Concrete Technology

2. Durability of Concrete
Durability of concrete can be defined
to mean ‘its resistance to
deteriorating influences, which
may reside inside the concrete
itself, or which are present in the
environment to which the
concrete is exposed’.

3. Features of Durable
Concrete
•A durable concrete is one that performs satisfactorily
in the working environment during its anticipated
exposure conditions during service.
•Durable concrete will retain its original form, quality
and serviceability when exposed to its environment.
•It should not disintegrate or show sign of wearing
under adverse conditions.

4. Significance of Durability
•In late 1930’s it was found that series of failures
of concrete pavements have taken place due to
frost attack.
•It is not always entirely true that the strong
concrete is always a durable concrete.
•A structure which is very strong when it is
constructed but can’t withstand for a long time
the effect of the environment in which it has to
exist is not really useful.

5. Causes of Lack of Durability
It is caused by external agents arising from the
environment or by internal agents within the concrete;
causes can be categorized as:
•Physical causes- action of frost (freezing and thawing),
difference between the thermal properties of aggregates
and of the cement paste.
•Mechanical causes- they are associated mainly with
abrasion (machinery and metal tyres).
•Chemical causes- it is associates with chemical
reaction in concrete (sulphate attack, acid attack by sea
water, chloride attack, alkali aggregates reaction,
carbonation).

6. Damage of concrete due to lack of
Durability
• Fine to wide cracks developed in concrete.
• Scaling (localized small patches) of concrete can
take place.
• Spalling of concrete can happen. (It is a result of
water entering brick, concrete or natural stone and
forcing the surface to peel, pop out or flake off)
• Disintegration of concrete takes place.
• Deposits of salts can take place, which is called
efflorescence of concrete.
• Complete structure failure can occur.

7.  Environment, freezing and thawing,
exposure to aggressive chemicals.
 Cover to embedded steel.
 Type and quality of constituent materials.
 Cement content and water-cement ratio.
 Workmanship to obtain full compaction and
efficient curing, and
 Shape and size of member.

8.  Curved members without adequate drainage and
very thin members are more vulnerable to
corrosion of reinforcement and cracking of
concrete.
 Adequate curing is essential to avoid the harmful
effects of early loss of moisture.
 Minimum cement content should be used, as
increase in cement content increase the risk of
cracking due to drying shrinkage and thermal
cracking.

9. Type of
Exposure Environment Description
Nominal
Cover
in mm
Mild Concrete surfaces protected against weather or aggressive conditions,
expect those situated in coastal area. 20
Moderate
Concrete surface sheltered from rain or freezing
Concrete permanently under water
Concrete in contact or buried non-aggressive soils.
Concrete surface sheltered from saturated salt air in coastal area.
30
Severe
Concrete surface exposed to severe rain, alternate wetting and drying
or occasional freezing.
Concrete completely immersed in sea water. Concrete exposed to
coastal environment.
45
Very
severe
Concrete surface exposed to sea water spray, corrosive fumes or
severe freezing conditions
Concrete in contact or buried aggressive soils.
50
Extreme Concrete surface exposed to abrasive action like sea water carrying
solids or chemicals (tidal zone). 75

10. For better understanding of durability following
issues have been explained in detail:
 Permeability of concrete.
 Resistance of concrete to freezing and thawing.
 Chloride and sulphate attack.
 Joints in concrete and cracks in concrete.

11. Nature of the Problem
Deteriorating Agencies
Chemical –
Sulphates,
Chlorides,
CO2,
Natural waters,
Mechanical –
Abrasion, Erosion, Cavitation,
Impact,
Temperature.

12. Exposure
Condition
Max. w/c
ratio
Min. cement
content,
kg/m3
Min. grade
of concrete
Min. cover,
mm
Moderate 0.45 340 M25 40
Severe 0.45 360 M30 45
Very Severe 0.40 380 M40 50
Extreme 0.35 400 M45 75

13.  This property is of interest in relation to the water-tightness
of liquid-retaining structures and to chemical attack.
( example - in sewage tanks, dams, pressure vessels in
nuclear reactor, foundation in aggressive soils, etc.)
 The attack by sulphates, acids, sea water, chlorides etc.,
which induce electro-chemical corrosion of steel
reinforcement. Since this attack takes place within the
concrete mass, the attacking agent must be able to
penetrate throughout the concrete mass, which therefore
has to be permeable.
 Many aspects of concrete durability are improved by
reducing the permeability of concrete.

14.  The cement paste consists of C-S-H gel, Ca(OH)2
and water filled or empty capillary cavities.
Although gel is porous to extend of 28%, the gel
pores are so small that hardly any water can
pass through normal conditions. The
permeability of gel pores is estimated to be
about 7 x 10-16 mls, i.e. approximately 1/100 of
that of paste. Therefore, the gel pores do not
contribute to the permeability of cement paste.
C-S-H Gel  C6A2S 2H34 or {Ca6[Al(OH)6]2·24H2O}(SO4)2[Al(OH)4]2

15.  The size and extend of capillary cavities
depend on the wIc ratio. It is one of the main
factors contributing to the permeability of
paste.
 At lower wIc ratio, not only extend of capillary
is less but the diameter is also small. The
capillary cavities resulting at low wIc ratio
will get filled up within a few days by the
hydration products of cement.

16.  Only unduly large cavities resulting from
higher w/c ratio (say more than 0.7) will not
get filled up by the products of hydration,
and will remain as unsegmented cavities,
which is responsible for the permeability of
paste.

18. Permeability of concrete is mainly
dependent on the:
 Water/cement ratio of concrete mix.
 The age of the concrete.

19. Apart from wIc ratio and age of concrete the other
factors which are responsible for increasing
permeability are as follows:
Use of Porous aggregate - Permeability of concrete
is generally of the same order when it is made with
normal weight aggregates, which have permeability
similar to that of the cement paste, but the use of a
more porous aggregate will increase the
permeability of concrete.

20.  Curing of concrete - Interruption of moist curing
for a period of drying will also cause an
increase in permeability because of creation
water passages by minute shrinkage cracks
around aggregate particles, especially the large
ones.
 Steam Curing of concrete - Permeability of
steam-cured concrete is higher than that of
moist-cured concrete.
 Gradation of aggregate - While a low W/C ratio
is essential for the concrete to have a low
permeability, it is not itself sufficient. The
concrete must be dense and therefore a well-graded
aggregate has to be used.

21. From durability point of view, it may be
important to achieve low permeability as
quickly as possible. This can be achieved by
taking following measures :
Use lower W/C mix - Adopt a mix with a low
W/C ratio because the stage at which the
capillaries become segmented is achieved
after a shorter period of moist curing.
Use pozzolanic materials - The use of
pozzolanic materials like fly ash, blast furnace
slag, silica fume, etc in optimum proportion
reduces the permeability of concrete.

22.  Do Proper Compaction - Proper compaction
of concrete should be done, so that dense
concrete is obtained.
 Provide adequate cover - Adequate cover to
the reinforcement is provided, so that cracks
will not develop and attacking agents do not
penetrate up to level of reinforcement easily.