The corrosion of reinforcement inside concrete is one of the factors affecting the performance of the structure. Here the presentation will enlighten you about its mechanism and other aspects.

1. CORROSION OF REINFORCEMENT, CARBONATION
OF CONCRETE AND IT'S DETERMINATION
Presentation by:
Rushikesh Katkar.

2. Content
 Corrosion of reinforcement
 Mechanism of corrosion
 Factors affecting
 Problems due to corrosion
 Causes of Corrosion
 Carbonation of concrete
 Reactions involve
 Effect of Carbonation
 Factors affecting carbonation
 Method of measuring carbonation
 Control of Carbonation

3. Corrosion of Reinforcement
Reinforcing steel in concrete is normally protected from corrosion by the passive film formed at the
steel/concrete border. Although steel’s natural tendency is to undergo corrosion reactions, the alkaline
environment of concrete (pH of 12 to 13) provides steel with corrosion protection. At the high pH, a
thin oxide layer forms on the steel and prevents metal atoms from dissolving. This passive film does
not actually stop corrosion
In the presence of chloride and other causing agents, the steel protective passive layer is locally
destroyed and unprotected steel areas dissolve.However, corrosion can occur when the passive layer is
destroyed.
The protective layer present around the steel bar in an alkaline medium

4. Corrosion of steel reinforcement is a major problem influencing the long-term
performance of reinforced concrete structures
It occurs due to attacks
of aggressive agents
Chloride ions
from marine
environments
De-icing salt
Chloride
contaminated
aggregates

5.  To maintain neutrality, the Fe2+ migrate through the concrete pore water to the
cathodic sites where they combine to form FeOH, or rust.
 This hydroxide tends to react further with oxygen to form higher oxides.
 The increases in volume as the reaction products react further with dissolved oxygen
leads to internal stress within the concrete that may be sufficient to cause cracking
and spalling of the concrete cover
The Mechanism of Corrosion
 Corrosion is an electrochemical process involving the flow of charges (electrons and
ions).
 The Anodic Reaction: At active sites on the bar, called anodes, iron atoms lose
electrons and move into the surrounding concrete as ferrous ions. The electrons
remain in the bar and flow to sites called cathodes, where they combine with water
and oxygen in the concrete
2Fe → 2Fe2+ + 4e-
2H2O + O2 + 4e- → 4OH-

6. The different factors causing
corrosion and its effect on
concrete
The mechanism of corrosion

7. Factors Affecting Corrosion
1. Permeability
 Permeability of concrete is mainly determined by the porosity of concrete and its
pore size distribution which are dependent on the ratio of w/c.
 Low w/c ,better compaction ,and use of mineral admixtures could lower the
permeability of the cover concrete, therefore they are the option to improve the
corrosion resistance of reinforced concrete.
A Permeable Concrete structure where Corrosion Inhibiters are applied

8. 2. Pore solution of concrete
 The pore solution in concrete is an electrolyte which is physically absorbed in the
pores of the concrete.
 It reacts with the steel reinforcement and under certain conditions can lead to the
corrosion damage at the steel surface.
Schematic illustration of the ingress of chloride ions from an
exposure solution (e.g. seawater) into a reinforced concrete
structure.

9. 3. Moisture
 The moisture of concrete has a complicated
influence on the corrosion of steel in the
concrete. The water absorption into concrete
from outside environment can rapidly increase
the rate of corrosion of reinforcing steel to the
level that will cause cracking and spalling.
 Presence of moisture is a precondition for
corrosion to take place because concrete can act
as electrolyte in electrochemical cell only if it
contains some moisture in pores.
 Corrosion can neither occurs in dry concrete or
in submerged concrete.
Accumulation of moisture at the sealing of a
warehouse

10. Problem Statement
The problem
of corrosion
the formation
of corrosion
products
substantial
volume
increase
expansive
stresses are
induced
loss of bond
between
steel/concrete
spalling of
concrete
cover
possible
cracking

11. Causes of Corrosion in Concrete Reinforcement
Causes
Carbonation
Reduction of
alkalinity
Destruction of
passive layer
Chlorination
Chloride
penetrates
protective layer
CORROSION

12. Carbonation
 Associated with the corrosion of steel
reinforcement and with shrinkage.
 The dissolution of CO2 in the concrete
pore fluid.
 CO2 reacts with calcium from calcium
hydroxide and calcium silicate hydrate
to form calcite Ca(CO)3.

13. Reaction involves
Ca(OH)2 + CO2 -------------------->CaCO3 + H2O
 A Physiochemical Reaction.
 Reaction between atmospheric carbon dioxide and the calcium hydroxide
generated in cement hydration.
 The precipitation of calcium carbonate reduces the pH level of concrete.

14. Effect of Carbonation
 Carbonation reduces the alkalinity of concrete which gives a protective coating to
the reinforcement against rusting. If depth of carbonation reaches up to steel
reinforcements, the steel becomes liable for corrosion.
 Carbonation of concrete also result in reduced permeability, possibly because water
released by carbonation promotes the process of hydration and also calcium
carbonate reduces the voids within the cement past
 Compressive strength of carbonated concretes slightly Increases in comparison
with non-carbonated concretes.
 Carbonation depth increases with an in- crease of carbonation time and higher
CO2 concentration has a higher carbonation depth

15. Factors affecting Carbonation
 Concentration of C0 2 gas in atmosphere Normally 3% but increasing annually
Higher in cities, due to motor vehicles and fossil fuel burning.
 Pore system of Hardened Concrete.
 Relative humidity (for dissolution of Ca(0H)2 ).
 Lower humidity , C0 2 can not dissolve.
 But in higher humidity, C0 2 can easily dissolve.

16. Methods to Measure
Carbonation Extent of Carbonation is measured in two ways:
 First way is to measure the concentration of CO2 absorbed by the concrete specimen.
 Second way is to carbonate the specimen in (a) natural or (b) laboratory environment
conditions and then break it and spray a pH indicator to know the extent of
Carbonation.

17. IR Spectrum Analysis
carbonation set-up consists of a close loop in which a mixture of air and carbon-dioxide
could be introduced at a certain RH. Due to the carbonation reaction, an amount of CO2
molecules will be immobilized reducing the concentration of carbon dioxide in the
circulating gas mixture. The CO2 concentration in the gas is measured using an IR
absorption device. A pump is used to circulate the gas while also temperature and RH are
measured.

18. Acid test
 The measurement of carbonation depth using the phenolphthalein solution.
 Spraying the indicator on the split surface of the concrete cylinder .
 The solution became a pink color in the carbonated concrete.
 It can be differentiated from the uncarbonated concrete.
 Carbonation depth up to an accuracy of 5mm
can be identified with the naked eye.

19.  The colorless acid-base indicator (phenolphthalein solution) monitoring
the carbonation depth is by capturing the depth at which the pH is about
9.2.
 It indicates the boundary at which the carbonated front meets with the
uncarbonated concrete, where concrete is alkaline .
 There is a partially carbonated zone where the pH value is not easily
detected using phenolphthalein indicator.
Mechanism- phenolphthalein solution
Compare in phenolphthalein indicator
method and FT-IR spectrum analysis
 FT-IR spectroscopic test can identify
a partial carbonation front more
readily than a phenolphthalein
indicator
 FT-IR Fourier transform infrared
spectroscopy

20. Carbonation Depth
 Carbonation depth is assessed using a solution of phenolphthalein indicator.
 Carbonation is slight and eventually comes to a stop, with the depth reached being
known as the maximum carbonation depth.
 Carbonation depth amounts to only a few millimeters and cannot extend as far as
the reinforcement. Carbonation protection (CO2 -proofing) is not necessary.
 Carbonation has nearly reached the reinforcement layer. Carbonation protection is
necessary in order to stop further progress.
 The majority of the reinforcement is located in the already carbonated zone of the
concrete. In this case, carbonation protection would be too late

21. Relationship between carbonation depth and compressive
strength
 The depth of carbonation decreases with an increase in compressive strength.
 Very logical, since both carbonation and compressive strength are significantly
controlled by the pore structure of concrete.

22. Control of Carbonation
To increase carbonation resistance of your concrete, you may try the following tips:
 Increase CO2 binding capacity: e.g. increase the available CaO by using low
replacement level of Portland cement; use higher binder-to-aggregate ratio
 Increase capillary condensation: i.e., reduce porosity (e.g. use lower water-to-binder
ratio, higher binder-to-aggregate ratio) and/or increase relative humidity, as such
lowering the diffusion of CO2.
 Surface coating. You may have already known these tips. To my opinion, if you are not
allowed to change the composition of your concrete. Surface coating by using less
expensive and concrete compatible coating agents may be the most efficient solution to
reduce the risk of carbonation

23. References
 Effect of Carbonation on Mechanical Properties and Durability by
Acceleration method by Jack M. , C.C.Yang . (2002)
 Response by Zhenguo Shi, Empa - Swiss Federal Laboratories for Materials
Science and Technology at www.researchgate.com
 Concrete technology theory and practice by M.S.Shetty,( June 2019)
 Article on corrosion of Embedded material (www.cement.org)