Concrete Slab - Problems, Leveling or Finishing

[Skavenger]

So I have nearly finished laying the floor for my garden workshop.

The concrete that was laid this morning is a little soft on the surface - scratch at it and it is loose. When I laid this to get it the correct height I did move it a lot and there was a lot of water on the surface. I have read that this could have caused a weak surface and if sprayed with water and left it may cure off?

I have just sprayed it with water and will leave it for another day and see what happens.

If it cures OK what do you guys n girls suggest to finish it off should I use some sort of a leveling compound or mix up a scree coat if so what would the mix be. I would like a smooth ish surface that I may paint with and epoxy paint (Grey).

Thanks

 

[KenGMac]

Skavenger, good evening.

Suggest you do nothing at all at present.

New laid Concrete less than 24 Hrs old will scratch, or even if you stand on a new slab and rotate your foot it will leave a mark. Even if the top surface of the new Concrete has been power floated

As far as moving wet Concrete around, generally absolutely no problem, in its fluid form Concrete is said to "Flow" Indeed some Concretes are designed to do just that, further a plastic flowable Concrete is desired for some uses, especially into complex shapes within a Shutter. Never mind Concrete specifically designed to be used in a Concrete Pump.

As far as the wet surface of the Concrete is concerned, it is said that the best condition to "Cure" Concrete is for the Concrete to set under water?

Surface water on a newly laid Concrete slab should be fine, the surface water prevents the top surface of the Concrete from drying out and halts the formation of cracks and Micro-Cracking of the top surface of the Concrete.

As an aside, the Concrete will probably be "Scratchable" tomorrow. It takes a minimum of 28 Days for Concrete to reach its "Design Strength" so obviously in days one, two and three, the Concrete is still surface soft, that is the nature of the beast, up here there is a phrase that states the Concrete is "Green" [slightly soft]

I would advise leave alone, keep the top surface WET!

Ken.

 

[Blagard]

It sounds like you may have "played" around with the concrete too soon with an excessively wet mix. Leave it for now but once it is cured (7 days should be fine) check the surface again. If the surface is still weak then you may need to remove the top surface completely by grinding or scabbling then you could screed it. If you don't get rid of the weak layer then that is what your finish will be bonding to. Expect it to flake off at some point if you don't bother. While I agree with Ken regards OPC concrete should normally reach design strength in 28 days, test cubes are sufficiently cured to test them at 7 days (in advance of a further 28 day test)

 

[Skavenger]

So if it goes hard I can apply a screed coat, do I need a leveling compound or is this in place of a screed coat? I'd like to paint it also.

 

[Blagard]

I am not sure if any levelling compounds are suitable because you plan on using it as a wearing surface. - So I would leave that to others to recommend. A sand cement screed would be fine subject to thickness - Avoid making it too thin.

 

[Doggit]

You can get wearable self levelling compounds, but you need to wait and see how the slab turns out first. Depending on it's thickness, you'll need to leave it a fair time before you add the eopxy, or it'll trap the moisture in, and delaminate the eopxy. But as the epoxy is a hard wearing surface, you may find you can get away with an ordinary SLC should you need to.

 

[KenGMac]

Scavenget, good evening.again

Can I ask what you did to the top surface of the Slab?

Was it trowel finished?

Ken.