Admixtures are normally provided as water based solutions and can be added
to the concrete at up to 5% on cement weight, although most types are added
at less than 2.0% and the majority are at less than 1.0%. Detailed information on admixture
types is available in Concrete Society Report 18 or on the CAA AS sheets, (see
publications page). The main admixture types can be summarised as follows:
Normal Water Reducing / Plasticising admixtures
Often lignosulphonate based, they are used to increase workability at constant
water content and / or reduce water by up to 10%. Used by most readymix companies
to optimise concrete performance for normal concrete.
High Range Water Reducing / Superplasticising
Based on sulphonated Naphthalene or Melamine formaldehyde condensates, Vinyl
polymers or Polycarboxylate Ethers. These admixtures give a much higher performance
than the normal plasticisers. They are used to give very high levels of workability
or water reductions from 12 to over 30%. They are used extensively on larger
projects where reinforcing steel requires high workability. Also used in precast
and on site where the large water reduction provides very high early strength
and improved durability.They are essential to the production of Self Compacting Concrete (SCC).
These admixtures slow the rate of cement hydration, preventing the cement from
setting before it can be placed and compacted. This type of admixture is mainly
used in hot conditions and climates or on very large pours.
Used to speed the rate of early hydration of the cement. Can accelerate the
setting or the early strength development. Used mainly in cold conditions or
where very early use of a concrete pavement is required to provide access.
Based on special surfactants, these admixtures cause tiny air bubbles < 0.3mm
in diameter to stabilise within the cement paste. This air helps to prevent
the concrete from cracking and scaling as a result of frost action. Air also
increases cohesion in the mix, reducing bleed water and segregation of the aggregate
before the concrete can set.
Water Resisting (waterproofing)
These water repellent admixtures block or impede the flow of water through the
natural capillaries in hardened concrete. Used in structures below the water
table or in water retaining structures.
Ready-to-use mortar admixtures
They increase the cohesion and retard the setting of mortar for masonry, allowing
it to be delivered to a building site by readymix in large volumes that can
be used over an extended period, usually of 1 to 2 days. Setting is initiated
when water/admixture is sucked out by the in-situ masonary unit
Sprayed concrete admixtures
Provide a very rapid set to concrete that is sprayed onto vertical and overhead
applications, preventing it from falling off before it has time to set. Mainly
used in tunnelling applications for early roof support. Dosage may exceed 5%
These admixtures work for many years after the concrete has set, increasing
the corrosion resistance of reinforcing steel to reduce the risk of rusting
steel causing the concrete to crack and scale.
Foamed Concrete / CLSM
Powerful surfactants that allow very large amounts of air to be formed in the concrete. Typically used to produce a a low strength, light weight material, particularly suitable
for filling large voids where strength is not an issue.
Polymer Dispersion admixtures
Polymer emulsions that film form within the concrete or mortar to
give enhanced bond and flexibility to the mix. Mainly used in thin
floors or for repair situations. Addition is often more than 5%.
Used to reduce pump pressure in long pump lines or where difficult aggregates
are being used. Especially effective with lightweight aggregates.
Segregation reducing, viscosity modifying admixtures.
Self-compacting concrete uses superplasticisers to give a low yield, highly
fluid mix but may also require a Segregation Control or Viscosity Modifying
admixture (VMA) to ensure that mix cohesion is maintained.
Precast, semi-dry concrete
Semi-dry or Earth-dry concrete used to make pavers or blocks requires special
admixtures to aid full compaction without effecting the properties required
for immediate demoulding.
Shrinkage reducing admixtures
Concrete shrinks, mainly due to loss of excess water. This causes internal stresses
that lead to cracking or curling, especially in slabs. These admixtures reduce
the shrinkage stress.
Truck Washwater admixtures
These environmentally friendly admixtures allow the re use of water and washings
used to clean out readymix trucks at the end of a day. The washings are treated
with the admixture and then combined back into the following days concrete without
any detrimental effects to the concrete quality.
Anti-washout / Underwater viscosity modifying admixtures
Concrete that is placed under water may be subject to washout during placing
and before it hardens. These admixtures stabilise the mix, increasing cohesion
and reducing the washout.