The reinforcing bars, surrounded by concrete, do not rust for many decades. This is due to the presence of alkaline medium in the concrete. However, if the reinforcement is not at sufficient distance from the external surface of the concrete structure, air and moisture can be directly accessed to the reinforcement bars, which inevitably leads to its corrosion and destruction. Due to the appearance of a rust film with the build-up of its layer, a thin outer layer of concrete also begins to destroy. Also, the incoming moisture freezes-thaws (for example, for external walls) and the effect of destruction is aggravated.
For this, the reinforcement bars in any concrete structure must be protected with a sufficient covering layer of concrete.
The minimal thickness of the protective layer should be in the range of 10 (for thin-walled and hollow slabs where it is impossible to provide a thicker protective layer, up to 120 mm for large reinforced concrete structures of foundations, supporting walls, dams, etc. For concrete surfaces exposed to moisture, wide temperature range, aggressive chemical substances, the size of the covering layer should be increased.
The covering layer from 25 to 60 mm is most often used. The size of the concrete covering layer is chosen by the designer or an experienced manufacturer of reinforced concrete works.
For more information please refer to wikipedia concrete cover
To fix reinforcing bars in the body of concrete structure, PTL plastic rebar spacers, manufactured by Plastic Technologies Ltd. are ideally fit to provide the necessary concrete covering layer protective layer between the reinforcing bar and the external surface of reinforced concrete structure.