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What is workability of concrete? Which test is generally performed at site for its determination? And what are its recommended values for different purposes?
Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration), and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increased bleeding (surface water) and/or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot be readily made more workable by addition of reasonable amounts of water.
Workability of concrete mixture is measured by:
a) Vee-bee consistometer test
b) Compaction factor test
c) Slump test
The first two tests are laboratory tests while the third test is the field test.
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| Determine the workability of concrete by slump test |
Workability is the ability of a fresh (plastic) concrete mix to fill the form/mold properly with the desired work (vibration) and without reducing the concrete's quality. Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (level of hydration), and can be modified by adding chemical admixtures, like superplasticizer. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increased bleeding (surface water) and/or segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. The use of an aggregate with an undesirable gradation can result in a very harsh mix design with a very low slump, which cannot be readily made more workable by addition of reasonable amounts of water.
Workability of concrete mixture is measured by:
a) Vee-bee consistometer test
b) Compaction factor test
c) Slump test
The first two tests are laboratory tests while the third test is the field test.
Slump Test
Concrete slump test (or simply the slump test) is an in situ test or a laboratory test used to determine and measure how hard and consistent a given sample of concrete is before curing.
Purpose
The goal of the test is to measure the consistency of concrete. Many factors are taken into account when satisfying requirements of concrete strength, and to make sure that a consistent mixture of cement is being used during the process of construction. The test also further determines the "workability" of concrete, which provides a scale on how easy is it to handle, compact, and cure concrete. Engineers use the results to then alter the concrete mix by adjusting the cement-water ratio or adding plasticizers to increase the slump of the concrete mix.
The slump test is suitable only for the concrete of high or medium workability.
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| Illustration of Slump Test |
This test is carried out with a mould called slump cone whose top diameter is 10cm, bottom diameter is 20 cm and height is 30 cm. the test may be performed in the following steps:
1. Place the slump mould on a smooth flat and non-absorbent surface.
2. Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water.
3. Place the mixed concrete in the mould to about one-fourth of its height.
4. Compact the concrete 25 times with the help of a tamping rod uniformly all over the area.
5. Place the concrete in the mould about half of its height and compact it again.
6. Place the concrete upto its three fourth height and then upto its top. Compact each layer 25 times with the help of tamping rod uniformly. For the second subsequent layers, the tamping rod should penetrate into underlying layers.
7. Strike off the top surface of mould with a trowel or tamping rod so that the mould is filled to its top.
8. Remove the mould immediately, ensuring its movement in vertical direction.
9. When the settlement of concrete stops, measure the subsidence of the concrete in millimeters which is the required slump of the concrete.
