Distributed fiber optic temperature measurements
The development of fiber optic temperature laser radar systems has opened up
new dimensions in dam monitoring. Among others the technology allows very
accurate and economic measurements of temperature distributions along fiber optic
cables. Generally, in dam engineering, the demands for structure monitoring are high,
and the applications for distributed fiber optic temperature sensors for heat of
hydration measurements and leakage detection purposes are promising.
Distributed fiber optic temperature measurements (DFOT) allow reliable
determination of temperature distributions in mass concrete. Usually, concrete
temperatures are monitored by conventional thermo-couples permitting only spot
measurements. In contrast, fiber optic cables provide the possibility of continuous
inline temperature measurements along the fiber cable embedded into the dam
structure.
In terms of any in-situ instrumentation, RCC is not very favorable due to the
damage likelihood of the instruments by heavy earthmoving equipment and other
vehicles involved in the rapid construction process (Figure 3). Therefore a cable type,
featuring high compressive and tensile resistance, was chosen (Figure 4).
The fiber optic cables can be installed during the continuous RCC placement
operation. The cable is laid out and fixed on the desired level shortly before the new
RCC layer is placed. Moreover, the installation of the fiber cables can be well
integrated into the rapid RCC construction process without causing any delays. The
DFOT can be used to monitor temperature with a spatial resolution of 0.25 m and an
accuracy of up to ± 0.2 K allowing detailed visualization of temperature gradients
within the RCC structure, which enables the evaluation of internal restraint or Eigen
stresses, respectively. Accurate monitoring of distributed temperatures inside young
concrete may also contribute to a better understanding of the evolution of the
mechanical properties (elastic modulus, tensile strength, tensile strain capacity),
necessary for the determination of distributed cracking potentials.
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