How to Determine Penetration Value of Bitumen?

Penetration is a measurement of the hardness or consistency of bituminous material. Penetration test of bitumen is the vertical distance traversed or penetrated by the point of a standard needle carrying a 100 grams of load for 5 seconds into the bituminous material at a temperature of 25 degrees.. This distance is measured in one tenth of a millimeter. This test is used for evaluating consistency of bitumen materials.

This test is not suitable for testing the road tar, because tar posses high surface tension that are exhibited by the materials and they contain relatively larger amount of free carbon.

Also Read: What is Camber in Road Construction?

Objective of Penetration Test on Bitumen

The main objective of penetration test on bitumen is to:

  1. Determine the consistency of Bituminous Material
  2. Asses the suitability of bitumen for its use under different climatic conditions and type of construction

Uses of Penetration Test on Bitumen

  1. The test helps to differentiate between different bitumen grades.
  2. Ensure the obtained bitumen for construction is off required grade and quality. 
  3. To check the sustainability of the bitumen in different weather and loading conditions.
  4. To measure softness, hardness and consistency of the bitumen.

Materials Requires for Penetration Test on Bitumen

1. Container

A flat bottom cylinder metal dish 55 mm in diameter and 35 mm in depth is required. If the penetration is of the order of 225 or more, a deeper dish of 70 mm diameter and 45 mm depth is required.

2. Penetration Needle (SS316)

A straight and highly polished needle  which is a cylindrical hard stainless steel rod of length approximately equal to 50 mm is used.

3. Water Bath

A water bath maintained at 25 ± 0.1 degree Celsius containing not less than 10 liters of water, the sample being immersed to a depth not less than 50 mm from the bottom of the bath.

4. Transfer Dish or Tray

It should provide support to the container and should not rock the container.

5. Penetrometer

Metal Base, Weight and a Carrier are the important parts of a manual penetrometer. 
The penetrometer, penetration apparatus should allow the needle to penetrate without much friction and is accurately calibrated to give results in one tenth of a millimeter.
It has a release knob. When we push the assembly is released, which lock when take our hand back.

Note * Manual Penetrometer requires stop watch to measure the time. An automatic penetrometer has a automatic load release arrangement. All the load application is conducted based on switch. It gives more precise measurement. In order to maintain the even surface, a spirit level is there at the base.

It contains 100 gram penetration carrier and graduated scale ( 1 in 10mm ) and a knob to release the weight carrier. Total weight of the carrier assembly and the penetration needle is 100 grams. 

6. Thermometer

The thermometer used must range 0 to 44 degrees and readable up to 0.2 degree Celsius.

7. Stop Watch

A time measuring device with ± 0.1 sec must be used.

Procedure for Penetration Test on Bitumen

The test procedure is as per IS 1203.

  1. Soften the material to a pouring consistency at a temperature not more than 60 degree celsius for tar and 90 degree celsius for bitumen above approximate softening point and stir it thoroughly until it is homogeneous and is free from air bubbles and water.
  2. Pour the melt into the container to a depth at least 10mm in excess of the expected penetration. Protect the sample from dust and allow it to cool in an atmosphere at a temperature between 15 degree celsius and 30 degree celsius for 1 hr. 
  3. Place this along with the transfer dish in the water bath at 25± 0.1 degree Celsius and allow it to remain for 60 to 90 minutes. The test is carried out at 25± 0.1 degree Celsius unless otherwise stated.
  4. Fill the transfer dish with water from the water bath to a depth sufficient to cover the container completely. Place the sample in it and put it upon the stand of the penetration apparatus.
  5. Clean the needle with benzene, dry it and load it with the weight. The total moving load required is 100 ± 0.25 g, including the weight of the needle, carrier and superimposed weights.
  6. Adjust the needle to make contact with the surface of the sample. This may be done by placing the needle point in contact with its image reflected by the surface of the bituminous material.
  7. Make the pointer of the dial to read zero or note the initial dial reading.
  8. Release the needle for exactly 5 seconds.
  9. Adjust the penetration machine to measure the distance penetrated.
  10. Make atleast 3 readings at points n the surface of the sample not less than 10 mm apart ad not less than 10 mm from the side of the dish. After each test return the sample and transfer dish to the water bath and wash the needle clean with benzene and dry it. In case the material takes a penetration greater than 225 mm, three determination of the two identical test specimens using a separate needle for each determination should be made, having the needle in the sample on completion of each determination to avoid disturbance of the specimen.

Precautions to be Taken For Penetration Test on Bitumen

  1. No movement of the container while the needle is penetrating into the sample.
  2. The sample taken for testing must be free from any extraneous matter. If present, sieve it through IS Sieve 30.
  3. The needle should be cleaned with benzene and dried before each penetration.

Record of Observations and Calculations

Actual Test Temperature = ____ oC

Experiment No: ____




Penetrometer Reading

a. Initial Reading

b. Final Reading




Penetration Value = Initial Reading –Final Reading







Mean Penetration value

(P1 +P2 + P3 )/3


Note * In most case initial reading is set to zero and the test is conducted for all the trials. The three trial values should not differ by more than the maximum value that is mentioned in below table.







150 TO 249





The result obtained from the test is Penetration value of the bitumen sample and it do not posses any unit.

Interpretation of Results

Penetration test is commonly adopted test on bitumen to grade the material in terms of its hardness. 
  1. 80/100 Grade Bitumen = Bitumen with penetration value between 80 and 100 at standard test conditions.
The grading of the bitumen helps to assess the suitability for use in different climatic conditions and types of construction. The penetration values of various types of bitumen used in pavement construction in India range between 20 and 225.

As per IRC, bitumen with Grades 30/40, 60/70, and 80/100 are best suitable and can be selected based on the construction type and the climatic conditions. 

In warmer regions, lower penetration grades are preferred to avoid softening whereas, higher penetration grades like 180/100 are used in colder regions so that excessive brittleness does not occur. The highest penetration value is used in spray application works.





30 TO 40


40 TO 50


50 TO 60


60 TO 70


80 TO 100


175 TO 225


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