An admixture is a material, usually a liquid, which is added to a batch of concrete during mixing in order to modify the properties of the fresh or hardened concrete in some way. Most admixtures benefit concrete by reducing the amount of free
water required for a given level of consistence, often in addition
to some other specific improvement. Permeability is thereby reduced and durability increased. There are occasions
when
the use of an
admixture is not only desirable
but also essential.
Because admixtures are added to concrete
mixes in small
quantities, they should be used only when a high degree of control can be exercised. Incorrect dosage of an admixture - either too much or too little - may adversely
affect the strength and other properties of the concrete.
BS EN 934-2 replaces BS 5075 in specifying the requirements for
the main types of admixture:
n Normal water-reducing
n Accelerating water-reducing
n Retarding water-reducing
n Air-entraining
n Super plasticizing / high range water reducing.
Water-reducing admixtures act by reducing the inter-particle
attraction between cement particles and produce
a more uniform dispersion of the cement grains. The cement paste is better 'lubricated', and hence the amount of water needed to obtain a given consistence can be reduced.
This effect may be beneficial
in one of three ways:
1. Added to a normal concrete
at normal dosage, they produce an increase in slump of about 50 mm. This can be useful with
high-strength concrete,
rich in cement, which would otherwise be too stiff to place.
2. The water content can be reduced while maintaining the same
cement content and consistence; this results in a reduced water/cement ratio (by about 10%), and therefore increased
strength and improved
durability. This can also be useful for
reducing bleeding in concrete
prone to this problem, or for
increasing the cohesion and thereby reducing segregation in concrete of high consistence class, or in harsh mixes sometimes
arising with angular aggregates, or low sand contents, or when
the sand is deficient
in fines.
3. A given strength and consistence class can be achieved with a reduced cement content. The water/cement ratio is kept constant, and with a lower water content the cement content can be reduced accordingly. This property should never be used if
the cement content would thereby be reduced below the minimum specified.
Overdosing may result in retardation and/or a degree of air- entrainment, but does not necessarily increase the workability
and therefore may not be of any benefit in fresh concrete.
Accelerating water-reducing
admixtures
Accelerators increase the initial
rate of chemical reaction between the cement and the water so that the concrete stiffens, hardens,and develops strength more quickly. They have a negligible effect
on
consistence, and 28-day strengths
are seldom affected.
Accelerating admixtures have been used mainly during cold weather when the slowing down of the chemical reaction between cement and water due to low temperature could be offset by the increased speed of reaction resulting from the use of the
accelerator. The most widely used accelerator for some time was
calcium chloride but, because the presence of chlorides, even in
small amounts, increases the risk of corrosion, the use of admixtures containing chlorides is now prohibited
in all concrete
containing embedded metal.
Accelerators are sometimes marketed under other names such as
hardeners, anti-freezes on frost-proffers, but no accelerator is a true anti-freeze and the use of an accelerator does not avoid the
need to protect the concrete
from the cold by keeping it warm
(with
insulation) after it has been placed.
NOTE: Accelerators are ineffective in mortars because the thickness of mortar, either in a joint or on a rendering,
is such that
any
heat generated by the faster reaction
is quickly dissipated.
Retarding water - reducing
admixtures
These are chemicals
that slow down the initial reaction between
cement and water by reducing the rate of water penetration to the cement and slowing down the growth of the hydration products.
The
concrete therefore
stays workable longer than it would
otherwise.
The length of time during which a concrete remains workable depends on its temperature, consistence class, and water/cement ratio, and on the amount of retarder
used. Although the occasions
justifying the use of retarders in the UK are limited, these
admixtures may be helpful when one or more of the following conditions apply:
n In warm weather,
when the ambient temperature is higher
than about 20°C, to prevent early stiffening ('going-off') and loss of work ability, which would otherwise make placing and finishing difficult
n When a large pour of concrete will take several hours and must
be
constructed without already placed concrete
hardening before subsequent concrete
is merged with it (i.e. without a
cold
joint)
n When the complexity of slip
forming demands a slow rate
of
rise
n When there is a delay of half an hour or more between
mixing and placing
- for example, when ready-mixed concrete is being used and when there may be traffic
delays and/or long hauls.
This
can be seriously aggravated during hot weather, especially
if
the concrete has a high cement content.
The amount of retardation can be varied - usually up to about four
to six hours - by altering the dosage,
but longer delays can be obtained for special purposes.
While the early strength
of concrete is reduced by using a retarder, which may affect formwork striking
times, the 7- and 28-day strengths are not likely to be significantly affected.
Retarded concrete needs careful proportioning to minimise
bleeding due to the longer period during which the concrete remains fresh.
Air-entraining admixtures
These may be organic vinsol resins or synthetic surfactants that
entrain a controlled amount of air in concrete in the form of small
air
bubbles. The bubbles need to be very small, about 0.05 mm
(50
microns) in diameter and well dispersed.
The main reason for using an air-entraining admixture is that the
presence of tiny air bubbles in the hardened
concrete increases its
resistance to the action of freezing and thawing, especially
when aggravated by the application of de-icing
salts and fluids.
Saturated concrete - as most external
paving concrete will be - can
be
seriously affected by the freezing
of water in the capillary
voids, which will expand and tend to burst it. But if the concrete is air-entrained, the small air bubbles,
which intersect the capillaries and
remain unfilled with water even when the concrete is saturated,
act as pressure relief valves and cushion the expansive
effect by providing voids into which the water can expand as it freezes, without disrupting the concrete.
When the ice melts, surface tension effects draw the water back out of the bubbles.
Air-entrained concrete
should be specified and used for all forms of external paving, from major roads and airfield
runways down to garage drives and footpaths, which are likely to be subjected to
severe freezing and to de-icing salts. These may either be applied
directly or come from the spray of passing traffic or by dripping from the underside of vehicles. Figure 4 shows the difference
in freeze/thaw resistance between air-entrained and non air-entrained concrete.
The volume of air entrainment
relates to maximum aggregate size
Dmax (see Table 7). With a reduction in Dmax, the specified air
content increases. Air content should be specified by minimum
value. Thus most heavy-duty concrete pavements, including roads
and
airport runways with their Dmax of 40 mm, require a minimum
air
content of 2.5%. The specified minimum air content increases
to 3.5% for 20 mm and 5.5% for 10 mm Dmax sizes. The maximum permitted air content is generally
4% greater than the minimum.
Air-entrainment also affects the properties of the fresh concrete. The minute air bubbles act like ball bearings and have a plasticizing effect, resulting in higher consistence. Concrete that is
lacking in cohesion, are harsh, or which tends to bleed excessively,
is
greatly improved by air-entrainment. Air-entrainment also
reduces the risk of plastic
settlement and plastic shrinkage cracks.
There is also evidence that uniformity of colour is improved and
surface blemishes reduced.
Table : Air contents of air-entrained concrete in accordance
with
BS EN 206-1 .
|
Nominal maximum
aggregate size, Dmax
|
Minimum volume of entrained air
|
Maximum volume of
entrained air
|
|
40 mm
|
2.5%
|
6.5%
|
|
20 mm
|
3.5%
|
7.5%
|
|
10 mm
|
5.5%
|
9.5%
|
One factor which has to be taken into account when using air-
entrained concrete is that the strength
of the concrete is reduced, by about 5% for every 1% of air entrained. However, the
plasticizing effect of the admixture
means that the water content of the concrete
can be reduced, which will offset most of the
strength loss which would otherwise occur, but even so some
increase in cement content is likely to be required
as well.
The amount of air entrained for a given dosage can be affected by
several factors: changes in sand grading, variation
in temperature and mixing time. These may call for adjustments to the dosage for uniformity of air content to be maintained. When pfa is present in
the concrete, a considerably increased dosage of air-entraining
admixture may be required. The measurement of air content in the
fresh concrete is described in BS 1881 : Part 106 and BS EN 12350-7.
Brief details are given on page 57 under Air
content test.