The Falling Weight Deflectometer (FWD) is a non-destructive measurement tool used to determine the in-situ structural performance of an existing pavement structure.

The use of the Falling Weight Deflectometer (FWD) is widely spread across the pavement design industry and is used to:

  1. Identify areas of weak pavement
  2. Back-calculate layer properties for use in overlay designs
  3. Estimate the remaining life of the structure

The process of the Falling Weight Deflectometer (FWD) is to apply a dynamic load to the pavement structure in the form of a standardised ‘drop weight’ and measure the deflection at various points longitudinally in the pavement to recreate the ‘deflection bowl’ of a wheel load.

This process is typically undertaken repetitiously so as to provide statistically validated deflections in order to develop ‘homogenous’ sections within the road and are correlated with the existing pavement compositions to identify the pavement performance.

The concept of identifying ‘homogenous’ sections is used so as to characterise a road section by its characteristic performance and consequently determines a representative performance value. When identifying these sections, consideration is given to the following:

Maximum Deflection

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The maximum deflection is typically measured at D0 (where the denoted number is the distance from the centroid of the drop weight) and represents the pavement stiffness and subgrade support beneath the pavement.

Curvature Function

The curvature function is the difference in the deflection at a nominated distance and D0 (i.e. D0 – D200) used to determine the shape or gradient of the deflection bowl and can be used to determine the susceptibility of fatigue of asphalt in a pavement configuration

Deflection Ratio

The deflection ratio is the ratio between the maximum deflection and a nominated distance (i.e. D0/D250) used to determine the structural response in the base material, whether it is rigid, bound or unbound.

A statistically validated homogenous section is typically represented by a ‘characteristic’ location that represents the entire section and is used to initiate further testing for the purpose of back-calculation layer analysis.

Back-calculation involves recreating the deflection bowl of the characteristic location and entering ‘seed’ layer properties based on the existing pavement profile followed by an iterative process to determine distinct layer properties.

The use of Falling Weight Deflectometer (FWD) as a design tool is highly beneficial to pavement design and its popularity is related to its reliability and a significant reduction in costly ‘destructive’ testing otherwise commonly adopted.

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