2.4 ADMIXTURE -
Present day concrete often
incorporates a fourth ingredient called admixtures, in addition to cement,
aggregates and water. Admixtures are added to the concrete mix immediately before
or during mixing, to modify one or more of the specific properties of concrete
in the fresh or hardened states. IS: 9103 lays down the procedures for
evaluation of admixtures for concrete and the changes in the properties of
concrete that should be expected when the admixtures are used.
The
different types of admixtures covered by the Indian Standard IS: 9103 are as
follows:
a)
Accelerating admixtures,
b)
Retarding admixtures,
c)
Water-reducing admixtures,
d) Air-entraining admixtures.
In addition, IS : 456 permits the
use of pozzolana like fly ash conforming to IS: 3812 (Part 11) or burnt-clay
conforming to IS : 1344 as admixtures for concrete. They are used mainly to
improve the workability of concrete and thereby reduce the water demand for a
given workability.
Before using an admixture in concrete, the performance of it should be evaluated by comparing the properties of concrete with the admixture and concrete without any admixture.
In addition, an admixture can
improve the desirable properties of concrete in more than one way. For example,
water-reducing admixtures can be used:
(a) To increase the workability of
concrete with the same water and cement contents,
(b) To increase the compressive strength of concrete without changing the workability by reduction of the water content in the concrete mix, when the cement content is unaltered, or (c) To effect saving in cement content by reduction in both the cement and water contents in the mix, while maintaining the same workability and compressive strength as in the reference concrete.
A viable water-reducing admixture
should allow a reduction in the water content between 5 to 12 % and
consequently cause an increase in the 28-day compressive strength of concrete
by nearly 10%, On the other hand, by modifying the concrete mix design in
conjunction with the use of a water-reducing admixture, the economy in the use
of cement can be of the order of 10 %.
Concretes made with admixtures when
compared with identical concrete made without admixtures should manifest
improved physical properties as given in Table 18
Some admixtures are likely to
contain water soluble chlorides and sulphates. These. if present' in large
quantities, may cause damage to the concrete structures during the course of
time. The chlorides may cause corrosion to the steel reinforcement whereas the
sulphates may cause disintegration of concrete by forming sulphoaluminates.
IS: 9103 for admixtures for concrete, stipulates that the chloride content of the admixtures should be declared by the manufacturers.
2.4.1 ACCELERATING ADMIXTURES –
These are substances which when
added to concrete increase the rate of hydration of cement,
shorten the setting time and
increase the rate of strength development. The chemicals that accelerate the
hardening of concrete mixes include soluble chlorides, carbonates, silicates, fluorosilicates
and hydroxides and also some organic compounds, for example, triethanolamine.
The most widely known accelerator is calcium chloride. Accelerating admixtures are used when concrete is to be placed at low temperatures. Indian Standard recommendations IS : 7861 (Part 11) for cold weather concreting envisages the use of CaCl2 up to a maximum of 1.5 % by mass of cement for plain and reinforced concrete works, in cold weather conditions.
The increase in the rate of
development of early strength of concretes containing ordinary and rapid
hardening Portland cement is shown in Fig. 14
At
normal temperatures, addition of 2% CaCI2
:
(a) Accelerates the rate of strength
of concrete containing ordinary Portland cement at early ages approximating
that of a rapid hardening Portland cement, and
(b) Increases the strength and abrasion resistance of concrete at all ages up to 1 year
However, addition of calcium chloride admixture may bring about reduction in flexural strength of concrete at ages of 28 days and beyond.
2.4.2 RETARDING ADMIXTURES -
A delay in the setting of concrete
is achieved by the use of retarding admixtures. Retarding action as exhibited
by sugar, carbohydrate derivatives, soluble zinc salts, soluble borates, etc.
Retarding admixtures are used in hot weather when normal setting time of cement
gets reduced due to high temperature. IS: 7861 (Part 1) envisages the use of
such admixtures to offset the accelerating effects of high temperature.
The retarding action of a set-retarder
to the penetration resistance of concrete is shown in Fig. 15.
A
small quantity of sugar (about 0.05% by mass of cement) delays the setting time
of concrete by about 4 hours.
The performance of such a retarder should be determined by trial mixes with the actual cement to be used in construction. A large quantity of sugar (say 0.2 to 1 % by mass of cement) prevents the setting of cement.
2.4.3 WATER-REDUCING ADMIXTURE -
The water-reducing admixtures are
usually based on calcium or sodium salts or derivatives of lignosulphonic acids
from the wood pulping industry. Modern admixtures based on lignosulphonic acid
derivatives arc formulated from the purified products, after removing the sugar
and other impurities.
Polycarboxylic acids, their salts,
modifications and derivatives also find some application as water-reducing
admixtures.
The increase in the workability of
concrete (in terms of compacting factor) using a water reducing admixture shown
in Fig. 16
The determination of workability is
an important factor In testing concrete admixtures. Rapid loss of workability
occurs during the first few minutes after mixing of concrete and gradual loss
ot workability takes place over a period from 15 to 60 minutes after
mixing". Thus the relative advantage of water-reducing admixtures
decreases with time after mixing. Therefore, IS: 9103 stipulates that the
workability of fresh concrete containing admixtures should be determined not
sooner than 15 minutes nor later than 20 minutes after completion of mixing of
concrete.
Water-reducing set-retarders belong
to the following two main groups:
a) Lignosulphonic acids and their
salts, and
b) Hydroxylated carboxylic acids and
their salts.
These admixtures increase the
setting time by about 2 to 6 hours during which concrete can be vibrated,
revibrated and finished.
This is particularly important in hot weather conditions or where the nature of construction demands a time 6~P between the placing of successive layers of concrete. It is possible to offset the set-retarding property of this kind of admixture, if the situation so demands, by incorporating an accelerator. for example, CaC12 or trtethanolamine
2.4.4 AIR ENTRAINING ADMIXTURES -
These admixtures cause air to be
incorporated in the form of minute bubbles in the concrete during mixing,
usually to increase workability and resistance to freezing and thawing. They
control the amount of air in fresh concrete and disperse properly sized air
bubbles throughout the concrete.
The
origins of air-entraining admixtures are as follows:
a) Natural wood resins;
b) Animal or vegetable fats and
oils;
c) Various wetting agents, such as
alkali salts of sulphated and sulphonated organic compounds;
d) Water soluble soaps of resin
acids and animal or vegetable fatty acids; and
e) Miscellaneous materials, such as
sodium salts of petroleum sulphonic acids, hydrogen peroxide. Aluminium powder,
etc.
The resistance of air-entrained
concrete in terms of dynamic modulus of elasticity and change in length,
against freezing and thawing is shown in Fig. 17.
The entrained air bubbles
(approximately 0.05 to 1.25 rom dia) reduce the capillary forces (the force
causing absorption of water by concrete) by restricting the effective length of
each capillary pore in concrete.
The capillaries are interrupted by
relatively large air voids in air-entrained concrete. The voids cannot fill
with water from the capillaries because of surface tension effects and,
therefore, under freezing conditions, they behave as 'expansion chambers' to
accommodate the ice formed. When the ice melts, surface tension effects draw
the water back into the capillary so that the air bubble acts as a permanent
safety valve, giving continuous protection to concrete against frost damage.
The resistance of air-entrained
concrete is thus attributed to a combination of reduced permeability of cement
paste, the breakdown of capillary action and the relief of pressure in the
concrete pores under the conditions of freezing and thawing.
Entrainment of small amount of air
results in concrete of insufficient durability, whereas with large amount of
air entrainment there is an excessive strength reduction in concrete.
Therefore, an optimum percentage of air giving a balance between compressive
strength and durability must be used in practice. Table 19 gives optimum air contents for concretes
of different maximum sizes of aggregate.
2.4.5 INFORMATION ON ADMIXTURES -
To facilitate approval of an
admixture, the following information is needed:
a) The trade name of the admixture,
its source, and the manufacturer's recommended method of use;
b) Typical dosage rates and possible
detrimental effects of under and over dosage:
c) Whether compounds likely to cause
corrosion of reinforcement or deterioration of concrete (such as those containing
chloride in any form as an active ingredient) are present and if so, the
chloride ions by mass or expressed as equivalent anhydrous calcium chloride by
mass of admixture; and
d) The average expected air content
of freshly mixed concrete containing an admixture which causes air to be
entrained when used at the manufacturer's recommended rate of dosage.
e)
The workability, compressive strength and the slump loss of concrete
with and without the use of admixtures shall be established during the trial
mixes before use of admixtures.
f) The relative density of liquid
admixtures shall he checked for cadi drum containing admixtures and compared
with till' specified value before acceptance.
g) The chloride content of
admixtures shall be independently tested fill each batch before acceptance.
h) If two or more admixture s art'
used simultaneously in the same concrete mi, data should be obtained to assess
their interaction and to ensure their compatibility.
