Concrete Technology Complete notes of unit 5

Creep and Shrinkage of Concrete,
Significance,
Types of Shrinkage and Their Control,
Factors Affecting Creep.
Durability of Concrete
Minimum & Maximum Cement Content,
Strength & Durability Relationship,
Exposure to Different Conditions,
Factors Contributing to Cracks in Concrete,
Sulphate Attack,
Alkali Aggregate Reaction (AAR),
factors affecting on AAR,
Deteriorating effects of AAR,
Chloride Attack,
Corrosion of Steel (Chloride Induced)

1. Unit-V Creep and Shrinkage By Dr.Prathik Kulkarni Assistant Professor Department of Civil Engineering Bajaj Institute of Technology Wardha -MH

2. Contents Creep and Shrinkage of Concrete, Significance , Types of Shrinkage and Their Control, Factors Affecting Creep . Durability of Concrete Minimum & Maximum Cement Content, Strength & Durability Relationship , Exposure to Different Conditions, Factors Contributing to Cracks in Concrete, Sulphate Attack, Alkali Aggregate Reaction (AAR ), factors affecting on AAR, Deteriorating effects of AAR, Chloride Attack , Corrosion of Steel (Chloride Induced) 20-06-2022 U-V Creep and Shrinkage 2

3. 20-06-2022 U-V Creep and Shrinkage 3 Creep Creep can be defined as “the time-dependent” part of the strain resulting from stress. We have discussed earlier that the stress-strain relationship of concrete is not a straight line relationship but a curved one. The degree of curvature of the stress-strain relationship depends upon many factors amongst which the intensity of stress and time for which the load is acting are of significant interest. Therefore, it clearly shows that the relation between stress and strain for concrete is a function of time. The gradual increase in strain, without increase in stress, with the time is due to creep. From this explanation creep can also be defined as the increase in strain under sustained stress.

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5. 20-06-2022 U-V Creep and Shrinkage 5 Measurement of Creep Creep is usually determined by measuring the change with time in the strain of specimen subjected to constant stress and stored under appropriate condition. A typical testing device is shown in Figure below. The spring ensures that the load is sensibly constant in spite of the fact that the specimen contracts with time. Under such conditions, creep continues for a very long time, but the rate of creep decreases with time.

6. 20-06-2022 U-V Creep and Shrinkage 6 It is generally assumed that the creep continues to assume a limiting value after an infinite time under load. It is estimated that 26% of the 20 year creep occurs in 2 weeks. 55% of 20 year creep occurs in 3 months and 76% of 20 year creep occurs in one year. If creep after one year is taken as unity, then the average value of creep at later ages are: 1.14 after 2 years 1.20 after 5 years 1.26 after 10 years 1.33 after 20 years and 1.36 after 30 years

7. 20-06-2022 U-V Creep and Shrinkage 7 There are many expressions to give the magnitude of ultimate creep in concrete member. Ross suggested the relation between specific creep (creep strain per unit stress) ‘c’ and time under load ‘t’ in the form. where ‘a’ and ‘b’ are constants. If a graph is drawn with t in the x-axis and t/c in the y-axis it shows a straight line of slope b and the intercept on the t/c is equal to a.

8. 20-06-2022 U-V Creep and Shrinkage 8 If a member is loaded and if this load is sustained for some length of time and then removed , the specimen instantaneously recovers the elastic strain. The magnitude of instantaneous recovery of the elastic strain is something less than that of the magnitude of the elastic strain on loading. With time, certain amount of creep strain is also recovered. It is estimated that about 15 per cent of creep is only recoverable. The member will have certain amount of residual strain. This shows that the creep is not a simply reversible phenomenon. Below figure shows the pattern of strain of a loaded specimen and the recovery of strain on unloading after some time.

9. 20-06-2022 U-V Creep and Shrinkage 9 Factors Affecting Creep Influence of Aggregate : Influence of Mix Proportions : Influence of Age Curing Cement Properties

10. 20-06-2022 U-V Creep and Shrinkage 10 Influence of Aggregate: Aggregate undergoes very little creep. It is really the paste which is responsible for the creep. However, the aggregate influences the creep of concrete through a restraining effect on the magnitude of creep. The paste which is creeping under load is restrained by aggregate which do not creep. The stronger the aggregate the more is the restraining effect and hence the less is the magnitude of creep. Below figure shows the effect of the quality of aggregate on the magnitude of creep.

11. 20-06-2022 U-V Creep and Shrinkage 11 The grading, the shape, the maximum size of aggregate have been suggested as factors affecting creep . But it is later shown that the effect of aggregate and their properties mentioned above do not effect the creep , but indirectly they affect the creep from the point of view of total aggregate content in the concrete . The modulus of elasticity of aggregate is one of the important factors influencing creep. It can be easily imagined that the higher the modulus of elasticity the less is the creep . Light weight aggregate shows substantially higher creep than normal weight aggregate.

12. 20-06-2022 U-V Creep and Shrinkage 12 Influence of Mix Proportions: The amount of paste content and its quality is one of the most important factors influencing creep. A poorer paste structure undergoes higher creep . Therefore , it can be said that creep increases with increase in water/cement ratio. In other words , it can also be said that creep is inversely proportional to the strength of concrete . Broadly speaking, all other factors which are affecting the water/cement ratio is also affecting the creep. The following table shows the creep of concretes of different strength.

13. 20-06-2022 U-V Creep and Shrinkage 13 Influence of Age: Age at which a concrete members is loaded will have a predominant effect on the magnitude of creep. This can be easily understood from the fact that the quality of gel improves with time.

14. 20-06-2022 U-V Creep and Shrinkage 14 Curing condition:  In view of the smallness of creep strains, the amount of water expelled during creep from the micro pores into the macro pores (or vice versa) must also be small, probably much less than 0.1% of the volume of concrete (since typically creep strains do not exceed 0.001, and even this is not due entirely to water but also to expelled solids).  Larger the curing smaller the creep.

15. 20-06-2022 U-V Creep and Shrinkage 15 Cement properties: The type of cement effects creep in so far as it influence the strength of the concrete at the time of application of load. Fineness of cement affects the strength development at early ages and thus influence creep. The finer the cement the higher its gypsum requirement so that regrinding of cement in laboratory without the addition of gypsum produces an improperly retarded cement, which exhibits high creep.

16. 20-06-2022 U-V Creep and Shrinkage 16 Effects of Creep: The magnitude of creep is dependent on many factors, the main factors being time and level of stress . In reinforced concrete beams, creep increases the deflection with time and may be a critical consideration in design In reinforced concrete columns, creep property of concrete is useful . Under load immediately elastic deformation takes place. Concrete creeps and deforms. It can not deform independent of steel reinforcement. There will be gradual transfer of stress from concrete to steel. The extra load in the steel is required to be shared by concrete and this situation results in employment and development of full strength of both the materials. However, in eccentrically loaded columns, creep increases the deflection and can load to buckling.

17. 20-06-2022 U-V Creep and Shrinkage 17 Shrinkage It has been indicated in the earlier chapter that concrete is subjected to changes in volume either autogenous or induced. Volume change is one of the most detrimental properties of concrete, which affects the long-term strength and durability. To the practical engineer , the aspect of volume change in concrete is important from the point of view that it causes unsightly cracks in concrete. We have discussed elsewhere the effect of volume change due to thermal properties of aggregate and concrete, due to alkali/aggregate reaction, due to sulphate action etc. Presently we shall discuss the volume change on account of inherenet properties of concrete “shrinkage”. One of the most objectionable defects in concrete is the presence of cracks, particularly in floors and pavements. One of the important factors that contribute to the cracks in floors and pavements is that due to shrinkage. It is difficult to make concrete which does not shrink and crack. It is only a question of magnitude.

18. 20-06-2022 U-V Creep and Shrinkage 18 Factors affecting Shrinkage Drying condition Time Water cement ratio Drying conditions:  The most important factor is the drying condition or the humidity in the atmosphere.  No shrinkage will occur if the concrete is placed in 100% relative humidity.

19. 20-06-2022 U-V Creep and Shrinkage 19 Time :  The shrinkage rate will decrease rapidly with time.  It has been documented that 14 to 34% of the 20 year shrinkage will occur within two weeks of it being poured.  Within one year of the concrete being poured, shrinkage will be about 66 to 85% of the 20 year shrinkage. Water cement ratio:  The water to cement ratio will influence the amount of shrinkage that occurs.  The concrete’s richness also affects the shrinkage.  The process of swelling and then drying affects the concrete’s integrity and the shrinkage.

20. 20-06-2022 U-V Creep and Shrinkage 20 The term shrinkage is loosely used to describe the various aspects of volume changes in concrete due to loss of moisture at different stages due to different reasons. To understand this aspect more closely, shrinkage can be classified in the following way: (a ) Plastic Shrinkage ; ( b) Drying Shrinkage; (c ) Autogeneous Shrinkage ; ( d ) Carbonation Shrinkage

21. 20-06-2022 U-V Creep and Shrinkage 21 Plastic shrinkage:  Plastic shrinkage happens soon after the concrete is poured in the forms.  The water evaporates and results in a reduction of volume, this causes the concrete on the surface to collapse.  The aggregate particles or the reinforcement comes in the way of subsidence due to which cracks may appear at the surface or internally around the aggregate or reinforcement  High water/cement ratio, badly proportioned concrete, rapid drying, greater bleeding, unintended vibration etc., are some of the reasons for plastic shrinkage.  Plastic shrinkage can be reduced mainly by preventing the rapid loss of water from surface.  It can be reduced by covering the surface with polyethylene sheeting immediately after it is poured.

22. 20-06-2022 U-V Creep and Shrinkage 22 Drying shrinkage:  Just as the hydration of cement is an ever lasting process, the drying shrinkage is also an ever lasting process when concrete is subjected to drying conditions.  The loss of free water contained in hardened concrete, does not result in any appreciable dimension change.  It is the loss of water held in gel pores that causes the change in the volume  Under drying conditions, the gel water is lost progressively over a long time, as long as the concrete is kept in drying conditions.  The magnitude of drying shrinkage is also a function of the fineness of gel.  The finer the gel the more is the shrinkage.  It has been pointed out earlier that the high pressure steam cured concrete with low specific surface of gel, shrinks much less than that of normally cured cement gel.

23. 20-06-2022 U-V Creep and Shrinkage 23 Autogeneous shrinkage:  In a conservative system i.e. where no moisture movement to or from the paste is permitted, when temperature is constant some shrinkage may occur . The shrinkage of such a conservative system is known as autogeneous shrinkage.  Autogeneous shrinkage is of minor importance and is not applicable in practice to many situations except that of mass of concrete in the interior of a concrete dam.

24. 20-06-2022 U-V Creep and Shrinkage 24 Carbonation shrinkage:  Carbonation shrinkage is a phenomenon very recently recognized and is very important.  Carbon dioxide present in the atmosphere reacts in the presence of water with hydrated cement.  Calcium hydroxide gets converted to calcium carbonate and also some other cement compounds are decomposed.  Such a complete decomposition of calcium compound in hydrated cement is chemically possible even at the low pressure of carbon dioxide in normal atmosphere.  Carbonation penetrates beyond the exposed surface of concrete only very slowly.  The rate of penetration of carbon dioxide depends also on the moisture content of the concrete and the relative humidity of the ambient medium.

25. 20-06-2022 U-V Creep and Shrinkage 25  Carbonation is accompanied by an increase in weight of the concrete and by shrinkage.  Carbonation shrinkage is probably caused by the dissolution of crystals of calcium hydroxide and deposition of calcium carbonate in its place.  As the new product is less in volume than the product replaced, shrinkage takes place.

26. 20-06-2022 U-V Creep and Shrinkage 26 Effects of Shrinkage  Shrinkage of concrete between movement joints causes joints to open or makes it wider . Therefore joints must be designed to accommodate the widening caused by shrinkage.  Where other materials, such as ceramic tiles, are fixed on top of concrete surface, shrinkage of the concrete causes relative movement between the different materials . The resulting stresses can cause failure at the interface.  If shrinkage is restrained, the concrete is put into tension and when tensile stress becomes equal to tensile strength, the concrete cracks.  Shrinkage of the concrete causes the concrete to grip reinforcing bars more tightly. This increases friction between concrete and steel and so improves bond strength, especially for plain bars

27. 20-06-2022 U-V Creep and Shrinkage 27  The deflection of flexural members is increased by shrinkage. This is because the lightly reinforced compression zone is free to shrink more than heavily reinforced tension zone.  Shrinkage causes a reduction in pre-stressing force. When calculating pre-stressing forces, designers take into account to ensure that residual stress is structurally adequate.

28. 20-06-2022 U-V Creep and Shrinkage 28 Prevention of Shrinkage Provide shun shades in case of slab construction to control the surface temperature. Dampen the subgrade of concrete before placement it is liable to water absorption but should not over damp . Try to start the curing soon after finishing Use chemical admixtures to accelerate the setting time of concrete.

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31. 20-06-2022 U-V Creep and Shrinkage 31 NOTES 1 Cement content prescribed in this table is irrespective of the grades of cement and it is inclusive of additions mentioned in 5.2. The additions such as fly ash or ground granulated blast furnace slag may be taken into account in the concrete composition with respect to the cement content and water-cement ratio if the suitability is established and as long as the maximum amounts taken into account do not exceed the limit of pozzolona and slag specified in IS 1489 (Part I) and IS 455 respectively. 2 Minimum grade for plain concrete under mild exposure condition is not specified.

32. 20-06-2022 U-V Creep and Shrinkage 32 Requirements for Durability (IS:456-2000) Shape and Size of Member Exposure Conditions General environment Abrasive Freezing and thawing Exposure to sulphate attack Requirement of Concrete Cover Concrete Mix Proportions Mix Constituents Chlorides in concrete Sulphates in concrete Alkali-aggregate reaction Concrete in Aggressive Soils and Water Drainage Compaction, Finishing and Curing Concrete in Sea-water

33. 20-06-2022 U-V Creep and Shrinkage 33 Alkali content is defined as  the content of sodium oxide (Na2O) and potassium oxide (K2O)  in cement . Alkali: Alkali aggregate reactions Alkali aggregate reactions (AAR) occur when aggregates in concrete react with the alkali hydroxides in concrete producing a hygroscopic gel which, in the presence of moisture, absorbs water and causes expansion and cracking over a period of many years . This alkali-aggregate reaction has two forms, namely:  Alkali-silica reaction  (ASR) and  Alkali-carbonate reaction  (ACR). It should be known that  no structure has ever collapsed due to alkali-aggregate reactions , but there are cases in which structural concrete members demolished due to the effect of alkali-aggregate reactions.

34. 20-06-2022 U-V Creep and Shrinkage 34 Types of Alkali Aggregate Reaction Alkali-silica reaction (ASR)  Random map cracking and closed joints and attendant spalling concrete are indicators of alkali-silica reactions. Petrographic examination can identify alkali-silica reactions. It occurs broadly because aggregates containing reactive silica materials are more common. Alkali-silica reaction generates enough expansive pressure to damage concrete. Cracking initiates in areas with a frequent supply of moisture, such as close to the waterline in piers, near the ground behind retaining walls, or in piers or columns subject to wicking action. It can be controlled using proper portions of supplementary cementitious materials like silica fume, fly ash, and ground granulated blast-furnace slag. Lithium compounds can be used to decrease alkali-silica reactions.

35. 20-06-2022 U-V Creep and Shrinkage 35 Alkali-carbonate reaction (ACR)  It is observed with certain dolomitic rocks. It may cause considerable expansion . Compare to alkali-silica reactions, ACR is fairly rare because aggregates susceptible to this phenomenon are less common . The use of supplementary cementing materials does not prevent deleterious expansion due to ACR. So, it is recommended that ACR susceptible aggregates not be used in concrete.

36. 20-06-2022 U-V Creep and Shrinkage 36 Sources of Alkalis in Concrete 1. Cement All ingredients of concrete may contribute to the total alkali content of the concrete, the major source of alkali is from cement. 2. Aggregate 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. 3. 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.

37. 20-06-2022 U-V Creep and Shrinkage 37 4. Water Water may contain a certain amount of alkali . 5. Alkalis from Outside Concrete In the areas of cold weather, de-icing salt containing sodium compounds which may increase the alkali content on the surface layer of concrete. Soils containing alkali may also increase alkali content on the surface of concrete.

38. 20-06-2022 U-V Creep and Shrinkage 38 Effects of Alkali-Aggregate Reaction Loss of strength , stiffness, impermeability Affects concrete durability a nd appearance Premature failure of concrete structures Consequently, life of concrete structure is declined Maintenance cost is increased Tests for Aggregate Reactivity Petrographic Examination (ASTM C 295, BS 812: Part 104) Chemical Test (ASTM C289) Accelerated Mortar Bar Test (ASTM C 1260, CSA A23.2­ 25A, DD 249: 1999) Concrete Prism Test (ASTM C1293, CSA A23.2­ 14A, BS 812: Part 123) Accelerated Concrete Prism Test

39. 20-06-2022 U-V Creep and Shrinkage 39 Chloride Attack Chloride Attack on Concrete Structures is one of the most important phenomena we consider when we deal with the durability of concrete. Among all sources of failure of concrete structures, the chloride attack accounts 40% contribution. The main effect of chloride attack is the corrosion of reinforcement that induces the strength of the structure drastically.

40. 20-06-2022 U-V Creep and Shrinkage 40 Causes of Chloride Attack on Concrete Structures The attack of chloride on concrete structures can be happened either from inside of  the concrete or through the ingress of chloride from outside to the inside of concrete structures . The chlorides exist in concrete during the casting process due to the following reasons: Use of seawater for the concrete mixing process Use of calcium chloride as an additive to increase the setting time Use of aggregates that contained chlorides which were not washed for mixing Aggregates with chloride content more than the limit stated in the specification The chlorides enter the concrete from the e xterior environment to concrete interior due to the following reasons: Exposure of concrete to seawater Use of salt to melt the ice

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42. 20-06-2022 U-V Creep and Shrinkage 42 Prevention of Chloride Attack on Concrete Structures Increasing the cover over the reinforcement bar . This is the simplest way to prevent chloride attack. Studies have shown that an increase in cover by one inch can increase the life period of the structure by double . The rate of deterioration of the reinforcement under extreme conditions of chlorides can be prevented by having a rebar coated by epoxy , having cathodic protection or by use of stainless steel-clad rebar . Another important way is to decrease the chloride ion ingress into the concrete by decreasing the permeability of the concrete. This will decrease the durability and the time, cost of expensive repairs.