At the Durrington Community Centre, 10th February. 1988,
DEVELOPMENTS IN STRUCTURAL ENGINEERING – CONCRETE by J.E.C. Farebrother.
Introducing, Mr. Farebrother said that while major developments took place in the 1950s – 1960s there has been little more than minor adjustments since. His own experience covering all this period began in his teens, working on design with various firms, and in 1963 forming his own consulting practice in the Midlands. His construction experience led him to have a more sympathetic attitude to contractors! More recently he was involved with arbitration and “expert witness” work.
Improvements resulted from the Cement and Concrete Association encouraging contractors and engineers to travel abroad as well as in re-search. This Association has disbanded because of lack of support of cement companies.
Of importance are pre-stressing and improvement of materials technology.
Pre-stressing eliminates tension stresses in the concrete when under load reducing the risk of cracking. Glass re-inforced concrete has now been developed.
Multi-storey buildings have just columns and plates – no beams – giving a flat soffit right across the building. Pre-cast panels are added.
Pre-stressing has affected the design of bridges – a technique considered for steel structures, but not pursued with any success. Pre-cast floor units, paving blocks, railway sleepers, telegraph poles have also been developed following its use in buildings. Larger applications are in Nuclear pressure vessels (heavily pre-stressed) and North Sea oil platforms.
Concrete mixing techniques have improved greatly the quality and reliability of the materials. Additives and the use of pulverised fuel ash are now widespread. Ready mix concrete prepared under well controlled conditions and pumped direct to the site as it is required has replaced on-site mixing. Concrete can now be produced that will take compression loads up to 40 tons per sq. inch.
Concrete shell construction enables large areas to be covered without obstruction under them. It also has a very smooth finish and thus less liable to collect dust; Barrel Vaulted roofs for Cadbury’s factories is a prime example. Hyperbolic parabaloids used for cooling towers and roofs are very strong.
Nerve an Italian, introduced Ferro-concrete – a thin layer of mortar over many layers of fine mesh wires. Use for boats, specially in China.
Housing in Tower Blocks largely using pre-cast panels has had major problems – the Ronan point disaster was due to a gas explosion in one flat “unzipping” the building! The in situ construction has failed because there is discontinuity in the structure. The joints are points of weakness difficult to inspect. Examination of demolished buildings proves this. Water ingress is also difficult to contain.
To obtain the high standard necessary for living accommodation: in situ construction is now normally adopted. This in fact is now cheaper. Much less is left to the foreman on site, very detailed design and construction specification being the order of the day. Even the shuttering is pre-designed.
Problems and limitations of concrete are –
Thermal insulation is lower than brick fcr living accommodation.
Concrete can shrink and creep under compression, so that beams begin to sag. It can also suffer a “cancer” due tc silicatious aggregates, reacting, causing cracking and expansion.
On the other hand it has been cheaper then steel, though the latter needs protection against deformation in a fire.
Nuclear reactor containers have improved radiation protection by using a high density basalt concrete.
