Shopping around for a compact solution
A new regional shopping centre at Milton Keynes is the first place where self-compacting concrete has been used on a commercial scale in the uk. The technique is widely used in Japan where the construction industry makes significant savings in construction time and cost as well as the improved on-site working environment and performance.
Japan has developed the material over the last decade to achieve durable concrete structures without the need for heavy vibration that requires extra manpower, generating both project delays and overcosts, and can also be a health hazard on site. Self-compacting concrete achieves compaction into every part of the mould or formwork simply by means of its own weight without any segregation of the coarse aggregate.
It differs from traditional concretes at the molecular interface between the cement compounds and the admixture polymers. Conventional plasticisers have worked on the basis of electrostatic repulsion, whereas newly developed plasticisers produce self-compacting concrete by reducing the steric hindrance of the material, allowing it to flow evenly throughout the mould or formwork.
The benefits led concrete contractor Byrne Bros (Formwork) to propose self-compacting concrete for the columns supporting the atrium roof of the new £65 million Midsummer Place shopping centre in Milton Keynes which is due to open in Autumn 2000.
The columns are elliptical and truncated, with extremely congested steel reinforcement. There is one row of six high tapering conical columns of 1300-900mm diameter, and an opposing row of six 10m-high elliptical columns tapering from 2050mm on the major axis to 1030mm over 8.5m and then to 900mm diameter over the last 1.5m. Byrne Bros proposed self-compacting concrete as a way to avert potential problems with finish and with compaction/ density that could have occurred when casting such unusually high, densely reinforced in-situ columns.
Byrne Bros presented the idea to the main contractor, Bovis Europe, which was impressed by the potential benefits. These fully support the increased buildability and efficiency ethos of the Egan Report.
rmc Readymix produced 150m3 of C40 self-compacting concrete. The pour- and-cure cycle per column was just three days, with rebar fixing going ahead independently. The structural design called for 40N/mm2 after 28 days. However, the actual strengths reached were 12N/mm2 at 24 hours, 50N/mm2 at seven days and an average of 70 N/mm2 at 28 days.
It is estimated that using self-compacting concrete was some 40 per cent faster than more traditional methods of placing and compaction. The cost savings are around 10 per cent, mainly due to the struck finish being of such high quality that minimum work was required to ensure the required super-fine finish. Using self-compacting concrete meant one less site operative as no-one was required for vibration. And of course having no vibration equipment on site meant it was considerably quieter.
Many European countries are now taking self-compacting concrete very seriously. The potential benefits are considerable as this project proves. The construction cost and time savings, together with the remarkable improved performance, make it an attractive structural solution.
London & Amsterdam Properties