Mechanistic-Climatic Characterization of Foamed Asphalt Stabilized Granular Pavements in Saskatchewan

Saskatchewan Department of Highways and Transportation (SDHT) is investigating alternate recycling and strengthening systems for in-service thin granular pavements. This research is being performed in an attempt to improve granular pavement structural integrity through recycling and stabilization, as well as reduce the dependence on new source aggregates. This paper summarizes the findings of a pilot project investigating the mechanistic-climatic laboratory characterization of two typical Saskatchewan thin granular pavements stabilized in the lab with foamed asphalt, cement and asphalt emulsion. The two granular systems evaluated as part of the study were Control Section 15-11 (C.S. 15-11), comprised of a conventional sealed granular base pavement, and Control Section 6-15 (C.S. 6-15), comprised of select silty sand borrow material with a thin asphaltic concrete wearing surface. The two granular systems evaluated as part of this study were chosen to characterize the typical range of select granular materials used in granular pavements constructed in central and northern Saskatchewan. The C.S. 15-11 in situ granular base was found to require strengthening because it is relatively high in fine sand fraction as well as contains a high fraction of plastic clay fines. These two physical properties are believed to be the primary causes for marginal performance of granular bases in the field. The C.S. 6-15 in situ select silty sand base was selected as a typical granular pavement requiring strengthening, as silty sand granular materials are a common road building material used in northern Saskatchewan. This research showed that conventional indirect tensile testing was relatively insensitive to the effects of foamed asphalt stabilization. In addition, conventional indirect tensile testing does not provide material constitutive relations across the full range of typical Saskatchewan field state conditions, including stress states and load frequencies. Triaxial frequency sweep characterization determined that foamed asphalt with cement stabilization significantly improved the mechanistic primary constitutive behaviour of the C.S. 15-11 granular base system; however, foamed asphalt only marginally affected the C.S. 6-15 silty sand. It was found that asphalt emulsion with cement significantly improved the mechanistic constitutive behaviour of both granular systems considered. This research also determined that foamed asphalt stabilization provided only marginal improvements in the unconfined compressive strength of post saturation and freeze-thaw climatic conditioning relative to the asphalt emulsion and cement stabilization system.