Even if visco-elastic materials like asphalt concrete relaxes, the effect leaves the interface in a somewhat disturbed condition that may affect the deflections as measured by the FWD.įig 1: A wheel load would leave the asphalt layer underside in a compressed state in the longitudinal direction of travel. This effect may sometimes result in a tiny gap between the interfaces, which will be exaggerated when the underside is cold as is a well-known fact for Portland cement concrete (PCC) slabs, see Figure 1.
When an approaching load such as from a truck gets to any arbitrary point the underside of the bound layer will be in a compression state, followed by expansion and finally go back to a compression state in the longitudinal direction as the load leaves.
#BACKCALCULATION STIFF BOTTOM LAYER FULL#
It has been found that full friction in the interface between layers is not always occurring in reality. However, the FWD is using a nominal load of 50 kN and a nominal load time history to set a standard for engineers to use.
Non-linearity and viscoelastic effects may not be accounted for. In many cases the less than realistic results depend on limitations of the model used. The derived backcalculated elastic modulus of the unbound base layer is sometimes very low, e.g. A finite element study confirms the predicted results of not dealing with layer slippage. The present paper looks upon the effects of faulty layer interface assumptions in the elastic layer analysis. However, sometimes assumptions in the model are violating factual conditions. Many engineers have developed a confidence in this method and its results are being used in the further reconstruction design of the road. Thus, it is possible to calculate a response on any given load that could be used in a mechanistic design procedure. Data from the tests are used for backcalculating elastic properties of layers in the road structure as well as the subgrade. The Falling Weight Deflectometer (FWD) is the most commonly used tool for non-destructive testing of roads and airfields in the world today. However, layer interface slippage can be backcalculated in favor of getting a better fit between measured and backcalculated deflection basins. If not accounted for, some critical strains determining the overlay design can be underestimated as well. If slippage occurs it leads to an underestimation of unbound base and subbase layers.
The present paper shows that full friction may not always be the case in reality. One is introducing full friction in layer interfaces. Some assumptions are made in the process. By using the non-destructive device the falling weight deflectometer, it is a proven method of achieving a sound mechanistic design of the new layer thickness. Johan Olsson, Chalmers University of Technology, Göteborg, Swedenīackcalculation of road layer elastic moduli is an important procedure in determining stresses and strains in road structures due for overlay design.
Lenngren, Chalmers University of Technology, Göteborg, Sweden Start > Contributions > Posters > Case Study: PrintĬarl A. International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003 Enhanced Backcalculation Techniques for Assessing Highway Structural Properties
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