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order of 15 μm and 100 nrad, during a standard data- acquisition period of tens of minutes.

The final design of the HQS was studied by computer modelling and laboratory tests to minimise all the pre- mentioned effects, by creating a monolithic, isolated and stable support with optimised thickness. The structure of the HQS, as illustrated in Figure 151, is composed of: Compacted existing soil (homogeneous alluvial material) with an ultimate bearing capacity of 50 MPa. A 600-mm-thick, roller-compacted concrete layer (dry mixture of gravel, sand, 5% cement and 3% water). A 50 to 100-mm-thick concrete base layer (mixture of gravel, sand, 12% cement and 7% water). A 2-mm-thick bitumen layer to allow the HQS to slide and reduce shrinkage stress. A 350-mm-thick steel reinforced concrete slab (specific mixture of gravels, sand, 14% cement, 6% water, additives and 35 kg/m2 steel).

Fig. 151: Structure of the high-quality slab (HQS).

The internal processes during the hydration and hardening of concrete are critical for the HQS. Once the concrete is poured, and throughout its lifetime, it undergoes complex volume evolutions, such as autogenous and drying shrinkage, creep, relaxation, thermal deformation, etc. Fast drying and shrinkage may lead to increased internal tensile stresses, which can result in greater cracking. To avoid degraded performance, a maximum value of 300 μm/m for shrinkage is one key parameter for the HQS construction. In order to achieve this value, a unique concrete formulation must be optimised, through testing, to allow for a slow and controlled hydration process. The high steel bar reinforcement ratio (35 kg/m2) and the bitumen layer also contribute to minimising cracking.

To reduce thermal deformation and shrinkage, the concrete temperature must be kept below 35°C during the hydration. Thermal sensors are installed inside the concrete to monitor it during the first few weeks; an air conditioning system is required to control this process. The temperature increases quickly during the first days of the hydration and then decreases progressively to meet the ambient temperature on its surface and the soil temperature at its base (Figure 152). After the first hydration phase, several less significant processes will continue to take place inside the slab (especially during the first two years), decreasing with time. Once finished, the slab has to be monitored to understand its behaviour in order for this to be taken into account for the operation of scientific equipment.

Fig. 152: Temperature evolution during the first week after the concrete pouring, measured by the sensors installed in the air above (RTD1) and at different levels inside (RTD2-3-4) the HQS.

AUTHOR

A. Ruiz Bailon. ESRF.

CONTRIBUTOR

T. Marchial. ESRF.

REFERENCES

[1] Science and technology programme 2008-2017. ESRF. [2] Experimental validation of the ESRF upgrade programme experimental hall prototype slab. E Bruas - D. Martin. ESRF. [3] EX2-APD-NE-RENDU-001-D-16-11-2010 - Note APS/APD. Project EX2. NECS.