The following tests are conducted to assess the suitability or quality of bitumen for specific use as a paving material and also for read making:
1. Specific Gravity:
Specific gravity of a binder material does not affect its bonding characteristics. However, its value is needed in bituminous mix design. This is determined at 27°C using a standard pyknometer. The bitumen sample is gently heated and transferred to the pyknometer, allowed to cool, and weighed. It is then filled with water in the remaining space and weighed again. The contents are emptied and the pyknometer is cleaned thoroughly Then it is filled with water and again weighed, The empty weight of the pyknometer is also obtained.
Let the empty weight of pyknometer be W1;
Wt. of pyknometer with bitumen W2;
Wt. of pyknometer + Bitumen + Water in the remaining space W3; and,
Wt. of pyknometer + Water W4
The specific gravity of bitumen, for road-making, ranges from 1.02 to 1.04.
Solubility in carbon disulphide, in naphtha 88 and in carbon tetrachloride is necessary for road-making bitumen. The IS specifications require a solubility of 99% in carbon disulphide. Insolubles indicate presence of mineral matter.
Some resinous oils mixed with bitumen dissolve in carbon disulphide, but are unsuitable for road-making. So, the solubility in naphtha 88 is important. ‘Petrolenes’ in bitumen are soluble in naphtha, but ‘asphaltenes’ are insoluble. If the petrolenes are sticky and not oily, the bitumen will have good binding property. The range of solubility in naphtha should be 60% to 75%.
Bitumen shall be soluble in carbon tetrachloride to an extent of at least 1.5% more than in CS2.
3. Spot Test:
This is useful for detecting overheating or cracked bitumen; this test is considered to be more sensitive than the solubility test for detection of cracking.
About 2 g of bitumen is dissolved in 10 ml of naphtha; a drop of this solution is placed on a filter paper – once after one hour and again after 24 hours. If the spot of the filter paper is uniform in colour, the bitumen is taken to be un-cracked. However, if the spots form a dark brown or black circle at the middle with an annular ring of higher colour around it, the bitumen is taken to be overheated or cracked.
4. Water Content:
It is desirable that the water content in bitumen is minimal to prevent foaming when heated above the boiling point of water.
The water content is determined by mixing a known weight of bitumen in a pure petroleum distillate free of water, and distilling off the water in the specimen being tested. The weight of water condensed and collected is determined and expressed as percentage by weight of original sample. The maximum water content in bitumen should not exceed 0.2% by weight.
5. Paraffin Scale:
The bitumen specimen should not contain more than 2.5% of paraffin scale so that its binding quality is not affected to such an extent that it is unsuitable for road work.
Viscosity is the property of a fluid that determines the resistance offered by the fluid to a shearing force under laminar flow conditions. It is therefore the opposite of fluidity.
Dynamic or absolute viscosity is the internal friction resulting from unit tangential force acting on planes of unit area separated by unit distance of the fluid, producing unit tangential velocity. The unit of measurement of dynamic viscosity in S.I. units is Ns/m2.
Kinematic viscosity is the quotient of dynamic viscosity and density of the fluid. The unit of kinematic viscosity is m2/s.
The determination of viscosity is usually done using efflux or orifice viscometers. The general procedure is to pour the specimen under test to a specified level in a container surrounded by water or oil bath for temperature control. The base of the container has a small orifice with a simple valve control.
After opening the valve, the time in seconds is recorded for a stated quantity of bitumen to discharge into a measuring container below.
The Standard Tar Viscometer (STV) with an orifice diameter of 10 mm is commonly used. The time required to discharge 50 ml is designated the ‘viscosity’ of the bitumen. (This does not bear any relation to the absolute viscosity).
The standard procedure is given in “IS: 1206-1978: Methods of testing tar and bituminous materials-Determination of viscosity-(Parts 2 & 3), Bureau of Indian Standards, New Delhi, 1978”.
The viscosity of tar is determined in seconds (for 50 ml to flow through 10 mm orifice) at 35° to 55°C. For cut-back bitumen, the value is got at 25° or 45°C. The orifice type is used for bitumens as well as tars. A sliding plate miro-viscometer may be used for bitumen of all grades.
An indirect measure of viscosity is the extent of penetration of a standard needle under standard conditions of load, time and temperature. The test result is a measure of the hardness or softness in terms of the penetration in tenths of a millimetre of the standard needle.
The needle of 1 mm diameter is allowed to penetrate under a total weight of 100g into a bitumen sample at 25°C for 5 seconds. The apparatus is known as the needle penetrometer.
Owing to limitations on the size of the needle and the weight, penetrations less than 2 and greater than 500 cannot be measured satisfactorily. If the logarithm is plotted for penetration against temperature, practically a straight line is obtained; this facilitates extrapolation. Penetration values cannot be correlated well with absolute viscosity.
A bitumen of penetration grade 80/100 means the range of penetration is 80-100 (in tenths of millimetre). The lower the value, the harder the grade of the bitumen.
8. Float Test:
Bituminous materials with a viscosity more than 140 and penetration more than 300 cannot be tested conveniently by the standard tar viscometer and the needle penetrometer, respectively. Such bitumen can be evaluated by the ‘float test’. The float test apparatus is shown in Fig. 6.68.
An aluminium float is fitted with a brass collar at the bottom. Hot bitumen is poured into the collar without any air bubbles and cooled gradually to 5°C. The collar is then screwed into the float-and-collar assembly and placed in a water bath maintained at the desired temperature (50°C).
The time in seconds required for the water to pierce into the float through the molten bitumen is noted; this is known as the ‘float test value’, expressed in seconds. As is obvious, a higher value denotes a harder bitumen and vice-versa.
9. Softening Point:
A viscous material like bitumen does not show a well-defined softening point. A standard test determines the temperature at which a standard ball passes through a disc of bitumen contained in a brass ring. The test is therefore called the ring-and-ball test. The apparatus is shown in Fig. 6.69.
A brass ring containing the bitumen sample is suspended in water or glycerine at the desired test temperature. A steel ball is placed on the bitumen disc. The liquid is heated at a rate of 5°C per minute. The temperature at which the softened bitumen touches the bottom of the metal plate, placed at a specified distance below the ring is noted as the softening point.
The hardest grade of bitumen is 30/40, which has a softening point of 50°C to 65°C. The softest bitumen for paving is the 180/200 grade, for which the softening point is 30°C to 45°C.
The property of ductility of a binder is an indication of its ability to deform under load without cracking. A cracked surface of a pavement is unsatisfactory for a number of reasons, the chief of which is that it allows water to penetrate into the courses of the pavement, affecting its stability.
Ductility is measured by the distance which a standard briquette of bitumen, with a cross- section of 1 cm x 1 cm at its neck, will stretch without breaking when elongated at the rate of 5 cm/min at 27°C.
The ability of the bitumen to spread and coat the surfaces of the aggregates completely to provide binding and interlocking is considered to be dependent on its ductility. Also, the ability to withstand repeated loading cycles under traffic, keeping the deformations within allowable limits, is partly dependant on its ductility.
The ductility test is covered by “IS: 73-2006: Indian Standard Paving bitumen: Specifications, Bureau of Indian Standards, New Delhi, 2006” and also by “IS: 1208-1978. Methods of testing bituminous materials – Ductility Test, 2002”.
The ductility machine functions as a constant temperature water bath, along with a pulling device, motorised to facilitate the calibration of the pulling rate to the desired standard.
The bitumen sample is heated and poured in the briquette mould placed on a plate. The excess material is cut off with a hot knife. After air cooling, the mould is placed in a water bath at 27°C for at least 90 min. The sides of the mould removed, the clips are hooked on the machine and the pointer on the scale set to zero. The pull is applied by means of the motor at a standard rate of 5 cm/minute. The elongation up to the point of breaking in centimetres is reported as the ductility value.
IS specifications suggest a minimum ductility of 75 cm for bitumen of grade 45 and more. However, a minimum of 50 is also considered adequate; the absolute minimum value specified in India is 15 cm. As the conditions are all arbitrary – the size, the rate of pulling, and so on – and do not practically correspond to field conditions, some authorities question the value of ductility test.
Some engineers feel that the ductility value at 4°C with a rate of pulling of 1 cm/minute may better reflect the field conditions.
In spite of these controversies, the test gives some idea of the ductility and its possible effects, desirable or otherwise.
11. Flash Point and Fire Point:
All bituminous binders produce or give out volatile vapours on heating.
The flash point of bitumen is that temperature at which it gives off vapours, which ignite with a flash in the presence of a fire momentarily, but do not continue to burn.
The fire point of bitumen is that temperature at which the vapours ignite in the presence of a flame and continue to burn for at least 5 seconds.
These temperatures are significant in preventing fire hazards during heating, mixing and compaction of bituminous mixes.
These tests are covered by “IS: 1209-1978: Methods for testing tar and bituminous materials- Flash point and Fire point, Bureau of Indian Standards, New Delhi, 2002”.
The ISI recommends the Pensky Marten’s Apparatus — it may be the open-cup type or the closed-cup type. A suitable quantity of bitumen is taken in a cup and heated at a rate of 5° to 6°C per minute, stirring the molten bitumen at 60 rpm. A thermometer is kept inserted in the molten bitumen to take temperature measurements wherever desired.
A small test flame is frequently brought near the surface of the molten bitumen. The temperature at which the earliest flash recurs is recorded as the flash point.
The test is continued by further heating, and when the test flame is brought near the surface of the molten bitumen, the surface ignites and burns at least for 5 seconds, at the temperature known as the fire point of bitumen, which is carefully noted. The minimum flash point recommended by ISI is 220°C.
Heating a bituminous binder leads to the loss of its volatile constituents, thereby causing it to harden. It also leads to loss in penetration and viscosity. This test helps in assessing the susceptibility of bitumen to heating when exposed to hot conditions during mixing operations for bituminous mixes.
A sample of bitumen, 50 ml, is placed in a steel pan of 140 mm diameter and 10 mm high. The pan with bitumen is weighed and placed on a rotating shelf mounted in a ventilated oven for 5 hours at 163°C. The shelf is rotated at 5 to 6 revolutions per minute. At the end of the test, the pan with bitumen is weighed and the loss of weight is determined. The bitumen residue is used for further testing to determine the change in the value of penetration.
IS specifications prescribe a loss of weight on heating of not more than 1% for all grades, except 180/200 for which it can be 2%. The loss in penetration on heating should not exceed 40%.