Effects of chemicals on concrete structures

1. Chemical Attack in Concrete
L.CRISPUS BRIGHTSTONE
M.Tech. (Structural Engineering)

2. BASIC TERMINOLOGIES
 Passive layer of concrete
 Corrosion

3. PASSIVE LAYER OF
CONCRETE
 The strongly alkaline nature of Ca(OH)2 (pH of
about 13) prevents the corrosion of the steel by
the formation of a thin protective film of iron oxide
on the metal surface. This protection is known as
passivity.
 If the concrete is permeable to such an extend
that soluble chlorides penetrate right up to the
reinforcement and water & oxygen is also
present, then the corrosion of steel will take
place. This layer can also be lost due to
carbonation.

4. Corrosion
For corrosion to occur, these elements must be present:
~There must be at least two metals (or two locations on a single
metal) at different energy levels
~ an electrolyte
~ a metallic connection
In reinforced concrete, the rebar may have many separate
areas at different energy levels. Concrete acts as the
electrolyte, and the metallic connection is provided by wire
ties, chair supports, or the rebar itself.

5. Chemical attacks:
 Chloride attack
 Sulphate attack
 Carbonation
 Alkali-aggregate attack

6. CHLORIDE ATTACK
• Chloride attack is one of the most important
aspects while dealing with durability of concrete.
• It primarily causes corrosion of reinforcement.
Statistics have indicated that over 40% of
failure of structures is due to corrosion of
steel.

7. Possibilities of Chloride
attack
•Chloride enters the concrete from the cement, water,
aggregate and sometimes from admixtures. This can also
enter by diffusion from environment if concrete is permeable.
•The Bureau of Indian Standard had specified the maximum
CHLORIDE CONTENT IN CEMENT AS 0.1%.
•The amount of chloride required for initiating corrosion is
partly dependent on the pH value of the pore water in
concrete.
•At a pH value less than 11.5 corrosion may occur
without the presence of chloride.

8. Corrosion of steel due to Carbonation
Carbon dioxide in air or dissolved in water reacts with hydrated
cement systems.
The passive layer can be lost by carbonation due to reduction in alkalinity
of the concrete.
In the presence of moisture and CO2 reacts as
CO2 +H2O  H2CO3 (Dil. Carbonic acid)
H2CO3 +Ca(OH)2  CaCO3 +H2O

9. Effects of Carbonation:
 Carbonation reduces alkalinity of concrete-
which in turn reduces the passivity of concrete
 The corroded volume occupies 6 times the
volume of the original volume.
 This trust on concrete results in
spalling,cracking or delamination of cover
concrete

11. Spalling

12. SULPHATE ATTACK:
 Present in soil in the form of
calcium,sodium,pottasium & magnesium
 Ammonium sulphate- water mixed with
fertilizers and sewage from industrial effluents
 Natural occurrence- Decay of vegetation and
shallow lakes

13.  Formation of sulpho-aluminates
C3A + 3 CS H2 + 26 H  C3A(CS)3H32
(sulpho aluminates increases in volume)
 Formation of calcium-sulphate:
NS + CH + 2H  NH + CS H2

14. Alkali-aggregate reaction
 Chemical reactions between aggregate
containing certain reactive constituents
and alkalis (sodium and potassium salts)
 Granite, granite gneiss and schist,
quartzite and sandstone, containing
strained quartz are among the reactive
rocks found in India.

16. THANK YOU..!!