Pick the most perishable substance: a) whole milk b) bakery bread c) newly mixed concrete.
If you answered, “c,” fresh concrete, then you probably already know a little about the rivers of beige-gray flowing around Chicago this summer.
Milk is good for about a week. Bakery bread, a day. But you have 90 minutes from when cement, aggregate and water are mixed into a batch of concrete before it starts to harden like, well, cement.
So if the hundreds of concrete mixers plying Chicago’s roads seem in a hurry, they are. Not only is the concrete in their drums setting, heating up as it does, but construction in Chicago is heating up too: After a long, harsh winter that was hard on the construction trade, the summer saw a jump.
“Business is definitely headed in the right direction,” said Tim Ozinga, a member of the fourth generation running Ozinga Bros., the city’s largest concrete company. “There’s a lot of pent-up demand in the marketplace. This year is very promising.”
From the reconstruction of the Ontario Street Bridge on the Kennedy to the Navy Pier Flyover to the expansion of the Chicago Riverwalk to the first of three towers at the billion-dollar Wolf Point project, concrete projects big and small are taking place all over the city.
Hiring is up in construction generally in Chicago — up 5.1 percent in the 1st quarter of 2014, according to Moody’s, compared to the same period in 2012, and the national cement market is rising 10 percent a year.
A note about those two terms, cement and concrete, since they’re often confused: Cement is a binding agent, made of powdered limestone and gypsum, that goes into making concrete, which is what you get when you add everything from gravel to fiber optics for translucent concrete, which is possible but pricey. Cement is to concrete as flour is to bread, or would be if bread were 10 percent to 18 percent flour.
Chicago is known for its concrete buildings, not only Marina Towers, but Lake Point Tower and Water Tower Place.
“Concrete has long been a major player in Chicago,” said Bill Baker, chief structural engineer at Skidmore, Owings & Merrill.
The Chicago region is peppered with concrete plants: Ozinga has 50 around Illinois, Indiana and southern Michigan; Prairie Materials has 22. The reason is so concrete can be mixed as close to the work site as possible; for really big jobs, mobile mixing plants are set up at the construction site.
“It’s a perishable product,” Ozinga said. “You can only go so far with it and maintain the quality and integrity.”
A generation ago, concrete was thought of as a single substance; contractors would call up and order a truck full of “mud.” But now, just as bread comes in hundreds of varieties and flavors, thousands of concrete mixture are used for all sorts of purposes — for setting underwater, weather resistance in roads, high strength in skyscrapers. There are ductile concretes that bend and porous concretes that allow water to flow easily through.
“We really should give it a new name,” Baker said. “We call it ‘concrete,’ but it’s the stuff you see on the sidewalk and the stuff you get delivered to the site — they’re both gray, but that’s about it.”
The surge in tall buildings and advances in concrete technology are directly related. Trump International Hotel and Towers and Dubai’s 2,722-foot-tall Burj Khalifa — both Skidmore, Owings and Merrill buildings — are concrete.
“Today’s concretes are not your father’s concrete,” said Lawrence Novak, director of structural engineering at the Portland Cement Association in Skokie. “Our ability to produce stronger and stronger concrete has really fueled the ability for us to go taller and taller with the high-rise buildings. If your concrete strength is low, you need bigger and bigger columns for support, and you wind up with a building that’s all column on the first floor. We can make columns smaller and buildings taller and lighter.”
Illinois is something of a center of concrete materials research, not only with Portland, which was once hired by NASA to make moon dust into cement, but the University of Illinois, renown for its concrete studies.
“Chicago for a long time has been a major leader in concrete technology,” Baker said. “Chicago got used to very, very high-quality concrete.”
“People are always drawn to use concrete in very flexible and creative ways,” said David Lange, a professor of civil engineering at U. of I., who explained how advances in concrete lead to design breakthroughs in architecture. “Another interesting trend is the emergence of self-consolidating concrete.”
Concrete has to flow into a form, and any kind of design or detail requires concrete to be packed or vibrated to fit into the crevice and avoid the formation of voids. That’s why concrete surfaces are typically, up to now, flat and smooth. Self-consolidating concrete “flows easily into formwork without vibration, which makes possible a lot of very complex shapes,” Lange said. “We seeing that come into the U.S. market.”
Chicago’s most notable recent buildings, like the Trump tower and Jeanne Gang’s undulating Aqua Tower, have been concrete, and more such buildings are on the way.
“There are about 18 high-rise residences planned for Chicago, all made out of concrete because concrete gives you the ability to form it in any shape, when hardened a unique combination of high strength and fire resistance,” Novak said.
Baker said changes in technology will allow more “fluid” designs to concrete buildings. “We’re looking at origami.”
The composition of the concrete isn’t the only way technology affects concrete building. Trump’s tower required some of the most powerful pumps ever developed for concrete construction to move a stream of it up 60 floors.
Usually, concrete is moved in fleets of trucks (a new McNeilus standard mixer costs $185,000). Ozinga has 500 trucks, 100 of them running on natural gas, though not so much because it’s cleaner but because it’s quieter, a consideration in the city. Also key is size; Ozinga is developing a smaller concrete truck to better navigate tight urban spaces.
Concrete heats up as it hardens, so much that venting the heat has to sometimes be figured into construction design, embedding water pipes that were later caulked in. Heat also sometimes required trucks to have ice poured into their hoppers to keep the contents cool. New chemical mix-ins help reduce that need.
At Ozinga’s main headquarters on the near Southwest Side along the Chicago River, trucks are filled with various mixtures of concrete for specific jobs. Getting the mix right is crucial, because there’s no point hauling concrete designed for highway pours to roadways to a high-rise.
The process is itself a mixture of high and low tech. Work orders come in through pneumatic tubes but are tallied on computer screens. An electronic gauge measures the resistance on the truck drum motors to determine each mix’s “slump” — the term used to measure the consistency of concrete. In the field, workers perform the slump test (Concrete InFocus magazine called it “a sacred rite”) by filling a foot-high cone with concrete, waiting, then removing the cone and measuring how many inches the pile of wet concrete slumps down.
What happens to those trucks that don’t get to work sites in time? Or find that workers aren’t ready? You can “kill” the load — sugar works in a pinch to render concrete unsettable. If there’s time, truck drivers hurry back to base where the concrete is poured into huge blocks, usually with a metal cable embedded in them to be moved by crane, and the blocks are used for ballast and various projects.
And yes, sometimes the concrete hardens in the drum.
What then? Somebody has to go in with a hammer and chisel to chip it out.