Volume 20, Issue 4 (December 1997)
Determining a Pavement Modulus from Portable FWD Testing
This paper assumes that there is room for an additional measuring device that, on the one hand, will serve mainly as an in situ device not requiring complicated back-calculation procedures and, on the other hand, will not involve large volumes of in situ materials as presently required by the falling weight deflectometer (FWD). Such a device is the new portable FWD (PFWD). Comparative tests with this device show no significant correlation between the central deflection measurements taken on the asphalt surface and the corresponding Benkelman-beam-measured deflections. This is due to the fact that the PFWD device “senses” only the upper layers of the pavement, down to no more than a small thickness, whereas the two other devices “sense” all pavement layers down to the subgrade medium, as well. A composite-pavement modulus of elasticity (Ep) can still be derived from the PFWD measurements, but only with the performance of additional measurements on the asphalt surface, followed by coring of the asphalt layers down to their far bottom. In this way, the above Ep value is calculated for the full structural thickness and for the standard radius of the contact area obtained from the PFWD testing, i.e., the modulus of elasticity (EA) of the asphalt layers and the modulus of elasticity (EG) of the granular layers. A comparison of the above Ep values calculated from the PFWD outputs, with the values derived from the conventional FWD measurements, led to the introduction of a correction factor that varies with the ratio of the thickness of the asphalt layers to their elastic modulus. The lower the ratio, the closer the correction factor approaches the value of 1.0.