5 Important Tests to Measure the Workability of Concrete

Workability of concrete is a complex property. Numerous attempts have been made by many research workers to quantitatively measure this important and vital property of concrete. But none of these methods are satisfactory for precisely measuring or expressing this property to bring out its full meaning. Some of the tests, measure the parameters very close to workability and provide useful information. The following tests are commonly employed to measure the workability of concrete :

1. Slump Test
2. Compaction Factor Test
3. Flow Test
4. Kelly Ball Test
5. Vee Bee Consistometer Test.

Before discussing these tests in detail, let’s understand what the workability of concrete is?

Workability Of Concrete Definition

Workability of concrete is the property which determines the amount of useful internal work necessary to produce full compaction.” Another definition which envelopes a wider meaning is that, it is defined as the “ease with which concrete can be compacted hundred per cent having regard to mode of compaction and place of deposition”.

Now let’s start discussing the tests to measure the workability of concrete.

1. Slump Test

Slump test is the most commonly used method of measuring consistency of concrete. It is not a suitable method for very wet or very dry concrete. It does not measure all factors contributing to workability of concrete, nor is it always representative of the placability of the concrete. However, it is used conveniently as a control test and gives an indication of the uniformity of concrete from batch to batch. Additional information on workability and quality of concrete can be obtained by observing the manner in which concrete slumps.

Apparatus

The apparatus for conducting the slump test essentially consists of:

• A metallic mould in the form of a frustum of a cone.
• A steel tamping rod 16 mm dia, 0.6 meter along with bullet end.

Procedure

1. The internal surface of the mould is thoroughly cleaned and freed from superfluous moisture and adherence of any old set concrete before commencing the test.
2. The mould is placed on a smooth, horizontal, rigid and non-absorbant surface.
3. The mould is then filled in four layers, each approximately 1/4 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.
4. After the top layer has been rodded, the concrete is struck off level with a trowel and tamping rod.
5. The mould is removed from the concrete immediately by raising it slowly and carefully in a vertical direction. This allows the concrete to subside. This subsidence is referred as SLUMP of concrete.
6. The difference in level between the height of the mould and that of the highest point of the subsided concrete is measured. This difference in height in mm. is taken as Slump of Concrete.

ASTM measure the centre of the slumped concrete as the difference in height. ASTM also specifies 3 layers.

2. Compaction Factor Test

The compaction 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 as are normally used when concrete is to be compacted by vibration. Such dry concrete are insensitive to slump test.

This 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 concrete fully compacted.

Apparatus

• Hooper
• Cylinder
• Balance

The diagram of the apparatus is shown in figure below:

Procedure

1. The sample of concrete to be tested is placed in the upper hopper up to the brim.
2. The trap-door is opened so that the concrete falls into the lower hopper.
3. Then the trap-door of the lower hopper is opened and the concrete is allowed to fall into the cylinder.
4. In the case of a dry-mix, 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 in motion.
5. 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. It is weighed to the nearest 10 grams. This weight is known as “Weight of partially compacted concrete”.
6. The cylinder is emptied and then refilled with the concrete from the same sample in layers approximately 5 cm deep.
7. The layers are heavily rammed or preferably vibrated so as to obtain full compaction.
8. The top surface of the fully compacted concrete is then carefully struck off level with the top of the cylinder and weighed to the nearest 10 gm. This weight is known as “Weight of fully compacted concrete”.

Compacting Factor = Weight of partially compacted concrete / Weight of fully compacted concrete

3. Flow Test

This is a laboratory test, which gives an indication of the quality of concrete with respect to consistency, cohesiveness and the 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 of concrete.

Apparatus

The apparatus consists of flow table, about 76 cm. in diameter over which concentric circles are marked. A mould made from smooth metal casting in the form of a frustum of a cone is used with the following internal dimensions. The base is 25 cm. in diameter, upper surface 17 cm. in diameter, and height of the cone is 12 cm.

Procedure

1. The table top is cleaned of all gritty material and is wetted. The mould is kept on the centre of the table, firmly held and is filled in two layers.
2. Each layer is rodded 25 times with a tamping rod 1.6 cm in diameter and 61 cm long rounded at the lower tamping end.
3. After the top layer is rodded evenly, the excess of concrete which has overflowed the mould is removed.
4. The mould is lifted vertically upward and the concrete stands on its own without support.
5. The table is then raised and dropped 12.5 mm 15 times in about 15 seconds.
6. The diameter of the spread concrete is measured in about 6 directions to the nearest 5 mm and the average spread is noted.
7. The flow of concrete is the percentage increase in the average diameter of the spread concrete over the base diameter of the mould.

Flow, per cent = [(Spread diameter in cm − 25)/25] x 100

The value could range anything from 0 to 150 percent.

4. Kelly Ball Test

This is a simple field test to measure workability of concrete, consisting of the measurement of the indentation made by 15 cm diameter metal hemisphere weighing 13.6 kg. when freely placed on fresh concrete. The test has been devised by Kelly and hence known as Kelly Ball Test.

The advantages of this test is that it can be performed on the concrete placed in site and it is claimed that this test can be performed faster with a greater precision than slump test. The disadvantages are that it requires a large sample of concrete and it cannot be used when the concrete is placed in thin section. The minimum depth of concrete must be at least 20 cm and the minimum distance from the centre of the ball to nearest edge of the concrete is 23 cm.

Apparatus

• Metal hemispherical ball
• Graduated scale
• Frame

Procedure

• Concrete is poured into a container up to at least 20 cm.
• The surface of the concrete is struck off level, avoiding excess working, the ball is lowered gradually at the center on the surface of the concrete.
• The depth of penetration is read immediately on the stem to the nearest 6 mm.
• The test can be performed in about 15 seconds and it gives much more consistent results than Slump Test.

5. Vee Bee Consistometer Test

This is a good laboratory test to measure indirectly the workability of concrete. This test consists of a vibrating table, a metal pot, a sheet metal cone, and a standard iron rod.

Procedure

1. Slump test as described earlier is performed, placing the slump cone inside the sheet metal cylindrical pot of the Consistometer.
2. The glass disc attached to the swivel arm is turned and placed on the top of the concrete in the pot.
3. The electrical vibrator is then switched on and simultaneously a stop watch started.
4. The vibration is continued till such a time as the conical shape of the concrete disappears and the concrete assumes a cylindrical shape.
5. This can be judged by observing the glass disc from the top for disappearance of transparency.
6. Immediately when the concrete fully assumes a cylindrical shape, the stop watch is switched off.
7. The time required for the shape of concrete to change from slump cone shape to cylindrical shape in seconds is known as Vee Bee Degree.

This method is very suitable for very dry concrete whose slump value cannot be measured by Slump Test, but the vibration is too vigorous for concrete with a slump greater than about 50 mm.

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