What is a Concrete Slump and how is it used?

Concrete is a construction material, traditionally made from a mixture of cement, water, sand, and gravel. A concrete slump is the measurement of the consistency or workability of a concrete mix.

Why perform a Concrete Slump Test?

This test is usually used to compare the quality of different batches of the same mixture. The result obtained from performing a laboratory or on-site slump test indicates the quality of the concrete and the best construction application area for it. The higher the slump rating, the higher the workability, vice-versa. A mix with a low concrete slump will not shape easily while a mix with a very high slump might have its ingredients settle outside the mixture, making it unfit for use. This test is widely used because not only is it simple, it is cheap, and provides instant results.

Factors influencing the result of a slump test …

It has been established that the slump test is used to determine the workability of a mix which indicates the water-cement ratio. However, other factors affect or determine the concrete slope and they are:

  • Material properties of the materials used in producing the cement, like the aggregates. These properties are; chemistry, size, texture, cleanliness, and moisture content.
  • Chemical admixtures properties like dosage, combination, type, interaction, effectiveness, and sequence of their addition.
  • The amount of water in the mix.
  • The temperature of the concrete mix
  • The air content of the concrete
  • The slump testing technique and the condition of the test equipment
  • The timeframe between mixing the concrete and testing time
  • Methods of batching and mixing the concrete, as well as transportation.

Equipment needed …

The following are the required equipment for carrying out a slump test:

  • Mould: This should come in the shape of a frustum of a cone. This cone is also called an Abrams cone or slump cone with a height of 30cm, base opening diameter of 20cm and a top opening diameter of 10cm.
  • Base plate: This is a steel, aluminium, or polymer base plate with an optional holding attachment.
  • A standard tape measure
  • Tamping rod: A steel tamping rod about 24 inches long and ⅝ inches diameter.

How to perform the test …

The following are the necessary procedure to carry out a slump test:

  1. Clean the internal surface of the mould and apply oil on the surface.
  2. Place the mould on a smooth, clean, horizontal, and non-porous base plate.
  3. Fill the mould with fresh concrete in three layers and tamp each later 25 times with a steel tamping rod. This tamping process should be done evenly.
  4. After the mould has been filled, level the surface of the concrete to remove all forms of excess concrete. Note that when the mould is filled with concrete, the base of the mould is held firmly by handles.

Lift the mould gently in a vertical direction and watch the unsupported concrete slump. A decrease in height at the centre point is measured to the nearest 5mm and is called “the slump”.

Precautions to take during the Concrete Slump test …

To achieve an accurate and reliable slump test result, the following precautions must be taken during the test:

  • Moisten the inside of the mould and base every time before a test. This is important to reduce surface friction.
  • Before you lift the mould, ensure the area around the base of the cone is clean and with no concrete residue.
  • The concrete sample should be very fresh and the test was done immediately after mixing.
  • The mould and the base plate should be non-porous.
  • The test should be performed in an area free from vibrations.

Interpreting the results …

When a slump test is done, the result of the test can come in three shapes; the true, the collapsed, and the shear shaped slump

  • A true slump: In a true slump, the concrete subsides and keeps to shape with little variation.
  • A shear slump: If half of the cone slides down in an inclined plane or the top portion of the concrete slips sideways, it is a shear slump. This result indicates an incomplete or inconclusive test.
  • A collapsed slump: This indicates that the mix is too wet, meaning the water-cement ratio is too high and the slump test is inappropriate. As a result, the concrete collapses totally.

Conclusions …

A slump test is used to determine the workability mix of freshly prepared concrete. It is encouraged by experts to carry out this test either in the laboratory or on-site before construction begins.