February 23, 2006, 2:00 – 3:00 p.m., Holmes Hall 244
Luis G. Diaz, PhD

Simplified Test Method for Stability Assessment of Asphalt Cold Mixes

The year-round need of small quantities of pavement repair materials for utility cut repairs and pothole patching make imperative the development of high quality materials to be used as needs arise. The quantities needed, and the times at which those materials are required, present cold mixes as the perfect candidate for these applications, particularly for utility-related pavement repairs. Given the fact that mix density plays an important role in the stability of asphalt mixtures, acceptable initial stability levels could be achieved in a properly designed mixture by controlling and adjusting compaction during construction. This situation makes initial compaction control extremely important for the stability of a cold mix repair, as its mechanical properties will improve with time as the solvents evaporate.

The objective of the study was to develop a test methodology to assess the stability of bituminous cold mixtures under repeated loading, potentially usable in the field as a quality control method. The methodology was developed by identifying the relationships between the parameters from a Laboratory Stability Test (LST), and the penetration rate (NPR) from a lighter version of the Dynamic Cone Penetrometer at various densification levels. The LST is a modified version of the laboratory uniaxial repeated loading test, commonly used to evaluate permanent deformation characteristics in Hot Mix Asphalt (HMA), and accounts for the initial instability present in non-cured asphalt cold mixes.

Multiple linear regression analysis (MLRA) was used to develop equations to estimate initial stability (εp100), stability under extended repeated loading (b), and number of load cycles to flow failure (Cf), as a function of NPR and other loose mix properties.

The equations presented are based on laboratory data from mixes commonly used by utility companies in their pavement repair operations. To cover the range of possible stability levels expected in materials of this type, mixes with different characteristics were included. The stability equations were satisfactorily validated after comparing the measured stability parameters from a mixture not used in the regression analysis, with the predicted values from the models.