Rust Vs Corrosion
What is the difference between Corrosion and Rusting?
Rust is the oxide that is formed by open air oxidation of iron. Rusting is caused by the reaction of air and
moisture, generally a slow process and is a very common phenomenon for most steel products and has
practically no effect on the functional quality.
Corrosion on the other hand is the electrochemical reaction between a metal and its environment. The
reaction causes formation of not only oxides but other compounds as well. Corrosion usually results in
accelerated and uneven erosion of metal surface and may be detrimental to its intended service
performance.
Background of corrosion/Driving force for the corrosion:
Oxidation of metal when exposed to atmosphere is rather a natural phenomenon and steel rebar is not an
exception. Iron in common steel grades has a tendency to react with oxygen /water present in the
atmosphere forming various types of oxides , brownish in color and is termed as rust. Process of rusting
may be slowed down through various protective measures but cannot be completely prevented. In fact for
most metals, the oxide is the most stable natural form as observed in the respective metallic ores in the
nature. Therefore any steel, with presence of oxygen, would have a tendency to go back to its natural form
of oxide.
Rebar is not meant for its aesthetic look but for its some critical properties to reinforce the concrete for
structural safety and these are mechanical strength, ductility, bond strength etc .
If we look at the above functional and essential properties of rebar, normal surface rusting as a result of
atmospheric oxidation, does not any way impact the above properties. This has been tested time and again
and the fact established. On the contrary, some amount of rusting makes the surface rougher and thereby
enhancing the bond strength which is beneficial.
This can be explained by following diagram:
Iron Ore in the form of
oxides – FeO, Fe2O3,
Fe3O4
Rebar with mill scale
Steel Making, Casting,
Rolling
Atmospheric rusting of rebar
Rusting of Rebar – Myth vs. Reality:
Myth- Rusted rebar will have weak bonding with concrete.
Realty-Controlled rust improves bond strength.
Surface rusting of rebar does not affect the performance of the rebar. In fact surface rusting can improve
the bond of the rebar with the concrete. This is because of the fact that compared to a smooth surface of
rebar which under load may have a tendency to slip out of the concrete, little rust increases the roughness,
offer more frictional resistance against any force to dis bond the rebar .
Myth-Rusting of rebar reduces its strength.
Realty-Rusting has no effect on rebar strength.
The strength of the rebar is achieved by the control of its chemistry and the thermal treatments through
which it was produced and rusting does affect neither of them. Chemistry and inherent strength remains the
same and so also its load bearing capacity.
However, severe rusting and pitting corrosion which can result from prolonged exposure to open
atmosphere/water and which reduces the section of the rebar heavily and create pits on the surface may
reduce the load bearing capacity. The simple example is the difference in load bearing capacities of rebar
of similar strength but of different sizes. Therefore prolonged exposure to open atmosphere which may
result in corrosion needs to be avoided.
Myth - Rust on rebar is not permitted as per rebar code/standard.
Realty- The rebar standard indicates that rust is a common expected phenomenon on rebar and not
a cause for concern.
IS 1786: 2008-Indian Code for rebar , Clause 4.3.1 States-“ All bars/ wires shall be well and cleanly
rolled and shall be sound and free from surface defects and pipe or other defects detrimental to its
subsequent processing and to its end use. Rust, seams, surface irregularities or mill scale shall not be
the cause for rejection provided a hard wire brushed test specimen fulfils all the requirement of this
specification”.
How to check rebar for its fitness for use?
One of the easy way to check a rebar for its application worthiness is to clean the surface with a wire brush
and observe whether there is any pit formation due to corrosion/over rusting which is not advisable to use.
Pits look like small localized depression on the surface
The other simple measure is to check whether the ribs on the surface has been washed out due to rusting
or not. If it has not shown any rib wash out, it can be used safely.
Severely rusted/pitted
rebar is detrimental
Did We Know?
1. In big construction projects like flyovers, high rise buildings, commercial complexes, and executed by
leading construction organizations and generally is spread over a longer time of construction than a
common house building, usage of rebar with reddish color is a very common phenomenon. Such rebar
would never been used /allowed to be used if there could be any detrimental effect.
2.There are some countries, known for their technological excellence, the construction engineers , by
design, do not use fresh rebar because they feel that it will not give the best bond strength with concrete.
At the site the rebars are kept in open atmosphere till some rust appears on the surface and then only
these are used.
Placement of rebar in big projects
Light coating of red rust is rather beneficial
Protection of Rebar against Corrosion:
While usage of little reddish rebar has its own advantages, it is important to protect it from progressive
corrosion. Corrosion is mostly dependant on the environment which the rebar is exposed to. Any acidic
atmosphere in presence of salts like chloride or gasses like C02 may promote corrosion .Corrosion can
also be encouraged if water is logged in some portion of rebar for long time. A common example of such
situation which may be created when a portion of rebar extends from already cast concrete members. This
is generally done when there is a plan to extend the construction in future.In such cases, the minimum
precaution to be taken is to cover such extended rebar with some coating like cement slurry. In absence of
such protection, it may lead to accelerated rusting /corrosion and should be avoided.
Protection of Rebar Protruding out of concrete for future construction:
In case the building has to be extended vertically in future, it is general practice that reinforcement bars in columns
are extended well above roof for lapping purpose. The reinforcement bars would eventually rust and then corrode if
the bars are left unprotected for longer durations.
If the extension of building is going to take place after some time , the following should be adopted:
1. Cleaning: Brushing with hard wired brush.
2. Cement Slurry Coating: Apply cement slurry by brush to the protruded reinforcement bars. Cement slurry
can be made by mixing cement with water generally in a ratio of one part of cement with 2 parts of water.
Extended rebar
Rebars coated
with cement
slurry
Roof/Slab
Wall
Column
Similar method of applying cement slurry should be adopted for rebars extended from roof slab/beams etc
If the extension of building is going to take place after longer time, the following should be adopted:
Weak concrete
Extended rebar
for lapping
50mm cover
Roof/Slab
Wall
Column
Protect the Protruding rebar with weak concrete
Weak Concrete:
Weak concrete is a M10 grade of concrete where cement: sand: coarse aggregates
have the ratio of 1:3:6 by weight proportion. This concrete cover can be broken before start of fres
construction
Storage of Rebar at Site:
The actual location of stacking place of reinforcement bars depends on the site conditions but it should be such that it
is protected against contact with water. In order to ensure that the reinforcement bars are kept in good condition
they should not be left in direct contact with the ground but they should be stacked on top of dunnages made
out of timber or say bricks and in particularly rainy season , should be covered. Where space is extremely limited,
bars may also be stacked vertically with adequate support with additional care of handling during use. Whatever
method of stacking is adopted the bars should be stacked in such a way that the various sections and lengths can be
easily identified. Section mix up being so common , it is advisable to use rebar which marks the section in rebar
clearly.
With such simple measures and understanding, usage of rebar for better structural safety and durability can be
ensured
X
Should not be stored in open yard
Points to be note:
 Rusting of rebar to an extent does not affect any functional quality rather improves its bond strength.
 However severe rusting and corrosion is not desirable and should be avoided.
 Storage and protection of rebar from direct contact with water needs a conscious attention.
 Protruded part of rebar from an ongoing construction site requires specific protection to avoid accelerated
rusting.