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Mixing up long-lasting, sustainable concrete pavements

How can we make our pavements more durable and sustainable while keeping them cost-effective? That’s the question the Illinois Center for Transportation and IDOT seek to answer in their joint project, “R27-180: Concrete Pavement Mixtures with High Supplementary Cementitious Materials Content” (vol. I, II and III).

Engineer of Concrete and Soils James Krstulovich leads the effort with University of Illinois Ernest Barenberg Professor in CEE Jeffery Roesler, U of I graduate research assistant Aniruddha Baral and Oklahoma State University professor Tyler Ley. Here the researchers explore one solution: Adding higher percentages of waste byproducts like coal fly ash, a material recycled from coal-burning electric plants, to concrete pavements.

Partially replacing cement in concrete pavements with these cement-like materials enhances pavement’s durability, reduces the carbon footprint of a cubic yard of concrete and reduces its overall cost. But that durability enhancement comes with a catch: adding large doses of fly ash may affect the pavement’s setting time or strength gain early in the construction process, increasing the time needed for construction.

Their goal? Develop tools and guidance to allow for higher volumes of cement-replacing materials, like fly ash, in concrete pavements while balancing the properties that affect concrete’s early strength gain or setting time. Key to this goal is evaluating the quality of the replacement fly ash, as the performance of fly ash can vary depending on when and where it is sourced.

The researchers established three main tools to evaluate and optimize the use of cement-replacing materials in concrete pavements: The isothermal calorimeter, digital foam index test and models to predict concrete’s various properties based on oxide contents present in the fly ashes.

The isothermal calorimeter helps detect adverse interactions between various mix design components by measuring the concrete’s hydration rates, which allows for determination of the setting characteristics and early strength gain of the concrete.

The digital foam index test, a computer-vision-based tool developed by the researchers, studies how fly ash and air entrainers – bubbles added to concrete to increase its durability and protect it from freezing and thawing damage – interact with each other. The novel technique will help determine how much air entrainers to add to high-volume fly ash concrete and help pioneer the use of computer-vision techniques in various concrete material applications.

“The digital foam index long-term effects on air entrainment of any concrete, even structural concrete,” Baral said. “So even though it was a project for pavement materials, the technique we developed can be expanded very easily to other structural concrete as well.”

They also developed models to predict properties of concrete with high volumes of fly ash more rapidly, such as compressive strength, resistivity and diffusion based on the oxide contents. Through their efforts, the research team not only provided tools for better quality control of fly ash, but also developed guidelines for mix design and testing protocols. They also verified that replacing cement with up to 40% fly ash in concrete can be used to achieve durable, sustainable and cost-effective concrete pavements.

“As the construction industry and DOTs like IDOT strive to become more environmentally sustainable, some of this project’s findings may be useful in providing a baseline or framework for evaluating alternative supplementary cementitious materials that are/might be emerging on the market sooner than later,” Krstulovich said.