Bitumen is used in road construction because of its varied properties and benefits it’s over alternative pavement construction materials. Bitumen gains sure distinctive properties that are integral in it throughout its manufacture. The hydrocarbon as a staple in versatile road construction and bitumen as a combination (composing other materials i.e. aggregates/ pozzolans) serves certain advantages, that prompt to use of bitumen wide in road construction.
- Bitumen is a petroleum product obtained by distillation of petroleum crude whereas Tar is obtained by destructive distillation of coal or wood.
- Both are similar in appearance, black in color though they have different characteristics, these materials can be used for pavement works.
- Tar is more temperature susceptible resulting in great variation in viscosity with temperature.
- The free carbon content is more in tar
- General requirements are Adequate Viscosity at the time of mixing, low susceptibility to temperature, and it should not strip off from aggregate.
TESTS ON BITUMEN
A) Penetration Test
It determines the hardness or softness of bitumen. Penetration value test on bitumen could be a measure of hardness or consistency of bituminous material. An 80/100-grade bitumen indicates that its penetration value lies between 80 & 100. Penetration value is that the vertical distance traversed or penetrated by the purpose of a customary needle into the bituminous material below specific conditions of load, time, and temperature. This distance is measured in one-tenth of a millimeter. A penetration test is employed for evaluating the consistency of bitumen. it’s not considered suitable to be used in connection with the testing of road tar as a result of the high surface tension exhibited by these materials.
- The sample is maintained at a temperature of 25 degrees.
- The penetrometer has the graduated dial to read penetration value to 1/10th of a millimeter
- Bitumen Grade is specified in term of penetration value 80-100 or 80/100-grade bitumen means that the penetration value of bitumen is in the range of 8 to 10 mm
- In hot climates lower penetration grade bitumen like 30/40 is preferred
a) Ductility Test
A ductility test is conducted to determine the number bitumen can stretch at a temperature below its softening point. A block having a cross-sectional space of one in2 is placed during a tester at 77 °F. plasticity values range from zero to over 150 depending on the kind of bitumen. The presence of ductility means that the formation of the film and coating would be proper.
- Ductility is expressed as the distance in centimeters to which a standard briquette of bitumen can be stretched before the thread breaks.
- The test is conducted at 37 degrees and at a rate of the pull of 50 mm per minute
- ISI has recommended a minimum ductility value of 75 cm for a grade of 45 and above
- The ductility value of bitumen vary from 5 to 100 for different grade of bitumen
B) Viscosity Test
- Viscosity test is the measure of resistance to flow
- Orifice type viscometer is commonly used
- Viscosity is measured by determining the time taken by 50 ml of the material to flow from a cup through test conditions
C) Float Test
- Consistency of the materials for which penetration test and viscometer are not suitable is measured by float test.
- The higher the float test value, the stiffer is the material.
- The test specimen is first cooled to a temp. of 5 degrees. The float assembly is floated in a water bath at 50 degrees and the time required in seconds for the water to force its way through the bitumen plug is noted as the float test value.
D) Specific Gravity Test
- The specific gravity of pure bitumen is in the range of 0.97 to 1.02 whereas Tars have value ranging from 1.1 to 1.25
E) Softening Point Test
- Higher softening point indicates lower temperature susceptibility and is preferred in a warm climate
- Ring and ball test is used to find out the softening of bitumen
- A steel ball is placed upon the bitumen sample in the ring which is kept in a liquid medium and then the entire assembly is heated at a rate of 5 degrees per minute. The temperature at which the ball passes the ring is noted
- The softening point of various bitumen grade used in paving jobs vary between 35 to 70 degree
F) Flash and Fire Point Test
- Flash Point- it is the lowest temperature at which the vapor of a substance momentarily takes fire in the form of a flash under the specified condition of the test
- Fire Point – it is the lowest temperature at which the material gets ignited and burns under the specified condition of the test
- Pensky Marten’s closed cup apparatus is used to conduct the test
- The minimum specified flashpoint of bitumen used in the pavement in Pensky Marten’s closed type test is a 175 degree
G) Solubility Test
- Pure bitumen is completely soluble in solvents like carbon disulphate and CCl4
- The insoluble material should be preferably less than 1%
H) Water Content Test
- The maximum water content in bitumen should not exceed 0.2% by weight
Grades of Tar
It has the lowest viscosity and used for surface painting under exceptionally cold weather
Recommended for standard surface painting under normal Indian climate condition
Used for surface painting renewal coats and premixing chips for the top course and light carpets
Used for premixing tarmacadam is base course for grouting purpose
It has the highest viscosity among the road other road tars may be adopted for grouting purpose