Semi-rigid pavements : a dead loss for backcalculation procedures using deflection bowls?

Non-destructive pavement testing by fway of dynamic deflection measurements with e.g. a Falling Weight Deflectometer (FWD) is a widely accepted procedure in the world. The measured deflection bowl can be used as an overall yardstick for the structural condition of the pavement. However, when the deflection bowl is used for the determination of the moduli, major errors can be made especially when using backcalculation procedures. In the case of semi-rigid pavements (or in general of pavements where the modulus of the base is of the same order as the modulus of the upper layer) the deflection bowl does not provide all the information required for the separate calculation of the right modulo for the asphalt layer and the bound base layer. It is recommended to determine the modulus of the asphalt layer or base layer by laboratory tests and to use this modulus as a fixed value in the backcalculation procedure.