NEW ADMIXTURE UNLOCKS THE POTENTIAL FOR CONCRETE
October 28, 2004

A recent new development in admixture technology has opened up remarkable new possibilities for the use of concrete in situations that were previously the preserve of specialist products, or where more radical solutions were needed, reports the UK Cement Admixtures Association (CAA).

The development of this new high fluidity, self-compacting, superplasticising admixture, based upon polycarboxylate ethers (PCE), will significantly increased the options available to engineers, not only in terms of design of new structures, but also in repairing and upgrading existing structures with concrete in order to meet current needs.

Polycarboxylate Ethers (PCE) are a new generation of superplasticisers. Not only are they chemically different to the older sulphonated melamine and naphthalene based products but their action mechanism is also different, giving cement dispersion by steric stabilisation, instead of electrostatic repulsion. This form of dispersion is more powerful in its effect and gives improved workability retention to the cementitious mix. Furthermore, the chemical structure of PCE allows for a greater degree of chemical modification than the older generation products, offering a range of performance that can be tailored to meet specific needs.

The availability of PCE admixtures has been essential to the development and acceptance of Self-Compacting Concrete (SCC). SCC offers the potential for improved and innovative methods of new construction, as well as repair or the upgrading of older structures where complexity of shape and difficult access may have previously prevented the use of traditional concrete. It offers a uniquely high flow in conventional concrete, without the problems of bleed and segregation that would normally be associated with very high levels of workability. SCC will flow long distances under minimal head, allowing it to fully penetrate voids that would otherwise be inaccessible. Furthermore, there is no requirement to vibrate the concrete in order to obtain full compaction, even where there are difficult shapes and/or heavy reinforcement. The complete lack of bleed means that full contact is maintained under soffits where the SCC concrete has flowed into a void created by formwork or an existing surface. In addition, SCC has been shown to produce excellent surface finish, free of significant air voids and other defects. This makes it particularly effective in repair or strengthening situations where the concrete will form the final finish on the structure.

As well as the admixture, SCC requires careful selection of materials and of concrete mix design, but most readymix companies can now provide this. The concrete can be placed directly from the back of the truck, or it can be pumped into place, including from the bottom of the shutter by upward displacement. SCC is cohesive, in spite of its very high fluidity and is relatively tolerant of imperfectly sealed small gaps in the shuttering, which can be a problem with repair or strengthening work. The lack of requirement for vibration can be a real benefit both in terms of noise reduction and access for personnel and equipment.

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