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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.

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.

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.

“If these ions are floating around, if they’re moving through honey instead of water, they’ll be significantly slowed down,” says Bentz. “The trick is to find the right chemical that will change the viscosity of the solution.”

The researchers took a cue from the food industry, which uses additives as thickeners in everything from salad dressings to carbonated drinks. Bentz searched for similar additives that would both increase the viscosity of the water solution found in concrete and slow ion diffusion; he even tried using food thickeners, including xanthum gum, which is used in sauces and ice cream.

After screening multiple additives in water solution in order to model the behavior of ions in concrete, the team found that those with a smaller molecular size were more successful at slowing the rate of ion diffusion. Additives that occur in small molecular chains, with branches of hydrogen and oxygen, were particularly good at increasing a solution’s viscosity. Bentz says this might be due to the fact that such hydrogen and oxygen branches can interact with water molecules to form a barrier against infiltrating ions, making it harder for them to penetrate.

The team also tested various additives within small cylinders of cement mortars–essentially, concrete without the aggregates. Bentz mixed the additives with cement, let the mortars dry, and placed each mortar into a chloride solution for up to one year. After removing the mortars from the solution, he and his team broke apart each mortar and analyzed how far chloride ions were able to penetrate. Compared with mortars without any additives, those with additives showed significant reduction in chloride diffusion.

However, the technique may not be quite ready for industry-scale application, mainly due to potential costs. Bentz says to get such results he had to make the additive as much as 10 percent of the cement solution. “The industry is comfortable with one percent, so there’s a cost factor, in that it’ll cost 10 percent more,” says Bentz. “We’ve demonstrated proof of concept, and now we would like to find an additive that works at 3 to 5 percent concentration.”

Jason Weiss, professor of civil engineering at Purdue University, works on improving concrete mixtures and increasing the material’s long-term performance. He says that such a technique may one day make bridges and roads less susceptible to corrosion. “This has an enormous potential,” says Weiss. “This would imply that a bridge that could last 30 years would now last 40 to 45 years under the same type of chemical attack.”

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