Process
CIR occurs within the roadway to be recycled and uses 100 percent of the RAP generated during the process. CIR treatment depths are generally from 3 to 4 inches (75 to 100 mm) with depths as thin as 2 inches (50 mm) possible with good underlying support and up to 5 inches (125 mm) provided proper compaction can be achieved. Greater depths are possible with a two layer system or using CCPR. Bituminous recycling agents are required and consist of foamed (expanded) asphalt or emulsified asphalt. Chemical additives, such as cement or lime may be used in addition to the recycling agent to improve early strength gain and resistance to moisture induced damage. New aggregate may be added to improve the recycled mixture properties. There are different types of CIR trains with different equipment configurations. CIR trains differ from one another in how the RAP is removed and sized, how the recycling agents and additives are added, how they are mixed and controlled and how the recycled mixture is placed. The CIR mat is typically open to traffic shortly after compaction (typically less than 1 hour). See CIR flowchart for detailed process.
Variations
Equipment Variations
Equipment Trains
There are different variations of trains used in CIR. These include Single Unit, Two Unit and Multi Unit trains.
Single Unit Trains mill the roadway and add new recycling agent immediately into the cutting head chamber and spread the mix with an attached screed. The forward speed of the machine determines the gradation of the material and the overall length of the train is shorter.
Two Unit Trains mill the roadway and add recycling agent in a mix paver where the materials are mixed and placed
Multi Unit Trains have a separate unit for some or all the milling, sizing, mixing, and paving. There is more flexibility at each stage (eg. can change screens to get different gradation; if the cold planer stops, the paver can keep moving)
Placement Variations
Some recycling trains transfer the mix directly to a screed or paver, allowing for a shorter overall length of the equipment train. Other trains place a windrow of processed mix on the road where a pickup elevator places it into the paver. This allows the paver to keep moving even if the recycler stops.
Road Thickness
The thickness of the roadway can be increased by adding additonal RAP or supplemental aggregate in front of the train and processing this material with the milled RAP into one homogeneous mixture.
Road Widening
This method can have a small cold planer ahead of recycling train to mill wider than 12.5 ft (3.8m), the width of drum of recycler. The small cold planer places the milled material in a windrow midlane where the CIR train can pick up & process the material and the paver can lay the mix to the wider width. Example: existing road has 2 x 12 foot (3.65m) lanes, new cross section has 2 x 12 ft (3.65m) lanes with a 3 ft (1m) paved shoulder on each side. CIR train can mill 5” deep and place 4” deep but wider. Do the math to make sure there’s enough material.
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 CIR. 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 CIR 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 millings is not ideal, corrective aggregate can be added via spreader immediately ahead of the recycling train. Coarse aggregate improves the skeleton of the mix and increases stability. Finer aggregate can help with the foaming process
Common Combinations
CIR + HOT MIX OVERLAY: To increase the structural capacity of the road, with less construction time, less trucking, and less cost, a hot mix overlay is commonly placed over CIR.
As a general rule, to achieve the same structure, a hot mix lift can be replaced by a CIR layer that is 15% to 25% thicker (eg. 1” hot mix ~1.25” CIR).
CIR + SINGLE OR DOUBLE CHIP SEAL: Where the existing hot mix is thick enough, but traffic is too low to justify new hot mix, a chip seal wearing course saves money.
A pavement design with appropriate traffic levels will tell you how thick the CIR layer needs to be.
CIR + MICRO OR SLURRY SURFACING: Where the CIR layer and existing pavement structure are sufficient to carry the anticipated traffic, but the traffic is too low to justify new hot mix, micro or slurry surfacing provides an alternative economical wearing surface.
To get the best bond, let the CIR cure for 3-10 days after construction (and 24 hours after any rainstorm) before applying the tack coat and final pavement surface.
CIR + CCPR: For deeper pavement distresses, the upper layer can be removed by cold milling and processed off-site via CCPR, while the lower layers can be Cold In-place Recycled. The CCPR mix is returned to site and placed, allowing thicker pavement sections to be recycled.
Note: CIR works best when there is 1” - 2” of existing asphalt pavement below the CIR layer. With thicker underlying asphalt , reflective cracks may come thru sooner. So check your boreholes to see if CIR+CCPR is right for you.
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