Concrete slump

Wet concrete is rather mushy, and it flows a little.  If you try to make a peak out of it, it slumps.

Slump is an important characteristic of freshly mixed concrete, and it is used as a specification for concrete.  It is a measurement of workability, or flowability, or viscosity.

Slump is measured using a specific standardized test.  The freshly mixed concrete is formed into a conical cylinder and consolidated, or tamped on.  The cone form is then removed, and the amount of slump is measured.

 

Here are the specifications for a slump test (taken from Wikipedia):

Principle   The slump test result is a measure of the behavior of a compacted inverted cone of concrete under the action of gravity. It measures the consistency or the wetness of concrete.^[

Apparatus
Metal mold, in the shape of the frustum of a cone, open at both ends, and provided with the handle, top internal diameter 102 mm, and bottom internal diameter 203 mm with a height of 305 mm. A 610 mm long bullet nosed metal rod, 16 mm in diameter

Procedure  The test is carried out using a mould known as a slump cone or Abrams cone. The cone is placed on a hard non-absorbent surface. This cone is filled with fresh concrete in three stages, each time it is tamped using a rod of standard dimensions. At the end of the third stage, concrete is struck off flush to the top of the mould. The mould is carefully lifted vertically upwards, so as not to disturb the concrete cone. Concrete subsides. This subsidence is termed as slump, and is measured in to the nearest 5 mm.

Interpretation of results    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. If a shear or collapse slump is achieved, a fresh sample should be taken and the test repeated. A collapse slump is an indication of too wet a mix. Only a true slump is of any use in the test. A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate.   Very dry mixes; having slump 0 – 25 mm are used in road making, low workability mixes; having slump 10 – 40 mm are used for foundations with light reinforcement, medium workability mixes; 50 - 90 for normal reinforced concrete placed with vibration, high workability concrete; > 100 mm.

Limitations of the slump test   The slump test is suitable for slumps of medium to high workability, slump in the range of 25 – 125 mm, the test fails to determine the difference in workability in stiff mixes which have zero slump, or for wet mixes that give a collapse slump. It is limited to concrete formed of aggregates of less than 38 mm (1 inch).

 

Differences in standards

The slump test is referred to in several testing and building codes, with minor differences in the details of performing the test.

United States

In the United States, engineers use the ASTM standards and AASHTO specifications when referring to the concrete slump test. The American standards explicitly state that the slump cone should have a height of 12-in, a bottom diameter of 8-in and an upper diameter of 4-in. The ASTM standards also state in the procedure that when the cone is removed, it should be lifted up vertically, without any rotational movement at all.  The concrete slump test is known as "Standard Test Method for Slump of Hydraulic-Cement Concrete" and carries the code (ASTM C 143) or (AASHTO T 119).

 

Typically, when you order ready mix concrete (delivered in a concrete truck), you’ll specify a slump.  It is usually anywhere from around 3 to 6 inches.  3 inch slump is pretty thick concrete, 6 inch slump is pretty smooth.  If you use superplasticizer, as discussed here, then the concrete is more workable, and will have a higher slump while still having a low water-to-cement ratio (indicated: w/c).  A low w/c is desirable, and makes for better concrete.

Sometimes a zero slump concrete is desirable.  For example, concrete block machines use zero-slump concrete.  This mix has low water content (usually 5-7%) and a correspondingly low w/c.  Because block are made so fast, they must be removed from their forming mold immediately after they are made.  These blocks should not slump at all, or they will be deformed.  Zero slump.