five main categories: plasticisers, superplasticisers, air-entrainers, accelerators, and retarders.
CEC: 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. Superplasticisers can reduce the required water content of the concrete mix by 15 – 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.
CEC: When did the construction industry become converted to the role that 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 scepticism 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.
CEC: 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), which is a highly flowable type of concrete that spreads into the form without the need for mechanical vibration. It is a nonsegregating 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 superplasticisers – produce fluid SCC that 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 superplasticiser supplied by Chryso.
CEC: What exactly was‘ new’ in Chryso’ s New Generation range of admixtures that 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( poly carboxylates). 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 for Chryso is that we produce our own raw materials for the formulation of our New Generation products.
CEC: 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.
CEC: 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 are 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, 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. nn
TECHNOLOGY
The towering pylons of the Nelson Mandela Bridge in Johannesburg were filled with self-compacting concrete, based on an admixture supplied by Chryso.
CEC April 2018- 23