Roundabouts

The geometry of traffic circles, combined with the speed at which they are used, means that they are subject to their own specific constraints.

• Shear stresses

Access roads are areas of braking and acceleration, resulting in axle-to-axle load transfers and high grip requirements.

On the annular pavement of roundabouts, increased stresses are caused by the rotational forces of vehicle axles on the pavement. This results in a significant increase in tangential shear stresses.

Added to this are the tangential forces resulting from centrifugal force, causing fatigue phenomena within the bituminous structure.

• Structural constraints

Roundabout speeds are lower than for straight traffic. As a result, load application time is proportionately longer than in straight road sections.

What's more, the extremely channelled traffic due to the particular conditions of the roadway geometry also adds to the constraints.

The effect of centrifugal force resulting directly from the bend unbalances the distribution of loads between the wheels on the same axle, causing the wheels outside the bend to be overloaded (+20 to 60%).

In addition to the shearing effect of the quasi-static pivoting of heavy vehicle wheels, tangential forces resulting from centrifugal forces also repeatedly stress the road surface.

This type of load is found mainly on roundabouts and short-radius bends. Heavy goods vehicles - the main contributors to road wear and tear - are particularly subject to these loads, due to their weight and associated geometry.
Their height, as well as the height at which the load they contain is placed (in addition to their unladen weight), considerably increases mass transfers to the outside wheels.These stresses, applied repeatedly as each truck passes over a pavement of this type, cause the asphalt layers to detach by fatigue, resulting in rutting and/or ravelling and leaving room for the associated cracking.

When a roundabout is to be rehabilitated, the absence of a detour option may mean that traffic has to be kept moving during the work.

As a result, the project is divided into phases and completed in a short timeframe to minimize disruption to users.

This imposed phasing represents a major risk of discontinuity in the structure, and is consequently a source of additional stress (differential settlement, opening of joints, etc.).

The high traffic density within these structures is a point to be anticipated in the management of a traffic circle reinforcement operation: minimizing the duration of work remains an essential point in the smooth running of a worksite in these busy areas.