Two IIT Hyderabad professors and their graduate students have come up with an innovative material to lay roads which reduces cost, is environmentally friendly and also stronger than asphalt roads. This material — reclaimed asphalt pavement (RAP) — uses a mix of fly ash (a coal-combustion by-product) with reclaimed asphalt concrete. It reduces use of virgin asphalt by about 30%. The team’s findings have been published in journals such as Construction and Building Materials and Journal of Materials in Civil Engineering.
Further, the material has also been tested on sections of the road built as “part of the State highway (SH 2213) connecting Nuzvid and Mylavaram [in Andhra Pradesh] designed for a traffic of 1,213 commercial vehicles per day,” Sireesh Saride, the Principal Investigator of the project, says in an email to The Hindu.
Reclaimed asphalt concrete is by itself not strong enough to lay roads with. Among the several additives that have been tried out to bolster its strength are limestone and industrial waste such as fly ash. Fly ash consists of fine particles of ash that mix and escape from chimneys of coal-fired boilers and is now collected by using electrostatic precipitators before they escape. “India is producing lot of fly ash which has very low calcium content. Low calcium fly ashes would not produce high pozzolanic reactions, meaning they are not highly cementitious materials. They need some kind of additives/activators to enhance their reactivity and cementation,” explains Prof. Saride.
In the method used by the researchers, fly ash is treated with sodium hydroxide, an alkali, before adding it to the RAP material. The treatment converts the fly ash into geopolymer that binds the RAP particles thereby rendering it stronger. “Activating fly ash is not very new, but using it for this application is very new. Generally, cement is used in place of fly ash to get better performance, but much more expensive,” Prof Saride adds.
The team found by testing that an 80:20 ratio of reclaimed to virgin aggregate mixture with 40% fly ash meets the standards stipulated by Indian Roads Congress as well as the American Association of State Highway and Transportation Officials (AASHTO). “We have managed to use RAP up to 60% (and also up to 80% in the lab conditions) with appropriate stabilisation techniques (fly ash geopolymer) and balanced between the performance and cost,” clarifies Prof. Saride.
Creating higher amount of geopolymer around the RAP particles lead to less voids in the matrix, which will reduce the permeability of water through the matrix. “We tried to optimise the alkali activator ratio (a ratio between the sodium silicate and sodium hydroxide) in the mixture to activate enough quantities of silica and alumina present in the fly ash. This way, an optimum matrix with fewer pores was created,” he adds. This combination gives high structural stability to the pavement for a longer duration.