Maximizing Ash Usage in Road Construction

Rishit Dalal
Designing roads and highways efficiently with the right materials will help achieve optimum construction cost, lower maintenance, and reduce life cycle costs. Ash from coal-fired thermal power plants can play a critical role in this.
Rishit Dalal, Director, Jaycee

As per a recent report, 59% of all freight in India is transported by roads, 35% by railways, 6% by waterways, and less than 1% by airways. Of this, highways carry 40% of the total road traffic – the reason why India is investing heavily in developing its transportation infrastructure, specifically highways.

The country is now averaging 37 km per day of highway construction and holds the world record for the fastest road construction. The Government has set a target of road construction worth `15 lakh crore (USD 225 billion) in the next two years, supported by the National Infrastructure Pipeline project.

Maximizing Ash Usage in Road Construction
Redesigning roads and highways such that they maximize ash usage can help reduce project costs up to 40% and significantly enhance durability while reducing maintenance and life cycle costs. Importantly, use of fly ash will conserve natural resources and reduce pollution.

India produced 226 million tons of fly ash in 2019-2020, of which only 9.2% was used in roads and flyovers.

Based on projects and applications in developed economies, here are the various ways in which we can maximize ash usage in the construction of roads and highways:

Fly Ash in Concrete
Fly ash reacts with the lime in cement to form an additional binder which enhances the durability and performance of concrete. Some of the benefits include higher long-term strength, improved workability, reduced bleeding, reduced heat of hydration, reduced permeability and resistance to chloride, sulphate and chemical attacks, increased resistance to alkali silica reaction, and reduced shrinkage.

Fly Ash in Stabilized Base Course
Fly ash and cement can be combined with aggregate to produce a stabilized base course, referred to as pozzolanic-stabilized mixture. The sub-grade can also be designed completely with bottom ash. This makes the base strong, more durable with recycled materials, and has increased energy efficiency and self-healing properties. Such a base can be placed with conventional equipment, which further reduces costs. It also increases flexibility and resistance to water damage. Closely controlled curing conditions are important as both time and temperature significantly affect strength.

Fly Ash in Flowable Fill
An optimum mix of fly ash, water and cement can be used to make a flowable fill, that flows smoothly, is self-levelling, and requires no compaction or vibration to achieve maximum density. It replaces conventional backfill materials such as soil, sand, or gravel. Flowable fill allows placement in any weather conditions, fills around/under structures that are inaccessible, increases soil-bearing capacities, prevents post-fill settlement issues, increases efficiency of backfill operations with less variability, reduces labor costs, and decreases excavation costs, and allows easy excavation at a later stage.

Fly Ash in Structural Fills & Embankments
Fly ash is typically used in the construction of structural fills and embankments that range from small fills for road shoulders to large fills for interstate highway embankments. It can also support highway buildings or structures when compacted. It is highly cost effective in bulk volumes, can be placed over low bearing strength soils, and is easy to handle, reducing construction time and equipment costs.

Fly Ash in Soil Improvement
Fly ash is an effective agent for chemical and mechanical stabilization of soils. It can improve soil density, water content, plasticity, strength performance and control shrink-swell. It can also help increase construction speed by improving excessively wet or unstable subgrades, and save cost by reducing required pavement thickness and eliminating the need for expensive natural aggregates in the pavement cross-section.

Fly Ash in Asphalt Pavements
Fly ash can be used as a mineral filler in hot mix asphalt pavements. It typically meets mineral filler specifications for gradation, organic impurities, and plasticity. Fly ash increases the stiffness of asphalt mortar, reduces potential for rutting and stripping due to hydrophobic properties, enhances the durability of the mix, and reduces costs.

Fly Ash in Grouts for Pavement Sub-sealing
Grouts are proportioned mixtures of fly ash, water, and other materials used to fill voids under a pavement system without raising the slabs (sub-sealing), or to raise and support concrete pavements at specified grade tolerances by drilling and injecting the grout under specified areas of the pavement or in mining applications. Grouts can fill voids without raising the slabs and develop higher ultimate strength. A major benefit is quick implementation with minimum disturbance to traffic.

Overall, maximizing ash usage in road construction can provide multiple technical advantages and reduce project costs significantly, while also providing a high-volume channel for beneficial utilization of ash from power plants in line with Government directives.
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