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Most construction equipment is easy to understand. Cranes move things up and down. Dump trucks load up, move out and unload. Bulldozers push and graders grade. The one exception to this is the humble cement mixer, beloved by children, hated by in-a-hurry drivers, and misunderstood by most people outside the cab of the 30,000-pound (13,608-kilogram) behemoths. While concrete has been around in one form or another since before the Romans built the Appian Way, the transit mixer is a child of the 20th century. But recent invention or not, the mixer is here to stay.
The misunderstanding begins with the name. What people refer to as a cement mixer is known in the construction industry as a concrete mixer and comes in a large number of types, sizes and configurations to handle the many tasks set before it each day. That need to fill so many roles means the machine is dynamic, changing shape and form as the needs of the people using concrete change as well.
In this article, let’s clarify the difference between cement and concrete. In baking terms, the difference between concrete and cement is the difference between flour and a loaf of bread. Concrete is a generic term for a mix of aggregate — usually stone or gravel, water and cement. Modern cement is a complex blend of finely ground minerals, and goes by the generic name of “portland.” Concrete is made by combining the three ingredients in a mixer, whether that mixer is stationary or driving down the road, and the water is absorbed by the cement, which then binds the aggregate together, creating concrete.
Mixer History--Stephen Stepanian developed and applied to patent the first motorized transit mixer in 1916, in an effort to replace the horse-drawn concrete mixer used at the time. Wooden paddles churned the mixture as the cart wheels turned, but the design was of limited use — it was cumbersome and slow. The same, however, could be said of the engines and trucks during that period. But by the 1940s, engines and truck-frame construction caught up to the need for a rugged vehicle capable of hauling thousands of pounds of wet, or unset, concrete. As the building boom following World War II went into full swing, mixer trucks came into their own.
The large drum mixer seen on roads today hasn’t changed much from Stepanian’s vision of a better concrete hauler. Mobile transit mixers are a mix-and-match selection of engine, truck frame and rotating mixer. The mixer is similar, though larger in scale, than the smaller ones found on construction sites. A large motor, separate from the engine, rotates the drum on the truck body, and a series of blades or a screw powered by the same motor keeps the aggregate, water and cement in constant motion. This keeps the premixed concrete from setting, though the clock is often ticking to get the load to the construction site, road section or parking lot. Most cement manufacturers suggest keeping the time between mixing and pouring to 90 minutes at most. It’s even better to get it to the site in less than an hour.
The Future of Cement Mixers--What the future holds for the concrete mixer is unclear. Like many industries, concrete is going “green.” The manufacturers of volumetric concrete mixers say their products are more environmentally friendly because they save fuel by mixing materials at the construction site — the trucks don’t have to run their engines to keep the concrete from setting. In addition, truck operators can create only the amount of concrete needed to finish a job — not only does this save materials, but this method also prevents dumping of excess concrete.
Truck manufacturer Peterbilt is experimenting with compressed air “push” systems. The truck would be started using compressed air, and when it reached a certain speed, the diesel engine would kick in. Since an engine requires a larger amount of fuel to overcome inertia, getting it up to speed with relatively inexpensive compressed air would show fuel savings over time. This developing technology is now aimed at inner-city delivery trucks rather than larger trucks like mixers.