Interactions between rheological propertiesand pumping of SCC

Fresh concrete properties

In the fresh state, self-compacting concrete (SCC) shows large differences compared to ordinary vibrated concrete. SCC is considered as a much more fluid material. On the other hand, the same equipment and the same standard rules as for ordinary concrete are applied during pumping. Does the different fluid behaviour influence the pumping procedure and are some new precautions needed? By means of full-scale testing, the influence of the fresh (rheological) properties on the pressure required during pumping of SCC were inves - tigated. The results show, in case of SCC, a very large influence of the viscosity of the concrete on the pumping pressures, and as SCC has in general a higher viscosity than ordinary concrete, it requires higher pressures to be pumped. The velocity profile of SCC flowing in pipes consists of a small plug with constant velocity in the centre of the pipe, a large velocity gradient near the pipe wall and a smaller velocity gradient in between. As a result, SCC, in contrast to ordinary concrete, does not flow at uniform velocity in a pipe. As a result, the flow in bends causes an additional pressure loss, but due to the very large scatter in the results, no conclusions can be drawn. The pumping process influences the fresh properties of the SCC due to two different actions: structural breakdown, which means that the concrete becomes more fluid due to high speed flowing, and an increase in air content. SCC with a high initial slump flow shows decreased segregation resistance after pumping, while for SCC with a low initial slump flow, the slump flow decreases even more. Both effects appear to have an increasing importance with increasing pumping velocities. As a result, it is advised to pump SCC at rather low velocities.

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