This Paper is Beneficial for FWD.

1. PURPOSE OF DEFLECTION TESTING
The primary purpose of deflection testing is to determine the structural adequacy of an existing
pavement and to assess its capability of handling future traffic loadings. As observed in the work
by Hveem, there is a strong correlation between pavement deflections (an indicator of the
structural adequacy of the pavement) and the ability of the pavement to carry traffic loadings at a
prescribed minimum level of service.(1) Early work attempted to identify maximum deflection
limits below which pavements were expected to perform well, and these limits were based on
experience and observations of performance of similar pavements. This concept quickly lent
itself to overlay design, in that required overlay thicknesses could be determined based on trying
to reduce maximum pavement deflections to tolerable levels.
When complete deflection basins are available, deflection testing can provide key properties for
the existing pavement structure through backcalculation of the measured pavement responses.
Specifically, for HMA pavements, the elastic modulus (E) of the individual paving layers can be
determined, along with the resilient modulus (MR) of the subgrade. For PCC pavements, the
elastic modulus (E) of the slab and the modulus of subgrade reaction (k or k-value) can be
determined. In addition, deflection testing conducted on PCC pavements can be used to estimate
the LTE across joints or cracks (see figure 2) as well as to identify loss of support at slab corners
These properties of the pavement layers and of the subgrade are used in pavement design
procedures or in performance prediction models to estimate the remaining life or load-carrying
capacity of the pavement. They can also be used in elastic layer or finite element programs to
compute stresses and strains in the pavement structure and are also direct inputs in many overlay
design procedures to determine the required overlay thickness needed for the current pavement
condition and the anticipated future traffic loadings.
Deflection data can also be used in a number of other ways to help characterize the condition of
the existing pavement. For example, plots of deflection data along a pavement project can be
examined for nonuniformity, which may suggest areas that require further investigation using
destructive means. In addition, daily or seasonal deflection data can provide insight regarding a
pavement’s response to environmental forces, including the effects of thermal curling, frozen
support conditions, and asphalt stiffening. Some agencies also use deflection criteria to establish
seasonal load restrictions for certain low-volume roads. Deflection testing has also seen some
limited use as a means of monitoring the quality of a pavement during construction.(3) Finally, a
few agencies conduct deflection testing at the network level to provide a general indication of the
structural capacity of the pavement structure.