Slump Test of Concrete

The slump test of concrete is a laboratory or field test that is used to determine the workability of concrete. The slump test is simply a measure of the wetness or consistency of the concrete mix.

Apparatus Used in Concrete Slump Test

The equipment or apparatus used to test the concrete slump is quite simple. It consist of 2 items – a tamping rod and a truncated cone that’s 300 mm tall, 100 mm in diameter at the top, and 200 mm in diameter at the bottom. You can use the image above to understand the structure of the mould.

You would also need some extra stuff such as a concrete mixing pan, measuring scale and weighing machine.

Procedure

• First, fresh concrete is poured into the mould in four layers, each 1/4th of the height, and tamped with twenty-five strokes of the tamping rod’s rounded end.
• The strokes should be evenly dispersed across the cross-section and should penetrate the underlying layer for the second and subsequent layers.
• Throughout its depth, the bottom layer is tamped.
• The concrete is struck off level with a trowel or the tamping rod after the top layer has been rodded, ensuring that the mould is completely filled.
• The mould is removed as soon as possible by slowly and carefully elevating it vertically.
• This allows the concrete to settle and the concrete slump is determined by calculating the difference between the mould’s height and the highest point of the specimen being evaluated.
• The slump is measured in millimetres of specimen subsidence.

Observation

• Slumping that occurs equally all around is considered a true slump.
• Shear slump occurs when one-half of a cone falls down along an inclined plane.
• Shear slump is caused by a lack of cohesiveness in the concrete, and the proportions of the concrete should be changed.
• Concrete mixes with a very stiff consistency have no slump, therefore no difference in workability may be seen in the very dry range.
• A real slump in a lean mix with a tendency to harshness can readily be altered to a shear type, or even collapse, with a non-uniform distribution of aggregates, especially coarse aggregates, and widely varied values of slump can be obtained in separate samples from the same mix.
• As a result, for lean blends, the concrete slump test is unreliable.

Below table shows the different workability values :

It should be noted, however, that the same slump can be recorded for several workabilities using different aggregates, as the slump has no unique relationship to the workability as established above.

Despite these restrictions, the slump test of concrete is quite useful on the work site as a check on the day-to-day or hour-to-hour variance in the materials dumped into the concrete mixer.

Points to be Noted While Performing Slump Test of Concrete

• The maximum size of aggregate utilised in the slump test should not exceed 38 mm, according to Indian standard guidelines.
• After tamping the cone with the rod, extreme caution should be exercised before raising the cone. It should be lifted without causing any damage to the internal concrete.
• If the side of the concrete is gently tapped with the tamping rod after the slump measurement has been completed, some indication of the cohesiveness and workability of the mix can be obtained.
• A well-proportioned concrete with an appreciable slump will gradually slump further, but if the mix has been poorly proportioned, it is likely to fall apart.
• Any slump specimen that collapses or shears off laterally yields false data, and the test is repeated at this point to ensure that only true slump is measured.