Just looked at the floor slab in the shower area that we laid 2 weeks back.

I was laying it with an assistant doing the mixing - basic C20 (1:2:4) mix.

Unfortunately some of the mixes went down too wet - seemed fine in the
barrow, but once it had been sitting on top of the DPM sheet and been
tamped down, a couple of mm of water appeared on top.

The end result is that the surface is quite weak - as in the surface can be
abraded with hard finger pressure.

The same mix was also used outside the DPM to fill some small voids under
the adjoining old floor slab and these bits are as hard as iron as
expected - I presume because the excess water could drain away into the
base sand.

Not really wanting to lay this again... Is this likely to be a surface
problem or is the whole slab likely to be weakened?

The aim is to put celotex on top then screed to 60-70mm - so 2nd question -
is any of this likley to matter that much? My default opinion is to wash
some dilute PVA over the top to firm up the surface then proceed as
planned. There is mesh in the slab and it's 100mm thick on a well packed
base so I can't actually see it going anywhere.

Opinions? Shame because I'd got it *very* level

Cheers

Tim

Tim S wrote:

The aim is to put celotex on top then screed to 60-70mm - so 2nd question -
is any of this likley to matter that much? My default opinion is to wash
some dilute PVA over the top to firm up the surface then proceed as
planned. There is mesh in the slab and it's 100mm thick on a well packed
base so I can't actually see it going anywhere.

Opinions? Shame because I'd got it *very* level

Given this is a sub floor I can't see it being a problem. Very little
point pressure will be placed on it through the celotex. You will also
find it gains in strength with time.

--
Cheers,

John.

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On 28 July, 02:10, John Rumm wrote:
Tim S wrote:
The aim is to put celotex on top then screed to 60-70mm - so 2nd question -
is any of this likley to matter that much? My default opinion is to wash
some dilute PVA over the top to firm up the surface then proceed as
planned. There is mesh in the slab and it's 100mm thick on a well packed
base so I can't actually see it going anywhere.

Opinions? Shame because I'd got it *very* level

Given this is a sub floor I can't see it being a problem. Very little
point pressure will be placed on it through the celotex. You will also
find it gains in strength with time.

--
Cheers,

John.

/================================================== ===============\
| * * * * *Internode Ltd - *http://www.internode.co.uk* * * * * *|
|-----------------------------------------------------------------|
| * * * *John Rumm - john(at)internode(dot)co(dot)uk * * * * * * *|
\================================================= ================/
Tim
My recent concrete laying exercises have 'always' tended to end up
with such a problem. In fact the workshop floor was laid from the end
of a batch on a concrete lorry (conveniently shot through the hedge
from the roadside) but it was a pretty wet batch and that, with the
slight drizzle that started as we levelled it, meant that there was
'loose' water on the surface once it was level and covered.

That was about a year ago and it set perfectly hard - maybe a little
loose dust but the surface is OK and is not showing any signs of
breaking up with machinery on it.

Rob

Rob G wibbled:

On 28 July, 02:10, John Rumm wrote:

Given this is a sub floor I can't see it being a problem. Very little
point pressure will be placed on it through the celotex. You will also
find it gains in strength with time.
snip

Tim
My recent concrete laying exercises have 'always' tended to end up
with such a problem. In fact the workshop floor was laid from the end
of a batch on a concrete lorry (conveniently shot through the hedge
from the roadside) but it was a pretty wet batch and that, with the
slight drizzle that started as we levelled it, meant that there was
'loose' water on the surface once it was level and covered.

That was about a year ago and it set perfectly hard - maybe a little
loose dust but the surface is OK and is not showing any signs of
breaking up with machinery on it.

Rob

Thanks John and Rob: I'll watch it and see what happens. Either way it stays
put but it will be interesting to see if it hardens up some more.

Cheers

Tim

On Jul 28, 9:03*pm, Tim S wrote:
Rob G wibbled:

On 28 July, 02:10, John Rumm wrote:

Given this is a sub floor I can't see it being a problem. Very little
point pressure will be placed on it through the celotex. You will also
find it gains in strength with time.

snip

Tim
My recent concrete laying exercises have 'always' tended to end up
with such a problem. *In fact the workshop floor was laid from the end
of a batch on a concrete lorry (conveniently shot through the hedge
from the roadside) but it was a pretty wet batch and that, with the
slight drizzle that started as we levelled it, meant that there was
'loose' water on the surface once it was level and covered.

That was about a year ago and it set perfectly hard - maybe a little
loose dust but the surface is OK and is not showing any signs of
breaking up with machinery on it.

Rob

Thanks John and Rob: I'll watch it and see what happens. Either way it stays
put but it will be interesting to see if it hardens up some more.

Cheers

Tim

Cement mixes take a few days to begin to toughen up. If its been a
while already, you can always pour cement slurry or pva on.


NT

NT wrote:
On Jul 28, 9:03 pm, Tim S wrote:
Rob G wibbled:

On 28 July, 02:10, John Rumm wrote:
Given this is a sub floor I can't see it being a problem. Very little
point pressure will be placed on it through the celotex. You will also
find it gains in strength with time.
snip

Tim
My recent concrete laying exercises have 'always' tended to end up
with such a problem. In fact the workshop floor was laid from the end
of a batch on a concrete lorry (conveniently shot through the hedge
from the roadside) but it was a pretty wet batch and that, with the
slight drizzle that started as we levelled it, meant that there was
'loose' water on the surface once it was level and covered.
That was about a year ago and it set perfectly hard - maybe a little
loose dust but the surface is OK and is not showing any signs of
breaking up with machinery on it.
Rob
Thanks John and Rob: I'll watch it and see what happens. Either way it stays
put but it will be interesting to see if it hardens up some more.

Cheers

Tim

Cement mixes take a few days to begin to toughen up. If its been a
while already, you can always pour cement slurry or pva on.



PVA will if laid on thin and in sufficient volumes, turn almost pure
sand into a reasonable surface!

The only time I have got crumbly surfaces after a time, is when too
little cement is in the mix. screeds will absorb enough moisture from
the air to fully cure in a week or two even if they had zero water in to
start with.

Viz any bag of cement left opened for a few weeks..


NT

The Natural Philosopher coughed up some electrons that declared:

NT wrote:
On Jul 28, 9:03 pm, Tim S wrote:
Rob G wibbled:

On 28 July, 02:10, John Rumm wrote:
Given this is a sub floor I can't see it being a problem. Very little
point pressure will be placed on it through the celotex. You will also
find it gains in strength with time.
snip

Tim
My recent concrete laying exercises have 'always' tended to end up
with such a problem. In fact the workshop floor was laid from the end
of a batch on a concrete lorry (conveniently shot through the hedge
from the roadside) but it was a pretty wet batch and that, with the
slight drizzle that started as we levelled it, meant that there was
'loose' water on the surface once it was level and covered.
That was about a year ago and it set perfectly hard - maybe a little
loose dust but the surface is OK and is not showing any signs of
breaking up with machinery on it.
Rob
Thanks John and Rob: I'll watch it and see what happens. Either way it
stays put but it will be interesting to see if it hardens up some more.

Cheers

Tim

Cement mixes take a few days to begin to toughen up. If its been a
while already, you can always pour cement slurry or pva on.



PVA will if laid on thin and in sufficient volumes, turn almost pure
sand into a reasonable surface!


One for google: Did this and it worked nicely