If pulverization is not sufficient to provide adequate structural support, a stabilizing agent (mechanical, chemical or bituminous) may be necessary. The selection of a stabilizing agent is a function of:
A wide range of bituminous and chemical stabilizing agents are currently available and are used to improve physical properties and/or moisture resistance of the reclaimed materials. Table 15-1 from the BARM provides recommended bituminous and chemical stabilizing agent selection criteria based on properties of the proposed reclaimed materials.
RAP and Underlying Granular Materials: In FDR, all asphalt layers are pulverized and mixed with some, or all, of the underlying materials (granular base, subbase or subgrade materials). The decision on how much of the underlying material to mix with the pulverized asphalt layers depends on the thickness of asphalt layers relative to the underlying material, gradation and physical properties of pulverized materials desired structural properties of the FDR section and subgrade stability.
Mechanical Stabilizing Agents: Mechanical stabilization is achieved with the addition of granular materials, such as new aggregate or recycled materials (reclaimed asphalt pavement (RAP) or crushed concrete) to the pulverized material. The amount and gradation of additional granular materials will depend on the existing roadway conditions and properties of the reclaimed material. The reclaimed material, and any granular materials which have been added, will behave similar to either an unbound granular base or a base with some cohesion, depending on the amount and type of RAP in the mixture.
Bituminous Stabilizing Agents: Bituminous stabilization is accomplished with the addition of emulsified asphalt or foamed (expanded) asphalt to the pulverized material. Bituminous stabilizing agents maintain a flexible structure and have little restrictions to the release of traffic. However, they have tighter gradation requirements and require FDR materials to be of lower plasticity.
Emulsified Asphalt Stabilizing Agents: Emulsified asphalts for FDR consist of engineered emulsions, cationic slow and medium set emulsions and high float emulsions. All can be made with or without polymer modification. Emulsified asphalts coat and bind the RAP particle together.
Foamed (Expanded) Asphalt Stabilizing Agent: Foamed asphalt occurs when a small amount of cold water, 60 to 77 °F (15 to 25 °C), is introduced into hot asphalt binder, 320 to 375 °F (160 to 190 °C) inside an expansion chamber. Foaming or expansion occurs as the water changes states from a liquid to a vapor, a process that is accompanied by an expansion of 8 to 15 times its original volume. In the foamed state, the asphalt binder’s viscosity is greatly reduced and its surface area is greatly increased, enabling it to be readily dispersed throughout the recycled materials. Foamed asphalt is well suited for mixing with moist materials at ambient temperatures. For adequate dispersion of the foam in FDR mixtures, additional fines, compared to emulsified asphalt, may be required.
Chemical Stabilizing Agents: Chemical stabilization is achieved with the addition of cement (portland or hydraulic), lime (hydrated or quicklime), self-cementing class C fly ash, class F fly ash (when used in combination with other additives), cement kiln dust (CKD), lime kiln dust (LKD), calcium chloride, magnesium chloride or proprietary products. Cementitious stabilizing agents work with a wide range of materials and can accommodate higher plasticity materials. There can be limitations on release of heavy traffic and special requirements to minimize shrinkage cracking. Lime stabilized materials are generally used with fine grained, highly plastic materials. They generally yield moderate strength gains and have some limitations with respect to release of traffic.
Additives: Stabilizing additives, such as cement or lime/LKD, in combination with bituminous stabilizing agent can significantly increase the retained strength and moisture resistance of FDR mixtures without adversely affecting fatigue properties, and act as a catalyst to increase early strength gain, allowing the roadway to be opened to traffic sooner. It is becoming common practice to use cement or lime/LKD as an additive in conjunction with bituminous stabilizing agents with marginal quality reclaimed materials. Additives are added in small amounts, typically 0.5 to 1.5% by weight of reclaimed material.