When time lapses between the laying of different bituminous layers, such as roadbase and base course, a tack coat is essential to enhance bonding between these materials. Without proper bonding, the layers would act independently, compromising their collective ability to withstand traffic loads.
The tack coat, typically a thin layer of bitumen emulsion or cutback asphalt, should be evenly sprayed onto the existing bituminous surface and allowed ample time to cure. When the hot bituminous layer is laid on top, it softens the tack coat, allowing it to partially fill voids in the existing layer. Emulsified asphalt tack coats are often diluted with water for a more uniform application and to prevent excessive asphalt usage. Subsequent compaction interlocks the tack coat with the bituminous materials, creating a unified and resilient structure.
It’s crucial to avoid applying an excessive tack coat, as this can lead to slippage or shear cracks due to the thick layer of material. In situations where there’s a delay in laying bituminous layers on the sub-base, a priming coat may be used instead of a tack coat.
A primer coat serves multiple purposes. It preserves the existing surface condition for a longer duration and creates an impermeable barrier against water intrusion and evaporation. It also fills surface voids, protecting the sub-base from adverse weather conditions. Additionally, it promotes adhesion between subsequent layers and hardens the surface.
In conclusion, tack coats and primer coats play distinct roles in ensuring the successful construction and performance of bituminous pavements. The appropriate application of these layers is crucial for creating a durable and long-lasting road surface.