Project : R&D Linac Facility Building, Elekta, Crawley
Value : £1.4m
Elekta develops state-of-the-art tools and treatment planning systems for radiation therapy, including brachytherapy and radiosurgery, as well as workflow enhancing software systems across the spectrum of cancer care.
The project required the construction of 5 new concrete bunkers within an existing portal framed building. The concrete structures are required to prevent any leakage of radioactive particles to anyone in the immediate vicinity of the enclosure both side or above.
It was fundamental to understand the performance of the concrete, rate of hydration, curing time, required strength and density after a certain time and the time required before striking the shuttering and lifting the formwork out of the existing structure before the other operations can take place.
We cast sample panels 2.4 x 2.4 x 1.8m thick prior commencement on site to prove the theory of the design. This was to assess the rate of hydration in differing lined and insulated conditions, including the requirement to install and monitor strain gauges and thermocouples. The results from the sample panels confirmed the requirement for the panels to be lined with insulation and ply. The insulation provided enough thermal shielding to prevent a temp differential from the core to the face exceeding 20 degrees. This was a stringent requirement of the design, to enable the warranty to be issued by the Engineer.
The 5 calls were completed over a period of 15 weeks. Due to the shielding requirements and properties of the design there could be no cold joints within the walls. The design of the pours were tailored to enable the controlled position of the cold joint to the tops of the walls, therefore the pours were tailored in such a way that walls would be competed over 2 days, giving a maximum single pour of 790m3. This was also due to the limitations of supply and the backup that could be incorporated into a pour of this size.
The formwork was set up in such a way that the cells could have technically been poured in a single pour, however, the limitations of concrete supply meant a guarantee of a maximum of 800m3 with suitable backups in place. The roof slab was the final pour; a total of 1900m3 with a maximum gap between pours of 12 hours over a period of 3 days.
Due to the issues with high heat gains in thick concrete walls and slabs, the mix was tailored to a 70%% GGBS. This reduced the initial heat gain and quick hydration which put additional load on the formwork as the fluid head of pressure has a longer setting time, therefore the rate of rise had to be monitored in line with the ambient concrete temperatures. Any deviation from this could have resulted in formwork failure.
As with the radiographic cell completed for Heatric, there were 180 thermocouples cast into the walls and slab, taking a temperature reading every 15 minutes, 24 hours a day over a period of 5 weeks. The results were collected every day and presented to the Structure Engineer for analysis. Once the required strength had been reached, the formwork was removed whilst leaving the ply and insulation in place. The insulation and ply was able to be removed once the engineer was satisfied the temp differentials had stabilized.