Precast Solutions - Winter 2009 - (Page 23) the calcium hydroxide content of the cementitious system. From an Rice hull ash for sustainable concrete Looking at the environmental impact of new construction, the use of sustainable building materials can certainly make a difference. This class of materials includes products that are usually defined as renewable, energy-efficient and nontoxic, and is chosen for the low impact of the products over the lifetime of the structure. In this scenario, driven by the idea of producing high-profit margin products from industrial wastes and developing eco-friendly and low-cost technologies for remediation of environmental pollutants, the ChK Group, in a project funded by the National Science Foundation, demonstrated the manufacturing of off-white rice hull ash (OWRHA). The mission of this small Texas-based company is to tailor materials for environmental, industrial and food applications. With these objectives, ChK Group designed and demonstrated the potential of an innovative process to produce OWRHA. The possibility to have an eco-friendly, off-white rice hull ash, with no graphitic carbon in it, generated the idea to combine this green material with white cement to make an off-white concrete. Previous studies have recognized that the use of pozzolanic materials as a partial replacement of portland cement is an effective way to improve the properties of concrete. Furthermore, the use of pozzolans such as fly ash, silica fume, palm oil fuel ash and ground rice husk ash as supplementary cementitious materials has represented a positive step forward in response to environmental concerns in sustainable building materials. However, the presence of graphitic carbon in some of these materials is a strong limitation in the off-white concrete mix design. OWRHA is a product of rice hulls, manufactured with a continuous process in a rotary furnace, designed expressly for concrete production. Off-white color with no graphitic carbon and absence of crystalline silicon dioxide (SiO2) and toxic metals are the principal OWRHA advantages. The product is defined as environmentally friendly and can be used to make off-white concrete. Pilot research conducted directly by ChK Group on pozzolanic activity of OWRHA indicated that, when used as a supplementary cementitious material (SCM) at dosages between 5 percent and 10 percent, OWRHA increases the ultimate strength of the concrete and reduces economical standpoint, the current market price of competing SCMs, (i.e., silica fume or metakaolin) is in the range of $400 to $580 per ton, while OWRHA may be sold at $275 per ton, which means that just in the first five years of production, this product could capture a significant portion of the market. Corrosion detrimental to white concrete aesthetics The use of this eco-friendly material in architectural precast applications of off-white concrete is the main goal of a study now underway at the University of Miami. It is evident that in applications where aesthetics is the driver, a great deal of attention needs to be devoted to the concrete mixture, but the reinforcing materials cannot be ignored. Corrosion of the steel reinforcing bars is a problem not only for the strength and serviceability of a reinforced concrete system, but also for its appearance. The study will attempt to scientifically evaluate the influence of reinforcement corrosion on the off-white concrete whiteness characteristics. The current literature in the civil engineering arena lacks information related to color analysis. Because of the direct connection between appearance of a product and its organoleptic properties, however, food engineering has become an unexpected but critical resource for this study (organoleptic refers to the sensory properties of a product, such as taste, color and feel). Several color scales have been used to describe color, and the most frequently adopted scale is CIELAB. CIELAB uses 3-D, X-Y-Z axis that scientifically describes how the average human eye sees color. The CIELAB color scale may be used on any object whose color needs to be measured. It provides a standard scale for comparison of color values and is used extensively in many industries. An experimental program will be conducted at the University of Miami Structural Laboratory in partnership with Artistic Stones, a precaster located in Miami, Fla. Several off-white concrete elements will be cast using different percentages of OWRHA (from 0 percent to 15 percent). The specimens will be subject to a cyclic aging test and, using a portable spectrophotometer, the color variation due to all factors including the corrosion www.precastsolutions.org Winter 2009 | Precast Solutions 23 http://www.precastsolutions.org
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