Top 8 test on bitumen for Pavement Construction
The common tests to asses the the properties and requirements of paving grade bitumen are the viscosity tests, penetration test, ductility test and the softening point test. Also specific gravity test and flash and fire point test are need for use in paving applications. Additional tests like the matter soluble in carbon disulfide, loss on heating and penetration test on residue may also be caring out.
Earlier the classification of bitumen was base on the penetration ad ductility test results. It was later observing that bitumen from different sources possessing same penetration value at a specifying temperature may exhibit entirely different viscosity characteristics Hence different temperature susceptibility characteristics at the applications and service temperatures. Therefore it is important to determine the viscosity property of the binder in terms of ‘absolute viscosity’ and ‘kinematic viscosity’ test results.
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Types of Tests on Bitumen
- Penetration test
- Viscosity tests
- Ductility test
- Softening point test
- Specific gravity test
- Flash and Fire point tests
- Loss on heating test
- Solubility test
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1. Penetration test on bitumen
The consistency of bitumen materials varies depending upon several factors such as constituents, temperature, etc. At temperature ranges between 25 and 50 degree celsius. Most of the paving bitumen grades remain in semi solid or in plastic state. Determination of absolute viscosity of bituminous materials is not so simple. Therefore the consistency of these materials is determining by indirect methods. Penetration test is one such indirect test to determine the consistency of paving grade bitumen, which is a very simple test.
- First of all, Sample of bitumen is softened to a pouring consistency, stirring thoroughly.
- Then, Pour into containers to a depth at least 15 mm in excess of the expecting penetration.
- The sample containers are then placing in a temperature control water bath at a temperature of 25 degree celsius for one hours.
- The sample with container is taking out.
- Place under Penetrometer.
- The needle is adjusting to make contact with the surface of the sample.
- The dial is set to zero or the initial reading is taking out.
- The needle is release for 5 seconds.
- Hence, The final reading is taking on dial gauge.
At least three penetration tests are make on this sample by testing at distances at least 10 mm apart. After each test, the needle is disengaged and wiped with benzene and dried. The depth of penetration is reporting in one-tenth mm units. The mean value of three measurements is reporting as a penetration value. It may be note that the penetration value is likely influence by any inaccuracy as regarding pouring temperature, size of needle, weight placing on the needle and the test temperature.
2. Viscosity Test on bitumen
Viscosity of a liquid is the property that retards its flow due to internal friction. It is a measure of resistance to flow of the liquid. The flow of liquid under an applying force will depend on its viscosity. Higher the viscosity, slower will be its movement or rate of flow.
Viscosity of liquid bituminous binders like bitumen emulsion and tar are determining by indirect method using orifice type viscometers. A specifying quantity of the binder is allowing to flow through specifying orifice size of the test-cup at a given temperature. The time taken in seconds is recording as the viscosity value. As per specifications of BIS, the viscosity values of bitumen emulsions are determine using ‘Saybolt Furol’ orifice viscometer at test temperature of 25 degree celsius and 50 degree celsius. The viscosity values of tar are determine using orifice viscometer termed as ‘Tar Viscometer’ using either 10 mm or 4 mm size orifice.
3. Ductility test on bitumen
In the flexible pavement constructions where bitumen binders are using, it is important that the binders from ductile thin films around the aggregates. The ductile film of binder improves the physical interlocking of the aggregate bitumen mixes. Under traffic loads, the bituminous pavement layer is subjecting to repeat deformation and recoveries. The binder material which does not possess sufficient ductility would crack and permit the surface water to enter into the pavement resulting in rapid deterioration and failure. Ductility test is carrying out on bitumen to test the adhesive property of bitumen and its ability to stretch. The bitumen may satisfy the penetration value, but may fail to satisfy the ductility requirements.
- The ductility value is expressing as the distance in centimetre (cm).
- In which the bitumen specimen of standard size can be stretch before the tread breaks.
- The standard briquette specimen has a minimum cross section 10 mm x 10 mm.
- The test is conducting at 27 degree celsius with a rate of pull of 50 mm per minute, until the stretch specimen breaks.
- The ductility machine functions as a constant temperature water bath with a pulling device at a Pre-calibrating rate.
- Two clips are thus pull apart horizontally at a uniform speed of 50 mm per minute.
- The bitumen sample is heat and pour in the mould assembly place on a plate.
- The samples along with the moulds are cooled in air.
- Then in water bath maintaining at 27 degree celsius.
- The excess bitumen material is trim.
- The surface is levelling using a hot knife.
- The mould assembly containing sample is replaced in water bath of the ductility testing machine for 85 to 95 minute.
- The sides of the mould are remove.
- The clips hook on to the machine and the pointer is adjusting to zero.
- The distance up to the point of breaking of thread is report as ductility value, in cm.
The ductility value gets seriously affecting by factors such as pouring temperature, dimensions of briquette, level of briquette in the water bath. Also, a presence of air pockets in the specimen briquettes, test temperature and rate of pulling.
The ductility values of bitumen generally vary from 5 to over 100 for different bitumen grades. A minimum ductility value of 50 to 75 cm is generally specifying for bitumen using in pavement construction.
4. Softening Point test on bitumen
In this, temperature at which the substance attains a particular degree of softening under specifying condition of test. It is usually determining by Ring and Ball test.
Generally higher softening point indicates lower temperature susceptibility and is preferring in warm climates.
- A brass ring containing test sample of bitumen is suspending in a breaker with liquid bath.
- Water is use as the bath if the softening point is less than 80 degree celsius.
- Glycerine is use for temperatures exceeding 80 degree celsius.
- A steel ball is place upon the bitumen sample.
- The liquid medium is then heat at a rate of 5 degree celsius per minute.
- The temperature at which the softened bitumen touches the metal plate place at a specify distance below the ring.
- It is record as the softening point of the bitumen.
- Harder grades of bitumen possess higher softening point than soft grade bitumen.
The softening point of various bitumen grades use in paving jobs vary between 35 degree to 70 degree celsius.
5. Specific gravity test
The specific gravity of a bitumen binder is a fundamental property. It is frequently use as an aid to classify the binders for use in paving jobs. In most applications, the bitumen is weighing, but finally when use with aggregate system, the bitumen content is convert on volume basis using density values. The density of bitumen is influence by its chemical composition. Increase amounts of aromatic type compounds or mineral impurities cause an increase in specific gravity.
The specific gravity of bituminous binder is termed as the ratio of the mass of a given volume of the binder to the mass of an equal volume of water, the temperature of both being at 27 degree celsius. The specific gravity is determining either by using a pyknometer. Then, preparing a specimen of cube shape in semi solid or solid state and by weighing in air and water. The specific gravity is obtaining by dividing the weight of the bitumen by weight of equal volume of water.
Generally the specific gravity of pure bitumen is in the range of 0.97 to 1.02. The specific gravity of cutback bitumen may be lower depending on the type and proportion of diluent use. Tars have specific gravity ranging from 1.10 to 1.25.
6. Flash and Fire point tests on bitumen
When a bituminous binder is heat continuously, above a certain temperature it starts emitting volatile vapours and these volatile vapours can momentarily catch fire causing a flash, though the binder itself does not catch fire and burn at this temperature. The temperature at which such behaviour occurs is found to differ for different types and grades of bituminous binders. This condition is very hazardous and therefore it is essential to determine the temperature at which the flash of fire can occur in each type and grade of bituminous binder.
Flash point test gives an indication of the critical temperature at and above which suitable precautions should be taken while heating the binder. In order to eliminate fire hazards during heating, mixing or application, the paving engineers should restrict the mixing and application temperatures well below this temperature.
The ‘flash point’ of a bituminous binder is defined as the lowest temperature at which application of a test flame causes the vapours of the binder to catch an instant fire in the form of flash under specified test conditions. Two types of test apparatus may be used for conducting flash point test on bitumen, namely the Pensky-Martens Closed Cup Tester and Open Cup Tester.
If the bituminous binder is further heat to a temperature higher than the flash point, the binder material itself catches fire and continues to burn, the lowest temperature causing this condition is termed as the ‘fire point’.
The fire point is always higher than the flash point of a material. The fire point is termed as the lowest temperature at which application of a test flame causes the binder material to ignite. And Burn at least 5 seconds under specifying test conditions. Pensky-Martens Open Cup Tester is made use of to determine the fire point of the bituminous binders.
7. Loss on heating test on bitumen
When bitumen is heat, it loses the volatiles and get hardened. To study the effect of heating, an accelerating heating procedure is adopting.
- About 50 grams of the sample is weighing.
- Then, heat to a temperature of 163 degree celsius for 5 hours in a special oven designed for this test.
- After the heating period, this specimen is cool and weighing again.
- Hence, The loss in weight is expressing as a percentage by weight of original sample.
Bitumen use in pavement mixes should not indicate more than one percent loss in weight. The penetration values of bitumen is 150-200 up to two percent loss in weight is allow. The residue after heating when subjecting to penetration test shows a reduction in penetration value. The reduction in penetration value should be less than 40% of the original penetration value of the bitumen.
8. Solubility test on bitumen
Pure bitumen is completely soluble in solvents like carbon disulphide and carbon tetrachloride. Hence any impurity in bitumen in the form of inert minerals, carbon, salts etc. It could be quantitatively analysing by dissolving the samples of bitumen in any of the two solvents.
- A sample of about 2.0 g of bitumen is dissolve in about 100 ml of solvent.
- The solution is filter and the insoluble material retaining is washing, drying and weighing.
- The insoluble material is expressing as a percentage by weight of original sample.
- Therefore, Insoluble material should be preferably less than 1%.
- In solubility test with carbon tetrachloride, if black carbonaceous residue is over 0.5%. The bitumen is considering to be ‘cracked’.
- Hence, The minimum proportion of bitumen soluble in carbon disulphide is specifying as 99%.