Q. Optimum binder content in bituminous pavement?

Amount of binder to be added to a bituminous mixture can't be too excessive or too little. Principle of designing the optimum amount of binder content is to comprise sufficient amount of binder so that aggregates are fully coated with bitumen and voids within the bituminous material are sealed up. The durability of bituminous pavement can be improved by the im-permeability achieved. Furthermore a minimum amount of binder is necessary to prevent the aggregates from being pulled out by abrasive actions of moving vehicles on the carriageway. 

Though the binder content can't be too high since it would lead to the instability of the bituminous pavement. Basically, resistance to deformation of bituminous pavement under traffic load is decreased by the inclusion of excessive binder content.