The quality and durability of roads surfaces depend on the adhesion strength of bituminous binders with mineral materials. If this condition is not met, then there are erosion, peeling, chipping and cracking. Pavement ceases to be water and frost-proof.
Adhesion is the interaction between contacting surfaces of different nature. In road construction, the first surface is bitumen coating and the second is the mineral material. It is clear that the stronger this bond the more durable the road is. Therefore, adhesion is a fundamental part of bitumen performance.
Formation of adhesive bonding
The adhesive bond is usually formed in two stages. First, bitumen is melted and bitumen molecules on the surface align in the direction of the substrate in the interphase layer (transport phase).
Then forces of various nature (chemical, van der Waals force, etc.) help bitumen interaction with mineral material . Adhesion process is completed by the intermolecular interaction of contacting phases, and bitumen completely hardens.
The problem of adhesion of bituminous materials
Adhesion can occur only with the formation of double electric field between bitumen film and mineral material. Therefore, the process is greatly influenced by polarity of asphaltenes and maltenes of bitumen, providing electrical conductivity in non-polar solvents.
By nature, road bitumen is hydrophobic, even though it contains some hydrophilic components (organic asphaltenes and naphthenic acids). They can have chemisorption interaction with mineral materials.
Other bitumen components (resins, polycyclic aromatics, heteroatom-containing compound) are not capable of providing the necessary adhesion of bitumen. Oxygenated compounds also have little effect on the interaction with the mineral part of asphalt concrete.
Solutions of the problem
Among possible ways to improve the quality of asphalt mixtures there is pre-processing of mineral components. Dealing with sand and gravel mixtures, bitumen adhesion can be improved by irrigation of mineral components with solutions of aluminum salt and iron. They act as cations that strongly connect with OH groups of silicate and firmly fix bitumen film. This significantly improves adhesion of bitumen to the surface. Due to this, the physical and chemical properties of asphalt concretes improve.
Adhesion can also be improved by using surfactants, which help to reduce the surface tension and enhance the processes of adsorption and chemisorption at the interface.
Often road workers use byproducts of chemical, food and wood-chemical industry as adhesive additives; the efficiency of these materials leaves much to be desired due to the instability of composition and inconsistent quality.