Damaged flat concrete roof

[Lobster]

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

[bigcat]

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.

A wire brush is my first thought.

yep, stone not necessary. Simple trowel job.

NT

[bigcat]

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

NT

[Harry Bloomfield]

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

[Lobster]

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 c*ck this up...

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

[bigcat]

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.

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

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

NT

[Space_Cowby]

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.