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)

enter image description here

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.


1 Answer 1


It's over kill, but yes it is fine.

I would just put to # 3 rebars dowelled inside the circles 6in OC. And fill the holes flush by a couple of bags of ready-mix concrete and wet the circumstances with acrilic glue.

  • $\begingroup$ Thank you, what does OC mean? I was tempted to fill the holes, using rebar fixed to inside of holes to support the cylindrical mass of concrete. However my FIRST concern was that the concrete would always be in tension, not ideal. The mass would be approx 32Kg of concrete per hole. SECOND concern is that a tiny amount of movement would break the waterproofing bond, and allow water to come through the holes. The problem with my way is that 100mm concrete would break the thermal insulation layer, causing possible cold bridge. Therefore I'm stuck $\endgroup$
    – hazymat
    Commented Jul 19, 2019 at 12:15
  • $\begingroup$ @hazymat, OC. Means distance from the center of rebar to the center of the next rebar, off-center. you drill holes into old concrete and use epoxy to glue the dowels so they become an integrated part of old concrete.. $\endgroup$
    – kamran
    Commented Jul 19, 2019 at 13:40
  • $\begingroup$ Ahh, I have drilled and resin-fixed rebar before, but didn't know what "OC" meant. Thanks for explanation. $\endgroup$
    – hazymat
    Commented Jul 19, 2019 at 13:47

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