Construction Knowledge Series: What is Concrete "Slump"

When it comes to concrete, there are countless varieties and mix designs used for different applications.

To determine what mix design might be applicable for a certain use, designers use common characteristics as the basis for design. Concrete strength, water to cement ratio, and workability are just a few factors to consider when choosing a concrete mix for any application.

Concrete “Slump” is a measure of a concrete’s workability and consistency as defined by a Slump Cone Test. To determine the slump of a concrete mix, concrete is placed into a 12” tall steel cone and consolidated with a rod to get rid of any air pockets. The cone is removed and the concrete slumps down. The distance that the concrete slumps down is measured as the “slump” in inches.

The workability/flowability of concrete is important. In cases with dense rebar within concrete, a low slump means that the concrete would be too stiff to full encapsulate all the rebar and there would likely be voids within the concrete. If the slump is too high, your concrete would be very soupy and difficult to properly finish.

The concrete in the attached photo has a slump of 3”. This is a fairly stiff mix that could be difficult to place within a wall form but might be ideal for a road paving project.