The project, for Doncasters Group, a leading aerospace and industrial gas turbine precision components and assemblies manufacturer, involved the construction of an X-ray area where Doncasters will test high tech, high precision components including gas turbine blades. The X-ray test facility uses a high powered linear accelerator (LINAC) to penetrate the metal components and detect any microscopic imperfections within them. To contain the high levels of radiation involved, the 17m long, 14m wide, 6.7m high structure used to house the accelerator, has concrete walls up to 2.8m thick.
The LINAC bay, designed by Consulting Engineer George Burfitt and project managed by Capita Symonds, was formed by using a special slow-curing mix to avoid temperature-induced cracking. “We designed the mix to keep the peak curing temperature as low as possible and reduce the risk of early thermal cracking”, explains Mr Burfitt. Concrete contractor Dancourt Limited cast the walls in a single 400 cu m pour taking more than 17 hours of continuous pumping. After the first pour had reached the required strength, sacrificial formwork was positioned on top, shuttering was installed and a 1.5m roof slab was cast with the second and third pour.
The design and construction of the test bay was quite different from standard concreting practice. There is no steel reinforcement in the concrete at all. Mr Burfitt explains: “It was not necessary because all the concrete was in compression – and concrete performs brilliantly when in compression”. The only reinforcement present in the structure is that contained in the sacrificial formwork upon which the roof slab was cast.
To withstand the high volume of the slow-curing concrete, Dancourt used SGB Formwork’s LOGIK® 60, a modular wall panel system that is quick and easy to assemble and is designed to withstand pressures of up to 60kN/m2.
LOGIK® 60 is a robust large-area wall formwork system manufactured to tight tolerances and assembled using a unique Align-Level-Lock (ALL) clamp which is tightened with a single hammer blow. “Our challenge on this project was to provide a formwork solution that would be quick and easy to erect and also strong enough to take the pressures” says Scott Fanning, SGB Formwork Account Manager.
To ensure radiological protection by the concrete wall it was specified that formwork contractors SGB abandoned the traditional practice of embedding the formwork tie-bars in plastic sleeves. Instead, they used specially imported end connectors screwed onto the ends of the tie-bars. When the concrete had cured, the connectors were twisted off, releasing the LOGIK® 60 panels and leaving the steel tie-bars embedded in the wall. The shallow conical hollow that remained was then simply grouted flush to the surface of the wall.
For maximum strength and rigidity, the design cleverly used small-format panels instead of the largest available, as would normally be the case. Jason Baker of Dancourt Limited commented, “It took three weeks to erect the formwork and prepare for the complicated pour, but in the end it was incredible – it went like clockwork” he says.
The new X-ray facility for Doncasters commenced testing operations in June 2009.