Role of the bonding of the road layers
A pavement structure is designed depending on a certain number of parameters (traffic, foundation layers, nature of the materials, etc.) and in such a way that each constituent layer is not stressed beyond its intrinsic mechanical limits.
When all the layers in the pavement are bonded together, they form a “monolithic” single structure. Each layer does not deform only according to its characteristics (modulus, capacity of deformation, thickness, …), but also according to those of the layers to which it is bonded.
The different techniques
Classic pure bitumen emulsion
The classic bitumen emulsion, made with a 70/100 or 160/220 penetration grade bitumen, used in surface dressing applications, to spreads out easily and evenly on the road surface.
If the breaking speed of conventional emulsions is inadequate to prevent pick up or damage from the tires of site construction traffic it leads to :
- a reduced and inhomogeneous residual binder film.
- soiling of roads near the construction site.
To overcome such shortcomings, new solutions are proposed by the road constructing companies. These are linked to the use of special emulsions which, on the one hand, allow a very fast breaking and, on the other hand, ensures a better adhesion to the surface layer than to the tires of the trucks or to the caterpillars of the asphalt finishers. This technique is called “clean emulsion” or “traffic resistant tack coat”.
Paver with integrated emulsion sprayer
A paver with an integrated emulsion spray bar allows simultaneous spreading of the tack or bond coat and the asphalt mixture in one single pass.The machine comprises all the functions of a conventional paver with, in addition, an emulsion tank with heating, feeding, dosing system for controlled emulsion addition and an adjustable spray bar. This spray bar is positioned so that the emulsion is applied right behind the paver caterpillar tracks.
Another special process is to simultaneously spread three products using a specially adapted spraying machine. It has three spray bars that apply successively :
- a product intended to improve the adhesiveness of the emulsion on the support,
- the tack-coat emulsion,
- a product that accelerates the breaking of the emulsion.
On thinner asphalt layers the interface is stressed more justifying the use of a modified binder emulsion. The modification can take place either by the addition of a polymer latex into the emulsion or by emulsification of a polymer modified bitumen. The emulsion is normally free of fluxing agent.
Slurry seal technology is sometimes used as a bonding layer. In particular, for open-graded surface asphalt mixtures the slurry reinforces the sealing role underneath to protect the existing pavement structure from surface water infiltration.