Plasticizers of Concrete

Super Plasticizers:

These are more recent and more effective type of water reducing admixtures also known ashigh range water reducer. The main benefits of super plasticizers can be summarized as follows:

Increased fluidity:

§  Flowing

§  Self-leveling

§  Self-compacting concrete

§  Penetration and compaction round dense reinforcement

Reduced W/C ratio:

§  Very high early strength, >200% at 24 hours or earlier

§  Very high later age strengths, >100 MPa or 15000 psi.

§  Reduced shrinkage, especially if combined with reduced cement content.

§  Improved durability by removing water to reduce permeability and diffusion.

The commonly used Super Plasticizers are as follows:

§  Sulphonated melamine formaldehyde condensates (SMF)

Give 16–25%+ water reduction. SMF gives little or no retardation, which makes them very effective at low temperatures or where early strength is most critical. However, at higher temperatures, they lose workability relatively quickly. SMF generally give a good finish and are colorless, giving no staining in white concrete. They are therefore often used where appearance is important.

§  Sulphonated naphthalene formaldehyde condensates (SNF)

Typically give 16–25%+ water reduction. They tend to increase the entrapment of larger, unstable air bubbles. This can improve cohesion but may lead to more surface defects. Retardation is more than with SMF but will still not normally exceed 90 minutes. SNF is a very cost-effective.

§  Polycarboxylate ether superplasticizers (PCE)

Typically give 20–35%+ water reduction. They are relatively expensive per liter but are very powerful so a lower dose (or more dilute solution) is normally used.

In general the dosage levels are usually higher than with conventional water reducers, and the possible undesirable side effects are reduced because they do not markedly lower the surface tension of the water.

3. Accelerators:

An admixture which, when added to concrete, mortar, or grout, increases the rate of hydration of hydraulic cement, shortens the time of set in concrete, or increases the rate of hardening or strength development.

Accelerating admixtures can be divided into groups based on their performance and application:

1.    Set Accelerating Admixtures,

Reduce the time for the mix to change from the plastic to the hardened state.

Set accelerators have relatively limited use, mainly to produce an early set.

2.    Hardening Accelerators,

Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least 130% at 48 hours. Hardening accelerators find use where early stripping of shuttering or very early access to pavements is required. They are often used in combination with a high range water reducer, especially in cold conditions.

Calcium chloride is the most effective accelerator and gives both set and hardening characteristics. However, is limited due to acceleration of corrosion of steel reinforcement and decrease resistance of cement paste in a sulfate environment. For this reason, it should not be used in concrete where any steel will be embedded but may be used in plain unreinforced concrete.

Chloride-free accelerators are typically based on salts of nitrate, nitrite, formate and thiocyanate. Hardening accelerators are often based on high range water reducers, sometimes blended with one of these salts.

Accelerating admixtures have a relatively limited effect and are usually only cost effective in specific cases where very early strength is needed for, say, access reasons. They find most use at low temperatures where concrete strength gain may be very slow so that the relative benefit of the admixture becomes more apparent.

In summary, a hardening accelerator may be appropriate for strength gain up to 24 hours at low temperature and up to 12 hours at ambient temperatures. Beyond these times, a high range water reducer alone will usually be more cost-effective.

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