Highway Engineering

210 Highway Engineering

varying penetration grades. This is the material used to bind and stabilise the

graded stone used in the top layers of a highway pavement.

A number of tests exist to ensure that a binder has the correct properties for

use in the upper layers of a pavement. Two of the most prominent are the pen-

etration test and the softening point test, both of which indirectly measure the

viscosity of a sample of bitumen. (The viscosity of a fluid slows down its ability

to flow and is of particular significance at high temperatures when the ability of

the bitumen to be sprayed onto or mixed with aggregate material is of great

significance.) The penetration test is in no way indicative of the quality of the

bitumen but it does allow the material to be classified.

The penetration test involves a standard steel needle applying a vertical load

of 100 g to the top of a standard sample of bitumen at a temperature of 25°C.

The depth to which the needle penetrates into the sample within a period of

5 seconds is measured. The answer is recorded in units of 0.1 mm. Thus, if the

needle penetrates 10 mm within the five second period the result is 100 and the

sample is designated as 100 pen. The lower the penetration the more viscous and

therefore the harder the sample. Figure 7.12 is a diagrammatic representation

of the penetration test.

100 g

100 g

Measured penetration

Test duration: 5 seconds

Temperature: 25∞C

Figure 7.12

Penetration test for

bitumen.

The softening point test involves taking a sample of bitumen which has been

cast inside a 15 mm diameter metal ring and placing it inside a water bath with

an initial temperature of 5°C. A 25 mm clear space exists below the sample. A

10 mm steel ball is placed on the sample and the temperature of the bath and

the sample within it is increased by 5°C per minute. As the temperature is raised,

the sample softens and therefore sags under the weight of the steel ball. The

temperature at which the weakening binder reaches the bottom of the 25 mm

vertical gap below its initial position is known as its softening point. An illus-

tration of the softening point test is given in Fig. 7.13.

Bitumen should never reach its softening point while under traffic loading.

The results from these two tests enable the designer to predict the tempera-

tures necessary to obtain the fluidity required in the mixture for effective use

within the pavement.