Shrinkage of Concrete
Presently we shall discuss the volume change on account of inherent 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.
Read More: Difference between shrinkage and creep
It is difficult to make concrete which does not shrink and crack. It is only a question of magnitude. Now the question is how to reduce the shrinkage and shrinkage cracks in concrete structures. As shrinkage is an inherent property of concrete it demands greater understanding of the various properties of concrete, which
influence its shrinkage characteristics.
It is only when the mechanism of all kinds of shrinkage and the factors affecting the shrinkage are understood, an engineer will be in a better position to control and limit the shrinkage in the body of concrete.
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.
Types of shrinkage of concrete
a) Plastic Shrinkage
Shrinkage of this type manifests itself soon after the concrete is placed in the forms while the concrete is still in the plastic state. Loss of water by evaporation from the surface of concrete or by the absorption by aggregate or subgrade is believed to be the reasons of plastic shrinkage.
The loss of water results in the reduction of volume. 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
b) Drying Shrinkage
Just as the hydration of cement is an everlasting process, the drying shrinkage is also an everlasting process when concrete is subjected to drying conditions. The drying shrinkage of concrete is analogous to the mechanism of drying of timber specimen.
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
c) Autogeneous Shrinkage
In a conservative system i.e. where no moisture movement to or from the paste is permitted, when the temperature is constant some shrinkage may occur. The shrinkage of such a conservative system is known as an autogenous shrinkage.
Autogenous shrinkage is of minor importance and is not applicable in practice to many situations except that of a mass of concrete in the interior of a concrete dam. The magnitude of autogenous shrinkage is in the order of about 100 x 10–6.
d) Carbonation Shrinkage
Carbonation shrinkage is a phenomenon very recently recognized. Carbon dioxide present in the atmosphere reacts in the presence of water with hydrated cement. Calcium hydroxide [Ca(OH)2] 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 the normal
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. 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
Factors affecting of Shrinkage of Concrete
One of the most important factors that affect shrinkage is the drying condition or in other words, the relative humidity of the atmosphere at which the concrete specimen is kept. If the concrete is placed in 100 percent relative humidity for any length of time, there will not be any shrinkage, instead, there will be slight swelling.
The magnitude of shrinkage increases with time and also with the reduction of relative humidity. The rate of shrinkage decreases rapidly with time. It is observed that 14 to 34 percent of the 20-year shrinkage occurs in 2 weeks, 40 to 80 percent of the 20-year shrinkage occurs in 3 months and 66 to 85 percent of the 20-year shrinkage occurs in one year.
Another important factor which influences the magnitude of shrinkage is water/cement ratio of the concrete. As mentioned earlier, the richness of the concrete also has a significant influence on shrinkage.
The grading of aggregate by itself may not directly make any significant influence. But since it affects the quantum of paste and water/cement ratio, it definitely influences the drying shrinkage indirectly. The aggregate particles restrain the shrinkage of the paste.
The harder aggregate does not shrink in unison with the shrinking of the paste whereby it results in higher shrinkage stresses, but the low magnitude of total shrinkage. But a softer aggregate yields to the shrinkage stresses of the paste and thereby experiences the lower magnitude of shrinkage stresses within the body, but the greater magnitude of total shrinkage.