Tests on Bitumen:

Bitumen as already discussed is available in various types and grades. I.S.I. and other agencies have suggested a number of tests to determine the behaviour and suitability of bitumens and cutbacks. These tests include penetration test , ductility test , softening point test and viscosity test. For classifying the bitumen and studying the performance of bituminous pavements the ductility and penetration tests are essential. The other tests like softening point and flash and fire point tests are more important to guide the paving technologists during field operations.

8.6.1. Penetration Test:

The penetration test determines the hardness or softness of bitumen by measuring the depth in tenths of a millimetre to which a standard loaded needle will penetrate vertically in five seconds. The sample is maintained at a temperature of 25⁰ C. The needle assembly weights 100 gms. The concept of penetration test is shown in fig 8.3. The mean value of three measurements is reported as penetration value. It may be noted that the penetration value is largely influenced by any inaccuracy as regards pouring temperature , size of needle , weight placed on the needle and the test temperature.

The bitumen grade is specified in terms of the penetration value. 80 - 100 or 80 / 100 grade bitumen means the penetration value of the bitumen is in the range of 80-100 at

standard test conditions. The penetration test is applied almost exclusively to bitumen. As road tars are soft , the test cannot be carried out on these materials. All other tests are used for tars , cut-backs and emulsions.

Fig 8.3. Concept of Penetration Test

The penetration values of various types of bitumens used in pavement construction in this country range between 20 and 225. 30 / 40 and 80 / 100 grade bitumen are more commonly used , depending on the construction type and climatic conditions. In hot climates a lower penetration grade bitumen like 30 / 40 bitumen is preferred.

8.6.2. Ductility Test:

The bituminous binder forms a ductile thin film around the aggregates. It improves the physical inter locking of aggregates. Under traffic loads the bituminous pavement layer is subjected to repeated deformations and recoveries. The binder material which does not possess sufficient ductility would crack and thus provide pervious pavement surface.

Ductility is carried out on bitumen to test this property of the binder. The test is believed to measure the elasticity and adhesiveness of the bitumen.

The ductility is expressed as the distance in centimetres to which a standard briquette of bitumen can be stretched before the thread breaks. The test is conducted at 27⁰C and a rate of pull of 50 mm per minute. The test set up is shown in Fig 8.4. The cross-section at the minimum width of the specimen is 10mm x 10mm.

The ductility machine functions as a constant temperature water bath and a pulling device at a pre-calibrated rate. Two clips are thus pulled apart horizontally at a uniform speed of 50mm per minute.

The ductility value gets seriously affected by factors such as pouring temperature , dimensions of the briquette , level of briquette in the water bath , presence of air pockets in the moulded briquettes , test temperature and rate of pulling.

The ductility values of bitumen vary from 5 to over 100 for different bitumen grades.

It is desirable that the ductility value of bitumen is not less than 50 , for satisfactory performance.

8.6.3. Viscosity Test:

Viscosity is inverse of fluidity. It is a measure of resistance to flow. Furol viscosity test is conducted on liquid bitumens (cut-backs , Tars and Emulsions). In this test , time in seconds is noted for 50ml. of the liquid bitumen at specified temperature to flow through an orifice of specified size. Higher the viscosity of the liquid , more will be the time required to flow out. For tar tests , orifice of 10mm is specified and is called tar visco meter.

Fig 8.4. Ductility Test

Fig 8.5. Viscosity Test

Degree of fluidity of bitumen at the temperature of its use greatly influences the strength and durability of the road pavements. For each aggregate gradation of the mix and

bitumen grade , there is an optimum value of viscosity. Bituminous binders of low viscosity simply lubricate the aggregate particles instead of providing a uniform thin film for binding action. Similarly high viscosity does not allow full compaction and resulting mix exhibits heterogeneous character and thus low stability values.

8.6.4. Float Test:

Bitumens which are more viscous cannot be tested either by Fural viscosity test or by penetration test. Such bituminous materials can be tested for consistency by float test.

Fig 8.6. Float Test

The apparatus used for float-test is a float made of aluminium. At the bottom of the float there is hole in which a brass collar can be screwed. The specimen of bitumen to be tested is filled in the collar cooled at 5⁰ C temperature and is then screwed to the float.

Completed float assembly is then floated in a water bath containing water. The temperature of water bath is maintained at 50⁰ C. The time required is seconds for water to force its way through the bitumen plug is noted. The higher the float value , the stiffer is the bitumen.

8.6.5. Softening Point Test:

The softening point is the temperature at which the substance attains a particular degree of softening under specified conditions. It is usually determined by the ‘Ring and Ball’ apparatus. Generally higher softening point indicates lower temperature susceptibility and is preferred in warm climates.

A brass ring containing the test sample of bitumen is suspended in liquid like water or glycerine at a given temperature. A steel ball is placed upon the bitumen and liquid medium is then heated at a rate of 5⁰ C per minute. The temperature at which the softened bitumen touches the metal frame placed at a specified distance below the ring is recorded as the softening point of a bitumen . Hard grade bitumens possess higher softening point than soft grade bitumens.

The softening point of various bitumen grades used in pavings jobs vary between 35⁰C to 70⁰C.

8.6.6. Flash point and Fire point Test:

This test give an indication of the critical temperature at and above which suitable precautions should be taken to eliminate fire hazards during heating of bitumen. The definition of flash and fire points as given by ISI are as follows:

Fig 8.7. Softening Point Test

Flash Point: The flash point of a material is the lowest temperature at which the vapour of a substance momentarily takes fire in the form of a flash under specified conditions of the test.

Fire Point: The fire point is the lowest temperature at which the material gets ignited and burns under specified conditions of the test.

Flash point test is carried out by Pensky - Martens closed cup or open cup apparatus.

The bituminous material , to be tested is filled in the cup upto the filling mark. The cover of lid should be put to close the cup in close cup system. Thermometer and other attachments are also suitably fixed. Sample is then heated at a rate of 5⁰C per minute. During heating , specimen is continuously kept stirred. When temperature of the specimen reaches about 15⁰C Less than the expected flash point , test flame is applied. The test flame is applied again and again at intervals of 3⁰C rise in temperature till the test flame causes a bright flash in the interior of the cup of a closed system.

The temperature at this instance is noted which is taken as ‘FLASH POINT’ . In the case of open cup system , flash may appear at any point on the surface of the specimen.

The material is further continued to be heated till the material gets ignited and continues to burn from five seconds. The temperature recorded is called the ‘FIRE POINT’.

For bituminous materials used in road pavements , the closed cup flash point should not be less than 175⁰C.

Fig 8.8. Flash and Fire point test 8.6.7. Specific Gravity Test:

The density of a bitumen binder is a fundamental property used as an aid in classifying the binders for use in paving jobs. In most applications , the bitumen is weighted out , finally when used with aggregate system. The bitumen content is converted into volume.

The specific gravity value of bitumen is also useful in bituminous mix design. Density of the binder is greatly influenced by the chemical composition. Increased amount of mineral impurities in the binder increase its specific gravity.

Specific gravity of a binder is the ratio of its weight at 27⁰ C temperature to the weight of an equal volume of water at the same temperature. The specific gravity is determined by standard displacement method for solid bitumen , by hydrometers for liquid bitumen and by specific gravity bottle for intermediate grade bitumen.

8.6.8. Solubility Test:

Pure bitumen is fully soluble in carbon-di-sulphide and carbon tetra chloride. If some quantity remains undissolved , it exhibits the quantity of inert materials present in the bitumen.

Dissolve 2 gms of bitumen in 100 cc of carbon-di-sulphide or tetra chloride. The solution is filtered and insoluble materials retained are washed and weighed and expressed as

a percentage of original sample. This percentage should not exceed by one. When solvent used is carbon tetra chloride and if black carbonaceous residue left is over 0.5% the bitumen is considered as CRACKED. The minimum proportion of bitumen soluble in Carbon - di-sulphide is specified as 99 percent.