Concrete roof garage

[axus]

Hi.
I have just inheritated a 14' x 20' garage with a concrete roof. I estimate about 70 years old.
The roof is in 3 sections with rsj's in between. Above the door is a 12' long RSJ supporting a 5' high 9" brick wall.
This RSJ is so badly rusted, the edge I can see is only about 9mm thick with a mass of flacking rust.
The other two RSJ's have rusted to the point they have pushed forward the two concrete slabs 50mm
I estimate the concrete to be 150mm thick with 18mm reinforcing. Can see a piece of reinforcing where the concrete has dropped away. Also there is many stalactites where rain water has seeped through.
My intention is to demolish it to the level 2 bricks below the concrete slab and replace it with a wooden roof.
I cannot get any plant near it so I will be demolishing it, brick by brick with an electric rock breaker / chisel as my main tool. Obviously well propt up with acrow props.
Anyone any idea if the concrete is likely to have one or two layers of reinforcing and will the concrete have deteriated at all being 70 years old.
Many thanks.

 

[Newboy]

The concrete may have deteriorated if there's water penetration and the rebar has started to rust but 70 year old concrete may have turned as hard as granite.

As far as reinforcing goes, I'd be inclined to disc cut an inspection hole through the slab to see how it was reinforced.

Whatever you do, do not underestimate the weight and the risk of collapse with the potential for serious injury.

Temporary propping appropriate for the weight is essential before starting to remove any of the structure - a rough estimate is that your concrete roof weighs somewhere between 9 and 11 tonnes.

 

[theprinceofdarkness]

So if your door is 6' and it has 5' of brickwork over it to the roof, the height of the concrete ceiling is 11' ?. This sounds a bit high for props. I would think that four in line across the width would support the balance of the roof as there will be some residual strength left in the beams or the roof would have fallen in. I would use a full width of 6" X 2" on top of the props with screws and washers to catch the edges of the top cross part of them. This because in the event of a large lump becoming dislodged, it will not bring any of the props down, as they will be acting as a "wall" across the width.
Put a prop "wall" just inside the concrete wall and another about 12" behind it. Take down the forward 6" section of the roof and wall. I would leave the lintel just to stabilise the front of the walls. If its gone to plan, you should have one prop wall holding the cut off bit of roof and another holding the remainder of the old roof. I would try to cut the old roof off in one foot lengths, at one foot wide and 6" thick a block will be heavy enough to shift.
Once the first strip has been cut off, move the front wall back to become the back wall, with a 12" gap to give you a clear cutting area. I think that if you do not support the strip to be cut off, it may bend and crash down possibly wrecking the top of the wall, not to mention the shear weight of it.
Frank