Rehabilitation of roads containing cobblestone pavements covered with a bituminous layer.

Resumé

Historically, many roads in Belgium were constructed using cobblestones. In
the past, cobblestone roads used by a limited amount of heavy traffic were sometimes covered
with a bituminous layer. For numerous reasons it is often recommendable to replace the
cobblestones when rehabilitating these roads. With their low budget or for urgent
interventions, road managers sometimes prefer replacing the bituminous layer by a new one.
One approach for the evaluation of structure rehabilitation was in use in Belgium since the
late 1970ies and consists of the computation of a theoretical single layer model equivalent to
the existing multilayer structure. Based on traffic expectations and an abacus of life-time
expectations of a standard road structure, the optimal thickness of a bituminous overlay can be
estimated. When the existing multilayer structure contains a cobblestone layer, the main
difficulty with this approach arises from the choice of the “equivalence factor” for the
cobblestone layer. When using back-calculation software, the main difficulty encountered is
the choice of realistic parameters modelling the cobblestone layer. Values for parameters such
as the layer thickness, the Poisson coefficient of the material, the degree of bounding with the
other layers, and the initial guess for the E-modulus of the layer have to be introduced in the
software for the cobblestones. Once the back-calculation delivered E-moduli for all layers in
the introduced road structure model, a direct computation can estimate life-time expectance
after repairs or the appropriate rehabilitation of the existing structure for a considered future
traffic load. Both approaches were used and compared in real cases of rehabilitation design. In
this contribution the limitations of road rehabilitation conserving a cobblestone layer below
the surface layer will be discussed. Examples will illustrate the difficulties experienced when
trying to estimate bearing capacity from FWD measurements and how they were dealt with.