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ا19. Does the presence of rust have adverse impact to the bond performance of bar
reinforcement?

In fact, the presence of rust in bars may not have adverse impact to the bond performance
and it depends on the types of bar reinforcement under consideration.

For plain round bars, the rust on bars improves the bond performance by the formation of
rough surfaces which increases the friction between steel and concrete.

However, for deformed bars, the same theory cannot apply. The presence of rust impairs
the bond strength because corrosion occurs at the raised ribs and subsequently fills the gap
between ribs, thus evening out the original deformed shape. In essence, the bond between
concrete and deformed bars originates from the mechanical lock between the raised ribs
and concrete. On the contrary, the bond between concrete and plain round bars derives from the adhesion and interface friction. With such differences in mechanism in bonding
the behaviour of bond between deformed bars and plain round bars in the presence of rust
varies. Reference is made to CIRIA Report 147.


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