Hot In-Place Recycling Vs. Cold In-Place Recycling
Hot In-Place Recycling Vs. Cold In-Place Recycling
Hot In-Place Recycling (HIPR) and Cold In-Place Recycling (CIPR) are two techniques used to extend the life of a pavement. HIPR consists of heating the asphalt surface, ripping, milling and mixing new asphalt material into the existing pavement structure while it is still hot.
The process will generally require less time than CIPR as well as minimal disruption to traffic since there is no need for excavation or replacement of the paved surface.
On the other hand, CIPR requires that the existing road pavement be milled and removed before new asphalt can be added back in. This usually takes more time than HIPR and may also cause more disruption to traffic due to excavation and reconstruction activities required during the process.
In the United States, various techniques for in-place pavement recycling have been utilized since the 1930s to repair and maintain roads. These sustainable methods have become increasingly popular in recent years due to cost-saving efforts and an emphasis on environmentally-friendly practices.
Both HIP and CIP recycling are in-place methods that allow for the reuse of a large portion of existing asphalt pavement, potentially up to 85%.
These methods can be cost-effective and resource-saving alternatives to traditional methods because they do not require the transportation, processing, or stockpiling of recycled asphalt pavement or the use of costly hot mix asphalt. It is important to carefully consider which method is the most appropriate for a specific project.
The HIP is a technique used to repair damaged pavements on site, without removing the surface layer. It is suitable for fixing surface issues such as cracking, erosion, low friction and ripples but it is not effective for fixing structural problems or for repairing damage deeper than 2 inches.
Cold-in-place recycling is a process that can be used to repair moderate- to low-volume roadways that have surface issues such as raveling, weathering, bleeding, corrugations, shoving, slippage, rutting, cracking and shallow potholes, but do not have major structural issues such as deep cracking, poor drainage, pumping, or saturated subgrade materials.
This method involves removing the damaged layers of pavement and replacing them with a crack-free layer, which can then be used as a base for a new hot mix asphalt overlay or surface course. It is typically performed on-site and is recommended for roadways that do not have severe underlying structural issues.
Cold-in-place recycling is a process that restores and reuses existing material, reducing the amount of new material that needs to be brought in and transported to the site. This can help improve the quality and strength of the road surface, as well as save money on the paving process.
CIP can also fix deep damage to the asphalt, such as ruts, fatigue cracks, and utility cuts that cannot be repaired with a surface treatment or overlay. It can also make small adjustments to the road’s profile, reduce the risk of reflective cracking, and cause minimal disruption to traffic. Because it is a cold process, it requires less energy to rejuvenate the asphalt compared to other methods.
To ensure success in this process, multiple samples should be taken from the road surface to check for any variations in the existing asphalt that may affect the added materials. This will create a more uniform and sturdy asphalt base when the material is repaved.
Since this is a cold process, the resulting asphalt may be more porous than hot mix asphalt. It is advisable to apply a surface treatment or overlay to the Cold In place recycling material to protect it from water damage and extend its lifespan.
Processes In Hot In-Place Recycling
HIR can be broken up into three categories:
1. Surface Recycling
Heater-scarification, also known as surface recycling, is a process used in hot-in-place recycling (HIR) of asphalt pavements. It involves heating the top layer of the pavement using indirect radiant or infrared heaters to a temperature between 230° F and 300° F.
This softens the material, which is then scarified and mixed with a recycling agent. The mixture is then compacted using a pneumatic roller or a static steel-wheeled or double-drum steel vibratory roller. Static rolling is preferred when compacting thin layers to avoid damaging the pavement.
The resulting compacted layer can be used as the wearing course on low-volume roadways or as a pretreatment for a hot mix asphalt (HMA) overlay or surface treatment.
2. Repaving
The repaving process involves heating and scarifying the pavement, applying a recycling agent, and then spreading and compacting a layer of recycled material and new hot mix asphalt (HMA). It is similar to the recycling process, but an additional layer of HMA is added on top of the recycled material.
3. Remixing
Remixing is a process used to repair and improve the quality of a roadway by heating the existing surface to a certain depth, scarifying it to loosen the material, and mixing it with other materials such as virgin aggregates or a recycling agent.
This mixture is then placed back onto the roadway and compacted. Remixing can be done in a single step or in a multistage process that involves the use of multiple pieces of equipment to heat and mill the roadway. It is typically used to change the gradation of the aggregate, improve skid resistance, and increase resistance to rutting.
Processes In Cold In-Place Recycling
Processes for CIR used include a single-unit, double-unit or a multiple-unit recycling train.
Single Unit Train
A single-unit train is a machine used for milling, sizing, and blending asphalt. It consists of a milling chamber and a cutting head, which can also apply asphalt emulsion or foamed asphalt using spray bars or foaming nozzles.
The processed asphalt is either placed in a windrow for pickup or directly into a paver for placement. One disadvantage of using a single-unit train is that it does not screen or crush the asphalt, which can make it difficult to control the particle size.
Therefore, it is not recommended for use on pavements with severe alligator cracking, where maximum particle size is important.
Two-Unit Trains
A two unit train is a combination of a milling machine and a pugmill mixer-paver. The milling machine grinds old pavement into small pieces and the pugmill mixes the ground pavement with asphalt emulsion and water to creates new paving material. This two unit train is efficient and can quickly produce a large amount of new paving material.
Multi-Unit Trains:
Multi-unit trains are made up of several pieces of equipment, including a milling machine, a portable crushing and screening unit, and a portable pugmill mixer. The mill is used to control the depth of the pavement, and the recycled asphalt (RAP) is fed into the crushing and screening unit to be consistently sized.
This provides better control over the gradation and mixing of the asphalt-treated base, resulting in a more uniform product compared to using single or double unit processes. Cold in-place recycling (CIR) mix can be placed in a windrow or directly into a paving machine.
However, the screed must be operated cold because using a heated screed can cause the emulsion to break and the mix to stick.
