DESIGN OF PAVEMENTS WITH LEAN-CONCRETE BASES

In the analysis approach to pavement design, structures with lean-concrete bases require certain distinctive considerations that arise from the incidence of cracks in the lean concrete attributable to thermal and shrinkage effects that influence stresses and strains in the rest of the structure. Details of a two-stage design process, for both conventional lean-concrete-base structures and for those that involve a "sandwich" layer of unbound granular material above the lean concrete, are permitted. Both types of structures incorporate 100 mm of rolled-asphalt surfacing; the conventional structures use an additional thickness of dense bitumen macadam as part of a composite base. In stage 1 of the procedure, the full uncracked value of modulus is used for the lean concrete, and pavement life, in terms of numbers of standard axle loads, is calculated to the point at which secondary (traffic-inducted) cracking occurs. In stage 2, the modulus of the lean concrete is substantially reduced as a consequence of this cracking, and additional life is calculated on the basis of asphalt fatigue cracking or permanent deformation, whichever is critical. The total life is the sum of stages 1 and 2. The results are compared with current recommendations and with the results of full-scale trials. Since the granular material can only develop a modest value of modulus in sandwich construction, that technique does not appear to be very effective unless it can be shown to reduce the incidence of reflection cracking in the asphalt surface. (Author)