Concrete slab roof queries

[Lobster]

I've just done a repair to my concrete slab flat roof over an outhouse, which had become spalled and had disintegrated at the corner - I made some shuttering around the corner and filled with cement; it worked very well. Howvever, I now want to finish the job with a protective layer of bitumen paint over the whole roof, having pressure-washed away all remnants of the previous layer during preparation for the repair.

So - how long do I need to wait before painting? The slab is about 4" thick, although most of the repair (other than the corner) is only about

1" to 1.5" deep. Given that the underside of the slab is exposed to the air, will it be OK just to wait a week or so, until the surface has gone "pale", and assume that the rest of the drying out will happen in a downwards direction? (ie will the moisture pass down through decades-old concrete?)

Supplementary question... on the underside of the roof you can see the some of the exposed bottom of the steel reinforcement to the concrete, which had obviously been far too close to the face of the concrete when it was poured. These areas have become rusted and need some form of treatment. Would a bit of Hammerite sloshed on do the job? Or would I need to cut the surrounding concrete back to expose clean unrusted steel, and do something else? (IIRC most proprietary rust treatments attack concrete like crazy, don't they??)

Cheers David

[bigcat]

concrete,

Rust converters use phosphoric acid, and acids eat crete. It should be sorted else the rust will expand and smash the slab. I wouldnt waste time with hammerite onto rust, aka pinholite.

I'd use a rotary wire brush to strip the rust off, then some decent metal paint.

NT

[The Natural Philosopher]

Remember that normal steel reinforced concrete doesn't suddenly rust and expand and smash the slab. And I have NEVER seen reinforcing rods that wern't a bit rusty whan laid.

Rusting needs water AND air.

You can stabilise it simply by excluding air. Poured concrete will do this.

[bigcat]

Right, but the OP's steel is exposed and rusting, so there is a problem. The rust will spread and smash it in time if not stopped. And since its on the underside, one cant pour on more crete.

NT

[Andy Dingley]

Hammerite is rubbish. If you have to, use a dark brown oxide primer like Finnegan's No 1, or else a good zinc-rich primer (Davids 182 is good).

No, they attack it slowly, if somewhat spectacularly. Get some glacial phosphoric acid (i.e. conc. as you can get it) from the dope-growing hydroponics shop and splash it around. The concrete will happily neutralise any excess and apart from being a little cleaner, you'll never notice the difference.

[Lobster]

Thanks to all for their answers to the second question; anyone care to comment on the first one, or has that got everyone stumped?! After 9 days, the whole repaired area is now dry, and very pale grey, although I realise it won't be fully cured deep down. Will it be Ok to slosh on my bitumen yet?

Cheers David