This picture shows an exposed reinforcing bar in
underside of concrete slab. The large grade aggregate (gravel) has locally
accumulated creating air pocket when the concrete was poured in form. This is
the outcome of conjunction of too thin spacers and too large aggregate
size.
The deficiency of this kind in a dry condition is
usually not a problem. I observed
35-stories-high pre-war building, where hand-made concrete cast slabs consisted of so many holes
perforating the slabs that it resembled more a net. The building was more than
70-years-old, yet the slabs shown no wear.
In a damp conditions the steel will corrode, the rust will expand, the concrete will spall and the failure will occur when the structural steel loss will have reached critical extent.
The photo below shows another condition of a poor rebar shielding. The rebar was placed too close to concrete formwork (the distance shall be fixed by spacers). The moisture easily penetrated a thin layer of concrete and caused rusting and spalling.
