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8/12/2019 Corrosion of Steel Reinforcement in concrete
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By,
R.Sabitha RajP. Nayana Sri
A.Swetha
K.Shanthi sri
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Corrosion is defined as the destruction of
materials due to chemical (or electro chemical)
reaction with the environment, and also the
loss of steel due to formation of rust.Corrosion is the depassivation of steel with
reduction in concrete alkalinity.
The formation of Ferric Oxide, occupies a
greater volume than steel and results in tensile
forces leading to cracks in the concrete around
the steel reinforcement.
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Corrosion
Reaction
withChloride
Carbonation
DissimilarMetal
Corrosion
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Carbonation is a process by which Carbon
dioxide converts free lime into Calcium
Carbonate and water.CO2 penetrates into the pores of concrete and
reacts with Ca(OH)2dissolved in pore water.
In good quality concrete, the carbonation
process is very slow.
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Chloride salts can enter concrete in two ways:
i. It may be present in the concrete itself
ii. It can penetrate into the hardened concrete
wherever it is permeable and reach thereinforcement at cracks.
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Chlorides may be
introduced
deliberately as an
accelerator or in theform of natural
ingredients found in
some aggregates.
Concrete made of
beach sand.
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The concentration ofchlorides in contact with
the reinforcing steel will
cause corrosion when
moisture and oxygen are
present.
As the rust layer builds,
tensile forces generatedcause the concrete to
crack and delaminate.
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Cracks in concrete permit
corrosive chemicals to enter the
concrete and access embedded
reinforcing steel.
Chlorides are not consumed in
the corrosion process, but
instead act as catalysts to the
process and remain in the
concrete.
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o The concentration of chlorides is greatly
affected by the concretes pH.
o It was demonstrated that a threshold level of
8000 ppm of chloride ions is required to
initiate corrosion when the pH was 13.2.o As the pH was lowered to 11.6 corrosion was
initiated with only 71 ppm of chloride ions.
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ACI 224-90 presents the following table of
tolerable crack widths in reinforced concrete:
Exposure condition Tolerable crack width
(In.)
Dry air, protective membrane 0.016
Humidity, moist air, soil 0.012
De-icing chemicals 0.007
Sea water and sea water spray;
wetting and drying
0.006
Water-retaining structures 0.004
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Corrosion can take
place in concrete
when two different
metals are cast into aconcrete structure,
along with an
adequate electrolyte.
This type of corrosionis also known as
galvanic corrosion.
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Different metals have different rates of electrochemical
activity. Below is a list of metals in increasing order of activity:
1. Zinc
2. Aluminum
3. Steel
4. Iron nickel5. Tin
6. Lead
7. Brass
8. Copper
9. Bronze10. Stainless steel
11. Gold
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1) Keep concrete always dry, so that there is no H2O
to form rust. If concrete is always wet, then there is no
oxygen to form rust.2) A polymeric coating is applied to the concrete
member to keep out aggressive agents. The
embedded epoxy-coating on steel bars provide a
certain degree of protection to the steel bars and,thereby, delay the initiation of corrosion. These
coatings permit movement of moisture to the steel
surface but restrict oxygen penetration such that a
necessary reactant at cathodic sites is excluded.
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3) Stainless steel or cladded stainless steel is used in
lieu of conventional black bars.
4) Using a Fly Ash concrete with very low
permeability, which will delay the arrival of
carbonation and chlorides at the level of the steel
reinforcement. Fly Ash is a finely divided silicarich powder that, in itself, gives no benefit when
added to a concrete mixture, unless it can react
with the calcium hydroxide formed in the first few
days of hydration. Together they form a calciumsilica hydrate (CSH) compound that over time
effectively reduces concrete diffusivity to oxygen,
carbon dioxide, water and chloride ions.
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5) A portion of the chloride ions diffusing through
the concrete can be sequestered in the concrete by
combining them with the tricalcium aluminate toform a calcium chloro-aluminate (Friedels salt).
It can have a significant effect in reducing the
amount of available chlorides thereby reducing
corrosion.
6) Electrochemical injection of the organic base
corrosion inhibitors, ethanolamine and
guanidine, into carbonated concrete.
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7) The rougher the steel surface, the better it adheres toconcrete. oxidation treatment (by water immersion andozone exposure) of rebar increases the bond strength
between steel and cement paste to a value higher than that
attained by clean rebars. In addition, surface deformationson the rebar (such as ribs) enhance the bond due tomechanical interlocking between rebar and concrete.
8) As the cement content of the concrete increases (for a
fixed amount of chloride in the concrete), more chloridereacts to form solid phases, so reducing the amount insolution (and the risk of corrosion), and as the physical
properties improve, the extent of carbonation declines, sopreventing further liberation of chloride from the solidphase.
9) Electrochemical Chloride Extraction (ECE) is arelatively new technology for which long-term servicedata are limited. This method employs a temporary anodethat is operated at current density
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