With base stabilization a stabilizing agent is added to an aggregate base material to improve the physical properties and/or improve the load carrying capacity of the aggregate base. 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 aggregate base materials. Table 15-1 from the BARM provides recommended bituminous and chemical stabilizing agent selection criteria based on properties of the proposed reclaimed materials for FDR. The same table is applicable for aggregate base stabilization.
A test strip on site should be used to verify that the contractor can produce a mixture that meets the requirements of the mix design and construction specifications
Aggregate Materials: Base stabilization consists of adding stabilizing agents to granular materials to improve the physical and/or load bearing properties of the materials. The aggregates can come from a quarry or pit for new construction or be from an existing paved or unpaved road.
Bituminous Stabilizing Agents: Bituminous stabilization is accomplished with the addition of emulsified asphalt or foamed (expanded) asphalt to aggregates. Bituminous stabilizing agents maintain a flexible structure and have little restrictions to the release of traffic. However, they have tighter gradation requirements and require aggregates to be of lower plasticity.
Emulsified Asphalt Stabilizing Agents: Emulsified asphalts for base stabilization 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 aggregate 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 aggregate mixture, 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 base stabilized 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.