Asphalt and Concrete
Pavements are classified as flexible (asphalt) or rigid (concrete) according to the way they distribute wheel loads. Flexible (asphalt) pavements consist of bituminous surface courses placed over granular or stabilized base and sub-base layers. The asphalt concrete is a mixture of mineral aggregate and bitumen binder; load is spread by grain-to-grain contact through successive layers so that the stress reaching the subgrade is low. Performance is governed by mix design (aggregate gradation, binder grade, air voids), layer thicknesses, and resistance to permanent deformation (rutting), fatigue cracking and thermal cracking. Rigid (concrete) pavements derive strength from the high flexural stiffness of a Portland-cement concrete slab. Loads are distributed over a wide area by the slab’s beam action. Joints (contraction, expansion, construction) control cracking caused by shrinkage and temperature changes; reinforcement or dowel bars may be used to transfer load across joints. Key design parameters are concrete flexural strength (modulus of rupture), slab thickness, joint spacing, and subgrade modulus (k-value). Selection between the two systems depends on traffic volume and loading, subgrade support, climate, construction speed, initial cost and expected maintenance.
Knowledge Check
1.What is the fundamental difference in load-distribution behavior between asphalt and concrete pavements?
2.Which material serves as the binder in conventional asphalt concrete?
3.Why are joints provided in jointed plain concrete pavements?
4.Which distress is most characteristic of asphalt pavements under heavy, slow-moving traffic in hot weather?
5.In rigid-pavement design, the modulus of subgrade reaction (k-value) represents:
6.What is the primary structural purpose of a granular base layer under an asphalt surface?
7.Dowel bars placed across transverse joints in concrete pavements are intended to:
8.Which volumetric property is critical in Superpave asphalt mix design to ensure durability and resistance to bleeding?
9.A concrete pavement is generally preferred over an asphalt pavement when:
10.Fatigue cracking in asphalt pavements typically initiates: