The Deflectograph is used to assess the structural condition of flexible pavements. It works on the principle that as a loaded wheel passes over the pavement, the pavement deflects and the size of the deflection is related to the strength of the pavement layers and subgrade.
The assessment procedure used depends on the type of pavement and its mode of deterioration. Some thick, well constructed flexible pavements with asphalt base have been found not to deteriorate in the conventional way and with timely attention to surface defects can have a long but indeterminate life.
These potentially long-life pavements are identified with deflection and thickness criteria. The structural condition of other flexible pavements is assessed in terms of residual life using a long-established Deflection Design Method based on deflection and traffic loading.
The Deflectograph is an automated deflection measuring system. It is a fully self-contained lorry-mounted system, whereby measurements of deflection are taken at approximately 4m intervals in both wheel-tracks while the machine is in motion. It is regarded by the Overseeing Organisation as the standard deflection measuring device for use on flexible pavements.
The Deflectograph measures the amount a flexible or flexibl-composite road pavement bends under the weight of a standard axle. This information can be intergrated with details of the pavement construction and present and future traffic flows to determine the residual life of the pavement and the recommended overlay.
Because the deflection is measured in the nearside and offside wheel tracks it is easy to determine if the whole width of the road requires strengthening or just repair of the (generally) nearside wheel track.
Equipment and procedures for its use in the measurement of road pavement deflection in the UK are described in the, Design Manual for Roads HD 29/04 and the original research that verified the use of the deflectograph on roads is published in Transport and Road Research laboratory Reports L.R. 833, L.R. 834 and L.R. 835.