RECYCLING BITUMINOUS MATERIALS IN HOT MIX PROCESSES

(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

THE FUNDAMENTALS OF RECYCLING BITUMINOUS MATERIALS
WHERE TO FIND DOCUMENTED INFORMATION
REPAVE / REMIX
HOT RECYCLING OF ROAD PLANINGS
THE STOCKPILING OF ROAD PLANINGS
ON SITE, HOT RECYCLING OF BITUMINOUS MATERIALS
COSTS AND PERFORMANCE
SITE SPECIFIC MATERIAL SPECIFICATIONS

RECYCLING BITUMINOUS MATERIALS IN COLD MIX (FOAMED BITUMEN) PROCESSES

THE FUNDAMENTALS OF RECYCLING BITUMINOUS MATERIALS

I do not wish to insult anybody, but a lot of people have not understood the basic principles of recycling bituminous materials.
A bituminous material is thermoplastic, meaning, when it is hot it is able to be worked, when it is cold it is solid, well, for practical purposes.  
A bituminous mixture used for conventional road surfacing consists mainly of aggregate and bitumen.
It is unlikely that if these two components where of good quality at the time of laying that they will have degraded much during the life of the pavement layer that they were contained in.
The surface course will have deteriorated most but only at the surface where the bitumen will have undergone some oxidisation, but material in the matrix of an impervious layer will be substantially the same in physical properties as at the time it was laid.
Note the previous sentence carefully because it is a fact that the most change in the physical properties of a bitumen binder will occur during the mixing process and the time of hot storage prior to laying, but temperatures of bituminous materials is another subject.
So, let me be even more blunt with the scenario, if you take two freshly, and correctly mixed batches of asphalt, and you leave one in the insulated lorry and you tip the other batch on a clean hard standing and deliberately "pull it out" so that it loses heat.

So, what will be the difference between the two batches in an hour ?
One will be hot and still in a plastic condition and able to be laid, the other will be cold and unable to be worked, and unfortunately this situation too often arrives when laying small tonnages. 
But other than this difference in temperature the two batches will be the same product.

But if you were able to reheat the cold material in a controlled procedure, so that it was heated through but not overheated, that bituminous material would be practically as good as new.

Note :- With correctly produced hot remixing you will have a complete break down and remixing of the individual constituent aggregate particles as the viscosity of the bitumen is lowered as the temperature increases.
This will not be the case with foam mixed recycling of reclaimed bituminous materials.
E.g. you could not take pieces of reclaimed bituminous surfacing, as shown in the picture above, and produce a usable consistent foamed bitumen product without first crushing the reclaimed material to an appropriate size range.  

If you have the services of a good Materials Engineer/Technician who knows the properties of the bituminous materials available for recycling the principles of this process has got to provide opportunities of a financial advantage in a number of situations as well as being environmentally friendly.

However in re-heating / re-mixing an existing bituminous material you MUST have a fairly accurate assumption of what the original reclaimed material was/is, 
i.e. is the bituminous mixture for recycling a hot rolled asphalt (HRA) with a stiff 50pen binder that can be reheated to a 160/180 degrees centigrade with little damage or hardening to the binder and associated fume production, 
OR was/is the material a bituminous mixture with a "soft", low viscosity binder, that must have a lower temperature mixing procedure to avoid damage to the binder, and prevent fume production as the volatile oils are driven off by overheating.
Of course it follows that a "softer" material will cost you less to recycle as it needs less heat to attain its workable temperature, but it would be unwise to recycle a stockpiled "soft" material on to a high stressed site.
I have already included a link to the temperatures of bituminous materials  but may I also suggest you take a look at bitumen viscosities for bituminous materials  because all the rules that apply to the production of new bituminous mixtures apply equally importantly to recycled product.
If you are working with bituminous materials you need to understand bituminous materials, that is why I keep saying, "consult your Materials Engineer", he is there to help prevent you making mistakes and show you the best way of working with the material you have to recycle, not to be an obstruction.

Note : It is necessary to point out that practically all requests that I received to attend hot remixers/recyclers that were producing fumes were solely due to overheating of reclaimed bituminous materials, and it is sad to say, often with the knowledge of the operators with the aim of producing bituminous materials that were very easy to lay. 
This not only produces unwanted fumes, it is also damaging the engineering qualities of the bitumen, just as it would if overheating occurred at a quarry production plant.
Why do you think we have maximum mixing temperatures for production of bituminous mixtures, that vary with the viscosity of the bitumen.
But again, I must stress that you must know the nature of the bituminous materials that you are remixing/recycling/heating.
This process is not "magic", it is a process based on engineering knowledge, if you do not have this knowledge you need to acquire it by educating your teams. 
It is also not a difficult process if you do know what you are doing.

Finally, I really do believe where at all possible bituminous materials should be reprocessed as bituminous materials because that is where the term "adding value" can really mean what it says.

I hope you find the information and references presented below are helpful to you, but I stress you need "materials" guidance with this process especially when you are recycling materials that are not of a known and consistent source.

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

DOT Specification for Highway Works : Vol.1 : Road Pavements - Bituminous Bound Materials : Clause 926

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

TRRL Research Report 305 : Assessment of the performance of off-site recycled bituminous materials

"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 - Recycling in Transport Infrastructure
An excellent report covering ALL aspects of undertaking recycling processes in road maintenance and construction.
It includes advice and guidance on all the necessary rules and regulations that must be complied with in any recycling process.  
The report also describes examples of types of recycling already being undertaken. 


Referring to,
BS 4987:Coated macadam for roads and other paved areas,  
Part 1:Specification for constituent materials and for mixtures & Part 2:Specification for transport, laying and compaction
and
BS 594 : 2002 : Hot rolled asphalt for roads and other paved areas,
Part 1 : Specification for constituent materials and asphalt mixes & Part 2 : Specification for the transport, laying and compaction of rolled asphalt. 
will give you an indication of the types of hot remixed bituminous mixtures you should be aiming to produce, and where and how they should be laid and compacted.

Note : From the 1st. of January the above standards will be superseded by, 
BS 4987 -------------> BS EN 13108-1 :2006 : Bituminous mixtures - Material Specifications - Part 1 : Asphalt Concrete
and
BS 594 ---------------> BS EN 13108-1 :2006 : Bituminous mixtures - Material Specifications - Part 4 : Hot Rolled Asphalt

But I would suggest that reference to the earlier documents may be more helpful/useful for practical working and the  mixtures that comply with BS 4987 and BS 594 will comply with appropriate tables/sections of the new BS EN standards.

REPAVE / REMIX

Repave

The Repave process is a recognised form of in-situ recycling and as such is specified and described in :-
DOT Specification for Highway Works : Vol.1 : Road Pavements -
Bituminous Bound Materials : Clause 926
and
DOT Design Manual HD 31/94 : Maintenance of Bituminous Roads

With Repave the immediate surface of the carriageway is scarified and heated to approximately 20mm. depth, and then it is reprofiled.
Material in excess of that required for the final line and level may be removed.
Fresh hot rolled asphalt material is laid on the top, and this and the scarified material are compacted together, using a highly specialised machine of considerable length, giving you a layer of new material over a layer of heated and scarified material.
Precoated chippings are applied to the new surface prior to rolling as for a conventional HRA wearing course.
The process will provide a new running surface to replace a surface which no longer has the required texture or anti-skid properties, but the existing wearing course material is itself in good condition, and capable of being part of the new surface.
Because of the size of the Repave machine the process is only really suitable and economical on fairly long lengths of carriageway with considerable overall width and the slightest of bends, the remix machine is of a similar size.

Remix

The Remix process is similar to Repave, the main difference from Repave being that the existing in-situ material after heating to 140 - 180 °C, and scarifying, is completely mixed with an appropriate amount of fresh material inside the machine.
The new material that is mixed with the existing material from the road is designed so that the resultant blend will be suitable for the site conditions and comply with the appropriate specification.
The newly mixed material is laid on a hot, scarified, level surface by the machine and precoated chippings are laid in the mat in a normal manner.
It is more economical to replace / overlay smaller areas of wearing course with new bituminous material and a normal paving machine.

For an indication of what a Remix/Repave operation entails, press -------------------------------------------------------------------> HERE

HOT RECYCLING OF ROAD PLANINGS

Bituminous planings are a premium material and their most cost effective use is to recycle them into fresh bituminous materials of varying types for use in highway works, as the established and specified Repave and Remix processes prove.

In order to be able to successfully recycle road planings as a hot bituminous material it is of the utmost importance to know what quality of planings you have, this is not difficult to establish with a few relatively simple laboratory tests.
Such tests as sample analysis to determine aggregate grading and binder content, and bitumen reclamation to establish the viscosity of the bitumen in the road planings.

If you are able to obtain large tonnages of planings of similar composition these can be utilised in the production of new mixed bituminous materials in quite high proportions in the production of basecourse and roadbase mixtures.
This usually means hot rolled asphalt wearing course planings to get large enough tonnages of the same material.

Be aware that stock piling road planings in large heaps, especially materials of higher binder contents and less viscous binders, can result in the problem of the materials recombining in the heaps. 
This can make it very difficult to "work" with these materials at a later time.
This situation can be a problem in warmer countries or during periods of hot weather in temperate areas.

Up to 10% of bituminous planings can be included in basecourses and roadbases by suppliers without seeking approval of the engineer, and up to 30% of bituminous planings can be included in these materials with the approval of the engineer.

The above figures have recently been increased, I suggest you consult the current Specification for Highway Works for confirmation of the current figures.

Of course the resultant materials must comply with the appropriate specification.

For a lot more information on road planings and "reclaimed asphalt pavement", RAP, Press, -------------------------> HERE

THE 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.
"People" have for some time realised the potential of good, consistent, surface course (wearing course) 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.

ON SITE, HOT RECYCLING OF BITUMINOUS MATERIALS

It is now actually possible to heat and re-mix quantities of bituminous materials on site with all the associated benefits of not having to transport materials long distances to be recycled. Mobile plants are now available that are capable of remixing amounts of up to 5.0 tonnes in a single batch. The reclaimed bituminous materials are remixed / heated in the mobile heater/mixers and the resulting "fresh" recycled material is used for appropriate small scale patching or footway refurbishment. The (re)mixers which perform this task are becoming more widely available as "environmental concerns" become a greater factor in all road maintenance issues.  However more significantly with the increase in the cost of bitumen and bitumen products  the mobile plants are being increasingly seen as a more cost effective option for some "small" tonnage work, i.e. 20/30 tonnes per normal working day, and this is driving development work to produce more sophisticated and still larger mobile plant. At the other end of the scale quite small remixer models are available specifically for producing small batches (50 /100kgs) of hot bituminous materials for emergency and planned  patching work, anywhere at anytime, and are able to be towed behind a conventional maintenance "works" lorry.
Freshly excavated bituminous material being placed in the remixer for heating and mixing, with the addition of a small amount of new binder, in this instance approximately 10% of estimated existing binder. E.g. Existing binder estimated to be 4.5% of 3000kgs., so this gives 135kgs. of existing binder, to rejuvenate this particular reclaimed material it was decided an addition of 10% of new 200pen. bitumen would be appropriate, so 13.5 kgs. of new binder was added prior to mixing.

It may be necessary to add a small amount of "rejuvenator" to the planings to produce the workability you require in your recycled material. Bitumen hardens during heating in mix production, and further stiffens by oxidation during the life of the road, this needs to be rectified when recycling to provide the grade of binder and workability for the nature of work intended. The "rejuvenator" can be a proprietary liquid, (they are now produced specifically for this market) or it can be something as simple as a cut-back oil, e.g. a kerosine type liquid, or a flux oil, e.g. a diesel type liquid, according to what properties you require in the recycled material. But in most cases it will be as simple as adding an appropriate amount of bitumen of the required viscosity, e.g. 300pen or 200pen.., it is unlikely anything stiffer would be required for most recycling work. The cut-back and flux options may be needed to modify stiff reclaimed bituminous materials e.g. hot rolled asphalt wearing course planings, into more workable materials for general "hand lay" patching and basecourse work.

Recycled bituminous material being discharge from the remixer after approximately 20 minutes

Recycled bituminous material discharged from the remixer being worked in a completely conventional manner as described in BS 4987

It is possible that this form of hot recycling may have higher production costs but the bituminous mixtures produced will replace the more expensive range of hot mix virgin materials and therefore achieve overall greater savings.  The materials produced will be of a high stiffness as soon as they reach ambient temperature, similar to the original reclaimed surfacing materials, and most importantly they can be used as the wearing course material when correctly formulated. Other environmental benefits, and cost savings are made because there are no lorry journeys to tip/recycling depot, no lorry journeys delivering/collecting fresh bituminous material, and no reprocessing/crushing of transported reclaimed material to produce granular or "foamed" recycled products.

CLEVER USE OF THE NEW "QUICK HITCH" METHOD OF SWAPPING "BUCKETS"

The recent development of being able to quickly change "buckets" on many fore-end loaders has prompted this gang to use two "buckets". One configured to break, scrape and load the remixer, and a second bucket to hold and spread the hot recycled bituminous material. The changing of buckets is now a quick process with not a lot of physical effort, mainly performed from the controls once the bucket is positioned correctly. It is obvious this facility allows most of the heavy work to be done by machinery, with a minimum amount of barrow, shovel and rake work to obtain correct levels and to work around ironwork and other obstructions in the footway or road. This allows the use of a smaller gang, and the gang works "smart" not hard, which I believe is always a good thing. It is especially important when you have the potential to produce and lay 30 tonnes of binder course (basecourse) material a day, on suitable sites.

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.

However there can be considerable cost of transport, stockpiling, crushing and blending, to facilitate the processing of reclaimed material off site 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.

SITE SPECIFIC MATERIAL SPECIFICATIONS

Many organisations have specifications for recycled material which allows the use of suitably processed reclaimed material (which may or 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.
This initiative is to be encouraged with the proviso that materials approved for sites experiencing lesser traffic loadings do not find there way on to sites with more demanding traffic conditions with claims that it is "an approved material".

For information on bituminous patching by insitu hot recycling, press, -----------------------------------------------------------> HERE

I try to refer to as few commercial sites as possible in compiling my site, but when a site offers particularly useful information about a subject I make an exception.

For further information regarding on site hot mix recycling press ---------> HERE ---------> HERE  and -------->  HERE

And, to access many excellent information sheets on recycling road materials, including bituminous materials,
from the recycling research body Viridis, a subsidiary organisation of TRL,  press --------------------------------->  HERE  

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