What is the function of prime coat in bituminous pavement?

The principal function of prime coat in
bituminous pavement is to protect the
subgrade from moisture and
weathering. Since the presence of
moisture affects the strength of
subgrade, the prevention of water
entry
during construction is essential to
avoid the failure of the pavement. In
cold countries, by getting rid of
moisture from subgrade, the danger
of frost heave can be minimized .
Prime coat is an asphalt which, when
applied evenly to the surface of sub-
base or subgrade, serves to seal the
surface to hinder the penetration of
moisture into subgrade. Vehicular
traffic should be avoided on the
surface sprayed with prime coat
because the traction and tearing
action of vehicles would damage this
asphalt layer .