Building green : development and evaluation of an environmentally friendly concrete
Roskos, Colter Eastman
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Concrete is the world's most used construction material, and although it offers some advantages over other building materials from an environmental perspective, its negative impacts in this regard are of growing concern as its use increases. This research investigated replacing 100 percent of the Portland cement and natural aggregates in structural grade concrete with self-cementing fly ashes (by-products of coal fired power plants) and pulverized glass (post-consumer glass from the container industry). Researchers at Montana State University identified 95 fly ashes from a screening of over 491 U.S. power plants that are potentially capable of replacing 100 percent of the Portland cement in conventional concrete. Samples were obtained for 15 of these ashes from which fly ash/glass concretes were made to evaluate their performance. Of these 15 fly ashes, 8 produced concretes with 28-day compression strengths of at least 3,000 psi, making these concretes potentially viable for standard construction applications. Two of these five ashes (from Wyoming and Kansas) were then used in scaled up bulk mixes to determine the mechanical properties (compressive and tensile strength, elastic modulus) and durability (ASR, abrasion, chloride permeability) of these "green" concretes. They exhibited satisfactory performance with some variances from the behavior of conventional concretes, while maintaining controllable set times and adequate workability. Reinforced concrete beams were further made with fly ash/glass concretes produced with the Wyoming and Kansas ashes, and then tested alongside those made with a previously studied Montana ash as well as with traditional Portland cement concrete. Three different reinforcing schemes were used with each of the four concretes to highlight separate failure mechanisms: yielding of the reinforcement, crushing of the concrete, and concrete beam shear. The failure behaviors were generally the same for elements with the same reinforcing scheme, independent of the concrete used. Some differences were seen, however, in the ultimate capacities of the fly ash/glass and traditional beams. Additionally, ultimate capacities predicted with the American Concrete Institute building code (ACI 318-08) were typically conservative compared to the measured capacities of the fly ash/glass beams (in the most unconservative case, the capacity was over predicted by only seven percent).