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Trying to determine some information regarding the reinforced concrete slabs (reinforced with mesh) in an apartment, by looking through the holes in the middle of the rooms for lights, I've come across the following situation:

  • mesh located at 40 mm from the bottom of the ceiling in the middle of the room (visible in a plastic tube used for the light cables) and mesh located at 17 mm from the bottom of the ceiling nearer the edges.

What I'm seeing might be a different level of reinforcement?

The reinforcement is in 2 directions, but this is the information I can get from what I can see in existing holes.

Details:

  • the slab height is 140 mm
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  • $\begingroup$ What is the thickness of the slab? $\endgroup$ – Solar Mike Jul 17 '19 at 12:54
  • $\begingroup$ 14 cm is the thickness. $\endgroup$ – Shared Jul 17 '19 at 13:26
  • $\begingroup$ 17 mm of cover sounds low to me, but I come from the bridge world. having the reinforcing low is good for bending, but not so good for fire protection or corrosion protection. Closer the reinforcing is to the surface the easier it is for a corroding wire to expand and spall off the concrete. $\endgroup$ – Forward Ed Jul 17 '19 at 18:44
  • $\begingroup$ @ForwardEd , and only in that room the reinforcing is at 17 mm. In other rooms it is even lower at about 12 mm. $\endgroup$ – Shared Jul 17 '19 at 19:25
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This just sounds like the mesh was sagging between supports during the pour.

Until you check across the width and length of the slab for the dimensions at several points there is not much else you can conclude.

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  • $\begingroup$ The weird thing is the OP states the mesh is lower near the supports, which is a pretty obvious place to put spacers. I'd've expected less cover at midspan, not the other way around. $\endgroup$ – Wasabi Jul 19 '19 at 2:05
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Depending on the direction of the pour, the mesh at the leading edge could have been dislocated without noticing. The 40 mm bottom cover thickness is about right for positive reinforcement. If there are two layers of reinforcement, this is nothing to worry about. Otherwise, you might find fine cracks on the top face of the slab area that having a 17mm bottom cover only.

For certainty, we need to know the slab is connected to its supports, whether it is simply supported or fixed along the edge.

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