ESRF lays "grey gold" paving in new experimental halls
Over a two week period, more than 200 concrete mixer lorries deposited over 1400 m3 of the precious composite that, once set, will provide a stable base for the two new experimental halls under construction as part of the ESRF upgrade programme.
The pouring of the concrete marked the final step in a lengthy process to find the ideal flooring for the two buildings. The technical constraints were so strict that at the outset of the project an international panel of experts from other synchrotrons in Europe (ALBA, PETRA III, Diamond, Soleil) was set up and consulted.
The nature of the experiments carried out at the ESRF means only a very low level of vibration is tolerated and any risk of amplifying background vibrations inherent to the geological composition of the site must be excluded. The slab also has to resist very heavy loads as the experimental hutches themselves can weigh up to 200 tonnes, even before the installation of granite tables and hefty detectors. Not in the least, the final design must also take into consideration the characteristics of the glacial morass underlying the ESRF, left over by glaciers more than ten thousand years ago.
The elaborate slab that results from this novel exercise in civil engineering is in fact a sandwich of different layers of concrete and bitumen. The foundation is a 65cm thick layer of “rollcrete”, a lean and fairly dry concrete, which was bull-dozed flat. Next a 10cm layer of concrete was poured, levelled and polished to a high level of flatness. Onto this was sprayed a coating of bitumen which allows the uppermost layer of the slab to slide as it contracts during the curing process. By doing this, pressure inside the concrete is reduced limiting the risks of cracking. The high quality slab topping is made from a 35cm thick layer of concrete reinforced by a relatively dense steel grid to resist cracking when the slab shrinks. The floor slab of each building is monolithic to ensure consistent behaviour throughout. With no joints, the concrete had to be poured in one session.
The steel grid in place ready for the final layer of "golden" concrete
The finished slab is a monolithic mass of 835 m3
More than 60 people from local contracting companies were involved in the operation and all team members were specifically trained for the task on a trial slab elaborated in the months preceding the final pouring. "Quality control was very high for the whole operation", says Paul Mackrill, one of the engineers overseeing the slab works. "It was crucial for the concrete to be delivered continuously during the pour of the final layer to ensure no effective joints were formed." To reduce a risk of interruption, two local production plants worked in unison to supply the concrete. "The operation ran very smoothly with self-motivated workers dedicated to the task underway."
Concrete strengthens over time and its characteristics can change depending on its shrinkage and potential curling. The micronic movement and vibration response of the ESRF’s slab will be closely monitored by in-house experts. The essential point is that the curling, which occurs naturally as the concrete loses its water content from the upper surface and starts to shrink differentially, does not alter the vibration characteristics over time.