Mikrotron camera peeks inside flowing concrete

November 29, 2021 – Concrete is the world’s most widely used building material, enabling the construction of high-rise buildings, super highways, and sophisticated bridges and dams.

Recent developments in 3D concrete printing have led to a growing interest in the prediction of its “rheological” properties, i.e. the deformation of hardened concrete and the flow behavior of freshly mixed concrete, which is difficult to measure due to the poor optical accessibility of concrete. . Lacking visibility, many assumptions have to be made and these assumptions are prone to error resulting in problems with placing, consolidating and finishing the concrete as well as hardened state properties such as strength and durability .

Engineers from the Institute of Mechanics and Fluid Dynamics in Freiberg, Germany, built an experimental facility1 called PulsaCoP (Pulsating Concrete Pump) which allows the optical study of dense granular suspensions during pulsed flow using a Mikrotron MotionBLITZ EoSens mini2 recording camera.

At the heart of the PulsaCoP system is a cylindrical borosilicate glass pipe surrounded by an octagonal section outer channel filled with glycerin. The refractive index of all materials – glycerin, borosilicate glass and concrete – is almost identical to allow undisturbed optical access to the entire flow domain.

The Mikrotron camera was tuned to a resolution of 15 µm per pixel with illumination provided by a transmitted light pattern. Images were taken at a frame rate of 100 Hz to 525 Hz, with an illumination time for all measurements of 10 s. By adjusting the focal plane, measurements could be made in the central plane of the tube only. Trigger signals were passed to the pressure measurement so that pressure and velocity could be correlated exactly.

Due to its optical accessibility, PulsaCoP can determine velocity data, flow profile, and pressure loss, all of which can be correlated with rheological data. These results provide valuable information for the practical application of concrete pumping and for granular materials in general. The Institute of Fluid Mechanics and Dynamics plans a more rigorous validation of the PulsaCoP experiment by comparison with data from large-scale pumping experiments with real concrete.

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