Cold Central Plant Recycling

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RAP: CCPR is different from other mixes in that it uses materials in existing RAP stockpiles or milled from an existing road.  CCPR RAP is not like new aggregate and is not produced to a tight specification band. Most times, the RAP can make a satisfactory CCPR mix, but sometimes the RAP is too soft or too fine and recycling additives are needed to improve its properties.

EMULSIFIED ASPHALT RECYCLING AGENTS: Emulsified asphalts for CCPR consist of engineered emulsions, cationic slow and medium set emulsions and high float emulsions. All can be made with or without polymer modification or being modified with an asphalt rejuvinator. If allowed by the specifications, the inclusion of a diluent, either pertoleum based or bio based, may improve the workability and coating of the mix. Emulsified asphalts coat and bind the RAP particle together. 

  • Engineered Emulsions: Engineered emulsions are gaining popularity as a recycling agent due their ability to be customized for particular projects. Customized properties include mixing and coating ability, breaking times, curing times, moisture resistance, softening ability of the emulsion and stiffness properties of the residual binder. Engineered emulsions are typically but not exclusively modified cationic slow set emulsions.  
  • Cationic Slow and Medium Set Emulsions: Cationic slow setting emulsions, with or without polymers, have long workability times to ensure good mixing and contain low levels of volatile solvents. Cationic medium setting emulsions have quicker breaking times but longer curing times than cationic slow setting emulsions due to higher solvent contents. Cationic slow set emulsions are often preferred over emulsions with higher solvent contents both from an environmental and performance standpoint.
  • High Float Medium Set Emulsions: High float emulsions are selected for their ability to soften old, aged binder and their ability to coat coarser aggregates. Coating tends to be selective with the smaller particles coated with a thick film of asphalt while the larger particles are partially coated. 
Foamed asphalt expansion chamber

FOAMED (EXPANDED) ASPHALT RECYCLING 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 CCPR mixtures additional fines, compared to asphalt emulsion, may be required.

CHEMICAL ADDITIVES: Chemical additives such as lime or cement are used with recycling agents to improve early strength gain, increase rutting resistance and improve moisture resistance. Lime is typically added at 1.0 to 1.5% by dry weight of RAP. Cement contents should be kept low, typically 0.25% to a maximum of 1%, to prevent brittle behavior of the mixture. The ratio of asphalt residue to cement should be a minimum of 2.5 – 3.0 to 1. 

CORRECTIVE AGGREGATE: Many CCPR projects are constructed without the addition of new or corrective aggregate. The decision to add new aggregates should not be based solely on the gradation of aggregate recovered from the RAP. There should be a quantifiable improvement in measured mix properties to justify the added expense. If the existing RAP is too fine or is lacking certain particle sizes, adding some ¾” stone can beef up the aggregate skeleton and increase mix stability. Adding ¼”-minus screenings can provide more fines to help the foamed asphalt process. The mix design process will determine what is needed.