These are chemicals that have a very great plasticizing effect on concrete. They are used for one of two reasons:
1 To increase greatly
the consistence of a mix so that 'flowing' concrete is produced that is easy both to place and to compact; some are completely self-compacting and free from segregation.
2 To produce high-strength concrete by reducing the water content
to a
much greater extent than can be achieved by using a normal plasticizer (water-reducing admixture).
Flowing concrete is usually obtained by first producing a concrete in S2 consistence class (50 mm - 90 mm slump) and then adding the
super plasticizer, which will increase the slump to over 200 mm
(see Figures 5 and 6). This high consistence lasts for only a limited
period of time; stiffening and hardening then proceed normally. Because of this limited duration of increased consistence, when
ready-mixed concrete is used it is usual for the super plasticizer to be added to the concrete on site rather than at the batching or
mixing plant.
Flowing concrete
can be more susceptible to segregation and
bleeding, so it is essential for the mix design and proportions to
take account of the use of a super
plasticizer.
As a general guide, if a conventionally designed
mix is modified by
increasing the sand content by about 5%, satisfactory flowing concrete
can be produced by the addition
of a super plasticizer. A high degree of control over the batching of all the proportions is
essential, especially the water, because if the consistence is not
correct at the time of adding the super
plasticizer, excessive
flow and segregation will occur.
The fluidity of flowing concrete
is such that little or no vibration is
required. Beams, walls and columns can be compacted manually
by
Roding, although it is desirable to have an immersion
vibrator (poker) available. For
slabs, the concrete is more easily moved using rakes or pushers than by conventional shovels, and the
surface can be finished with a skip float drawn across it. Excessive vibration may cause segregation and bleeding
and, accordingly, some formulations of super plasticizer contain a viscosity
modifier to produce a self-compacting concrete that, in the right mix, is free from segregation.
The use of flowing concrete is likely to be restricted to work where the
advantages in ease and speed of placing offset the increased cost of the concrete
- considerably more than for other admixtures. Typical examples are where the reinforcement is
particularly congested, making both placing
and vibration difficult,
and where large areas such as slabs, would benefit
from a flowing, easily placed concrete.
The fluidity of flowing concrete increases the pressures on formwork, which should be designed to resist full hydrostatic pressure. Guidance on design pressures is given in CIRIA Report 108.
When used to produce high-strength concrete, a reduction in water content of as much as 30% can be obtained using a super plasticizer, compared with a water reduction of only about 10% when using a normal plasticizer; 1-day and 28-day strengths can be increased by as much as 50%.
High-strength water-reduced concrete containing a super plasticizer is used both for high performance in-situ concrete construction
and for the manufacture of precast units where the increased early
strength allows earlier demoulding.
Other admixtures
There are a number of other admixtures that may occasionally be used for special purposes. These include bonding aids, pumping aids, expanding agents, damp-proofing and integral water
proofers, fungicidal admixtures
and corrosion inhibitors.
For details of these
reference should be made to specialist literature.
Storage of admixtures
Most admixtures are stable, but they may require
protection against
freezing, which can permanently damage them, and may also require stirring. The manufacturer's instructions should be
followed.
Dispensing
Because admixtures are usually added in small quantities, generally 30 -1000 ml per 50 kg cement, accurate
and uniform dispensing is essential. This is best done using manual or
automatic dispensers so that the admixture is thoroughly
dissolved in the mixing water as it is added to the concrete.
Super plasticizers for flowing concrete, however, are usually added
just before discharge and the concrete should then be mixed for a further
one
minute per m but not less than five minutes, in
accordance with BS EN 206-1 .
Trail
mixes
Preliminary trials are essential
to check that the required
modification of the concrete
property can be achieved. The use of
an admixture
is likely to require some adjustment of the mix proportions. For example,
when using an air-entrained concrete, the additional
lubrication produced
by the small air bubbles
permits a reduction
in the water content, and it is usually
advantageous at the same time to reduce the sand content by about 5%. The correct adjustments can be determined only by trial mixes.
Although the admixture
manufacturer's instructions will usually
include recommended dosages, the optimum dosage will often
depend on the cement type, the mix proportions, the grading of the fine aggregate and the temperature.
The programme for trial mixes should include some with deliberate
double and treble over-dosages to determine
the effect on both the fresh and hardened concrete so that the dangers arising from
mistakes can be appreciated by all concerned
The properties of concrete are too many and varied to be dealt
with
fully in this publication: further information is available in
specialist textbooks. Therefore, only the main properties of concrete
in the fresh, hardening and hardened states are
considered here. Fire resistance, elasticity and other properties, which may be essential in some circumstances, have been omitted.
For methods of testing concrete,
refer to the section titled Testing concrete and concreting materials on page 52 and to relevant
Standards, in particular
BS 1881, BS EN 12350 for testing fresh
concrete and BS EN 12390 for hardened
concrete.
Fresh concrete
It is essential
that the correct level of workability is chosen to match the requirements of the construction process. The ease or difficulty of placing concrete
in sections of different sizes, the type of compaction equipment, the complexity
of reinforcement, the size and skills of the workforce
are amongst the items to be considered.
In general,
the more difficult it is to work the concrete, the greater should be the level of workability. But the concrete must also have some cohesiveness in order to resist segregation and bleeding. Concrete needs to be particularly
cohesive if it is to
be pumped, for example, or allowed to fall from a great height.
Workability and cohesion cannot be considered in isolation
because they are affected by each other: in general,
more workable concrete requires extra care to be taken with the mix
design if segregation is to be avoided.
The workability of fresh concrete is increasingly referred to in
British and European standards as consistence. It is useful to think
of consistence as a combination
of workability with cohesion.
Although cohesiveness cannot at present be measured, some of the test methods indicate whether a concrete
is likely to segregate.
The slump test is the best-known method for testing consistence
and
the recognized slump classes are listed in Table .
Table : Consistence classes in BS EN 206-1 for slump tests
conforming to BS EN 12350-2.
Slump class
|
Range of slump (mm )
|
S1
|
10 - 40
|
S2
|
50 - 90
|
S3
|
100-15 0
|
S4
|
160-21 0
|
Three further test methods
are recognized in BS EN 206-1 , all with
their unique consistence classes. They are the Vebe, degree of compactability and flow
tests conforming to BS EN 12350 : Parts 3, 4 and 5 respectively. It should be noted that the compactability
test to BS EN 12350 : Part 4 is totally different
from the compacting factor
test to BS 1881: Part 103.