Robert C Briggs

Using the Multidepth Deflectometer to Verify Modulus Backcalculation Procedures

This paper describes the use of a multidepth deflectometer (MDD) for determining the resilient modulus of pavement layers. The MDDs are installed in specially drilled holes, and up to six modulus may be placed in a single hole. This device measures the relative deflection of each layer with respect to an anchor point located approximately 2.1 m (7 ft) below the pavement service. In this paper results are obtained from two instrumented pavement sections at the Texas Transportation Institutes Research Annex. By obtaining MDD deflections under falling weight deflectometer (FWD) loadings, two independent procedures are available for estimating layer moduli. A procedure for automatically calculating layer moduli from MDD measurements is described and the results obtained are compared with those obtained from three backcalculation schemes using FWD surface deflection data. When a semi-infinite depth subgrade was specified, all three procedures predicted subgrade moduli approximately 50% higher than those estimated from MDD measurements. When a finite depth was specified, a rock layer at 6 m (20 ft), all three procedures calculated subgrade moduli values very similar to the MDD values. The ELSDEF program with finite subgrade also produced comparable values for the moduli of base and subbase layers.

Seasonal Effects on a Low-Volume Road Flexible Pavement

Alaskas Parks Highway is a 323-mi, asphalt-paved, predominantly two-lane, low-volume roadway that links the states two largest cities (Anchorage and Fairbanks). The effects of thawing on this low-volume roadway pavement were assessed, and seasonal adjustment factors for the different pavement layers were developed. Falling weight deflectometer (FWD) testing was carried out weekly during springtime at three temperature data probe sites. The purpose of the FWD tests was to monitor the pavement structural conditions and moduli variations during the spring thaw and recovery periods. When the pavement began to thaw, how quickly thawing progressed, and whether the pavement returned to an equilibrium summertime state were also determined. Observations indicated that profound changes in backcalculated modulus values could occur within 1 week of thaw onset. In addition, moduli were used to develop modular ratios for the subgrade and base course layers. Results showed that while thawing, the subgrade and base course layers reached about 30% to 40% and 50% to 70%, respectively, of their summertime reference moduli. These ratios were lower than those currently used in Alaskan pavement design. Additional testing should be conducted to verify the proposed seasonal adjustment factors. Surface moduli values, calculated and plotted without the need for the pavement layer thicknesses, are useful in instantly identifying whether partial thawing has occurred.

Procedure for Optimum Pavement Deflection Test Spacings for Network-Level Applications

Structural models in pavement management systems range from the simple to the relatively complex. The simplest models use deflections or deflection basin parameters to characterize subgrade and pavement structural properties, while the more complex ones use pavement layer moduli (derived from deflections) and pavement layer thicknesses and material types to calculate pavement response, which is then used to predict failure, much like project-level pavement design analysis. Any pavement management system using the latter, more complex approach would undoubtedly need more defection information. In any case, deflection data collection is expensive and time-consuming. Most states and local agencies have few falling weight deflectometers, which are used mainly to collect project-level deflection data for scoping maintenance and rehabilitation work at the project level and for research purposes. This study explores the use of models to estimate the error associated with the choice of different deflection test spacings that can be used to optimize data collection for more efficient and economical applications in pavement management systems.