Permeability of concrete, chemical attack, acid attack, efflorescence, Corrosion in concrete. Thermal conductivity, thermal diffusivity, specific heat. Alkali Aggregate Reaction

1. Durability of Concrete
Structures
Dr. Vinay Kumar B M
1

2. Introduction
 A long service life is considered synonymous with
durability.
 Designers are mainly concentrated on strength
characteristics.
 Since durability under one set of conditions does not
necessarily mean durability under another.
2

3. Definition
“Durability of Portland cement concrete is defined as
its ability to resist weathering action, chemical
attack, abrasion, or any other process of
deterioration”
3

4.  No material is inherently durable.
 A material is assumed to reach the end of service life
when its properties under given conditions of use have
deteriorated to an extent that the continuing use of
the material is ruled either unsafe
4

5. Materials related failures
 The use of inappropriate materials.
 Poor construction practices.
5

6. Environmental related causes
 Temperature.
 Moisture.
 Physical factors.
 Chemical factors.
 Biological factors.
6

7. Durability problems related to environmental causes
include the following:
 Steel corrosion
 Cracking
 Carbonation
 Chemical attack
 Abrasion and cavitation.
7

8. Impact of W/C ratio on durability
 Permeability is an main contributory factor for volume
change.
 Higher w/c ratio---permeability---volume change---cracks-
--disintegration.
 For durable concrete – use of lowest w/c ratio is a
fundamental requirements.
 Low w/c ratio --- less sensitive to carbonation, chemical
attack and other detrimental effect
8

9. Permeability
 Micro cracks usually occurs at transition zone.
 Initially micro cracks are small---permeability is less.
 Propagation of micro cracks --- drying shrinkage,
thermal shrinkage, and externally applied load ---
increases permeability.
9

10. Permeability of cement Paste
 C-S-H gel , Ca(OH)2 and water is filled or empty capillary
cavities.
 Gel is porous---hardly any water can pass through under
normal conditions.
 The permeability of gel pores is estimated to be about
 Gel pores do not contribute to cement paste.
10

11.  Size of capillary cavities depends on w/c ratio.
 Lower w/c ratio---capillary cavities less---filled up
with hydration products in few days.
 Higher w/c ratio---will not get filled up by hydrated
products----unsegmented cavities.
11

12. Permeability of cement paste with w/c=0.7
12

13. Reasons for higher permeability
 Formation of micro cracks—long term drying
shrinkage and thermal stress.
 Micro cracks at the time of transition zone.
 Higher structural stresses.
 Due to volume change.
 Existence of entrapped air
13

14. Permeability of concrete
 Compared to neat cement paste, concrete with same w/c
ratio---lower coefficient of permeability.
 Larger size aggregate increases permeability.
 Development of micro cracks at the transition zone.
 Drying shrinkage, thermal shrinkage, and externally applied
may cause cracks in transition zone.
 Size of cracks is much bigger than the capillary cavities.
14

15. Interrelation b/n Permeability, volume change
and cracking
 Permeability of concrete referred as the root cause
for lack of durability.
 Volume change-due to heat of hydration or internal
manifestation can crack concrete.
 Micro cracks in transition zone makes concrete
permeable.
15

16. Thermal shrinkage
 Solids expand on heating and contract on cooling.
 The strain associated with change in temperature will
depend on the coefficient of thermal expansion of the
material.
 Except under extreme climatic conditions, ordinary
concrete structures suffer little from changes in
ambient temperature.
16

17.  In massive structures-the combination of heat produced by
cement hydration and relatively poor heat dissipation.
 Results in a large rise in concrete temperature within a few
days after placement.
 Subsequently, cooling to the ambient temperature often
causes the concrete to crack.
 Shrinkage strain from cooling that is more important than
the expansion from heat generated by cement hydration.
17

18. Thermal properties of concrete
1. Thermal Conductivity
2. Thermal diffusivity
3. Specific Heat
4. Coefficient of thermal expansion.
18

19. Thermal Conductivity
 This measures the ability of material to conduct heat.
 Measured in joules/second/square metre of area of
body when the temperature deference is 1
degree/meter thickness of the body.
 influenced by the mineralogical characteristics of
aggregate, moisture content, density, and
temperature of concrete.
19

20. Thermal conductivity of concrete made with
different aggregate
20

21. Thermal diffusivity
 Represents the rate at which temperature changes
within the concrete mass.
 Diffusivity is simply related to conductivity by the
following equation;
Where,
C-Specific heat
P-Density of Concrete.
Range of diffusivity of concrete is b/n 0.002 to 0.006
m2/h
21

22. Specific Heat
“Defined as the quantity of heat needed to raise the
temperature of a unit mass of a material by one degree”.
 The specific heat of normal weight concrete is not very
much affected by the type of aggregate, temperature and
other parameters.
 The common range of values for concrete is b/n 840 and
1170 j/kg/degree Celsius
22

23. Coefficient of thermal expansion
“Defined as the change in unit length per degree of
temperature change”.
 Depends on mix proposition.
 Coefficient of thermal expansion of cement paste
varies b/n
 Coefficient of thermal expansion of aggregate vary
b/n
23

24.  limestones and gabbros have low coefficient of
thermal expansion.
 sandstones, natural gravels, and quartzite have high
coefficient of thermal expansion.
 Selecting an aggregate with a low coefficient of
thermal expansion when it is economically feasible and
technologically acceptable.
24

25. Corrosion
 Chemical interactions between aggressive agents present in
the external environment and the constituents of the
cement paste.
 Chemical corrosion results from the breaking down and
dissolution of the cement paste.
 The nature, concentration, and rate of replenishment of
aggressive agents affect the rate of concrete deterioration.
25

26. What is corrosion of steel ?
“The chemical and electro chemical reaction between the
material (usually steel) and its environment that results
in deterioration of materials and its properties”
 Corrosion results in formation of rust, which has 2 to 4
times the volume of the original steel.
26

27. Why is corrosion of steel is a concern?
 Reinforced concrete uses steel to provide tensile
properties that are needed in structural concrete.
 Prevent failure of concrete structures which are
subjected to tensile and flexure stresses.
 Reinforcement corrodes, the formation of rust leads
to loss of bond between concrete and steel.
 Reduction in c/s area reduces strength capacity.
 40% failure is due to the corrosion of steel.
27

28. Causes of Corrosion
The two most common causes of reinforcement corrosion:
1. Localized breakdown of the passive film on the steel
by chloride ions.
2. General breakdown of passivity by neutralization of
the concrete, predominantly by reaction with
atmospheric carbon dioxide.
28

29. contributing factors leading to corrosion
 Loss of Alkanity due to Carbonation.
 Loss of Alkanity due to Chlorides.
 Cracks due to Mechanical Loading.
 Corrosion of steel reinforcement due to atmospheric
pollution.
 Moisture Pathways.
 Water-Cement Ratio
29

30. 30

31. Mechanism of corrosion
 Corrosion of steel in concrete is an electrochemical
process.
31

32.  When there is a difference in an electrical potential
along the steel reinforcement in concrete, an
electrochemical cell is set up.
32

33.  Corrosion doesn’t takes place if concrete is dry or
below relative humidity of 60% because enough water
is not there to promote corrosion.
 And also if concrete is fully immersed in water
because diffusion of oxygen does not takes place into
the concrete.
 The optimum relative humidity for corrosion is 70-
80%.
33

34. Corrosion control
 Good quality of concrete through good construction
practices.
 Use of lowest possible w/c ratio.
 Proper mix design-use of right quality and quantity of
cement for different exposures.
 Use of supplementary cementitious materials like fly ash.
34

35.  Metallurgical methods
 Corrosion inhibitors
 Coatings to reinforcements
 Cathodic protection
 Coatings to concrete
 Design and Detailing
35

36. Metallurgical methods
 Altering metallurgical process.
 Rapid quenching of hot bars by series of water jets
 By keeping the hot steel bars for a short time in a
water bath.
36

37. Corrosion inhibitors
 By using certain corrosion inhibiting chemicals such as
nitrites, phosphates, benzoates etc.
 Added to concrete during mixing.
 Typical dosage is b/n 10-30 litres per cubic-meter of
concrete.
 Hence the steel is protected by a passivating layer of
ferric oxide.
37

38. Coatings to reinforcements
 Coating to Steel bar is to provide a durable barriers
to aggressive materials such as chlorides.
 Coating should withstand-fabrication of
reinforcement, pouring of concrete and during
compacting.
38

39. 39

40. Steps involved in process are:
1. Derusting
2. Phosphating
3. Cement coating
4. Sealing
5. Fusion bonded epoxy coating
40

41. Cathodic protection
 Comprises of application of impressed current to an
electrode laid on the concrete above steel reinforcement.
 Electrode serves as anode and the steel reinforcement act
as cathode.
 External anode subjected to corrosion and Cathodic
reinforcement is protected against corrosion.
41

42. Coatings to concrete
 Surface coating increases durability of concrete.
 Serves the dual purposes-protection and decoration.
 Bridges, flyovers, industrial buildings, and chimneys.
 Epoxy coating is avoided.
 Acrylic based polymer is more preferred.
42

43. 43

44. Chemical Attack on Concrete
 Carbonation
 Sulphate Attack
 Acid attack
 Alkali-Aggregate Reaction
 Concrete in sea-water
44

45. What is carbonation?
“Carbonation is the formation of calcium carbonate
(CaCO3) by a chemical reaction in the concrete.”
The creation of calcium carbonate requires three
equally important substances :
1. Carbon dioxide (CO2)
2. Calcium phases (Ca)
3. Water (H2O).
45

46. 46

47.  The first reaction is in the pores where carbon
dioxide (CO2) and water (H2O) react to form carbonic
acid (H2CO3):
 The carbonic acid then reacts with the calcium
phases:
47

48.  Once the Ca(OH)2 has converted and is missing from the
cement paste, hydrated C-S-H (Calcium Silicate Hydrate
- CaO•SiO2•H2O) will liberate CaO which will then also
carbonate:
48

49. Rate of carbonation
 The level of pore water.
 Grade of concrete.
 Permeability of concrete.
 Whether the concrete is protected or not.
 Depth of cover.
49

50. Sulphate Attack in concrete
 Solid salts do not attack concrete, but when they are
in the form of a solution, they can directly react with
the hardened cement paste.
 soils contain alkali, magnesium and calcium sulphates.
 sulphates come into contact with groundwater, they
form a sulphate solution.
50

51. sulphate reaction with the Ca(OH)2 and the calcium
aluminate hydrates.
51

52.  The rate of a sulphate attack increases with an
increase in the strength of the sulphate solution.
 The concentration of the sulphates is expressed as the
number of parts of the weight of S04 per million parts
of the solution (ppm).
 1000 ppm is considered to be a moderately severe
sulphate content
52

53.  2000ppm is considered to be a very severe sulphate
content, especially if MgSO4 is the predominant
constituent.
 Concrete that is attacked by sulphates has a
characteristically whitish appearance.
 Sulphate damage usually starts at the edges and
corners of the concrete member and is followed by a
progressive cracking and spalling of the concrete.
53

54. 54

55. Methods of controlling sulphate attack
 Use of sulphate resisting cement.
 Quality of concrete.
 Use of air-entrainment.
 Use of Pozzolana.
 Use of high alumina cement.
55

56. Use of sulphate resisting cement
 The most efficient method of resisting the sulphate
attack is to use cement with the low C3A content.
 It has been found that a C3A content of 7% gives a
rough division between cement of good and poor
performance in sulphate waters.
56

57. Use of high alumina cement
 The cause of great resistance shown by high alumina cement
to the action of sulphate is still not fully understood.
 However, it is attributed in part to the absence of any free
calcium hydroxide in the set cement, in contrast to Portland
cement.
 High alumina cement contains approximately 40% alumina, a
compound very susceptible to sulphate attack, when in
normal Portland cement.
57

58.  But this percentage of alumina present in high alumina cement
behaves in a different way.
 The primary cause of resistance is attributed to formation of
protective films which inhibit the penetration or diffusion of
sulphate ions into the interior.
 It should be remembered that high alumina cement may not
show higher resistance to sulphate attack at higher
temperature.
58

59. Acid Attack on concrete
 Almost all types of mineral acids will have a destructive
effect on concrete.
 The rate of an acid attack is determined by some factors
such as:
1. The amount and concentration of acid,
2. The cement content.
3. The type of aggregate and
4. The permeability of the concrete.
59

60. When the hydrated cement reacts with an acid,
 The lime in the cement tends to neutralize the acid.
 If the concrete is made with a siliceous aggregate,
neutralization can only be affected by the breaking down
of the cement binder.
 If a calcareous aggregate, such as limestone, is used in the
concrete, the aggregate is also active in the neutralization
of the acid.
60

61.  Concrete can be attacked by liquids with PH value less
than 6.5
 Severe if it is less than 5.5
 Very severe ,if it is less than 4.5
 If acids or salt solution are able to reach
reinforcing steel, then corrosion occurs.
61

62. 62

63. What is efflorescence?
“Efflorescence is the formation of salt deposits,
usually white, on or near the surface of concrete
causing a change in appearance.”
63

64. 64

65.  Primary efflorescence is efflorescence occurring during
hardening of the concrete.
 Secondary efflorescence is the efflorescence resulting
from the weathering of the hardened concrete.
 Efflorescence is most obvious in the winter but may be
observed throughout the year after a heavy rain and a
drop in temperature.
65

66. How efflorescence occurs?
Chemical processes
Formation of efflorescence can be the result of a
reaction of concrete constituents with carbon dioxide
and/or sulphurous gases.
Physical processes
The formation of efflorescence depends on a number
of physical processes involving both salt and water
transfer in and out of concrete or masonry.
66

67. Alkali aggregate reaction in concrete
“Chemical reaction between the hydroxyl ions in the pore
water within concrete and certain types of rock minerals”
 Reaction results in hygroscopic gel which expands.
 Gel expansion causes cracking in the concrete.
 The number of structures affected by AAR is relatively
small.
67

68.  Most of the structures severely cracked by AAR are
underground structures, gets contact with damp soil.
 Apart from the moisture, high content of alkali in the
concrete is also essential.
 when there are sufficient moisture and alkali,
maximum expansion of concrete due to AAR occurs
68

69. Sources of alkalis in concrete
1. Cement
2. Pozzolans
3. Aggregate
4. Admixtures
5. Water
6. Alkalis from outside the concrete
69

70. Cement
 The major source of alkali is from cement.
 The chemical composition of cement is usually
expressed in terms of oxides.
 In relation to AAR, alkali content in cement is
determined from Na2O and K2O.
70

71. Pozzolans
 A pozzolan is a siliceous or siliceous and aluminous
material which react with lime forming a compound
possessing cementitious properties.
 Common pozzolanic material used in concrete include;
fly ash, silica fume, and GGBS.
71

72. Aggregates
 Aggregate containing feldspars, some micas, glassy
rock and glass may release alkali in concrete.
 Sea dredged sand, if not properly washed, may contain
sodium chloride which can contribute significant alkali
to concrete.
72

73. Admixtures
 Admixture in the context of AAR in concrete means
chemical agents added to concrete at the mixing stage.
 These include accelerators, water reducers (plasticizers),
retarders, superplasticizers, air entraining, etc.
 Some of the chemicals contain sodium and potassium
compounds which may contribute to the alkali content of
concrete.
73

74. Water
 Water may contain certain amount of alkali.
74

75. Alkalis from outside the concrete
 In area of cold whether, de-icing salt containing
sodium compounds may increase alkali content on the
surface layer of concrete.
 Soils containing alkali may also increase alkali content
on the surface of concrete.
75

76. Alkali content and AAR
 Research show that when the total alkali content, in terms
of equivalent sodium oxide, is less than 3 kg/m3.
 Damage expansion due to AAR is unlikely to happen.
 No reliable universal testing method have been established
for the determination of reactivity of an aggregate.
 Limiting alkali content in concrete has become the most
widely used approach for the control of AAR.
76

77. General types of AAR
1. Alkali-silica reaction.
2. Alkali-silicate reaction.
3. Alkali-carbonate reaction
77

78. Alkali-silica reaction
 Alkali-silica reaction is a reaction between alkali
hydroxides and free silica in aggregate form a alkali-
silica gel
78

79. Alkali-silicate reaction
 Alkali-silicate reaction is the same as alkali-silica
reaction except that in this case the reactive
constituent is not free silica, but it is in the
combined form of phyllosilicates.
79

80. Alkali-carbonate reaction
 Alkali-carbonate reaction occurs in concrete when
alkalis react with certain lime stones containing clay.
 Reaction causes cracks allowing water to enter which
causes the clay to swell and disrupt the aggregate
80

81.  Majority of the structures affected by AAR is found
due to alkali-silica reaction.
 Alkali-silicate and alkali-carbonate reaction is
relatively rear.
81

82. Concrete in sea water
Effect of seawater on concrete deserves special attention:
 First, coastal and offshore sea structures are exposed to
the simultaneous action of a number of physical and
chemical deterioration processes.
 Second, oceans make up 80 percent of the surface of the
earth; therefore, a large number of structures are
exposed to seawater either directly or indirectly.
82

83.  Most seawaters are fairly uniform in chemical composition,
which is characterized by the presence of about 3.5 percent
soluble salts by weight.
 The ionic concentrations of Na+ and Cl− are the highest,
typically 11,000 and 20,000 mg/liter.
 For aggressive action to cement hydration products, sufficient
amounts of Mg2+ and SO2
− 4 are present, typically 1400 and
2700 mg/liter.
 The pH of seawater varies between 7.5 and 8.4
83

84. Table shows the concentration of major ions
in some of the world seas.
84

85. Concrete exposed to marine environment may deteriorate
as a result of combined effects of:
 chemical action of seawater constituents on cement
hydration products.
 Alkali aggregate expansion.
 crystallization pressure of salts within concrete.
 corrosion of embedded steel in reinforced.
 physical erosion due to wave action and floating objects
85

86. Requirements for Durability as per
IS 456:2000
1. Shape and Size of Member.
2. Exposure Conditions.
3. Requirement of Concrete Cover
4. Concrete Mix Proportions
5. Mix Constituents
6. Concrete in Aggressive Soils and Water
7. Compaction, Finishing and Curing.
8. Concrete in Sea-water
(Refer Page no.17 to 21 0f IS 456:2000)----------8marks
86