SHRINKAGE IN CONCRETE:-
Concrete is subjected to changes
in volume either autogenously 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 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. 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. 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.
Types of Shrinkage in Concrete:-
To understand this aspect more closely,
shrinkage can be classified in the following way:
(a) Plastic
Shrinkage
(b) Drying
Shrinkage
(c) Autogenously
Shrinkage
(d) Carbonation Shrinkage
The Types of shrinkage are explained as
below:
i). 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 sub grade 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. In case of floors and pavements where the
surface area exposed to drying is large as compared to depth, when this large
surface is exposed to hot sun and drying wind, the surface of concrete dries
very fast which results in plastic shrinkage. Sometimes even if the concrete is
not subjected to severe drying, but poorly made with a high water/cement ratio,
large quantity of water bleeds and accumulates at the surface. When this water
at the surface dries out, the surface concrete collapses causing cracks.
Plastic concrete is sometimes
subjected to unintended vibration or yielding of formwork support which again
causes plastic shrinkage cracks as the concrete at this stage has not developed
enough strength. From the above it can be inferred that high water/cement
ratio, badly proportioned concrete, rapid drying, greater bleeding, unintended
vibration etc., are some of the reasons for plastic shrinkage. It can also be
further added that richer concrete undergoes greater plastic shrinkage.
Plastic
shrinkage can be reduced mainly by preventing the rapid loss of water from
surface. This can be done by covering the surface with polyethylene sheeting
immediately on finishing operation; by fog spray that keeps the surface moist;
or by working at night. Use of small quantity of aluminum powder is also
suggested to offset the effect of plastic shrinkage.
Similarly, expansive cement or
shrinkage compensating cement also can be used for controlling the shrinkage during
the setting of concrete.
ii). 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. Under drying conditions, the gel water is lost
progressively over a long time, as long as the concrete is kept in drying
conditions. Cement paste shrinks more than mortar and mortar shrinks more than
concrete. Concrete made with smaller size aggregate shrinks more than concrete
made with bigger size aggregate. The magnitude of drying shrinkage is also a
function of the fineness of gel. The
finer the gel the more is the shrinkage.
iii).
Autogenously 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 autogenously shrinkage. Autogenously 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.
iv). Carbonation Shrinkage:-
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 normal atmosphere. Carbonation penetrates beyond the exposed surface
of concrete 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 place.
Carbonation of concrete also
results in increased strength and reduced permeability, possibly because water
released by carbonation promotes the process of hydration and also calcium
carbonate reduces the voids within the cement paste. As the magnitude of
carbonation shrinkage is very small when compared to long term drying
shrinkage, this aspect is not of much significance
Factors Affecting Shrinkage:-
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 per cent relative humidity for any length of time,
there will not be any shrinkage; instead there will be a slight swelling. The
typical relationship between shrinkage and time for which concrete is stored at
different relative humidity’s is shown in Figure. The graph shows that 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 per cent of the 20 year shrinkage occurs in 2 weeks, 40 to 80 per
cent of the 20 year shrinkage occurs in 3 months and 66 to 85 per cent 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. The richness
of the concrete also has a significant influence on shrinkage. Aggregate plays
an important role in the shrinkage properties of concrete. The quantum of an
aggregate, its size, and its modulus of elasticity influence the magnitude of
drying shrinkage.
Harder aggregate with higher
modulus of elasticity like quartz shrinks much less than softer aggregates such
as sandstone. Moisture Movement Concrete shrinks when allowed to dry in air at
a lower relative humidity and it swells when kept at 100 per cent relative
humidity or when placed in water.
Just as drying shrinkage is an
ever continuing process, swelling, when continuously placed in water is also an
ever continuing process. If a concrete sample subjected to drying condition, at
some stage, is subjected to wetting condition, it starts swelling. It is
interesting to note that all the initial drying shrinkage is not recovered even
after prolonged storage in water which shows that the phenomenon of drying
shrinkage is not a fully reversible one.
Just as the drying shrinkage is due to loss of
adsorbed water around gel particles, swelling is due to the adsorption of water
by the cement gel. The water molecules act against the cohesive force and tend
to force the gel particles further apart as a result of which swelling takes
place. In addition, the ingress of water decreases the surface tension of the
gel.
