TOM 4 styczeń-luty 2000 r. nr 1
K la u s M . D ie d r ic h * , B e r n a r d J. B u r n s* *
Pavements from Asphalts Modi
fied with Ground Tire Rubber (GTR)***
High quality pavements can be prepared by mixing o f trans-polyoctemer (TOR) with ground tire rubber (GTR) in asphalt. As small as 3-6% o f TOR causes efficient dispersibility o f fillers, compatibility and crosslinking in asphalt GTR mixtures. Mixtures preparation methodes and ways o f their usage were described in details. Besides, practical examples o f the cost calculation for pavements in numerous roads were presented.
Received results illustrate in which way TOR can contribute to a solution o f a great ecological problem - the recycling o f GTR.
Key words: asphalt modified, TOR, ground tire rubber
Nawierzchnie asfaltowe modyfikowane zmieloną gumą oponową (GTR)
Nawierzchnie o wysokiej jakości można przygotowywać w wyniku zmieszania polioktenameru (TOR) ze zmieloną gumą oponową (GRT) i asfaltem.
TOR użyty w niewielkich ilościach, od 3 do 6%, spełnia rolę skutecznego dyspergatora napełniaczy, kompatybilizatora i środka sieciującego. Omówiono szczegółowo metody otrzymywania mieszanin oraz sposoby ich użycia. Ponadto podano praktyczne przykłady obliczeń kosztów nawierzchni ułożonych na kilku drogach. Uzyskane rezultaty ilustrują w jaki sposób można wykorzystywać TOR w rozwiązywaniu znaczącego problemu ekologicznego jakim jest recykling GTR.
Słowa kluczowe: asfalt modyfikowany, polioktenamer (TOR), mielona guma oponowa
Introduction
Western Europe alone generates about 1.8 to 2 million tons of used tires annually. The quantity of used technical rubber goods may be of a sim ilar amount, with almost no sources available as yet to show where these products end up. In the United States every year 275 m illion old tires, one for every living breathing
* Degussa-Hiils AG, Business Unit CREANOVA, 45764 Marl, Germany
** Creanova Inc., 220 Davidson Avenue, SOMERSET, NJ 08873, USA
*** Presented at the International Conference ECO -R U BBER ’99
“Technical and econom ical options o f rubber recycling”, Warsaw, October 21, 1999, Poland
Am erican, are thrown away. The weight of these tires is estimated to be 3.3 million tons of which around 80 % of the total are passenger car tires. Additionally more than 800 million tires are piling up around that country.
The recycling of this large amount of used rub
ber, and used tires in particular, is a challenge for both, the technologists and the politicians. Com m on recyc
ling m ethods, like tire derived fuel (substitution of fuel with ground tire chips in pow er stations, cement kilns or paper mill furnaces) or production of pellets and powder, use up only 60 to 70 % of the used tires. Due to the downgrading of quality, the use of ground tire rubber (GTR) in new products is lim ited but there is a large m arket potential.
S f a a t M t e n y nr 1 styczeń-luty 2000 r. T O M 4
A nother large m arket potential is the application of GTR in rubberized asphalt. Trials started in the United States m ore than 10 years ago with more or less success. The political pressure to use GTR in asphalt pavem ents during the last years has been in
creased despite of a different opinion of the asphalt lobby. But m eanw hile many tests in different regions of the USA and C anada show the benefits of using GTR in asphalt pavem ents.
Use of GTR in asphalt
In the past GTR was added in many cases to the asphalt term inal-blending tank. That often gave pro
blem s w ith stirrin g o f the m ix as GTR is very stic ky at h ig h er tem p eratu res and tends to separate and to d ep osit at the b o tto m of the tank. This process continues during the tran sp o rtatio n o f the asphalt concrete m ix to the paving site and therefore it is not guaranteed to hav e a reg u lar d istrib u tio n o f the GTR all overth e pav em en t length. A nother problem can be the rem ain ing m ix in the term in al-blending tank after fin ish in g the p avem ent w ork. In this case the m ix has to be h eated and strongly m ixed over night to avoid a G T R sep aratio n and ag g lom era
tion. A ll these d isa d v a n tag e s o f p rocessin g G TR in the „w et” asp h alt m ixing p rocess and the p ro b le
m atic h andling o f the m ixes led to hig h er costs and bro ug ht the use o f G T R in d isrepute.
O ther a p p lic atio n s o f G TR are m ade in the ,,dry”asp h alt m ixing p ro cess. The h andling o f the
m ixes up to the p av ing site are less d ifficu lt but a problem is the stick in ess o f the hot m ix laid dow n on the street. It causes a co n sid erab le am ount of asphalt pick up on the steel rollers. T his gives im p erfection s w hich can be filled w ith w ater during the w inter. T hese w ater filled depressions result in freeze thaw d e te rio ra tio n on the road surface. In consequence, the steel ro llers have to w ait a few hours until the asph alt has cooled dow n to a tem p e
rature w hich does not resu lt in pick up of m aterial on the rollers. T his effect add ition ally increased the costs of the G TR process. O ften GTR #80 m esh was used w hich is m uch m ore expensiv e than #40 m esh or # 20 m esh m aterial.
In oth er tests im p ro p er test co nditions w ere used. F or exam ple, in the p avem ent o f a test road, sponsored by the N ew York D ep artm ent o f T ran s
port, only 2 % o f G T R w ith a particle size o f 6.3 m m (2.5 inches) w ere added to the asphalt concrete pavem ent. T hese large iso lated p articles w ere e asi
ly ripped out and the street surface show ed dam a
ges already a fte r one year.
The aforem entioned results and some other inci
dents, e.g. G TR-m odified asphalt started to burn, tur
ned out to be disadvantageous for the use of GTR in asphalt concrete and the applications were limited.
Only in the last years the am ount of used GTR incre
ased due to the political pressures of several D epart
ments of Transport (DOT). That resulted in a larger amount of test roads and easier acceptance in the DOTs to try new products and applications in this field.
Fig. 1. Synthesis ofVestenamer®
TOM 4 styczeń-luty 2000 r. S(#Ato*K & U f, nr 1
Application of trans-polyoctena- mer (TOR) in asphalt concrete
Due to experiences with Trans-polyoctenam er, TOR, in recycling of GTR in conventional rubber pro
cesses, in which it im proved the properties of the final products [1], this m aterial was tried in the production of rubberized, asphalt and pavement. The product is produced and m arketed since 1980 by Degussa-H uls AG with the tradem ark VESTENAM ER® . TOR is de
rived from butadiene by dim erization to cyclooctadie- ne-1,5, followed by a selective hydrogenation to cyc- looctene and finally a metathesis polymerization of this cyclic m onom er to Trans-polyoctenam er, TOR (Figu
re 1). This polym er with its at least 25 wt.% of macro- cyclic m olecules is well-known in the rubber industry [2,3]. Due to its com bination of unique and exceptio
nal properties it is a real rubber but also acts as a pro
cessing aid being active in all m ixing and shaping pro
cesses.
At first, let’s review the physical properties of TOR which lead down the path to its’ use in rubberi
zed asphalt applications. The TOR polym er has a m el
ting point of 54°C (130°F) which is far below the nor
mal asphalt processing tem peratures o f from 160 - 170°C (320 to 340° F). Therefore it becom es a liquid almost instantly at these asphalt concrete m ixing tem peratures. It can also be seen that the polym er has a Tg of -65°C (-85°F) which makes it ideal for low tem pe
rature applications. Its’ M ooney viscosity of < 10 at 100°C (212°F) allows it to thoroughly mix GTR into the asphalt cement. Studies have shown that 30% TOR can be dissolved into processing oils at tem peratures as low as 100°C (212°F). Finally, the TOR is supplied as 114 inch pellets, which are very easy to handle and weigh.
The characteristics of GTR are of significance.
There are two different main processes for m anufac
turing GTR, the am bient and the cryogenic processes.
In the ambient process, it is difficult to get very fine (80 to 200 mesh) GTR unless a wet grinding process is used since the heat generated is a lim iting factor in how long one can grind the rubber before it starts to get sticky. This am bient grinding process results in a very high surface area rubber crum b since the rubber polymers are literally tom apart leaving stringy thre
ads on the surface of the crumb.
In the cryogenic process the rubber is cooled with liquid nitrogen during the grinding process which cau
ses it to be very brittle. In this process the rubber is fractured into very fine crumb but it has very smooth fracture surfaces and therefore less surface area. This is obviously an im portant factorfor large mesh crumb rubber particles (for instance 5 to 20 mesh) and less of a factor for smaller m esh sized particles.
From the bond energies in Table 1, it can be seen that in a grinding operation, S-S bonds (226.1 kJ/m ole or 54 kcal/m ole at 25°C) will break before C-S bonds (272.1 kJ/m ole or 65 kcal/m ole at 25°C) which will break before C-C bonds (345.8 kJ/m ole or 82.6 kcal/
mole at 25°C). Therefore, once the waste/tire rubber has been ground into crumb rubber, the surface now has reactive sulfur atoms on its surface that can under
go vulcanization reactions. This GTR can be molded into rubber goods with as little as 3 parts of TOR per 100 parts of GTR. Since the resultant products are real rubbers after the vulcanization process used to make them, they are in and o f them selves recyclable. This was the rationale that lead to investigate the effects of TOR + GTR in asphalt paving where a road surface is wanted that can be recycled.
Tabele 1. Bond energies at 25°C
s - s
226.1 kJ/m ole 54.0 kcal/m ol3C - S 272.1 kJ/m ole 65.0 kcal/m ole
C - C 345.8 kJ/m ole 82.6 kcal/m ole
The actual com position of asphalt is constantly changing due to the feedstock and to the rapid im pro
vements in refinery technology which allows them to extract more and more of the lower boiling com po
nents of oil leaving a product - asphalt - which is stif- fer and m ore difficult to use. If this trend to extract out everything of value in an oil source progresses, the com position of asphalt will have to be increasingly m odified in order to be able to use it. In addition, there is no uniformly consistent source of asphalt to this day.
The percentages of asphaltenes, m altenes, sulfur and other components vary from refinery to refinery and from oil well to oil well.
One of the advantages of TOR is its unique abi
lity to be a very efficient dispersant of fillers in diffe
rent sorts of polymers as well as having the ability to disperse polar and non-polar polymers into one ano
ther. This unique behavior makes it very useful as a dispersant, com patibilizer and cross-linking agent in asphalt GTR mixtures.
In order to have some sort o f data to present to the industry, some prelim inary laboratory work has
S fa ć& M t& itf nr 1 styczeń-luty 2000 r. TOM 4
been perform ed by an outside laboratory. This data showed that adding GTR plus TOR to asphalt gave 30% better Bending Beam Rheom eter (BBR) values than adding either TO R or GTR alone to the asphalt.
From those studies it was apparent that the sulfur on the surface of the GTR reacted with the isolated do
uble bonds in the TOR to produce a crumb that now had double bonds on the surface o f the crumb rubber. Now this TOR modified GTR crumb could react with the ad
ditional sulfur in the asphalt to cross-link this mixture into a rubber like matrix. This „Polymer Modified Rub
berized Asphalt” has been used in road construction in Canada where it has shown excellent crack resistance and water repellant characteristics without loss of „grip”.
Although TOR does reduce „tack” in rubber compounds and in rubberized asphalt, it does not reduce „grip”. Evi
dence of this is shown by the fact that the TOR is used in rubber golf club grips where you get this very desirable non-tack but positive grip scenario.
In September 1998 the first test road was put down in Grey County, Ontario (Route # 7 project). On this road was tested a combination of TOR + GTR (using 7% of 10 mesh GTR) + 58-28 asphalt that contained 13 % of 30 mesh GTR. The 30 mesh GTR had been premixed in a terminal-blending tank for 3 hours. A conveyor belt was used to add the TOR and 10 mesh GTR directly into the pugmill. This GTR modified asphalt was transported to the concrete mixing plant in a tank truck that had a constant recirculating pump on it.
Two different sections were put down on Route
#7 North. The first section going N orth had the above mix design for approxim ately 800 meters. The second section going N orth for 200 m eters and South for 1000 meters side by side with the North section had the above mix design w ithout the TOR. In this way the two mix designs were tested with the exact same amount of traffic going North and the exact road elevations go
ing South. This road pavem ent was laid at 5 cm (2 inches) and com pacted down to 3.8 cm (1.5 inches).
That is where the real savings in cost is obtained when normally an asphalt m at is laid at 9-10 cm (3.5 to 4 inches) and reduced to no less than 7.6 cm (3 inches) in order to get the required perform ance from the road.
The road is located in a m ountainous area. It saw 3 m (120 inches) o f snow last w inter and experienced tem peratures from -25 to +35° C (-17° F to + 95° F). The road was constantly plow ed, salted and sanded in or
der to keep it open for the resort traffic. To date the TOR +GTR m odified section of the road not only has no cracks, but has no hairline cracks as well. W here
as, the other section w hich had no TOR only GTR has shown prelim inary raveling due to the fact that when it was first applied last year, there was a considerable am ount of asphalt concrete pick up on the steel rol
lers. This caused imperfections that filled with water during the winter. These water filled depressions resul
ted in freeze thaw deterioration on the road surface.
On the other hand, the section with TOR has had no concrete pick up on the steel roller which could work near behind the spreader. The small am ount of TOR (4.5% based on the total amount of GTR added to the cem ent mix or 0.06% of the asphalt concrete mix), reduced the tack of the rubberized asphalt and gave a sm oother and m ore com pact road surface. The difference betw een the two sections of the street can be seen a few m inutes after a rain shower. It can be seen how the w ater beads up on the TOR treated side and sank into the non -TOR treated section. In addi
tion, it is well known that rubberized asphalt will re
duce ice build up because the ice will flake off the flexible road surface under traffic conditions. Also, it will stay free o f ice longer because of the dark black color, which absorbs heat faster and holds it longer due to the rubber.
In June 1999 three other test strips have been laid down in Canada. These mix designs will hopefully give a better picture of how much TOR and GTR is needed to get the maximum benefits for the least cost, which is the constant demand from any road construction project.
For these test strips the „all-dry” mixing process was used where the dry #10 mesh GTR + TOR were shot into the pugm ill by way of a conveyor belt as the aggregate was being added. The TOR + GTR were m ixed for 10 seconds with the aggregate and then the 58-28 asphalt was added. This final mixture was fur
ther mixed for an additional 46 seconds for a total mix tim e of 56 seconds. The asphalt concrete was dumped into the waiting truck. It takes time to get the truck weighed and transported to the job site where it sits around waiting its’ turn to dump its’ load into the spre
ader hopper. The asphalt concrete was further mixed in the spreader as it was being applied to the road sur
face. This process gave a very uniform road surface with very good durability at a very low cost compared to other m odified asphalt processes.
It is recom m ended that the com pletely m ixed rubberized asphalt concrete be allowed at least 10 to 20 minutes for the GTR to com pletely absorb the low m olecular weight oils in the asphalt before applying it to the road surface. In our experience it took at least
TOM 4 styczeń-luty 2000 r. S la a to tH & iy nr 1
20 minutes from the tim e the truck had its last batch added to it and the tim e it was applied to the road sur
face since the site was 15 miles away.
C ost c a lc u la tio n s fo r u sin g TOR + GTR in asphalt
In the following, a calculation on the additional costs for the production of the rubberized asphalt for the Route #7 project is given in table 2.
Tabele 2. Typical mix
The ag gregate is m ixed in a pug m ill w ith 20 Lbs. o f G TR + 3 Lbs. o f TO R for 15 seconds and then m ixed w ith 395 Lbs. o f the 58-28 asphalt/G T R
#30 m esh blend for 45 ad ditio n al seconds.
Tabele 3. Modification costs fo r 5000 Lbs. o f concrete 65 Lbs. of GTR x $ 0 .1 5 /L b . = $ 9.75
3 Lbs. of TOR x $1 .85/Lb. = $ 5.55 Cost fo r 5000 Lbs. of concrete $15.30
= $6.12 fo r a 2000 Lbs. ton = $6.75 fo r a 2205 Lbs. MT A lternatively, the agg regate is heated in a co n tinuous aggregate mixer and the GTR and TOR are ad
ded through the rap collar or through the fines bag return system. In either of these systems, the final mixing step in the asphalt concrete spreader completes the mixing process. Since the TOR has been sitting in the truck at temperatures above 150°C (300°F) it is completely liquefied by the time it reaches the job site. After final mixing in the spreader as it is moved onto the road surface it will cross-link on the road surface within 10 minutes as the steel rollers compact the aggregate + GTR + TOR together.
This results in a 7% asphalt cem ent m odified concrete where the asphalt cem ent is m odified with 1 8 .6 % GTR and the GTR is m odified with 4 .6 % TOR based on the weight of the GTR.
1. Sand (1 /4 ” ) 2 420 Lbs. GTR #10 mesh 20 Lbs.
2. #2 gravel (3 /8 ” ) 610 Lbs. TOR 3 Lbs.
3. #3 stone (9 /1 6 ” ) 940 Lbs. GTR #30 mesh 45 Lbs.
4. #4 stone (1 ,1 /1 6 ” ) 612 Lbs. 7% (58-28) Asphalt 350 Lbs.
Total aggregate 4 582 Lbs. Total paving concrete m ix 5 000 Lbs.
Tabele 4. Calculations o f the necessary amount o f TOR fo r the tree different test roads County Road No. 1 2 1 2 , Asphalt Concrete M ix (3% TOR):
Length = 2.4 Km, Quantity = 1920 MT = 4,224,000 Lbs. asphalt concrete
Using 7 % asphalt cem ent 295,680 Lbs. 58-28 asphalt cem ent
Using 20% total GTR 59,136 Lbs. GTR
Using 3% TOR 1,774 Lbs. TOR
County Road No. 42, A sphalt Concrete M ix (6% TOR):
Length = 1.0 Km, Quantity = 810 MT = 1,782,000 Lbs asphalt concrete
Using 7 % asphalt cem ent 124,740 Lbs. 58-28 asphalt cem ent
Using 20% total GTR 24,948 Lbs. GTR
Using 6% TOR 1,497 Lbs. TOR
County Road No. 13, Asphalt Concrete M ix (4% TOR):
Length = 1.43 Km, Quantity = 1473 MT = 3,240,600 Lbs. asphalt concrete
Using I % asphalt cem ent 226,842 Lbs. 58-28 asphalt cem ent
Using 20% total GTR 45,368 Lbs. GTR
Using 4% TOR 1,815 Lbs TOR
For 4.83 km street (9,246,600 Lbs. asphalt concrete) 647,262 Lbs. 58-28 asphalt cem ent
129,452 Lbs. GTR
and only 5086 Lbs. TOR were used.
S(*At<U 4tenty nr 1 styczeń-luty 2000 r. TOM 4
The calcu latio n s o f the n ecessary am ount of TO R for the th ree d ifferen t test roads o f 1999 are given in table 4.
W ith the new process the pre-weighed GTR + TOR can be added to the conveyor belt and then to each batch in the pugm ill. Alternatively, using a set of screws or conveyors or com bination of both, the TOR and GTR can be continuously added through the rap collar of a continuous drum mixer. Either way, there is the elim ination of the tim e and energy consuming pro- cess of prem ixing GTR with the asphalt in a „wet”
addition process, w hich adds considerable costs to the paving project; in addition to the fact that #80 mesh GTR is far more expensive than #10 mesh. Since the TOR and GTR can be added at an asphalt concrete mixing plant, this allows the paving contractors to receive a much lo
wer cost modified asphalt since they are eliminating the whole terminal blending operation cost.
Quality aspects
As with any process these days, everyone is con
cerned with „Q U A LITY ” . F or decades, the asphalt industry has shown that in such large scale projects such as asphalt concrete road construction, you still need to control the w eight o f the ingredients being m ixed together in the m ixing operations no m atter whether they use pug m ills or continuous drum m i
xers. However, you also need a simple fast m ethod for getting all the ingredients into the asphalt trucks, which keep com ing every 3 to 5 m inutes.
W ith the elim ination o f the whole term inal blen
ding operation the asphalt producers do not have to worry about having the GTR settle out in the term inal blending tank or having it settle out in the tank truck.
Either scenario would give a non-uniform amount of GTR over the expanse o f the road, and unpredictable perform ance.
The basic tenant o f ISO 9000 is the fact that if a process meets the control specifications, the product so produced will also m eet the desired specification.
Therefore, once the m ixing param eters are set in the mixing plant the product produced should meet speci
fications. The experiences in Canada demonstrated that laying asphalt concrete paving is not brain surgery. It is for sure that the weights of all the components which were added were not plus or minus 1 gram but then again when the mix is 8,000 pounds per batch it does not need to be that accurate.
There are a num ber o f ways one can m onitor the road project such as taking samples from the truck or from the laid road surface. These samples can be run in extraction studies or D SR (Dynamic Shear Rheom e
ter), DSR on PAY (Pressure Aging Vessel), BBR (Ben
ding Beam R heom eter) stiffness and Slope tests to determ ine if the project meets the original m ix design or to obtain a PG grade on the mix. To date no one is found in the industry who can predict from ju st one test, exactly how an asphalt concrete road will per
form. Everyone has their own experiences with diffe
rent asphalt PG rated m aterials and their own biases based on these experiences.
Summary
In conclusion, the new and m eanwhile patented TOR + GTR process [4] gives you:
♦ The ability to use a low cost raw m aterial - #10 m esh GTR is about $0.12/Lbb versus about $0.55 to $0.80 for fine GTR
♦ The ability to apply the asphalt concrete at 4 cm (1.5 inches) versus 8 cm (3 inches) which is a huge cost savings with getting at the same time the requ
ired perform ance of the road
♦ The ability to reduce processing costs since there is no term inal m ixing tim e or m ark up
♦ A sim ple process which only needs a conveyor belt or screw in addition to the normal mixing equip
m ent
♦ Roads paved with rubberized asphalt are more du
rable than roads with unm odified asphalt
All of which leads to a high performance low cost m odified rubber asphalt road surface.
A dm ittedly, there have been some rubberized road test trials that have failed in the past. Unfortuna
tely, just one failure causes a huge backlash in the in
dustry. It is very easy to do nothing new since there is little risk involved in tried and true processes. That is why there has been a lot of opposition to trying our new TOR + GTR process in the USA. L et’s hope that we can avoid these m istakes not only in the USA but also in Europe and will be more successful here with the new process.
References
1. a) D. M ahlke, Kautsch. Gum m i, Kunstst. 46 (1993) 889
b) D. M ahlke, G. H uhn, Paper presented at the N ordic R ubb er con feren ce, H eisingor, M ay
1993
2. a) Draexier: a) Kautsch. Gummi, Kunstst. 34, 185 (1981); Reprint: Elastom erics 114 16 (1983).
b) Kautsch. Gummi, Kunstst. M , (1983) 1037.
3. K. M. Diedrich, Kautsch. Gummi, Kunstst. 42.1130 (1989), and literature quoted there
4. US Appi. (CREANOVA Inc., B. J. Bum s)
