The bitumen is available in a variety of types and grades. The various tests have been developed to judge its suitability as a binding material.
Following are the usual physical tests of the bituminous materials:
(1) Ductility Test:
In order to determine the ductility of bitumen, the sample is cast in standard briquette mould whose cross-section at the minimum width is 10 mm x 10 mm.
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The ductility value is expressed as the distance in centimetres to which a standard briquette of bitumen can be stretched before the thread breaks, as shown in fig. 17-1. The test is conducted at a temperature of 27°C ± 0.5°C at a rate of pull of 50 mm ± 2.5 mm per minute.
The ductility values of bitumen may vary from 5 to over 100 for different grades of bitumen. For satisfactory performance, the desirable ductility value of bitumen should not be less than 50.
(2) Flash and Fire Point Test:
The terms flash point and fire point are defined by the B.I.S. as follows:
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(i) 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 test.
(ii) The fire point is the lowest temperature at which the material gets ignited and burns under specified conditions of test.
It is found that different bituminous materials have quite different values of flash and fire points. It should be seen that when bituminous material is heated before mixing or application, the temperature remains well below the flash point.
If this precaution is not taken, it may lead to serious fire hazards during the heating of bituminous material for use in road or in shipping them during transport.
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The test is carried out by Pensky-Martens Closed Tester and the minimum value of flash point specified by the B.I.S. for all the grades of bitumen by this test is 175°C.
(3) Float Test:
This test is carried out to define the consistency of the bituminous material and it is helpful when the material has the consistency range which cannot be effectively defined either by orifice viscometer test or penetration test. The test is carried out by floating an apparatus as shown in fig. 17-2.
It consists of a float made of aluminium and a brass collar filled with the specimen material to be tested and it is screwed to the float. The material is cooled to a temperature of 5°C and the entire assembly floated in a water bath having temperature of 50°C.
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The time required in seconds for water to force its way through the bitumen plug is noted and it indicates the float value. The higher the float value, the stiffer is the material.
(4) Loss on Heating Test:
When bitumen is heated, the volatiles present in it get evaporated and the bitumen becomes hard.
The test is carried out as follows:
(i) The sample of about 50 gm is taken and it is weighed.
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(ii) It is heated for 5 hours at a temperature of 163°C in a special oven.
(iii) The specimen is taken out and it is weighed again.
(iv) The loss in weight due to heating is expressed as a percentage by weight of original sample.
The bitumen to be used in pavement mix should not indicate more than one per cent loss in weight on heating.
(5) Penetration Test:
The penetration test is carried out to know the hardness or softness of bitumen used in road construction by measuring the distance in tenths of millimetre to which a standard needle will penetrate vertically into a sample of bitumen under the stipulated conditions of temperature, loading and time. Fig. 17-3 shows the concept of penetration test.
The test is carried out with a standard penetrometer as shown in fig. 17-4 with a needle loaded with 100 gm weight. The sample to be tested is maintained at a constant temperature of 25°C and at the start of the test, the needle is kept in such a way that its point just touches the surface of sample.
The needle is then released and is allowed to penetrate into the sample under the full action of the load. The penetration at the end of 5 seconds is measured in tenths of millimetre which gives the penetration number of the sample.
The penetration test is almost applied exclusively for bitumen and it is not intended for softer materials like tar or cutback which are usually Standard penetrometer graded by the viscosity test.
The bitumen grade is specified in terms of penetration value. For instance, the bitumen of grade 80/100 means that the range of penetration value of the material is between 80 and 100. Depending upon the climatic conditions and type of construction, bitumens of different grades are used.
The I.R.C. has recommended bitumen grades 30/40, 60/70 and 80/100 for the bituminous macadam and penetration macadam. In general, it can be stated that for hot climates, lower penetration grades of bitumen are preferred and for cold climates, higher penetration grades of bitumen are preferred.
(6) Softening Point Test:
There is no particular instant or definite temperature at which a bituminous material softens. As the temperature of the material rises, there is a gradual and imperceptible change from a brittle or exceedingly thick and slow-flowing condition to a softer and less viscous state. Hence, in order to determine the softening point, resort is made to an arbitrary test.
The softening point of bitumen is usually determined in the laboratory by the ring and ball test and the test procedure in its simplest form is as follows:
(i) The material is melted and stirred thoroughly to avoid the incorporation of air bubbles in the mass. It is then poured in a hollow brass ring placed on a brass plate. The depth of ring is 6.4 mm and it’s inside top and bottom diameters are respectively 17.5 mm and 15.9 mm. The outside diameter of the ring is 20.6 mm.
(ii) The ring is filled with enough material so as to leave an excess material on cooling. In order to avoid the sticking of the bitumen to metal plate, the coating of a solution of glycerine and dextrine is applied.
(iii) After the material has cooled, the portion above the top of the ring is cut off cleanly with a slightly heated putty knife or spatula.
(iv) The apparatus is in the form of a heat resistant glass container of 85 mm diameter and 120 mm depth. It also contains a frame for supporting the thermometer and the ring in the proper positions during the test. It is filled with freshly boiled and cooled distilled water at a temperature of 5°C for depth of 82.5 mm.
(v) The ring containing the bituminous material is lowered in the glass vessel so that its lower surface is 25 mm above the bottom of the vessel. A steel ball of 9.5 mm diameter and weighing 2.5 gm is picked up by means of forceps and placed in the centre of the upper surface of the test sample in the ring.
(vi) The heat is then applied in such a way that the temperature of water is raised at the uniform rate of 5°C per minute.
(vii) The material in the ring softens and sags under the weight of the ball and the temperature at which it touches the bottom of the vessel is taken as the softening point.
If desired, more than one ring can be put in the vessel at the same time and an average value of the softening point can be obtained. The liquid in vessel is water for materials having softening point below 80°C and for materials having softening point more than 80°C, the glycerine is used instead of water. The range of softening point of various bitumen grades used in road work is from 35°C to 70°C.
(7) Solubility Test:
The pure bitumen is completely soluble in solvents like carbon disulphide or carbon tetrachloride. Hence, the solubility test is carried out to know the amount of pure bitumen in a sample of bitumen.
A weighed sample of the material is dissolved in an excess quantity of the solvent. The solution is then filtered for the removal of any insoluble material which may be present. This insoluble material is then washed, dried and weighed. It is expressed as a percentage of the original weight of sample and it should preferably be less than one per cent.
(8) Specific Gravity Test:
The specific gravity of bitumen is defined by the I.S.I, as the ratio of the mass of a given volume of the substance to the mass of an equal volume of water, the temperature of both being specified as 27°C ± 0.1°C.
The specific gravity is determined by the following two methods:
(i) Pycnometer method
(ii) Balance method.
(i) Pycnometer Method:
A pycnometer is simply a glass flask, known as specific gravity bottle. The test is carried out by weighing the empty bottle, bottle filled with distilled water, bottle filled with the material upto half depth and bottle filled with material and distilled water in equal heights. The specific gravity is then worked out by the following expression –
(ii) Balance Method:
In this method, the specimen is weighed in air and then in water. The specific gravity is calculated by the following expression –
The specific gravity of bitumen assists in classifying the material for use in paving jobs and for designing the bituminous mix. The specific gravity of pure bitumen is usually in the range of 1.01 to 1.03. For cutback bitumen, it is lower and depends on the type and proportion of diluent used. For tars, the specific gravity ranges from 1.10 to 1.25.
(9) Spot Test:
This test is made for detecting overheated or cracked bitumen.
The test is carried out as follows:
(i) About 2 gm of bitumen is dissolved in 10 ml of naphtha.
(ii) A drop of solution is taken out after one hour and it is placed on a filter paper.
(iii) A second drop of solution is taken out after 24 hours and it is placed on a filter paper.
(iv) If the stains of the spots on the paper are of uniform colour, the bitumen is treated as un-cracked. On the other hand, if the spots form dark brown or black circle in the centre with an annular ring of lighter colour, the bitumen is considered to be overheated or cracked.
(10) Viscosity Test:
The resistance to flow by liquid is known as viscosity and it is thus the inverse of fluidity. At high fluidity, the viscosity is low and at low fluidity, the viscosity is high.
The degree of fluidity of binder plays a great role in deter-mining the strength characteristics of the resulting paving mixer. If the viscosity is high, the compaction is poor and resulting mix is heterogeneous in character.
On the other hand, if the viscosity is low, the bituminous binder simply lubricates the aggregate particles instead of providing a uniform film for the binding action. Thus for each aggregate gradation of the mix and bitumen grade, there is an optimum value of viscosity.
The viscosity at any specified temperature is measured by recording the time in seconds for a given quantity of the product at the same temperature to flow through an orifice of standard dimensions into a receiver.
The standard test temperatures have been specified for the various grades of cutback and tar. The higher the viscosity of binder, the more time will be recorded and the lower the viscosity of binder, the less time will be recorded.
(11) Water Content Test:
If bitumen contains water, there will be foaming of bitumen when it is heated above the boiling point of water.
This test is carried out to know the content of water in a sample of bitumen and the test procedure is as follows:
(i) The specimen of known weight is mixed in a pure petroleum distillate free from water.
(ii) It is then heated and the water is distilled off.
(iii) The condensed water is collected and its weight is recorded.
(iv) The water content of the specimen is expressed as a percentage of the weight of condensed water to the weight of the original sample.
The maximum water content in bitumen should not exceed 0.2 per cent by weight.