By: Andrew Gamble
Corrosion of rebar has proven to be one of the leading
causes of premature failure of concrete structures. Moisture wicks its way into
just about anything, and concrete is no different. Galvanized rebar is an
economical and effective way to eliminate this problem.
How failure occurs is quite simple. As moisture and the
environment attack the rebar, iron oxides form, taking up more space than the
rebar did initially. This creates stress cracks in the concrete, which in turn
accelerates the corrosion, creating more stress. Eventually spalling and
failure will occur.
Three layers of protection
The galvanizing process is equally simple and easy to
understand. It's primarily a cleaning process, because zinc will not form
a bond with a surface with even the slightest amount of impurities evident.
Once pickled clean, the rebar is slowly lowered into a
liquid zinc bath. It remains in the zinc until it reaches kettle temperature,
which is 850°F. It's then cooled in a water and chromate bath to seal
the fresh surface. This process leaves 3-4 mills of zinc and zinc-iron alloy
layers on every surface of the steel.
Galvanizing uses three different methods to protect steel
from corroding. First, the zinc acts as a barrier, totally encapsulating the
product in zinc, which keeps moisture and the ravages of the environment from
the steel. If the environment can't reach your steel, it can't
rust.
It takes over 2,500 psi to remove galvanizing, while most
paints come off at 250 psi or less. This tough barrier is developed in the
kettle. Molten zinc is 850°F, and with steel melting at 2,200°. There
is a lot happening at the surface of the steel when it's in the kettle.
Three distinct zinc-iron alloy layers are formed, each of which is harder than
the base piece of steel. This tremendous barrier is "grown" on
every surface of a galvanized piece of steel, inside and out.
The second way in which galvanizing protects steel is by
providing cathodic protection to the steel. When a steel surface is exposed,
zinc corrodes sacrificially so that the steel will not rust. This is an
electro-chemical process, some describe as self-healing, in which scratches up
to 1?4 in. wide are protected from rusting. This is a very common way of
preserving steel. Many types of cathodic protection are offered to rebar users
but none provides the longevity, reliability and ease of use that galvanizing
does.
The third way zinc protects steel is by protecting itself.
As the zinc layer oxidizes, its byproducts form a carbonate that inhibits
further oxidation.
It can go anywhere
Galvanized rebar also has other beneficial properties for
use in concrete structures. The bond that develops between galvanized rebar and
concrete is similar to and often stronger than that of black and epoxy-coated
rebar. This superior bond results in shorter development lengths, less cost and
stronger structures. The chromate used in quench tanks further passivates the
zinc surface when it goes into the fresh cement, further promoting bond
strength. Storage, installation and general handling of galvanized rebar is
less of a problem because galvanized rebar needs no special precautions, the
surface of the bar is harder than the steel under it.
One of the greatest misconceptions surrounding galvanized
rebar is that it cannot be fabricated after galvanizing. Rebar can be bent
after galvanizing with a radius of no less than 6 to 10 times bar diameter,
depending on bar size. Any coating discontinuities can be repaired using
zinc-rich paint, according to ASTM standard A767.
Another advantage galvanizing has over the widely popular
epoxy-coated rebar is coating durability, before and after it's in place.
No "field guide to handling rebar" is available or necessary.
Additionally, rebar that is refabricated onsite can be
repaired by simple methods, eliminating work stoppages due to field changes.
Field cutting and welding does little to degrade the long-term protection
afforded by galvanizing.
Zinc has been used successfully for decades to protect rebar
from corrosion. Galvanizing has been used in many different climates, from the
subtropics to the far north with equal success. Recently an elevated roadway
project was built near Pulaski, Pa., on Rte. 56 using galvanized rebar
exclusively.
The reasons for this were many. Storage space was limited
and, knowing that materials were going to be stacked up, the specifier chose
galvanizing because it could stand up to the abuse of being moved around often.
Because of the superior bond strength of zinc to concrete, development lengths
could be reduced, which also improved storage problems. All of these benefits
offset the cost of the galvanizing. High-performance concrete, which has been
known to crack, was used in this application. Galvanizing was again chosen to
provide long-term protection from road salts and the environment that might
find an easy path to the rebar. Another reason was longevity. With a goal of 40
years to first maintenance, many parts of this structure received a galvanized
coating, not just the rebar. The
cost increase of a fully galvanized project vs. an epoxy rebar and painted
bridge is minimal compared to the cost of maintaining or replacing a corroding
structure.
Crash tested
Highway guardrail is one of the most commonly galvanized
products there is and if there was a better and cheaper way of protecting steel
from corrosion, it would definitely be used on guardrail. Few coatings or
materials can match the durability of galvanizing in this hostile environment.
Galvanizing can provide the same extension of useful life
for rebar that it does for guardrail at a minimal cost premium over epoxy bars.
This will save the owner or DOT money.
