A comparison of devices for measuring stiffness in situ.

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.