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Elemental understanding

Concrete has always excited architects. Now Dutch researchers suggest possibilities for the next generation of the material

Cornelis van Eesteren, architectural doyen of the De Stijl movement, gave a lecture in 1928 in which he said that the urbanist of the future must fully understand the elements that he works with.'Architects must thoroughly acquaint themselves and know as much as a painter knows his paint and an engineer his materials, ' he said.He should have added that the architect must also be aware of the more esoteric aspects of the material to be used.

Whether the cities of the future are ecologically sustainable or comprise a cluster of mile-high towers, it will be even more important that the design limits of our materials are fully understood.One of these materials, which will survive long into the future, will be concrete. Unfortunately only a few engineers and their increasingly dependent architects have any idea about what really takes place within concrete's metabolic structure and how the material can be better utilised.

For centuries, brick and stone were the materials of first choice. In Amsterdam Hendrick Petrus Berlage articulated the principles of structural rationalism with his load-bearing brick-built Stock Exchange (1897-1903) while the internal roof was supported by curved cast-iron trusses. Berlage's bricklayers were encouraged to be artists, to give visual expression to handcrafted material.

His vast urban housing blocks still give a sense of community on the south side of Amsterdam. As with other architectural touchstones, we are reminded of the new materials of the industrial revolution in Guimard's iron and glass Art Nouveau Paris Metro entrances and our own examples of cast-iron church-like structures in Leeds' glass-covered, double-tier internal markets.

Structural triumph

As new political revolutions began to shake the fabric of world society, new construction materials were also deemed to be revolutionary. The invention of Bessemer-produced steel, reinforced concrete, air-conditioning and the electric elevator were essential to the art of the skyscraper which became the new urban tool of emerging, fast-track industrial capitalism. In Europe the new architectural Modernists divided along political lines.

Sant' Elia, the Futurist proto-fascist, on the right; with Walter Gropius, the Bauhaus and Russian Constructivists, such as Berthold Lubetkin, on the left. Le Corbusier hovered between the two.

Mies' early approach to skyscraper construction was to render glass as a complex reflective surface, but later he saw that the primary expressive material was reinforced concrete.According to Kenneth Frampton, Mies demonstrated 'decidedly Hegelian overtones' when he wrote that 'architecture is the will of the age conceived in spatial terms' and insisted that maximum effect comes from minimum effort using concrete, steel and glass.

Concrete became a communicator.

Among some people in the 1930s, 'Tecton' had the same street-cred as today's buzz words such as 'electronic communication'. The word Tecton was derived from 'Tektonika' from the Constructivist circles of Vladimir Tatlin and Alexander Rodchenko, and the Greek word meaning 'builder'. Tecton was the architectural practice of Berthold Lubetkin, born in Tiflis, Georgia in 1901. His architectural ideas were forged on the anvils of the Russian avant-garde; his building and concrete technical knowledge was developed at Berlin's Bauschule.He moved to Warsaw and finally to the Ecole Speciale d'Architecture in Paris where until 1931 he designed many of the Futuristic constructions for the new Soviet state.

In 1932, Lubetkin began a long association with Ove Arup and a whole series of reinforced concrete structures were built.

The Gorilla House at London Zoo was a 'machine for living' with its drum construction and fairfaced concrete. Using sliding and revolving screens it created stringent environmental control to prevent cross-infection from humans to apes.

But concrete was still an unknown quantity. It was at this point that Ove Arup entered the stage to begin what John Allan, Lubetkin's consummate biographer, describes as 'one of the most creative and fruitful collaborations in modern architecture'.

There followed other milestones of concrete architectonic structures including the performance art of the zoo's Penguin Pool and the splendid example of the Highpoint blocks of flats in Highgate, London.

Learning curve

Many architects and engineers have enjoyed the plasticity of concrete and recognised its potential as a material that could be factory made, poured on site, or used in kit-form to construct social housing on a scale never previously conceived.

From the start, reinforced concrete was assumed to last indefinitely.

In adopting a system of climbing shutters, Arup - who in the early 1930s was seeking new architectural forms through the use of structural concrete - became innovative to a point of innocence. At Highpoint One structural elements were reduced to minimum thickness - beyond the point where structural efficiency should give way to constructional prudence.

Allan dealt with Tecton's construction with a critical steeliness: 'Such unwarranted faith in the longevity of thin concrete structure seemed all the more puzzling in a practice otherwise so conscientious in its investigative approach to technique and detail.' He continues: 'The process of concrete deterioration was not fully understood.

Little or nothing was known about the phenomenon of carbonation, whereby the alkalinity of fresh concrete is neutralised by atmospheric carbon dioxide, allowing an acidic and potentially corrosive environment to develop in the thickness of the material.'

But Allan is writing with 60 years of hindsight; these issues were not understood at the time. Mistakes and failures multiplied, especially with social housing when post-war political expediency was linked to making a fast buck. The adoption of inappropriate systems and unrealistic expectations resulted in poor design.

Having learned the hard way, Arup and Partners, which is now one of the world's largest construction consultants, has developed its own scientific investigations into materials.According to Robert Cather, a materials scientist and associate director of Arup and Partners, there was a twofold problem: poor design and an 'unsuitable mix'of concrete aggregates and chemical admixtures which resulted in concrete being perceived as grey, damp and of poor quality.

Cather is using the concrete research facilities at Imperial College, London.

Here the development of new techniques of electron microscopy are enabling material technologists to see and fully understand what actually takes place within mixed concrete. These and new Dutch investigations will enable new concrete products to be designed and created to meet our future construction needs.

Dutch breakthrough

If future architects are to build higher buildings and use their materials more ambitiously, then it is imperative that we find the secrets of all the materials that we specify. Professor Jan van Mier of the Technical University, Delft is doing just that.

More than £2.5 million has been spent on developing the university's Microlab - 2,000m 2of laboratory space for concrete and material technology research. In October, six PhD students and two postdoctorate students beefed up a staff of five technicians in an attempt to develop what van Mier describes as a new world of 'designer concrete'.

They will be investigating concrete composites in a laboratory environment where thin 'bacon' slices of concrete impregnated with fluorescent epoxy can be scanned, atomically zapped, and altered to produce site-specific materials. These will then be exhaustively tested in demonstration buildings.

Cather is certain that Microlab will help extend the use of concrete, enabling future architects to specify a time-honoured material that is even more flexible, fire- and weather-resistant, as well as stronger and more durable than ever.

Reinforced concrete broke the old architectural traditions. It became a material of Modernism, Brutalism and fascist monumentalism, frightening the political conformists of both the left and right. Its mass facilitated thermal storage, structural rigidity and visual delight.

Now, however, we are in a new age. The different alchemy being discovered suggests, says van Mier, that concrete can have a longer life-span and that its strengths and flexibility can be assured. In recent experiments mixing concrete with fibres has given tensile strengths of 30N/mm 2(30 megapascals) - about 10 times higher than plain concrete.

This would mean that we could start considering concrete structures without any main reinforcement. Discovering the secret of concrete could lead to hybridfibre concrete eventually replacing conventional reinforced concrete.

ARTICLES

'Visual concrete part one: exposing the concrete facts' AJ18.11.92

'Visual concrete part two: vibration, curing and striking' AJ25.11.92

'Visual concrete part three: making good' AJ2.12.92

'Visual concrete part four: surface treatments' AJ9.12.92

GUIDANCE

BS1881:1983 - Methods of testing concrete, part 108: method for making test cubes from fresh concrete Examination of the principles of cube testing.

BS1881:1983 - Methods of testing concrete, part 116: method for determination of compressive strength of concrete cubes Detailing the engineering requirements of concrete performance and accountability.

BS 5075:1982 - Specification for concrete admixtures, part 1: accelerating admixtures, retarding admixtures and waterreducing admixtures Specifies performance and uniformity tests and requirements, information to be provided by the manufacturer, certificates and marking. Appendices for test methods and guidance on use.

BS 5328:1997- Concrete, part 1: guide to specifying concrete Guide to specifying concrete, gives guidance on the selection of materials for concrete.

BS 6089:1981 - Guide to the assessment of concrete strength in existing structures Guidance on planning and conducting investigations including a description of test methods and interpretation of results.

BS EN 12350:2000 - Testing fresh concrete, part 1: sampling

BS EN 12615:1999 - Products and systems for the protection and repair of concrete structures - test methods - determination of slant shear strength Examining cement loads and stresses.

Behaviour of reinforced concrete flat slabs CIRIA report, study of the design of reinforced concrete flat slabs with regard to flexure, punching shear, and deflection.

Information drawn from Technical Indexes, tel 01344 426311

BOOKS

'Design and Construction of Concrete Floors' George Garber, Edward Arnold,1991 Good practice guidance on concrete floor structures in logical sequence. Includes an introduction on the philosophy of floor design.

'Concrete detail design' The Concrete Society, Architectural Press, 1986 This book offers expert guidance to the architect and engineer on remedial work for in-situ, reinforced concrete-frame buildings.

Includes simple, concise line drawings.

'Repair of concrete structures' Edited by R T L Allen & S C Edwards, Blackie, 1987 A very good reference book on lightweight concrete for civil and structural engineers. In addition to basic repairs, it covers leak sealing, tanking and surface coatings.

SPECIFYING SLUMP

Many concrete technologists are beginning to wonder if office-based specifiers know what slump is, or what it is for, writes John Dransfield .

A 50mm slump is where a 300mm high cone of concrete falls by 50mm to stand 250mm high from the baseplate when tested. But why is this level of slump built into everyday specification?

If a typical specifier was re-laying their own drive on a Saturday morning, which concrete specification would they choose? A 50mm or a 230 mm slump? Which do they think would be better compacted and free of entrapped air voids? Not only is the 50mm slump stiff, heavy and difficult to move around, it also quickly loses workability. After as little as 30 or 40 minutes it is down to 25mm slump and even harder to use.

Faced with a backbreaking problem, it does not take the average concrete gang long to find a way round such restrictions. Any concrete supplier will tell you - off the record - that, if they can get away with it, water addition to deliveries of 50mm slump is standard practice; even though they know that too much water in concrete is usually a bad thing.

Increases in the water content reduce the concrete's strength and permeability rises, so the concrete is less resistant to abrasion, water ingress, carbonation, sulphate attack and chloride ingress. Concrete suppliers also know that more water can result in bleed and segregation with sand runs and grout loss at joints. But specifiers mistakenly believe that high initial slump means there is too much water.

About 70 per cent of UK readymix concrete contains an admixture, but most of this is employed to maintain workability at a 50mm slump.

At a nominal additional cost, higher dosages or different types of plasticising and water-reducing admixtures can be used to increase slump while maintaining, or even further reducing, water content.

Most of these admixtures also enhance cohesion, eliminating problems of bleed at higher workability. A 230mm slump is not necessary for most jobs. A slump of 125mm at delivery will usually ensure that the concrete can be easily placed and well compacted - reducing the temptation to add water on site.

However, workability will fall if there is a placing delay, especially during warmer weather. Under these conditions a slightly higher initial slump of about 150mm may be more appropriate.

Admixtures are now used in the construction of all major structures in the UK and overseas and European standards EN480 and EN934 for admixtures will shortly replace BS5075. Modern dispensing equipment and batch plants, coupled with refined chemicals and quality-assured production of both the admixtures and the concrete, means that concerns about overdosing and retardation and about unworkable 50mm slump concrete should be consigned to the last century.

John Dransfield is secretary of the Cement Admixtures Association, tel 01564 776362

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