Bitumen – Characteristics and Tests

The term ‘bitumen’ originated in Sanskrit where the words ‘Jatu’ meaning pitch and ‘jitu-krit’ meaning pitch creating, referred to the pitch-producing properties of certain resinous trees. The later Latin terms of ‘gwitu-men’ and ‘pixtu-men’ became shortened into ‘bitumen’ when passing via French to English. The origin of bitumen as an engineering material dates from 3800-300 B.C when in the Euphrates and Indus Valleys it was used as mortar for masonry and water proofing. Bitumen is one of the man’s oldest engineering materials, and it is the responsibility of engineers to ensure its effective used and avoid waste of this value resource.

The manufacture of bitumen involves distillation, blowing and blending. Atmospheric distillation is used to refine the crude oil by fractional distillation to separate gas, gasoline, kerosene, gas oil and long residue (heaviest fraction consisting of a complex mixture of high molecular weight hydrocarbons. The long residue is then re-distilled under vacuum at 350 to 400C to produce short residue, which is the feedstock used in the manufacturing of different grades of bitumen. In many cases, however, vacuum residues are further processed by air ratification (blowing) to produce harder penetration grade bitumen which can then be blended with softer bitumen to produce intermediate grades.

The behaviour of bitumen is complex and to describe its properties over a wider range of operating conditions (temperature, stress, strain rate) would require a large number of tests. To avoid this and simplify the situation, the mechanical behaviour of bitumen has traditionally been described using empirical tests and equations. The two consistency tests required to characterise different bitumen grades are the Penetration Test and Softening Point Test. Viscosity tests are usually used in the ‘high’ temperature range, i.e. 60C and above, whereas the Penetration and Softening Point tests are usually used in the ‘low’ temperature range, i.e. 60C and below.

Penetration Test

This is an empirical test. The test suffers some disadvantages due to its empirical nature but has been shown to be approximately equivalent to a measurement of ‘stiffness’ at a loading time of 0.4 seconds.

The penetration is the consistency expressed as the distance in tenths of a millimetre that a standard needle of 100 grams, will penetrate vertically into a sample of bitumen at 25C in 5 seconds. Other combinations of load, temperature and loading time may also be used, e.g. 200g, 4C and 60 seconds. However, meaningful comparison of penetration of a bitumen over a range of temperature is only possible if the same load and loading times are used consistently.

Softening Point Test

In this test, two steel balls are placed on two discs of bitumen contained within metal rings. These are raised in temperature at constant rate (5C/min) in a water bath. The softening point is the temperature (in C) at which the bitumen softens enough to allow the balls enveloped in bitumen to fall a distance of 25mm and hit a plate. The test is commonly referred to as the Ring and Ball test.

The American Society for Testing Materials (ASTM) also specifies a similar Softening Point test. The only difference is that the BS/IP method uses a stirrer in the liquid bath. Consequently, the ASTM test gives values for softening point about 1.5C higher than the BS/IP method.

The softening point test determines an equi-viscous temperature. In other words, regardless of the grade of bitumen tested, its consistency will be the same as any other grade when the ball hits the plate. Van der Poel has shown that the softening point is approximately the temperature at which the penetration is 800. Hence the notation T800pen is commonly used to describe softening point.

Viscosity

Viscosity of penetration grade bitumen can be measured with a variety of devices. The ASTM requires control of both an absolute and kinematic viscosity in their specifications for ‘asphalt cement’, the equivalent of the British term ‘penetration grade’. Specifications are based on absolute viscosity ranges at 60C and a minimum kinematic viscosity at 135C. A minimum penetration at 25C is also included in most specifications. Absolute and kinematic viscosities are measured with capillary viscometers. The absolute viscosity is measured by ‘pulling’ the bitumen through the viscometer with a vacuum, whereas for the kinematic viscosity the bitumen flows under its own weight.

Sliding plate and rotational viscometers are used for determining viscosity at temperature below 60C. The sliding plate type monitors force and displacement on a thin form of bitumen contained between parallel metal plates at varying combinations of temperature and loading time. Rotational or torsional viscometers monitor the torque and rotational displacement on a small bitumen sample contained between concentric cylinders or between cone and plate or plate and plate arrangements. These rotational devices can be used in a dynamic oscillatory-type testing mode to examine a wide range of rheological characteristics. They are known as Dynamic Shear Rheometers (DSR’s) and allow the viscoelastic nature of bitumen to be determined over a wide range of temperature and loading times. These DSR’s are being implemented for routine specification testing in the UK following completion of the Strategic Highway Research Program (SHRP), which invested $50 million in asphalt research during the period 1988 to 1993.

Efflux viscometers such as the standard tar viscometer (STV) are used in Britain for characterising tars and liquid bitumen. They determine an empirical viscosity, i.e. the time of flow for a measured amount of bitumen to flow through an orifice of standard size at a controlled temperature. Such devices may be used for penetration grade bitumen, but the temperatures used would need to be high i.e. above 100C. The empirical measurement of viscosity obtained from efflux viscometers can be converted to fundamental units using conversion charts.

The Frass Breaking Point

The Frass test is an empirical means of obtaining an estimate of the temperature at which a thin film of bitumen might crack. The test involves flexing the sample of bitumen contained on a spring steel plaque at successively lower temperatures until it cracks. The temperature at which the sample cracks is termed the breaking point and represents an equi-viscous temperature.

The Frass breaking pint is used in bitumen specifications in many European countries and is standardised by IP 80/53. It is also attracting attention as a potential test to evaluate modified binders having enhanced low temperature properties.