Team pounds pavement
with concrete calculations
Posted June 1, 2009
Concrete is a $100 billion business in the United States. The material is used for all manner of structures, from roads and bridges to buildings. So it's hard to imagine that such a common and ubiquitous material could still hold secrets.
Yet knowledge about concrete flow is typically based on construction crews' practical experience preparing a mixture of local ingredients: cement, sand and rock. These components can vary widely in shape and chemical composition in different regions, making concrete mixtures diverse.
One common on-site assay, the slump test, measures how far a conical pile of fresh concrete spreads over time. The mixture is adjusted on the fly, like adding milk to cake batter so it stirs faster, says Nicos Martys of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.
Martys, a physicist in the Materials and Construction Research Division, is part of a NIST team formulating a theory on how concrete components interact as they flow.
With a renewal award from the Department of Energy's Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program, NIST researchers are using 750,000 processor hours to simulate viscosity, yield stress and other fundamental concrete flow questions.
Toward greener building materials
Accurate concrete flow simulations and a better understanding of the effects of different additive materials could lead not only to sturdier buildings but also a better environment.
Concrete's strength depends on the amount of cement used, but cement processing from limestone accounts for up to 4 percent of the carbon dioxide humans release into the atmosphere.
Although finished concrete structures absorb some carbon dioxide and reduce the overall carbon footprint, concrete companies would like to find ways to build strong concrete using less cement and perhaps incorporating industrial waste materials such as fly ash from coal combustion or slag from steel production.
NIST researchers suggest that whatever ingredients end up in a more earth-friendly concrete, they have found a formula for assessing a mixture's strength by combining physical theory, powerful computers and, not least, a way to see inside their simulations
One of those researchers, Ed Garboczi, says fundamental concrete research remains a juggling act, sitting at a juncture where basic science meets engineering applications.
"Working on concrete, we're sort of in a gray area," says Garboczi, leader of NIST's Inorganic Materials Group of the Materials and Construction Research Division. "We need to do fundamental science to understand concrete because it's so complex, but most of the world thinks concrete is something that you mix in the bag and dump in a hole."
