3 METHODS OF DETERMINING WORKABILITY OF CONCRETE

Different Test Methods for Workability Measurement

Depending upon the water cement ratio in the concrete mix, the workability may be determined by the following three methods.

  1. Slump Test
  2. Compaction Factor Test
  3. Vee-bee Consistometer Test

1. Slump Test

Suitability

This method is suitable only for the concrete of high workability.

This test is carried out with a mould called slump cone whose top diameter is 10 cm, bottom diameter is 20 cm and height is 30 cm.

Slump-Apparatus
Slump-Apparatus

Procedure

The test is performed in the following steps:

  1. Place the slump mould on a smooth flat and non-absorbent surface.
  2. Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water in it.
  3. Place the mixed concrete in the mould to about one-fourth of its height.
  4. Compact the concrete 25 times with the help of a tamping rod uniformly all over the area.
  5. Place the mixed concrete in the mould to about half of its height and compact it again.
  6. Similarly, place the concrete upto its three-fourth height and then up to its top. Compact each layer 25 times with the help of tamping rod uniformly. For the second and subsequent layers, the tamping rod should penetrate into underlying layer.
  7. Strike off the top surface of mould with a trowel or tamping rod so that the mould is filled to its top.
  8. Remove the mould immediately, ensuring its movement in vertical direction.
  9. When the settlement of concrete stops, measure the subsidence of the concrete in millimeters which is the required slump of the concrete.

Recommended Slump Values for Various Concrete Works

Type of Construction Recommend slump in mm
Minimum Maximum
Pavements 25 50
Mass concrete structure 25 50
Unreinforced footings 25 75
Caissons and bridge decks 25 75
Reinforced foundation, footings and walls 50 100
Reinforced slabs and beams 30 125
Columns 75 125

Limitations of Slump Test

Following are the limitations

  • Not suitable for concrete containing aggregates larger than 40 mm.
  • Not suitable for concrete of dry mix.
  • Not suitable for very wet concrete.
  • Not reliable because slump may be of any shape.

2. Compaction Factor Test

According to this test, the workability may be defined as the amount of applied work required to compact the concrete to its maximum density.

Suitability

This method is adopted for determining the workability of concrete mix in laboratories. It gives fairly good results for concrete of low workability.

Procedure

The apparatus required for performing the compaction factor test is shown below.

Compaction-Factor-Test-Apparatus
Compaction-Factor-Test-Apparatus
  1. The test is performed in the following steps:
  2. Clean and dry the internal surface of the mould.
  3. With the help of hand scoop, place the concrete in upper hopper A.
  4. Open the trap door of hopper in order to facilitate the falling of the concrete into lower hopper B. the concrete sticking to the sides of the hopper A, should be pushed downward with the help of a steel rod.
  5. Open the trap door of the hopper B and allow the concrete to fall into cylinder C.
  6. Remove the surplus concrete from the top of the cylinder with the help of a trowel. Wipe and clean the outside surface of the cylinder.
  7. Weigh the cylinder with partially compacted concrete nearest to 10 g.
  8. Fill in the cylinder with fresh concrete in layers not exceeding 5 cm in thickness and compact each layer till 100 percent compaction is achievd.
  9. Wipe off and clean the outside surface of the cylinder and weigh the cylinder with fully compacted concrete nearest to 10 g.
  10. Calculate the value of compaction factor using the following formula.

Compaction factor = weight of partially compacted concrete/weight of fully compacted concrete

Recommended Values of Workability for Various Placing Conditions

Conditions Degree Values of Workability
Concreting of shallow sections with vibrations Very low 20 – 10 seconds Vee-Bee time

or 0.75 to 0.80 compacting factor

Concreting of lightly reinforced sections with vibrations Low 10 – 5 seconds Vee-Bee time or

0.80 to 0.85 compacting factor

Concreting of lightly reinforced sections without vibrations or heavily reinforced sections with vibrations Medium 5-2 seconds Vee-Bee time or

0.85 to 0.92 compacting factor or

25 – 75 mm slumps for 20 mm aggregates

Concreting of heavily reinforced sections without vibrations High Above 0.92 compacting factor or 75 – 125 mm slump for 20 mm aggregates.

Advantages of Compaction Factor Test

Following are the advantages:

  • Suitable for testing workability in laboratories
  • Suitable for concrete of low workability
  • Suitable to detect the variation in workability over a wide range
  • Its results are more precise and sensitive.

3. Vee-bee Consistometer Test

The apparatus used in this method of test is shown below.

Vee-Bee-Consistometer
Vee-Bee-Consistometer

Suitability

This method is suitable for dry concrete having very low workability

Procedure

The test is performed as given described below

  1. Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water.
  2. Pour the concrete into the slump cone with the help of the funnel fitted to the stand.
  3. Remove the slump mould and rotate the stand so that transparent disc touches the top of the concrete.
  4. Start the vibrator on which cylindrical container is placed.
  5. Due to vibrating action, the concrete starts remoulding and occupying the cylindrical container. Continue vibrating the cylinder till concrete surface becomes horizontal.
  6. The time required for complete remoulding in seconds is the required measure of the workability and it is expressed as number of Vee-bee seconds.

COMPARISON OF WORKABILITY MEASUREMENTS BY VARIOUS METHODS

Workability Description Slump in mm Vee-bee Time in Seconds Compacting Factor
Extremely dry 32 – 18  
Very stiff 18 – 10 0.70
Stiff 0 – 25 10 – 5 0.75
Stiff plastic 25 – 50 5 – 3 0.85
Plastic 75 – 100 3 – 0 0.90
Flowing 150 – 175 0.95

 

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