Why is cover important?

Answer 1

The most important reason for "Depth of Cover" requirements, is to afford protection to the steel reinforcement embedded in the concrete. Certain tension, compression and bending moments also play a role, when the structural engineer designs for loads of the intended use, however the need to prevent corrosion [rust] from forming on the steel is the primary objective.

New concrete has pH of 13-13.2 when placed. This high pH forms a "Passivating Layer" on the steel. The deeper placement of the steel [ Cover ] delays the degradation of the pH to a level that supports and fosters formation of corrosion cells. Chloride ions [Salts] are the most common contaminates in concrete that commence a corrosive environment. At a pH of 13.2, a chloride level of 8000 ppm are needed to begin rusting steel ! When that pH drops to a level of 11.5 the chloride threshold is lowered to 70 ppm ! Drastic difference !

Rusting Re-Bar expands in size as rust forms on it's surface. This expansion swells the re-bar size to the point that the concrete surrounding the steel cracks away [spalls] and the bond is lost, weakening the concrete in tension. Now cracked or spalled, these cracks allow a new path for contaminates and moisture to enter the matrix and speed the corrosion process up significantly. This corrosion leads to unsightly rust stains, loss of designed strength and very expensive repairs to the concrete. Left to continue a complete failure will occur.

Although several other conditions may contribute to the corrosion cycle, by far, Depth of Cover is the most Important. Knowledgeable structural designers and contractors should also call for a specification to "Waterproof the Concrete". Doing so will slow the loss of pH and decrease the oxygen and moisture content at the level of the steel inside. Corrosion does not occur if moisture and oxygen are not present!