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(Keywords: workability of concrete definition, workability of concrete test, slump test, compaction factor test, Vee-Bee Consistometer test)
Workability of Concrete
Workability is the quantum of work required to bring into being full compaction or the ease of working with concrete.
The important facts in connection will workability are:
It results into concrete of low strength and poor durability, if more water is added when trying to attain the required degree of workmanship.
If the strength of concrete is not to be affected, the degree of workability can be obtained:
By slightly changing the proportions of fine and coarse aggregates, in case the concrete mixture is too wet and,
By adding a small quality of water-cement blend in the proportion of original mix, in case the concrete mixture is too dry.
The workability of concrete is also affected by the maximum size of the coarse aggregates to be used in the mixture
The workability of concrete is affected substantially by water content, water-cement ratio and aggregate-cement ratio
Commonly used tests for measurement of workability of concrete
1. Slump test (IS:1199)
Stamp test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work.
It is not a suitable method for very wet or dry concrete and stiff mix.
It does no measure all factor contributing to workability.
The diameter of the rod is 16mm and its length is 60cm. The strokes to be given for ramming usually falls between 20 to 30 but most commonly 25 times is preferred.
SLUMP TEST DEMONSTRATION VIDEO
Procedure
The mould is placed on a smooth, horizontal, rigid and non-absorbent surface.
The mould is then filled in four layers after ramming 25 times at each layers, and each layer has approximately 1/4th of the height of the mould.
The mould is removed from the concrete by raising it slowly and carefully in vertical direction without disturbing the concrete test sample.
This allows the concrete to settle down.
This subsidence of concrete after removing the mould is referred as “SLUMP” of concrete.
The difference in level between the height of the highest point of the subsided concrete is measured.
The difference in height in mm is taken as slump of concrete.
2. Compaction Factor Test
In the compaction factor test, it offers a direct and reasonably reliable assessment of workability of concrete since it measures the degree of compaction imparted to concrete for a standard amount of work.
A compaction factor of 0.95 represents flowing concrete having high workability concrete; 0.92 plastic concrete having medium workability concrete; 0.85 stiff plastic concrete having low workability and a compaction factor greater than 0.70 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 also.
It is more precise and sensitive than the slump test and was developed in U.K. It is particularly useful for concrete mixes of medium and low workabilities and which is normally used when concrete is to be compacted by vibration like, such as dry concrete which is insensitive to 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 actually achieved in the test to density of same fully compacted concrete.
The sample of concrete to be tested is place in the upper hopper up to the brim.
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 blend, it is likely that 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 to fall down.
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.
COMPACTION FACTOR TEST DEMONSTRATION VIDEO
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 10gm.
The weight is 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 layer are heavily rammed or preferably vibrated so as to obtain full compaction.
The weight is known as weight of fully compacted concrete.
C.F. = Weight of partially compacted concrete/ Weight of fully compacted concrete
3. Vee-Bee Test/Vee-Bee Consistometer test
This is carried out in such a manner that the specimen concrete in the test receives more or less same treatment in respect of the method of placing as it would in actual execution of the work. This test is preferred for finding 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 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.
Slump test as described earlier is performed, placing the slump concrete inside the sheet metal cylindrical pot of the consistometer.
The glass disc attached to the arm is turned and place on the top of the concrete in the pot
The electrical vibrator is then switched on and simultaneously a stop watch 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 stop watch 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 cannot be measured by slump test.
VEE-BEE CONSISTOMETER DEMONSTRATION VIDEO
Conclusion
In this blog post we discussed about the measurement of workability and its types of test used in the industry and brief procedures for the tests. The test includes-
Slump Test
Compaction Factor Test
Vee-Bee Consistometer Test
Having discussed about the measurement of workability in this blog post we will again introduce topics like factors affecting workability of concrete in the same post, so stay tune for more updates!
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