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Dating buildings: concrete The final article in our three part series focuses on concrete, with case studies to demonstrate the art of judging a building's age

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technical & practice

With the fall of the Roman Empire the knowledge of concrete making was largely lost until the late 1700s when lime concrete was rediscovered in the uk. But this became obsolete by the 1880s with the advent of Portland concrete.

In 1854, W B Wilkinson patented reinforced concrete. In 1865 he built what was probably the first reinforced concrete house, which stood in Newcastle until demolished in 1955.

Wilkinson's invention did not apparently progress. Reinforced concrete technology was developed by continental engineers and transferred back to the uk in the 1890s.

It became attractive to engineers and architects alike, for its freedom of form, fire resistance, and high tensile strength.

Apart from Ordinary Portland Cement (opc) Concrete, other concrete mixes have been used throughout the last hundred years for specific properties, such as lightness or rapid hardening. They can sometimes be distinguished by their colour and texture (see table below).

Mass components

From the advent of Parker's Roman cement in 1796 until the inception of reinforced concrete in the 1890s, mass (unreinforced) concrete was used for components of buildings, such as footings, 'fireproof' flooring, underpinning, and artificial stone bay-windows. With a few exceptions1, mass concrete was not used to construct whole buildings.

Terrible fires in warehouses and factories during the industrial revolution produced a demand for fireproof floors. Mass concrete seemed the ideal non-combustible replacement for timber. Systems evolved from the jack- arch floors of the 1790s, (which were not truly fireproof as they had unprotected joist-flanges), through proprietary systems such as Fox & Barrett, King, Homan & Rogers, Fawcett, and filler-joist floors up to the 1930s.

Proprietary reinforcement

The 1890s until the First World War was the era of proprietary reinforcing systems for cast in-situ concrete. A large number of systems were patented2. Those of Truscon and Considiere were widely used, but the Hennibique system was most popular, and was used in 35,000 buildings and other structures.

Hennibique's design method for reinforced concrete (rc) floor beams patented in 1897 was based on plain mild steel round bars with fish-tailed ends, and distinctive stirrups of flat strip.

The first uk Hennibique rc building was the now-demolished 1897 Weavers Mill in Swansea. An extant example is the 1909 Royal Liver Building in Liverpool, then the tallest concrete building in the world at 94m including its Liver birds. It was designed by the architect W Aubrey Thomas, with an entirely reinforced concrete frame.

Many more modest Hennibique buildings survive today, such as the 1909 Kings College Hospital in London, designed in a restrained Neo-Georgian style by the architect, William Pite.

To the untrained eye, most of these early rc concrete buildings could be mistaken for concrete cased steel-framed buildings of the period.

Unfortunately concrete is opaque, preventing its dating by the very distinctive patented reinforcement within, unless the concrete cover is broken away. One visible exception is the use of expanded metal as soffit reinforcement from 1890 until the Second World War.

In 1915, the lcc introduced the first British reinforced concrete regulations, which put an acceptable method of designing reinforced concrete into the public domain. Thus proprietary patented systems could be circumvented, and their era drew to a gradual close.

Expressionist use

After the First World War, rc-framed buildings which aped their steel counterparts, continued to be built but concrete's freedom of form also enabled shells, arches and spirals to be created.

Notable examples include the 1921 Wembley Stadium by architect Maxwell Ayrton and engineer Owen Williams, the 1926 New Royal Horticultural Hall in Westminster by architect Easton and Robertson and the 1933 Penguin Pool at London Zoo by architect Lubetkin and Tecton and engineer Arup and Samuely.

Flat slabs were first used in the uk in the early 1930s, thanks to design development in the usa and mainland Europe. A notable example is Owen Williams 1932 Boots 'wets' factory at Beeston near Nottingham.

Guidance on flat slabs was included in the first British code of practice for reinforced concrete in 1934. The code also noted the developments in concrete materials, including high alumina cement (hac) first distributed in the uk by Lafarge in 1923.

hac was initially used for its strong resistance to chlorides and sulphates. After the Second World War hac was more widely used because of its rapid- hardening qualities, for the manufacture of precast and prestressed components. It was discredited in 1974 by roof beams which collapsed at Sir John Cass School in Stepney, but it is still manufactured today for non-structural uses.

As well as developments in cement in the 1920s, coarse aggregates for making lightweight concrete were also changing. Supplies of clinker were becoming scarcer, and other lightweight aggregates were being developed which came into volume use from 1950 onwards, including Aglite, Leca, and latterly Lytag. They act as lightweight replacements for stones in the concrete mix, and do not have the problematic acidity of clinker.

Economy and speed

During the Second World War, half a million houses were totally lost due to bombing, and another three and a half million houses were damaged; one third of the total stock. Britain ended the war virtually bankrupt with an enormous national debt.

There was a great shortage of building materials, and problems with distributing whatever there was. The government aimed to build 200,000 permanent dwellings per year, but only managed 550,000 from August 1945 to December 1949. By 1951, Britain was still short of 1.5 million dwellings, and 10 million households were still living in overcrowded, substandard or unfit houses.

New forms and methods of construction were needed to make available materials go further, and faster.

Concrete played its part. At first reinforcing steel was rationed, but pre-stressing strand was not. This encouraged the use of pre-stressed concrete, and no-fines mass concrete construction. Between 1945 and 1986, Wimpey and the Scottish Special Housing Association alone produced 300,000 no-fines dwellings.

After 1950, precast concrete floor systems, and large panel system blocks of flats burgeoned. Calcium chloride and hac were used to speed up curing of the concrete, until the mid 1970s when their detrimental effect upon durability and strength became widely known.

Quality construction

By 1976, despite the post-war baby boom the housing shortage had been reduced to 2.7 million households. The pressure to build fast and cheap had abated, and there was time to reflect upon the shortcomings of the desperate post-war years.

Not only were Calcium Chloride and hac discredited, but also autocaved aerated concrete slabs were found to sag due to creep; and woodwool slab permanent shutters were banned by conveyancing solicitors, for causing grout-loss and poor compaction of concrete.

The use of precast large panel system buildings drew to a close in the early 1980s after concerns mounted about their durability and safety (initiated by the collapse of Ronan Point in 1968), and also due to their social problems.

Concrete floor systems continued to evolve. Profiled steel permanent shuttering for concrete slabs gained popularity and post-tensioned slabs were introduced.

Clive Richardson is a structural engineer and technical director of Cameron Bedford Consulting. Contact him at clive.richardson@camerontaylor.co.uk. He and Gary Powell are contributing to a series of one-day conferences being run around in London, Leeds and Manchester, 'Dating Buildings and Diagnosing Defects from their Ages'. For more information, please call Kerry Leech on tel 0118 959 1590

Footnotes

1 Concrete Through the Ages, British Cement Association, 1999, pp 8-9

2 M N Bussell, 'The Era of the Proprietary Reinforcing Systems', Proceedings Institution, Civil Engineers Structures & Buildings, 1996, 116, Aug/Nov, pp 295-316

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