mobile asphalt recycling techniques for in situ footway repair · the rsl mobile asphalt recycler...

JMU Sustainable Aggregates Conference February 2009

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Paper 30

LCMT 2009 8th Annual International Conference Sustainable Aggregates, Pavement Engineering & Asphalt Technology Design Construction, Management, Performance and Rehabilitations

18th - 19th February 2009, Liverpool, United Kingdom

Mobile asphalt recycling techniques for in situ footway repair Andy Ward – Innovations and Development Manager (Enterprise plc / Staffordshire Highways)

Abstract Footways form a conduit for pedestrians as well as providing the service void for many utilities, and it is this dual function that leads to many issues over footway condition and repair. With the current culture of rising insurance claims it is essential that footways are maintained in good order in the most sustainable way.

Construction methods for footways vary considerably, typically employing low-cost materials that are sourced locally. The principle of in situ recycling is to re-use the existing material if suitable or to import a blend or replacement of the bitumen-coated stone. As part of its strategy for footway repairs, Staffordshire Highways (a partnership between Staffordshire County Council and Enterprise plc) has implemented a method of evaluating the condition and constituent materials of footways, together with a programme of refurbishment work. This has included use of the RSL Mobile Asphalt Recycler (MAR) for in situ repairs, which recycles the wearing and binder course materials, supplemented with additional binder or by blending in carriageway planings to form a new binder course. The MAR technique has been further developed to use recycled planings for the wearing course. The combination of reduced waste, removal of the need to purchase any new materials and the reduction in vehicle movements offer a 15-20% saving over traditional methods and a major reduction in carbon emissions.

Introduction A number of systems developed for the in situ refurbishment of footways have had limited success due to the quality of the residual material. Working with Staffordshire Highways, RSL Ltd has developed a methodology that removes the risk of dealing with unsuitable material, together with a flexible batch process that can utilise a wide range of new and recycled materials.


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Within Staffordshire the annual footway refurbishment programme is made available in the year prior to the work being carried out. Decisions on suitability of the process are made based on size of scheme, location, timing, working environment and core samples. In parallel to this process is the carriageway resurfacing programme, which indicates the tonnage and locations from where usable planings will be generated. If all parameters are acceptable then core samples are taken to confirm suitability of the existing construction, and footway repairs are then recycled using either the existing material or planings from an adjacent scheme.

Process developers During the past five years Staffordshire Highways has worked closely with Enterprise plc and RSL Ltd. Enterprise plc, one of the UK’s largest public service providers, is the term maintenance contractor who provides the service in the form of plant, labour and materials. RSL Ltd is a fourth-generation, family-run engineering company based in Leicestershire and specialising in quarry and asphalt plant equipment, and has been responsible for the invention, design and manufacture of the MAR system over the past six years.

In delivering best value to the county, Staffordshire Highways has direct control over the design, planning and specification of maintenance works. The forward programme of footway refurbishment works is established via a Joint Planning Unit (JPU), whose members collate the job register of priority works. The methodology to be adopted for the works is set by the Specification Group, who produce guideline documents for the various materials and processes to be used.

Forward planning Staffordshire Highways produces a 12-month forward plan for footway refurbishment across the county. All repair sites in the plan are examined against a set of criteria to establish their suitability for in situ recycling:

1. Location: rural or urban

2. Size of works: typically above 200m2

3. Environment: road width, traffic management, conservation area

4. Footway type: width, grass verge, trees, replacement paving

5. Timing: to coincide with school holidays, events, utility works

These five criteria are used to form a matrix from which the final decision is made to take core samples from the footway to establish the method of original construction. This is an essential process, as the quality of the original construction may vary considerably between locations. Previous core samples have indicated that lower layers constructed from a wide range of non-bituminous materials are unsuitable for the process. Such materials previously used across the county have included power station ash, 30mm gravel and crushed house bricks, with varying depths of surface course.

The specification for footway construction varies considerably across the UK, and was traditionally influenced by the value of locally-arising materials. The use of open-


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graded macadam in Staffordshire was historically common, and the oxidation of binder in this material is now a major cause of footway failure.

When the list of footways suitable for recycling has been finalised, it is then cross-referenced with the pavement surfacing programme in order to identify the locations where suitable asphalt planings will be produced. The two plans are then overlaid and an appropriate source of planings identified for footway core samples indicating that they will require supplementing or replacing with this high-value material. Stocks of planings are then placed adjacent to footway sites for use in their reconstruction.

Ongoing work is being carried out by Staffordshire to identify, select and segregate carriageway surface course and binder course materials for use in specific footway applications. One option is to plane off the surface course separately and use this material as a recycled surface course on rural roads and footways.

The best example of effective planning of footway recycling goes to the project where the resurfacing programme required an adjacent footway level to be lifted by 100mm, and the planings from the carriageway were used to reconstruct the footway following the installation of new kerbs.

The process has now been developed to the extent where selected planings in rural locations are used in two composite layers to include the surface course. In this type of scheme the surface of the recycled planings is rolled and then finished with a 3mm sealing grit. The footway can then be surfaced dressed at a later date with local gravel or recycled chippings.

Plant description (MAR)

Background The initial design used to prove the concept of in situ recycling was a machine with a two-tonne capacity, mounted on a four-wheel chassis. Subsequent refinements to the loading, heating and safety aspects of the original design have improved productivity, reliability and environmental performance. These have included three-tonne and four-tonne drums as well as a semi-static unit, the first of which was commissioned in the USA in 2006.

General The latest incarnation of the Mobile Asphalt Recycler (MAR) comprises a five-tonne capacity rotating steel drum mounted on a six-wheel chassis cab. At the open end of the drum is a charging chute and cyclic feeder with an oil burner fitted beneath. The drum has a number of steel plates fitted to the inside surface which fold and turn the breakout material as it is heated by an oil burner adjacent to the drum void. This action, combined with careful control of the drum rotation speed, minimises the volume of particulates produced.

Ignition of the bituminous material is avoided by the typical 15-20% moisture content creating a steam blanket within the drum. The MAR is self-contained, with a donkey engine and generator set providing the electric and hydraulic power. The heating cycle is automated via a control panel and the operator is provided with hand controls for discharge of the hot material.


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In the true spirit of a recycling plant, many of the MAR units are assembled on used chassis-cab units, as the mileage covered is by each unit is comparatively low. This helps to reduce the capital cost of the units, which in turn leads to lower operating costs.

Working practices

Traditional practice The labour and plant used to carry out the in situ recycling process is the same as the conventional method in terms of material removal, disposal and replacement with new. This comprises a four-man team using a wheeled excavator (JCB 3CX), a small two-tonne lorry, road roller and hand tools.

Some 200-300m of footway would typically be excavated at one time, and the material taken from site to be recycled into sub-base or disposed of as landfill. Levels in the footway sub-base would be regulated to allow for the currently specified 70mm bound construction thickness, and any adjustments to iron work made. This would typically take place over a few days, dependant upon local circumstances. New binder course material would be supplied from a coating plant in 20-tonne loads, with an average of 60 tonnes laid per day. The footway would then be brought up to finished level with new surface course material.

The VEB5000 is 9.1m long, 2.3m wide and 2.7m high. It has an unladen weight of

10.8 tonnes and produces 16 tonnes per hour of hot recycled asphalt


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The MAR in situ process The MAR is designed to work adjacent to an asphalt footway where the old (oxidised) base and wearing course materials are broken out using a wheeled excavator and loaded together into the charging hopper. The aperture of the charging hopper is 400mm square and the machine driver is required to lift and break the material into a size suitable for loading, from where a hydraulic ram on the cyclic feeder then pushes the breakout material into the drum. An area of approximately 20m x 1.8m of footway is typically excavated and loaded into the drum for each batch, dependant upon the width of the footway and the depth of re-usable material in the construction. This process is a significant part of the whole cycle and can take up to 15 minutes, dependant upon the skill of the machine operator. Once the first batch has been loaded the machine driver can then continue to excavate the footway, preparing the material for quicker loading into subsequent batches.

Overfilling is prevented by load cells and a weight indicator fitted to the drum, which alert the loading machine driver when the drum is full. The oil burner is lit during the loading process to ensure that heat is transferred to the material at the back of the drum. The heating time varies from around 20 minutes for a cold drum on startup to around 12 minutes for a repeat cycle on a warm day. Cycle time is also affected by the type of material used, with large pieces of breakout material (around 100mm-250mm) allowing effective transfer of hot gases through the feedstock, whereas finely-planed carriageway material (from 40mm down to dust-size) reduces convection heating and may also hold more moisture, and hence has a longer heating cycle.

The MAR control panel incorporates stop/start controls for the donkey engine, fan and burner controls, plus an adjustable timer for the heating cycle. Overheating of the drum is prevented by a temperature sensor located in the exhaust flue of the waste gas stream.

Both surface and binder course materials are planed out from the carriageway and

stockpiled for charging into the MAR. The machine operator breaks the material into pieces smaller than 400mm square to pass through the hopper


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On the first batch the operator will set a heating time of 20 minutes, to be started during the charging cycle. The emissions from the flue during this period will gradually change from white steam to a more clear efflux, indicating that the majority of the water vapour has been evaporated. At this point the drum is raised on two hydraulic rams to discharge the material from the rear lower hopper. A small amount is then discharged into a wheelbarrow and a digital optical thermometer used to measure the temperature of the test sample. Heating will continue until the optimum temperature of 160-165°C is reached. The programmable timer can be set in increments of one minute, so once an optimum heating time for a warm drum has been established, this can easily be repeated throughout the day.

Dependant upon the bitumen content of the excavated material, a judgement is made on the workability of the re-heated material. If the footway was badly depleted with a low bitumen content, the operator can supplement this using a quantity of 160/220 pen bitumen. Experience has shown that there is a large variation in both the depth and quality of footway asphalt, consequently there is no formula at present for adding bitumen to rejuvenate the recycled material and much depends upon the skill of the operator.

Following excavation of the footway, the area is prepared by regulating and rolling to compact the surface, at which point the depth of construction may vary significantly from the conventional specification for footway works. The batch that has been recycled is then discharged into wheelbarrows or the machine bucket and tipped onto the prepared sub-base. Traditional skills take over at this point, with the material being raked and rolled into the formation. During this time the excavator is used to break out and load another section of footway material into the recycler, the oil burner is switched on and the heating cycle is repeated, during which time the newly exposed area is regulated and rolled ready for resurfacing.

The main difference to conventional working is that the team are busy carrying out lighter, short-term tasks throughout the day, compared with the usual demand to work 60 tonnes of hot asphalt in a single day. The MAR in situ recycling process utilises a five-tonne cyclic batch process whose efficient application depends upon the team working effectively together.

Daily output from the MAR process can vary significantly between jobs, but experience has indicated that an average of six batches are usually achievable in a normal working day. Based on a nominal charge weight of five tonnes per batch, this is equivalent to 30 tonnes of footway material being lifted, loaded, recycled and laid per day. With effective traffic management and flexible working, as many as 14 batches (70 tonnes) have been produced and laid in a 12-hour period.


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Material from the footway is loaded into the hopper and fed into the drum via a

reciprocating feeder. The oil burner is mounted beneath the hopper

This method of working has the advantage that only a short length of footway is excavated at any one time. Each batch of excavated material is removed from site to create the new work area and the next batch is then excavated and loaded into the machine for recycling and laid into the footway. This means that only 30-40m of footway is undergoing reconstruction at any one time, which allows easier traffic management and hence improved site safety. This is also of benefit to householders in maintaining access to their property during the works and improved traffic flow on the adjacent carriageway. There is also no dependence on the local quarry to supply coated material, which leads to greater flexibility of working. The process normally produces surplus material equal to the volume of the new surface course, so vehicle movements from site to landfill or secondary recycling sites are minimised.


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Hot material is discharged from the chute and placed in the footway using

wheelbarrows. This method of hand-laying minimises compaction on placement and allows for easier raking and level control

If the material in the footway is not suitable for in situ recycling, it is excavated and recycled into sub-base in a local highways depot. The footway is then reconstructed using the MAR from a supply of local planings, which is sometimes a preferred option for sites with unsuitable binder course material. The logistics are critical for the importation and stocking of planings, as this can adversely affect the carbon footprint of the scheme. Feeding the MAR with planings offers an 80% reduction in loading time compared to footway breakout material. Also, the control of the bitumen content is far easier if BS 4987 dense binder course planings are used.

Recent experience has shown that recycled planings have been used to reconstruct rural footways by rolling and compacting a 50mm binder layer followed by a 50mm surface layer. Improved surface texture has been achieved by the addition of sharp sand, which also helps to improve workability and reduce air voids in the asphalt. The thicker surface layer is used to ensure compaction of the various aggregate sizes in the planings.


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The footway in this moorlands village was reconstructed using planings up to finished

level and dressed with 3mm grit stone

Technical performance Staffordshire Highways Laboratory has tested the recycled material by taking cores from completed works. Results are available to show grading, binder content and air void levels. The major advantage of the MAR process and the reason why it is approved by the SH Specification Group is that the material is fully fit for purpose:

The reheating and addition of bitumen binder to restore workability produces a hot material that compacts well and minimises voids to around 3%. This is a major improvement over conventional working practices without the additional cost overhead of using hot boxes.

Although the addition of material from the wearing course modifies the grading of the binder layer, the changes are minimal and the overall specification is still well within acceptable limits.

The table below indicates the range of results obtained from a number of footway cores.


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The table above, extracted from Staffordshire Engineering Services Laboratory,

shows the benefit of early compaction of the hot recycled asphalt by the low air void values achieved

Material grading The combination of binder and wearing course aggregates does not conform to any MCHW Volume 1 specification, however the overall grading is within a reasonable tolerance for footway works. Analysis of the grading and retained bitumen content is not significantly different to the specification as shown in the extract from the laboratory report shown below.

The binder content is typically higher than the original material, prior to recycling

Staffordshire Highways Laboratory has conducted a number of trials on new material supplied from local coating plants by early morning collection. It is usual to find that the temperature of this material has dropped significantly by midday and that any material laid after that time will be difficult to compact. Cores taken from this material have shown to be poorly compacted due to product stiffness, and when tested have had voids in excess of 20%. With the ingress of water and frost this porosity leads to early life failure.


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Environmental impact It is important that a process offering the environmental benefits of MAR does not itself cause significant pollution to the environment in which it operates, and the developers have recently made significant progress in the control of particulate matter and smoke from the machine. To minimise emissions from the recycling process, which essentially tumbles and oxidises asphalt and burns a mineral-based oil to reheat and recycle it, the drum is maintained under negative pressure during the firing cycle. Airborne particulates are collected from the exhaust gases in a cyclone separator, which is emptied on a regular basis, and modifications to the drum design and rotational speed have further reduced emissions.

Noise control Exposure of operatives to noise is covered by the Control of Noise at Work Regulations 2005, which became law in the UK in 2006. They place a duty on employers to reduce the risk of hearing damage from exposure to noise to the lowest reasonably practical level. For the purpose of these regulations, daily noise exposure levels and peak sound pressure levels for an operative are:

1. Lower exposure level: LEX,8h = 80dB(A).

2. Upper exposure level: LEX,8h = 85dB(A).

3. Exposure limit value: LEX,8h = 87dB(A).

From noise measurements taken by competent authorities, the machine operator standing by the MAR control panel has been assessed to have an LEX,8h of 83dB(A) during the time the burner is firing. The operator is part of the construction team and hence does not stand by the burner or control panel for long periods of time. Operatives working five metres from the machine when the burner is firing are subject to a noise level of 72dB(A). The donkey engine is fitted with a soundproof enclosure which reduces the noise level in the immediate vicinity to 70dB(A).

Particulate and smoke emissions Tests have been carried out by the Swindon Laboratory of Scientifics Ltd to establish particulate and smoke emissions levels for the MAR unit. The results produced by this analysis are too lengthy to be included in this paper, and extracts have not been included as the whole report has to be read in context. The findings are available on request from RSL Ltd.

Operating costs and carbon savings Most clients have definite climate change agendas, but there is also a need to realise direct cost savings associated with the recycling process. The MAR process saves the cost of purchasing new material from the coating plant as well as the cost of disposal of the depleted asphalt excavated from the footway. It has the added flexibility of removing the constraints on operating times imposed by the reliance on purchased materials, and can use any suitable feedstock. It utilises the same plant, labour and equipment as conventional construction methods, with an overall saving of 15-20%.


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Typical savings It is difficult to calculate the total savings for a footway refurbishment scheme, however some of the main advantages can be priced to give an indication within the current volatile market. On the assumption that all labour and plant are the same, the major saving is made from avoiding the purchase of new materials from the coating plant. Additional costs are associated with the heating oil and bitumen added to the drum:

The amount of oil used to heat a five-tonne batch of material is 25 litres, which at £0.35 per litre totals £8.75

If the material is badly depleted, a 20kg keg of bitumen is added per batch, at a cost of £18 per keg

The formula is: cost of new material – cost of oil + cost of bitumen + cost of hire

Taking a typical day’s production of six five-tonne batches producing 30 tonnes recycled and laid, and assuming that 150 litres of oil and 120kg of bitumen is used per day, savings at December 2008 prices would be:

30 tonnes x £58 per tonne of new material = £1740.00 - £52.50 oil + £108 bitumen + £350 hire = £1229.50 saving per day

Material prices vary considerably in each locality and are influenced by both transport costs and availability. All prices used above are particular to the Staffordshire contract.

To maximise savings, Staffordshire Highways plans the footway programme to give continuity of work, thus avoiding non-productive hire time. Experience has also shown that due to the flexibility of the MAR process, footway schemes are completed earlier than planned, with a further major influence on cost.

Carbon process calculator Staffordshire Highways has developed a number of carbon process calculators to measure the actual savings produced from its everyday recycling activities. These savings include depot-based recycling of sub-base, infrared pothole and patch repair as well as mobile asphalt recycling. Carbon savings from the MAR process are the most significant of these, as they completely avoid the purchase of new coated stone.

An example of the carbon calculator is included in Appendix 1 and uses the following data:

1. Embedded carbon per tonne of new asphalt.

2. Energy used to transport new asphalt to site.

3. Energy used to heat recycled asphalt.

4. Energy from depot to site.

5. Energy from site to landfill.

6. Embedded carbon per tonne of waste to landfill.

The calculator, reproduced below, uses the standard Defra conversion factors for the production of asphalt, type of vehicle used, heating oil and disposal to landfill. It can


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be used for individual jobs to show the projected reduction in carbon emissions from using the MAR process.


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The green boxes shown on the process map are the fixed Defra conversion factors and the yellow boxes can be set to match the distances from the quarry, depot, landfill to the site and vehicles used on the job. The oil used to reheat the material is factored into the calculation and based on the assumption of the waste disposed to landfill, from which total carbon savings are calculated.

In the example shown above, the annual tonnage recycled is calculated with some average distances travelled from a highways maintenance depot in Staffordshire. Further work is being done to develop the tool to show the affect on carbon emissions when the waste material is taken to a recycling facility and processed into sub-base.

Clients Although RSL has carried out the majority of its development work with Staffordshire Highways in the areas of health and safety and working practice, the MAR process is not unique to Staffordshire Highways. As well as being used by councils in Leicestershire, Cornwall and Oxfordshire, a number of machines are operated by the private sector in the UK, USA, South Africa and Australia.

Awards The MAR process was presented with a Silver Gilt Award from the Staffordshire Environmental Business Network for recycling performance in 2006/07. For carbon emission reduction the process was awarded first place in the Staffordshire Sentinel Business Environmental Awards 2007/08, sponsored by Michelin.

Future developments This paper refers to the advances made in working practices for the Mobile Asphalt Recycler used for in situ footway refurbishment. There is a growing trend for contractors in the industry to use the same technology as a semi-static facility where breakout material and planings are brought to a central site for processing. The benefits in using a recycling hub to blend and grade the materials are in the quality control of aggregate size and bitumen content. The first installation of this type was commissioned in the UK in December 2008. This is in contrast to the quarry-based static plant, which produces new stone coated with bitumen to specific recipes. The fundamental difference between the two processes is in the way the stone is pre-heated in the quarry plant and added to hot bitumen in the mixing drum. The stone in a quarry-based plant is heated to around 200ºC before the addition of the bitumen and hence has a high residual heat content. Material in the MAR is heated solely in the drum while the surface temperature of the bitumen is raised to 165ºC, therefore the aggregate will remain at a slightly lower temperature. There is an interesting area of research to be undertaken in the field of bitumen additives to ensure comparative workability.


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The first semi-static plant commissioned in the UK in December 2008. A similar plant was commissioned in Barcelona in January 2009

References MCHW Volume 1 - Specification for Highway Works, Nov 2008.

Control of Noise at Work Act 2005.

Environmental Protection Act 1994.

mobile asphalt recycling techniques for in situ footway repair · the rsl mobile asphalt recycler...

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