How Cement Mixers Work


Concrete mixers are needed for many jobs, and have to get there quickly to make sure the mixture is poured before it sets.
Concrete mixers are needed for many jobs, and have to get there quickly to make sure the mixture is poured before it sets.
iStockphoto/Thinkstock

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 we'll examine some of the major types of mixers, from the traditional drum-shaped ready-mix transit mixer to the less-common but growing in popularity volumetric mixer, essentially a concrete plant on wheels. How cement mixers work and why they work the way they do is a fascinating combination of old and new technology. You'll never see a cement mixer the same way again.

But before we begin, 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.

As technology has changed, so has the basic mixer design. While many transit mixers still have rotating drums, most don't simply pick up a load of wet cement and transport it. The few that still do head mostly to road sites where it's possible to pour the mixture immediately.

Most transit mixers have a separate water tank in the truck. The spinning drum keeps the dry ingredients, aggregate and cement mixing during most of the trip. When the driver is within a few miles of the site, water is added to create concrete fresh for delivery.

This is considered "batch" delivery of ready-mixed concrete, mixing ingredients off site and trucking them where they're needed. Advances in technology have made it possible to mix concrete at the job site, though transit mixers are still the workhorse of the field.

Volumetric and metered mixers are becoming more common. Both types are essentially on-site custom concrete plants. Separate holding tanks of aggregate, cement and water are contained in one truck with a computer hooked to augers and pumps. At the site, the customer can order a specific type of concrete (there are more than a dozen) that can be mixed by the truck.

Volumetric and metered mixers are often used during high-rise construction and can be paired with pumper trucks to deliver concrete more than 15 stories above the ground.

Concrete's Toll on the Mixer

While concrete comes in bewildering array of types, one thing is certain -- it's heavy. A large batch of concrete can weigh more than 30,000 pounds (13,608 kilograms), not counting the weight of the truck itself, anywhere from 10,000 to 30,000 pounds (4,536 to 13,608 kilograms). For a truck to haul that weight, it has to be powerful. And to get that load over the rough terrain of a construction site, the truck has to be tough.

The trucks come in three separate parts -- engine, frame and mixer. Most truck companies provide the engine and frame, with amenities ranging from sleeper cab to computer navigation. The mixer, or volumetric plant, is added on at a later time. The mix-and-match approach to building trucks is aimed at giving a company -- spending anywhere from $30,000 to more than $100,000 -- a new truck built to order. Each company has specific wants and needs and requires a truck tailored to those. For example, some may need a truck with a heavier engine and a lighter drum, which could be removed at a later time and turned into a trash hauler with a few modifications.

Most truck engines range from 250 to 300 horsepower, depending on the application. Some companies offer engines with more than 400 horsepower. Horsepower is a measure of power, an engine's "oomph," in other words. The "oomph" is usually supplied by a diesel engine, most commonly manufactured by Cummins or Caterpillar. Diesel engines produce more torque at lower engine RPM than a similar gas engine, making them ideal for low-speed, high-power applications like towing or hauling. Diesels are also preferred for their longevity -- many can go for a million miles (1,609,000 kilometers) or more with routine maintenance -- as well as their ruggedness.

Unlike gas engines, diesel engines operate using compression ignition and require a heavy engine block to withstand the tremendous forces at play inside them. That same compression ignition means the engine function with a higher compression ratio within the cylinders, thereby producing more power. That power is translated to torque, or rotational power, through special gearing in the transmission -- mixers have anywhere from 7 to 18 gears and can be manual or automatic, and differentials.

Most concrete trucks produce anywhere from 1,000 to 3,000 foot-pounds of torque. What this means, in layman's terms, is a concrete truck will never beat a street car at a quarter-mile race, but it will be able to break through the concrete crash barrier at the end without a blip in engine RPM and keep going.

Gas engines develop torque at higher RPM than diesel engines. Anyone who has ever towed a boat or trailer behind a gas-powered vehicle has experienced the need to press the gas pedal to make it up a hill. Diesel engines, because of their design, actually produce better torque at lower revolutions per minute. So slowing on a hill actually provides more torque.

But even a truck with the most power and torque can't control a 60,000-plus-pound load with ease. That's where the truck's axles come in, and these play a larger role than simply keeping the wheels and tires in their correct places.

Most of the newer trucks are equipped with live axles. Live axles are generally non-drive axles and are equipped with air brakes. Those brakes can be used to help steer the truck. There are usually at least three axles behind the cab, though some larger volumetric mixers can have up to six. Some of those axles are lifted and lowered to help distribute the weight of the truck and load.

The defining factor in truck type is a given state's bridge laws. Each state puts bridge crossing weight restrictions on construction vehicles. Contractors who break those laws face fines and penalties, and each state has different legal requirements, including many that require a bridge axle, or an extra axle off the back of the truck used to further distribute weight when making crossings.

Types of Cement Mixers

All mixers leverage power, torque and weight distribution to get the concrete to the site, but getting it out of the truck is where you'll see differences in truck designs.

Most of the mixer fleet, especially older batch model trucks, uses a simple tip-and-pour method to get the concrete out of the mixer. A chute attaches to a port and the concrete oozes (or pours, depending on its consistency) out of the mixer to the project. Usually, the driver of the truck operates the machinery and aims the chute. Many transit mixers are coupled to a hydraulic lift bed that can tip up the drum, similar to a dump truck, if needed.

Other trucks, many of them volumetric and the newer transit mixers, use a pump to move the concrete from the truck to the project. The pumps, usually reciprocating piston pumps, can be mounted on the front or the rear of the truck. Having the pump in front allows the driver to maneuver to a section of the work site and direct the concrete from inside the cab. The controls can be mechanical, electro-mechanical, hydraulic or purely electronic. Newer trucks are employing more on-board computers to monitor pumps and other components of the mixers.

Before the concrete is pumped or poured, a number of simpler machines act in concert to keep the concrete from setting, and even mix the concrete at the site. Some of the older portions of the fleet, like early mixers, used paddles to stir the concrete and keep it from "settling out," or separating into its component pieces. This technology has been largely replaced by the use of augers and fins. Inside a traditional batch mixer is a concentric series of fins with a slight corkscrew pattern. The direction of the drum's spin squishes the wet concrete into the back of the mixer. When the mixer arrives at the site, the driver reverses the direction of the machine to push it out of the mixer to the chute. From there, gravity does the rest.

Volumetric mixers use augers to move concrete. These are similar to the blades in the batch mixer but smaller. Inside the mixer, an operator feeds data into the mixer and several augers feed aggregate and cement together. Water is added to the mix and larger augers blend the components.

Challenges to Maintaining Cement Mixers

Concrete is so viscous and sets so quickly that it leaves residue after every load. Eventually someone with a jackhammer must climb into the drum to clean it out.
Concrete is so viscous and sets so quickly that it leaves residue after every load. Eventually someone with a jackhammer must climb into the drum to clean it out.
iStockphoto/Thinkstock

"Breaking up is hard to do" is more than a song lyric to concrete-truck owners. The simple truth of the matter is, once the last of the concrete trickles out of the drum or dribbles out of the pump, the truck isn't empty. A small amount of concrete always remains inside the truck, especially in drum mixers. You can wash the truck out, run water through the drum while it rotates and even pressure-wash the inside. But some concrete is always left. And it dries. The next trip a little more is left. And it dries. Over time, this becomes a problem, reducing the volume of the mixer and its capacity.

Several companies offer pre-treatments, chemical and acid baths and high-pressure washes for drums and interiors. But eventually, the concrete will build to a point where the only viable alternative is mechanical removal. Many smaller companies handle this on their own, sending in a worker with a jackhammer to chip out the hardened concrete. This job, largely unknown outside of the construction field, got its 15 minutes of fame during an episode of the Discovery Channel's "Dirty Jobs" series, which sent host Mike Rowe to work with Jim's Chipping Service, one of the few companies specializing in cleaning hardened concrete.These highly specialized vehicles are essential to the construction business and it doesn't look like they're going away any time soon. But what will cement mixers look like in the future? Read on to find out.

These highly specialized vehicles are essential to the construction business and it doesn't look like they're going away any time soon. But what will cement mixers look like in the future? Read on to find out.

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 [source: Modern Contractor Solutions].

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.

IVECO Trucks of Australia is working to perfect a compressed natural gas (CNG) engine for heavy truck use. The CNG trucks have comparable mileage, but release 40 percent less carbon dioxide than diesel and the natural gas is less expensive than diesel fuel [source: IVECO]. Like many of the new generation engine systems, CNG is paired with state-of-the-art electronics designed to get the most out of every tank of fuel.

For more on construction equipment and related technologies, get the concrete facts from the links on the next page.

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Sources

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  • Flesher, Patti, spokesperson for the Portland Cement Association. Personal interview conducted Oct. 15, 2008.
  • Giroux, David, director of communications for the Peterbilt Motors Corporation. Personal interview conducted Oct. 15, 2008.
  • HIT Entertainment. "Bob the Builder. 2008. (Jan. 25, 2012) http://www.bobthebuilder.com
  • HIT Entertainment. "Thomas & Friends." 2008. (Jan. 25, 2012) http://www.thomasandfriends.com/usa/Thomas.mvc/Home
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  • Jackson, Kirk, owner of Oregon-based Tanner Equipment Corporation. Personal interview conducted Oct. 14, 2008.
  • Modern Contractor Solutions. "Volumetric Mixers: The Green Solution for Concrete." September 2008. (Jan. 25, 2012) http://www.moderncontractorsolutions.com/articlesdetail.php?id_articles=334&id_artcatg=3
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  • Peterbilt Motors Company. (Jan. 25, 2012) http://www.peterbilt.com
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  • Truck Mixers Manufacturers Bureau. (Jan. 25, 2012) http://www.tmmb.org
  • Volumetric Mixer Manufacturers Bureau. (Jan. 25, 2012) http://www.vmmb.org