Admixtures. What they do and when to use them
Admixtures are materials (often in the form of fluid or powders) which are added to concrete. These materials allow the concrete to gain characteristics which would not be obtainable with plain concrete mixes. As a standard, admixtures tend to be added at the time of mixing and generally no more than 5% (by mass of cement) is added, any more than this can cause concrete deterioration or poor workability.
It’s important to note that admixtures should only be used if stipulated as part of your original design specification. If an admixture is proposed, where it was not stipulated as part of the design, a Structural Engineer must confirm that the admixture is necessary and appropriate.
- Plasticisers – These improve the workability of concrete (especially when pumped). They can also improve the concretes adhesion, useful when the concrete is reinforced.
- Air Entraining Agents – These increase the air void volume of concrete, which helps produce a surface much more resilient to cold weather. Concrete mixed with air entraining agents is used for outdoor conditions where cold weather and exposure is high (e.g. pathways and roads).
- Accelerators - These provide a quicker curing time, caution should be taken however as a reasonable time is still required to properly ‘finish’ the concrete.
- Retarders - These slow the hydration of the concrete and are most often used in large or difficult pours where partial setting before the pour is completed in undesirable.
- Pigments - These are used to change the colour of concrete.
- Corrosion Inhibitors - These are used to reduce the corrosion of steel and steel bars often used to reinforce concrete.
- Bonding Agents - These are used to help create bonds between older concrete and new concrete generally they have wide temperature tolerance as well as being resistant to corrosion.
- Pumping Aids - These help improve the pumpability of concrete, thickening the paste and assisting in reducing separation and bleeding.
Admixtures in Cold Weather
Though you can use admixtures in cold weather, they don’t generally help in preventing your concrete from freezing. It is therefore important not to rely upon admixtures to compensate for freezing conditions. Please follow our guidance for using concrete in cold weather in these instances.
Admixtures and Reinforcements
It is important to note that admixtures containing chloride will cause increased corrosion to reinforcements, it’s advised not to use them in any concrete containing reinforcing.
Please note that we do not accept fibre reinforced concrete for structural elements.
For further information on Admixtures you can download Chapter 2 of our Technical Manual:
Please note the information used in this article is taken from Version 8 of the LABC Warranty technical Manual and is provided as guidance in meeting our technical standards. If working on an LABC Warranty site please check which standards apply.
Please Note: Every care was taken to ensure the information in this article was correct at the time of publication on 5.3.2018. However, for the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.