RECYCLING OF ASPHALT PAVEMENTS- A Necessity

Dr. Ambika Behl

Principal Scientist & H.O.D Flexible Pavement Division, CSIR-CRRI

In any country, the infrastructure construction sector- a highway built up is considered an important economic factor and resource-consuming sector. As economic growth demands a good road network with well connectivity to remote areas, a lot of energy, natural resources, funding are also required. As per the current scenario, depleting resources of aggregates has introduced the use of recycled road in the construction sector & recycling of the road material is becoming an essential part to fulfil the requirement of the new road construction materials in sustainable way. Use of RAP (reclaimed asphalt pavement)material in new road construction leads to elimination of disposal of old asphalt pavements which reduces adverse environmental impacts on landfills, conserves new asphalt binder & natural aggregates, saving in energy which is required in terms of production of new binder, drying, and heating of aggregates, production of mix and transportation of materials. Benefits of Recycling of pavement is not only limited to environmental & economic, but it is also cost-effective without compromising in performance characteristics of the pavement.

While the industry mayinitially start using RAP for the economic benefits it provides, RAP is just one way the asphalt industry is pushing to become more sustainable, as it has economic, environmental, and social benefits- it leads India towards its mission of Swachch Bharat &Sashakt Bharat. RAP technology offers both the economic and environmental benefits, such as reductions in virgin material cost, carbon emissions, and use of nonrenewable natural resources.Some of the challenges that arehindering the advancement of higher RAP quantities in mix designs are the lack of guidelines related to mix design and processing as well as the availability of field performance data to show how these mixtures behave in the field.

Recycling Methods:

There are mainly three methods of Asphalt Pavement Recycling: Hot Recycling, Cold Recycling and Full Depth Reclamation.

Hot mix asphalt recycling is the process in which reclaimed asphalt pavement (RAP) materials are combined with new materials, sometimes along with a recycling agent, to produce hot mix asphalt (HMA) mixtures.The RAP material can be obtained by milling or ripping and crushing operation. The mix placement and compaction equipment and procedures are the same as for regular HMA. The depth of treatment varies between 20 to 50 mm.

Cold in-place recycling (CIR) involves reuse of the existing pavement material without the application of heat. An asphalt emulsion or foamed bitumen is added as a recycling agent or binder. The emulsion/foamed bitumen is proportioned as a percentage by weight of the RAP. Fly ash or cement or quicklime may also be added. The depth of treatment is typically from 75 to 100 mm. The advantages of cold in-place recycling include significant structural treatment of most pavement distress, improvement of ride quality, minimum hauling and air quality problems, and capability of pavement widening.

Full depth reclamation (FDR) is a recycling method where all of the asphalt pavement section and a predetermined amount of underlying base material is treated to produce a stabilized base course. It is basically a cold mix recycling process in which different types of additives such as asphalt emulsions and chemical agents such as cement, fly ash, and lime, are added to obtain an improved base. The four main steps in this process are pulverization, introduction of additive, compaction, and application of a surface or a wearing course.

Selection of Recycling Method

In order to decide whether hot recycling, cold recycling or full depth recycling should be adopted for a pavement, or to what depth of pavement should be recycled; a detailed pavement condition assessment is necessary. The condition of the existing pavement is assessed through: Pavement History, Pavement Condition/Distress Survey, Pavement Strength Evaluation, Surface- Base and Subgrade Analysis, Surface and Subsurface Drainage Review. It is very important to know the depth and extent of deformation in the pavement and accordingly the depth and method of recycling is decided.

Design of Cold Recycled Asphalt Mixes

Cold recycled and bitumen stabilized materials can be produced by using either bitumen emulsion or foam bitumen. Foamed Bitumen is produced by injecting air and water droplets under a high pressure into hot liquid bitumen, resulting in spontaneous foaming.

While designing foam bitumen cold recycled mixes, two parameters are very important: expansion ratio and half life. The expansion ratio is a measure of the viscosity of the foamant, it will determine how well the bitumen will disperse in the mix. It is calculated as the ratio of the maximum volume of foam relative to the original volume of bitumen.The half-life is a measure of the stability of the foam and provides an indication of the rate of collapse of the foam during mixing. It is calculated as the time taken in seconds for the foam to collapse to half of its maximum volume. One of the dominant factors influencing the foam properties is the water that is injected into the bitumen to create the foam as illustrated in figure 1.

TG2 guidelines are followed to design cold recycled asphalt mixes. TG2 is the Technical Guideline for Design and Construction of Bitumen Emulsion and Foamed Bitumen Stabilized Materials Published by the Asphalt Academy. Following steps are to be followed:

• Obtain samples of RAP from field
• Determine RAP gradation, binder content, gradation of extracted aggregate, and aged binder
• Select amount and type of additional aggregate if required
• Select type and grade of recycling additive (Emulsion or Foam bitumen)
• Estimate recycling additive demand
• Determine pre-mix moisture content for coating
• Test trial mixtures: moisture sensitivity, modulus
• Establish job mix formula

Design of Hot Recycled Asphalt Mixes

Mix design of RAP mix is done the same way as a conventional asphalt mix but some additional steps are to be followed before mix design. The design of the Recycled bituminous mix is followed as per MS-20 and MS-2. The primary steps in the design of RAP mixes include the determination of material properties of RAP and new materials, the selection of an appropriate blend of RAP and virgin aggregate to meet gradation, the requirement to add a rejuvenating agent to soften the existing binder, and the need to comply with stability, flow, and air voids requirements.Additional virgin aggregates may be required to satisfy gradation requirements to improve stability and to limit the RAP content in recycled hot mixes. A viscosity blending chart is prepared, which enables the designer to determine the percentage of rejuvenating agent (or soft bitumen) to add to the total binder in order to achieve a desired value of absolute viscosity for the recycled binder.The final mix design proportions for the recycled hot mix paving mixture will be determined by completing mix design testing using standard procedures to satisfy applicable mix design criteria.

RAP material characterization is very important and crucial step in designing RAP mix, one must characterize the basic RAP aggregate properties and bitumen content and its properties. Frequent RAP samples should be tested in order to capture the variability,at least 10 tests per stockpile should be done to understand the material variability. Detailed evaluation of recovered binder and aggregate properties is very important for achieving a good RAP design mix.

To produce uniform high-quality recycled hot mix asphalt, systematic quality control of the RAP should be done. Parameters like RAP moisture content, gradation, and binder content should be monitored throughout the project.  Recovered binder should be tested for softening point, penetration and viscosity and failure temperature regularly to monitor the RAP characteristics to enable appropriate adjustments to the mix design.The same field testing procedures which are used for conventional hot mix asphalt mixes should be used for mixes containing reclaimed asphalt pavement.

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