Thermal Activity of Base Course Material Related to Pavement Cracking (18-2)

Preliminary studies into environmental deterioration of pavements indicated that low-temperature cracking of the asphalt concrete surface was not likely for the west Texas area. Likewise, shrinkage cracking due to moisture loss in the base course was not a likely mechanism. An initial computer study using a heat transfer program showed that freeze-thaw activity during a winter, for Perryton, Texas, was centered mainly in the base course for a typical pavement. Comprehensive freeze-thaw testing showed the base course to be more active, thermally, than asphaltic concrete. The thermal activity is composed of a freeze deformation and a residual deformation. Both quantities are related to the soil moisture suction of the material. The soil moisture suction which develops in a sample is a function of the grain size, moisture, pore structure and clay mineralogy. Clay mineralogy determinations were done using x-ray diffraction to obtain the relative percentages of the clay minerals present. A direct relationship is established between the freeze deformation and clay mineralogy of the material tested. The data collected indicate a mechanism of particle reorientation may be causing the measured volume changes and be responsible for observed loss of strength in roadbed material after undergoing several freeze-thaw cycles. The data also demonstrate the validity of using clay mineralogy properties as a design quantity for base course material.