Case Studies Using Falling Weight Deflectometer Data with Mechanistic-empirical Design and Analysis

Abstract

The need to accurately characterize the structural condition of existing pavements has increased with the recent development, release, and ongoing implementation of the Mechanistic-Empirical Pavement Design Guide (MEPDG), developed under National Cooperative Highway Research Program Project No. 1-37A. A number of different material inputs are required in the procedure, and it is important that these be adequately characterized and defined so that competent structural designs can be developed. The analysis of deflection data collected with a falling weight deflectometer (FWD) provides a quick and reliable way of characterizing many of the parameters of the existing pavement layers. This paper summarizes how deflection data are incorporated into the MEPDG and describes two case studies, one with a flexible pavement and one with a rigid pavement. Significant findings and recommendations from the evaluated flexible pavement case study include the following: surface-down cracking is critical in the design of the hot mix asphalt (HMA) overlay, correction factors should be used for adjusting backcalculated layer moduli to laboratory determined values, and an FWD testing frequency of 30 Hz should be used for estimating the existing HMA modulus. For rigid pavements, the case study found that the thinnest overlay produced from the MEPDG was a bonded Portland cement concrete (PCC) overlay, whereas the HMA overlay was unreasonably thick. Within the design procedure, the manually entered k-value is used for unbonded and bonded jointed plain concrete pavements but does not appear to be used by the program in the HMA overlay design. The backcalculated dynamic (or static) elastic modulus should be used for the PCC layer, and the dynamic k-value should be used for the supporting layers. The backcalculated k-value representing the composite stiffness of all layers beneath the slab does not appear to have a significant influence on the design thickness for the pavement structure analyzed.

Sensitivity Analysis of Determining the Material Parameters of an Asphalt Pavement to Measurement Errors in Backcalculations

“…The influence of measurement errors on the calculation results has been analyzed. Four variants of measurement errors for deflections were considered: the normal distribution of errors is assumed with zero mean value and a standard deviation of the error equal to (2 µm + 0.01 u i )/3 (2 µm + 0.01 u i —value of maximum error comes from [ 39 ])—error calculated separately for each i -th measurement point of a given basin—variant denoted as D1S; the normal distribution of errors is assumed with zero mean value and a standard deviation of the error equal to (2 µm + 0.01 u i )—error calculated separately for each i -th measurement point of a given basin—variant denoted as D1B; the normal distribution of errors is assumed with zero mean value and a standard deviation of error equal to (2 µm + 0.01 u 1 )/3—error was firstly calculated for deflection in the load axis; the error for deflection in i -th measurement point is equal to the ratio of deflections u i /u 1 multiplied by error in a first measurement point—variant denoted as D2S; the normal distribution of errors is assumed with zero mean value and a standard deviation of error equal to (2 µm + 0.01 u 1 )—error was firstly calculated for deflection in the load axis; the error for deflection in i -th measurement point is equal to the ratio of deflections u i /u 1 multiplied by the error in a first measurement point—variant denoted as D2B. …”

“…the normal distribution of errors is assumed with zero mean value and a standard deviation of the error equal to (2 µm + 0.01 u i )/3 (2 µm + 0.01 u i —value of maximum error comes from [ 39 ])—error calculated separately for each i -th measurement point of a given basin—variant denoted as D1S;…”

Evaluation of solid-polymer-modified asphalt mixtures : phase 1 : construction and performance testing of field pavement sections

“…To compare the modulus values of asphalt overlay layers among four sections, this study treated the asphalt layer as two individual layers in the backcalculation analysis; the top layer is a thin asphalt overlay, and the second layer is the old existing asphalt layer. However, a thin asphalt layer less than 75 mm is not recommended due to the associated high error (Pierce et al 2017). Therefore, the current study made several attempts to minimize the associated errors to estimate a reasonable modulus value for each layer.…”

“…Asphalt Layer: The seed modulus value for the asphalt overlay layer was considered to be 500,000 psi (3447 MPa) at 20°C based on the dynamic modulus master curve conducted in the laboratory. The seed value for the old asphalt layer was assumed to be 150,000 psi (1034 MPa) at 20°C and treated as a fixed layer in the backcalculation analysis (Pierce et al 2017).…”

Seasonal and diurnal variations of back-calculated layer moduli in flexible pavement and FWD testing guidelines

“…Pierce et al 2017): a. Temperature data should be collected during FWD testing on flexible pavements.…”