Minnesota Department of Transportation

511 Travel Info

Materials and Road Research

Materials and Road Research | Pavement Design

Falling Weight Deflectometer

The Falling Weight Deflectometer (FWD) is a device that is used to evaluate pavement and pavement layer stiffness. It is a trailer-mounted device that operates by dropping a weight on to the pavement and measuring the resulting pavement deflections. Various computations may be performed on the deflection data to evaluate the pavement and the stiffness of it and its constituent layers.

Data Analysis

FWD data may be analyzed several different ways for different purposes. The following options are available to analyze FWD data.

TONN Method

The TONN Method, implemented with the TONN Program, calculates the recommended seasonal load restriction based on pavement deflections collected with a FWD. The TONN Method applies only to HMA pavements, either on aggregate base or on subgrade.

The TONN Program is an Excel spreadsheet that is used by District personnel to automate the TONN Method. When FWD testing is completed, the District will receive the data as an Excel spreadsheet from the Pavement Design Section. District personnel may then use the TONN Program, and information provided by the District, to calculate:

  • The recommended Seasonal Load Restriction (SLR) rating (known as the “TONN Rating”).
  • The estimated overlay (if any) required for the road to meet a 10 ton TONN Rating.
  • The remaining life of the pavement in ESALs.
  • The R-value of the subgrade.
  • Estimated GE value of the entire pavement section.

Download the TONN program.

ELMOD Back-calculation

ELMOD is a program operated by personnel at the Office of materials & Road Research that can calculate the modulus of each pavement layer from FWD data. The modulus of the subgrade can be converted into a subgrade R-value. The thicknesses of the existing layers is required to be able to process FWD data with ELMOD.

PCC Load Transfer Efficiency

This testing is used to determine how well a PCC joint or crack transfers a load across it. This testing is performed by dropping the FWD weight on one side of the joint and recording the subsequent deflections at the location of the weights impact and the deflections on the other side of the joint. The ratio of the unloaded deflection to the loaded deflection is reported as the load transfer efficiency.

Request Testing

FWD testing is normally performed in the summer and early autumn months when the pavement is unaffected by frost or thaw-weakening. In the northern districts (D1,D2, D3, D4) testing is normally performed from June 1 to Oct. 15. In the southern districts (Metro, D6, D7, D8) testing is normally performed June 1 to Nov. 1.

Typically, the districts will be asked to file requests for FWD testing in the winter before the testing season so that the operators can be most efficiently scheduled. However, testing may be requested at any time.

FWD Testing History (Excel), Updated April 11, 2019

FWD Calibration Center

The MnDOT FWD Calibration Center is located at the Materials & Road Research Laboratory in Maplewood. Prior to traveling for calibration, please complete the Calibration Checklist (PDF).