Sridhar Raju, Associate Professor and Bhanuprasad Katla, Research Scholar Department of Civil Engineering, BITS Pilani Hyderabad Campus

India has the second largest road network in the world, spanning a total of 5.89 million km, transporting 64.5% of all goods in the country. Nearly 90% of India's total passenger traffic uses the road network to commute. The road transportation in India has gradually increased over the years with improvement in connectivity between cities, towns and villages (www.ibef.org). Due to these massive construction activity, there is an increased demand for aggregates and bitumen. The bituminous pavements which were constructed during late 90's and early 2000 requires an overlay or a resurfacing as a part of maintenance activity. The upgradation or the maintenance activities requires a huge quantity of aggregates and bitumen. It is necessary to adopt the process of reusing the existing bituminous material for maintenance activity and/or for new construction, as this is an environmental-friendly technology. Also, providing an overlay as a maintenance measure results in rise in the road level with respect to the adjoining area, especially in the city roads. To avoid such rise in the road level and to save our natural resources, there is a need for reusing the existing road materials. The bituminous material obtained from the distressed pavement is called as Reclaimed Asphalt Pavement (RAP).

During the service life, the pavement gets stiffened due to binder aging and starts showing the signs of distresses with the movement of heavy vehicles. The pavement distresses generally observed are ravelling, rutting and cracking. The distressed pavement causes the discomfort to the road users leading to an increased vehicle operation cost leading to a need for maintenance. Instead of resurfacing or overlaying with virgin aggregates and virgin bitumen, it is economical to use RAP. The use of RAP in a new bituminous mixture is considered to be a sustainable solution and has gained popularity in the last few years. The use of RAP not only saves money but also helps in saving our natural resources for the future generation. Figure 1, shows the variation in the pricing of bitumen from 2000 to 2020 and this helps us in assessing the saving in cost with the increase in percentage of RAP material in a bituminous mixture.

Bitumen pricing from 2000 till dateFigure 1: Bitumen pricing from 2000 till date (https://mdasphalt.org/asphalt-index/)

Benefits of using RAP
The application of RAP in new bituminous mixtures presents environmental and economic benefits.

Economic benefits
The economic benefits either refer to savings in the cost of materials, which derive from the reduction in the amount of virgin aggregates and binders used in new mixtures, or to savings in the costs associated with the transportation of virgin materials to plant sites. Figure 2, presents the reduction in costs with the increased RAP content in the new bituminous mixture. The bitumen has the highest portion of the cost in constructing conventional bituminous mixtures at almost $40 per ton. This is followed by aggregate cost which is roughly $20 per ton. Using 50% RAP materials resulted in decreasing the construction cost by almost $20 per ton. Figure 2, also, shows the costs of RAP processing, testing, using rejuvenator, pollution control, and cost of RAP material. Overall, by utilizing 100% RAP material the construction cost was reduced by 50–70% which is a considerable amount (Zaumanis et al., 2014).

Material costs of hot mix recyclingFigure 2: Material costs of hot mix recycling (Zaumanis et al., 2014)

Environmental benefits
Environmental benefits are associated with lesser fuel usage and reduced level of emission. There will be a decrease in the demand for non-renewable resources like aggregate and bitumen. Transportation costs will be reduced as these RAP materials can be reused on the same road instead of being transported as a landfill material for disposal at a distance. Chiu et al. (2008), carried out a life cycle cost analysis and showed that the use of RAP material can save energy up to 23% when compared to conventional bituminous mixtures. Figure 3, shows a mountain being depleted due to quarrying of aggregates for infrastructure projects.

Quarrying of aggregates for infrastructure projectsFigure 3: Quarrying of aggregates for infrastructure projects

RAP usage: Global and Indian scenario
The use of RAP in bituminous pavements have been adopted in the United States, Canada, France, Germany, Spain, Australia, Japan, etc. Most countries are using RAP in bituminous layers to reduce pollution, increase the pavement life and to reduce the construction cost. Among the countries who are using RAP in construction of roads, Japan is utilizing the maximum percentage of RAP in practice. On an average, Japan is using 47% of RAP material in the bituminous mixtures, while the maximum RAP used is 76% in the surface layer (Hirama, 2014). In the US, most of the states use 20 to 30% RAP material in the design of bituminous mixture, while they consider 30 to 40% as the maximum RAP. According to Hansen and Copeland (2013), 68.3 million tons of RAP material was reused in pavements during the year 2012 in the US. According to a survey conducted during 2011, though more than 40 states in the US utilized 30% RAP in bituminous mixtures, the average utilization was still less than 20% (Copeland, 2011). The Indian Roads Congress restricts to 30% use of RAP in bituminous mixtures (IRC: 120-2015) but recommends up to 50% with the use of rejuvenators. There are a few trial sections reported in India with 15 to 20% RAP in the binder layer but there are many more trial sections yet to be reported. The National Highway Authority of India (NHAI) also recommends the use of RAP to ensure conservation of energy in addition to the saving in construction cost. There are circulars being issued by MoRT&H and NHAI encouraging the use of higher percentages of RAP in bituminous mixtures. Ms. Ashoka Buildcon Limited has completed a road project between Kharar to Ludhiana on NH-95 (New NH-5) in Punjab (see Fig. 4). They have utilized 50% RAP material by adding a rejuvenator and were constructed using a HRC Plant.

Kharar to Ludhiana Road on NH-95 (New NH-5) with 50% RAP (Courtesy Ms. Ashoka Buildcon Limited)Figure 4: Kharar to Ludhiana Road on NH-95 (New NH-5) with 50% RAP (Courtesy Ms. Ashoka Buildcon Limited)

Challenges to use higher percentages of RAP
There are challenges being posed by the contractors for utilizing higher percentages of RAP, say 50 to 60% by weight. The probable reasons for avoiding higher percentages of RAP material are:
  • Variability of the RAP material (especially the fine content)
  • Blending process between virgin and RAP binder
  • Stiffness of the RAP binder
To increase the RAP in bituminous mixtures, it is necessary to follow the best practices. In Japan, the RAP to be used in bituminous layers should have a minimum bitumen content of 3.8%, penetration value of ≥ 20 dmm and the material passing 0.075 mm should be less than 5%. If the RAP fails to meet these 3 requirements, it can be used in unbound base layers. Generally, 5 to 10% rejuvenators by weight is recommended to restore the physical characteristics of the RAP binder.

Variability of RAP material
The RAP material will be inhomogeneous in terms of gradation and the binder content. This inhomogeneity arises due to the stockpiling of different sources of RAP material in a single stockpile, bitumen aging, milling from multiple layers. The variability in RAP materials does not have a considerable influence on bituminous mixtures with up to 15% RAP, but higher RAP contents in the mixture design can significantly influence the overall performance of the bituminous mixture. The RAP content is limited to 30% in bituminous mixtures to reduce variability in aggregate gradation. Also, it is necessary to separate the fine and coarse particles in the RAP material to have a control over material passing 0.075 mm. IRC specifies that the material passing 0.075 mm shall be less than 5% (as followed in Japan). If the variability in RAP leads to segregation (see Figure 5), it can be minimized by loading the bituminous mixture in multiple heaps (see Figure 6). The other problems caused due to variability in RAP cannot be minimized only by loading in multiple heaps. Hence, there is a need to arrive at the mitigation measures to minimize the variability of RAP.

Segregation in a DBM LayerFigure 5: Segregation in a DBM Layer

Multiple heaps to reduce segregation at siteFigure 6: Multiple heaps to reduce segregation at site

Blending process between virgin and the RAP binders
The major concern about the incorporation of the higher percentage of the RAP in the bituminous mixtures is how much stiffer binder can be mobilized during the mixing operation. During the short mixing time, the aged RAP binder is expected to attain the required viscosity, to blend with the virgin binder, and to form a homogenous film thickness around RAP and virgin aggregate. Several agencies have followed this assumption in the design of mixtures and have given full credit to RAP binder contribution in the mixture design.

It is a difficult task to identify the quantity of RAP binder that blends with the virgin binder in a bituminous mixture. In general, the blending process between RAP and the virgin binders can be assumed to be homogenous after mixing. It is vital to know the blending process to produce a high RAP content bituminous mixtures. Blending is a complex process and this is found to be the governing phase in understanding the rheological behaviour of the RAP mixture. As a part of NCHRP Project 9-12, RAP and virgin binders blending process was explained through an experiment. They had considered three scenarios of blending as shown in the Figure 7. The first scenario was 'the black rock scenario', where there was no contribution from the RAP binder. In this scenario, only the RAP was considered as an aggregate only with no blending between the RAP and the virgin binders. In the second scenario, RAP was mixed with the virgin bitumen and the virgin aggregate. In this scenario some part of the RAP binder was blended with the virgin binder and major portion of the RAP binder was acting like a stiffer layer coated to the RAP aggregate. The second scenario was referred to as 'the actual practice'. In the third scenario, bitumen and aggregate were mixed with RAP material and were allowed for a complete blending during the mixture design process. The construction agencies assume that scenario 3 occurs during the mixing and production of RAP added mixtures. Hence, they end up producing a mixture with lower effective binder content which results in premature pavement failure (Zaumanis et al., 2015). However, from their study it can be understood that only second scenario with partial blending takes place in the field.

Blending process (Mohammadafzali, 2019)Figure 7: Blending process (Mohammadafzali, 2019)

Stiffness of the RAP binder
The incorporation of higher percentage of RAP stiffens the bituminous mixtures leading to poor workability. Poor workable mixture is difficult to compact in the field and may lead to premature failure (Mogawer et al., 2012). One of the reasons for the reluctance of utilising higher RAP content is the concern with the stiff mixtures, resulting in less workability, challenges during compaction, and resulting in premature distresses (cracking, ravelling, etc.). The in-service thermal and ultraviolet oxidation of bitumen for many years leads to excessive ageing of RAP binders. Zaumanis et al. (2013) stated that the stiffer bitumen with low stress relaxation and low workability is the main reason for avoiding higher percentage of RAP in bituminous mixtures. According to the researchers, the stiffness increases by 49% for 40% RAP while it increases by 60% with 55% RAP. Though stiffer mixtures may lead to premature cracking or ravelling but will result in a rut resistant mixture. Hence, there is a need to reduce the stiffness of the bituminous mixture for utilizing higher percentage of RAP.

Mitigation Measures

For variability of RAP

To increase the RAP content and to reduce the variability it is necessary to separate the fine and the coarse particle in to 2 or 3 fractions considering the critical sieves. The process of separating the fine and the coarse particle in RAP is called as 'Fractionation'. It also, helps in arriving at the desired gradation by blending virgin aggregate and the RAP. The fractionated RAP can be stockpiled in different sizes (see Figure 8), thus allowing more flexibility in the design of RAP added mixtures.

Fractionation unit in a hot mix plant site (Source: West 2015)Figure 8: Fractionation unit in a hot mix plant site (Source: West 2015)

For the blending process
Exposing the RAP material directly to high temperature flames and combustion gases might prove detrimental due to excessive oxidation. The steam gets built up inside the plant due to moisture in the RAP material. To avoid any accidents at the plant site, there is a need for modifying the plant for using higher percentage of RAP. If a batch plant is used, the RAP material gets mixed with the super-heated aggregates and the heat transfer happens between superheated virgin aggregates and the RAP material. Once the RAP added bituminous mixture leaves the pug mill and moves to the storage silos it can attain and maintain the equilibrium temperature in the storage silo. In Japan they use a separate parallel drum for drying and heating the RAP material. The parallel heating helps in increasing the RAP content and also, avoids the steam cloud formation. However, the direct heating of RAP to 165 °C in a parallel-flow dryer further oxidizes the RAP binder and creates significant smoke. In order to avoid these issues, the Japanese plants diverts the RAP dryer exhaust through an afterburner. The afterburner burns the residual hydrocarbons in the drum and the afterburner exhaust passes through the virgin aggregate dryer to reuse the heat energy (West and Copeland, 2015).

The degree of blending between the RAP and the virgin binders depend on the type of plant being used for producing the bituminous mixture. The mixture can be produced either in the batch mixing or a continuous drum mixing plants. In a batch mixing plant, the virgin aggregates are super-heated in the dryer drum to get sufficient energy for drying and heating the RAP. Generally, when the RAP is incorporated at an ambient temperature, it is necessary to increase the mixing time to obtain a higher quality recycled mixture (Howard et al. 2009). In the US, they use a continuous drum mixing plants with additional arrangements for incorporating higher percentage of RAP (30 to 40%). To increase it further, to an extent of 40 to 70%, it is necessary to have a parallel drum arrangement for heating the RAP material and mixing in the batch mix plant (as followed in Japan).

For reducing the stiffness
To get over these challenges of stiffer mixtures, the road contractors take the shelter of the rejuvenators. Generally, rejuvenators are bitumen additives used for softening the oxidized RAP binders. Rejuvenators typically have a high proportion of maltenes, which balances the chemical composition of aged binder, and then lose the maltenes during the construction and service phases. By adding rejuvenators, the RAP binder can be recovered and reach the desired binder grade. It can also improve the cracking resistance of asphalt mixtures without any undesirable effects on its rutting resistance.

Rejuvenator type must be selected carefully in order to fulfil both short-term and long-term criteria. Table 1 gives the list of a few rejuvenators used for modifying the RAP binder and there could be many more in the market. In short-term criteria, a rejuvenator must defuse rapidly into the RAP binder and mobilize the aged bituminous mixture. This is necessary to avoid the reduction of friction and decrease the permanent deformation (rutting) susceptibility of rejuvenated mixture by constructing uniformly coated mixture. In addition, complete diffusion of rejuvenator into RAP binder is difficult to achieve and this can contribute to non-uniform properties and distribution in recycled mixture.

Table1. Types of rejuvenators (NCAT 2014)
Category Examples Description
Paraffinic Oils Waste Engine Oil (WEO)
Waste Engine Oil Bottoms (WEOB)
Valero VP 165®
Storbit®
Refined used lubricating oils
Aromatic
Extracts
Hydrolene®
Reclamite®
Cyclogen L®
ValAro 130A®
Refined crude oil products with polar aromatic oil components
Triglycerides &
Fatty Acids
Waste Vegetable Oil
Waste Vegetable Grease
Brown Grease
Oleic Acid
Derived from vegetable oils, Has other key chemical elements in addition to triglycerides and fatty acids
Tall Oils Sylvaroad™ RP1000
Hydrogreen®
Paper industry by products, Same chemical family as liquid antistripping agents and emulsifiers

Conclusions
  • It is necessary to adopt the process of Fractionation to increase the percentage of RAP in a bituminous mixture and to avoid the variability in the RAP gradation. Fractionating RAP provides flexibility in meeting the volumetric properties of the RAP added mixture.
  • It is suggested to add rejuvenators with the hot RAP after parallel heating. The optimum percentage of rejuvenator shall be arrived based on the RAP binder property. This is to allow the aged binder in RAP to get activated with sufficient mixing time inside the drum or a pug mill. This process may result in lower production capacity but still results in environmental and economic benefits.
  • The specifications adopted in Japan can be followed in India for utilizing higher percentage RAP (40 to 70%).
References
  1. Chiu, C. T., Hsu, T. H., & Yang, W. F. (2008). Life cycle assessment on using recycled materials for rehabilitating asphalt pavements. Resources, conservation and recycling, 52(3), 545-556.
  2. Copeland, A. (2011). Reclaimed asphalt pavement in asphalt mixtures: State of the practice (No. FHWA-HRT-11-021). United States. Federal Highway Administration. Office of Research, Development, and Technology.
  3. Hansen, K. R., Copeland, A., & National Asphalt Pavement Association. (2013). Annual asphalt pavement industry survey on recycled materials and warm-mix asphalt usage: 2009-2012 (No. IS-138). National Asphalt Pavement Association.
  4. Hirama, A. (2014). The Actual Situation of Recycled Asphalt Mixture in Japan. Presentation at Seminar on Pavement Technology Exchange Between U.S.A. and Japan, 4 December 2014, Tokyo.
  5. Howard, I. L., Cooley, L. A., & Doyle, J. D. (2009). Laboratory testing and economic analysis of high RAP warm mixed asphalt (No. FHWA/MS-DOT-RD-09-200). Mississippi State University. Dept. of Civil Engineering.
  6. IRC:120-2015 on “Recommended practice for recycling of bituminous pavements” Indian Roads Congress, New Delhi.
  7. McDaniel, R. S., & Anderson, R. M. (2001). Recommended use of reclaimed asphalt pavement in the Superpave mix design method: technician's manual (No. Project D9-12 FY'97). National Research Council (US). Transportation Research Board.
  8. Mogawer, W. (2012). “Development of Balanced & Eco-Friendly Thin Lift Asphalt Mixtures Incorporating Sustainable Materials.” International Pavement Preservation Confernce, University of Massachusetts Dartmouth.
  9. Mohammadafzali, M., Ali, H., Sholar, G. A., Rilko, W. A., & Baqersad, M. (2019). Effects of rejuvenation and aging on binder homogeneity of recycled asphalt mixtures. Journal of Transportation Engineering, Part B: Pavements, 145(1), 04018066.
  10. NCAT (National Center for Asphalt Technology). (2014). Researchers explore multiple uses of rejuvenators.
  11. West, Randy C. Best practices for RAP and RAS management. No. QIP 129. 2015.
  12. Zaumanis, M., & Mallick, R. B. (2015). Review of very high-content reclaimed asphalt use in plant-produced pavements: state of the art. International Journal of Pavement Engineering, 16(1), 39-55.
  13. Zaumanis, M., Mallick, R. B., & Frank, R. (2013). Evaluation of rejuvenator's effectiveness with conventional mix testing for 100% reclaimed Asphalt pavement mixtures. Transportation research record, 2370(1), 17-25.
  14. Zaumanis, M., Mallick, R. B., Poulikakos, L., & Frank, R. (2014). Influence of six rejuvenators on the performance properties of Reclaimed Asphalt Pavement (RAP) binder and 100% recycled asphalt mixtures. Construction and Building Materials, 71, 538-550.
  15. Zhang, K., Wen, H., & Hobbs, A. (2015). Laboratory tests and numerical simulations of mixing superheated virgin aggregate with reclaimed asphalt pavement materials. Transportation Research Record, 2506(1), 62-71.
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