Strengthening Concrete

Nano particles could prevent buildings and bridges from suffering serious structural damage.

For professionals whose job it is to evaluate infrastructure, it's clear that the country's vast system of roads and bridges is in urgent need of repair. In 2007, officials at the Federal Highway Administration rated 25 percent of US bridges "structurally deficient or functionally obsolete." And just this year, the American Society of Civil Engineers released its annual infrastructure report card, giving the overall state of bridges a "C" and roads a "D-".

Cementing concrete: A new technique makes concrete less susceptible to corrosive agents such as road salt. At the top of this X-ray image, the barely visible blue-green area shows that very few chloride ions (in green) have penetrated the treated concrete (blue). Red particles indicate grains of sand mixed with cement. Credit: NIST

The majority of these structures are made of concrete, many erected in the 1940s and 50s. Today, these bridges and roadways are crumbling into disrepair, partly due to age and partly because of winter de-icing. While road salt melts ice from surfaces, it can also work its way into the many micropores in concrete, thawing the water molecules within. This rapid thawing can cause the concrete to expand and crack from within, taking years off its service life.

Now engineers at the National Institute of Standards and Technology (NIST) have developed and patented a new technique, called VERDiCT (Viscosity Enhancers Reducing Diffusion in Concrete Technology), that could potentially double the lifespan of a piece of concrete. By mixing a nano-sized additive with cement, they devised a method that slows the infiltration of road salt. They reasoned that the longer it takes for deteriorating agents to penetrate, the longer concrete will last without cracking.

In conventional concrete manufacturing, dry cement--typically consisting of limestone, clay, and other minerals--is mixed with water to make a paste and combined with aggregates, such as rocks or sand. As it dries, the paste glues the aggregates together into a concrete slab. Recently there have been efforts to create stronger, high-performance concrete, mainly by increasing the material's density. To do this, researchers either add various strengthening chemicals or grind the dry materials used to make cement so that they are even finer than those found in conventional mixes. Once combined with water, the paste and resulting slab is much denser and stronger than traditional concrete.

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However, scientists have found a major downside to such high-performance alternatives. "In fast-track construction, everyone is going for early-strength concrete because they want to get traffic back up and running," says Dale Bentz, a chemical engineer at NIST and lead investigator on the project. "To get that strength, you might grind concrete finer [to make it] more reactive, but that also generates more heat, and when it cools down and contracts, it could cause cracking. So you get high-performance concrete between the cracks, which is not what you want."

Bentz and his colleagues took a nano-scale approach to improving concrete instead. They recognized that within concrete there are millions of tiny micropores filled with water molecules. It is known that chloride and sulfate ions from road salt penetrate concrete by diffusing into this water solution, so they hypothesized that increasing the viscosity of the solution within these micropores might slow the penetration of road salt and other deteriorating agents, and extend concrete's lifespan.