Concrete roof add on weight

[PeteC]

OK, a question for you structural engineers out there. Picture a concrete in ground tank about 480cm long x 170 cm wide. The roof of the tank is comprised of 9-10cm of concrete. Some rebar in there but size/thickness is unknown. I'm assuming it is the thin diameter, inexpensive wire type.

What we want to do now is add about 4-5cm of reinforced concrete on top, place a 100 x100cm marble/granite decorative slab in the middle weighing about 60kgs, then finish it all off with a layer of stonewash about 1 1/2-2cm thick.

What are the chances of the whole think collapsing, and any tips on how to do it correctly by possibly adjusting the thickness or reinforcement of what we're planning to lay on top. Thanks for any advice. Pete

EDIT: The roof of this think will be walked on as well, but not by more than 2 people at once, so add in a temporary moving weight of about 180 kilos max.

[splitlid]

as im sure you are aware, it is the steel inside the concrete that is important and it alone is responsible for the tensile strength of the concrete.

i would possibly cut a small hole in the existing 'roof' and see what steel has been used. if you have been walking on this roof for sometime, and or, the steel found in the test hole runs both ways and is spaced at say 150-200mm.then relax.
if it passes one of these test then i wouldnt worry too much and would just lay the marble and sandwash on top, no otter steel or concrete required.
because of its width then im inclined to so it will be ok, but without seeing it i cant say for sure.
do you have some original drawings of the house which shows the tank?

[PeteC]

Thanks SL. No drawings as it's a water holding tank for an overflow pool and the pool company let's say is having problems so no one of any experience or memory there at the moment. I've found nothing in my files, just drawings for the pool itself.

To date we've been walking on a raised wood deck with boards running width wise, laid on top of support timbers running length wise. They seem to only be attached to the concrete roof beneath at the ends and perhaps at a few places along the length. We don't have the wood off yet to tell exactly. Everything starts on Monday.

The reason to add a 4-5cm concrete layer is to bring the tank top surface up close to the existing surface of where the top of the boards were, to bring the entire tank close in height to the pool perimeter walkway. I'll post a pick of what it looks like later on today. Pete

[PeteC]

Ok, here are some photos.

#1 That concrete collar around the access hatch is 18-19 cm. The actual concrete roof of the tank starts 9cm below the top edge. My 480cm length number in post #1 is only the far end to the top end of the hatch.

#2 View of the concrete roof beneath the boards.