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December Newsletter: Page2

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Professor Peethamparan Characterizes Mechanical Properties of Sustainable Concrete Materials

Alkali-activated concrete is a promising sustainable alternative to ordinary Portland cement concrete. The global production of ordinary Portland cement concrete, which totals over ten billion metric tons per year, is responsible for more than 5-7% of anthropogenic carbon emissions. Instead of Portland cement, which requires the high-temperature, energy-intensive firing of quarried virgin minerals, alkali-activated concrete makes use of readily-available recycled materials like fly ash, a byproduct of coal combustion, and slag cement, a byproduct of pig iron refinement. Recent estimates indicate that the use of alkali-activated concrete in place of ordinary Portland cement concrete could reduce the carbon footprint of concrete by one-half or more.

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Figure 1: Tensile strength in alkali-activated fly ash (AAF), alkali-activated slag cement (AASC), and ordinary Portland cement (OPC) concrete showing improved tensile strength in alkali-activated concrete.

The concept of alkali-activation was first introduced in 1908, but alkali-activated concrete did not emerge as a viable alternative to Portland cement until around the turn of the century. Since that time, many important developments have been made with regard to characterizing the properties and performance of this emerging construction material. Professor Sulapha Peethamparan and her research group are working on overcoming several remaining barriers which prevent widespread commercialization of alkali-activated concrete. In particular, they are working to characterize the mechanical properties of alkali-activated concrete in order to provide practitioners with the knowledge necessary for designing alkali-activated concrete structures. They have recently published one of the first and most comprehensive studies of the mechanical properties of alkali-activated concrete. The reference paper is provided at the end of this article.

In general, concrete is very weak in tension. The tensile strength of Portland cement concrete is normally less than 15% of the corresponding compressive strength. Alkali-activated concrete (Figure 1) repeatedly exhibits higher tensile strengths than Portland cement concrete, meaning that existing models used in design must be re-evaluated for alkali-activated concrete. The modulus of elasticity of concrete is also estimated by a function of the compressive strength. Professor Peethamparan’s research indicates that the modulus of elasticity of alkali-activated concrete is not very different from that of Portland cement concrete, but similar revisions are necessary to provide practitioners with the most accurate information possible.

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Figure 2:  Stress-strain curves for alkali-activated fly ash (AAF), alkali-activated slag cement (AASC), and ordinary Portland cement (OPC) concrete showing higher brittleness in alkali-activated concrete.

The stress-strain behavior is another important property for concrete design. In general, concrete is very brittle material, which affects design in terms of allowable failure modes and applied safety factors. Brittle materials fail suddenly, and so high safety factors must be used to prevent brittle failure in structures. Dr. Peethamparan’s research indicates that alkali-activated concrete is even more brittle than Portland cement concrete. This is shown in Figure 2, which compares stress-strain curves for alkali-activated fly ash, alkali-activated slag, and Portland cement concrete. Higher brittleness is indicated by a steeper slope in the post-peak region of the curve. This could indicate a need for further changes in existing concrete design procedures. Now that the group has identified the need for revisions in design procedures, they will focus on identifying how to implement those changes into the existing design code.

Reference paper: Thomas, R.J., & Peethamparan, S. (2015) Alkali-activated concrete: Engineering properties and stress-strain behavior. Construction and Building Materials, 93, 49-56.


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