The variability between devices that measure the stiffness of a road foundation in situ and their
accuracy are important considerations for the introduction of such field assessment methods into the
construction monitoring process. The aim of this paper is to present the significant findings of recent research
into the comparability of four such stiffness devices. Results have been obtained from commercial
construction sites and large-scale field trials covering a wide range of material type and stiffness. In
addition, controlled tests on a synthetic rubber were carried out to investigate repeatability, as well as a study
to investigate the stress dependency of the computed stiffness values.
The results show significant variations in the correlation coefficients, which were shown to be dependent
upon material type and construction methods. Conclusions are drawn with regard to the efficacy and
accuracy of the four stiffness measuring devices and proposals made concerning their suitability for the range
of site construction conditions and materials
