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Structural design

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A road structure generally comprises the following pavement courses, each composed of typical materials and having a specific function in structural design:

  • the subbase layer: acts as a buffer between the subgrade and the base layer and most often consists of a sand bed or crushed stone;
  • the base layer: may be either bound or unbound. Crushed stone is often used for unbound bases. Bound bases are obtained by adding a hydraulic binder such as cement;
  • the bound surfacing consists of bituminous layers, cement concrete, or a combination of both.

There are tree types of road pavement. Flexible pavements consist of a subbase layer, a crushed stone base and one or more bituminous surface layers; rigid pavements of a subbase layer, a lean concrete base and cement concrete at the surface; and semi-rigid pavements of a sand bed, a lean concrete base and bituminous surface layers.

The object of structural design is to ensure adequate service life, during which the road will require no structural adaptations. The layers in the structure should be designed with a thickness such that traffic loads are adequately distributed over the various courses, as each load causes a deflection in the pavement.

Three phenomena are important to consider in the structural design of roads.

Fatigue: the (small) deflections in the pavement generate strains that may lead to cracks in the surfacing or in a bound base layer, under repetitive traffic loads. Deflections should, therefore, be limited.

Permanent deformation: repetitive traffic loads cause the entire pavement to deflect. This results in vertical deformation under the subbase layer, at the level of the subgrade.

Since the bearing capacity of the subgrade is also strongly affected by moisture content, it is recommended to keep the subgrade dry to a depth of at least 1 m below the road surface. In addition, the structure must be protected from frost.

BRRC’s activities in the field of structural design take the forms of:

  • assistance in design calculations, as part of a technological guidance service. This assistance is based on experience and on the use of manual calculation methods (standard structures) and design software packages. The latter allow input of freely chosen parameters for traffic, climate, pavement structure and materials, resulting in the most accurate estimate possible of the (residual) life of a road;
  • participation in international congresses and symposiums, to keep abreast of the most recent developments in structural design.