Viscosity characterizes the consistency of bitumen at different temperatures of use. It is easier to determine bitumen viscosity than penetration or softening temperature. Let us see the change in the state of bitumen on the example of the BND 90/130 brand:
- At temperatures below 40 ° C, it is in a solid state.
- In the range from 40 to +140 °C — in the form of a structured liquid.
- At temperatures above 140 ° C — in the form of a true (Newtonian) liquid.
There are several ways to determine the viscosity of bitumen. Note that this value is not directly regulated by GOST, so manufacturers rarely need to calculate this parameter specifically. Most often, the penetration index is determined, which indirectly determines the viscosity of bitumen. We will discuss how the tests are conducted.
Determining the specific viscosity of bitumen
Special viscometers are used for testing. The procedure is described in GOST 11503-65 and the interstate standard EN 13302-2013. The method is based on the calculation of the time during which a certain amount of material will flow through a special hole in the cylinder of the device, provided the constantly maintained temperature.
Tests to determine the viscosity of bitumen are carried out in several stages:
Thus, the viscosity of bitumen is determined by the number of seconds during which the bitumen brought to the test temperature will expire in an amount of 50 cm3. As a rule, two tests are carried out, and the arithmetic mean is taken as the desired value.
- Bitumen is heated for a mobile state. If necessary, excess liquid is removed by filtration. Either calcium chloride or sodium salt is used as a filter.
- The bitumen composition is passed through a sieve. The laboratory technician carefully stirs the material, preventing air bubbles from remaining in it.
- The viscometer is installed horizontally. The vapour bath is filled with water. In order for the tests to be accurate, it is recommended to heat it to a temperature several degrees higher than necessary to determine the specific viscosity of bitumen.
- The material is cooled to room temperature and left to cool for an hour. The bitumen is then heated to a temperature a few degrees above the temperature assigned by the laboratory technician for the test. The liquefied composition is poured into a working cylinder. The shutter closes. It is important to observe the condition: air bubbles should not form during transfusion.
- Bitumen is mixed with a thermometer.
- As soon as the test temperature is reached, the thermometer is removed from the material. After lifting the shutter, the substance begins to move. As soon as the bitumen level in the measuring tank reaches the mark of 25 cm3, the laboratory technician turns on the stopwatch. It turns off at a value of 75 cm3.
Thus, the viscosity of bitumen is determined by the number of seconds during which the bitumen brought to the test temperature will expire in an amount of 50 cm3. As a rule, two tests are carried out, and the arithmetic mean is taken as the desired value.
How is bitumen penetration determined?
The penetration determines the depth to which the needle will penetrate the bitumen at a certain temperature and loads. It is determined by a penetrometer according to GOST11501-78. Calculations are performed at temperatures of 0°C and 25° C, and the unit of measurement is the penetration depth of the needle by 0.1 mm. The penetration index (PI) is determined is calculated using another important parameter — the softening temperature. The penetration index determines the plastic properties, temperature dependence, and colloid features of the compositions. For road bitumen, the PI should be in the range from -1 to +1.