What is Slump in Concrete?

 

In concrete terminology, slump is used to describe a measure of consistency in a specific batch of fresh concrete.  To determine the slump, a simple test is done to measure that the proper amount of water has been added to the mix.

The test consists of a steel cone known as an Abrams cone that is 12” in height with an 8” diameter on the bottom and a 4” diameter at the top of the cone.  The cone is placed on a level, impermeable surface and filled in three stages of equal amounts of concrete.  Each layer is tamped down using a special rod to make sure the concrete is compacted.        

At this point the cone is removed and the concrete settles or somewhat “slumps.”  Placing the cone alongside the pile of concrete, the height of the concrete is compared to the top of the slump cone and measured to the nearest 5mm.  Wikipedia states, “The slumped concrete takes various shapes, and according to the profile of slumped concrete, the slump is termed as true slump, shear slump or collapse slump.”  

A true slump is a situation in which the concrete simply subsides but generally maintains its shape whereas in a shear slump the top portion of the concrete shears off.  A collapse slump is as the name indicates and typically means the mix is too wet, or it may be a case of a high workability mix that needs to have a flow test done rather than a slump test. 

Since much of the concrete produced today is a blend of admixtures, fibers and polymers, the aggregate is not the only factor to influence the amount of water added.  Other considerations are taken into account to determine the proper consistency.

 

Sources:

concrete.org.uk/fingertips_nuggets.asp?cmd=display&id=559
en.wikipedia.org/wiki/Concrete_slump_test

 

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