(Editor’s note: “CEG” will provide a similar in-depth article on asphalt in an upcoming issue.)
It’s being considered as the main material for the first structures on the moon.
Several million tons of it have gone into the construction of New York City’s Third Water Tunnel, the largest public works project in the city’s history.
It makes a good skyscraper, especially if you live on an upper floor, because it doesn’t sway like steel. And a pretty good house, except that you can’t hammer a picture hook into the wall. Not a very good ship, though some have sailed the oceans. Sturdy bridges arching the waters, and many smooth, white, durable highways lacing the countryside are among its greatest testimonials. The “Big Dig” tunnel in Boston, includes an estimated 4 million cu. yds. of this material. The Hoover Dam between Nevada and Arizona includes 3.25 million cu. yds.
We’re talking concrete, the most widely-used structural material in the world, employed on almost all construction jobs. Here’s an overview of this construction material, how it’s made and used, and how leaders in the industry view it.
Begins in Fiery Kiln
Though it’s a common part of our culture, concrete is far from ordinary. It begins with large flame-filled rotary kilns producing cement “clinker” from a raw material like limestone as temperatures reach 3,000F. After the clinker is ground into fine powder, water, sand, course aggregate like crushed stone or gravel, and chemical additives are mixed with the cement in a batch plant at the construction site or in the familiar trucks with rotating barrels.
Then miracles are made. The resulting wet concrete mix is put in place and becomes rock-hard as foundations for homes, as sections of highways and bridges, as piers and beams of soaring skyscrapers, or in tunnels, dams, stadiums, shopping malls, parking lots, airport landing strips and thousands of other applications.
Make Concrete Structures on the Moon?
Back in 1986, the National Aeronautics and Space Administration (NASA) sent a 40-gram sample of soil from the moon to Construction Technology Laboratories (CTL) a subsidiary of the Portland Cement Association in Skokie, IL. Could the lunar soil be used as an aggregate to make future structures on the moon?
CTL combined the sample with portland cement and water, created a small cube of concrete, and tested the cube by electron microscope to determine properties. It then said the tests “provide convincing evidence that lunar material will be ideal for building concrete structures on the moon,” adding that the tests suggested that compressive strengths of more than 10,000 psi could be obtained.
CTL returned the concrete cube to NASA, which stored it for approximately 20 years in a facility that simulates the cold, airless, near-vacuum lunar conditions. NASA then returned the cube to CTL for further testing, including a compression test to failure.
CTL has now informed the space agency that the cube’s physical properties are still the same, that the glassy lunar material, used as an aggregate, would not react adversely with cement, and that the latest test results “suggest that lunar concrete should be stable.”
Of course, you would still need cement, which comprises approximately 15 percent of concrete, for the lunar mixture.
A separate CTL project has determined that lunar soil also could be used as a source of cement-like material as well as an aggregate.
“On the moon, you cannot build castles out of sand; we had developed a composition of cement that can be produced right on the moon from lunar minerals and which could be used for future construction,” said Alex Mishulovich, now a CTL consultant in Skokie, IL, who had worked on the project in the 1990s. “You cannot make portland cement on the moon but you can make something that is capable of cementing particles together.”
Not that it would be easy. Mishulovich pointed out that cement is made in high temperature kilns.
“You can’t use a rotary kiln, with all its needs for fuel and oxygen, on the moon,” he told Construction Equipment Guide (CEG). “You need some other energy source, which we conceive as a solar furnace using solar energy. If NASA decided to fund this project further, we would pursue this in conjunction with the University of Chicago.”
And you would still need water to make the lunar concrete. A mineral called ilmenite is present on the moon, and can be used to produce oxygen. CTL has therefore concluded that “at least theoretically, only hydrogen would need to be transported to the moon in order to produce mixing water for lunar concrete.”
Back here on earth, concrete is growing stronger. As a result, it can be employed in many more applications. A recent article in the New Yorker Magazine estimated that New York City adds concrete to itself at the rate of approximately 6 million tons of concrete, every 18 months.
Thomas Edison built concrete houses with compressive strength of approximately 3,000 psi (pounds per square inch), but 20,000 psi concrete is now common in high rises while 4,500 to 5,000 psi concrete is typical for highways. Some lab projects are developing 100,000 psi concrete.
Robert Garbini, president of the National Ready Mix Concrete Association in Silver Spring, MD, said that “If the market demands, strength of concrete will go up; we have the technology now; it’s a matter of practical application.”
Garbini said high-strength concrete is being increasingly used in place of steel for building high-rise structures.
“Rather than using a large steel column, you benefit from a much smaller column with a higher compressive strength,” he explained. “That’s an advantage in terms of the amount of forming, and amount of material, which is required. This can allow you to build taller buildings because the concrete provides high compressive strength which is needed in the lower columns.”
(Using 12,000 psi reinforced concrete, the Trump World Tower in New York City is 860 ft. high.)
Garbini believes that you’ll see more concrete buildings because engineers, designers and architects view concrete as being more versatile than steel in terms of strength, coloring, and texture.
“If you employ curvilinear structural steel in a building, you need to put some sort of covering or facade on it. If you use concrete, you can just leave it exposed, with a smooth, aggregate, or other finish. Certainly you can cover a building faster with concrete than with steel. You can put the cladding on a steel structure until all the connections are tightened. As you cap concrete — as it reaches its strength — you can start putting the cladding on in a shorter period of time.”
Discussing versatility, Garbini said, “When they expanded the Dulles Airport outside Washington, D.C., for instance, in the 1990s, they could exactly replicate the color and texture of the spires which were cast 30 years earlier to support the roof of the original structure built in the 1960s. They replicated the old weathered appearance.”
Can you build a taller high-rise with concrete than with steel?
“You can’t go any taller with steel than concrete, let’s put it that way,” Garbini said. “Building height is really a question of economics rather than materials. It depends on people’s use of the building.”
Behind the scenes, the concrete industry is always working on new, more sophisticated mixtures to add to the basic mix in order to augment its properties.
“The add mixtures in the industry have become very sophisticated,” Garbini said. “They can accelerate, or instance, or slow down, the process of setting the concrete. They can allow the concrete to meet different temperature requirements. Many new advances are coming from the chemical companies that service the concrete industry.”
Proponents of concrete high rises say they also sway less in the wind. Garbini cited three large apartment buildings in Detroit, “The first was made of steel, which was very flexible. In high winds, the tenants on the upper floors were complaining that they could feel the building moving, and see water swishing in the toilets. For that reason, the owner went to concrete structures for the other two buildings.”
Cites World Trade Center
Garbini said concrete is more fireproof than steel, “Since the collapse of the World Trade Center in the terrorist attacks, people are starting to see concrete as a balanced material in firefighting systems. Fire-damaged concrete may not be serviceable, but it’s not going to collapse. I think everyone agrees that, when the planes hit the buildings, the impact jarred off the fireproofing. The connections that supported the floors to the columns were sheared off. With cast-in-place concrete, on the other hand, the connections are all integral. They are all cast together. Steel structures, even pre-cast structures, don’t provide that benefit.”
One of the main uses of concrete, of course, is in highways. Here, too, concrete appears to be growing in use. And it’s lasting longer, with high performance concrete beginning to appear on the scene.
“A lot of designs [for highway use] are 20-year designs, but concrete in many cases lasts much longer than that,” said Bill Davenport, a spokesperson for the American Concrete Pavement Association (ACPA) in Skokie, IL. “We’ve seen cases of concrete pavements that are on the ground for 40 years, carrying far more traffic than their original design.”
Davenport said there has been a “significant growth” in the use of concrete in highway applications over the past three years.
“We actually measure concrete usage with a proprietary system that separates out anything that is not purely a highway pavement item,” he told CEG. “Concrete is now used on about 29.5 percent of the National Highway System. That is up sharply — about 15 percentage points nationwide since 1998.”
Asked to explain his statement, Davenport replied, “Number one, we are promoting concrete a lot more aggressively. Many state agencies are now looking at its long-term features and benefits. This has done a lot in terms of clearing up some misinformation about concrete, lingering misconceptions like high costs, that it’s difficult to repair, or that it just takes too long. I think we have addressed that. If the agency needs to have a pavement open to traffic soon, we can put down a pavement between rush hours and open it to traffic, using high-early-strength concrete.”
The general understanding is that concrete costs more than asphalt, but lasts longer. Not necessarily true, Davenport said, “We’ve seen some applications where the initial cost is about equal. On a project-by-project basis, there’s probably some truth to the claim that asphalt is less expensive on first cost, but life-cycle costs are almost always in favor of concrete. A lot of agencies, looking at pavement as part of their asset management programs, are becoming convinced that concrete is the better buy.”
Costs of Concrete
The increased cost of steel, due in part to demand from China, could be expected to increase the market for concrete.
“China has such a huge building program that it will take much of the world’s steel,” Garbini said. “Obviously the cost of steel will impact the types of buildings which are selected. It’s a good thing for the marketing of concrete, though China is also sucking up a lot of the cement in the world, too.”
China continues to dominate the global cement picture, accounting for approximately 37 percent of worldwide production. India is second, followed by the United States.
After 10 years of expansion, consumption of portland cement (often for concrete) in the United States declined by 4 percent in 2002, reflecting the effects of recession, and tightened state budgets, but has been on the upswing in a recovering economy.