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:
- Tests and specifications are intended for bitumen "binders," which include both modified and unmodified bitumens.
- The physical properties measured by Superpave bitumen tests are directly related to field performance by engineering principles rather than just the experience.
- A long-term bitumen aging test, which simulates aging of bitumens during 5-10 years in service, was developed and included for the first time.
- Tests and specifications are designed to eliminate or minimize three specific 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.
- As shown in Figure 1, the entire range of pavement temperature experienced 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.
|Figure 1: Superpave performance grade bitumen testing is conducted over 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 PG 82. 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.
|Figure 2: Schematic of equipment for testing fine aggregate angularity (FAA)|
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.
Mix DesignMarshall 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|
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 (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 asphalt cross-section||Fig. 4b Conventional hot mix asphalt cross-section|
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 (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.
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.
Warm Mix Asphalt
|Figure 5: Lanes on the right have OGFC and lanes on the left have dense graded hot mix asphalt on a interstate in the US. Note absence of standing water and splash/spray on the lanes on the right side.|
The development of WMA was initiated in Europe in the late 1990s primarily in response to the need for greenhouse gas reduction under the Kyoto Protocol. WMA technologies such as Aspha-min, WAM Foam, and Sasobit were developed during that time. New WMA technologies such as Evotherm, Rediset WMX, REVIX, LEA (Low Energy Asphalt) and Double Barrel Green were later developed within the US.
Warm mix asphalt offers the following significant advantages:
- Energy savings. The most obvious benefit of WMA is the reduction in fuel 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 non visible) 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 odour both 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 binder takes 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.
|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 right containing warm mix asphalt at 250 F (121 C). (Photo courtesy: Matthew Corrigan, FHWA)|
- Extended paving season. By producing WMA at normal HMA temperatures, 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 may be 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.
- Increased amount of RAP. Research has shown that the percentage of reclaimed asphalt pavement (RAP) can be increased in WMA compared to HMA during hot recycling.
- Generation of carbon credits for India. Developing countries like India 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.
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.
Recycling of Bituminous PavementsRecycling 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:
- When properly used, recycling can result in substantial savings over the use 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.
- Recycling can help in conservation of natural resources by reducing the need 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.
- Recycled materials have proven to be equal or even better than new materials 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 the surface course.
- Recycling can maintain pavement geometrics as well as pavement thickness. 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.
- Recycling can save considerable amount of energy compared to conventional construction techniques. This factor is of significant importance during an energy crisis like the one experienced during the 1972 Arab oil embargo.
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.
|Figure 7: Hot mix asphalt recycling in a batch plant|
Hot in place recycling (HIR) consists of a method in which the existing pavement is heated and softened, and then scarified or hot rotary mixed to a specified depth. New HMA or recycling agent may be added to the RAP material during the recycling process. HIR can be performed either as a single pass or a multiple pass operation. In single pass operation, the restored RAP material is combined with new material. In multiple pass operation, the restored RAP material is recompacted first, and a new wearing surface is applied later. The depth of treatment varies between 20 to 40 mm (3/4 in to 1½ in). The Asphalt Recycling and Reclaiming Association (ARRA) has identified three HIR processes; (a) surface recycling, (b) repaving, and (c) remixing. In a surface recycling operation the existing asphalt surface is heated and scarified to a specified depth. The scarified material is combined with aggregate and/or recycling agent. The mix is then compacted. A new overlay may or may not be placed on the recycled mix. In the second type of HIR method, repaving, the surface recycling method is combined with a simultaneous overlay of new hot mix asphalt (HMA). Both the scarified mix and the new HMA are rolled at the same time.
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.
SummaryThis 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.
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- Kandhal, P.S. Warm Mix Asphalt Technologies: An Overview. Journal of the Indian Roads Congress, Volume 71-2, 2010.
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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 members from 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