Ductility of bitumen and centrifuge extraction test

Introduction

Asphalt binders are categorized by chemical and physical properties. Asphalt is a chemical mixture of polar and non-polar complex organic molecules. The micro-structure of these molecules tends to govern asphalt’s physical behavior. Since chemical knowledge and testing is limited, asphalt is generally defined by its physical features. Over the years many tests have been established to completely characterize asphalt’s physical attributes. Super pave tests measure specific asphalt binder physical properties that are directly related to field performance by engineering principles. In this post some of these tests will be inspected, namely ductility and centrifuge extraction tests.

Fig-1: Using bituminous materials in isolation

Objectives

• Part A To determine the ductility of samples of bitumen.
• Part B To determine the bitumen percentage in bituminous mix using centrifuge extractor.

Theory

Part A (Ductility Test)

Ductility can be defined as: Bitumen property that allows it to undertake deformation and elongation, it can be defined as the distance in cm when a sample of the material can undertake when it's pulled and elongated without breaking at a specified temperature and a specified speed. This test is to determine the ductility of bitumen as per IS: 1208 - 1978.

The ductility value affected by factors such as test temperature, pouring temperature rate of pulling etc. A minimum ductility value of 75 cm has been definite by the BIS.


Part B (Centrifuge Extraction Test)The following formula was used to determine the percentage of bitumen in bituminous mix:

Percentage of bitumen =

Where,

A= weight of sample before segregation

B= weight of sample after segregation

C= weight of filter paper

Apparatus

Part A (Ductility Test)
1) Standard mold

2) Water bath
3) Testing machine
4) Thermometer – Range 0 to 44^oC, Graduation 0.2^oC

Part B (Centrifuge Extraction Test)

1. A sample of asphalt
2. Benzene
3. Filter paper
4. Centrifuge Extractor with the following apparatus: 

a) Removable aluminum rotor bowl and have capacity of 1500 gm

b) This shaft and the bowl is manually rotated fast by the closed gears in the cast body and handle

c) Solvent is introduced during the test through the holes in the cap of the housing

d) The drain was used in order to collect the dissolved bitumen thats coming out of the rotating bowl.

e) Shafts rotated by an electric motor and gear (substantial care is taken to prevent solvent entering into the rotor of electrical motor)

f) Operates on 230 Volts A.C. single phase. 

Procedure

Part A (Ductility Test)

1) The bitumen sample was heated and poured out into the mold. These samples with molds were cooled in the air and then cooled another time in water bath at 27o C temperature.

2)  The excess bitumen was cut and the surface was leveled using a hot knife.

3) The assembly which contains the sample was kept in water bath of the ductility machine for like 90 minutes.

4) The sides of the molds were removed, the clips were hooked on the machine and the machine was operated.

5) The distance was reported up to the point of breaking of thread is the ductility value in cm.

Part B (Centrifuge Extraction Test)

1) A sample of 575gm of asphalt was taken

2) The sample of was placed in the centrifuge extractor. It was covered with benzene and the filter paper was put on it with the cover plate to fit on the bowl.

3) The centrifuge extractor was started and the speed was gradually increased until the solvent ceases flow from the outlet.

4) The centrifuge extractor was stopped. Benzene was added again and the procedure was repeated.

5) The procedure was repeated so that the extract was clear and not darker than the light straw color and the volume of total extract in the graduated vessel was recorded.

6) The filter paper was removed from the bowl and dried in the oven at 110 + 5^oC.

7) After 24hours, the weight of the extracted sample was taken.

 

Data and Calculations

Part A (Ductility test)

Table-1: The maximum distance of bitumen samples before separating under tension.

Sample No.	Distance (cm)
1	3.4
2	3.3
3	3.2

Avg. Distance =

Part B (Centrifuge extraction test)

Table-2

The Weight of the sample before segregation (gm)	515.4
The Weight of the sample after segregation (gm)	476.9
The Weight of the filter paper (gm)	20.4
The weight of bitumen in the sample (gm)	38.5
The percentage of bitumen in the sample	7.47%

The Temperature of water: 25͒ C

Percentage of the bitumen =

Where,

W₁= weight of sample before segregation (gm)

W₂= weight of sample after segregation (gm)

 

Results and Conclusion

Part A (Ductility test):

Table (1) shows the results obtained for the ductility of a sample of bitumen.

We notice that the three determinations are within ± 0.5 percent of their mean value, so we conclude that our test is successful.

Sources of errors:

The distance reading might not be taken at the moment when the sample thread had broken.

Part B (Centrifuge extraction test):

Table (2) shows the results obtained for the percentage of bitumen in a sample of bituminous mix.

The percentage of bitumen in the asphalt is usually between 4-8%, so our percentage (7.47%) is accepted.

Sources of errors:

1.      Incomplete washing of the asphalt from the mix.

2.      Non-consistent torques applied to the bowl cover plate.

3.      Loss of mineral aggregate particles when transferring from extractor bowl and filter papers to pan.

4.      Overheating the aggregate during drying.

Engineering use

The primary use of asphalt is in roads and highway construction, where it is can be considered as the glue or binder mixed with aggregate particles to create asphalt concrete. Its other main applications are for bituminous waterproofing products, including production and isolation (water isolation) of roofing felt and for sealing flat roofs.

In flexible pavement construction, bitumen binders are used. It is important that the aggregates be covered by bituminous material which forms ductile thin film around it to serve as a binder. It is important to do the ductility tests on bituminous material the binder material not of appropriate ductility renders pervious pavement surface and leads to cracks in the pavement.