Admixtures
Admixtures are chemicals added in very small amounts to the
concrete to modify the properties while the concrete is still fluid
and also after it has hardened and is in service. The quantity
added is less than 0.2% and usually less than 0.1% of the concrete
weight but even at this low level, admixtures have a very
significant effect on the concrete properties. These effects
include:
- Increased fluidity: reduces noise and energy requirements
during placing.
- Optimised mix design: reducing embodied carbon dioxide
and energy by enhancing the effectiveness of the cement
component.
- Reduced permeability: increases the durable life of the
concrete.
- Reduced damage from harsh environments: including marine,
freeze-thaw and sub zero situations.
- Improved quality: better finish and reduced service life
repair.
There are five main classes of admixtures, and these are classified
according to function. These five classes are:
- Air-entraining
- Water-reducing
- Retarding
- Accelerating
- Plasticizers (superplasticizers)
All other varieties of admixtures fall into the Specialty category
- its functions include corrosion inhibition, shrinkage
reduction, alkali-silica reactivity reduction, workability
enhancement, bonding, damp proofing and colouring.
More information on the main classes of
admixture and speciality admixtures.
Although all admixtures are chemically based, many are derived from
sustainable raw material sources such as corn or wood. In the
latter case the chemicals are produced from a by-product of paper
pulp manufacture that was in past years a waste material that had
to be disposed of. The admixtures themselves are generally harmless
and safe to handle, requiring no hazard labelling but all are
supplied with safety data sheets detailing what to do in the event
of personal contact, spillage or other incidents.
Manufacture is under factory-controlled conditions in mixing
vessels. In most cases heating is not required so energy
requirements are low. By purchasing raw materials in bulk,
packaging waste is reduced to a minimum and mixer cleaning water
can usually be recycled so process waste is also almost zero.
By supplying to concrete manufacturers by tanker into fixed
storage facilities, admixture packaging is also minimised. Most
deliveries are also optimised by a 'milk round ' top up
system.
Stringent testing has shown that admixtures are bound into the
concrete and do not leach out into the environment in significant
quantities during the life of the concrete. Admixtures have been
approved for use in concrete in contact with drinking water
following tests to show that they do not migrate from the concrete
to taint, colour or otherwise affect it.
Testing for admixtures in an end of life scenario have shown
that even when the old concrete is crushed and stockpiled, the
admixture leaching rate is so slow that the admixtures biodegrade
quicker than they can reach significant concentrations in the
natural environment below the stockpile.
The admixture manufacturing process has been subject to an
environmental inventory that has enabled Environmental Product
Declarations (EPD) to be produced covering over 80% of admixture
production in the European Union.
There are eight significant admixture suppliers in the
UK, seven of which are CAA members. All produce in the UK and all
are certified to ISO 9000. CAA estimates that its members hold an
85% + market share. About half of CAA members are ISO 14001
certified.
