Slump test is a laboratory or at site test used to measure the consistency of concrete. Slump test shows an indication of the uniformity of concrete in different batches. The shape of the concrete slumps shows the information on the workability and quality of concrete. The characteristics of concrete with respect to the tendency of segregation can be also judged by making a few tamping or blows by tapping rod on the base plate. This test continues using since 1922 due to the simplicity of apparatus and simple procedure. The shape of the Slump cone shows the workability of concrete.
Principle of Slump test
The slump value of concrete is just a principle of gravity flow of surface of the concrete cone that indicates the amount of water added to it, which means how much this concrete mix is in workable condition.
Apparatus for Slump test
Followings apparatus are used in the slump test of concrete:
- Metallic mould in the shape of a frustum of cone having bottom diameter 20 cm (8 in), top diameter 10 cm (4 in) and height 30 cm (12in).
- Steel tamping rod having 16 mm (5/8 in) diameter, 0.6 m (2 ft.) long with bullet end.
Procedure of Slump test
During Slump test following steps are followed:
- First of all, the internal surface of the mould is cleaned and free from moisture and free from other old sets of concrete.
- Then place the mould on the smooth horizontal, rigid, and non-absorbant surface.
- The mould is then filled with fresh concrete in four layers with taping each layer 25 times by taping rod, and level the top surface with a trowel.
- Then the mould is slowly pulled in vertical and removed from concrete, so as not to disturb the concrete cone.
- This free concrete deform all the surface to subside due to the effect of gravity.
- That subsidence of concrete in the periphery is a SLUMP of concrete.
- The height difference between the height of subsidence concrete and mould cone in mm is ‘slump value of concrete’.
Recorded slump value of a sample is = ……… mm
The development of the slump is shown in fig.3.
Precaution during test
- The internal surface of the mould should be cleaned and free from moisture.
- The base plate or surface should be free from vibrations or shocking.
- This test is done just after sampling nearly after 2 minutes.
Uses and Drawbacks of slump test
This test does not give good results for very wet and dry concrete. Also for stiff-mix, it is not sensitive. The table below shows the various values of slump with the workability of concrete.
Following chart shows the Slump Value of concrete for different Degree of workability for various placing conditions:
|Degree of workability||Placing Conditions||Slump(mm)|
|Very Low||Binding concrete (member of concrete by spreading, shallow sections, Pavements using pavers (mixer with spreading arrangements)||Compaction factor 0.75 – 0.8|
|Low||Mass concrete, lightly reinforced slab, beam, wall, column sections, canal lining, strip footing (ling wall with smaller width )||25 – 75|
|Medium||Heavily reinforced sections in slab, beams, walls, columns. Slip formwork (slope concrete), pumped concrete.||50-100|
|High||Trench fill, in-situ piling||100-150|
|Very high||Tremie concrete (concreting in water by using water tight pipe to pour concrete.)||Flow test.|
Much research shows that for the “very high” category of workability, the flow test is more suitable for workability measurement.
For large construction projects, a slump test is a very useful tool to check day-to-day or hour-to-hour variation of the quality of the concrete mix, and by observing slump reading, we can easily change the moisture (water) content and grading of concrete. Due to that reason, it is the most common method of test.
Shape of concrete slump
When the metal mould is removed from the concrete cone, the slump takes the following shapes:
- True Slump: True shape of a slump is only a verified slump. This shape is measure as the difference between the top layer of the cone to the top of the slump concrete.
- Collapse Slump: It shows that due to a high water-cement ratio, the shape of a slump is not a clear dimension. It means, concrete is very high workability, for which slump test is not suitable.
- Shear Slump: This shape of slump is the same as shear failure of soil. This is an indication of a lack of cohesion of the concrete mix ingredients. So, a fresh sample is taken and the test is repeated.
Factors influencing Slump Cone:
Followings factors influence concrete Slump value:
- Water –cement ratio of concrete.
- The quality of coarse and fine aggregates, their shape, moisture content, texture, and grading.
- The use of plasticizer, superplasticizer admixture, and the sequence of their mixing.
- The void ratio of concrete and air content of concrete.
- The time of the test after mixing of concrete.
Standards for Slump test:
- US – standard: In the United States, this test is known as “ Standard Test Method for Slump of Hydraulic – Cement Concrete” and flow the code ASTM C143 OR (AASTO T119).
- United Kingdom & Europe: The older standard for British was first (BS 1881–102). But, now they use European standars (BS EN 12350-2).
- Indian Standard: Indian standard is : IS 1199-1959
For fast testing, a new apparatus called ‘K-Slump Tester’ is developed. This device can measure slump value within a minute after it is inserted into fresh concrete. And it can also measure relative workability.
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