Federal Highway Research Institute


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Dimensioning of road surface structures

The road is a multi-layer construction of differing building materials. It is referred to as a road surface structure. In order to permanently withstand the impact of traffic and the weather, the right choice of road surface structure, the determination of the layer thicknesses as well as the quality of the building materials, their processing and the quality of the installation are critical. The task of dimensioning is to determine the sequence and thickness of the layers comprising the road surface structure subject to the building materials selected and their properties as well as the traffic load.

The road surface structure must satisfy two key criteria in this regard: on the one hand it must be sufficiently stable, while on the other hand the road surface structure must exhibit a sufficient protection against frost. Sufficient stability means that the road will be able to withstand a forecast traffic load for a determined period of use – usually 30 years – without suffering structural damage. Sufficient protection against frost is provided if the water is drained via the unbound layers away from the road surface structure in such a manner that no water remains in it, which can freeze and thus result in damage at low temperatures.

The dimensioning of road surface structures ensues in accordance with the "Directives for the Standardisation of Road Surface Structures of Traffic Infrastructures" (RStO) or the "Directives for the Calculational Dimensioning of Traffic Infrastructures with Asphalt Wearing Courses (RDO Asphalt) or the "Directives for the Calculational Dimensioning of Traffic Infrastructures with Concrete Surfaces" (RDO Beton).