Recycling of Pavement as Sustainable Development

In most of the developing countries, the dismantled or secondary materials are either wasted or dumped off-site due to following problems:

• Lack of appropriate standards, specifications and design guides
• Concerns about experience and ability of contractors with new techniques
• Lack of quality control due to dearth of certified laboratories and trained staff
• Concerns about quality and variability of recycled materials
• Concerns about long term durability of new materials and methods
• Logistical difficulties in making changes when most works are small scale and geographically spread out.

Benefits of Using Recycled Materials

The use of recycled materials has following social, economic and environmental benefits:

• Increased local employment
• Business development opportunities
• Reduction in nuisance
• Reduced waste disposal
• Reduced haulage
• Less expenditure on imported materials
• Reduce the time required to complete work
• Conserve natural resources
• Less carbon emission
• Use of binders that are industry by-products
• Value of social and community gains
• Conservation of finite resources
• Reduced energy consumption and emission from transport
• Maintenance of biodiversity.

Recycling Asphalt

Recycling of asphalts can be divided into two categories – in-situ recycling and off-site recycling.

In-Situ Recycling

In-situ recycling can be further sub divided into four processes known as ‘repaving’, remixing’, ‘re-tread’ and ‘deep recycling’.

Repaving and remixing are hot in situ recycling processes whereas re-tread and deep recycling are cold processes.

The repave process involves heating and scarifying the existing surface to a depth of about 20 mm. Approximately 20 mm of new asphalt is laid directly onto the hot scarified material and then the material is compacted. As both layers are hot when compacted, a good bond is achieved between the recycled material and the new material.

In the remixing process, a hot scarified material is mixed with new material in the pugmill mixer of a machine, and the blended mixture is paved and compacted on the scarified surface.

Re-tread, a cold in situ process consists of scarifying the existing road to a depth of approximately 75 mm, breaking down the scarified material to the required size and reshaping of the road profile. The material is then over sprayed two or three times with bitumen emulsion. After each application of emulsion, the material is harrowed to distribute the emulsion. When harrowing has been completed, the material is compacted with an 8 to 10 tonne dead weight roller and finally the surface is sealed with a surface dressing.

Deep recycling is a cold in situ process for full depth reconstruction of pavement. The road is pulverised, graded, if necessary and mixed with cement as stabiliser and bitumen, usually with compaction between each stage of the process. The binder can be foamed bitumen, produced in-situ by injecting a very small proportion of water and additives into the bitumen causing it to expand to 10 to 15 times its original volume. The resulting low viscosity makes it easy for the binder to mix with cold or damp aggregates. Alternatively, a bitumen emulsion can be used. Deep recycling is suitable for treating material depth between 125 and 300 mm.

The advantages of the in-situ recycling are:

• The existing road material is reclaimed thus reducing the demand for primary aggregate
• The process can be completed rapidly
• Cost savings over traditional construction methods should be possible
• Disruption to traffic and local communities is reduced in comparison to traditional methods by the shorter duration of the works and few movements of vehicles carrying materials
• Although manholes and other chamber covers have to be lowered and sealed prior to the process and subsequently raised, the disruption is generally less than that associated with total road construction
• Process works within the width of the machine permitting work to be done on single carriageways within traffic management safety
• Process can take place in urban areas without re-laying kerbs and gullies
• There is no significant height difference between the section of road under reconstruction and the normal running surface, allowing ease of traffic management and access to adjacent properties.

The disadvantages of the process are:

• Low take-up can lead to high mobilisation costs.
• Shallow services can be disrupted
• Large site is required in order for the process to be worthwhile
• Limited experience in the country of use, on which to base performance criteria and design parameters
• Contractors include a risk element with any unfamiliar process to allow for possible remedial works, downtime and use of a specialist sub-contractor
• Some clients dislike cement because of the long-term problems of reflective cracking but are uncertain of the performance of bitumen stabilisation.
• Complaints from residents as a result of the heavy compaction plant necessary to achieve the high (95%) percentage of refusal density and low air voids necessary for structural and durability criteria in full depth in-situ recycling.

Ex-Situ or Off-Site Plant Recycling

In this process, material removed from the surface of an existing road is transported to a hot mix plant where it may be stockpiled for future use or processed immediately. Both batch and continuous plants have been successfully converted to produce recycled mixtures utilising a range of methods to heat the reclaimed material prior to mixing. This success manifests itself in the fact that recycling can now be undertaken without excessive fuming and blue smoke emissions during manufacture and laboratory performance tests on asphalt containing a proportion of recycled material were found to be identical to mixtures using virgin components.

Recycling in batch plants is achieved by superheating the virgin aggregate and then adding the material to be recycled either immediately after the drier or directly into the pugmill. Heat transfer takes place during the mixing cycle. Although this method successfully overcomes the problem of blue smoke, it entails keeping aggregate in the heating drum for longer, which means that the output from the plant is reduced.

As it is not possible to obtain adequate heat transfer with high percentages of recycled material, it is widely accepted that the maximum quantity of recycled material that can be economically added is between 25 and 40%. Other disadvantages using batch plants for recycling are high heating costs and accelerated wear and tear on the drum and dust collectors due to the higher manufacturing temperatures.

Recycling using continuous mixers involves introduction of the reclaimed material into the drum itself. During early trials using this type of plant, environmentally unacceptable blue smoke was produced. Blue smoke is produced when vapourised bitumen condenses. The condensate takes the form of particles that are too small to be removed by the conventional emission control equipment and so escape through the stack. Bitumen vaporises at about 450C and as gas temperatures in the drum of a continuous mixer can reach 2000C, the introduction of reclaimed material has to be carefully controlled. By various modifications, continuous mixers that can take up to 60% recycled material without exceeding statutory pollution standards have been designed. The entry point for the recycled material is approximately halfway down the drum. The flights in the drum have been modified to produce a homogeneous mixture of virgin and recycled material prior to the addition of the bitumen and filer. In addition, the redesigned flights shield the recycled material from the intense radiant heat originating from the heating unit.

Ex-situ recycling has the following characteristics/advantages:

• The recycling plant is easily transported and can be established on site in a few hours
• The plant is smokeless, odourless and is quiet in operation
• Recycling plant can be located in surroundings appropriate to the nature of the operation where environmental impacts can be mitigated
• Environmental impacts at the reconstruction site are reduced
• A wide range of materials can be processed including road planings, crushed and screened to fit a predetermined grading envelope before being mixed with binder
• All materials are processed in a controllable environment resulting in the production of a quality-controlled product
• The graded material can be stockpiled for the later incorporation of a binder
• Bound material, if correctly stored, can be used for up to four weeks after production
• The excavation of the road and its replacement can be undertaken using conventional plant and equipment.

Use of Recycled Materials in Pavement Structural Layers

The surfacing and particularly the wearing course, represents only a small proportion of the total depth of construction. However, it accounts for a high proportion of the total cost of the pavement. Specifications for surfacing therefore impose more stringent requirements than is the case for the lower layers of the road pavement. The potential for using alternative material in surfacing is therefore very restricted.

From a structural aspect, the road base is the most important layer of a flexible pavement. It is expected to bear the burden of distributing the applied surface loads so that the bearing capacity of the subgrade is not exceeded. Since it provides the pavement with added stiffness and resistance to fatigue, as well as contributing to overall thickness, the material used in a road base must always be of reasonable high quality. For this reason, the scope for using alternatives to the naturally occurring materials is restricted.

Unbound and Bound Subbase Course and their Specifications

Unbound aggregates are described in Clauses 803 and 804 of the Specification which give, respectively the requirements for granular sub-base material Type 1 and granular sub-base material Type 2. Type 1 granular materials include ‘crushed rock, crushed slag, crushed concrete or well burnt non-plastic shale’. Type 2 granular materials include the materials permitted for Type 1 and also natural sands and gravels.

Type 1 materials, which exclude the use of sands and gravels, are expected to be all-weather sub-base aggregates; Type 2 materials have lower specifications and are not expected to give good performance under construction in wet weather. Unbound subbases are not permitted in rigid pavement construction; they may be used in flexible pavement construction, but Type 2 materials are excluded if the design traffic loadings are more than 400 commercial vehicles per day.

*CBR assumed to be adequate

In addition to the other requirements unbound material used within 450 mm of the road surface also have to be no-frost susceptible when tested by the BS frost heave test (BS 812 1988)

Cement bound materials may be used as an alternative to unbound granular materials under flexible construction and are the only permitted materials under rigid construction. The specifications contain clauses for a family of cement-bound materials referred to by their initials CBM1, CBM2, CBM3 and CBM4. All could be used at subbase level but the superior quality of CBM3 and CBM4 means that they are used principally for base construction or for subbase construction. The materials permitted for use for CBM1 and CBM” are not specified as such, it being assumed that as long as they satisfy all the requirements of the specification they may be regarded as suitable. The materials to be used for the superior quality CBM3 and CBM4 are specified as natural aggregates complying with the requirements of BS 882 or air-cooled blast-furnace slag complying with the requirements of BS 1047, which complies with the quality and grading requirements of BS 882.