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Heading: 1.1slump loss through slick line pump
I have a Ready Mix customer who is looking for any literature / publications / technical bulletins that explain why concrete looses slump when pumped. My customer is going to be pumping his mix through a slick line pump for a long distance and the contractor is nervous about slump loss. There seems to be some disagreement between my customer and the contractor as to the causes of slump loss through a pump. I have looked in the usual places and haven't found much written about the subject. Any information you can share would be greatly appreciated.

John Cherne

There are several reasons why concrete loses slump with time, e.g. loss of water due to:

• aggregate water absorption
• evaporation (a function of temperature, humidity and wind speed)
• chemical reaction with the cement.

Additionally, slump is lost during pumping because concrete is partially compacted as it moves through a pumpline. (It’s easy to envisage the effect of compaction on slump – just imagine measuring the slump of a concrete as it is being discharged from a truck then remeasuring it after the concrete has been compacted or partially compacted.)

However, pumps are often consider as being as being good quality control devices as a concrete mix is generally considered to be satisfactory, in terms of workability, if it can be pumped. And remember that slump does not measure workability; rather, it is a measure of consistency, useful in detecting variations in the uniformity of a particular mix.

Loss of air content in pumped concrete is normally considered more important than loss of slump – see NRMCA CIP 21.

See ACI 304.2R Placing Concrete by Pumping Methods for a more comprehensive understanding of the properties of pumpable concrete.

Alan Kirby
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