Shrinkage of hardened concrete is influenced by various factors in a similar manner as that of creep under drying conditions.
These factors are: 1. Aggregate Content 2. Quality of Cement Paste 3. Water Content 4. Properties of Cement 5. Chemical Composition of Cement 6. Effect of High Alumina Cement 7. Entertainment of Air 8. Effect of Admixtures 9. Effect of Curing and Storage Conditions 10. Effect of Relative Humidity of the Medium 11. Effect of Volume/Surface Ratio and a Few Others.
Factor # 1. Aggregate Content:
The most-important influence on the shrinkage of concrete is exerted by the aggregate, which restraints the amount of shrinkage of the cement paste. For a constant w/c ratio and at a given degree of hydration, the ratio of shrinkage of concrete Sc to the shrinkage of neat paste Sp. depends on the aggregate content ‘a’ in the concrete.
This relation can be written as:
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Sc = Sp (1 – a)n …(1)
where,
a = relative volume concentration of aggregate in concrete
n = a constant, whose experimental value varies from 1.2 to 1.7.
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There is some variation due to the relief of stress in the cement paste creep. Fig. 16.6 shows a typical relation and gives the value of n = 1.7, but the value of n depends on the modulus of elasticity and poisson’s ratio of the aggregate and concrete. The volume of fractions will have some influence on the total shrinkage.
(а) Max Size and Grading of Aggregate:
The maximum size and grading of aggregate directly do not have any influence on the magnitude of shrinkage of concrete with a given volume of aggregate and a given w/c ratio. But larger size aggregate permits the use of a leaner mix at a constant water/cement ratio. Thus the use of larger aggregate results in lower shrinkage. It has been observed that changing the maximum size of aggregate from 6.3 mm to 152 mm will raise the aggregate content from 60 to 80% of the total volume of concrete, resulting the decrease of shrinkage to 1/3rd of the original.
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Also it has been found that increase in the aggregate content from 71 to 74% has resulted in the reduction of shrinkage by 20% Fig. 16.6. The reduction in shrinkage by the addition of aggregate is due to internal cracking of the paste due to restraint of the aggregate. For aggregates upto the size of 6.3 mm the shrinkage has been observed uniform indicating that for these aggregates no internal cracking takes place.
(b) Type of Aggregate:
Type of aggregate or strictly speaking the modulus of elasticity of aggregate influences the shrinkage of concrete and determines the degree of restraint. The use of steel aggregate reduces the shrinkage to one third less and expanded shale one third more than normal or ordinary aggregate.
The relation between the shrinkage and modulus of elasticity of concrete depends on the compressibility of the aggregate used. Even in ordinary range of ordinary aggregates there is a considerable variation in the shrinkage due to the variation of modulus of elasticity of aggregate as shown in Fig.16.7.
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The aggregate in order of increasing shrinkage are as follows:
Sand stone has the highest shrinkage followed by gravel, then Basalt, Granite, 1600 lime stone and quartz. Thus light weight concrete exhibits higher shrinkage than normal weight concrete. A charge in the modulus of elasticity of aggregate is reflected by a change in the value of n of equation 1.
Sc = Sp (1 – a)n …(1)
Factor # 2. Quality of Cement Paste:
The quality of cement paste influences the magnitude of shrinkage. The quality of cement pate is dependent on the water cement ratio. Higher the w/c ratio, greater the shrinkage, thus it can be said that for a given aggregate content the shrinkage of concrete is a function of the water-cement ratio. The effect of water-cement ratio on the shrinkage is shown in Fig. 16.8.
The shrinkage of concrete made with different aggregate cement-ratio and water-cement ratio is shown in Table 16.1 below.
Factor # 3. Water Content:
Though water is not believed to be a primary factor to influence the shrinkage, but the water content affects the water-cement ratio hence higher the amount of water, greater the shrinkage as higher the water-cement ratio.
Factor # 4. Properties of Cement:
The properties of cement have little effect on the shrinkage of concrete. Latest studies have shown that fineness of cement have no influence on the shrinkage of concrete. However it increases the shrinkage of cement paste. Particles coarser than 75 micron hydrate comparatively little and have a restraining effect similar to aggregate.
Factor # 5. Chemical Composition of Cement:
The chemical composition of cement has been observed as not having any effect on the shrinkage of concrete. However cement deficient in gypsum exhibits a greatly increased shrinkage. The range of gypsum contents which significantly affects the shrinkage is narrower than that affects the setting time.
Factor # 6. Effect of High Alumina Cement:
The shrinkage of concrete made with high alumina cement is of the same magnitude as that of concrete made with Portland cement, but in case of high alumina cement shrinkage takes place much more rapidly than when Portland cement is used.
Factor # 7. Entertainment of Air:
Entertainment of air has been found to have no effect on shrinkage of concrete.
Factor # 8. Effect of Admixtures:
Addition of calcium chloride increases the shrinkage of concrete generally between 10 to 50% due to the formation of finer gel and possibly due to greater carbonation of the more mature specimen with calcium chloride.
The effect of other admixtures on the increase in shrinkage of concrete is of varying amount, but the percentage influence on shrinkage is constant for an admixture, whatever the aggregate is used. The plasticizers which reduce the water-cement ratio of the concrete, their net effect on shrinkage is negligible.
Factor # 9. Effect of Curing and Storage Conditions:
Shrinkage takes place for a long period. Some movements have been observed even after 28 years, but a part of long term shrinkage may be due to carbonation. The prolonged moist curing delays the development of shrinkage, but its effect on the magnitude of shrinkage is small and complex. In case of neat cement paste, the prolonged curing induces greater shrinkage. Further it has been observed that a well cured concrete shrinks more rapidly. In general it can be said that the length of the curing period is not an important factor in the shrinkage of concrete.
Mostly the magnitude of shrinkage is independent of the rate of drying except that transferring the concrete directly from water to a very low humidity, which can lead to fracture. Rapid drying does not allow a relief of stress by creep and may lead to more pronounced cracking.
Factor # 10. Effect of Relative Humidity of the Medium:
The relative humidity of the medium surrounding the concrete greatly affects the magnitude of the shrinkage. If the concrete is placed in dry air, it shrinks, but if it is placed in water or air with 100% humidity it swells. This indicates that vapour pressure within the cement paste is always less than the saturated vapour pressure. Thus the paste would be in hydrous equilibrium at some intermediate humidity.
If the concrete is placed in 100% humidity for any length of time, there will not be any shrinkage of concrete, instead the concrete will swell. A typical relationship between the shrinkage and time for which concrete is stored at relative humidities is shown in Fig. 16.9. The graph shows that shrinkage increases with time and also with the reduction in relative humidity, the rate of shrinkage decreases rapidly.
Factor # 11. Effect of Volume/Surface Ratio:
Actual shrinkage of a given concrete member is affected by its size and shape. However the influence of shape has been observed as negligible. Thus shrinkage can be expressed as a function of the ratio of volume and exposed surface. The relation of volume/surface and shrinkage with time has been shown in Fig. 16.10. From this fig it will be seen that the relation between the logarithm of ultimate shrinkage and volume/space ratio is linear.
Factor # 12. Effect of Size and Shape on Shrinkage of Concrete:
As the drying takes place at the surface of the concrete the magnitude of shrinkage has been observed to vary considerably with the size and shape of the concrete specimen being a function of the surface/volume ratio. In case of small specimen the effect of carbonation is more pronounced. Thus in case of small specimens a part of the size effect may be due to carbonation. Hence for practical purposes the shrinkage cannot be considered purely as an inherent property of concrete without reference to the size of the concrete member.
Many researchers have indicated the influence of the size of the specimen on the shrinkage. It has been found that the observed shrinkage decreases with the increase in size of the specimen, but above some value, the size effect is small initially, but later on increases substantially. As stated above shrinkage is a function of surface/volume ratio of the specimen.
The relation between the surface/volume ratio and the logarithm of shrinkage seems to be linear as shown in Fig. 16.10. Further the ratio of surface volume is linearly related to the logarithm of the time required for the half shrinkage to be achieved. This latter relation is applicable to concretes made with different aggregates so that while the magnitude of the shrinkage is affected by the type of aggregate used, the rate at which the final value of shrinkage is reached is not influenced.
Further on the basis of his work HOBBS says that theoretically shrinkage is independent of the size of the concrete elements, but for realistic periods, it must be accepted that for larger sections or elements shrinkage is smaller.
Factor # 13. Effect of Shape:
The effect of shape is secondary shaped specimens have been found to exhibit 14% less shrinkage than cylindrical specimen of the same volume surface ratio. This difference is not significant for design purposes.
Factor # 14. Effect of Moisture Movement:
The rate of moisture movement under the given conditions of storage depends to a great extent on the size and shape of the concrete member. A large mass of concrete is subjected to a greater variation of moisture content from point to point than a smaller specimen. These variations will produce a non-uniform state of volume change with in the large mass.
Under drying conditions shrinkage near the surface develops tensile stresses, which are in equilibrium with compressive stresses near the centre. These stresses may cause plastic yielding of the concrete, permanently elongating the tensile fibres and shortening the compressive fibres. Thus the maximum contraction of a large mass might be appreciably less than for a small specimen.
In order to determine the loss of moisture at various distances from the exposed surface at 50% humidity Carlson carried out experiments and measured shrinkage distribution on a 60 cm long prism dried from one end only. During this study he found that the specimen was dry upto its middle point even after 600 days.
Next he carried out experiments on concrete slabs varying in thickness from 15 cm to 120 cm. He exposed these surfaces to dry air from both sides for drying and computed the drying distribution. During this study he observed that 15 cm thick slab took 12 months for 80% evaporation, 30 cm thick slab took 10 months for 40% evaporation and in 120 cm thick slab only about 10% evaporation took place during a period of about 12 months Fig. 16.12.
Factor # 15. Effect of Reinforcement:
The reinforcement produces restraints on the movement of concrete, resulting less shrinkage than the plain concrete.