Question

In case if there is a delay of bituminous laying on top of sub-base, have to tack coat be applied on the top surface of sub-base?

Answer

When there is a delay between bituminous laying of different bituminous layers such as roadbase, base course. A tack coat is applied on top of bituminous layers as it helps to improve better bonding between bituminous materials. If there is not enough bonding between adjacent bituminous layers, they perform as separate independent layers which are able to barely resist traffic loads. When applying tack coat, it have to be sprayed uniformly on bituminous surface and allowed for enough curing. Hot bituminous material laid on top of coat would soften it, enabling tack coat to partly fill voids in bituminous materials. For emulsified asphalt category tack coats, they are usually diluted with water in order to achieve an additional uniform application without excessive usage of asphalt. After subsequent compaction is carried out, coat would be interlocked with bituminous materials. On other hand, care should be taken to make sure that excessive coat will not be laid, or else slippage or shear cracks in bituminous material would occur due to relative thick layer of tack coat applied.

Though, for sub-base surface, priming coat in its place of tack coat might be applied in event of a delay in laying of bituminous layer on top of sub-base layer. Function of primer serves to maintain existing surface condition for a longer period and it also provides an impervious surface to stop ingress of water or water beating by evaporation.

Besides, it fills surface voids and protects sub-base from adverse weather setting. In adding up, it and helps to promote adhesion between adjacent road layers and to harden surface.