Evaluation of different ultrasound transmission parameters to monitor microstructure development in fresh concrete or mortar

Title: Evaluation of different ultrasound transmission parameters to monitor microstructure development in fresh concrete or mortar
Author(s): Nele De Belie, Christian Grosse
Paper category : conference
Book title: 2nd International RILEM Symposium on Advances in Concrete through Science and Engineering
Editor(s): J. Marchand, B. Bissonnette, R. Gagné, M. Jolin and F. Paradis
Print-ISBN: 2-35158-003-6
e-ISBN: 2351580028
Publisher: RILEM Publications SARL
Publication year: 2006
Nb references: 24
Language: English

Abstract: Non-destructive ultrasound transmission measurements allow monitoring microstructural changes of concrete at early age in a continuous way in contrast to conventional methods. In the current research project, the effect of different chemical admixtures (setting accelerators) and supplementary cementing materials (fly ash and blast-furnace slag) on setting and hardening properties of concrete and mortar was examined with the FreshCon equipment.
The velocity of the ultrasonic waves is sensitive to the effect of additions and accelerating admixtures on the setting and hardening behaviour of concrete and mortar. The point where the pulse velocity starts to increase steeply, corresponds to a solid percolation threshold, where the system changes from a suspension to a more solid state. In this study, the percolation threshold increased with an increasing content of fly ash or blast-furnace slag. In mortar with different type and dosage of accelerator, no such threshold could be noticed. Once all the particles are connected, the pulse velocity increases more slowly and follows the evolution of the total solid fraction of the paste.
Also the evolution of the ultrasound energy versus concrete age can be investigated with the FreshCon software. The energy change of the transmitted ultrasound wave is a parameter that has been less discussed in literature, mostly because of the difficulties to reproduce the energy of the ultrasound emitter. The rate of energy increase depended on the level of fly ash replacement, blast-furnace slag content or accelerator dosage. The maximum energy was reached between the final setting time (pulse velocity of 1500 m/s) and the maximum velocity.
Finally, the change in frequency content of the transmitted ultrasound signal in time was studied. Comparison of the results of two different transmitter-receiver systems showed the influence of the sensor characteristics on the measured frequency spectra. At very early ages, when the cementitious sample can be considered as a suspension, no clear spectrum was noticed. At a certain point that could correspond to the percolation threshold mentioned above, lower frequencies started to appear in the spectrum. The frequency content increased rapidly and the spectrum broadened. Later on, the frequency spectrum stagnated. As with the velocity and the energy curves, the rate of these changes depended on the type of mortar or concrete mixture.

Online publication: 2006-08-02
Classification: 3.2 Theme 2: From Fresh to Hardened Concrete
Publication type : full_text
Public price (Euros): 0.00
doi: 10.1617/2351580028.056