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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.
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Longitudinal cracks in bituminous pavement are usually caused by fatigue failure under repeated traffic loading. In thin pavements, cracking starts at the bottom of the bituminous layer where the tensile stress is the highest and then it spreads to the surface as one or more longitudinal cracks. In thick pavements, the cracks usually commence from the top because of high localized tensile stresses from tire-pavement interaction. After repeated loading, the longitudinal cracks develop into a pattern similar to the back of an alligator.
Paver, steel-wheeled roller and pneumatic tire roller compact bituminous material by using the following principles:
(i) The static weight of the paving machines exerts loads on the bituminous material and compresses the material directly beneath the machine. The compacting effort increases with the period of contact
and larger machine weight.
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The use of bituminous surfacing over concrete structures (e.g. existing concrete roads) is widespread to improve the skid-resistance and the general appearance of roads on one hand, and to avoid the pre-mature
failure of concrete surface by frost spalling in cold countries on the other hand.
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The choice of compacted thickness is closely related to the nominal maximum size of aggregates of bituminous materials. Based on the recommendation by Dr. Robert N. Hunter, the rule of thumb is that the compacted layer thickness should exceed 2.5 times the maximum size of aggregate. If the layer thickness is less than 1.5 times the nominal maximum size of aggregates, the mechanical properties of bituminous material is impaired by the possible crushing of larger sizes of aggregates.
