Update on concrete From classic uses to experimental forms, concrete technology is developing as rapidly as the guidance which regulates it
Concrete is a remarkably versatile material which can be made almost anywhere: it is a concept rather than a material. Its advantages let it be taken for granted as a simple material which anyone can use. But developments in the material and our expectations should change how it is used.
Interest is growing in self-compacting concrete whose technology derives from pre-stressing grouts and underwater concreting. In Japan, it has been seen as a solution to the problem of getting site labour, whereas its use in the uk is often based on whether its cost, twice that of conventional concrete, can be justified by the savings in labour, improved health and safety, and reduced environmental impact.
Its strength characteristics are the same as conventional concrete. It is quicker to place, which is not necessarily reflected in the overall construction time, and it is easier to achieve good concrete in heavily reinforced sections. An important benefit is eliminating vibrators, which cause noise pollution and risk operators developing white hand.
There should be more use of testing techniques which target the con struction process, such as temperature-matched curing. With this, test cubes are cured under the same conditions as the structure and tested to justify the removal of formwork, achieving faster turn-around. Other techniques are the estimation of maturity by a time/temperature summation and the use of mechanical tests (eg pull-out tests).
Better testing should lead to quicker identification of problems. At present, time spent testing and analysing can be so protracted that the removal of defective concrete becomes difficult and disruptive. There is no immediate prospect of a test to establish, simply and accurately, the strength of suspect concrete.
Time spent on preparation and trials results in less defective work and fewer arguments. Such arguments frequently concern appearance but a set of specification definitions for architectural concrete is under development. A set of reference samples for the current BS 8110 definitions now exists, but its effectiveness is not yet proven.
Time to recycle
Reduced noise pollution from self-compacting concrete has already been mentioned. High-strength concrete reduces the amount of materials - both concrete and others - because less space is occupied by the structure, thereby reducing the dimensions of a building in plan and elevation.
We are familiar with the use of 'waste' materials such as pfa and blast- furnace slag in cement. We need the same approach for aggregate, which makes up a bigger percentage of concrete. Crushing concrete for re-use is well established. Reinforcement, itself predominently recycled, is melted down while crushed concrete aggregate reduces the extraction of raw materials, transportation, waste disposal and, critically, avoids landfill tax. Recycled concrete is often used as road base or fill, but with the cost of natural aggregate five times that of recycled, a lot may be gained from its use in concrete.
The bre has a recycling project in place with the precast industry which has a constant supply of trial pieces and rejects which can provide aggregate of known quality.
Current practice is to use a high-quality natural aggregate, as well as Portland cement, in all applications, whereas what we ought to be doing is selecting the constituents which are more in accordance with real needs. Thus substructure, for example, could utilise different materials from superstructure.
Improved durability of concrete reduces maintenance, prolongs useful life and hence reduces environmental impact.
The specification of durability by key parameters such as cement type and water/cement ratio has been almost unchanged for 100 years, but a quantitative approach is being developed based on models of the process of decay. This could allow a flexible design approach which is more specific than the current, rather generalised description of exposure and concrete to resist it. So, for example, a quantitative assessment could be made of the effects of changing cement content or reinforcement type.
The reinforcement plays an important role in durability. Stainless-steel reinforcement is well established. A new development is glass-reinforced epoxy reinforcement, which should be considered as a new material with particular properties rather than a substitute for steel reinforcing bars. It could be used in bar form, but also as plates or as structural permanent formwork.
Cement free of macro defects is derived from work using the science of fracture mechanics to produce inherently stronger materials. A demonstration of its properties is the concrete spring. Although it uses the same raw materials as standard concrete it is not a construction material.
Micro-silica is high-purity silica derived from flue precipitates generated by the manufacture of ferrosilicon and silicon. It is used in high-strength concrete and chemically resistant concrete. It is also used in some non- structural applications, such as floor toppings.
Of perhaps greater interest to the engineer is rpc (reactive powder concrete). Micro-silica cement and crushed quartz aggregate with a tuned particle size are cast under pressure with heat treatment to give strengths of 250-300 N/mm2. This is effectively a new material with great potential for special structures, but is not a site-cast material.
John Thornton is a director and Bob Cather an associate director at Ove Arup & Partners