DESIGN OF FLEXIBLE PAVEMENTS IN CZECHOSLOVAKIA: RECENT RESEARCH WORKS

In 1961 a recommendation for the design of flexible pavements was presented for the approval of the Central Road Administration in Slovakia. The method contains design principles used for selection of the structure and the total thickness of the pavement as well as the material and the thickness of the different layers. The total thickness is determined taking into consideration the protection of the pavement and subgrade against frost effects. In the design of the structure of the pavement the following factors are considered: (1) the influence of traffic during the whole service lief of the pavement; (2) the bearing capacity of the subgrade; (3) the characteristics of materials used in the pavement structure; (4) the mechanics of the behavior of the pavement under a load; and (5) the method of construction. The designed pavement is assessed with regard to the required performance characteristics (the service life) and the stresses to which the various layers will be subjected. The design criterion for serviceability is the deflection of the pavement. The stresses in the pavement are assessed according to the tensile (bending) stress arising from the action of a design load at the bottom of the surfacing or at the bottom of one of the base courses. In this case the criterion is the permissible tensile (bending) stress of the material. When calculating the deflection and the stresses, the real pavement is simulated by an ideal three-layer system consisting of perfectly bonded layers of flexible materials. The elastic properties of the various materials are considered in design (table) values; the elastic modulus of the subgrade is derved from a CBR value established in the laboratory. The correctness of the design can be checked by load bearing tests performed on the finished pavement. In view of the fact that the structural design uses design values instead of real (instantaneous) values of the deformation properties of the materials in the pavement and the subgrade, the deformations measured in the bearing test will be lower than the calculated ones and similarly, the real stresses in the pavement will differ from those established by a calculation.