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Shape of things to come

Concrete Technology

Richard Day asks whether concrete has anything new to offer construction.

Concrete, a conglomerate of hydraulic binder and graded aggregate, might be considered to be a mature technology - after all, it was used by the Romans and possibly by much older civilisations. It came into its own as a construction essential when Joseph Aspdin patented Portland cement in 1824, and with the advent of embedded steel reinforcement soon afterward concrete was recognised as a serious structural material.

So for a material that, in its modern form, is over 170 years old, what is actually new in the world of concrete technology? Is it the material itself or the way it can be used to improve the construction process?

The answer may be that the novelty comes in concrete's ability to be manipulated to enable more efficient and complicated structural forms while remaining an economic construction medium.

A huge number of conference papers, research reports and magazine articles have been published in the last few years on self-compacting concrete.

Indeed, the second international conference devoted to the subject is to be held in Reykjavik in November. Many of the papers will concentrate on the rheological properties of concrete mixes made with a multitude of constituent materials and chemical admixtures. By contrast, few will develop the applicability of the flowable material to influencing site processes.

This suggests the construction industry needs to influence the direction of research into self-compacting concrete for its own purposes. Think about it - a concrete that, assuming the constituents are carefully proportioned, can take virtually any form, require no vibration for compaction, is stronger than traditional designs and is capable of a good surface finish - is this not worth serious consideration?

Precast concrete manufacturers in the Netherlands and several factories in the UK, notably Trent Concrete and Tarmac Precast, have adopted self-compacting concrete and revolutionised their production processes, not just in terms of efficiency but also to provide quieter and safer working environments.

Adding fibres, both plastic and steel, is becoming an accepted technique for improving properties of fresh and hardened concrete. Industrial ground floor slabs built on piled foundations are a prime example.

After extensive consultation with client, end user, designer, contractor and academia, the UK's Concrete Society has brought together design guidance on such floors in its latest edition of Technical Report 34 Concrete industrial ground floors - a guide to design and construction. This covers the practical use of fibres to enhance concrete's properties and to make the process of construction easier.

Another approach is to densify the matrix. The incorporation of condensed silica fume (microsilica), small hard aggregate and superplasticisers to reduce water content serves to increase strength to 150MPa.

Macro defect free concretes, using polymers to fill the pores, have been developed. In both cases the matrix is strong in compression but brittle.

Ultra high performance fibrereinforced concrete attempts to improve ductility by including fibre mixtures. The downside is that mixing and placing start to become a problem.

Several proprietary concretes have been developed, the key property being the improved tensile strength, which, unlike in conventional concrete, can be taken into account in design.

The Footbridge of Peace project in Seoul, South Korea, is a fine example. It is exceptionally slender and elegant - the deck of the 130m span arch has a constant depth of only 1.3m, one third that of similar bridges.

Ductal, developed by Lafarge, was essential to design realisation. Combining steel and plastic fibres in a cementitious matrix, without conventional steel reinforcement, produced flexural strengths 10 times greater than normal concrete. Its high postcracking strength makes it a ductile material - a property other concretes do not possess.

Resistance to abrasion, freezethaw and acids and low water permeability give potentially excellent durability.

Richard Day is divisional director - technical at the Concrete Society

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