Rust & Protective Coatings

Rust and Reinforcement

Accepting or rejecting a bar or mesh with visible rust is a decision often facing
site engineers and superintendents. The criteria for acceptance or rejection is
usually not known by the decision maker. When does the reinforcement have
excessive rust that is detrimental to its performance?

A moderate coating of rust is not detrimental to the reinforcement and can
actually improve its bond strength. The improved bond due to moderate rusting
is well documented and is included in the commentary to AS3600. Rust
usually appears first at the bends of reinforcement, where the steel has
undergone some cold working.

Rust is only excessive if the cross-sectional area of the steel is reduced
below the minimum tolerance permitted for a bar or wire. To check the area,
take a rusty piece of steel, wire brush it to remove the rust, measure its
length and then weigh it. The area is calculated taking the density of steel as
7850 kg/m³.

Rust due to exposure to salt water can be detrimental to the reinforcement.
The chloride ions in the salt water cause pitting of the steel, and this
reinforcement should not be used without rigorous testing of yield stress,
uniform elongation, tensile strength and cross-sectional properties. Even
reinforcement that has only had mild exposure to salt water should be washed
prior to use to remove any salt from the steel surface.

Reinforcement that has been fixed for some time before concrete placement
may, after rain, show lines of iron oxide (rust) on the forms. If the forms are not
cleaned and the staining removed prior to pouring the concrete a rusty looking
line will be visible on the concrete soffit. This is an aesthetic problem, not a
structural or durability problem.

Mill Scale

Mill scale on hot rolled products, in the levels found on Australian produced
reinforcement, is not detrimental to the reinforcement. Wire and mesh are free
of mill scale.

Types of Coating

The two principal protective coatings are hot dipped galvanising and fusion bonded
epoxy coating or FBE. The former is generally available in most major centres,
although there may be a physical limit to the size of the zinc bath in some
localities. Fusion bonded epoxy coating in Australia is not easily obtainable.

Note 9 to AS3600-2009 Table 4.3 states "Protective (concrete) surface
coatings may be taken into account in the assessment of the exposure
classification". AS3600 does not allow any reduction on cover when a
protective coating is added to the reinforcement.

Hot Dip Galvanising of Reinforcement

Galvanising of reinforcement is to AS4680 and AS4534. AS3600-2009
requires galvanised bars to be bent about a 5 d_b pin if 16 mm diameter or less
and an 8 d_b pin for larger bars. This is regardless of whether the bar is to be
galvanised before or after bending.

The Galvanising Process

1. Preparation – Mill scale, rust, oil and dirt are removed from the reinforcement.
It is then placed in a pickling bath of hydrochloric acid. After pickling the
reinforcement is rinsed.
2. Fluxing – The pickled reinforcement is immersed in a solution of zinc
ammonia chloride at about 65° C.
3. Galvanising – The reinforcement is immersed into a molten zinc bath at
445° C to 465° C. The molten zinc reacts with the steel to form layers of
zinc – iron alloys.

The galvanised coating of zinc improves the reinforcement’s corrosion
resistance. The zinc forms a sacrificial coating about the reinforcement. Minor
breaks in the coating, such as may be caused by bending of the reinforcement,
are not detrimental to the corrosion protection offered by the galvanising.

Embrittlement of reinforcement is rare in steels below 1000 MPa, however it
must be considered when galvanising reinforcement. The major factors
affecting embrittlement of reinforcement are the length of time the steel is in
the pickling bath, the heat of the galvanising process and the presence of cold
working, particularly at bend locations. A detailed explanation of this is given
in the May 1994 edition of Corrosion Management, “Designing for Galvanizing
– Avoiding Embrittlement”.

Galvanised reinforcement should not be in contact with stainless steel,
aluminium or copper and their alloys as the zinc corrodes preferentially
to these metals.

More detailed information regarding hot dip galvanising can be obtained from
the Galvanizers Association of Australia. Their publication, “After-Fabrication
Hot Dip Galvanizing”, provides an excellent overview of this subject.

Galvanising of Reinforcement – AS/NZS4680:2006

AS/NZS4680, Hot-Dipped Galvanised Coatings on Fabricated Ferrous
Articles, includes provisions for galvanising wire, bar and fabric in Section
5 General Articles. As a general requirement for reinforcement, the
minimum average coating is 600 grams per square metre, or approximately
0.085 mm thick.

Limits for the molten metal and finished appearance are given, together with
test requirements for coating mass and adherence.

It should be noted that a smooth finish on reinforcing products cannot be
expected. The deformation on the surface of bars does not allow a particularly
pleasing appearance but this does not detract from the overall performance.
The steel should be reasonably free of dags of surplus zinc.

Appendix D of AS/NZS4680, Properties of the Steel to be Coated, which can
affect or be affected by hot-dip galvanising, gives an excellent overview of
steel embrittlement. Reinforcement that has been galvanised should not be
bent on pin diameters smaller than those given in AS3600-2009, should not
be heated or bent on site and should not be welded.

Appendix E of AS/NZS4680, Renovation of Damaged or Uncoated Areas,
states that exposed steel situated within 1 mm of a substantial zinc layer,
should receive sacrificial protection. This implies that cut ends of pregalvanised
bar or fabric should be repaired.

Galvanising of Welded Wire Mesh - AS/NZS4534:2006

AS/NZS4534, Zinc and Zinc/Aluminium-Alloy Coatings on Steel Wire
addresses galvanising of welded wire mesh. As a general requirement for
reinforcement mesh, the minimum average coating is 610 grams per square
metre, or approximately 0.085 mm thick.
Appendix C of AS/NZS4534 gives an overview of hydrogen embrittlement.
Appendix F is a comprehensive guide to coating thickness selection for
corrosion protection.

Epoxy Coating

An excellent reference is ‘The Epoxy Coated Rebar CD-Rom’, produced by the
Concrete Reinforcing Steel Institute, 9333 North Plum Tree Grove, Schaumber,
IL, 60173, USA.