Corrosion
protection provided by galvanising:
In areas where the reinforcement
may be exposed accidentally due to thin
or porous concrete, cracking, or damage
to the concrete, the galvanised coating
provides extended protection. Since the
corrosion product of zinc occupies a smaller
volume than the corrosion products of
iron, any small degree of corrosion which
may occur to the galvanised coating causes
little or no disruption to the surrounding
concrete mass.
Studies were made at the
Structural Engineering Materials Laboratory,
University of California, Berkeley California,
of the effects of corrosion on reinforced
concrete test prisms.
Prisms 300 x 100 x 100
mm were axially-reinforced with 19 mm
diameter galvanised or black steel bars.
A 12.5 mm deep notch was cut at the mid
section of each prism to enforce formation
of a crack at the notch should corrosion
products exert sufficient disruptive stresses.
Prisms were placed in loading frames and
the steel reinforcing bars stressed to
140 MPa. Prisms were then subjected to
alternate immersion/drying cycles in a
4% NaCl solution.
Cracks occurred in test
prisms reinforced with uncoated steel
bars in less than ten months exposure.
Large crack areas had developed by about
18 months and were still increasing at
24 months. No cracks were observed in
prisms reinforced with galvanised bars
until almost 16 months exposure. These
crack areas were very small compared to
those in prisms reinforced with uncoated
steel bars and crack development ceased
after a further two and a half months
exposure.
