4.1 General

Frequently, local base and subgrade materials do not meet the material and engineering properties required for good pavement foundation performance. A large portion of pavement construction performed today consists of rehabilitating existing roads, which frequently contain reclaimed subgrade, base, and surfacing material layers that are inadequate for current or future traffic loading demands. In order to achieve the needed engineering properties, the base or subgrade frequently requires treatment. In rehabilitation projects, treatment is typically road- mixed in place in accordance with the
Standard Specifications for Construction and Maintenance of Highways, Streets and Bridges
, Items 260, 265, or 275 (road-mixed) to expedite construction and save cost, but better material uniformity is achieved by plant-mixing. For new and total reconstruction jobs, the designer may choose plant-mixed materials governed by the Standard Specifications, Items 263, 276, 292, 340, 341, or 344.
Where treatment is necessary to meet the engineering requirements, most materials can be made suitable by incorporating chemical additives, such as asphalt, cement, fly ash, or lime. Each of these additives is effective when the material is designed and applied properly. Proper design and application of materials with additives will minimize premature failures of the material and pavement structure. Base and subgrade materials are treated with chemical additives to achieve one or more goals when available materials do not meet project-specific requirements. Reasons for treatment include the following:
  • increase strength to provide long-term support for the pavement structure,
  • reduce the required pavement thickness,
  • reduce moisture susceptibility and migration,
  • allow for the use of local materials, and
  • bind salvaged materials of differing composition for pavement rehabilitation projects.
In addition, goals of subgrade treatment include the following:
  • reduce shrink/swell of expansive soils or existing materials, and
  • provide a working platform for construction of subsequent layers by drying out wet areas and/or temporarily increasing strength properties.
“Guidelines for Modification and Stabilization of Soils and Base in Pavement Structures”
is a document outlining the proper methodology of selecting, designing, and evaluating treated soils and base courses for pavement structures. This document also provides some basic knowledge on the various treatment methods, the goals of treatment, and the mechanisms each treatment method employs.
When soils and base contain soluble sulfates, use the
“Guidelines for Treatment of Sulfate Rich Soils and Bases in Pavement Structures”
to identify the feasibility for treatment and construction considerations for incorporating chemical additives.
Typically, permanent treatment is desired for base and subgrade materials, especially over the projected useful life of the pavement structure where modulus values are assumed constant in the design process. Structural credit above the intrinsic capability of the raw base and subgrade material should not be assigned to a treatment process that does not have proven lasting effectiveness. It is incumbent upon the design team to evaluate representative samples of the proposed base and subgrade materials under test methods appropriate for the additives being considered. Follow the recommendations in the
“Guidelines for Modification and Stabilization of Soils and Base in Pavement Structures”
or the applicable specification. Furthermore, strength testing under dry and wet conditions should be used to ensure the required properties are reliable under varying environmental conditions.