The phase stability of crude oil is dependent on a multitude of factors, including temperature, pressure, and component fractions, especially long chain paraffin and polar asphaltene fractions. Paraffins precipitate out of the crude oil during pipeline transportation due to solubility limits, and form paraffin-oil gel deposits on the pipe walls. The presence of asphaltenes in crude oil is postulated to affect the formation of these paraffin gels. To quantify this effect, a controlled stress rheometer was used to study the gelation temperature and the yield stress of a model paraffinoil system. It was observed that the addition of asphaltenes in small proportions (∼0.1 wt %) resulted in a significant decrease both in the gelation temperature and the yield stress of the model system, indicating that the presence of asphaltenes hinders the gelation mechanism. Addition of higher amounts of asphaltenes resulted in macroscopic phase separation: a deposit consisting of asphaltenes and paraffins separated out of the liquid. The effects of operating conditions and the asphaltene polarity on the gelation process were also studied. Polarized light microscopy and nuclear magnetic resonance spectroscopy were used to obtain insights into the rheometric results.

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The asphaltic overlay design process considers various input parameters. These parameters are probabilistic in nature, but the current design process adopts deterministic values. This study proposes a comprehensive probabilistic approach for asphaltic overlay design to accommodate the variations in pavement layer thicknesses, pavement layer moduli, vehicle damage factor (VDF), lane distribution factor (LDF) and vehicle growth rate (r). An analytical method, first order second moment (FOSM), is used in developing the proposed design approach. A design example based on real world falling weight deflectometer (FWD) data of a 30 km long highway stretch is worked out to illustrate the proposed design approach. The results indicate that the current deterministic approach estimates an overlay thickness with approximately 95% reliability. Detail analysis of results from the proposed probabilistic approach denotes that choosing marginally lower design reliability could significantly reduce the overlay thickness. A sensitivity analysis of pavement layer moduli indicated that the asphaltic overlay thickness is sensitive to resilient modulus of base layer for fatigue and subgrade layer for rutting. The probability based design approach can accommodate variability of field material properties and construction practices in the asphaltic overlay design process.