Last year we poured a RC suspended slab according to designs of a structural engineer. (Note we are just DIY home builders but have taken professional advice throughout.)
The slab was cast with three little 300mm diameter port-holes which we had planned to use for light, but for various reasons we wish to cover these up now.
Of course, a steel plate over the top would be fine, but for waterproofing purposes I'd like to set a concrete square over the top, using a slurry to bond new to old concrete, and waterproofing the whole lot.
My concrete square would perform two functions: to provide an upstand to waterproof, and to cover the hole.
From a structural point of view, a piece of plywood would (hypothetically) be adequate, because over the top we will add 100mm insulation then hard finish like paving.
But I'm aware I may need to consider the mass of the concrete itself, so will need to add rebar.
There are other possible solutions, like trying to seal a steel plate, then screeding over the top - but as we will be applying quite an expensive liquid membrane, then setting hard finish which I hope to last for 50 years on top, I'd like this to be a bullet-proof solution in terms of waterproofing. The fact they stick out of the ground is not a problem, as we will be adding insulation over the top of the remaining area before the paving stones. Although my idea would mean a reduction in area of insulation which is not idea.
Here's a pic of the formwork (not complete) which shows square sides and the circle inside the hole is propped from underneath (NOTE: I have not yet adjusted prop height, when I do, the circles will be flush with the top of the concrete)
I have also cut and bent rebar into 400mm lengths, with a right angle bend at each end and a further 100mm that could be doweled into the existing concrete to provide both reinforcement and prevent movement. I was going to use 4x lengths laid parallel per hole.
I've made no calculations, as I think this may be a trivial "onsite solution". But I'm aware there will be an (albeit small) amount of suspended concrete here.
Can someone tell me if this is a sensible plan so far?
Another option might be to place a steel plate over the holes then add concrete as per my plan above.