The slump cone test is the most commonly used method to determine the workability of the fresh concrete mix.
Before we get further into the test procedure, one should have sufficient knowledge about the term workability, so let’s take a short look at the term workability.
What is the workability of concrete?
In layman’s terms, the term workability can be defined as the ease with which the fresh concrete can be mixed, transported, and placed in its place in a homogeneous state.
But it is often confused with the term consistency. Consistency is the general term used to indicate the degree of fluidity or mobility of concrete.
But there is a major difference between these two terms.
A batch of concrete that has high consistency (thus more mobility), need not be of the right workability for a particular job.
Each job requires a different workability. For example,
i) A hunk of concrete that is considered workable for a mass concrete foundation may not workable to be used in roof construction.
ii) Similarly, a concrete that is considered workable when used in a thick section may not be workable to be used in a thin section.
Therefore, the word workability is difficult to explain since it assumes the full significance of the type of work, the thickness of the section, the gradation of aggregates, the extent of reinforcement, and the mode of compaction.
Another definition of Workability
Some research works tried to define the word workability in terms of compaction energy.
How? Let’s say,
A good workable concrete has a higher degree of mobility which means it exhibits very little internal friction between its own particles and even it overcomes the frictional resistance offered by the surfaces of reinforcement and the formwork with just a small amount of compaction energy.
A highly workable concrete mix requires just a small amount of compaction energy to achieve full compaction whereas a poor workable concrete mix requires a high amount of compaction energy to achieve full compaction.
The Road Research Laboratory in the UK defined workability “as the property of concrete which determines the amount of useful internal work necessary to produce full compaction.”
Measurement of Workability
There are various tests available to measure the workability of the concrete mix. They are,
- Slump Test
- Compaction Factor Test
- Flow Test
- Kelly Ball Test
- Vee Bee Consistometer
Among these tests, let’s discuss the slump cone test of concrete today.
Slump Cone Test Of Concrete
It is the most commonly used method of measuring the workability of concrete. It can be employed either in the laboratory or at the work site.
Even though the slump cone test does not consider all the factors contributing to workability such as aggregate gradation, w/c ratio, mix proportions, etc, it is conveniently used as a control test and gives an indication of the uniformity of concrete from batch to batch.
Repeated batches of the same mix are checked for the same slump. If there is a change in the slump value, then it may indicate that there is a change in the moisture content of the mix. It gives a warning to correct the causes for the change in slump value.
The Slump cone test is very useful on the site to check the hour-to-hour variation in the quality of the mix.
Slump Cone Test Apparatus
The apparatus for the slump cone test essentially consists of,
i) Metallic mould in the form of a frustum of a cone having the following dimensions (Internal)
- Top diameter – 10 cm
- Bottom diameter – 20 cm
- Height – 30 cm
ii) Steel Tamping rod (16mm diameter and 0.6m length)
iv) Tools and containers for mixing concrete
Slump Cone Test Procedure
1. The internal surface of the mould should be cleaned thoroughly. It should be free from any old hardened concrete. Damp the internal surface of the mould and place it on a flat, moist rigid surface.
It shall be held firmly in place during the concrete filling by standing on the two foot pieces.
2. The mould is then filled with concrete in four layers, each approximately 1/4th of the height of the mould.
3. Each layer is tamped 25 times by the tamping rod. Uniformly distribute the strokes over the entire cross-section of each layer.
4. After filling and tamping the top layer, the excess heap of concrete above the top of the mould should be scrapped off with a trowel.
5. Remove the mould immediately from the concrete by raising it steadily and carefully in the vertical direction without any lateral or torsional movement. The mould should be removed fully within 5 ± 2 seconds.
6. As the mould is removed, the concrete inside may subside in its height. Measure the difference in level between the top of the mould and that of the highest point of the subsided concrete. The height difference is taken as the slump of the concrete. The slump shall be reported in mm to the nearest 5mm.
The entire test from the start of the filling to the removal of the mould should be completed within 2.5 mins without any interruption.
After the completion of the test, the concrete sample shall be used for casting any cube or concrete specimens for future testing.
NOTE: ASTM specifies that the concrete should be filled in 3 layers. It indicates that the slump should be measured from the top of the mould to the center point of the subsided concrete.
Pattern of the Slump
As the concrete subsides, it exhibits a definite pattern of slump. The pattern of the slump indicates the characteristics of the concrete sample.
If the concrete slumps evenly, then it is called a True slump.
If one-half of the cone slides down, then it is called a Shear slump. In the case of a shear slump, the slump value is measured as the difference in height between the top of the mould and the average value of the subsidence. The shear slump also indicates that the concrete is non-cohesive and shows the characteristic of segregation.
If the concrete slumps more heavily, then it is called a Collapse slump.
Slump Cone Test Values
Suitability of Slump Cone Test
Just because the slump test gives values for all degrees of workability, it doesn’t mean all the values are correct and accurate.
In the case of a dry mix that has very low workability, the slump value is often not satisfactory. So, in that case, a Vee-Bee consistometer is used to measure the workability of the mix.
In the case of a wet mix that has very high workability, the slump value does not often exhibit the true behavior of the mix. In that case, a flow test is used to measure the workability of the mix.
The slump cone test is suitable for concrete of Medium and high workability.
- IS codes: IS 1199 – 1959: Method of sampling and analysis of concrete, IS 7320 -1974: Specification for concrete slump test apparatus
- American Standard: ASTM C143
- European Standard: EN 12350-2
Hope you understand everything you need to know about the slump cone test of concrete. If you found this article helpful, let us know in the comment section.