COLD RECYCLING BITUMINOUS MATERIALS - FOAMED BITUMEN, (Including additions of cement, lime and other hydraulic binders) 

(Please note I have now made a separation between the use of reclaimed bituminous material recycled in hot or cold processes to avoid confusion.)

CONTENTS

WHERE TO FIND DOCUMENTED INFORMATION
RETREAD (Now often referred to as Linear Quarrying)
STOCKPILING OF ROAD PLANINGS
FOAMED BITUMEN RECYCLING OF ROAD PLANINGS
COSTS AND PERFORMANCE
SITE SPECIFIC MATERIAL SPECIFICATIONS

RECYCLING BITUMINOUS MATERIALS IN HOT MIX PROCESSES

WHERE TO FIND DOCUMENTED INFORMATION

BS 6543 : Guide to the use of industrial by-products and waste materials in building and civil engineering

HD 31/94 : Maintenance of Bituminous Roads, Design Manual for Roads and Bridges, Vol.7, Pavement Design and Maintenance

County Surveyors Society Report ENG/1-94, Use of recycling for road pavement construction and maintenance

"Alternative Materials in Road Construction", published by Thomas Telford is a very useful book on recycling and the use of alternative materials in highways construction and maintenance.

TRL REPORT 386 : Design guide and specification for structural maintenance of highway pavements by cold in-situ recycling,
by L.J.Milton and M.Earland

TRL Report - Recycling in Transport Infrastructure

Manual of Contract Documents for Highway Works - 
Volume One (and associated Notes for Guidance) of the Specification for Highway Works

RETREAD (Now often referred to as Linear Quarrying)

The Retread process has been around for 20/30 years, and the process has improved with the availability of better designed machinery with increased power to better carry out the work.
Broadly speaking Retread processes involve assessing the existing road to see if it is suitable for treatment, if the existing road construction is suitable it is pulverized to a suitable depth to produce a uniformly graded material.
Aggregate can be added to modify the in-situ grading if necessary.

recycling plant capable of pulverising existing road pavement also adding and mixing binder

Foamed bitumen, bitumen emulsion, and/or cement, or a mixture, is combined with the pulverized aggregate to act as the binding agent, and after final shaping the material is rolled with a heavy roller.
With truly minor roads a surface dressing may be all that is needed to seal the road surface.
To produce a more durable and better running surface a conventional new bituminous wearing course can be laid on the recycled road construction, but this adds considerably to the cost, and I think defeats the object of recycling, but it may be the the most acceptable option in residential areas.

In a rural situation, where finished road levels are not critical, it is probably far less expensive to lay an increased thickness of a bituminous material over the weak existing road construction, than to retread and use a new bituminous surface course (wearing course).
In effect using the existing weak road pavement as a relatively strong sub-base layer.

The cost effectiveness and environmental benefits of the Retread process is often determined by the haulage distance for fresh materials to be brought to site.
New materials that have to be brought a long distance from the quarry or mixing plant will be expensive and also contribute a polluting effect to the environment.

STOCKPILING OF ROAD PLANINGS

It might be appropriate to say right at the outset that bituminous road planings have a habit of "disappearing", and I am fairly confident in saying that for the past thirty years I have been in the industry all road planings that have been generated in reasonable quantities have been recycled.
It may have been an unofficial form of recycling that would no longer be permitted under modern regulations, but there has been many a farm drive or factory car park that has benefited from the use of road planings.
If you do not make provision to know what tonnage of planings you are creating from the works and instigating some form of payment to transport for tonnage delivered to a secure stockpile you are going to have a lot less planings than you thought you would have.

There can also be a problem in stockpiling bituminous planings with the recombining of the material into large blocks which are subsequently very difficult to handle.
The problem will occur more often with less viscous materials, and of course long periods of hot weather will aggravate the problem.
This is another reason why hot rolled asphalt wearing course and other stiffer bituminous materials are more favoured for large scale recycling.
This problem is more or less eradicated when bituminous planings are part of a more general recycling operation which includes concrete and brick waste.

FOAMED BITUMEN RECYCLING OF ROAD PLANINGS

There are now a number of, mainly, proprietary processes where suitable bituminous road planings are being plant mixed with foamed bitumens to produce proprietary bituminous products suitable for specific purposes.

I specifically mention road planings because this material will mainly be in a well granulated form allowing the particles to be coated with the foamed bitumen. However it may be necessary to perform screening on the planings to remove any large lumps of material because it is unlikely this process will reduce the size of large pieces of material and these pieces could be a problem in laying.
The removed larger pieces can be crushed/granulated and re-introduced into the process. 

Foamed bitumen binder is an emulsified base bitumen which is then processed to achieve the "foaming".
This causes a huge increase in binder volume and enables the binder to more successfully coat the aggregate particles with bitumen.

The foaming process normally takes place immediately prior to mixing, you do not have very large containers of foam about the place.
This process can be performed cold, (as with the on-site "Retread" process), but it is more usual to use in plant processing to involve low level heating to aid mixing and subsequent workability and compaction on site.

When the process is a truly cold process the foamed bitumen can be described as more accurately "glueing" together granulated pieces of reclaimed bituminous material, rather than causing a complete remix of the bituminous mixture as with a genuine hot mix process.

The mixes produced with a foam bitumen are normally of a less viscous nature (less stiff) and tend to be used on less demanding sites with respect to weight and amount of traffic, and are rarely used as a surfacing material.

These materials have been used successfully as base (roadbase) and binder course (basecourse) in road pavements that do not require a high degree of stiffness.
This problem is usually overcome by overlaying with a conventional, fairly stiff, hot-mix bituminous wearing course.
The stiffness of materials produced using foamed bitumen can be improved by including a hydraulic binder such as cement, lime or pulverised fuel ash into the process, but the stiffness will still not be immediate, it will take considerable time to develop.

There does appear to be a developing wide variety of recycled materials that have a foamed bitumen component, and the various products will have differing properties that must be defined.

In my opinion the two ends of the spectrum of recycled materials, containing reclaimed bituminous mixture content, tend to be dependent on the proportion and bitumen content of the reclaimed bituminous material.

Uniform, Bitumen Rich Reclaimed Material
The reclaimed materials for processing that are all bituminous material, and predominantly surface course, and particularly high bitumen content hot rolled asphalt surface course, lend themselves to reprocessing using only foamed
bitumen and perhaps a small amount of hydraulic binder such as limestone filler.
This is because there is plenty of bitumen binder in the reclaimed material and by thoroughly mixing with a suitable amount of foamed bitumen, which will coat all the already bitumen coated particles this will enable the material to bond together with suitable compaction, and with time produce a satisfactory base or binder course for appropriate road classifications.
This material is likely to be plant mixed off site to produce a uniform product, and in time,will be similar in its final properties to a conventional bitumen bound product.

Suitably Graded Less Bitumen Rich Reclaimed Materials that may also contain other Suitable Reclaimed Material 
The reclaimed materials for processing that have low bitumen contents, e.g. base (roadbase) and binder courses (basecourse), or which also contain a significant proportion of other suitable, but "dry", reclaimed material tend to contain a higher proportion of hydraulic binders, such as cement and pulverised fuel ash, as well as foamed bitumen.
This will give the recycled material a closer to resemblance to a cement bound material than a bitumen bound product.

In between these two opposites will be many blends whose engineering properties when incorporated in a road pavement need to be understood.
Both types of correctly formulated material are suitable for use in a road pavements, but the differing engineering properties will be a factor in where and how they should be used.

The support of a qualified and experienced Materials Engineer, with a suitably equipped materials laboratory, is going to be particularly important when many types of material are going to be offered by contractors and suppliers for incorporation in to a road pavement. 
The variability of the engineering parameters of these materials could be quite wide, and I believe appropriate sampling and testing should be a part of ALL road construction and maintenance.

The pictures below show the use of a plant mixed material that incorporated foam bitumen in the mixing process, but which I believe can be regarded more as a cement bound material than a bitumen bound mixture, because of the  significant proportions of cement and limestone filler that were added.
The material contains sufficient water for the hydration of the hydraulic binders, but it is not "wet".
On a warm dry day as it was, there may have been a case for sealing the surface with an emulsion spray to avoid moisture loss before hydration was complete. 
But this would have resulted in an increase in cost that may not have been appropriate to the site, and as you can see it is a lightly used rural road, and it was subsequently overlaid with a thin (30mm.) surfacing of BS 4987 - 10mm. Size, Close Graded Macadam. 

COSTS AND PERFORMANCE

It is my belief that the cost of recycled material MUST be no greater than the cost of conventional material, and the performance of the recycled product MUST be equal to that of conventional / new materials or it is very difficult for an engineer with a limited budget to make out a case for specifying recycled material to the customer or authority employing him.

With the recent introduction of Land Fill Tax there is now quite a large cost incentive to recycle a greater amount of suitable material, rather than pay for it to be taken to tip.

There is however the considerable cost of transport, stockpiling, crushing and blending, to facilitate the processing of reclaimed material to make it suitable for inclusion in highway works.

The considerable amount of paperwork involved in gaining planning position for recycling sites , and the health and safety regulations which are associated with this type of work adds even further to the cost.

The supplier of recycled material should also have a suitable level of laboratory backup to ensure the quality of his product.

Having said all the above I believe there is significant potential for recycling in highways maintenance on appropriate sites with appropriate materials. These processes will need to be correctly engineered with specifications and technical support relevant to the materials produced so that it can be seen that recycling is a sustainable option.  

SITE SPECIFIC MATERIAL SPECIFICATIONS

Many organisations have specifications for recycled material which allows the use of suitably processed reclaimed material (which may not meet existing D.O.T. or British Standards) into low stressed situations, e.g. lightly trafficked rural roads, residential estate roads and footways.
The recent Volume One, of the Specification for Highway Works, especially the 700, 800 and 900 Series contains an increasing amount of information and guidance on the specification and use of reclaimed and recycled construction materials for use in road pavements. 

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