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Posted: Mon May 12, 2014 7:21 pm
by r896neo
how did you finish it? did you swipe it with a steel trowel as you went?
It looks a bit like it was trowelled while there was a lot of bleed water on the surface
Posted: Mon May 12, 2014 8:11 pm
by lutonlagerlout
that is what i said at the start neo
we know not to touch it while there is water on it
TBH it looks a fairly strong mix,but if it was towelled too soon it would have pressed the aggregate down
making the top inch that bit weaker
the inclusion of a DPM would also inhibit the curing
have you struck the shutter yet?
be interested to look at the sides
cheers LLL
Posted: Tue May 13, 2014 10:18 am
by Tony McC
Sometimes, I'm so in awe and immensely proud of the incredible expertise that is available completely free of charge in the Brew Cabin. This is *exactly* the sort of thing I had in mind when I set this up all those years ago.
Well done lads - you have proved your worth! 
Posted: Fri May 16, 2014 4:03 pm
by djaychela
Yeah, I'd have to agree with that Tony, it's great to have the expertise, and at least I know better for next time, if there is one!
I've removed the shuttering today, and have to say that it looks OK to me - seems pretty strong lower down, it now appears that the top coating (which is maybe 5mm thick) is weak, and below that it's pretty strong, although it did take a LONG time to cure/set - I rubbed a groove in it with my finger that was about 15mm deep over a week after the slab was laid (it was done on 22 April), but there's no chance of that now.
Anyway, I've taken a couple of shots of the side of it:
One of the short sides.
The longer side along the front - the DPM was against the shuttering here, giving a smoother, darker surface.
Certainly seems hard enough here to me?
So, moving forward, I take it that I should build the shed now, and then once I have the slab covered I can treat it with Sika Pro Seal? Is there anything else I can do to get a smooth surface that I can work on?
Posted: Fri May 16, 2014 4:19 pm
by engcon
Does the shed have a floor or the concrete the floor? Just wondering if you were using the concrete as the floor you could put a soleplate around and do a 50MM floor screed then build the shed
Posted: Fri May 16, 2014 4:23 pm
by djaychela
It's my own design - the idea is to have the floor as the concrete slab. I was hoping it wouldn't get too high due to the limits on building height, etc - it's already 4" above the ground, although I plan on raising the level of the lawn now...
Posted: Sat May 17, 2014 11:59 am
by Brucieboy
The concrete appears to be reasonably well compacted.
With regard to the top 5mm of the slab still being quite weak, does it dust a lot if swept with a stiff broom?
If you removed all the obvious scaly / loose material and applied a hardener such as Pro Seal (which should beef up the surface to minimise further dusting), would you be able to live with a slab that's not perfect but still adequate as workshop?
If a total no no, there are options open such as thin toppings (15 to 20mm) but these usually require scabbling of the surface to ensure a good bond and are not that easy to lay for the inexperienced and also quite costly.
If it's an overlay with good quality concrete, the minimum thickness is 75mm otherwise you risk curling at the edges.
As said by others, the problem has been caused by over-trowelling whilst bleed water is still evident (exacerbated by the likelihood of a blended cement being used, probably with flyash in view of the dark appearance of the concrete - although there are potentially other reasons).
If you can live with an imperfect surface appearance, a hardener is probably the way forward without too much hassle or additional cost. If the slab is more than a month old, there's no reason why you can't apply it now but best check the manufacturers recommended application guidelines.
Posted: Sat May 17, 2014 12:06 pm
by djaychela
Yes, it does dust a lot when brushed.
Now I've got over it a bit, I can live with it being not perfect - after all, it is adequate as a workshop floor. When I originally posted it, I was concerned the whole thing would be that weak, but it's hardened considerably in that time; the place where I made the deep groove with only my finger is now hard.
Obviously I've learned a lesson from this (or a number!) - that I should have left it until the water had bled off, but I guess experience is something you get just after you needed it. I think the hardener route will be the way to go - I think I can remove all the loose/flaky top without too much effort, and from the areas I've done that in the surface is 'acceptable' underneath; my initial worry was that it was all that weak, which doesn't appear to be the case. I'll build the shed and then get the hardener on it as then I'll be able to make sure nothing gets on the surface (it's under trees, and we are in England!)
Once again, thanks to everyone who's given their time and knowledge on here, it's hugely appreciated.