CHEMICAL ADMIXTURES – THE FIFTH CONSTITUENT
MATERIAL FOR DURABLE CONCRETE
Abstract :
Concrete is one of the most
widely used construction materials from time immemorial. Since the day of its advent, concrete has
been undergoing changes as a material and technology.
Of late, due to the growing
needs of the performance and the durability of concrete, there has been a
continuous search for upgrading the properties of concrete. High performance concrete, fibre reinforced
concrete, self-compacting concrete are only a few examples for the outcome of
the same.
In this paper, an attempt is
made to find out as to how chemical admixtures have become the indispensable
component of modern day concrete, their effect on its properties both in
plastic and hardened states and durability aspects. Besides, new developments in the field of
chemical admixtures and expectations from the construction industry are also
discussed.
Introduction:
The history of our
civilisation reveals that concrete, in some form or the other was in use when
man started ‘construction’. As the time
passed by, concrete has undergone different changes and the ‘evolution’ of
concrete is still continuing.
The simple form of modern
day concrete in use worldwide is a mixture of cement and water as binder
materials and coarse and fine aggregates as filler materials. But this simple form of concrete is no longer
sufficient to cater the challenging needs of construction industry. Newer and more effective materials are being
developed. Concrete admixtures are the
results of such an innovation, which has become the integral part of today’s
durable concrete.
Parameters
for durability:
In simple terms concrete must be
placeable and durable. Durability
includes consideration of appropriate mechanical properties at given
environmental conditions. Once the
concrete is mixed and placed in the formwork in the structure, the focus shifts
to the durability of concrete. But durability
cannot be achieved in the hardened concrete, if necessary attention is not paid
in the mix design stage and the problems encountered in the placing stage are
not solved. Aspects of permeability
long-term strength and shrinkage as well as other factors related to the
materials used as the environment of the structures are all important. In pursuit of high performance in one aspect
must not compromise performance in others.
Properties such as cohesion, density, stiffening, shrinkage temperature
rise and total cost may all need consideration.
Workability and water-cement ratio may affect the other properties. The revised Bureau of Indian Standards code
of practice IS: 456 also specify maximum water cement ratios for various
exposure conditions, and minimum cement contents. In order to achieve durability, a
specification may require the use of a particular admixture type, for example a
superplasticiser even though it may be possible to do the job with a normal
plasticiser.
Aggressive
exposure conditions:
Water is a material, which
is essential to make and hydrate concrete.
But water, for reinforced concrete in the hardened state will be highly
aggressive. The other aggressive agents
such as chlorides, sulphates, carbon di oxide, oxygen etc. will become more aggressive in the presence
of water. Hence the specifications should
be available to control these agents apart form other ingredients like alkali
levels.
Water penetrates concrete
and in so, doing may bring other dissolved materials with it. If it
fully penetrates in concrete then the
structure will leak. This can lead to
leaching of other materials from the concrete.
Reducing water-cement ratio slows down the rate at which water
penetrates the concrete, extending the life of the structure.
Carbon dioxide reacts with
the alkalinity in concrete, neutralising it and reducing the pH. As the carbonated layer penetrates deeper
into the concrete, eventually the cover to the reinforcement will be fully
carbonated and will no longer passivate the reinforcing steel, which will start
to corrode. Reducing water-cement ratio
slows down the rate of penetration of the carbonation front, giving extended
protection for the same depth of cover.
Sulphate ions also attack
cement, reacting with the products of aluminate hydration. This is an expansive reaction, which can
disrupt the concrete matrix, causing crumbling and easier penetration by other
aggressive materials. Reducing
water-cement ratio will reduce the rate of sulphate penetration, but cannot
prevent sulphate attack. The use of
sulphate resisting cement or blends of Portland cement and cement replacement
materials is advised in such cases by standards. Sulphate resisting cements have very low
levels of aluminate phases. The problem
of sulphate attack will only occur if the total expansion is above a certain
limit. The lower aluminate levels in
sulphate resisting cement ensure that it will not produce enough expansion to
be a problem, however much sulphate is present in the environment.
Chloride ions are
detrimental to the steel reinforcement.
They can cause corrosion to start even if the pH of the surrounding
concrete is still nominally high enough to maintain the steel in a passive
state. Reducing the water-cement ratio
cuts down the rate of penetration and again increases the life of the
structure. Of course, if there is no
embedded steel there is no risk of corrosion.
Use
of Superplasticisers :
The principle purpose of
using superplasticisers in the concrete mixes from the durability point of view
are as follows either individually or collectively:
·
To increase workability in order to achieve easy
placeability
·
To reduce the quantity of water in the mix without
reducing the workability. Reduced water
cement ratio means increased strength and durability.
·
To reduce both water and cement at a given
workability and strength in order to save cement and reduce creep, shrinkage
and thermal strains caused by heat of hydration.
Mechanism
of action:
Superplasticisers
in concrete, cause uniform dispersion of cement particles avoiding the
otherwise agglomerated particles of cement. Due to the dispersion, the
workability of concrete increases. The
dispersion effect is attributed to the development of negative charges on the
surface of cement particles. These like
charges repel each other and disperse the cement particles in the matrix
breaking the agglomerates, releasing the water.
This water is effectively used in increasing the workability of the mix.
Cement-admixture
incompatibility:
The cement-admixture
incompatibility is a phenomenon in which the concrete with the admixture not
exhibiting the intended effect with some cements. The type of cement used influences the effect
of plasticising admixture. The chemical
composition of cement is found to have direct relevance with the fluidizing
effect of superplasticiser. C3A
content and the fineness of cement play a major role in the plasticising action
of an admixture. Higher the C3A
content and the cement fineness, the lower the fluidizing effect. The type of gypsum used in the cement
performs much differently with some type of superplasticisers. The fluidizing effect is much larger with
dihydrate of gypsum than with hemihydrate.
Advantages:
Improving the effectiveness of water in concrete
The dispersion of cement
particles in the concrete mix enables using the mixing water effectively in the
mix. The practical effect of this is
that at any target workability a mix containing a plasticising admixture needs
less water than a plain concrete. How
much less depends on the mix concerned and the type of admixture used. Water reductions of up to a 30%, and
sometimes higher, are possible. Needing
less water to get a particular workability means that less cement is needed for
the same strength and durability.
Allowing workability to be
obtained at a lower water content provides greater flexibility in mix design,
allowing low water-cement ratios to be combined with good workability. This higher workability brings benefits to
concrete performance and profitability.
Using a higher starting workability means that the concrete is earlier
to place, allowing the use of more efficient automated methods and lower labour
costs. Improved workability produces a
greater ease of compaction, reducing the risk of defects in the structure such
as honeycombing. Finally, being able to
start at a higher workability allows an extended working life to the concrete,
providing a degree of protection against on-site delays and making a further
contribution to efficient working.
Workability
retention:
Unlike retardation,
workability retention is not easily defined.
Mix workability drops with time because of the slow reduction in free
water in the concrete, due to absorption into aggregate, evaporation loss or
cement hydration. Workability loss is
important because concrete must have sufficient workability to be placed. If
the workability is too low at the time of placing, then poor quality concrete
will result. The rate of workability
loss is not the most important thing, what is needed is the right level of
workability at a particular time.
Retardation slows cement
hydration, but adding a pure retarding admixture has surprisingly little effect
on workability loss. Workability will
typically fall at the same rate until a slump of approximately 50 to 75 mm is
reached. It is only at around this level
and below that the retarder affects workability loss. The greatest benefit comes from starting at a
higher workability. This increases the
time before workability falls below the target value, because there is more
workability to lose. Adding more water
to the initial mix will do this, but affects strengths or requires increased
cement contents. Addition of water
reducing or superplasticising admixtures is a better method, allowing higher
workabilities to be obtained for the same water content. Specially formulated workability retention
admixtures are available with optimised combinations of retarding, plasticising
and other properties. These materials
can give the best solutions to problems of workability loss.
An alternative
admixture-based solution may be appropriate, particularly for more extended
working life requirements. A combination
retarding and water-reducing admixture is used at the concrete mixer. The starting workability is adjusted to
ensure that the concrete arrives at the place and time of use at typically 50
to 75mm slump. A second, plasticising or
superplasticising admixture is added to the concrete when it arrives and the
concrete is re-mixed. The dosage of the
second admixtures is adjusted to give the desired level of workability. When the second admixture is added the
workability must be high enough for re-mixing, which must be continued until
the concrete is homogeneous. The second
admixture should be a standard, non-retarding, type as there is no requirement
for further retardation if the concrete is to be placed immediately.
Other methods can be used
for more control over workability at the point of placing. Controlled re-dosing of plasticising
admixtures can be a very useful technique.
Temperature control for the concrete also often found to be beneficial.
Reducing
shrinkage and cracking:
Reducing
cement content reduces the cost of the concrete per cubic meter, an immediate
contribution to better productivity, but there are further benefits. Three of these are reductions in shrinkage,
heat evolution and alkali content. The
latter is only really a problem if alkali aggregate reaction is a
possibility. The cement contributes the
greatest levels of alkalis in a concrete mix, so reducing the cement content
gives the greatest benefit.
The chemical reaction
between cement and water creates heat and increases the temperature of the
concrete. The amount by which the
temperature rises depends on the cement content and the degree of cooling
experienced. As the general environment
does not gain temperature it has a cooling effect. However, at the center of large blocks of
concrete, such as foundation rafts, the temperature rise can approach 13 deg. C
for every 100 kg of cement per cubic meter.
Large temperature rises, in rich mixes, create high temperature
differentials between the interior and surface of the concrete. This causes differential rates of expansion
and contraction and can lead to severe cracking. As has already been seen, the use of a
plasticising admixture allows a particular combination of workability and
strength to be obtained with less cement.
Therefore, there is less risk of temperature cracking.
The reaction between cement
and water is the cause of further problems.
This is because the reaction productions are, on average, slightly
denser than the original materials.
Therefore there is a loss in volume, or shrinkage, experienced as
concrete sets. Aggregate minimises this
effect, just as it minimises temperature rise, but the more aggregate that can
be included the more that the shrinkage can be reduced. Again, the reduced cement content from the
use of a plasticiser minimises the problem.
Reducing
water permeability:
Durability specifications require low
water-cement ratios. Without the use of
admixtures, concrete to meet these specifications must have high cement
contents and still tends to be fairly low in workability and difficult to
place. Using admixtures, water-cement
ratios of well below 0.40 are obtainable without excessive cement contents and
still usable workabilities. At these
levels of water-cement ratio, most requirements for durability can be met.
Typical values from measurements of
water penetration under pressure show the level of improvement in permeability
coefficient that can be obtained through reductions in water-cement ratio. The
test method uses disks of concrete, 100mm in diameter and 50mm thick. These are cured for 28 days and then dried to
constant weight at 35% humidity.
Water at a pressure of 10 bar is applied
and the time taken for the water to fully penetrate the sample is
measured. This time is used to calculate
the permeability coefficient by the Valenta method. The rate of flow of water is then measured
and used to calculate a value for the permeability coefficient use Darcy’s
equation. At low water-cement ratios it
may take a considerable time for water to fully penetrate the specimen. In this case the sample can be broken open
and the depth of penetration measured and used in the Valenta calculation.
Using accelerating and retarding admixtures:
The term acceleration and retardation
can often be confusing. There are great
differences between an extension of the workable life of a concrete mix and a
delay in its stiffening time. However,
the term retardation is often used for either effect, particularly at higher
temperatures. At lower temperatures
there can be similar confusion between acceleration of stiffening time and
early strength gain.
When retardation and acceleration are
referred to in admixture standards, it mainly concerns effects on stiffening
rate. This does not mean that the other
areas are less important. Indeed, often workability retention or early strength
gain is really what is needed on site.
When specifications are drawn up or enquiries made all parties must be
aware of the properties needed. Too
often two sides understand different things.
Stiffening:
Retardation is measured by the time
taken for concrete to reach a particular value of penetration resistance. It is therefore a measure of the development
of internal structure in the concrete.
Also important is that retardation is the difference between the
stiffening rate of a control mix and a mix containing the admixture.
Stiffening does not start until the
workability of the concrete has ended.
Obviously, the working life of the concrete will affect the stiffening
time, but it will not automatically have the same effect in all mixes. Changes to stiffening time may be desirable
for a number of reasons. In large volume
of concrete, cold joints between adjoining truckloads cannot be accepted.
Retardation of stiffening can avoid this problem. Scheduling requirements, such as in floor
finishing or slip forming, may also require acceleration or retardation of
stiffening.
Changes in temperature change the rate
at which plain concrete stiffens.
Accelerating or retarding admixtures are used to offset these effects
and produce a setting time that meets the requirements of the overall construction
process. Dosages can be varied for the
desired effect. Retarders are not
restricted to use in summer months, nor are accelerators to winter. They are suitable for use whenever the rate
of stiffening needs changing. Retarders
may be used to control the rate of stiffening of individual loads in a mass
pour so that it gains and loses temperature as a whole, minimising temperature
differential even in the cooler months.
Retarders do not reduce the peak
temperature at the center of a concrete pour, as this depends on the cement
content. They delay the start of the
main heat evolution, but once started it continues at a fairly constant
rate. Accelerators, in contrast,
slightly increase the peak temperature because they cause the hydration
reaction to occur more rapidly, leaving less time for heat to be
dissipated. If the peak temperature must
be reduced, cement replacement materials can be used or water-reducing
admixtures can reduce cement content.
Strength acceleration:
After concrete has stiffened, it
develops strength. Although it is
possible to consider retardation of strength development, in practice this is
virtually never required. Once concrete
has hardened, compressive strength is usually wanted quickly.
Accelerators, as defined in
international standards, will increase early age strengths. However, in many situations,
superplasticisers used to produce low water-cement ratios may be more cost
effective and give further benefits of improved durability and long term
strengths.
It may not be possible to predict
exactly whether an accelerator or superplasticiser will be the best solution in
a given situation. Where acceleration of
both strength and stiffening is needed, accelerating admixtures must be used,
possibly in combination with superplasticisers.
Usually, superplasticisers are more effective at ages beyond 12 to 15
hours. Accelerators may be preferable at
early ages and low temperatures. Even in
these situations, careful selection of a combination of steam curing and use of
a superplasticiser may still produce better-precast results.
Conclusions:
The advent of admixture has become an
indispensable ingredient of the modern reinforced concrete. The multifold
advantage of concrete admixture not only help in manufacturing and placing a
good quality and high performing concrete, but also enhance the durability of
the r.c. structures. The technology is in the developing phase and new break
through in terms of high performing admixtures is foreseen.
References:
1.
Neville AM (1981), “Properties
of concrete”, ECBS Publications.
2.
Peter J Egan, “Economy and
Durability with Admixture”, Fosroc International Limited, UK
3.
S Collepardi, L et al.
“Superplasticisers – Types, Composition and Properties”
4.
V M Malhotra and V S
Ramachandran (1995), “Superplasticisers” in Concrete admixture handbook
properties, types and technology.
