Cold Recycling of Bituminous Pavement With Foamed Bitumen

Prof. (Dr.) Dharamveer Singh & Purbayan Ghosh Mondal, Post-doctoral Fellow, Department of Civil Engineering, IIT Bombay, give insights on Cold Recycling of Bituminous Pavements, particularly with foamed bitumen (FB), which has emerged as a promising solution to address both environmental concerns and budget constraints.

Cold Recycling of Bituminous Pavements

Introduction

The global expansion of road infrastructure and its environmental implications underscore the urgent need to prioritize the construction of sustainable and environmentally friendly pavements. In the last few decades, pavement maintenance and rehabilitation have emerged as significant challenges confronting highway agencies. This is particularly because of the acute depletion of raw materials and the increased price of road-building materials (such as bitumen) over the last decade.

In this context, road agencies are seeking construction technologies that offer both cost-effectiveness and environmental sustainability. As a result, there is a significant push towards embracing technologies that incorporate recycling and utilize low-energy production processes. Cold recycling of bituminous pavement is one such approach that aims to improve the structural capacity of distressed pavements by utilizing existing pavement materials with minimal addition of fresh materials. In this technique, known as cold recycling, Reclaimed Asphalt Pavement (RAP) material is mixed with virgin aggregate (if necessary) at ambient temperature, along with a small amount of foamed or emulsified bitumen and active filler. While both emulsion and foamed bitumen (FB) technologies present their own sets of advantages and drawbacks, the widespread adoption of FB has become prevalent in rehabilitating flexible pavements owing to its ability to offer rapid, cost-efficient, and eco-friendly construction processes.

Foamed Bitumen Technology and its History

Foamed bitumen (FB), a composite of bitumen, air, and water, is produced by heating bitumen to temperatures typically ranging from 160 to 180°C. This heated bitumen is then mixed with a small quantity of water (around 2-3% by weight of bitumen) and pressurized air. Upon contact with the hot bitumen, the water absorbs heat energy, causing the bitumen to cool while simultaneously increasing the temperature of water. Eventually, the water reaches its boiling point, transitioning from a liquid to a vapor state. During this phase change, the vapor becomes trapped within numerous tiny bitumen bubbles, resulting in the expansion of bitumen volume by nearly 5-20 times, thus forming FB. In its foamed state, bitumen exhibits notably low viscosity and a high surface area, facilitating improved dispersion within a cold and damp aggregate matrix.

However, the workable duration of FB is short, lasting only a few seconds. As the foam cools to ambient temperature, the steam within the bubbles condenses, leading to bubble collapse and foam deterioration. Consequently, much of the water evaporates as steam, leaving behind residual bitumen with properties similar to the original bitumen.

The FB technology is not new; in fact, it was invented more than 60 years ago by Dr. Ladis Csanyi at Iowa State University. Motivated by the availability of marginal aggregates and scarcity of quality aggregates in Iowa, Csanyi devised a method to stabilize marginal aggregates using FB. Initially, he achieved foaming by injecting steam directly into bitumen, demonstrating its effectiveness in stabilizing marginal aggregates.

producing FB in modern recyclers and mix production plantFigure 1: System in modern foaming machines (Wirtgen 2021)
However, the complexity of injecting steam into bitumen and equipment requirements, such as steam boilers, hindered the widespread adoption of the technology in pavement recycling projects. In 1968, ‘Mobil Oil Australia’ revolutionized the process by substituting steam with water injection into hot bitumen. This modification streamlined implementation, especially with the integration into field equipment like cold in-place recycling machines, significantly expanding its utilization. Further advancements occurred in the mid-1990s when ‘Wirtgen’ refined the technology by incorporating both water and air injection into hot bitumen. This enhanced method has since become the standard approach utilized in laboratory foamed bitumen units, in-situ recyclers, and mix production plants, leading to a new era of efficiency and effectiveness in road construction and maintenance. Figure 1 shows the commonly used system for producing FB in modern recyclers and mix production plant.

Foamed Bitumen Stabilized Materials (BSM-foam): Laboratory Production

the Indian Roads Congress (IRC)Figure 2: Laboratory bitumen foaming plant and pug-mill mixer (Wirtgen 2021)
In 2015, the Indian Roads Congress (IRC) introduced guidelines (IRC:120-2015) for recycling bituminous pavement, incorporating specifications for cold recycling with FB. Like any mix design guidelines for foamed bitumen stabilized materials (BSM-foam), IRC:120 entails three primary steps: (1) determining the foam characteristics of bitumen concerning maximum expansion ratio (ERm) and half-life (HL), (2) selecting the optimal foaming conditions (bitumen temperature and foaming water content), and (3) optimizing the FB content. Evaluating the foam characteristics of bitumen is among the initial steps in BSM-foam design. For the production of FB from hot bitumen, the Wirtgen WLB 10S, a laboratory-scale bitumen foaming plant is used, which is depicted in Figure 2. During the foaming process, FB is dispensed directly into a foam measuring bucket, where the ERm is determined by physically observing the maximum expansion achieved by the FB in the bucket using a dipstick, while the HL is measured with a stopwatch. After assessing the foam characteristics at different foaming conditions, the optimal foaming temperature is selected based on achieving the best FB characteristics in terms of ERm and HL. The IRC:120 recommends a minimum ERm of 8 and HL of 6 seconds for selecting the optimal foaming water content (FWC). Consequently, the optimum FWC is determined as the average of the two water contents necessary to meet these minimum criteria.

Before producing BSM-foam, the aggregate is pre-wetted through the addition of mixing water content (MWC). As per IRC:120 specifications, the optimal MWC is determined as 50% of the optimum moisture content (OMC), derived from modified proctor compaction, plus an additional 1%. Prior to the addition of FB, 1% cement (by weight of aggregate) is added as an active filler to the moistened aggregate.

Subsequently, FB with varying contents is sprayed onto the damp aggregates and mixed using a twin-shaft pug-mill mixer (WLM 30) to prepare BSM-foam. After mixing, additional water is added to reach 90% of OMC, followed by further mixing. Following this, samples are prepared by compacting with a standard Marshall compaction effort of 75 blows on both sides of the specimen. The entire mixing and compaction processes are carried out at ambient temperature. The compacted specimens are then left in their respective molds for 24 hours to allow sufficient strength development before extrusion, followed by curing in an oven at 40°C for three days (72 hours). Determining the design FB content of BSM-foam typically involves a parametric test coupled with a moisture susceptibility test parameter. Hence, Indirect Tensile Strength (ITS) tests are performed on the cured specimens under both dry and wet conditions to assess whether the prepared mixes meet the minimum strength criteria (225 kPa ITS-dry and 100 kPa ITS-wet) as outlined in IRC:120 guidelines. Wet conditioning is achieved by immersing the specimens in water for 24 hours at 25°C. The test results (ITS-dry and ITS-wet) are then plotted against FB content, with the FB content yielding the desired properties deemed as the design FB content.

Foamed Bitumen Stabilized Materials (BSM-foam): In-situ Production

Cold recycling with FB can be executed either in-situ or in-plant. Although several factors influence the choice among the two options, however, the popularity of in-situ recycling (CIR) has been on the rise primarily because of the emergence of more robust recycling machinery, enabling higher production rates at reduced expenses. In CIR with FB, the distressed pavement is rehabilitated without utilizing heat at any phase of the process. Initially, the top layer of the existing pavement is milled or scarified to a predetermined depth to remove the surface distresses, such as cracks, potholes, and rutting. The milled material is then combined with a small percentage of new aggregate to adjust the target gradation.

Depending on specific project requirements, active fillers such as cement or lime may be added to improve the strength and durability of the recycled mix. FB and compaction water content are then introduced in the mixing chamber of the recycler via independent spray system, where the blended material is mixed properly so that the FB disperses thoroughly within the aggregate skeleton to produce BSM-foam. In CIR, the milling and mixing is a single-pass operation and is accomplished simultaneously using a train of equipment as shown in Figure 3.

Cold Recycling of Bituminous PavementsFigure 3: (a) Cold in-situ recycling train – recycler, bitumen tank, and water tank; (b) In-situ recycling process (Wirtgen 2021)

This is followed by compacting the BSM-foam in two stages: primary or initial compaction and secondary or final compaction. Initial compaction is generally completed using a pad foot vibrating (sheep-foot) roller (Figure 4) to ensure the full depth of material is adequately compacted and achieves the target density which is 98% to 100% of maximum dry density (MDD) obtained by modified proctor compaction.

The secondary or final compaction is undertaken with a smooth-drum vibrating roller, followed by a pneumatic-tired roller (PTR) (Figure 4). The smooth-drum roller is used to compact the upper horizon of the recycled layer, whereas the PTR is used to obtain a tightly-knit surface finish. Once the compaction and surfacing of BSM-foam layer are completed, water starts evaporating, and the layer gradually gains strength. However, the rate of drying of the surface and the rate of gaining strength are the two most dominating factors that control the performance of the BSM-foam layer. If the rate of drying exceeds the rate of gaining strength, then shrinkage cracks will develop at the surface.

Cold Recycling of Bituminous PavementsFigure 4: Rollers used in compacting BSM-foam layer (Wirtgen 2021)

To avoid this problem, the surface of the BSM-foam layer must be prevented from drying out for a period of at least seven days. Drying can be prevented by frequently spraying water on the surface or alternatively, a temporary seal can be applied as a curing membrane to minimize the detrimental effect of rapid drying. If dilute asphalt emulsion is sprayed onto the surface during or after the final rolling, then little or no further water spraying will be necessary. After the initial seven days, the layer is left to dry back until the moisture content reduces to 50% of the OMC after which the surface layer is placed.

Challenges and Way Forward

While cold recycling technology with FB offers significant advantages, its adoption is constrained by certain limitations. Designing and manufacturing BSM-foam presents a greater complexity compared to Hot Mix Asphalt (HMA). The technology behind BSM-foam demands specialized equipment and a high level of proficiency and expertise. Achieving mixes that meet performance standards can prove challenging, particularly if the project personnel lack experience and expertise. Consequently, highway authorities may need to invest resources in training and capacity-building programs to equip their workforce with the necessary skills and knowledge essential for effectively managing this technology.

In the Indian context, the mix design process for BSM-foam still lacks the clarity and standardization seen in HMA design. Although BSM-foam technology was introduced in India in the early 2000s and has been utilized in various projects since then, the absence of systematic guidelines hindered its widespread adoption. To address the issue, the IRC released guidelines (IRC:120-2015) in 2015, incorporating specifications for cold recycling with FB. However, the specification is yet to achieve standardization. Consequently, gaining experience in BSM-foam mix production proves challenging. Therefore, there is a pressing need for the research community to concentrate on establishing a standardized and consistent mix design procedure for BSM-foam, particularly within the Indian context.

Another limitation pertains to the ability of BSM-foam to maintain long-term performance, specifically, regarding their moisture susceptibility. The presence of non-continuous bond due to partially coated nature of aggregate and high void content (typical range 10–15% after curing) heightens the potential for moisture damage to emerge as a critical failure mode for the BSM-foam. Therefore, mitigating the moisture susceptibility problems is one of the primary challenges for pavement engineers.

A proper design of the mix may help to alleviate the problem to a certain extent. It is imperative for practitioners to formulate a mix design that carefully considers the proportions of FB, aggregate, and additives, aiming to boost the resistance against moisture damage. Furthermore, the choice of bitumen type for mix preparation should be made judiciously. Opting for higher viscosity grade bitumen such as VG30 or VG40, which exhibit excellent foam characteristics, is recommended to ensure the production of moisture-resistant BSM-foam.

Conclusion

Despite a few drawbacks as discussed above, the use of FB technology in road construction delivers a multifaceted solution, providing enhanced pavement performance, sustainability, and economic benefits. Its versatility, coupled with ongoing research and development, ensures its continued relevance and effectiveness in meeting the evolving needs of transportation infrastructure. By embracing FB technology, stakeholders can pave the way for durable, eco-friendly, and cost-efficient road networks essential for sustainable development.

References

  • Csanyi, L.H., 1957. Foamed Asphalt in Bituminous Paving Mixtures. Highway Research Board Bulletin 160.
  • IRC:120, 2015. Recommended Practice for Recycling of Bituminous Pavements, Indian Roads Congress
  • Wirtgen, 2021. Cold Recycling Application Manual, Wirtgen GmbH
NBM&CW - JUNE 2024
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