CEMENT & CONCRETE
How admixtures influence concrete designs Over the past three to four decades, admixtures have become increasingly popular. Bryan Perrie outlines the rationale for, and type of, admixtures most commonly found in the mix.
A
dmixtures are chemicals added to a concrete mix to modify some of its properties; however, they should never be regarded as a substitute for good mix design, sound workmanship, or use of the correct materials. The most common reasons for using admixtures in concrete are to: • increase workability without changing water content • r educe water content without changing workability • achieve a combination of the above • adjust setting time • reduce segregation and/or bleeding • improve pumpability • accelerate or retard the rate of strength development at early stages • i mprove potential durability and reduce permeability
• reduce the total cost of the materials used in the concrete • compensate for poor aggregate properties. Below are various types of admixtures worth gaining an understanding of.
Plasticisers When added to a concrete mix, plasticisers (water-reducing agents) are absorbed on the surface of the binder particles, causing them to repel each other and de-flocculate. This results in improved workability and provides a more even distribution of the binder particles through the mix. The main types of plasticisers are lignosulfonic acids and their salts, hydroxylated carboxylic acids and their salts, and modifications of both. Plasticisers usually increase the slump of concrete with a given water content and can reduce the water requirement of a concrete mix for a given workability by about
Bryan Perrie, managing director, TCI
10%. The addition of a plasticiser makes it possible to achieve a given strength with a lower cement content, and it may also improve pumpability. However, some plasticisers contain a retarder and can cause problems if overdosed. While some entrain varying amounts of air, others are reasonably consistent in this. Where plasticisers are used to increase workability, shrinkage and creep will invariably be increased.
An admixture played an important part in the construction of the Nelson Mandela Bridge in Johannesburg. Selfcompacting concrete, containing a superplasticiser supplied by Chryso SA, was used to build the towering pylons
Superplasticisers
These admixtures are chemically different from normal plasticisers and their action is more marked. When used to produce flowing concrete, a rapid loss of workability can be expected and, therefore, they should be added just prior to placing. Superplasticisers are usually chemical compounds, such as sulfonated melamine formaldehyde, sulfonated naphthalene formaldehyde, modified lignosulfonates, and polycarboxylate-based materials. Superplasticisers are used to the best advantage in areas of congested reinforcement and where a self-levelling consistence facilitates placing. For highstrength concretes, they decrease the watercement ratio by reducing the water content by 15% to 25%.
62
IMIESA July 2020
