HIGH RANGE WATER REDUCING ADMIXTURES

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 otheadmixtures. 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, generall30 -1000 ml per 50 kg cement, accurate and uniform dispensinis 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 babout 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 botthe 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 propertieswhich 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 concretmust 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 othe 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 : Part3, 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.

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