There are a few ways to test the workability of concrete at a construction site, but three of the most common methods are the Vee-Bee test, slump test and compaction factor test. Different concrete mixes will have various values for these tests, depending on their applications. So, what is the workability of concrete?
The workability describes the ease or difficulty of the handling of a concrete mix. It is the dryness or liquidity of concrete that determines how easy it is to mix, transport, pour and place in formwork. The workability is an important factor for a concrete mix for a couple of reasons.
If the mixture is too wet, the coarse aggregates will sink and settle at the bottom of the concrete structure. As a result, the concrete will be non-uniform and have weakened integrity. If the concrete mixture is too dry, it will be hard to mix and place in position. It will also be prone to cracking.
Tests for the workability of concrete
As mentioned before, there are three common tests for concrete workability. These include:
- Vee-Bee consistometer test
- Slump test
- Compaction factor test
The slump test is the only one that can be performed on-site. The Vee-Bee test and compaction factor test are usually performed in a laboratory or specialised facility.
How to perform a concrete slump test
A concrete slump test is performed on-site to determine the correct workability of a concrete mix. Contractors will use a cone-shaped mould that is 30cm high with a top diameter of 10cm and a bottom diameter of 20cm. The test is performed in the following steps:
- Place the slump mould on a smooth, flat, non-absorbent surface.
- Mix the dry ingredients of the concrete and then add the required amount of water, according to the instructions on the bag of cement.
- Fill one quarter of the slump mould with the concrete mix.
- Compact the concrete 25 times using a tamping rod.
- Place some more concrete in the mould, filling it half-way. Compact it again.
- Place more concrete in the mould, filling it three-quarters of the way. Compact again, 25 times.
- Fill the mould completely and compact again. The tamping rod should penetrate all the layers to ensure maximum compaction and no lines of separation between layers.
- Scrape off any excess concrete from the top of the mould, using a trowel.
- Flip the mould over and place the concrete on the flat surface. Remove the mould in an upwards direction.
- Watch the concrete slump and settle. Once it has stopped moving, measure the subsidence (or how much the concrete sinks from its original 30cm height) in millimetres. This will tell the contractor how much the concrete will settle and how easy it will be to handle on-site.
Recommended values for the slump test
The slump test should only be used for concrete that is supposed to be of high or medium workability (i.e. stiff or semi-dry mixes, not dry mixes). The following table outlines the recommended slump value for various concrete applications:
|Road construction||20 to 40 mm|
|Walls, slabs, piers, curbs||40 to 50 mm|
|Normal reinforced cement concrete (RCC) work||80 to 150 mm|
|Mass concrete||20 to 50 mm|
|Concrete to be vibrated and compacted||10 to 25 mm|
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