The term clear cover defines the space given to the reinforcement from exposing. If reinforcement gets exposed to air and water, oxidation process happens which reduces the strength of the steel (reinforcement) resulting rust. Whenever steel is used there will be clear cover varying from 20-75 mm which acts as protection layer for steel.
In practice, we use cover blocks which are made up of hard plastic, concrete, PVC etc,. Normally these cover blocks are available in market in different sizes such as 20mm, 25mm,40mm,50mm etc.
Cover blocks are usually tied to steel (reinforcement), so that at the time of vibration/ compacting the concrete it will not have any movement.
How many number of coverblocks should I use?
When I entered construction field after completion of my degree many doubts use to arise regarding this issue . But in short , in very few books this detailing is mentioned as reference (its not a bench mark).
Spacer positioning is based primarily on acceptable deflection at maximum loading. More the thinner rods more will be the deflection.
For beams and column(longitudinal)
Cover block helps us to achieve required cover for reinforcement which enhances the life of structure without adding extra cost.