New technology mixes green building with custom concrete colors.
By Rossella M. Ferraro and Antonio Nanni
Not too long ago, when architects and engineers mentioned concrete, ordinary people rarely got excited: concrete meant a dull, grey, visually unappealing appearance. Today, thanks to new technological advances, precast concrete can be cast in a variety of finishes and colors, offering architects and the construction industry a material that is both eminently practical and aesthetically pleasing. Looking beyond appearance and with the sustainable management of resources in mind, white concrete becomes an attractive construction material for wide use around the world.
The trick is in the availability of white cement. From an architectural standpoint, with the inclusion of pigments or additives to the mixture constituents, white cement allows for a broad spectrum of concrete colors from bright whites and pastels to saturated colors in the final precast concrete product, which make this materia
l really attractive. But color is not only about aesthetics, it can also offer important practical benefits in terms of safety (i.e., reflection in the dark) and environmental impact. Based on the U.S. Green Building Council certification practice, the Leadership in Energy and Environmental Design Green Building Rating System for New Construction and Major Renovation (LEED-NC) considers that the reflective quality of white surfaces may help to reduce temperature fluctuations from solar radiation and improve lighting efficiency, resulting in lower heating and cooling with a reduction of related-energy costs.
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 the calcium hydroxide content of the cementitious system. From an 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 of the steel reinforcement will be monitored. In particular, a statistics-based model will be developed to describe the relationship between corrosion of the steel reinforcement and color variation of the offwhite concrete over time.
As an alternative to traditional steel reinforcing bars, the use of glass fiber-reinforced polymer (GFRP) reinforcement will also be considered in the research program. The primary advantage of GFRP reinforcing for this type of application is the lack of corrosion. This means stability in color and less stringent requirements on concrete cover and consequently lighter elements.
Precasters need consistent color in manufacturing
The experience of the precast manufacturer was the major motivation for the investigation on color analysis: Artistic Stones Inc. explained how difficult it can be to deal with white concrete in an industrial setting. Often a change in brand of any constituent, particularly cement, can result in a change in color. If the study can jointly develop a field technology to predict and control color variation of concrete, it will have achieved a milestone for the precast industry in its quest to address costumers’ needs and expectations.
The authors gratefully acknowledge the National Science Foundation for financially supporting the project. The authors also would like to thank Artistic Stones Inc. for assistance and collaboration.
Rossella M. Ferraro is a Graduate Research Assistant at the Department of Civil, Architectural, and Environmental Engineering at the University of Miami. Her research focuses on the use of advanced composite material for sustainable construction.
Antonio Nanni is the Fisher Endowed Scholar, Professor and Chair of the Department of Civil, Architectural, and Environmental Engineering at the University of Miami. His research interests include the evaluation, repair and rehabilitation of concrete structures.