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Durability and Permeability of
Concrete
130605
Unit-5
Syllabus
• Durability and permeability of concrete:
Definitions, causes, carbonation, cracking.
Durability
• Durability is the ability to last a long time
without significant deterioration. A durable
material helps the...
Permeability
• Permeability is defined as the property that
governs the rate of flow of a fluid into a
porous solid.
Durability of Concrete
• Durability of concrete: ability to resist
weathering action, chemical attack,
abrasion, or any pr...
Durability of Concrete
• A durable concrete is one that performs
satisfactiorily under anticipated exposure
conditions dur...
Durability of Concrete
Durability of Concrete
• One of the main characteristics influcing durability of concrete is its
permeability to the ingre...
Durability of Concrete
• Role of Water-Cement Ratio
• The volume change in concrete results in cracks and
the cracks are r...
Durability of Concrete
• Role of Water-Cement Ratio
• It is generally recognized that quality of hydration product and
the...
Durability of Concrete
• Role of Permeability
• The capillary pores in concrete serve as a conduit or provide
transport sy...
Capillary Pores in Concrete
Durability of Concrete
• Permeability of Cement
• Cement paste consists of C-S-H (gel), Ca(OH)2 , and both water filled
an...
Durability of Concrete
• Permeabilty of Concrete
• Introduction of aggregate, particularly large size of
aggregate, increa...
Durability of Concrete
• Effect of Mineral Additives
• Concrete containing cement with 35 % fly ash has
been found to be 2...
Durability of Concrete
• Effect of Air-Entrainment
• An air-entrainment upto 6 % can make the
concrete more impervious. Th...
Air-Entrainment
Factors Affecting Durability
• The factors affecting durability are broadly
divided into two groups, namely external
facto...
Factors Affecting Durability
External factors Physical, chemical, or
mechanical
Environmental, such as extreme
temperature...
Physical causes of deterioration of
concrete
Cracking Surface wear
Structural
loading
Overloading
and impact
cyclic
loadin...
Requirement for Durability
• We shall discuss the requirements for durability
under following heads
• Exposure conditions
...
Requirement for Durability
• Requirement of concrete cover
• Shape and size of member
• Type and quality of constituent ma...
General Environment
• The general environment to which the concrete
will be exposed during its working life is
classified ...
General Environment
Effect of Weathering-Freezing and
Thawing
• Lack of durability of concrete due to freezing and thawing
of frost is not so ...
Freezing and Thawing
Sea Water Attack
• Sea water contains sulphates and hence attacks concrete in a
manner similar to the sulphate attack.
• T...
Sea Water Attack
Steps to Improve Durability of
Concrete in Sea Water
• The use of pozzolana or slag cement is advantageous
under such cond...
Abrasion, Erosion and Cavitation
• Abrasion refers to wearing of the surface by friction. Erosion refers
to wearing away o...
Abrasion, Erosion and Cavitation
Shape and size of member
• The shape or design details of exposed structure should be such as to
promote good drainage of ...
Requirements of Concrete Cover
• The protection of the steel in concrete against
corrosion depends upon an adequate thickn...
Requirements of Concrete Cover
Exposure Nominal cover in mm (minimum)
Mild 20 mm
Moderate 30 mm
Severe 45 mm
Very Severe 5...
Type and Quality of Constituent
Materials
• 1) Cement Mix proportions:
• The water/ cement ratio is an important factor
go...
Type and Quality of Constituent
Materials
• 3) Chloride in Concrete:
• Due to high alkalinity of concrete protective oxide...
Chloride attack
Sulphates in Concrete
• Sulphates are present in most cement and in
some aggregate. Excessive amount of sulphate
attack on...
Sulphates in Concrete
Permeability of Concrete
• For completing hydration of cement about 38
% of water by weight of cement is rquired to
fill t...
Importance of Permeability
• In reinforced concrete, ingress of water and air will
result in corrosion of steel leading to...
Factors Affecting Permeability
• The main factors affecting permeability are:
• Water/ cement ratio
• Properties of cement...
Factors Affecting Permeability
• Water/ Cement ratio:
• For the pastes hydrated to the same degree, the permeability is lo...
Factors Affecting Permeability
• Absorption and homogenity of concrete:
• The volume of pore space in concrete is measured...
Factors Affecting Permeability
• Curing: Continued hydration of the cement
paste results in the reduction in the size of t...
Factors Affecting Permeability
• Use of admixtures: Use of water proofing admixtures
reduces permeability of lean mixes. I...
Measurement of Water Permeability
• The measurement of permeability in the
laboratory is very simple. The side of test
spe...
Where, of flow of water m3/s
A= Cross –Sectional area of the sample (m2)
L= thickness of sample
K= Coeffecient of permeabi...
Carbonation
• Concrete is an alkaline substance and provides excellent
protection to reinforcement embedded inside. The al...
Carbonation Damage
Process of Carbonation
• The carbondioxide present in the atmosphere reacts in
presence of water with the concrete surface...
Process of Carbonation
Advantages and Disadvantages of
Carbonation
• Carbonation of concrete improves several characteristics of ordinary
concret...
Cracks in Concrete
• The cracks in concrete is an inherent feature which
cannot be completely prevented but can only be
co...
Cracks in Concrete
Cracks in Concrete
• Causes of Cracks
• Use of unsound materials, bad
workmanship, use of high water/cement
ratio, bad joi...
Cracks in Concrete
• All the concrete structures crack in some form
or the other. Most buildings develop cracks in
their f...
Cracks in Concrete
• Cracking of concrete structures can never be
totally eliminated, but the practitionar should be
aware...
Causes of Cracks in Concrete
• Cracking of Plastic Concrete
• When the exposed surface of freshly placed concrete are subj...
Plastic Shrinkage Cracks
Causes of Cracks in Concrete
• Cracking in Hardened Concrete
• The moisture induced volume changes are characteristics of ...
Cracking in Hardened Concrete
Causes of Cracks in Concrete
• Thermal Cracking
The temperature difference within a concrete structure result in different...
Thermal Cracking
Causes of Cracks in Concrete
• Cracking Due to Chemical Reactions
• The most important constituent of concrete is cement w...
Cracking Due to Chemical Reactions
Causes of Cracks in Concrete
• Cracking Due to Chemical Reactions
• When the sulphate bearing water come in contact with t...
Causes of Cracks in Concrete
• Cracking Due to Weathering
• The environmental factors that can cause cracking
include (i) ...
Cracking Due to Weathering
Causes of Cracks in Concrete
• Cracking due to Corrosion of Reinforcement
• The corrosion of steel produces iron oxide and...
Cracking due to Corrosion of
Reinforcement
Causes of Cracks in Concrete
• Cracking due to Poor Construction Practices
• Poor construction practices, such as adding w...
Cracking due to Poor Construction
Practices
Causes of Cracks in Concrete
• Cracking due to Construction Overloads.
• The loads induced during construction can be far
...
Cracking due to Construction
Overloads.
Causes of Cracks in Concrete
• Cracking due to Errors in Design and Detailing
• The design and detailing errors that may r...
Thanks
References
• Concrete Technology by: R.P. Rethaliya
• Concrete Technology by . M.S. Shetty
• Internet websites
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Durability and Permeability of Concrete

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Durability and Permeability of Concrete

  1. 1. Durability and Permeability of Concrete 130605 Unit-5
  2. 2. Syllabus • Durability and permeability of concrete: Definitions, causes, carbonation, cracking.
  3. 3. Durability • Durability is the ability to last a long time without significant deterioration. A durable material helps the environment by conserving resources and reducing wastes and the environmental impacts of repair and replacement.
  4. 4. Permeability • Permeability is defined as the property that governs the rate of flow of a fluid into a porous solid.
  5. 5. Durability of Concrete • Durability of concrete: ability to resist weathering action, chemical attack, abrasion, or any process of deterioration.
  6. 6. Durability of Concrete • A durable concrete is one that performs satisfactiorily under anticipated exposure conditions during its life span. The material and mix proportions used should be such as to maintain its intigrity and, if applicable, to protect embeded metal from corrosion. Even though concrete is a durable material requiring a little or no maintenance in normal environment but when subjected to highly aggressive or hostile environments it has been found to deteriorate resulting in premature failure of structures or reach a state requiring costly repairs.
  7. 7. Durability of Concrete
  8. 8. Durability of Concrete • One of the main characteristics influcing durability of concrete is its permeability to the ingree of water, oxygen, carbon dioxide, chloride, sulphate and other potential deleterious substances. • Most of the durability problems in the concrete can be attributed to the volume change in the concrete. Volume change in concrete is caused by many factors. The entire hydration process is nothing but an internal volume change, the effect of hydration, the pozolanic action, the sulphate attack, the carbonation, the moisture movement, all type of shrinkages, the effect of chlorides, corrosion of steel, comes under the aspects of volume change in concrete. • The internal and external restraints to volume change in concrete results in cracks. It is the crack that promotes permeability and thus it becomes a part of cyclic action, till such time that concrete deteriorates, degrades, disrupts, and eventually fails.
  9. 9. Durability of Concrete • Role of Water-Cement Ratio • The volume change in concrete results in cracks and the cracks are responsible for disintegration of concrete. • Permeability is the contributory factor to the volume change with higher water-cement ratio being the fundamental cause of higher permeability. Therefore, use of higher water-cement ratio- permeability- volume change-cracks- disintegration- failure of concrete is a cyclic process in concrete. Therefore, for a durable concrete, use of lower possible water-cement ratio is the fundamental requirement to produce dense and impermeable concrete.
  10. 10. Durability of Concrete • Role of Water-Cement Ratio • It is generally recognized that quality of hydration product and the micro-structure of the concrete in case of low water- cement ratio is superior to the quality of micro-structure that exists in the case of higher water-cement ratio. • The lower water-cement ratio concretes are less sensitive to carbonation, external chemical attack and other detrimental effects that cause lack of durability of concrete. • However, in lower water-cement ratio concretes, there is not enough water available to fully hydrate all cement particles, only surface hydration of cement particles takes place leaving considerable amount of unhydrated core of cement grains. This unhydrated core of cement grains constitute strength in reserve.
  11. 11. Durability of Concrete • Role of Permeability • The capillary pores in concrete serve as a conduit or provide transport system for deteriorating agents. However, it may be mentioned that the micro-cracks in initial stage are so fine that they may not increase the permeability. But propogation of micro-cracks with time due to drying shrinkage, thermal shrinkage, and externally appied loads will increase the permeability of the system.
  12. 12. Capillary Pores in Concrete
  13. 13. Durability of Concrete • Permeability of Cement • Cement paste consists of C-S-H (gel), Ca(OH)2 , and both water filled and empty capillary cavities. The gel has porosity to the extent of 28 % with permeability of the order of 7.5 x 10-16 m/s which is about one-thousandth of that of cement paste. Therefore, contribution of gel pores to the permeability of cement paste is minimal. The extent and size of capillary cavities or pore depend upon water- cement ratio. At low water-cement ratio the extent of capillary cavities is less and cavities are very fine which are easily filled up within few days by hydration product of cement. Only unduly large cavities resulting from high water-cement ratio (of the order of 0.7) will not get filled up by product of hydration and will remain unsegmented and are responsible for the permeability of the paste.
  14. 14. Durability of Concrete • Permeabilty of Concrete • Introduction of aggregate, particularly large size of aggregate, increase the permeability considerably. As explained above, the increase in the permeability is due to the development of micro-cracks in the weak transition zone at early age. The size of cracks in the transition zone is reported to be much bigger than that of capillary cavities present in the cement paste.
  15. 15. Durability of Concrete • Effect of Mineral Additives • Concrete containing cement with 35 % fly ash has been found to be 2 to 5 times less permeable than concrete manufactured with OPC or blast-furnace slag cements. Moreover, concretes made using pozzolanic cements have a better flexural/ compressive strength ratio and tendency to cracking than cement made using OPC.
  16. 16. Durability of Concrete • Effect of Air-Entrainment • An air-entrainment upto 6 % can make the concrete more impervious. The steam curing of concrete using pozzolana has been reported to decrease the permeability due to formation of coarser C-S-H gel, lower drying shrinkage and accelerated conversion of Ca(OH)2 into cementing product.
  17. 17. Air-Entrainment
  18. 18. Factors Affecting Durability • The factors affecting durability are broadly divided into two groups, namely external factor and internal factor.
  19. 19. Factors Affecting Durability External factors Physical, chemical, or mechanical Environmental, such as extreme temperatures, abrasion, and electrostatic action. Attack by natural or Industrial liquid and gases Internal factors Permeability of Concrete Alkali aggregate reaction Volume changes due to difference in thermal properties of the aggregate and cement paste.
  20. 20. Physical causes of deterioration of concrete Cracking Surface wear Structural loading Overloading and impact cyclic loading Exposure to temperature Fire Freezing Thawing action Volume changes due to Temperature Humidity De-icing salts Abrasion Cavitation Erosion
  21. 21. Requirement for Durability • We shall discuss the requirements for durability under following heads • Exposure conditions • General environment • Freezing and thawing • Exposure to sulphate attack • Acid attack • Sea water attack • Abrasion, erosion and cavitation • Carbonation • Fire resistance
  22. 22. Requirement for Durability • Requirement of concrete cover • Shape and size of member • Type and quality of constituent materials • Concrete mix proportions • Maximum cement content • Chloride in concrete • Sulphate in concrete • Alkali aggregate reaction • Compaction, finishing and curing of concrete.
  23. 23. General Environment • The general environment to which the concrete will be exposed during its working life is classified to five levels of severity, namely, mild, moderate, severe, very severe, extreme.
  24. 24. General Environment
  25. 25. Effect of Weathering-Freezing and Thawing • Lack of durability of concrete due to freezing and thawing of frost is not so important in Indian conditions. But is of great importance in cold countries. • As the temperature of saturated hardened concrete is lowered, the water held in the capillary pores in the concrete freezes in a similar manner to the freezing in the capillary in rock and expansion of concrete takes place. Hence, if concrete mass is subjected to alternate cycles of freezing and thawing, it has detrimental effect on the strength of concrete. • Hence, while concreting in cold weather, the temperature of the fresh concrete should be maintained above O0 C.
  26. 26. Freezing and Thawing
  27. 27. Sea Water Attack • Sea water contains sulphates and hence attacks concrete in a manner similar to the sulphate attack. • The deterioration of concrete in sea water is often is not characterized by the expansion, as found in concrete exposed to sulphate attack. Attack of sea water causes errosion or loss of constituents of concrete without undue expansion. Calcium hydroxide and calcium sulphate (gypsum) are considerable soluble in sea water, and this will increase the leaching action. • Incase of reinforced concrete the absorption of salt results in corrosion of reinforcement. The accumulation of the corrosion product on the steel, causes rupture of the surrounding concrete. So that effect of sea water is more sevee on reinforced concrete than on plain concrete.
  28. 28. Sea Water Attack
  29. 29. Steps to Improve Durability of Concrete in Sea Water • The use of pozzolana or slag cement is advantageous under such condition. • Slag, broken brick bat, soft limestone, or other poros or weak aggregate shall not be used. • As far as possible, preference shall be given to precast members, plastering should be avoided • Sufficient cover to reinforcement, preferable 75 mm shall be provided • Care should be taken to protect reinforcement from exposure to saline atmosphere during storage, abrication and use. It may be achieved by treating the surface of reinforcement with cement wash or by suitable methods.
  30. 30. Abrasion, Erosion and Cavitation • Abrasion refers to wearing of the surface by friction. Erosion refers to wearing away of surface by fluids, The cavitation refers to the damage due to non-linear flow at velocities more than 12m /s • Concrete floors and pavements are subjected to abrasion and impact, which results in the wear of the surface during use. In case of hydraulic structure, the action of the abrasive materials carried out by water leads to errosion. • The higher the compressive strength of concrete, the higher is the resistance to abrasion. Hardness of course aggregate is also important to abrasion resistance. Graded aggregate improves the wear resistance. • Wear resistance of concrete can be improved by adopting mixes of lower water/cement ratio and lowest practicable slump. Uniformity of the concrete also increases the wear resistance.
  31. 31. Abrasion, Erosion and Cavitation
  32. 32. Shape and size of member • The shape or design details of exposed structure should be such as to promote good drainage of water and to avoid standing pools and rundown of water. Precaution should be taken to minimize any cracks and may collect or transmit water. Adequate curing is nessary to avoid the harmful effect of early loss of water. Members shall be designed and detail in a way to ensure flow of concrete and proper compaction during concreting. • Concrete is more vulnerable to deterioration due to chemical or climatic attack when it is in thin section, in section under hydrostatic pressure from outside only, in partially immersed section and edges of elements. The life of the structure can be increased by providing extra cover to steel reinforcement, by chamfering the corners or by using circular cross-sections or by using surface coating which prevent or reduce the ingress of water, CO2 or aggressive chemicals.
  33. 33. Requirements of Concrete Cover • The protection of the steel in concrete against corrosion depends upon an adequate thickness of good quality concrete. • Minimum nominal cover to meet durability requirements for normal weight aggregate concrete which should be provided to all reinforcement, including links depending on the different conditions of exposure.
  34. 34. Requirements of Concrete Cover Exposure Nominal cover in mm (minimum) Mild 20 mm Moderate 30 mm Severe 45 mm Very Severe 50 mm Extreme 75 mm
  35. 35. Type and Quality of Constituent Materials • 1) Cement Mix proportions: • The water/ cement ratio is an important factor governing the durability of concrete and should always be the lowest as possible. • 2) Maximum Cement Content: • Cement content excluding fly ash and ground granulated blast furnace slag should not be used unless unless special consideration has been given in design to the increased risk of cracking due to drying shrinkage in thin section to the increased risk of damagedue to alkali silica reactions.
  36. 36. Type and Quality of Constituent Materials • 3) Chloride in Concrete: • Due to high alkalinity of concrete protective oxide film is formed on the surface of steel reinforcement. This protective laye can be lost to carbonation and presence of chloride in the concrete. The action of chloride in inducing corrosion of reinforcement is more serious than any other reasons. • Chloride enters the concrete from cement, water, admixtures and aggregate. When there is chloride in concrete, there is an risk of corrosion of embedded metal. The higher the chloride content, the greater the risk of corrosion all constituents may contain chloride and concrete may be contaminated by chlorides from external environment. To minimize the chances of deterioration of concrete from harmful chemical salts, the level of such salts in concrete coming from cement, water aggregate and admixtures should be limited.
  37. 37. Chloride attack
  38. 38. Sulphates in Concrete • Sulphates are present in most cement and in some aggregate. Excessive amount of sulphate attack on concrete can cause expansion and deterioration of concrete. To prevent this the total water soluble sulphate content of concrete mix, expressed as SO3, should not be exceeded 4 % by mass of the cement in the mix.
  39. 39. Sulphates in Concrete
  40. 40. Permeability of Concrete • For completing hydration of cement about 38 % of water by weight of cement is rquired to fill the gel pores. If more than 38 % of water is used, than excess water will cause undesirable capillary cavities and the concrete becomes porous. Porous concrete has a higher permeability.
  41. 41. Importance of Permeability • In reinforced concrete, ingress of water and air will result in corrosion of steel leading to expansion, cracking, and disruption of concrete. • The penetration of deleterious material in solution may adversely affect the durability of concrete. Ca(OH2) leaches out and aggressive liquids attack the concrete. • If concrete becomes saturated with water due to permeability, it is more vulnerable to frost action. • The permeability is very important in case of liquid retaining structures like water tanks and dams where water-tightess is necessary.
  42. 42. Factors Affecting Permeability • The main factors affecting permeability are: • Water/ cement ratio • Properties of cement • Aggregate • Absorption and homogeneity of concrete • Curing • Use of admixtures • Age of concrete
  43. 43. Factors Affecting Permeability • Water/ Cement ratio: • For the pastes hydrated to the same degree, the permeability is lower with lower water/ cement ratio or higher cement content. • Properties of Cement: • The permeability of concrete is affected also by the properties of cement. For the same water/ cement ratio, coarse cement tends to produce a paste with higher porosity of cement than a finer cement. In general, higher the strength of cement paste, the lower will be the permeability. • Aggregate: The permeability of aggregate affects the behaviour of the concrete. If the aggregate has a very low permeability its presence reduces the effective area over which flow can take place. • For a given water/ cement ratio, greater the maximum size of aggregate greater is the permeability. This is because of the relatively larger voids. Well graded aggregate reduces the permeability.
  44. 44. Factors Affecting Permeability • Absorption and homogenity of concrete: • The volume of pore space in concrete is measured by absorption. Absorption is a physical process by which concrete draws water into its pores and capillaries. The absorption depends upon the structure of the concrete. Non homogenity affects the permeability. The defects in concrete due to cracks in the structure, void spaces due to segregation or honeycombing increases the absorptions. The permeability can be reduced by workable mix so that segregation is avoided.
  45. 45. Factors Affecting Permeability • Curing: Continued hydration of the cement paste results in the reduction in the size of the voids which decreases the permeability. Proper curing of concrete decreases the permeability of concrete. • Permeability of steam cured concrete is generally higher than that of wet-cured concrete.
  46. 46. Factors Affecting Permeability • Use of admixtures: Use of water proofing admixtures reduces permeability of lean mixes. In general, the use of extra cement will be more effective in reducing the permeability. In case of porous concrete surface treatment decreases permeability. • Age of Concrete: The permeability of cement paste also varies with the age of concrete or with the degree of hydration. In fresh paste the flow of water is controlled by the size, shape, and concentration of the original cement grains. With the progress of hydration, the permeability decreases rapidly because the gross volume of gel is approximately 2.1 timess the volume of the unhydrated cement. Gel gradually fills the original water filled space.
  47. 47. Measurement of Water Permeability • The measurement of permeability in the laboratory is very simple. The side of test specimen are sealed and water under pressure is applied to top surface only. The quantity of water flowing through a given thickness of concrete in a given time is measured and the permeability is expressed as a coefficient of permeability k given by Darcy’s equation.
  48. 48. Where, of flow of water m3/s A= Cross –Sectional area of the sample (m2) L= thickness of sample K= Coeffecient of permeability (m/s) Measurement of Water Permeability Ah= drop in hydraulic head (m)
  49. 49. Carbonation • Concrete is an alkaline substance and provides excellent protection to reinforcement embedded inside. The alkaline evironment forms a protective oxide film which passivates the steel and protects it from corrosion. Concrete initially has pH value of about 12 to 13. Due to leaching, carbonation and defective construction practices the pH value drop rapidly. Once pH value of concrete in the concrete drops below 10, corrosion of steel reinforcement is inevitable and therefore concrete durability is at stack. This is however dependent on the quality concrete and its porosity mainly in the cover area. A dense concrete cover offers good protection to steel embedded in it. It is also essential to produce concrete using low water-cement ratio so that it has minimum unblocked capillary pores. Since the concretes of higher strength have lower water-cement ratio they are preferred.
  50. 50. Carbonation Damage
  51. 51. Process of Carbonation • The carbondioxide present in the atmosphere reacts in presence of water with the concrete surface and concrete gets carbonated or in other words turns acidic. This chemical reaction starts at the surface and gradually proceeds inside the concrete mass and is generally measured as depth of carbonation. As hydrated calcium silicates and aluminates are less stable than calcium carbonates, concrete carbonation cannot be avoided.
  52. 52. Process of Carbonation
  53. 53. Advantages and Disadvantages of Carbonation • Carbonation of concrete improves several characteristics of ordinary concrete but can also affect the durability of reinforced concrete significantly. If the concrete is dense and well compacted, carbonation reduces the total porosity, specific surface of cement pastes as well as water permeability which inturn increases resistance to sulphate and aggressive chloride ion penetration. However, in reinforced concrete these beneficial effects are accompanied by large decrease in alkalinity or drop in pH value. • On carbonation, the concrete loses its pH value from around 13.5 to 8.3. therefore steel is no longer passivated by the alkaline concrete around it. Oxidation of reinforcement steel therefore takes place in the presence of moisture and oxygen, and corrosion occurs. The corrosion increases the volume of steel and ultimately results in cracking and spalling concrete.
  54. 54. Cracks in Concrete • The cracks in concrete is an inherent feature which cannot be completely prevented but can only be controlled and minimized. • Concrete being a material having very low tensile strength, readily cracks, when such tensile stresses beyond the tensile strength of concrete occurs in the structures. Cracking in concrete has become one of the inherent defects with which we have to live with. However, a better understanding of concrete gives us an insight into the various parameters responsible for causing cracking.
  55. 55. Cracks in Concrete
  56. 56. Cracks in Concrete • Causes of Cracks • Use of unsound materials, bad workmanship, use of high water/cement ratio, bad jointing techniques, freezing and thawing, thermal effects, heat of hydration, shrinkage stresses, structural stresses, alkali aggregate reaction, sulphate action are some of the major causes of development of cracks in concrete.
  57. 57. Cracks in Concrete • All the concrete structures crack in some form or the other. Most buildings develop cracks in their fabric which are superficial and occur soon after the construction. Cracks, even if harmless, may have an adverse psychological effects
  58. 58. Cracks in Concrete • Cracking of concrete structures can never be totally eliminated, but the practitionar should be aware of causes, evaluation techniques, and the methods of repair. • The cracks in a structure are broadly classified in two categories: superficial cracks, and structural cracks. • Before any repair work is taken in hand, the cause of damage must be clearly identified, for which careful investigation is required.
  59. 59. Causes of Cracks in Concrete • Cracking of Plastic Concrete • When the exposed surface of freshly placed concrete are subjected to a very rapid loss of moisture caused by low humidity, wind, and/ or high temperature, the surface concrete shrinks. Due to restraint provided by the concrete below the drying layer, tensile stresses develop in the weak, stiffening plastic concrete, resulting in shallow cracks that are usually short, discontinious running in all directions and very seldom extend to the free edge. • In presence of reinforcement the patterns may be modified. • Plastic shrinkage usually occurs prior to final finishing before curing starts. • Plastic shrinkage cracks can be cantrolled by reducing the relative volume change between the surface and the interior concrete by preventing a rapid moisture loss due to hot weather and dry winds. • This can be achieved by using fog nozzles to saturate the air above, use of plastic sheets to cover the surface, wind breakers and sunshades to reduce the surface temperature are also helpful.
  60. 60. Plastic Shrinkage Cracks
  61. 61. Causes of Cracks in Concrete • Cracking in Hardened Concrete • The moisture induced volume changes are characteristics of concrete. A loss of moisture from cement paste results in volume shrinkage by as much as 1 % On the other hand, an increase in moisture of concrete tends to increase its volume. If these volume changes are restrained, the tensile stresses develop. When the tensile strength of concrete is exceeded, it will crack.In massive concrete elements, tensile stresses are caused by differential shrinkage between the surface and interior concrete. • The extent of shrinkage cracking depends upon the amount of shrinkage, degree of restraint, modulus of elasticity, and amount of creep. The shrinkage decreases with the increase in the amount of aggregate, and reduction of water content. • Therefore drying shrinkage can be reduced by using the maximum possible aggregate and lower usable water content in the mix. Shrinkage cracking can be controlled by using properly spaced contraction joints and proper steel detailing. The shrinkage cracking can also be controlled by using shrinkage compensating cement.
  62. 62. Cracking in Hardened Concrete
  63. 63. Causes of Cracks in Concrete • Thermal Cracking The temperature difference within a concrete structure result in differential volume change. When the tensile strain due to differential volume change exceeds the tensile strain capacity of concrete, it will crack. The temperature differentials associated with the hydration of cement affects the mass concrete such as in large column, piers, footings, dams, etc. whereas the temperature differetials due to change in ambient temperature can affect any structure. • The liberation of heat of hydration of cement causes the internal temperature of concrete to rise during the initial curing period, so that it is usually slightly warmer than its surroundings. In thick sections and with rich mixes the temperature difference may be considerable. As the concrete cools it will try to contract. Any restraint on the free contraction during cooling will result in tensile stresses which are proportional to the temperature change, coefficient of thermal expansion, effective modulus of elasticity and degree of restraint. The more massive the structure, the greater is the potential for temperature differential and degree of restraint. Thermally induced cracking can be reduced by controlling the rate of cooling and increasing the tensile strength capacity of the concrete.
  64. 64. Thermal Cracking
  65. 65. Causes of Cracks in Concrete • Cracking Due to Chemical Reactions • The most important constituent of concrete is cement which is alkaline; so it will react with acids or acidic compounds in presence of moisture, and in consequence the matrix will become weak and its constituents may be leached out. • The concrete may crack, as a result of expensive reaction between aggregate containing silica and alkalis derived from cement hydration, admixtures, or external sources. The alkali silica reaction results in the formation of a swelling gel, which tends to draw water from other portions of concrete. This causes local expansion and accompanying tensile stresses which if large may eventually result in the complete deterioration of the structure. • The alkali-carbonate reaction occurs with certain limestone aggregates and usually results in formation of alkali-silica product between aggregate particles and surrounding cement paste. The problem may be minimized by avoiding reactive aggregates, use of smaller aggregates, and use of low alkali cement.
  66. 66. Cracking Due to Chemical Reactions
  67. 67. Causes of Cracks in Concrete • Cracking Due to Chemical Reactions • When the sulphate bearing water come in contact with the concrete, the sulphate penetrates the hydrated paste and reacts with hydrated calcium aluminate to form calcium sulphoaluminate with the subsequent large increase in volume, resulting in high local tensile stresses causing the deterioration of concrete. The blended or pozolana cement impart additional resistance to sulphate attacks. • The calcium hydroxide in hydrated cement paste will combine with carbondioxide in the air to form calcium carbonate which occupies smaller volume than the calcium hydroxide resulting in the so called carbonation shrinkage.
  68. 68. Causes of Cracks in Concrete • Cracking Due to Weathering • The environmental factors that can cause cracking include (i) freezing and thawing (ii) wetting and drying (iii) heating and cooling. Except in trophical regions, the damage from freezing and thawing is the most common weather related to physical deterioration. • The control measures include the use of lowest practical water/ cement ratio, and total water content, durable aggregate and adequate air-enterant. Adequate curing prior to exposure to freezing conditions is also important.
  69. 69. Cracking Due to Weathering
  70. 70. Causes of Cracks in Concrete • Cracking due to Corrosion of Reinforcement • The corrosion of steel produces iron oxide and hydrooxides, which have a much greater volume than the original metallic iron. The Increase in volume causes high radial bursting stresses around reinforcing bars and results in local radial cracks. These splitting cracks may propagate along the bar, resulting in the formation of longitudinal cracks parallel to the bar,Cracks provide easy access to oxygen, moisture, and chloride and thus even a minor split can create a condition in which corrosion continues and causes further cracking. • Increased concrete cover over the reinforcement bar is effective in delaying the corrosion or traversing tension. In very severe conditions, additional protective measures, such as coated reinforcement, sealers or overlays on concrete, and corrosion- inhibiting admixtures can be adopted.
  71. 71. Cracking due to Corrosion of Reinforcement
  72. 72. Causes of Cracks in Concrete • Cracking due to Poor Construction Practices • Poor construction practices, such as adding water to concrete to improve workability, lack of curing, inadequate form support, inadequate compaction, and arbitrary placement of concrete joints, can result in cracking in concrete structures.
  73. 73. Cracking due to Poor Construction Practices
  74. 74. Causes of Cracks in Concrete • Cracking due to Construction Overloads. • The loads induced during construction can be far more severe than those experienced in service. Unfortunately, these conditiones may occur at the early age when the concrete is most susceptible to damage and often result in permanent cracks. • Damage from unintentional concrete overloads can be prevented only if the designers provide information on load limitations for the structure and if the constructional personnel heed to these limitations.
  75. 75. Cracking due to Construction Overloads.
  76. 76. Causes of Cracks in Concrete • Cracking due to Errors in Design and Detailing • The design and detailing errors that may result in unacceptable cracking include use of poorly detailed re-enterant corners in walls, precasts members and slabs; improper selection and/ or detailing of reinforcements; restraints of members subjected to volume changes caused by variations in temperature and moisture, lack of adequate contraction joints, and improper design of foundations resulting in differential settlement within the structures. An inadequate amount of reinforcement may result in excessive cracking. Improper foundation design may result in excessive differential movement within a structure. Special care need to be taken in the design and detailing of structures in which cracking may cause a major serviceability problem.
  77. 77. Thanks
  78. 78. References • Concrete Technology by: R.P. Rethaliya • Concrete Technology by . M.S. Shetty • Internet websites

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