Procedure for Use of Falling Weight Deflectometer to Determine AASHTO Layer Coefficients

Abstract

The AASHTO pavement design model is one of the most widely used empirical design models for flexible pavement design. A factor complicating its utility is the use of an abstract quality, the structural number (SN), to quantify the strength of the total pavement structure. A consequence of the use of SN is the need for structural layer coefficients to characterize the component materials of the pavement structure. These layer coefficients are difficult to assess directly. However, a method for determination of layer coefficients with a falling weight deflectometer (FWD) that are representative of the in situ behavior of the pavement materials is presented. This method is based on a model provided in the 1993 edition of the AASHTO Guide for Design of Pavement Structures for assessment of the effective SN. The Vermont Agency of Transportation (VTrans) has established a procedure that uses FWD data and the AASHTO effective SN model to characterize layer coefficients for Vermont pavement materials. VTrans found that the layer coefficients determined for unbound subbases were reasonable, whereas layer coefficients estimated for asphalt cement concrete materials were generally 25 to 35 percent higher than AASHTO’s implied maximum of 0.44. However, a statistical analysis indicates considerable support for the predictive qualities of FWD-derived layer coefficients to approximate the in situ layer coefficients estimated by an elastic layer simulation.