Tests for Workability of Concrete
Workability test of concrete done from the following:
- Slump Test
- Compaction Factor Test
- Vee-Bee Consistometer Test
- Flow Test
- Kelly Ball Test
1. Slump test
the slump test is the most commonly used method of measuring the consistency of concrete which can be employed either in the laboratory or at the site of work.
It is not a suitable method for very wet or very dry concrete and stiff mix.
It does not measure all factors contributing to workability.
The diameter of the rod is 16 mm and its length is 60 cm. The strokes to be given for ramming vary from 20 to 30.
- The mould is placed on a smooth, horizontal, rigid and non-absorbent surface.
- The mould is then filled in four layers, each layer approximately 1/4th of the height of the mould.
- Each layer is tamped 25 times by the tamping rod taking care to distribute the strokes evenly over the cross-section.
- The mould is removed from the concrete immediately by raising it slowly and carefully in the vertical direction.
- This allows the concrete to subside.
- This subsidence is referred to as a slump of concrete.
- The difference in levels between the height of the mould and that of the highest point of the subsided concrete is measured.
- The difference in height in mm is taken as a slump of concrete.
Advantages of slump test:
- it grants the facility to easily detect the difference in water content of successive batches of concrete of the same identical mix.
- The apparatus is cheap, portable and convenient to be used at the site.
Disadvantages/limitations of slump test:
- there is no direct relationship between the workability and the value of the slump.
- It is not suitable for concrete in which the maximum size of the aggregate exceeds 40 mm.
- There are chances of many shapes of a slump to occur and it is difficult to decide which one is giving the correct value.
- Slump occurs only in the case of plastic mixes. It does not occur in the case of dry mixes.
2. Compaction factor test
- In the compaction factor test, the degree of workability of concrete is measured in terms of internal energy required to compact the concrete thoroughly.
- A compaction factor of 0.95 represents flowing concrete having high workability: 0.92 plastic concrete has medium workability: 0.85 stiff plastic concrete has low workability and a compaction factor of 0.75 represents stiff concrete having very low workability.
- The compacting factor test is designed primarily for use in the laboratory but it can also be used in the field.
- It is more precise and sensitive than the slump test and is particularly useful for concrete mixes of very low workability and is normally used when concrete is to be compacted by vibration like dry concrete which is insensitive to the slump test.
- The test works on the principle of determining the degree of compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height.
- The degree of compaction called the compacting factor is measured by the density ratio i.e., the ratio of the density achieved in the test to the density of the same fully compacted concrete.
- The sample of concrete to be tested is placed in the upper hopper up to the brim.
- The trap door is opened so that the concrete falls into the lower hopper.
- Then the trap door of the lower hopper is opened and the concrete is allowed to fall into the cylinder.
- In the case of a dry mix, likely, the concrete may not fall on opening the trap door. In such a case, a slight poking by a rod may be required to set the concrete in motion.
- The excess concrete remaining above the top level of the cylinder is then cut off with the help of plane blades supplied with the apparatus.
- The outside of the cylinder is wiped clean.
- The concrete is filled up exactly up to the top level of the cylinder.
- It is weighed to the nearest 10 gm.
- This weight is the known weight of partially compacted concrete.
- The cylinder is emptied and then refilled with the concrete from the same sample in layers
approximately 5 cm deep.
- The layers are heavily rammed or preferably vibrated to obtain full compaction.
- This weight is known as the weight of fully compacted concrete
C.F. = weight of partially compacted concrete/weight of fully compacted concrete
3. Vee-Bee test or Vee-Bee consistometer test
- This is carried out in such a manner that the specimen concrete in the test receives the more or less same treatment in respect of the method of placing as it would in the actual execution of the work. This test is preferred for finding the workability of stiff concrete mix having very low workability.
- In this test, a vee-bee time of 3 to 5 seconds represents stiff plastic concrete having medium
workability, 10 to 15 seconds represents stiff concrete of low workability and vee-bee time of 18 to 10 seconds represents very stiff concrete having very low workability.
- This is a good laboratory test to measure indirectly the workability of concrete
- this test consists of a vibrating table, a metal pot and a standard iron rod.
- The slump test as described earlier is performed, placing the slumped concrete inside the sheet metal cylindrical pot of the consistometer.
- The glass disc attached to the arm is turned and placed on top of the concrete in the pot
- the electrical vibrator is then switched on and simultaneously a stopwatch is started.
- The vibration is continued till such a time as the conical shape of the concrete disappears and the concrete assumes a cylindrical shape.
- Immediately when the concrete fully assumes a cylindrical shape, the stopwatch is switched off.
- The time required for the shape of concrete to change from slump concrete shape to cylindrical
shape in second is known as vee bee degree.
- This method is very suitable for very dry concrete whose slump value can not be measured by a slump test.
4. Flow test
this is a laboratory test which indicates the quality of concrete with respect to consistency, cohesiveness and proneness to segregation.
In this test, a standard mass of concrete is subjected to jolting.
The spread or the flow of the concrete is measured and this flow is related to workability.
Flow, percent= (spread diameter in cm – 25)/25 * 100
the value can range anything from 0-150%.
It can be realized that the compacting factor test measures the inherent characteristics of the concrete which relates very close to the workability requirements of concrete and as such it is one of the good tests to depict the workability of concrete.
5. Kelly ball test
this is a simple field test consisting of the measurement of the indentation made by a 15 cm diameter metal hemisphere weighing 13.6 kg. When freely placed on fresh concrete. The test has been devised by kelly and is hence known as the kelly ball test.
This has not been covered by the Indian standards specification
the advantage of this test is that it can be performed on the concrete placed on site and it is claimed that this test can be performed faster with greater precision than the slump test.
It cannot be used when the concrete is placed in a thin section.
The minimum of concrete must be at least 20 cm and the minimum distance from the centre of the ball to the nearest edge of the concrete is 23 cm.