The following tests are conducted on tar to check or assess its suitability or quality for specific uses in road construction:
1. Specific Gravity:
For determining the specific gravity of tar, the same procedure as for bitumen using a pyknometer is applicable.
A distillation flask of about one litre capacity is filled with a weighed quantity of tar for about 1/2 to 2/3 capacity. It is then subjected to the process of distillation. A thermometer of range 80°C to 330°C with a precision of 1°C is fitted to the flask.
The products of distillation up to 300°C shall be condensed by cooling with a cold condenser separately fractioned in the following temperature ranges- below 170°C, 170°C to 270°C, 270°C to 300°C. Above 300°C, the condenser tube without the cold water jacket shall be used.
The distillate below 170°C shall be collected and weighed. The volume of water or ammoniacal liquor shall be read in the cylinder, and their percentages by weight shall be calculated, assuming 1 ml weighs 1g. These are called light oils. Similarly, the fraction between 170°C and 270°C is separated; this is kept separately for the determination of phenols and naphthalene. This fraction constitutes middle oils.
The fraction between 270°C and 300°C constitutes heavy oils. The fraction above 300°C is the residual pitch. The combined weights of all these fractions should not be less than 99% of the weight of tar taken.
Crude tar acids or phenols are determined by bringing the whole of the distillate between 170°C and 270°C to a temperature of 40°C to 50°C, adding about 20% of caustic soda (sp. gr. 1.20), shaking vigorously every 5 minutes. The mixture is poured into a tap-funnel, the caustic soda solution which settles being run off into a measuring cylinder.
This is repeated with the same amount of fresh caustic soda that is run off into the measuring cylinder containing the first extract. Gradual addition of hydrochloric acid to the measuring cylinder liberates phenols which are read off and their percentage by volume of the tar taken is calculated.
The distillate between 170°C and 270°C after separation of phenol, is weighed, warmed and agitated, and a small part weighing about 25 g is taken. This is cooled to 15°C and kept for half-an-hour. The settled naphthalene is filtered off with the aid of a pump and pressed between the folds of a filter paper until all the oil has been removed by the filter paper. The collected naphthalene is then weighed and its percentage by weight of the tar taken is calculated.
5. Free Carbon:
Matter in the tar which is insoluble in benzol of the commercial 90% grade is reported as free carbon (provided that not more than 10% of this insoluble matter is retained on a sieve with 32 holes in a linear centimetre).
Two grams of the tar is mixed with cold benzol and after the free carbon has settled, the benzol is decanted through a balanced pair of filter papers. After several washings with benzol on decantation, the free carbon is passed on the filter paper and washed with 500 ml of hot benzol. (The total quantity of benzol used in the extraction is restricted to 1 litre.) The filter paper is dried and the free carbon weighed. (Care should be exercised to prevent occurrence of fire since benzol is inflammable.)
Viscosity is the time taken by the Hutchinson’s tar tester poise (or Viscosity Gauge No. 2) to sink from the lower to the upper ring of the gauge when placed in tar at a temperature of 25°C in a cylindrical vessel of 100 mm internal diameter (Fig. 6.73).
Alternatively, the standard tar viscometer, already dealt with in the previous section while discussing the tests for viscosity of bituminous materials, may be used for determining the viscosity of tar.
7. Equi-Viscous Temperature:
In all the methods of determining viscosity of a bituminous material, there are two variables, i.e., time and temperature.
The equi-viscous temperature (EVT) is defined as the temperature at which bituminous material of 50 ml volume flows through the orifice of the standard tar viscometer in 50 seconds.
This concept of equi-viscous temperature is used only for tars. Since temperature susceptibility of all tars is similar, this property is made use of in testing tar viscosity (it is not so for bitumens). Thus, all tars from very fluid consistency to hard pitches can be represented on a single scale; this is because the EVT of a mixture of two components can be got from the EVT of the components by simple proportioning.
The EVT may be measured using an ‘EVT Viscometer’, as shown in Fig. 6.74.
The apparatus consists of two coaxial cylinders between which 10 g of tar can be contained. The outer cylinder is fixed, and the inner one is suspended on a torsion wire through a flywheel. The cylinders with tar between them are immersed in a bath, the temperature of which can be raised gradually. The torsion head is given a twist of 180° due to which the flywheel will also swing, the amount of swing depending on the viscosity of tar.
As the temperature of the bath is raised gradually, at a particular temperature, the inertia will carry the wheel 45° beyond 180°. The temperature at which this occurs is designated the equi-viscous temperature (EVT). It is apparent that at EVT, all the tars have the same viscosity.
EVT may also be determined from knowledge of viscosity obtained from the standard tar viscometer. A correlation between the ETV and the viscosity from STV is given in “IS: 1207- 1978, 2002” for viscosity, in the form of a table giving correction to the STV-values to obtain EVT, which may be positive or negative. This table may be used directly for viscosity values ranging between 33 and 75.
For viscosity values less than 33 and more than 75, the approximate EVT value may be obtained from the table. Then, the viscosity is again determined by STV at a temperature nearest to the approximate EVT, which is a multiple of 5. From this viscosity value so obtained, the EVT is again got from the table; this may be in error compared to the correct EVT by about 10%.
For the correct EVT-value, the viscosity shall be determined at two different temperatures (differing by 5° to 10°C), such that the viscosity values are within the range of 25 to 100. From these values, the correct EVT value is calculated with the help of the correlation table between STV and EVT. If the two values are not identical,
Where, T1 and T2 = values of EVT calculated from the two test temperatures,
d1 and d2 = respective correlations to be applied to the temperatures of test to give the EVT.
A nomographic chart for this interpolation is also available in ‘IS: 1207-1978, 2002’. “IS: 1201 to 1220:1978, 2002″ covers all the tests on tar and bitumen.