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Raw materials & technologies, Technologies

Predicting the behaviour of new concrete formulas

Friday, 23 January 2015

Just because concrete is the most widely used building material in human history doesn’t mean it can’t be improved.

A study has led to a new way to predict concrete’s flow properties from simple measurements. Source: Roman Milert/Fotolia

A study has led to a new way to predict concrete’s flow properties from simple measurements. Source: Roman Milert/Fotolia

A study conducted by an international research group has led to a new way to predict concrete’s flow properties from simple measurements. Concrete begins as a thick pasty fluid containing innumerable particles in suspension that can, ideally, flow into a space of nearly any shape, where it hardens into a durable, rock-like state. Its initial flexibility combined with its eventual strength has made it the material of choice for building everything from the ancient Roman Colosseum to the foundations of countless modern bridges and skyscrapers.

Concrete is not without its problems

For example, when concrete is pumped, it can jam in pipes, leading to time and cost overruns during construction. The particles can settle out, leading to structural problems after the concrete hardens. And a significant amount of energy is needed to create the cement that reacts with water to produce hardened concrete. This critical binding agent is manufactured at high temperatures in a kiln, a process that generates a great deal of carbon dioxide, a greenhouse gas. According to the World Business Council for Sustainable Development, worldwide cement manufacture is estimated to account for at least 5% of humanity’s carbon dioxide emissions.

Reducing the manufacture energy

The industry can develop less energy-intensive concrete mixtures by replacing some of the cement with alternative materials like fly ash. However, these alternatives can require expensive chemical additives, and they also can have a range of effects on concrete flow. Ideally, the industry would like to tailor the use of these chemical additives, thus helping to assure the greatest use of alternative materials. "We’d like to be able to design concrete that performs better on the job and doesn’t demand so much energy to manufacture,” says NIST computer scientist William George. "But what should we make it from? And what can we replace cement with? The answers will affect its properties. So we realised we needed to learn more about how suspensions work.” While it’s a simple goal to describe, accomplishing it demanded some complex math and physics, and at the same time, an enormous amount of computer power to study how all the particles and fluid react as they are mixed. The supercomputers allowed them to simulate how a suspension would change if one or more parameters varied - the number of suspended particles, for example, or their size.

Suspensions have a remarkable property

Plotting two parameters - viscosity vs. shear rate (the latter refers to how neighboring layers of the fluid change velocity as it flows through a pipe) - always generates the same shaped curve as plotting them for the suspending fluid alone without added particles. This is true no matter what fluid is used. The curve just sits on a different location on the X-Y axis, as though someone had pushed it upwards or off to the side without otherwise altering its shape. What the team unexpectedly found was the amount that the curves had to be shifted could be predicted based on the microscopic shear rates that existed between neighboring particles. Experiments at the University of Strasbourg confirmed the simulated results, which allowed the team to come up with a general theory of suspensions’ properties.

Predicting how fresh concrete will behave

"So now if you have a suspension that is made with a fluid that behaves a bit differently, you can still predict what its properties will be,” a researcher says. "You just have to measure the properties of the fluid that the particles are placed in, and you predict how the fresh concrete will behave.” The results should help accelerate the design of a new generation of high-performance and eco-friendly cement-based materials by reducing time and costs associated with R&D, he adds.

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