A short explanation about the deterioration of concrete due to chemical attack.

1. PREPARED BY:-
SUBASH KUMAR SAH (PAS076BCE130)
SUBHASH SUBEDI (PAS076BCE131)
SUDHIR PANGENI( (PAS076BCE132)

2. One of the most prevailing reason for the failure of concrete
structures.
A chemical attack involves dissolution of substances or chemical
reactions between substances and components of the concrete.
Reaction products might cause serious problems, due to dissolution
or expansion.
Any type of concrete structure is prone to Chemical Ataacks.
Moreover, The chemical attack can cause cracking, volume changes
and deterioration on concrete.
Due to this, the life of structure will reduce gradually; ultimately
leading to the failure of structure.

3.  Sulphate Attack
 Alkali-Aggregate Reactions
 Acid Attacks
 Attack By Sea-Water.
 Carbonations
 Chloride Attack
 Corrosion of Steel(Chloride Induced)

4.  Sulphate attack refers to the chemical breakdown of
components of hardened cement paste by sulphate ions
(which can be prominently found in natural soil, ground
water, sea water, sewers, etc).
 The sulphate ions react chemically with the hydrated lime
and hydrated calcium aluminate of concrete to form
Calcium Sulphate(Gypsum) and Ettringite respectively,
both products occupying larger volume than compounds
they displace.
 It causes expansion and cracking of concrete which
decreases the durability of concrete.
 Concrete attacked by sulphates have characteristic whitish
appearance.

5. Methods of Controlling Sulphate Attack
1. Use of Sulphate Resisting Cement:
Cement with low C3A content.
2. Quality Concrete:
3. Use of air-entrainment:
4. Use of pozzoloana : Pozzolana converts
leachable Ca(OH)2 into insoluble non-
leachable cementitious product. The removal of
Ca(OH)2 reduces the susceptibility of concrete
to attack by MgSO4.

6.  Various types of aggregates, containing silica (in
presence of water) react with alkalis in concrete
(K2O and Na2O derived from cement) producing an
expansive gel that creates extensive cracks in
concrete and damages the structural members.

7. Factors Influencing the Alkali-Aggregate Reaction
1. Alkali Content: Cement containing more than 0.6% Na2O
when used in A combination with an alkali reactive
aggregate can cause significant expansion.
2. Admixtures: Admixtures may contribute to the total alkali
content in concrete.
3. Aggregate: Aggregates containing high silica content are
more reactive.
4. Environment: Concrete exposed to fluctuating conditions
of moisture content and temperature, continual dampness
or continuous wetting and drying over a long period are
vulnerable to AAR.

8.  The gases like CO2, NO2, SO2, etc. present in the
atmosphere combine with the atmospheric moisture to
form acids.
 These acids fall to the concrete structure in the form of
rain which dissolves and removes a part of hydrated
cement paste leaving behind a weak mass.
 The acid attack is generally encountered in industrial
areas.
 Rate of attack also depends on the ability of hydrogen
ions to be diffused through the cement gel (C-S-H) after
Ca(OH)2, has been dissolved and leached out.

9.  Sea water contains sulphate which causes sulphate
attack forming calcium sulphate and Ettringite.
 These compounds are more soluble in a chloride solution
than in water, so that they can be easily leached by sea
water.
 Expansion can take place as a result of the pressure
exerted by the crystallization of salts in the pores of the
concrete resulting reduction of strength.
 Concrete between tide marks, subjected to alternating
wetting and drying, is severely attacked, while
permanently immersed concrete is attacked least.

10.  Carbonation of concrete is a process by which CO₂
from the air penetrates into concrete and reacts with
calcium and hydroxide to form calcium carbonates. This
results in small shrinkage.
CO2+ Ca(OH)2 CaCO3+H₂O
 CO₂ by itself is not reactive. In the presence of
moisture, CO, changes into dilute carbonic acid which
attacks the concrete and reduces alkalinity of concrete.
 Higher the concentration of CO₂, higher is the chance of
carbonation. So, more carbonation occurs in city than in
village. In tunnels, carbonation is even more.

11.  The pH value of pore water in hardened concrete is
between12.5 to 13.5 which prevents the steel
reinforcement from action of oxygen and water. This
highly alkaline condition which prevents corroding is
known as passivity.
 Carbonation reduces the alkalinity of the pore water
which results in the corrosion of steel reinforcement.

12. Factors Affecting the Rate of Carbonation
i. Level of pore water, i.e., relative humidity: high rate at 50-
70% relative humidity.
ii. Grade of concrete: Slower in stronger concrete.
iii. Permeability of concrete: Higher in more permeable concrete.
iv. Concrete Protection: Higher in unprotected concrete.
v. Depth of cover: Higher in concrete having less depth of cover
vi. Time: Increases with time.

13.  The strongly alkaline nature of Ca(OH)2 prevents the corrosion
of steel reinforcement by the formation of thin protective layer
of iron oxide on the metal surface. This protection is known as
Passivity.
 However, Chloride ions present in the cement paste
surrounding the reinforcement reacts at anodic sites to form
hydrochloric acid which destroys the passive protection film on
the steel. The surface of the steel then becomes activated
locally to form the anode, with the passive surface forming the
cathode; thus ensuring corrosion in the form of localized
pitting.

14. The passive iron oxide layer is destroyed when PH falls
below 11 and carbonation lowers the pH to about 9.
The formation of rust results in increase in volume of the
steel reinforcement so that swelling pressures will cause
cracking and spalling of the concrete.
FeFe+++2e- (anodic reaction)
4e-+0₂+ 2H2O 4(OH)- (cathodic reaction)
Fe+2(OH)- Fe(OH)2 ( ferrous hydroxide)
4Fe(OH)2+2H2O+ O2 4Fe(OH)3, (ferric hydroxide)

15. Preventive Measures of Corrosion
I. Provide adequate cover to reinforcing bars.
II. The maximum chloride content of cement should be in
accordance with the type of concrete. It should never be
more that 1% by mass of cement.
III. Use galvanized steel as reinforcement.
IV. Provide protective coating to concrete.
V. Use slag cement or portland pozzolana cement.

16. References
i. Theconstructor.org
ii. Wikipedia.com
iii.Concrete Technology Theory and Practice by M.S. Shetty