Material witness

Jan. 28, 2009

Engineering a better concrete pavement might someday involve manipulating cement grains at the molecular level. Nanotechnology is now being explored by the Federal Highway Administration (FHWA) and others in the industry hoping to determine the nanoscopic properties required to create the desired macroscopic concrete characteristics, such as resistance to water or chloride penetration, strength, stiffness and durability.

Engineering a better concrete pavement might someday involve manipulating cement grains at the molecular level. Nanotechnology is now being explored by the Federal Highway Administration (FHWA) and others in the industry hoping to determine the nanoscopic properties required to create the desired macroscopic concrete characteristics, such as resistance to water or chloride penetration, strength, stiffness and durability.

The National Nanotechnology Initiative supports nanoscale science and engineering research and development at 13 federal agencies, including $1 million to the FHWA. Advanced infrastructure research at the FHWA focuses on nanoscience technology and computational structural mechanics to improve highway materials and structural performance, according to the FHWA’s Office of Infrastructure R&D within the Office of Research, Development and Technology.

In fact, nanoscience—science on the scale of a nanometer, a billionth of a meter—has already contributed to knowledge about concrete. The FHWA used a technique called nuclear resonance reaction analysis to study how water and portland-cement powder react during the setting process. (See the sidebar for a more detailed description of the experiment.)

Nanotechnology might someday help produce a pavement that has both great strength and flexibility, Leif Wathne, P.E., vice president of highways and federal affairs at the American Concrete Pavement Association, told Roads & Bridges.

Peter Taylor, Ph.D., P.E., associate director of the National Concrete Pavement Technology Center at Iowa State University, was skeptical of nanotechnology’s ability to deliver short-term breakthroughs.

“We’re really only just getting out of the ground with that,” he told Roads & Bridges.

Taylor said the basic materials of concrete had not changed much in the past few years, except that to meet environmental regulations, electric power producers had begun intentionally adding carbon, ammonia and mercury to the fly ash produced at their power plants. Those additives may be good for power plant emissions, but they are generally bad for the concrete the fly ash might go into as an admixture.

Carbon in fly ash makes it harder for concrete to entrain air, according to Taylor. “Ammonia doesn’t do much to the concrete, but if you’re doing flat slabs it’s potentially not pleasant for the operators of the equipment.” And mercury is more of a health hazard than anything else.

Mix mastery

One of the more unusual admixtures to concrete is titanium oxide. Italcementi Group developed a concrete product called TX Active that includes titanium and has the ability to extract smog out of the air around it. The concrete has been used in the Umberto I tunnel in Rome and the pavement of a street in Paris. It can reduce air pollution by up to 50%, according to Italcementi Group. Another attractive property of TX Active is that it maintains its natural white color over time.

“The titanium oxide acts as a catalyst,” Wathne said, “so that when sunlight hits it in the presence of smog particles in the air, it basically binds it so it turns into essentially a particle that sticks to the concrete, and water washes it away.”

One of the more recent mixtures is silica-fume concrete. It has been around for a while but has not yet reached its full potential. Silica fume is an extremely fine powder.

“It tends to plug up the pores that normally would be voids,” David Fowler, a professor of civil engineering at the University of Texas, told Roads & Bridges. “You end up with a denser concrete” that is more impervious to water or salt intrusion.

Wathne said one of the prime objectives of ongoing concrete research is mixture optimization. With the various ingredients that can go into concrete in various quantities and the complicated interactions between ingredients, optimizing the mix for the desired properties, such as strength, durability and imperviousness, is a tricky task.

Metal a la carte

The same type of complications go into designing steel for an application such as bridge building. There are 13 different elemental components besides the basic iron that go into HPS 100W, the high-performance weathering steel made by ArcelorMittal. None of those 13 elemental additives makes up more than 1.5% of the mix, but they have a significant effect on the macroscopic characteristics of the steel plate.

“There are empirical formulas that help guide us in predicting weldability and strength from the chemical composition that we use, but quite often we take an existing steel and try to improve it,” Alex Wilson, manager of customer technical service at ArcelorMittal USA, told Roads & Bridges.

Strength can be increased by adding carbon, but adding carbon also decreases the metal’s toughness and weldability. For improved toughness, which means resistance to crack propagation, a metallurgist can reduce levels of the impurities sulfur and phosphorous. Modern steel making is better than ever at controlling the process to get exactly the desired amount of alloying additives such as manganese, nickel, chromium and molybdenum.

HPS 100W has improved toughness and weldability over the lower-performance steel of the same yield-strength grade of 100 ksi, or 100,000 psi.

The high-performance steel is more expensive, because of the alloy additions, but it can still be cost effective.

“What we have found with the HPS 70W,” a high-performance steel in the 70-ksi strength grade, said Wilson, “even though that steel is more costly, you can end up with a more cost-effective bridge, because you need less of it,” and because it is easier and less costly to weld.

ArcelorMittal is currently researching more corrosion-resistant steels. The company already has developed A1010, a stainless steel with less chromium (12%) and therefore less cost than traditional stainless steels, which contain about 18% chromium.

“If an owner is concerned about life-cycle costs and has a very aggressive environment near the ocean or on an overpass structure where they use a lot of salt during the winter,” they might consider A1010, said Wilson. “It probably won’t dominate the marketplace, but it will be available for significant use in long-span bridges where you need the higher-strength steels.”

Zinc-rich primers are still the gold standard for painting steel to protect it from corrosion, but the topcoats have changed over the years from vinyl to epoxy, urethane, polyaspartics and other chemicals. Topcoats add pigments besides the gray of zinc. Topcoats also can add protection from ultraviolet light, which fades colors, water penetration and impacts from wind-blown debris and any other sources.

People are working on new generations of zinc-rich coatings, Eric Kline, executive vice president/senior consultant at KTA-Tator Inc., told Roads & Bridges. “There are different ways to put zinc in paint that might make it perform a little bit better. There are new barrier coatings that are going to come along shortly . . . that will be beneficial. There are two-coat systems that have come out in the last four or five years that have been tested and found to be as good as three coats.” There is even a search going on for a one-coat paint that would have the characteristics of a two- or three-coat system. That might be possible in five to 10 years.

Out to justify

Nanotechnology has the potential to reduce CO2 emissions from cement production, Gerald Voigt, P.E., president and CEO of the American Concrete Pavement Association, told Concrete Pavement Progress (Aug. 31, 2007).

Nanotechnology also has the potential, because of its sophisticated nature, to significantly add to the cost of building concrete structures.

“If we can change the chemistry of cement or change the surface of aggregates or markedly improve the performance of admixtures, that’s where I see it going,” said Taylor. “If we can get a shrinkage-reducing admixture that really works a whole lot better and is cost effective, then we’ll be rich and famous.

“When we’re playing with a commodity that’s sensitive to nickel and dime price increases, can we justify the cost of the nanomaterial?” he continued. “Maybe, maybe not. If we get an order of magnitude improvement in performance, it may be cost justified.”


Cement gets a reaction analysis

In research to study how water and portland-cement powder react during the setting process, the FHWA used a technique called nuclear resonance reaction analysis. The research was described in the January 2003 issue of Research & Technology Transporter.

A beam of nitrogen atoms was focused on a reacting cement grain to locate hydrogen atoms, a marker for water, or its reaction products. The results over time show the water’s rate of penetration into the cement grain.

The researchers found that a 20-nanometer-thick surface layer acts as a semipermeable barrier that allows water to enter the cement grain and calcium ions to seep out, but traps the large, silicate ions within the surface layer. The silicate forms a layer of gel beneath the surface of the grain. The gel swells and eventually breaks down the surface layer. The silicate gel is released into the surrounding solution, where it reacts with calcium ions to form a calcium-silicate hydrate gel, which binds the cement grains together and sets the concrete.

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