Process
Cold Central Plant Recycling (CCPR) is the process in which the asphalt recycling takes place at a central location using a stationary cold mix plant. The stationary plant could be a specifically designed plant or a CIR train minus the cold planing machine set up in a stationary configuration. RAP used in CCPR is commonly obtained by cold planing or removal by backhoe, excavator or front end loader. The material is then transported to a central plant where it is sized and mixed with a bituminous recycling agent. The recycling agent type, grade and application rate are determined through evaluation of the RAP and the mix design process.
The CCPR plant has a belt scale, a computer controlled recycling agent system, a chemical additive system, if necessary, and a pugmill. The CCPR mixture can be immediately used as a recycled pavement or it can be stockpiled for later use. For temporary storage or loading of haul trucks, a surge hopper may be used. A conveyor stacker is used if the CCPR mixture is stockpiled. The CCPR mixture is hauled to the job site with conventional haul/dump trucks or belly dump trucks if a windrow elevator is used. Placement of the CCPR mixture is with conventional asphalt pavers. If smoothness is not a factor, a motor grader could be used.
Variations
Equipment Variations
PROCESS VARIATIONS
- Onsite CCPR: RAP is milled from the roadway to be reconstructed, hauled to a processing site, processed and the CCPR mix returned to the same roadway for placement and compaction
- Imported CCPR: RAP is milled from one roadway, hauled to a processing site, processed and retuned to a different roadway for placement and compaction
- Central Facility CCPR: RAP is imported from various projects to a central location and stockpiled for future use. The RAP is processed and hauled to various roadways for placement and compaction
CCPR mixes can be made using a specifically designed plant. The CCPR plant usually has a single cold feed bin/hopper for the RAP. The central plant has a RAP feed conveyor belt with a belt scale linked to a computerized liquid recycling agent, water and additive system that accurately meters the materials into the pugmill based on the weight of the RAP. Dry additives are typically transferred via silo augers or separate transfer devices tied to the belt scale.
The central plant may require a screening/crushing unit to control the maximum size of RAP or a scalping screen to remove oversize RAP.
CCPR is often produced using the screening/crushing unit and mixer from a CIR train set up in a stationary configuration.
If new aggregates or RAP fractionation is desired, cold feed bins require separation chambers or multiple cold feed bins.
Material Variations
Recycling Agents
Bituminous recycling agents are required and consist of foamed (expanded) asphalt or emulsified asphalt.
Recycling Additives
Cement and lime have been successfully used as additives in CCPR. They can be added dry or in slurry form. These chemical additives are used with recycling agents to improve early strength gain, increase rutting resistance and improve moisture resistance of CCPR mixtures. 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 3 to 1 for emulsified asphalts and 2.5 to 1 for foamed asphalt.
Corrective Aggregate
If the gradation of the RAP is not ideal, corrective aggregate can be added through a separate cold feed bin. Coarse aggregate improves the skeleton of the mix and can increase stability. Fine aggregate can help with the foaming process.
Common Combinations
CCPR + Hot Mix Overlay: To increase the structural capacity of the road and to apply a wearing surface to protect the CCPR mix, a hot mix overlay is commonly placed.
As a general rule, to achieve the same structure, a hot mix lift can be replaced by a CCPR layer that is 15% to 25% thicker (eg. 1” hot mix ~1.25” CCPR)
CCPR + Single or double chip seal: Where traffic volumes are low and the CCPR layer and existing pavement structure are sufficient to carry the anticipated loads, a single or double chip seal provides an economical wearing surface.
A pavement design with appropriate traffic levels will tell you how thick the CCPR layer needs to be
CCPR + Micro or slurry surfacing: Where traffic volumes are low and the CCPR layer and existing pavement structure are sufficient to carry the anticipated loads, micro or slurry surfacing provides an alternative economical wearing surface.
To get the best bond, let the CCPR cure for 3-10 days after construction (and 24 hours after any rainstorm) before applying the tack coat and the final pavement surface
CCPR + CIR: For deeper hot mix distresses, the upper hot mix can be milled off and processed off-site via CCPR, while the lower layers can be recycled in-place. The CCPR mix is returned to site and placed, so thicker pavements can be recycled as well.
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