Norman Seymore, head of leading admixture producer, Chryso Southern Africa, explains the now virtually indispensable role of admixtures in concrete design.
What are admixtures?
NS: Admixtures, basically, are chemicals used to modify – and enhance – concrete’s freshly mixed, setting or hardened properties. They are added to the batch before or during mixing.
What led to the development of admixtures?
NS: There has been a change in building practice over the past few decades with more expected of concrete than ever before. The trend now is to use fluid concrete that can be fitted and moulded into any shape. However, concrete strength is non-negotiable and that it why chemicals such as admixtures were developed to enhance concrete without sacrificing strength.
How do admixtures improve concrete performance?
NS: Admixtures can improve workability, develop high early strength, increase ultimate strength, compensate for temperature fluctuations, and improve resistance to corrosion, shrinkage, or freezing and thawing. They are also used to overcome placement emergencies such as equipment failure and other unexpected delays. Admixtures also reduce the cost of the concrete mix and compensate for poor aggregate properties.
What are the main categories of admixtures used in concrete mixes today?
NS: Admixtures can be classified into five main categories: plasticisers, super-plasticisers, air-entrainers, accelerators, and retarders.
Briefly, what are the individual characteristics of these admixtures?
NS: When plasticisers are added to the concrete mix, they enhance workability by repelling binder particles and distributing the particles more evenly throughout the concrete. Super-plasticizers can reduce the required water content of the concrete mix by 15 to 20%. Air-entrainers introduce bubbles into the concrete mix to provide maximum protection against freezing and thawing and increase workability when the mix lacks fines. Accelerators speed up the chemical reaction of the cement and water and consequently the rate of hydration improving the setting or early gain in the strength of concrete. Retarders, on the other hand, slow the chemical reaction of cement and water, providing longer setting times and slower initial strength gain.
When did the construction industry become converted to the role admixtures can play to enhance performance and durability? Was there substantial early resistance?
NS: It was during the 1980s that admixtures really started to be regarded as an essential part of concrete mix design. In the early years, there was skepticism from some quarters but as the industry has advanced – and been able to really measure the benefits of admixtures – there are very few projects on which admixtures are not included in the concrete mix design today.
Is there a particular facet of the construction operations that particularly benefit from the use of admixtures in the concrete?
NS: Many precast operations now use self-compacting concrete (SCC). Self-compacting concrete is a highly flowable type of concrete that spreads into the form without the need for mechanical vibration. It is a non-segregating concrete that is placed by means of its own weight.
With careful mix design, SCC admixtures produce a concrete that is fully flowing and self-compacting and can be placed with no vibration. SCC functions in the same manner as superplasticised, flowing concrete but does not need any compacting effort. It is also less susceptible to bleed and segregation.
Admixtures – specifically superplasticizers – produce fluid SCC which is cohesive with high flow retention.
SCC is also extremely pumpable. The towering pylons of the Nelson Mandela Bridge in Johannesburg were, for example, filled with SCC based on a super-plasticizer supplied by Chryso.
What exactly was ‘new’ in Chryso’s New Generation range of admixtures which have been around for a few years now?
And how radically did these relatively ‘green chemicals’ change the fortune of admixtures not only in this country but worldwide?
NS: The new generation admixtures are based on specific raw materials or polymers (polycarboxylates). These raw materials and formulations radically improve the performance of the admixture in terms of water reduction /workability retention and strength development compared to traditional or old generation admixtures. A key benefit of Chryso is that we produce our own raw materials for the formulation of our New Generation products.
Which sectors of the construction industry are the main users of your admixtures?
NS: Ready-mix concrete users mainly use admixtures to adjust the workability retention of concrete; precast users use admixtures to speed placing and setting times so that they can get faster turnaround time of the moulds and improved surface finishes, and for on-site mixing for new construction, different admixtures are used to meet a project’s specific needs.
How important is the advisory role and after-sales service in the admixture industry?
NS: Accurate dosing is essential to fully capitalise on the benefits of admixtures. Dosage rates and admixture selection is determined through a series of trial mixes to optimise concrete properties. This is why it is vital for suppliers of admixtures to provide detailed guidelines to assist with the design of mix proportions and admixtures dosages.
As for after-sales service is concerned, Chryso has recently substantially expanded its laboratory testing facilities with the opening of a new Centre of Excellence in Jet Park. The Centre features ultra-modern equipment and temperature control systems to offer cement, concrete, and construction systems technology as an added-value service to customers.
The Centre’s testing and advisory service to customers is the most comprehensive of its kind available in the South African admixtures industry and can provide tailor-made solutions to match specific applications and customer requirements. This service is provided free of charge to Chryso customers.