There are two basic balcony configurations: those with cantilevered slabs and those with slabs supported at the edges by shear walls.
Slabs falling under the first category rely on the top matt of reinforcing steel for structural capacity. Slabs supported at the edges span side to side and require concrete that is reinforced with steel in the bottom of the slab.
In theory, therefore, balcony-slab-edge deterioration should really only be an issue on side-supported slabs. In reality, corrosion also occurs in cantilevered slabs. This is because designers and forming contractors often put ‘temperature’ steel—whose purpose is to prevent cracking rather than provide structural capacity—right near the outside bottom edge of even cantilevered slabs.
To avoid corrosion in cantilevered slabs, designers should consider placing this temperature steel further back from the edge of the slab, or closer to the midline of the slab to increase the cover over the steel, particularly near the drip slot. This minor change in design and construction practice could save each building hundreds of thousands of dollars in repair work and all the disruption that goes along with it.
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