Shrinkage of concrete can be classified into the following categories: 1. Plastic Shrinkage 2. Drying Shrinkage 3. Autogeneous Shrinkage 4. Carbonation Shrinkage.
1. Plastic Shrinkage:
Plastic shrinkage takes place, soon after the concrete is placed in the form work, while the concrete is still in plastic stage. The cement paste at this stage un-goes a volumetric contraction. The magnitude of this volumetric contraction is of the order of 1% of the absolute volume of the dry cement. The plastic shrinkage is caused by the loss of water by evaporation from the surface of concrete or by suction by dry concrete below. The contraction in volume induces tensile stress in the surface layers due to the restraint caused by non-shrinking inner concrete. As the concrete is very weak in plastic state, plastic cracking takes place at the surface.
Plastic shrinkage is directly proportional to the loss of water i.e., plastic shrinkage is greater, the greater the rate of evaporation of water, which in turn depends upon the air temperature, the concrete temperature, relative humidity of the air and wind velocity etc. The rate of evaporation should not be greater than 0.5 kg/m2 per hour of the exposed concrete surface to avoid plastic cracking of the surface. A complete prevention of evaporation from the concrete surface immediately after casting reduces plastic shrinkage.
As stated above, that the loss of water from the cement paste is responsible for the plastic shrinkage. The plastic shrinkage has been found greater, for the larger cement content in the mix and lower for larger aggregate content. The effect of cement aggregate ration is shown in Fig 16.1.
2. Drying Shrinkage:
Withdrawal of water from hardened concrete stored in unsaturated air develops drying shrinkage.
A part of this movement is irreversible. This part should be distinguished from the reversible part or moisture movement. In case of reversible moisture part, if the concrete allowed to dry in air of a given relative humidity is placed in water or at a higher humidity later on, the cement paste will absorb water and win swell. This phenomenon has been shown in Fig. 16.2 (a).
The reversible moisture movement represents 40 to 70% of the drying shrinkage but it depends on the age before the start of first drying. If concrete is fully hydrated before exposing to drying the reversible moisture movement will form the greater proportion of the drying shrinkage. On the other hand if drying is accompanied by extensive carbonation, the cement will not allow movement of moisture content and irreversible shrinkage will be larger.
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However not all the initial drying shrinkage is recovered even after a prolonged storage in water. For usual range of concrete, the pattern of moisture movement under alternating wetting and drying is a common occurrence in the practice. It has been shown in Fig.16.2 (b). The magnitude of this cyclic moisture movement depends upon the duration of the wetting and drying periods, but drying is very much slower than wetting. Thus the influence of long dry weather can be reversed by a short period of rain.
The movement of moisture also depends upon the following factors:
1. Range of relative humidity.
2. Composition of the concrete.
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3. Degree of hydration at the time of initial drying.
Concrete made with light weight aggregate has a higher moisture movement than normal weight aggregate concrete.
The irreversible part of shrinkage is associated with the formation of additional chemical and physical bond in the cement gel when adsorbed water has been removed.
When concrete dries, first all the free water present in capillaries or pores evaporates and gets lost i.e., water present in concrete pores or capillaries which is not physically bound is lost. Thus under drying conditions, the gel water is lost progressively over a long time, as long as concrete is kept in drying conditions. This loss of free water does not cause any significant volumetric contraction of the cement paste. Due to the loss of free water, an internal relative humidity gradient is induced within the cement paste structure.
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As the time passes water molecules are transferred from the large surface area of calcium silicate hydrate into empty pores or capillaries and then out of the concrete. This phenomenon causes contraction in cement paste. This reduction or contraction in cement paste is not equal to the volume of water removed due to the internal restraint caused to contraction by the calcium silicate hydrate structure and also due to the negligible effect of free water loss on the change of volumetric contraction.
The loss of free water from the hardened concrete does not cause any appreciable change in volume. It is the loss of water held in gel pores that causes change in the volume of concrete. Fig. 16.3 shows the relationship between loss of moisture and shrinkage. Under drying conditions the gel water is lost progressively over a long time as long as the concrete is kept in drying conditions.
Theoretically it is estimated that the total linear change due to long time drying shrinkage is of the order of 10,000 x 10–6, but actually this change has been observed upto 4,000 x 10–6. Fig. 16.4 shows a typical apparatus for the measurement of shrinkage.
Further it has been observed that cement paste shrinks more than mortar and mortar shrinks more than concrete. Concrete made with smaller sized aggregate shrinks more than the concrete made with bigger size aggregate. The magnitude of drying shrinkage is also a function of the fineness of gel, the finer the gel, greater the shrinkage. The high pressure steam cured concrete with low specific surface of gel, shrinks much less than that of normally cured concrete.
Magnitude of Shrinkage:
If no other reliable data is available, the magnitude of shrinkage can be estimated by the formula suggested by Schorer.
εs = 0.00125(0.90 – h) …(16.1)
where.
εs = shrinkage strain
h = relative humidity expressed as a fraction. If the average humidity is 50%, then h = 0.50. If average humidity is 100%, then h = 1.0.
In these conditions, shrinkage is given as:
(i) If h = 0.5, then εs = 0.00125 (0.90 – 0.5) = 0.00125 x 0.4 = 0.0005
(ii) Uh = 1.0, then εs = 0.00125 (0.9 – 1.0) = – 0.00125.
-ve sign indicates swelling.
The rate of shrinkage decreases with time. It has been observed that 14 to 34% of 20 years shrinkage takes place in two weeks’ time. Whereas 40 to 70% in 3 months and 66 to 80% in one year.
3. Autogeneous Shrinkage:
If no movement of water to or from the set paste of concrete is allowed, then the shrinkage developed is known as autogeneous shrinkage. This shrinkage is caused by the loss of water consumed or used up in the hydration of cement. Autogeneous shrinkage is not distinguished from shrinkage of hardened concrete due to the loss of water to the outside except for massive structures as interior of concrete dams. The magnitude of this shrinkage is very small of the order of 50 x 10-6 to 100 x 10-6. Hence is not of much significance.
4. Carbonation Shrinkage:
In addition to drying shrinkage, concrete also undergoes carbonation shrinkage. Many experimental data include both types of shrinkage, but their mechanism is different.
Carbonation is the reaction of carbon dioxide CO2 present in the atmosphere, with the hydrated cement minerals in the presence of moisture. The action of CO2 takes place even in small concentrations such as present in rural air, where the content of CO2 is about 0.03% by volume. In an un ventilated laboratory the CO2 content is about 0.1%, where as in the atmosphere of big cities, the carbon dioxide content exists about 0.3% and in exceptional cases it may go up to 1.0%. The rate of carbonation increases with the increase in the concentration of CO2 especially at high water/cement ratio.
In the presence of moisture, CO2 forms carbonic acid, which reacts with Ca(OH)2 to form calcium carbonate (CaCO3). Other cement compounds are also decomposed, producing hydrated silica, alumina and ferric oxide The complete decomposition of calcium compounds in hydrated cement is chemically possible even at low pressure of CO2 in normal atmosphere, but carbonation penetrates beyond the exposed surface of concrete extremely slowly. The simultaneous reaction of CO2 with hydrated cement minerals in concrete induces contraction of concrete, which is known as carbonation shrinkage.
Factors Affecting the Rate of Carbonation:
The actual rate of carbonation depends on the permeability of concrete, its moisture content and on the content of CO2, relative humidity of the ambient medium and size of the specimen , grade of concrete, depth of core, whether the concrete protected and time. As the permeability of concrete is governed by the water/cement ratio, the inadequately cured concrete will be more prone to carbonation i.e., the depth of carbonation of inadequately cured concrete will be more. The effect of following factors has been found more pronounced.
i. Effect of Relative Humidity:
The highest rate of carbonation occurs at a relative humidity of 50 to 70%.
ii. Effect of Time:
The rate of carbonation depth is approximately proportional to the square root of time. It doubles between 1 and 4 years and then again doubles between 4 and 10 years. This process repeats upto about 5-years. However periodic wetting of concrete by rain slows down significantly the progress of carbonation.
iii. Effect of w/c Ratio:
The depth of carbonation is directly proportional to the increase of water/cement ratio i.e., the depth of carbonation increases with the increase in water/cement ratio. It has been observed that depth of carbonation at w/c ratio 0.4 is half that of at w/c ratio 0.6 and at w/c 0.8, it is 50% higher than that at w/c ratio 0.6.
iv. Effect of Cement Content:
It has been observed that higher the cement content lesser the depth of carbonation. The depth of carbonation is found about 50% with cement content of 500 kg/m3 of that having cemented content of 310 kg/nr. At cement content of 180 kg/m3 it has been found twice that of a cement content of 310 kg/m3.
The extent of carbonation can be determined easily by treating freshly broken surface with phenolphthalein. The portion containing free Ca(OH)2 turn pink by soaking it in Phenolphthalein, whereas carbonated portion remains unaffected.
Effects of Carbonation on Concrete:
Following effects of carbonation are observed:
Carbonation of concrete is accompanied by:
(a) Increase in the weight of the concrete.
(b) Shrinkage in concrete known as carbonation shrinkage. Carbonation shrinkage most probably is caused due to the dissolving of crystals of Ca(OH)2, while under a compressive stress imposed by the drying shrinkage and depositing CaCO3 in the voids of cement paste. Thus the compressibility of cement paste is increased and the carbonation of the hydrates present in the gel does not contribute to the shrinkage.
(c) Carbonation results in increased strength.
(d) Carbonation results in reduced permeability. These changes possibly are due to the water released by carbonation, which promotes the process of hydration. The calcium carbonate produced reduces the voids with in the cement paste. This applies to concrete made with Portland cement only. In case of super sulphated cement the strength of concrete decreases with carbonation. However this decrease in strength is not structurally significant.
(e) The most important effect of carbonation is the neutralization of alkaline nature of hydrated cement paste. The pH value reduces from 12 to 8. This reduction in pH value provides protection to steel from corrosion. However, if full depth of reinforcement cover is carbonated and oxygen and moisture can penetrate into the concrete then corrosion and cracking of concrete may take place. This is a very significant development. Fig. 16.5 shows the drying shrinkage of mortar specimen dried in CO2 free air at different relative humidities and also the shrinkage after subsequent carbonation.
It has been observed that carbonation increases shrinkage at intermediate humidities, but not at extreme limits of 100 and 25%. In the latter case there is no sufficient water present in the pores with in the cement paste for CO2 to form carbonic acid. On the other hand at 100% humidity when the pores are full of water, the diffusion of carbon dioxide into paste will be very slow.
Thus carbonation is greater in concrete protected from direct rain, but exposed to moist air, then concrete periodically exposed fully to rain.
The sequence of drying and carbonation, greatly affects the total magnitude of shrinkage. Simultaneous drying and carbonation produces lower total shrinkage than when drying is followed by carbonation. Carbonation shrinkage of high pressure steam cured concrete is very small. Concrete subjected to alternate wetting and drying in air containing CO2, carbonation shrinkage becomes progressively more apparent.
The total shrinkage at any stage is greater than drying the concrete in CO2 free air. Thus carbonation increases the magnitude of irreversible shrinkage and crazing of exposed concrete mass occurs. However carbonation of concrete before exposure to alternate drying and wetting reduces moisture by about 50%.