Chapter 11: Non-destructive deflection testing

A simple and convenient method to assess the structural integrity of pavements is to apply a load to the surface and measure the resulting deflection. In this chapter a variety of methods for measuring pavement deflections to obtain information about the structural condition and load-carrying capacity of pavement systems is examined.

Static deflection measurements under a loaded plate were, from the 1930s, used in the application of basic soil mechanics principles to develop bearing capacity evaluations for the design of building foundations. This process was used in the early stages of pavement engineering, as attempts were made to develop a rationale for assessing the carrying (bearing) capacity of the pavement structure. Very slow-moving load deflection measurement, which originated with the development in the late 1940s of the Benkelman beam used at the Western Association of State Highway Organizations (WASHO) Road Test in the early 1950s in the USA (WASHO, 1954), represented a progression from the plate bearing test. Following the introduction of the Benkelman beam, pavement testing could be undertaken under a loaded wheel which allowed complete pavement structures (foundation and bound layers) to be tested, rather than just the foundation/soils.