Utilizing RAP Materials in Roller Compacted Concrete

Depletion of good quality virgin aggregates, an increasing amount of RAP stockpiles, and restricted allowable RAP usage in Portland Cement Concrete (PCC) motivates the use of reclaimed asphalt pavement (RAP) as a virgin aggregate replacement, for production of low-cost paving mixes like roller-compacted concrete (RCC) pavements. Savings associated with RCC over PCC are primarily due to reduced cement content, reduced forming and placement costs, and reduced construction times. Moreover, RCC needs no forms or finishing, and there are no dowels, tie rods, or steel reinforcement.

RCC is a zero-slump concrete, and its consistency is stiff enough to remain stable under vibratory rollers, yet wet enough to permit adequate mixing and distribution of paste without segregation. Unlike conventional PCC pavements (e.g., slip-form), RCC has higher fine aggregates that provides better aggregate-to-aggregate interlocking and contributes to its initial load-carrying capacity. RCC can be used as the base layer of PCC pavements but may also serve as the surface layer material if the concrete has a compressive strength of 27.6 MPa or higher after 28 days of water curing age.


The materials and curing requirements for RCC pavements are similar to that of the conventional PCC pavements while it has similar aggregate gradation and construction methodology as that of the conventional asphalt pavements. The typical machines and equipment used for construction of RCC pavements is shown in Figure 1.

One of the major hindrances towards the utilization of RAP in concrete is the reduction in the mechanical strength that is due to the presence of oxidized asphalt layer engulfing the RAP (see Figure 2). Irrespective of RAP type, the inclusion of RAP may reduce the compressive strength by up to 68% and the flexural strength by up to 37% when compared to RCC mixes made with virgin aggregates only1. This reduction is mainly attributed to the weak interfacial bonding between the RAP and cement paste (see Figure 3). The physical and chemical bonding between organic asphalt film and inorganic cementitious materials is not sufficient, fails primarily due to asphalt-cohesion, and hence reduction in the mechanical strength of RCC mixes.

Experience has shown that the mechanical performance of the RAP incorporated RCC mixes may depend on the type, quality, and quantity of RAP used. A highly-oxidized RAP will generally have lower asphalt content, lesser agglomerated particles and more clean patches of aggregates surface than the RAP which is less-oxidized2. This in turn can lead to better interfacial bonding between the RAP and cement matrix, leading to better mechanical performance in the case of mixes made with highly-oxidized RAP (figure 4). In addition, the percent of RAP aggregate surface covered with asphalt has significant influence on the concrete strength. A linear relationship between the average asphalt coating and RAP asphalt content is generally said to co-exist. The thicker the binder film, the higher the overall asphalt binder content, and the greater the strength reductions. The RAP replacement level also has a great influence on the concrete strength. The higher the replacement level, the greater the strength reductions3. Experiences have shown that a 50% RAP replacement level can lead to compressive strength reduction of about 47% while a 100% RAP replacement level can lead to greater reduction of about 68%.

Asphalt oxidation of RAP using chemicals such as HNO3, H2SO4, HCl, NaOH, H3PO4, and H2CrO4 is an effective way to improve the interfacial interaction between the RAP and cement paste and this may lead to improved mechanical performance for RAP concrete; however, this remedial measure may not be cost-effective since the chemicals are often expensive. On the contrary, mechanical treatment techniques like the Abrasion & Attrition Method4 can be an economical way to produce punctured RAP aggregates that can be used to cast concrete with relatively higher strength than the concrete made with untreated RAP, but requires validation on cost, energy, and field feasibility.

Another way to improve the compressive strength is by increasing the cementitious content. Supplementary cementitious materials (SCMs) like silica fume (SF), fly ash (FA), and sugarcane bagasse ash (SCBA) can be utilized as a partial replacement to portland cement to improve the interfacial bonding between the aggregate and cement matrix. However, available literature did not show any significant improvement on the mechanical strength of RAP incorporated RCC mixes when SCMs were used5,6. Only the inclusion of SF may be expected to exhibit a marginal improvement on the mechanical strength.


On the contrary, the use of SCMs can provide a good degree of pore refinement and improve the durability of the RAP-RCC but may not improve the mechanical strength due to poor aggregate-cement interfacial characteristics. A sustainable way is to determine the optimal proportion of RAP aggregates with or without SCM that satisfies the 28-day’s criterion of compressive strength and flexural strength. The asphalt content, agglomerated particles, type of recycling techniques employed, highly-oxidized/less-oxidized RAP, all these factors can significantly affect the overall performance of the RCC pavement mixture.

Now, the question arises, is there a need to use RAP for construction of concrete roads or other infrastructures in India? What lessons can be learned from developed countries like the USA? About 88% (~85.1 million tons) of stockpiled RAP remained unused in 2015 by the contractors in the USA. By the end of 2018, the National Asphalt Pavement Association (NAPA) reported nearly 110 million tons of RAP stockpiled in the USA. A similar figure may be considered for Indian conditions as the road density is similar to that of the USA. With an enormous generation of RAP materials on an annual basis and even after excess RAP remaining after effective utilization for construction of asphalt pavements, there is still scope for its wider application particularly in terms of concrete pavements. Based on literature and our experience, the coarser fraction of RAP aggregates up to 50% can be utilized to produce concretes of grade M30 or even higher without significantly affecting its strength and durability. Concretes made with high percent of RAP can be cheaper by 30-40%, produce 19% less greenhouse gases, 20% less conventional air pollutants, and consume 64% less energy7.

Now, another question arises, what could be the potential application of RAP incorporated concretes? To validate our translational hypothesis, we participated in the National Science Foundation (NSF) I-Corps program (technology transfer: LAB TO MARKET) in 2021. We developed a novel concrete – A high-performance and sustainable material made with higher percent of reclaimed asphalt pavement material and recycled steel fibers from scrap tyres. The technology is patented and owned by Texas A&M University, USA. We interviewed people from industry, academia, and government in the USA and India to explore the value of the technology in the market. Our team members were Dr. Xijun Shi from Texas State University USA, Dr. Zachary Grasley from Texas State University, Dr. Solomon Debbarma from IIT Bombay (formerly Texas State University), and Dave Angelow from Texas State University. The interviews revealed several interesting findings as summarized below.

• Some Department of Transportation (DOTs) like CalTrans are very interested in recycled materials and environmental issues.
• Recycling rate of RAP in many DOTs is very low at the range of 20-30% only.
• RAP is more available in metropolitan areas like New York and New Jersey.
• RAP can be owned by the state or contractors.
• New York and New Jersey own a large portion of RAP. New Hampshire can own only 20%.
• Most contractors are looking for less time-consuming and cost-effective product, and hence the demand for recycled materials like RAP cannot be neglected.
• Use of RAP in concrete roads or allied structures in India seems possible only after recommendations made by relevant IRC/MoRTH standards.

In spite of several proven studies in developed countries like USA, the use of RAP for concrete roads and allied structures in India is still a long way. There exist several questions that needs to be answered and proven through several comprehensive laboratory and field investigations by premier institutes like the IITs and NITs. Some critical questions that still needs to be answered are listed below.

• Is RAP material/aggregate the answer to the shortage of virgin materials for road construction?
• How much of RAP can we really use?
• Are there any changes in the mix design?
• Are there any constructability problems?
• What is the design life of the recycled pavement?

Some of the major conclusions and recommendations are:

• the type and age of RAP, concentration of asphalt and agglomerated particles, gradation of RAP, and replacement levels of RAP can significantly affect the hardened properties of RCC.
• it is recommended to utilize highly-oxidized RAP over less-oxidized RAP to achieve better mechanical strength.
• the use of SCMs can improve the durability properties but does not have a significant effect on the mechanical strength of RAP-RCC.
• the prevalence of asphalt-cohesion failure acts as a barrier for significant strength improvement even when SCMs were utilized.
• the use of RAP and SCMs can not only lower the cost of cubic meter of concrete but can also provide environmental benefits like reducing or even elimination of RAP stockpiles.

References:

1. Debbarma, S., Singh, S., & RN, G. R. (2019). Laboratory investigation on the fresh, mechanical, and durability properties of roller compacted concrete pavement containing reclaimed asphalt pavement aggregates. Transportation Research Record, 2673(10), 652-662.
2. Debbarma, S., Ransinchung, G. D., & Singh, S. (2019). Feasibility of roller compacted concrete pavement containing different fractions of reclaimed asphalt pavement. Construction and Building Materials, 199, 508-525.
3. Debbarma, S., & Ransinchung, G. D. (2021). Achieving sustainability in roller compacted concrete pavement mixes using reclaimed asphalt pavement aggregates–state of the art review. Journal of cleaner production, 287, 125078.
4. Singh, S., Ransinchung, G. D., & Kumar, P. (2019). Feasibility study of RAP aggregates in cement concrete pavements. Road Materials and Pavement Design, 20(1), 151-170.
5. Debbarma, S., Ransinchung RN, G. D., & Singh, S. (2021). Suitability of various supplementary cementitious admixtures for RAP inclusive RCCP mixes. International Journal of Pavement Engineering, 22(12), 1568-1581.
6. Debbarma, S., Ransinchung, G., & Singh, S. (2020). Improving the properties of RAP-RCCP mixes by incorporating supplementary cementitious materials as part addition of portland cement. Journal of Materials in Civil Engineering, 32(8), 04020229.
7. Shi, X., Mukhopadhyay, A., & Zollinger, D. (2018). Sustainability assessment for portland cement concrete pavement containing reclaimed asphalt pavement aggregates. Journal of Cleaner Production, 192, 569-581.

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