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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.
In general, bituminous materials are also broadly classified into two types, namely bitumen macadams and hot-rolled asphalts. During compaction, the increase of temperature causes the reduction of viscosity of binder. The binder acts as a lubricant among aggregates particles because it is mobile in a fluid state under high temperatures. The internal resistance between the bituminous materials is drastically reduced resulting in the formation of a mixture with better aggregate interlock.
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The amount of binder to be added to a bituminous mixture cannot be too excessive or too little. The principle of designing the optimum amount of binder content is to include sufficient amount of binder so that the
aggregates are fully coated with bitumen and the voids within the bituminous material are sealed up. As such, the durability of the bituminous pavement can be enhanced by the impermeability achieved. Moreover, a minimum amount of binder is essential to prevent the aggregates from being pulled out by the abrasive actions of moving vehicles on the carriageway.
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The skid resistance of wearing course in a bituminous pavement is contributed by the macrotexture (i.e. the general surface roughness) and the microtexture (i.e. the protruding from chippings) of the wearing course.
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The air void content of bituminous materials is an important control parameter for the quality of bitumen being laid and compacted. If the air void content is too high, it allows for intrusion of air and water. Moreover, it also increases the rate of hardening of binders which produce premature embrittlement of pavements. In addition, too high a void content will also lead to differential compaction subject to traffic loads and result in formation of ruts and grooves along the wheel track.
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