Seasonal Variations of Pavement Layer Moduli Determined Using In Situ Measurements of Pavement Stress and Strain

Pavement design procedures have advanced a great deal in recent years, changing from empirical equations based on road tests in the 1950s to mechanistic-empirical design procedures developed in the past few years. The resilient moduli for the asphalt and soil layers of pavement sections are important properties necessary for pavement design, and an accurate method for determining moduli under different conditions is necessary. The stiffness of pavement section layers changes with the season, and typically, a road section will be the weakest during spring thaw due to loss of frozen soil stiffness, and increases in water content. This is critical to consider for roadways that are traveled by heavy truck traffic, where weight limits are implemented to reduce spring thaw damage. Resilient modulus is a form of the elastic modulus of soil. The value can be calculated using a variety of methods. AASHTO has a procedure for laboratory determination of resilient modulus, and correlations exist to estimate modulus based on other soil properties. The most widely used method of calculating pavement layer moduli is the backcalculation of moduli from deflection data obtained using a Falling Weight Deflectometer. The goal of this project was to collect in situ stress and strain data in an attempt to calculate resilient modulus directly in the field. Temperature data was also collected to help quantify the effect of freezing and thawing cycles on changes in modulus. A section of Rt. 15 in Guilford, Maine was instrumented with strain gages, stress gages, and climate related gages during the reconstruction of the roadway. Strain gages and thermocouples were installed in the asphalt layer, and strain gages, pressure cells, thermocouples, resistivity probes, and moisture gages were installed in the subbase and subgrade layers. A data acquisition system was set up on site to collect both high speed stress and strain responses, and static temperature, moisture, and resistivity responses. Data was collected during the winter, spring, and summer of 2006. Stress and strain responses were recorded for traffic loading due to normal truck traffic and controlled loading with a MaineDOT dump truck with a known weight. A Falling Weight Deflectometer was also used to acquire data for modulus backcalculation. Asphalt strain responses were used to estimate the value of Nf, the number of loading cycles required to cause fatigue cracking. Predicted and measured values of strain in the asphalt and the soil were compared. In situ moduli were calculated using recorded stresses and strains and related to FWD backcalculated moduli. These initial results from the instrumented site were used to observe the effect of freezing and thawing on pavement responses