This document prepared by the
Materials Engineering and Research Office is available to answer questions regarding
the MTO specifications for stainless steel reinforcing bar and to provide
examples of acceptable and unacceptable material.
MTO has used stainless steel
reinforcement since 1996; first on an experimental basis and once its
effectiveness was demonstrated, in concrete structures on corridors with the
highest traffic volumes. Current policy introduced in the fall of 2000 extends
the use of stainless steel reinforcement to structural components with the most
severe exposures in terms of likelihood of corrosion damage occurring and
resulting in damage to the structure.
Designers specify stainless
steel when corrosion resistance is of primary concern. By specifying stainless
steel, the intent of the bridge designer is to achieve a long design and
maintenance-free life and may be the preferred option for bridges that are
inaccessible for future maintenance (i.e. high traffic areas).
The advantage of stainless
steel is the extremely slow rate at which it corrodes in a concrete/chloride
environment. Minimal corrosion damage is anticipated during a service life of
75 years. Although it is much more expensive to purchase than black or
epoxy-coated reinforcing steel, it is more cost-effective over the long term
because the corrosion-induced damage typically seen with black steel and coated
steel will not occur.
Because of the premium cost of
stainless steel reinforcement, and the very long term performance expected from
the stainless steel reinforcement, it is very important to ensure at the time
of arrival of the steel on site that it meets all specification requirements
and that all the factors important for long-term performance are checked
carefully. Severely damaged or contaminated stainless steel will compromise the
longterm durability and will not achieve the goal of the designer or the bridge
owner.
Surface Contamination and
Mechanical Defects
The Special Provision 905S04
states "Reinforcing stainless steel bars at the time the concrete is
placed shall be free of mud, oil and other contaminants that adversely
affecting bonding strength, and deposits of iron and non-stainless steel.
Special Provision 114S03 states "Fabrication of reinforcing stainless
steel bars shall be such that the bar surfaces are not contaminated with
deposits of iron and non-stainless steels, or damage due to straightening from
coil.
This is a different kind of
problem from the surface discolouration described above, and may be harder to
spot. The defects are generally a result of careless or improper
transportation, handling or fabrication procedures. Defects may be localized
and occur only in one part of a bar or at intervals along the bar.
For example, if stainless
steel bars are bent on the wrong kind of equipment or improperly handled, steel
from the bending equipment or the environment may be pressed into the stainless
steel surface. What you will see, by the time the bar gets to the site, looks
like “rust” on the bar. Since stainless steel itself does not rust, the sight
of rust tells you that there is contamination on the bar. If the amount of rust
contamination is excessive, or occurs frequently along the length of the bar,
the reinforcing bar should be rejected.
The reinforcing bar should be
rejected if:
i.
any
area of contamination of the stainless steel by iron exceeds 100 mm in length
ii.
two or
more areas of iron contamination greater than 25 mm in length occur along the
length of the reinforcing bar
iii. there are frequent
small occurrences of rust contamination along the full length of the bar.
Contamination of stainless
steel is not just a “cosmetic” problem; in the long term, those contaminants on
the bar can cause localized damage (pitting) that can be very harmful. This
problem has already been seen on a number of contracts and is likely to occur
again; the long term effectiveness of the stainless steel, and its value to us,
is reduced if this kind of damage is present.
If reinforcing bars have been
rejected due to excessive iron contamination, it may be possible for the
contractor to have the bar treated to remove the contamination. This can be
accomplished by mechanical cleaning with a (stainless steel) wire brush, by use
of a polishing machine or even by chemical treatment (pickling) if the
contamination is excessive or other approved methods are not successful.
Another problem that may occur
is when mechanical damage to bars occurs during bending or straightening
operations. For example, stainless steel coming from some suppliers may be
supplied in the form of large coils, which are then straightened out and cut
into bars by the fabricator. Sometimes this straightening is not done very
well, leaving the bar “twisted” or damaging the deformations (i.e. the
longitudinal rib may twist around the bar, or and rather than running straight
along it) or flattening them out, and often leaving very sharp tears and edges
along the bar. This is not acceptable, since the pattern of deformations on the
bar may be destroyed or badly distorted. Handling of such steel with sharp
projections may cause injury.
The attached Figures 3 and 4,
illustrating unacceptable reinforcement that has been observed on MTO
contracts, are intended to provide guidance to field staff in inspection and
acceptance of stainless steel reinforcement. Captions on each figure describe
the deficiencies.
There may be other types of
contamination or damage that have not yet appeared in MTO work. If you have any
doubt as to whether stainless steel supplied to an MTO contract is acceptable,
contact your Regional Quality Assurance Section or the Concrete Section of the
Ministry for assistance.