Section 4: Treated Subgrade and Base Courses
Anchor: #i10489024.1 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:
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- increase strength to provide long-term support for the pavement structure, Anchor: #NYGQVTWU
- reduce the required pavement thickness, Anchor: #TTAQVXSO
- reduce moisture susceptibility and migration, Anchor: #SHSUPLOQ
- allow for the use of local materials, and Anchor: #UVDQKETE
- bind salvaged materials of differing composition for pavement rehabilitation projects.
In addition, goals of subgrade treatment include the following:
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- reduce shrink/swell of expansive soils or existing materials, and Anchor: #PTMMHEDU
- 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.
Anchor: #i10500504.2 Cement, Lime and Fly Ash Treatments
These materials have a long history of use in various parts of the state and are generally the most cost efficient method of treating base and subgrade materials. Construction concerns when using these additives include the extended time necessary prior to opening the roadway to traffic. Additional concerns include inconsistent distribution of additives or mixing additives to the wrong depth when using “road-mixed” procedures. Too much additive, particularly cement, may result in extensive shrinkage cracking that may reflect through the surface of flexible pavements. A mitigation procedure to reduce shrinkage cracking in cement treated base is to microcrack the treated layer with a vibratory roller 24-48 hours after completing compaction and finishing.
Specific design strength requirements when using cement. To ensure long-term strength and stability of cement treated base (CTB) layers, a mix design must be completed to ensure sufficient amounts of cement are added to the mixture to achieve the target strength. Item 276, Cement Treatment (Plant-Mixed), currently designates three classes of cement-treated flexible base, based on 7-day unconfined compressive strength. Class M is intended for use with flexible pavements, Class L is intended for use with rigid pavements, and Class N may be used if the district has successful long-term experience with other strength values. When specifying Item 275, Cement Treatment (Road-Mixed), the laboratory target strength should be comparable to Item 276 requirements when selecting the appropriate cement content.
Anchor: #i10502594.3 Asphalt Treatment (Plant-Mixed) Bases
Asphalt-treated base (ATB) is also known as asphalt-stabilized base (ASB), or simply “black base.” For the HMA options, lower layers are allowed to contain higher percentages of recycled products and a lower quality aggregate with less stringent material specification requirements as compared to surface mixtures. Standard mix design, production, placement, and field acceptance procedures govern these base mixtures in accordance with the applicable specification. These materials are ready to be trafficked as soon as compaction is completed and temperature is below 160°F, but the cost is high compared to other treated base options.
Where long-term moisture susceptibility of HMA and asphalt-treated base (ATB) is a concern, using a plan note to increase the target laboratory density or decrease Ndesign (and thus increase asphalt content) may be beneficial.
Anchor: #i10503204.4 Emulsion and Foamed Asphalt Treatments
Item 314 and various one-time use special specifications allow the use of these additives. These treatment procedures are all intended for reclamation of in-place pavement structures. Application is typically accomplished using a reclaimer/recycler coupled to one or more bulk tankers that supply the liquid treatment agents. Recycling machines have become increasingly sophisticated and more powerful over the years; up to 12-in. of existing pavement depth can be processed with much improved distribution and uniformity of water and bituminous treatment agents. The emulsion content is typically below 4.0%, with the residual typically around 2.5%. Small percentages of lime or cement (typically ≤ 1.5%), when required by the mix design, are usually spread on the surface of the existing roadway ahead of the recycling machine. These additives are lifted and mixed together with the reclaimed roadway materials and liquid agents in the on-board mixing drum in a single pass. The lime or cement additives serve the following functions ( Wirtgen Cold Recycling Technology, 2012):
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- improves adhesion of the bitumen to the aggregate, Anchor: #MJTFLUWO
- improves dispersion of the bitumen in the mix, Anchor: #RTCHIFHH
- modifies the plasticity of the natural materials (reduces PI), Anchor: #JTFNLKUX
- increases the stiffness of the mix and rate of strength gain, and Anchor: #YHXRVXEK
- accelerates curing of the compacted mix.
Alternately, a lime or cement slurry can be injected into the recycler mixing chamber through a separate spray bar attachment. These processes result in a product that locks up finer particles by encapsulating them in bitumen, causing them to adhere to larger particles that are not coated. The result is a material that remains more flexible than other chemically-treated materials, but significantly reduces the moisture sensitivity. A limitation for using these treatment methods is the lack of knowledge and experience for many in the mixture design and placement operations of these materials. Designing a foamed asphalt mixture requires a specialized laboratory foaming unit. Contact CST’s Geotechnical, Soils and Aggregates Branch for assistance.
A significant precaution for these types of treatment options is dredging up high PI subgrade material and mixing it with the reclaimed pavement materials. The high PI materials will not disperse properly and cannot be adequately coated with bitumen; hence the material will remain moisture susceptible and prone to permanent deformation (rutting). This is more likely to occur with thin existing structures and structures with highly variable sectional thickness/composition along the project. Where this condition is anticipated and high PI soils are likely, the design process should include a preliminary recycling pass using lime, or install a lift of flexible base before the recycling pass to include this better material into the mix and to avoid dredging up the subgrade material. Curing emulsion treated layers to achieve strength and stability in a timely manner may be problematic where moisture loss is delayed by environmental conditions, such as high humidity or unanticipated high in-place moisture contents. On the other hand, curing foamed asphalt treated layers can often be achieved in approximately 2 hours, and the roadway can be reopened to traffic.