Cold In-Place Recycling

This environmentally friendly is very effectively for treating full-depth cracked asphalt pavements. It is a more cost effective alternative to partial or total reconstruction of the cracked pavement.

The process involves milling the mixed pavement layers to a maximum depth of 500 mm. The milled pavement is then remixed, with or without new aggregate materials, together with a binding agent before it is laid back. Depending on the condition of the pavement, binding agents such as cement, conventional or modified bitumen emulsion and foam bitumen are used.

Background

The current concept of Cold In-place Recycling (CIR) of bituminous pavement was introduced in Eastern Canada in 1989. The benefits associated with the CIR process are significant when compared with traditional pavement rehabilitation methods from both cost and performance perspectives. The CIR method is an environmentally friendly process as it reuses in-place bitumben and aggregates which reduces the overall fuel consumption while minimizing hazardous emissions.

Process

CIR is based on the principle that the in-place bituminous pavement is a source of materials that may be used to build a new bituminous layer. The process is carried out in-place and reuses the existing bituminous material to a depth between 65 and 125 mm. The existing bituminous pavement is reclaimed, transformed into a bituminous aggregate, which is then mixed with an emulsion, laid down and compacted to the specified density. After a curing period specific to the emulsion used, the recycled material is surfaced with a wearing course.

Project Selection

CIR may be considered wherever cracking is present. It may also be considered when permanent deformation and/or loss of integrity in the existing bituminous pavement occurs. Structurally sound and well-drained pavements are the best candidates.

Rutting, shoving and flushing may be indicative of an excess of bitumen in the existing mixture. The addition of a corrective aggregate may be required to reduce the ratio of total bitumen/mineral aggregate and to improve the strength of the mineral skeleton of the recycled material.

When the pavement is severely deformed, CIR may require one of the following additional corrective operations; profiling the road with a milling machine before the CIR operation, adding an aggregate from an external source and/or using additional wearing course material.

Bituminous Mixture and Pavement Design

A sequence of tests is performed on trial specimens of bituminous aggregate with varying emulsion and water contents. When required, the trial mixtures also include a corrective aggregate. The testing provides the necessary information to select an emulsion and water content for proper mix density, adequate air voids, and optimum stability. The typical emulsion content for standard CIR ranges from 1.5 to 2.2 % and the typical water content ranges from 2.3 to 3.0 %.

The structural capacity of CIR recycled material is dependent on the nature of the in-place bituminous material, the added binder and the curing/fluxing time. Many US road agencies assume an AASHTO layer coefficient of 0.3 and a Granular Base Equivalency (GBE) of 1.8 for the CIR process.

A minimum depth of CIR treatment is required to mitigate reflective cracking. As a rule of thumb, whenever the depth of the treatment is 100 mm or 70 to 80 % of the full depth of bituminous pavement, the potential occurrence of reflective cracking is greatly reduced.

Pavement Performance

CIR is considered the most effective process to mitigate reflective cracking in a cold environment. In the Municipality of Ottawa, a pavement usually requires rehabilitation when the cracking frequency is approximately 170 cracks per kilometre. It is estimated that standard hot bituminous mixture overlay reaches such cracking frequency after 10 years of service while pavement rehabilitated using CIR will not reach such cracking frequency until 15 to 20 years of service.

Exceptional performance was observed where a “Superpave” hot bituminous surface course was placed over a cold in-place recycled pavement on a project completed in 1996. Ten years after recycling, the pavement is still in excellent condition with only a few minor cracks while extensive cracking was observed on the adjacent section of the same road rehabilitated the following year using a traditional mill and resurfacing method.

Economics

CIR is a cost effective rehabilitation alternative to traditional methods. Based on the life cycle cost of pavement rehabilitation, Municipalities have reported that the annual cost of CIR projects may be as low as 80 % of a traditional method.

source: http://www.hcme.com.my/v4/index.php?option=com_content&task=view&id=106&Itemid=131
: http://www.millergroup.ca/pavement/cir.html