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Advances in Bituminous
Road Construction
Prof. Prithvi Singh Kandhal, Associate Director(Emeritus), National Center for Asphalt Technology(NCAT), Auburn University, Alabama, U.S.A.
Introduction
An ambitious road construction plan is underway in India, which primarily
involves bituminous pavements. At the present time, Ministry of Road
Transport & Highways (MORTH) Specification for Road and Bridge Works, 2001
Edition is used for construction of all roads including national highways.
Advances in bituminous construction technologies are made in the world almost
every year. This paper describes such advances in terms of materials, mix
design, special bituminous mixes, and recycling. There is a need to incorporate
these advances in MORTH specifications which are about 10 years old, to keep
abreast of latest technologies.
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Materials
Paving Bitumen
Prior to July 2006, penetration graded road paving bitumen such as 60/70 was
used in India. This grading system was based on empirical penetration test,
which is conducted at 25oC. The penetration test measures the consistency of
bitumen. Although two 60/70 penetration bitumen samples from different
refineries may have similar consistency (stiffness) at 25oC, but one may be
softer than the other when tested at 60oC, which is close to the highest
pavement temperature on a hot summer day. Bitumen which is very soft at
high temperature is undesirable because it can cause rutting in bituminous
pavement under heavy loads. Therefore, the Bureau of Indian Standards (BIS)
adopted a viscosity grading system for paving bitumen in July 2006 by issuing
standard IS:73:2006. This system is based on viscosity testing at 60oC.
Penetration graded bitumen 60/70 was deleted and substituted with viscosity
graded VG-30. Similarly, penetration graded bitumen 80/100 was deleted and
substituted with viscosity graded VG-10.
Although the preceding advancement has been made, there is a need to
advance further by adopting performance graded (PG) bitumen, especially for
national highways. The viscosity grading system gave excellent performance
results in the US for over 20 years. However, the viscosity grading system,
although more rational than the penetration grading system, was still based on
experience. A 50-million dollar, 5-year Strategic Highway Research Program
(SHRP) was undertaken from 1987 to 1992 to develop a performance based
grading system for bitumen, which was based on engineering principles to
address common asphalt pavement distress problems. The so-called Superpave
(acronym for Superior Performing Pavements) performance grading system
includes new bitumen tests and specifications with the following salient
features:
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1. Tests and specifications are intended for bitumen "binders," whichinclude both modified and unmodified bitumens.
2. The physical properties measured by Superpave bitumen tests aredirectly related to field performance by engineering principles rather than
just the experience.
3. A long-term bitumen aging test, which simulates aging of bitumensduring 5-10 years in service, was developed and included for the first
time.
4. Tests and specifications are designed to eliminate or minimize threespecific types of asphalt pavement distresses: rutting, fatigue cracking,
and thermal cracking. Rutting typically occurs at high temperature,
fatigue cracking at intermediate temperature, and thermal cracking at
low temperatures.
5. As shown in Figure 1, the entire range of pavement temperatureexperienced at the project site is considered. New testing equipments
were developed/adopted for testing bitumens for this purpose. A
rotational viscometer is used to measure the bitumen viscosity at 135oC.
A dynamic shear rheometer is used to measure the viscoelastic
properties of the bitumen at two temperatures: high temperature
corresponding to the maximum 7-day pavement temperature during
summer at the project site, and intermediate temperature corresponding
to the average annual temperature of the pavement at the project site. A
bending beam rheometer and a direct tension tester are used to
measure the rheological properties of the bitumen at the lowest
pavement temperature during winter at the project site.
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Figure 1: Superpave performance grade bitumen testing is conductedover the entire range of temperature experienced at the project site
The Superpave performance grade (PG) bitumen is based on climate. For
example, PG 64-22 bitumen is suitable for a project location, where the
average 7-day maximum pavement temperature is as much as 64oC, and the
minimum pavement temperature is 22oC.
The high temperature grades are PG 52, PG 58, PG 64, PG 70, PG 76, and PG82. The low temperature grades are 4, 10, -16, -22, -28, -34 and so forth.
Both high and low temperature grades are in increments of 6 Celsius degrees.
Example: A project location in Rajasthan has a maximum record 7-day
pavement temperature of 70oC in summer and a minimum record pavement
temperature of 3oC. A PG 70-4 bitumen will be specified for paving that
project.
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Aggregate
Many advances have been made in characterization of
coarse aggregate, fine aggregate and mineral filler used
in bituminous construction. However, one simple test for
characterizing particle shape & surface texture of fine
aggregate (sand) passing 4.75 mm sieve can be
implemented easily in India. It is desirable to have
angular fine aggregate particles in mix so as to resist
rutting in bituminous pavements. Whereas angularity of
coarse aggregate (retained on 4.75 mm sieve) can be
evaluated by naked eye, it is not easy to do so in
case of fine aggregate particles. The schematic of
the test equipment for measuring fine aggregate
angularity (FAA) is shown in Figure 2. It can be
fabricated easily in India. FAA test procedure has been adopted as standard
AASHTO Test 304.
A calibrated cylindrical measure is filled with fine aggregate of prescribed
grading by allowing the sample to flow through a funnel from a fixed height
into the cylindrical measure. The fine aggregate is struck off at the rim, and its
mass is determined by weighing. Uncompacted void content in the fine
aggregate is calculated as the difference between the volume of the cylindrical
measure and the absolute bulk volume of the fine aggregate collected in the
measure. Bulk volume of the fine aggregate is calculated from its mass and its
bulk dry specific gravity.
This test is based on the concept that round particles pack closer than angular
particles and therefore produce lower uncompacted void content, that is, lower
FAA value. A FAA value of 45 or more is desirable to ensure that the fine
aggregate is angular and does not contain any natural sand, which normally
has rounded particles.
Figure 2: Schematic
of equipment fortesting fine aggregate
angularity (FAA)
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Mix Design
Marshall Mix design is currently used in India for designing bituminous mixes.
In any mix design, it is desirable to compact laboratory specimens to a density
which is expected to be achieved in the bituminous course after 2-3 years of
densification under traffic. For designing bituminous mixes for heavy traffic, 75
blows each are applied with a Marshall impact hammer on both sides of the
specimen. This laboratory compaction level worked well in the past. However, it
was observed in the US during the 1980s that the field density of in-service
bituminous pavements was significantly higher than the laboratory design
density obtained with 75 blows. This was attributed to increased truck tyre
pressures and new tyre designs with stiffer side walls. Therefore, 75-blow
compaction level appeared inadequate. Increasing the number of blows was not
desirable because it merely caused degradation (breakage) of aggregate
particles in the specimen.
Figure 3: Schematic of Superpave gyratory compactor
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During SHRP, a new Superpave mix design method was developed in the US. A
Superpave gyratory compactor (SGC) was developed which compacted the
laboratory specimen with gyratory action (see schematic of SGC in Figure 3)
rather than impact compaction as is done with Marshall hammer. Gyratory
compaction also simulates field compaction with rollers in terms of aggregate
particle orientation. Depending on the traffic level in ESALs (equivalent single
axle loads) expected on the highway, desired compaction level can be obtained
in SGC by varying the number of gyrations without causing any significant
degradation of aggregate in the mix.
Another advantage of SGC is that a densification curve (number of gyrations
versus compacted density of specimen) is obtained during the compaction
process. At least three different gradations of the proposed mix are evaluated
in the Superpave mix design to select the gradation which has the strongest
aggregate skeleton.
Special Bituminous Mixes
Stone Matrix Asphalt
Stone matrix asphalt (SMA) was developed in Germany in the mid 1960 and it
has been used very successfully by many countries including US as a highly
rut-resistant bituminous course, both for binder (intermediate) and wearing
course for heavy traffic roads. SMA is tough, stable, rut-resistant mix that
relies on stone-on-stone contact to provide strength and a rich mortar binder to
provide durability.
Fig. 4a Stone matrix asphaltcross-section
Fig. 4b Conventional hot mix asphaltcross-section
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Figure 4 provides a cross-sectional representation of the difference between
SMA and dense-graded conventional bituminous mix. In a conventional dense
graded mix, coarse aggregate particles (retained on 4.75 mm sieve) are
separated by fine aggregate matrix, which primarily carries the load. In SMA,
coarse aggregate particles have stone-on-stone contact forming a stone
skeleton which carries the load.
Since SMA mix has relatively higher bitumen content, cellulose fiber is added to
the mix to minimize drain down of bitumen in trucks during transportation of
the mix from plant to project site. Although the cost of SMA is typically about
25-30% higher than the cost of dense graded bituminous mix, it is still
economical considering life cycle costs.
SMA has been widely used in the US since 1991 for heavy-traffic roads. It must
also be used in India for heavy corridors especially when overloading is also
common. Indian Roads Congress (IRC) has recently published a tentative
specification for SMA (IRC:SP:79-2008), which was drafted by the author to
facilitate its use in India. A manual containing detailed guidelines for designing
and constructing SMA mixtures was developed by the author in the US for
practicing engineers.
Open Graded Asphalt Friction Course
Open graded asphalt friction course (OGFC) is an open graded hot mix asphalt
mixture with interconnected voids that provide improved surface drainage
during rainfall. The rainwater drains vertically through the OGFC to an
impermeable underlying bituminous layer and then laterally to the day lighted
(exposed) edge of the OGFC onwards to shoulder. In addition to minimizing
hydroplaning potential during rainfall and providing improved friction values on
wet pavements, the OGFC offers the following advantages compared to other
dense graded surfaces: (a) reduced vehicle splash and spray behind vehicles,
(b) reduced tyre-pavement noise, (c) enhanced visibility of pavement
markings, and (d) reduced night time surface glare in wet weather.
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Numerous states in the US currently using OGFC have experienced excellent
performance in terms of safety (improved wet pavement surface friction) and
durability. This has been accomplished by one or more of the following: use of
polymer modified asphalt binders, relatively higher bitumen content (by using
cellulose fibers), and/or relatively open gradations.
Figure 5 shows an interstate highway in the US, where OGFC was used in the
lanes on the right side and dense graded bituminous mix was used in the lanes
on the left side. Note the dramatic difference: there is no standing water and
absence of splash/spray on the lanes on the right side during rain.
A manual giving detailed guidelines on design, construction and maintenance of
OGFC was developed by the author for use by practicing engineers in the US.
Due to economic considerations, OGFC should be used in India selectively in
regions with heavy rainfall and stretches of roads prone to accidents resulting
from skidding on wet pavement.
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Warm mix asphalt offers the following significant advantages:
Energy savings. The most obvious benefit of WMA is the reduction infuel consumption. Fuel is used to dry and heat the aggregate. Studies
have shown that lower plant mix temperatures associated with WMA can
lead to as much as 30 percent reduction in energy consumption.
Decreased emissions. WMA produces emissions (both visible and nonvisible) from the burning of fossil fuels at a significantly reduced level
compared to HMA (Figure 6). This would permit asphalt plants to be
located in and around non-attainment areas such as large metropolitan
areas that have air quality restrictions.
Decreased fumes and odour. WMA produces lower fumes and odourboth at the plant and the paving site compared to HMA. This would also
result in improved working conditions at both places.
Decreased binder aging. Short-term aging of liquid asphalt bindertakes place when it is mixed with hot aggregate in pug mill or mixing
drum. This aging is caused by the loss of lighter oils from the liquid
asphalt binders during mixing at high temperatures. It is believed that
the short-term aging of the binder will be reduced significantly because
the loss of lighter oils will be less at relatively lower mixing
temperatures. This may enhance asphalt pavement durability.
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Figure 6: Emission can be seen from the hot mix asphalt at 320 F(160 C) in left truck. No emission is visible from the truck in rightcontaining warm mix asphalt at 250 F (121 C). (Photo courtesy:
Matthew Corrigan, FHWA)
Extended paving season. By producing WMA at normal HMAtemperatures, it may be possible to extend the paving season into the
colder months of the year or in places located on high altitudes since the
WMA additives or processes act as a compaction aid. Further by
narrowing the difference between compaction temperature and ambient
air temperature the rate of cooling is decreased. WMA may also be
transported over longer distances as compared to HMA with reduced loss
of mix temperature in the hauling units. This advantage should facilitate
the Indian Border Roads Organization (BRO) in constructing asphalt
roads in high altitude and/or remote areas far away from hot mix plants.
Compaction aid for stiffer mixes. WMA additives and processes maybe used to improve the compactibility of stiff mixes when mix is
produced closer to typical HMA production temperatures. Smaller
reductions in temperature may also be possible. There is extensive
experience with the use of certain types of WMA with SMA in Europe.
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Increased amount of RAP. Research has shown that the percentage ofreclaimed asphalt pavement (RAP) can be increased in WMA compared to
HMA during hot recycling.
Generation of carbon credits for India. Developing countries likeIndia can earn CERs (Certified Emission Reductions) or popularly known
as carbon credits under the Kyoto Protocol if technologies such as WMA
are introduced and implemented.
At the present time a mix is considered warm mix in the US if the mix produced
at the plant has temperature exceeding 100C but significantly below that of a
normal hot mix. WMA has a wide range of production temperatures ranging
from slightly over 100C to about 20 to 30C below typical HMA temperatures.
WMA technologies are also applicable to mixes made with polymer modified
asphalt binders.
WMA technologies can be classified broadly as (a) those that use water, (b)
those that use some type of organic additive or wax, and (c) those that use
chemical additives or surfactants.
Technologies which introduce small amounts of water to hot asphalt binder,
take advantage of the phenomenon: when water turns into steam at
atmospheric pressure it expands in volume by a factor of 1,673. This causes
tremendous increase in the volume of asphalt binder which not only helps in
coating the aggregate easily but also lowers the mix apparent viscosity.
Processes to introduce water into the asphalt binder consist of foaming nozzles,
use of hydrophilic material such as zeolite or use of damp aggregate. Asphalt
binder temperature typically is the same as that used for hot mix asphalt.
Technologies that use organic additives or waxes lower the asphalt binder
viscosity above their respective melting points. It should be ensured that their
melting points are above the in-service pavement temperatures during hot
summers so that permanent deformation or rutting does not become a
problem.
Technologies that use some chemical additive and /or surfactants produce a
variety of different mechanisms to coat the aggregate at lower temperatures.
It appears WMA technology is about to take off in India. There is a need to
incorporate WMA specifications in MORTH specifications.
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Recycling of Bituminous Pavements
Recycling of existing asphalt pavement materials to produce new pavement
materials results in considerable savings of material, money, and energy. The
specific benefits of recycling can be summarized as follows:
1. When properly used, recycling can result in substantial savings over theuse of new materials. Also, the cost of haulage can be avoided if
recycling is performed in place. The need for economic consideration is
felt now more than ever, because of tightening budgets and ever
increasing cost of materials.
2. Recycling can help in conservation of natural resources by reducing theneed for new materials. This translates to substantial savings in
aggregate resources and demand for asphalt binder (bitumen),especially during supply interruptions. Even though there may be an
abundant supply of aggregates, the distribution of these sources does
not always coincide with the location of need.
3. Recycled materials have proven to be equal or even better than newmaterials in quality. Hot mix asphalt (HMA) overlay on recycled base is
expected to perform better than an HMA overlay on the existing surface,
even though they have the same thickness, because the former can
substantially reduce the potential of reflective cracking through thesurface course.
4. Recycling can maintain pavement geometrics as well as pavementthickness. The existing pavement structure can be strengthened by
recycling without adding substantial overlays. In some cases, the traffic
disruption is lesser than that for other rehabilitation techniques.
5. Recycling can save considerable amount of energy compared toconventional construction techniques. This factor is of significant
importance during an energy crisis like the one experienced during the1972 Arab oil embargo.
Over the years, recycling has become one of the most attractive pavement
rehabilitation alternatives. With the continuous accumulation of performance
data, field and laboratory evaluations of recycled mixes, and with the
simultaneous development of realistic performance oriented guidelines it is
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expected that recycling will continue to be the most attractive rehabilitation
technique.
Different recycling methods are now available to address specific pavement
distress and structural needs. A brief description of these recycling methods
follows.
The Asphalt Recycling and Reclaiming Association define five different types of
recycling methods: (1) Cold Planing; (2) Hot Recycling; (3) Hot In Place
Recycling; (4) Cold In-Place Recycling; and (5) Full Depth Reclamation.
Cold planing is described as an automatic method of removing asphalt
pavement to a desired depth and restoration of the surface to a desired grade
and slope and free of humps, ruts and other distresses. This method can be
used for the roughening or texturing of a pavement to improve frictional
resistance. Cold planing is performed with a self propelled rotary drum cold
planing machine with the reclaimed asphalt pavement (RAP) transferred to
trucks for removal from the job site. The resulting pavement can be used
immediately by regular traffic and overlaid at some future time or left as a
textured surface.
Hot recycling or hot mix recycling is the process in which reclaimed asphalt
pavement (RAP) material is combined with new materials, sometimes along
with a recycling agent, to produce hot mix asphalt (HMA) mixtures. Both batch
and drum type hot mix plants are used to produce recycled mix. The RAP
material can be obtained by milling or ripping and crushing operation. RAP at
ambient temperature when introduced in weigh hopper of the batch plant
(Figure 7) or drum of the drum plant is heated by superheated virgin
aggregate. If the amount of RAP exceeds 15-20 percent, a softer asphalt binder
is used to rejuvenate the aged asphalt binder in the RAP. The mix placement
and compaction equipment and procedures are the same as for regular HMA.
Typical RAP to new aggregate ratio varies from 10:90 to 30:70 with a
maximum of 50:50 (drum plant). The advantages of hot mix recycling include
significant structural improvement, equal or better performance compared to
conventional HMA, and capability to correct most surface defects, deformation,
and cracking.
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In the case of remixing, the scarified RAP material is mixed with virgin HMA in
a pug mill, and the recycled mix is laid down as a single mix. The advantages
of hot in place recycling are that surface cracks can be eliminated, ruts and
shoves and bumps can be corrected, aged asphalt binder is rejuvenated,
aggregate gradation and asphalt content can be modified, traffic interruption is
minimal, and hauling costs are minimized.
In cold in place recycling (CIR), the existing pavement material is reused
without the application of heat. Except for any recycling agent, no
transportation of materials is usually required, and, therefore, haulage cost is
very low. Normally, an asphalt emulsion is added as a recycling agent. The
process includes pulverizing the existing pavement, sizing of the RAP,
application of recycling agent, placement, and compaction. The use of a
recycling train, which consists of pulverizing, screening, crushing, and mixing
units, is quite common. The processed material is deposited in a windrow from
the mixing device, where it is picked up, placed, and compacted with
conventional hot mix asphalt lay down and rolling equipment. The depth of
treatment is typically from 75 to 100 mm (3 to 4 in).
The advantages of cold in place recycling include significant structural
improvement, treatment of most pavement distress, improvement of ride
quality, minimum hauling and air quality problems, and capability of pavement
widening.
Full depth reclamation has been defined as a recycling method where all of the
asphalt pavement section and a predetermined amount of underlying material
are treated to produce a stabilized base course. It is basically a cold mix
recycling process in which different types of additives such as asphalt
emulsions and chemical agents such as calcium chloride, Portland cement, fly
ash, and lime, are added to obtain ail improved base. The four main steps in
this process are pulverization, introduction of additive, compaction, and
application of a surface or a wearing course. If the in place material is not
sufficient to provide the desired depth of the treated base, new materials may
be imported and included in the processing. This method of recycling is normally
performed to a depth of 100 mm to 305 mm (4 to 12 in). The advantages of full depth
reclamation are that most pavement distresses are treated, hauling costs are
minimized, significant structural improvements can be made (especially in base),
material disposal problems are eliminated, and ride quality is improved.
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Summary
This paper describes recent advances in bituminous road construction in terms
of materials, mix design, special bituminous mixes, and recycling. These
include performance grading system for paving bitumen; measuring particle
shape of fine aggregate; Superpave mix design; stone matrix asphalt (SMA);
open graded asphalt friction course (OGFC); warm mix asphalt (WMA); and
four types of asphalt pavement recycling.
References
Kandhal, P.S. An Overview of the Viscosity Grading System Adopted inIndia for Paving Bitumen. Indian Highways, Volume 34, No. 4, April
2007. Roberts, F.L., P.S. Kandhal, E.R. Brown, D.Y. Lee, and T.W. Kennedy.
'Hot Mix Asphalt Materials, Mixture Design and Construction.' NAPAEducation Foundation, Lanham, Maryland, Second Edition, 1996.
Kandhal, P.S. and F. Parker. 'Aggregate Tests Related to AsphaltConcrete Performance in Pavements.' Transportation Research Board,National Cooperative Highway Research Program Report 405, 1998.
Kandhal, P.S. 'Aggregate Tests for Hot Mix Asphalt:' State of thePractice. Transportation Research Board Circular No. 479, December,1997.
Kandhal, P.S. Design, Construction, and Maintenance of Open-GradedAsphalt Friction Courses. National Asphalt Pavement Association
Information Series 115, May 2002. Kandhal, P.S. Designing and Constructing Stone Matrix Asphalt Mixtures
State-of-the-Practice. National Asphalt Pavement Association QualityImprovement Publication QIP-122 (Revised Edition), March 2002.
Kandhal, P.S. Warm Mix Asphalt Technologies: An Overview. Journal ofthe Indian Roads Congress, Volume 71-2, 2010.
Kandhal, P.S. Recycling of Asphalt Pavements: An Overview. Associationof Asphalt Paving Technologists, Asphalt Paving Technology, Vol. 66,1997.
Kandhal, P.S. and R.B. Mallick. Pavement Recycling Guidelines for Stateand Local Governments. Federal Highway Administration Publication No.FHWA-SA-98-042, December, 1997.
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About the Author
Prof. Prithvi Singh Kandhal is Associate Director (Emeritus) at the National
Center for Asphalt Technology (NCAT) based at Auburn University, Alabama,
U.S.A. NCAT is the largest asphalt (bitumen) road technology center in the
world. Prior to joining NCAT in 1988, Kandhal served as Chief Asphalt Engineer
of the Pennsylvania Department of Transportation for 17 years. He is the first
person born outside North America, who has held the following three very
prestigious positions in the asphalt technology area:
President, Association of Asphalt Paving Technologists (with membersfrom all continents in the world)
Chairman, American Society for Testing and Materials (ASTM)International Committee on Road Paving Standards (responsible for over
200 highway standards used worldwide)
Chairman, Transportation Research Board Committee on Asphalt Roads,U.S. National Academy of Sciences
Prof. Kandhal has published over 120 technical papers and has co-authored the
first ever textbook on asphalt road technology, which is used by more than 25
universities in the U.S.
Courtesy : NBMCW March 2011 Issue