Bitumen is a mixture of organic liquids that are highly viscous, black, sticky, entirely soluble in carbon disulfide, and composed primarily of highly condensed polycyclic aromatic hydrocarbons. Naturally occurring or crude bitumen is a sticky, tar-like form of petroleum which is so thick and heavy that it must be heated or diluted before it will flow. At room temperature, it is much like cold molasses. Refined bitumen is the residual (bottom) fraction obtained by fractional distillation of crude oil. It is the heaviest fraction and the one with the highest boiling point, boiling at 525 °C.
On June 3, 2002 the first lot of paving bitumen of the quality set according to the European Union standards was produced in Mazeiku Nafta. One of the requirements of the EU, during the preparation for the membership, was replacing the “local” standards by the respective EU standards. Another aspect is that according to the new standard the quantity indicators are determined using more modern and precise methods.
The European standard showing the quality indicators of paving bitumen and their values for bitumen used in Lithuania corresponds to the requirements of the Lithuanian standards LST 1505 adopted in 1997.
The main technical parameters of bitumen:
Bitumen was the first oil product used by people as a construction material as far back as 3800 BC. At present the bitumen of AB Mazeikiu Nafta is produced by means of oxidation: by oxidising tar which gets from the fuel oil deep processing complex by the air in the columns. The process occurs in the bitumen producing equipment. By changing oxidation modes, the paving, roofing and construction bitumen types are obtained, which are later used to build roads, to produce roofing and in construction.
The quality of bitumen is stated by a large number of indicators. The main indicators are penetration, viscosity and the softening temperature. The bitumens which in the case of certain penetration have maximum possible viscosity and high softening temperature as well as good adhesive properties are regarded the best. The other bitumen quality indicators are flash temperature, viscidity, fragility temperature, density, cohesive and adhesive properties.
Penetration. It is determined by sticking a standard needle into semihard and semi-liquid products according to the set regime. It indicates the firmness of bitumen. The unit of penetration is considered to be the depth of the penetration of a needle by 0,1 mm at the temperature of 25 °C and the load of 1N. The time of the load operation is 5 seconds. The lower is the penetration of bitumen at the set softening temperature, the higher is the heating stability of bitumen.
Adhesion. The adhesive properties of bitumen determine its viscidity and adhesive properties. The paving bitumen must have great adhesivness in the conditions of a wide temperature ranging order to prevent the road-metal from deterioration when affected by motor wheels. Good grip with dry and wet surfaces of different materials is especially important while building roads.
Fragility temperature. At this temperature bitumens deteriorate from short-term load. The lower is the fragility temperature, the better is the bitumen quality. It is agreed to regard such temperature as an indicator at which the slot forms on the layer of bitumen which covers a steel plate.
Viscosity. The viscosity of bitumens is characterised by the distance at which the bitumen can be stretched into a thread until it breaks. This indicator shows the consistence (body) and viscidity of bitumens. The lower is the temperature, the lower is the viscosity.
Softening temperature. Oil bitumens do not liquefy and have no melting point – they get softer when heated. At the softening temperature bitumens change from the relatively solid state into the liquid state. The softening temperature is determined by the method of a ring and a ball. As the softening temperature increases, the properties of bitumen to resist the changes of temperature improve.
Density. For oxidised bitumens this indicator increases as the oxidation time increases. According to the requirements laid down by the standards, the density of bitumen must be larger than 1 gr/cm3 at the temperature of 25°C.
Viscidity. Characterises the consistence of bitumen in different temperatures. The viscidity of bitumens is determined by several methods: dynamic viscidity (at 60°C), kinematic viscidity (at 135 °C). It is desirable that on condition all the other indicators for the maximum temperature remain the same, the viscidity of bitumen should be maximal.