Understanding Concrete Admixtures – Concrete admixtures are a complex subject but it is very important to understand what admixtures are available and what they do.
Admixtures are the ingredients in concrete which are other than the hydraulic cementitious material, water, aggregates or fiber reinforcement that are used as ingredients of a cementitious mixture to modify its freshly mixed, setting or hardened properties and that are added to the batch before or during mixing.
Water-reducing admixtures improve concrete’s plastic (wet) and hardened properties, while set-controlling admixtures are used in concrete being placed and finished in other than optimum temperatures. Both, when used appropriately, contribute to good concreting practices.

In the modern construction industry ,below are the most widely used concrete admixtures .
●Water reducing concrete admixtures
●Superplasticising concrete admixtures
●Set Retarding concrete admixtures
●Accelerating concrete admixtures
●Air-entraining concrete admixtures
●Water Resisting concrete admixtures
●Retarded, ready-to-use Mortars
●Sprayed Concrete admixtures
●Corrosion inhibiting concrete admixtures
●Foamed Concrete admixtures

Water reducing concrete admixtures
Water-reducing admixtures are water-soluble organic materials, which reduce the amount of water needed to achieve given workability without affecting the air content or curing of the concrete. They perform three functions:
●Increase strength and rate of strength gain.
●Economies in the mix design and reduced carbon footprint.
●Increased workability.

Superplasticising concrete admixtures
High range water reducing admixtures are called Superplasticising admixtures are synthetic, water-soluble organic chemicals, usually polymers, which significantly reduce the amount of water required to achieve a given consistency in plastic concrete.
They reduce water content without reducing strength for high workability requirements. They also improve durability.
High range water reducing admixtures function in a similar way to ‘Normal Water Reducing Admixtures, but they are more powerful in their cement dispersing action and can be used at higher dose without unwanted side effects such as air entrainment or retardation of set.

Set retarding concrete admixtures
Set retarding admixtures are water-soluble chemicals that delay the setting of the cement. They do not plasticise significantly and have little or no effect on the water demand or other properties of the concrete.
Set retarding water-reducing admixtures not only delay the setting of the cement but also increase initial workability by plasticising the concrete or reduce its water demand. The majority of commercially available retarding admixtures are of this type.
Retarding water-reducing and retarding high range water reducers are used to:
●Delay the setting time of concrete
●Prevent the formation of cold joints
●Increase initial workability 

●Improve workability retention to the concrete Increase ultimate strength.
●Produce economies in mix designs
It should be noted that while a retarder is needed for retention of slump. Addition of a retarding admixture does not in itself produce slump retention and other changes to the mix will probably be needed.

Accelerating concrete admixtures
Accelerating admixtures can be used either to increase the rate of stiffening/setting of the concrete or to increase the rate of hardening and early strength gain to allow earlier de-moulding and handling. Most accelerators primarily achieve one rather than both of these functions.
Accelerators are most effective at low temperature.Set accelerators are a very effective way of controlling the setting time of such concretes, even those containing cement replacements.
Accelerators are also used to reduce the risk of damage by freezing when concreting in cold weather and to allow the earlier removal of form work but it should be noted that they are not an anti-freeze. The exposed faces of struck concrete must still be protected and properly cured.
At normal temperatures, a technically better way of enhancing early strength is to use a high range water reducer.
Significant reductions (greater than 15%) in the water cement ratio can more than double compressive strength at ages less than 24 hours. Accelerators can be used in conjunction with superplasticisers (< 0.35 w/c ratio) where very early age strength is required. Especially at lower temperatures. If required, the use of accelerators can be combined with high range water reducers to further enhance early strength development in both low & normal temperatures.
Other applications for accelerating admixtures include urgent concrete repairs and in sea defence work to ensure early stiffening of concrete in the tidal zone.

Air-entraining concrete admixtures
Air Entraining admixtures are surface active chemicals which cause small stable bubbles of air to be formed uniformly through a concrete mix. The bubbles are mostly below 1 mm diameter with a high proportion being below 0.3 mm.
The benefits of entraining air in the concrete include:
●Increased resistance to the action of freezing and thawing

●Increased cohesion resulting in less bleed and mix segregation.
●Improved compaction in low workability mixes.
●Gives stability to extruded concrete
●Gives improved cohesion and handling properties to bedding mortars.
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Water resisting concrete admixtures
Water resisting admixtures are more commonly called ‘waterproofing’ admixtures and may also be called permeability reducing’ admixtures. Their main function is to reduce either the surface absorption into the concrete and / or the passage of water through the hardened concrete. To achieve this, most products function in one or more of the following ways:
●Reducing the size, number and continuity of the capillary pore structure 

●Blocking the capillary pore structure 

●Lining the capillaries with a hydrophobic material to prevent water being drawn in by 
absorption / capillary suction

These ‘waterproofing’ admixtures reduce absorption and water permeability by acting on the capillary structure of the cement paste. They will not significantly reduce water penetrating through cracks or through poorly compacted concrete which are two of the more common reasons for water leakage in concrete structures. 

Water resisting admixtures have been shown to reduce the risk of corrosion of reinforcing steel in concrete subject to aggressive environments but this is subject to appropriate admixture types or combinations of types being used.

Water resisting admixtures have other uses including the reduction of efflorescence, which can be a particular problem in some precast elements. 


Retarded, ready to use mortars
Retarded Ready-to-use Mortars are based on a combination of a mortar plasticiser (air entraining/plasticising admixture) and a mortar retarder. This combination is adjusted to give extended retention of consistence, typically for 36 hours. However, when the mortar is placed between absorbent masonry units, setting is accelerated and the mortar sets normally.
These properties facilitate the provision of mortar to building sites by ready-mix suppliers and offer the following primary benefits:
●Quality Assured control of mix proportions
●Consistent and stable air content
●Consistence (workability) retention (for up to 72 hours.)
●Increased productivity
●Eliminates the need for mixers and storage of materials on site
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The restrictions on the use of retarded ready-to-use mortars for non-absorbent masonry and rendering, detailed in clauses 4.6 and 4.7, should be noted.

Sprayed concrete admixtures
Sprayed concrete is pumped to the point of application and then pneumatically propelled into place at high velocity. The applications are frequently vertical or overhead and this requires rapid stiffening if slumping or loss by concrete detaching from the substrate under its own weight is to be avoided. In tunnelling applications, sprayed concrete is often used to provide early structural support and this requires early strength development as well as very rapid stiffening.
Admixtures can be used in the fresh concrete to give stability and hydration control prior to spraying. Then by addition of an accelerating admixture at the spray nozzle, the rheology and setting of the concrete are controlled to ensure a satisfactory build up on the substrate with a minimum of un- bonded material causing rebound.
There are two processes:
●The dry process where the mix water and an accelerator are added to a dry mortar mix at the
●spray nozzle.

●The wet process where the mortar or concrete is premixed with a stabiliser / retarder prior to
●pumping to the nozzle where the accelerator is added.

The wet process has become the method of choice in recent times as it minimises dust emissions, the amount of material rebound and gives more controlled and consistent concrete.

Corrosion inhibiting concrete admixtures
Understanding Concrete Admixtures – Corrosion inhibiting admixtures increase the passivation state of reinforcement and other embedded steel in concrete structures. This can inhibit the corrosion process over extended periods when passivation would otherwise have been lost as a result of chloride ingress or carbonation.
Corrosion inhibiting admixtures added to concrete during production are called “integral” corrosion inhibitors. Migratory corrosion inhibitors are also available which can be applied to the hardened concrete but these are not admixtures.
The most common cause of reinforcement corrosion is pitting corrosion due to the ingress of chloride ions through the covering concrete and subsequent diffusion down to the embedded steel. Although corrosion inhibitors can raise the corrosion threshold of the steel, they are not an alternative to producing impermeable, durable concrete which limits chloride diffusion.
Carbonation of the concrete leads to a lowering of the alkalinity around the steel and this causes a loss of passivation that can also result in general reinforcement corrosion. Corrosion inhibitors can help to guard against this form of attack.
Corrosion inhibitors can significantly reduce maintenance costs of reinforced concrete structures throughout a typical service life of 30 – 40 years. Structures especially at risk are those exposed to a maritime environment or other situations where chloride penetration of the concrete is likely. Such structures include bridges, tunnels, industrial plants, jetties, wharves, mooring dolphins and sea walls. Highway structures can be affected by the application of de-icing salts during winter months, as can multi-storey car parks where salt laden water drips off cars and evaporates on the floor slab.

Foamed concrete admixtures
Understanding Concrete Admixtures – Foamed Concrete Admixtures are surfactants that are diluted with water before passing the solution through a foam generator which produces a stable pre foam, similar to shaving cream. This pre foam is then blended into a cementitious mortar in a quantity that produces the required density in the foamed mortar (more usually called foamed concrete).
Low Density Fill Admixtures are also surfactants but are added directly into a sand rich, low cement content concrete to give 15 to 25% air. This low density fill; also called Controlled Low Strength Material (CLSM), has good flow properties and finds use in trench filling applications and other similar low strength void filling jobs. 

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