The selection of bitumen is determined by the scope of application, accepted technology of works and technical-economic requirements to the structure built. Process conditions of bitumen application at the stage asphalt mix preparation mainly depend on the bitumen grade. As the optimum viscosity of bitumen during preparation of bitumen-concrete mixes is about 0.2 Pa•s, its heating temperature when mixed with mineral materials is accepted such as the viscosity conforms to optimal. For bitumen of various grades with different consistency the temperature at which the specified viscosity is reached will be different. The recommended heating temperature at mixing with mineral materials is accepted as 150-160 °C for bitumen of grades BND 40/60, 140-150 °C – for BND 60/90, 130-140 °C – for BND 90/130, 110-120 °C – for BND 130/200, 100-110 °C – for BND 200/300..
Bitumen shall be held at this temperature for no more than 5 hours to avoid ageing processes. At temperature not above 80 °C viscous bitumen may be held for 12 hours at most, and liquid bitumen of MG class — at temperature not above 60 °C for 12 hours at most. The optimum viscosity of bitumen when laying the asphalt concrete mix is equal to 20 Pa•s. The temperature of asphalt concrete mixes at laying should be not below 120 °C when the bitumen-based mixes with penetration depth 40–130 are laid; and not below 80 °С if the penetration depth is 130-300. Bitumen should meet a certain set of requirements presented to physical-chemical, mechanic, and adhesive properties, as well as steadiness of these properties in operation.
Requirements to physical-chemical properties of bitumen are set based on the general requirements to operational reliability of structure, area of construction, and the type of construction layer where the bitumen is used. The diversity of climatic conditions in Russia and diversity of operating conditions of bitumen in asphalt concrete coatings depending on the location of the road and position of layer in the road paving structure determine the need to differentiate requirements to paving bitumen depending on the scope of its application. The basic rheological parameters determining operational behavior of bitumen in specific utilization conditions include the plastic range, consistency (viscosity), and limit ductility.
For road paving construction, it is necessary to use such grades of bitumen the plastic range of which conform to the actual range of temperatures characteristic of this structural layer in operation of road paving. In the general case the bitumen consistency should be selected so that the mid-point of plastic interval is the same as the mid-point of the range of operation temperature.
The analysis of construction and operation of asphalt concrete pavement in different regions of the country allows concluding that depending on climatic conditions binding agents with plastic range 60-90 °C and conventional melting temperature 12–30 °C should be used for arrangement of upper layers of road paving. The conventional melting temperature at which the bitumen plasticity is equal to 0.5. Binding agents with plastic range 50-70 °C and conventional melting temperature 0–15 °C may be used for arrangement of lower layers of pavement. Binding agents with plastic range 35-55 °C and conventional melting temperature -5–+5 °C may be used to arrange road foundations.
For top layers of pavement it is necessary to differentiate selection of bitumen as by the value of plastic range as by consistency, whereas the differentiation by consistency is sufficient for foundation layers as the required plastic range is almost equal. At fluctuation of asphalt concrete viscosity along the layer thickness and to ensure equal consistency of material with increase in depth, bitumen of 5–8-time lower viscosity may be applied every 5 cm of pavement. In cloudy weather and in winter the temperature gradient in the pavement layer is insignificant, so setting recommendations for selection of bitumen grade it is necessary to take into account the specified duration of operation time with various values of temperature gradient. At the same time, as the stress analysis of material in pavement shows, the compression stresses occurred in it are decreasing with depth, and the tension stress in the bottom layer is as low as low the hardness of asphalt concrete in the bottom layer is. This suggests the use of low-viscous and liquid bitumen for bottom layer bordering on unbound layers of road structure.