Precast components are usually produced under almost factory like environment even if it is produced on site. Thus the quality control are much better than in situ components. Problems like insufficient cover for precast components shows a very lacking quality control by the precaster.
Concrete covers are provided for two main reasons, namely fire and durability. For fire resistance, if there is insufficient concrete cover after hardening, the Code allows additional protection like plaster or sprayed fibre.
However, insufficient cover will also cause durability problems, especially on long term exposure. The most common method to repair is by breaking up the exiting surface to provide bonding and to grout with high strength concrete up to the required depth. Another possible solution is to place protective coatings to the elements. However, these elements may need to be re-coated after a certain interval as it is not a long term solution to the problem.
The cheapest solution is to ensure good cover before concrete casting.