THE
FUNDAMENTALS OF RECYCLING BITUMINOUS MATERIALS IN A HOT MIX PROCESS| [ Top ] | Hot Rolled Asphalt and Bituminous Macadam |
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RECYCLING RECLAIMED BITUMINOUS MATERIALS USING A HOT MIX PROCESSES
THE
FUNDAMENTALS OF RECYCLING BITUMINOUS MATERIALS IN A HOT MIX PROCESS
I
do not wish to insult anybody, but a lot of people have not understood the basic
principles of the hot recycling/remixing 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 hot mixed 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.
What will be the difference between the two batches
in an hour ?
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.
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.
| BEFORE REMIXING | AFTER REMIXING |
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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, rather than "adding cost".
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 binder course (basecourse) and base (roadbase) bituminous mixtures.
This usually means hot rolled asphalt surface course (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 surface course (wearing
course), and up to 50% in binder course (basecourse)
and base (roadbase) by suppliers with the appropriate approval of the engineer,
lesser amounts of bituminous planings can be included in
these materials without the approval of the engineer.
The above figures may change, I suggest you consult the Specification for Highway
Works for confirmation of the current figures.
Of course the resultant materials must comply with the appropriate specification
that they are supplied to.
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
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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. |
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| 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.
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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. It may be necessary to add a small amount of "rejuvenator" to the planings to produce the workability you require in your recycled material. It can be a proprietary cut-back oil, e.g. a kerosene type liquid, or a flux oil, e.g. a diesel type liquid, according to what properties you require in the recycled material, usually to lower the viscosity (stiffness) of the recycled bituminous mixture. These products have been available for many years and are used in main stream bituminous mixture production. The "rejuvenator" can be a proprietary liquid or a pelletised product, these are now produced specifically for this market but tend to be expensive. 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. |
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Recycled bituminous material
being discharge from the remixer after approximately 20 minutes |
NOTE - You must pay strict attention to remixing
temperatures when adding cut-back and flux oils, it my opinion it is good
practice to put ALL heat into the reclaimed material before addition of
cut-back or flux, i.e. mix the added cut-back or flux oil at the correct heat level already
contained in the mixture. Pay careful attention to the instructions appropriate to the particular cutback (volatile) oil that you are purchasing and using, e.g. maximum mixing temperatures. |
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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"
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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. |
Note, below the bucket is reversed from the normal "digging" position to a "holding" and spreading position.
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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, and seem to put
unnecessary barriers in the way of this extremely innovative way of recycling
good quality bituminous planings and arisings on site.
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 in situ 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-----> 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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