I've got an old brick shed with a concrete slab roof; watertight at the
mo, but in one corner (external) the surface of the concrete has broken
up under the influence of weather; probably an inch deep? in a 4-5 inch
slab. Think it's got visibly worse over the winter.

If this was an indoor repair, I'd be brushing out the loose stuff,
treating with PVA solution, put some shuttering around the corner, and
then fill and seal with mortar or concrete; however PVA presumably won't
cut the mustard here.

So how would this best be repaired? I'm sure some form of treatment of
the old surface would be in order, but what? And can I use just a
sand/cement mix (easier) or would concrete be needed?

Cheers
David

Lobster wrote:
I've got an old brick shed with a concrete slab roof; watertight at
the
mo, but in one corner (external) the surface of the concrete has
broken
up under the influence of weather; probably an inch deep? in a 4-5
inch
slab. Think it's got visibly worse over the winter.

If this was an indoor repair, I'd be brushing out the loose stuff,
treating with PVA solution, put some shuttering around the corner,
and
then fill and seal with mortar or concrete; however PVA presumably
won't
cut the mustard here.

No need for pva, just wet the crete first to stop it sucking the new
mix dry. Some pva in the mix would be a definite advantage.


So how would this best be repaired? I'm sure some form of treatment
of
the old surface would be in order, but what?

A wire brush is my first thought.

And can I use just a
sand/cement mix (easier) or would concrete be needed?

yep, stone not necessary. Simple trowel job.


NT

Lobster wrote:
I've got an old brick shed with a concrete slab roof; watertight at
the
mo, but in one corner (external) the surface of the concrete has
broken
up under the influence of weather; probably an inch deep? in a 4-5
inch
slab.

3:1 is the strongest non-shrinking mix, and just a little pva makes it
adhere better.

NT

explained on 13/03/2005 :
Lobster wrote:
I've got an old brick shed with a concrete slab roof; watertight at the
mo, but in one corner (external) the surface of the concrete has broken
up under the influence of weather; probably an inch deep? in a 4-5 inch
slab.

3:1 is the strongest non-shrinking mix, and just a little pva makes it
adhere better.

NT

Then once properly dry, consider adding a waterproof membrane to
prevent water getting into and freezing in the concrete again.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org

Harry Bloomfield wrote:
explained on 13/03/2005 :

Lobster wrote:

I've got an old brick shed with a concrete slab roof; watertight at the
mo, but in one corner (external) the surface of the concrete has broken
up under the influence of weather; probably an inch deep? in a 4-5 inch
slab.

3:1 is the strongest non-shrinking mix, and just a little pva makes it
adhere better.

Thanks guys. Are we definitely 100% sure about the PVA thing? :-) I
always thought it was a complete no-no outdoors. and wouldn't want to
cock this up...

Then once properly dry, consider adding a waterproof membrane to prevent
water getting into and freezing in the concrete again.

How do you mean, paint-on bitumen type stuff? Aquaseal? Or felt of some
sort? Wouldn't just adding waterproofer to the cement mix do the job
(or would that interact with PVA?)

Thanks
David

Lobster wrote:
Harry Bloomfield wrote:
explained on 13/03/2005 :
Lobster wrote:

I've got an old brick shed with a concrete slab roof; watertight
at the
mo, but in one corner (external) the surface of the concrete has
broken
up under the influence of weather; probably an inch deep? in a
4-5 inch
slab.

3:1 is the strongest non-shrinking mix, and just a little pva
makes it
adhere better.

Thanks guys. Are we definitely 100% sure about the PVA thing? :-)
I
always thought it was a complete no-no outdoors. and wouldn't want to

cock this up...

The PVA I've got gives instructions for using it for this. It says mix
pva and water 50/50 and use that for the liquid in the mortar mix.

Yet it also says do not use in constantly damp situations.

I've come across this puzzle before, heres what I know:
1=2E its used in mortar for copings to increase adhesion and tensile
strength.
2=2E 3:1 sand:cement is used as a mix that is not water damageable, due
to its minimum permeability. Weaker mixes are much more permeable and
do get damaged by a wet freeze.
3=2E PVA is no good used alone as glue in damp, but in mortar it seems to
work.
4=2E Your wall mortar shouldnt be constantly saturated anyway, once its
set.


Then once properly dry, consider adding a waterproof membrane to
prevent
water getting into and freezing in the concrete again.

How do you mean, paint-on bitumen type stuff? Aquaseal? Or felt of
some
sort? Wouldn't just adding waterproofer to the cement mix do the job


it would, but for 3:1 nothing is needed.


(or would that interact with PVA?)

no idea, personally, dont use them. Why pay =A38 for what you can buy
for 13p.


NT

"Lobster" wrote in message
...
I've got an old brick shed with a concrete slab roof; watertight at the
mo, but in one corner (external) the surface of the concrete has broken
up under the influence of weather; probably an inch deep? in a 4-5 inch
slab. Think it's got visibly worse over the winter.

If this was an indoor repair, I'd be brushing out the loose stuff,
treating with PVA solution, put some shuttering around the corner, and
then fill and seal with mortar or concrete; however PVA presumably won't
cut the mustard here.

So how would this best be repaired? I'm sure some form of treatment of
the old surface would be in order, but what? And can I use just a
sand/cement mix (easier) or would concrete be needed?

Cheers
David

Knock off any loose concrete

Wire brush any steel you can seal and paint it. With hammerite or similar if
you have some.

erect your formwork.Not forgetting there may be a 'throat' on the underside
to stop water tracking back to the walls.

mix sand, cement and grano with diluted PVA in the water.

Spay some WD40 on the inside of your formwork and fill with the grano mix.

Tap sides of formwork as you fill and trowel of level with roof.

When dry, 48 hrs. Strip form.

Clean rest of roof and apply bitumen type sealant to whole roof.