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Concrete Workability

The slump test is used to measure the workability and assess the consistency of fresh concrete.

Generally it is used to check that the correct volume of water has been added to the mix, however, there are a number of other variables that can affect the test – including:

1. Use of chemical admixtures – dosage, type, combination, effectiveness, etc.
2. Air content of concrete
3. Temperature
4. Sampling of concrete & test equipment condition
5. Time since mixing of concrete at the time of testing
6. Other material properties – including particle size distribution, moisture content, chemistry, fineness, etc.

To find out more about workability we've written a useful guide on concrete workability and how it can effect your construction.

What Equipment Do I Need To Test Concrete Slump?

• A mould for the test (typically a slump cone)
• Measuring scale
• Non porous base plate
• Tamping rod

Slump Test Procedure

There are several steps you need to go through to test for slump:

1. The cone is positioned on the base plate with the smaller aperture uppermost
2. Freshly supplied concrete is poured into the cone to roughly one third of its depth (100ml)
3. The concrete is tamped using 25 strokes of the steel rod
4. Further concrete is added to fill the cone to about two thirds depth (another 100mm of concrete)
5. The concrete is tamped again using 25 strokes of the rod just penetrating the layer below
6. The cone is filled to the top and tamped using a final 25 strokes with the steel rod
7. Using the tamping rod slid across top of the cone the surface of the concrete is "struck off" level with the top of the cone
8. The cone is carefully lifted upwards, clear of the concrete and placed, upside-down beside the concrete
9. After about a minute, the unrestrained concrete will settle downwards or "slump" due to gravity
10. The steel rod is used to span the inverted cone and towards the slumped concrete
11. The height difference between the steel cone and the slumped concrete is measured. This difference, measured to the nearest 10 millimetres, is "The Slump".

Slump Test Results

• True Slump – the only slump that can be measured in the test. The measurement is taken between the top of the cone and the top of the concrete once the cone has been removed.
• Zero Slump – Zero slump is the indication of a very low water to cement ratio, which results in dry mixes. This type of concrete is largely used for road construction.
• Collapsed Slump – this indicates that the water-cement ratio is too high, for instance - the concrete mix is too wet, or it is a high workability mix.
• Shear Slump – indicates an incomplete result, and the concrete needs to be retested.