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Thermal slab bending study on ID03, ID14 and BM18: first insights

Most experiments at the ESRF require both physical and temporal stability, and stability requirements for the EBS beamlines are more stringent than ever before. It is generally assumed that the floor and ground on which an experiment is installed is stable; however, studies show that both are constantly moving.

There are two main types of movement that affect the high- quality concrete floor slabs of the ESRF experimental halls. The first are long-term deformations of the ESRF site due to local groundwater variations, the adjacent rivers, and the underlying geomorphology and geology of the Grenoble valley and surrounding area. They are significant over a period ranging from months to years. The second main mechanism is due to slab curling, which is significant over periods ranging from minutes up to years; but generally, most significantly over a period of 24 hours.

There are two slab curling mechanisms: one driven by thermal gradients through the slab; and the other by humidity gradients through the slab [1]. Ignoring the initial hydration period that lasts a few days, humidity gradients driving slab drying shrinkage (the volume reduction that concrete undergoes due to moisture migration when exposed to a lower relative humidity environment after the initial hydration, i.e., the hardening of concrete during the first few days) and creep (the time-dependent strain that occurs due to a constant stress) dominate the slab curling for the first year or two of the slab s lifetime. The drying shrinkage and creep last the lifetime of the slab, but their effect diminishes rapidly, asymptotically approaching a final dried state. The time it takes to reach 90% of the final state is dependent on the slab composition, but, for example, it took roughly one year for the EX2 experimental

hall slabs. Today, drying shrinkage and creep for both the EX2 (2013) and EXPH (1991) experimental hall slabs are insignificant.

Thermal gradients are by far the dominant mechanism driving curling for all slabs older than one year at the ESRF. A model developed by the high-quality slab designer NECS for the EX2 experimental hall (Figure 148) shows how thermal slab bending works qualitatively. It is important to note that both variations in air temperature above the slab, and long-term ground variations below the slab, help drive slab thermal bending. Simplistically, temperature changes above the slab are diurnal and mainly a function of the air-conditioning system; while variations under the slab are annual and influenced by deep ground temperature changes.

Recently, two Hydrostatic Levelling System (HLS) studies were made on ID03 (July 2020 to April 2021) and ID14 (March to June 2020). As a reminder, the HLS is used extensively at the ESRF. It is based on water-filled pots linked via a tube system. The water provides an equipotential reference. A capacitive sensor measures relative displacements of the water surface when the pot/ sensor ensemble, which is attached to the floor, moves up or down relative to the other pots and sensors in the system. Initially, it was thought that EXPH slab thermal bending would be relatively homogeneous, but different slabs behave quite differently from one another. On ID03, maximum slab thermal bending was found to be roughly 10 µm/°C, whereas on ID14 it was double this.

The slab curling mechanism has been studied for a long time at the ESRF. Indeed, it was first observed in a study made on BM18 between 2009 and 2011. This study provided the background data for the EX2 slab thermal bending model shown in Figure 148. The BM18 data was revisited and it was found that the maximum slab bending

Fig. 148: Characteristic slab bending (a) driven by temperature gradient variations through the slab (b).