What Is Hot In-Place Recycling?

What Is Hot In-Place Recycling?

Hot In-Place Recycling is a method for repairing damaged pavement by heating and scarifying the existing asphalt, adding a recycling agent, and laying the new mixture over the old surface. This process is used to fix surface issues, rather than problems with the underlying structure.

After the recycled asphalt has been laid, a top layer of micro surfacing, chip seal, or hot mix asphalt may be applied.

Hot in-place recycling is a method used to repair surface defects in pavement made of hot mix asphalt. It is not suitable for fixing deeper structural problems such as base course or subgrade failure.

To be a good candidate for HIR, the existing pavement layer should be at least 3 inches thick and should not have a geotextile interlayer within the recycling depth.

The presence of surface treatments, rubberized crack sealants, thermoplastic paint stripes, and specialty mixes may cause difficulties and are not ideal for hot in-place recycling.

The Hot in-place recycling process involves heating and softening the current hot mix asphalt pavement structure, removing and scarifying the surface material, and mixing it with a soft asphalt or recycling agent and additional aggregate.

The mixture is then placed and compacted onto the roadway as a new recycled asphalt surface. This process can be divided into three categories.

1.      Surface Recycling

The process of surface recycling, also known as heater scarification during HIR, involves using indirect radiant or infrared heaters to heat the top layer of a paved surface. To maximize the effectiveness of the heat, multiple heating units are positioned closely together.

The material is heated to a temperature between 230° F and 300° F to soften it, and then it is scarified and a recycling agent may be added if necessary. After this, compaction should be performed using either a pneumatic roller in breakdown mode or a static steel-wheeled or double-drum steel vibratory roller.

It is generally better to use static rolling when compacting thin layers in order to prevent damage to the surface. This compacted layer can be used as the top layer on a low-volume roadway, or it can be applied as a base layer before adding a Hot Mix Asphalt (HMA) overlay or surface treatment.

2.       Repaving

The process of repaving involves heating and scarifying the pavement, then adding a recycling agent and spreading the material to create a level surface. This is similar to the recycling process. However, in repaving, a layer of new hot mix asphalt (HMA) is also added on top of the recycled material and compacted.

3.       Remixing

Remixing is a process in which the top layer of a roadway is heated to a depth of 1.5 to 2 inches, scarified, and the softened material is collected and mixed with virgin aggregates, a recycling agent, or new hot mix asphalt (HMA) if necessary in a pugmill.

The resulting mixture is then laid down as a new layer of HMA. This method is used when repaving alone is not enough to address issues with the existing roadway, such as changing the aggregate gradation, improving skid resistance, or increasing rutting resistance.

Remixing can also be done in a multistage process that consists of one or more pre-heaters, two heater-milling units, a pugmill mixer and a standard paver and compaction equipment.

The multistage process recycling train can treat pavements 1-in. to 1-1/2-in. on the first pass while the second unit follows closely behind, picking up the milled material from the first unit.

Advantages Of Hot In Place Recycling System

1. Recycling 60 to 65% of bitumen and aggregates helps protect the environment.
2. The cold recycling method does not require handling of recycled asphalt pavement (RAP), unlike the hot in-place recycling (HIPR) method.
3. HIPR using the AR 2000 train is also environmentally friendly because it uses hot air instead of propane or other combustible materials for heating, making it suitable for use in urban areas.
4. Both cold recycling and HIPR can increase the life and improve the quality of the road, with better rut resistance in the case of recycled roads.
5. These methods can also result in cost savings of 15% to 25% due to the reduced need for fresh aggregates.

Application Of Hot In-Place Recycling Process

Hot in-place recycling is a technique that is only suitable in certain situations. The existing asphalt must have a high enough air void content to accept the rejuvenating asphalt binder, and the technique can only be used to repair shallow surface damage (less than 50 mm or 2 inches).

Pavements with delaminations in the top 50 mm (2 inches) should not be considered for hot in-place recycling, and pavements that have been rutted, heavily patched, or chip-sealed are not suitable for this technique.

Design Approaches Of The Hot In-Place Recycling Technology

Before beginning the HIR process, it is important to thoroughly investigate and analyze the current state of the road. This includes collecting design and construction data about the old pavement, including its structure and materials, as well as inspecting the pavement for damages such as cracks and potholes.

It is also necessary to assess the smoothness and skid resistance of the pavement, as well as its surface strength through deflection testing. In addition, core samples of the pavement should be taken and analyzed, and the mixture of the old pavement should be sampled and tested.

It is recommended to do this sampling under simulated HIR conditions and to collect at least one sample per kilometer if there are discrepancies in the pavement structure, building time, or maintenance history.

If these factors are consistent throughout the road, one representative sample may be sufficient. If the road includes reclaimed asphalt pavement (RAP), it should be inspected before the HIR process begins.