully everything has been done in a fairly gradual manner, with the only mistake being that we turned on the heating only 10 weeks after the screed was poured, whereas it probably would have been best to wait to 13 or 14 week point.

 

So given the stove and steel table top is not that heavy (150kg in total and supported on three sides by concrete plinths that were cast a few months ago), I think the conclusion is to proceed with the steel tabletop and stove installation next week, but to delay switching on the heating till a week or so after that. Is that right?

 

 

[SimonD]

I can't seem to find actual dates. When was the microcement done? How many days has it been now? General rule of thumb for my microcement was allow to dry for 48 hours with no use. Light use only for 7 days. Do not cover and wet for 14 days. Fully cured in 28 days. Recommended Minimum application temp 11 C. Drying times are based on certain humidity/temps only.

 

HTH

[SteamyTea]

11 hours ago, Gus Potter said:

What happens is that during the curing / drying process the top of the slab shrinks more than the bottom and this causes it to bend up, particularly the corners

This is giving away the secrets of successfully making composite plastic components. A combination of chemical changes that can cause uneven shrinkage, mechanical alignment that can warp components, making them seemingly 'grow' in place, evaporation that is uneven due to varying matrix thickness, or shape. Mismatch between the matrix components.

The list goes on.

As a mate of mine who worked for International Paints (industrial division) once said '(expletive deleted) I have it easy, just tell the (expletive deleted) to not put it on too thick, and no drips'.

[Adsibob]

1 hour ago, SimonD said:

I can't seem to find actual dates. When was the microcement done? How many days has it been now? General rule of thumb for my microcement was allow to dry for 48 hours with no use. Light use only for 7 days. Do not cover and wet for 14 days. Fully cured in 28 days. Recommended Minimum application temp 11 C. Drying times are based on certain humidity/temps only.

 

HTH

Microcement installation started on 22nd March and was finished by 25th/26th. I think all the microcement itself was finished on 25th, the 26th was just sealant. But we've not been able to follow the guidance to keep the room above 18C. We have kept it uncovered with no traffic on it for first 48h and only light traffic since. I won't turn heating on for another couple of weeks, but my doubt remains whether I should postpone the installation of the stove. The plinths of concrete which support the steel tabletop which support the stove, are also covered in microcement, installed at same time as floor.

Edited March 31, 2022 by Adsibob

[Huckleberrys]

How did this all go? Any issues with the microcement? I am thinking of a similar finish but wanted some real world people's experience. Thanks