Bitumen extraction test is conducted to determine the percentage of bitumen present in an asphaltic pavement. This test is important because various pavement properties such as durability, compatibility, resistance to various defects and failure such as ravelling, rutting, bleeding etc depend upon the content of bitumen present as the binder for the aggregates.
Generally, in asphalt pavement, bitumen is used as a binder for the filler aggregates. The mix design of asphalt must be done as per the standard by using hit and trial method for determining the actual binder content. Hence, at the site, it is necessary to check whether the bitumen content is as per the mix design or not before the final payment is made to the contractor. There are two types of method used for determining the bitumen content which includes:
i. Centrifuge Method
ii. Extraction Bottle Method
The most commonly used method among these two methods is the centrifuge method. It is also commonly referred to as cold solvent extraction method.
i. Centrifuge Method
The apparatus required for conducting the bitumen extraction test are as follows:
1. Centrifuge apparatus.
2. Thermostatically controlled oven with a capacity of 2500 degree Celsius.
3. Balance with a capacity of 500 grams and sensitivity of 0.01 grams.
4. Beaker or bowel extraction sample for the collection of the extracted sample.
5. Bitumen Sample
Procedure of bitumen extraction test by Centrifuge Method
The procedure for determining the bitumen content by using centrifuge method involves two primary steps i.e. preparation of the sample and centrifuging the sample. These steps are further explained in brief as follows:
A. Preparation of the sample
The test sample can be collected either from the dump truck or the asphalt plant present at site depending upon the site condition. Since the age of the sample can affect the test result, the best result is obtained when the sample is obtained directly from the mixtures of pavement immediately after their preparation. In such a case, the sample may be scraped off using a spatula. The sample must be soft enough so that it can easily be scraped off else it must be placed on a flat pan and heated in the oven at a temperature of about 110 degree Celsius till it can be separated easily.
Then, the representative sample is collected from the heated sample as per the size of the aggregate mix and the following steps are carried out:
1. The sample is taken and weighed to the nearest of 0.05% of its mass and is recorded as W1. It is then placed in the bowl.
2. The sample is then immersed in a solvent for about an hour. The solvent used may be petrol, trichloroethylene or benzene.
3. A filter ring that has been oven-dried is taken and is weighed. It must be noted that the filter ring has been oven-dried at a temperature of about 110 degree Celsius.
4. The bowl is taken and its edge is covered by this filter ring.
5. Then, the bowl is covered with a steel cover and is clamped tightly. It is then placed in the apparatus.
6. A beaker or a well cleaned and dried container is placed below the drain outlet of the centrifuge test apparatus to collect the extracted mixture of the bitumen sample and the solvent.
B. Centrifuging the sample
After the completion of the above-mentioned steps, the next step is to centrifuge the apparatus. The sample is then centrifuged which involves the following series of steps:
1. The bowl is placed in the apparatus and the machine is started to revolve. The speed of the revolution is gradually increased until it attains a maximum speed of 3600 rev/min.
2. The machine is continuously revolved until the flow of mixture ceases from the drain outlet of the centrifuge apparatus.
3. The machine is then stopped and more solvent is added depending upon the quantity of sample previously taken.
4. The process is continued and the solvent is added for about three cycles until the colour of the obtained extract is not clear and darker than light straw colour.
5. The filter paper is then gently removed from the bowl or container that is previously placed below the centrifuge apparatus along with the obtained residue.
6. The residual aggregate collected is then oven-dried at a constant temperature of 1050 C to 1110 C and cooled down.
7. The aggregate fragments are carefully scrapped and weighed as well as the weight of filter paper is recorded with an accuracy of 0.01 gm.
Observations and Calculations
|Sl No||Observation||Sample 1||Sample 2||Sample 3|
|1||Weight of mix taken before extraction (W1)|
|2||Weight of filter paper before extraction (B)|
|3||Weight of mix after extraction (W2)|
|4||Weight of filter paper after extraction (D)|
|5||Weight of filler collected in filter paper (B-D) =W3|
The formula to calculate the percentage of binder content is given by-
The precautions that must be taken during the bitumen extraction test can be listed as follows:
1. The apparatus and materials must be handled with care to ensure safety.
2. The balance must be calibrated using reference weight at least once in every
twelve months to maintain its accuracy.
3. Since the extraction may result in the emission of fumes, it must be taken care that the place where the test is being carried out is well ventilated. If required external fans may be provided.