Asphalt & Bitumen West Co .

Penetration of Bitumen

Asphalt Penetration

One of the most significant features of asphalt cement is temperature susceptibility that is obtained and evaluated in terms of penetration index (PI). Penetration and softening point tests are used to determine the penetration of asphalts.

The asphalt phase change from solid to liquid due to the increase in temperature is not sudden, and it happens gradually so that the time of conversion from solid to liquid is not the same for various asphalts. 

For instance, the asphalts which have similar penetration grades in 25ºC may have different stiffness values in higher temperatures; hence the determination of their penetration grade is essential.

For example, the 40-50, 60-70,etc. grading that is used for these asphalts indicates the range of penetration depth of the test needle into the sample. It should be noted that there is a direct relationship between viscosity and penetration grade so that the increase in viscosity leads to an increase in the softening point and the decrease of the penetration grade.  

Penetration grade test is used to determine the relative stiffness of asphalts and is one of the most applicable asphalt tests. One of the grading systems that is used for various asphalts is the penetration grade system. 

Since the asphalt has a viscoelastic nature, its behavior depends on the change of temperature and loading per unit of time.


Asphalt behaves as a viscose liquid in hot climates, and in these conditions, permanent deformation and less recoverable ruts are produced in it. In cold weather, the asphalt acts as a rigid and elastic material, it deforms when the vehicles pass the road and comebacks to its initial condition when it is unloaded, but it may be brittle and get cracked while loading. 

Asphalt's common tests are often run at 25 ◦C, and this subject does not indicate the asphalt behavior in other temperatures. 

The viscosity test only gives information about the viscose behavior of asphalt in a specified temperature range and does not show its behavior in higher or lower temperatures.

The most important application of the penetration grade is to find the appropriate region for asphalt use. In hot climate areas with high traffic loads, asphalts with higher penetration grades are used and lower penetration grade asphalts are utilized in cold climate areas. 

Therefore, as the asphalt becomes harder, the penetration grade decreases, and as a result, softer asphalts have higher penetration grades.  

This test is run according to the ASTM D5, and its description is as follows. 

Penetration Test Definition

This test is carried out to measure the asphalt consistency as the vertical penetration length of a standard needle into a sample in the unit of one-tenth of a millimeter (0.1mm) under a specified condition.

Penetration Test Objective

This test aims to determine the penetration grade of asphalt and asphaltic materials in rigid and semi-rigid conditions.

This test measures the asphalt consistency so that higher penetration grade shows lower consistency, and lower penetration grade indicates higher consistency.


Penetration Test Condition

The length of the needle is 50mm for standard needles and 60mm for longer ones. The specifications of the asphalt sample container are as follows: 

For penetration grades lower than 200, the container diameter and internal height should be 55 and 35 mm, respectively.

 For penetration grades between 250 and 350, the container diameter and its internal height should be 55-75 and 45-70 mm.

For penetration grades between 350 and 500, the container diameter and its internal height should be 55 and 70 mm, respectively.

The water bath for this test should have the capability to keep the water in test temperature with 0.1◦C accuracy.

The total weight of the needle with its belongings is 100 grams, and its penetration time into the asphalt sample is 5 seconds.

Penetration Test Description

In this test, the asphalt or asphaltic material is poured into the sample container so that its surface is completely level to the container edge after cooling down.

After pouring the asphalt sample into the container, it should be left at room temperature for 1 to 1.5 hours to cool down, and then it is placed in a 25◦C water bath for the same period.

Then, the sample with the water container is placed under the apparatus that measures the penetration grade, and the penetration test is started in the following manner.

The sample container is placed under the measurement apparatus, and the needle is put on the surface of the sample. Then the releasing button of the test needle is pushed, and it is allowed to penetrate the asphalt sample for 5 seconds.

At least, three needles are used as a hypothetical triangle in the penetration grade test (the distance of each needle from the edge of the container is 1cm).

The test result is reported as the average penetration depth of three needles into the asphalt sample.

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