What Is Penetration Test of Bitumen?
The penetration test of bitumen measures the hardness or softness of bitumen by measuring the depth of penetration of standard loaded needle in five seconds while maintaining bitumen sample temperature at 25 °C. The more viscous the bitumen, the less distance needle is able to penetrate. Hence, the penetration value for viscous bitumen is less.
Also, the softening point of viscous bitumen is more. Thus, a bitumen sample with a low penetration value has a high softening point. The road pavement comprises of stone chippings attached together with the help of a binder like bitumen.
Bitumen is available in the market in various grades. The suitable grade of bitumen is selected as per the surfacing type and climatic conditions as well.
The grading of the bitumen is done on the basis of the consistency of bitumen. The consistency, in turn, is measured by the penetration value obtained from the penetration test of the bitumen.
To determine the penetration value of asphaltic bitumen, fluxed native asphalt, and blown type bitumen.
In this practical, the consistency of the sample is determined by measuring the distance in one-tenth of a millimeter the distance by which the penetrating needle penetrates in the sample under certain loading (100 g) at 25 °C for 5 seconds.
Important Terms related to Penetration Test
Penetration: It is the distance travelled by a standard needle of 1 sq. mm cross-section and comprising specific size, weight, and shape under predetermined conditions of temperature as well as time in terms of one-tenth of a millimetre.
IS code for Bitumen penetration test is (IS:1203-1978)
Following apparatus required for penetration test
Penetrometer: It is the testing machine for bitumen penetration test. It allows fixation of penetration needle, plunger, weight, and a dial. It allows the needle to penetrate into the sample placed in the cup without considerable friction. It should be accurately calibrated so that it gives the results in one-tenth of a millimeter.
There are two types of penetrometer–
1. Standard Penetrometer:The knob is to be pressed for five seconds manually so that the released needle can penetrate the sample.
2. Automatic penetrometer: The penetration needle is automatically released for five seconds with the help of a timer and then stops.
3. Standard Penetration needle: It is straight, highly polished, and cylindrical made up of stainless steel. It has a 3 mm diameter shank into which it is fixed such that its movement is restricted. The taper is pointed while the point is made blunt by grinding to a truncated cone.
Total length of the needle = 50 mm
Diameter of needle = 1.0 mm
Diameter of the pointed end of needle = 0.14-0.16 mm
Weight = 50 gram
4. Container: The material is either metal or glass and it is cylindrical in shape. Its bottom is flattened. The dimensions of the container are mentioned below:
For penetration value < 225
Diameter = 55 mm
Internal Height = 35 mm
For penetration value > 225
Diameter = 70 mm
Internal Height = 45 mm
5. Water bath: It should contain a minimum of 10 litres of water. Bitumen specimen should be immersed to at least 100 mm depth from the top. Moreover, the specimen should be supported over a perforated shelf which should be above 50 mm from the bottom of the bath. The water bath is maintained at 25 ± 0.1 °C temperature.
6. Transfer Dish: It is a small dis or tray It helps in keeping the container fully immersed in the water when placed in the water bath. It has some means to ensure a firm bearing so that the container does not shake.
7. Thermometer or Temperature indicator– To measure the temperature of water in the water bath. Its range should be from 0 to 44 °C. Its least count is 0.2 °C.
8. Time device: If a standard penetrometer is used, a suitable time device like a stopwatch, electric timer, or another spring-actuated device may be used which is graduated 0.1 second or less. The timing device should have an accuracy of ± 0.1 seconds for a 60-second interval. Even an audible seconds counter where one beat corresponds to one second can also be used alternatively.
Other accessory apparatus include:
Following materials required test,
- Bitumen of known grade
Following precautions should be taken while performing the penetration test on bitumen,
- For a sample having extraneous matter, the sample should be sieved through IS sieve 30
- Ensure no air bubbles or water remain entrapped after heating the sample
- An air oven can be used to avoid overheating of the bottom of the container. Or a sand bath can also be used.
- The needle is so adjusted that it just touches the bitumen sample- it should not penetrate on the bitumen surface beforehand
- If there is any movement of the container while the needle is penetrating, the test should be discarded.
Procedure for Penetration Test of Bitumen
- Take the bitumen sample in a beaker and heat it to a temperature above its approximate softening point. For tars and pitches, it is heated up to 60 °C while bitumen is heated up to 90 °C.
- Allow the bitumen to melt until it achieves a pouring consistency. Keep stirring the sample so that the sample is homogenous and is free from air bubbles and water.
- Pour the melted bitumen into the container of 35 mm depth until it is filled up to a depth of 10 mm more than the expected penetration.
- Place the container on the transfer dish and allow it to cool in the atmosphere at temperature in-between 15-30 °C for 1.0-1.5 hours when a 35 mm deep container is used. The time period is increased to 1.5-2.0 hours when a 45 mm deep container is used.
- Place the whole assembly with container and transfer dish in a water bath, which is maintained at 25 ± 0.1 °C for about 1.5-2.0 hours when 35 mm deep container is used. The time period is reduced to 1.0-1.5 hours when a 45 mm deep container is used.
- When cutback bitumen or Digboi type bitumen is to be tested, the residue left after the distillation process is used for this test.
Testing of the Specimen:
1. Remove the transfer dish from the water bath and fill the water from the water bath in the dish to such a depth that the container is covered completely.
2. Place the sample container in this transfer dish and place this assembly under the penetration needle on the penetrometer.
3. Clean the needle with cotton dabbed in benzene and allow it to dry. Load the needle to the specified weight.
4. Adjust the needle so that its tip just touches the surface of the bitumen sample. Finely adjust the needle by raising the platform slightly with the help of adjusting the screw. Check the contact of the needle to the surface of bitumen by observing the image of the needle reflected by the surface of the material.
5. Clamp the needle in this position.
6. Load the needle holder with a weight such that the total moving weight is equal to 100 ± 0.25 grams.
Moving weight = weight of needle + weight of carrier + superimposed weights
7. Note down the initial reading of the dial of the penetrometer. Or bring the pointer to zero.
8. In a standard penetrometer, release the needle for exactly 5 seconds by pressing the knob. Measure the time with the help of a stopwatch.
9. In an automatic penetrometer, release the needle for exactly 5 seconds by pressing the start button.
10. Rotate the knob on the dial and note down the final reading from the dial as the penetration value in one-tenth of a millimeter.
11. Clean the needle with benzene after every test and allow it to dry.
12. Take at least 3 penetration values on the surface of the sample. (For a sample with penetration value > 225 mm, leave the needle in the sample and use another needle to take another reading so that the sample is not disturbed.)
Distance between two test points ≮ 10 mm
Distance between the test point and side of the dish ≮ 10 mm
13. Take out the average of the three values and note it as the penetration value of the sample.
14. If the penetration ratio is to be determined: Carry out the entire test procedure at 4 °C. Tolerance of 0.1 °C can be allowed.
In this test,
Take the total weight on the penetration needle = 200 ± 0.25 gram.
Increase the time of penetration to 60 seconds.
In the penetration test, as the needle of the penetrometer is released, it penetrates the sample to some distance.
The distance by which the needle penetrates in the bitumen sample is to be noted down for all the three samples.
A sample observation table is drawn below for bitumen penetration test,
|Actual test temperature (in °C) =|
|Penetrometer Dial Reading||Test 1||Test 2||Test 3|
|Initial Reading (a)|
|Final Reading (b)|
|Penetration Value (b-a)|
Penetration Value of Bitumen = Average Value of Penetration of Sample – 1, 2 & 3
Penetration at 4 °C – Wt. 200 gm – time 60s / Penetration at 25 °C – Wt. 100 gm – time 5s) x 100
In the penetration test of bitumen, the average of the penetration values obtained from the three tests is the penetration value of the given bitumen specimen.
The penetration values of the three readings should not differ from each other beyond certain limits. These limits are mentioned in the table below:
|Penetration Value||Maximum difference between readings|
|0 – 49||2|
|50 – 149||4|
|149 – 249||6|
Minimum penetration values standardised by Bureau of Indian Standard for Paving Bitumen Type1 for different grades are tabulated below:
|Grade of Bitumen Type-I||Ductility (in cm)|
|S – 35||30 – 40|
|S – 45||40 – 50|
|S – 55||50 – 60|
|S – 65||60 – 70|
|S – 90||80 – 100|
|S – 200||175 – 225|
Conclusion of Penetration Test of Bitumen
By performing the above penetration test on bitumen, we can determine whether the given bitumen sample meets the specification requirements for penetration as given in the standards.
Penetration value is one of the measures of the quality of bitumen along with other measures such as ductility value and so on.
The penetration value found out by the penetration test of bitumen is compared to the standard value and thus it can be concluded whether the sample meets the penetration criteria.
Following table show step by step procedure of bitumen penetration test with picture
|The bitumen sample is heated in a beaker up to 60-90 °C- above the softening point of bitumen till it achieves a pouring consistency.|
|The sample is stirred thoroughly so that a homogenous mix is obtained which is free from air bubbles and water.|
Melted bitumen is poured into the 53 mm deep container till it is filled 10 mm excess to an approximate penetration depth.
|The container along with the sample is placed in the transfer tray and is allowed to cool in the atmosphere between the temperature range- 15-30 °C for 1-1.5 hours (for 35 mm deep container).|
|The whole assembly is placed in the water bath at 25 ± 0.1 °C for 1.5-2 hours (for 35 mm deep container).|
|Transfer dish is removed from the water bath and filled with water from water bath till it covers the container completely.|
The sample container along with the transfer dish is placed on the platform of the penetrometer under the needle.
The needle is cleaned with cotton dabbed in benzene.
|Lower the needle assembly so that the tip of the needle just touches the surface of bitumen.|
|Adjust the needle finely with the help of adjusting the screw.|
The contact of the tip of the needle to the bitumen surface is checked by observing the image of the needle reflected on the surface.Or it can also be checked by a mirror.
|The needle assembly is clamped in this adjusted position carefully.|
The needle holder is loaded with a weight such thatTotal moving weight = 100 ± 0.25 grams.
The pointer of the dial is set to zero.Or the initial reading is noted down.
The needle is released for exactly 5 seconds by pressing the knob.The time is measured from a stopwatch with specifications as discussed in the apparatus.
Automatic PenetrometerThe needle is released for exactly 5 seconds by pressing the start button.
|The knob on the dial is rotated to take the reading for the test.|
|The difference between final and initial reading is noted as penetration value in one-tenth of a millimetre.|
|The needle is cleaned with benzene and allowed to dry after each test.|
At least, three penetration values are taken for a sample.
Distance between these points ≮ 10 mm.Distance between the testing point and edge of container ≮ 10 mm.
|The average of the three values is noted down as the penetration value of the sample.|
To Determine Penetration Ratio:
The above procedure is followed and the test is carried out at 4 ± 0.1 °C
Total weight on the penetration needle = 200 ± 0.25 gPenetration time = 60 sec
|Penetration value of the sample||The average penetration value taken at 3 distinct points. (The values should not differ by ± 2-8 % of the mean value depending on the penetration value obtained).|
|Penetration Ratio||Penetration at 4 °C – Wt. 200 gm – time 60s / Penetration at 25 °C – Wt. 100 gm – time 5s) x 100|
Image Courtesy: Image-1
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