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.

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.

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 ( , 2012):

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.