Buzz
Cement mixers are crucial for various construction tasks, and they need to arrive swiftly to ensure the concrete is poured before it begins to set.
iStockphoto/ThinkstockMain Points to Remember
- Cement mixers are indispensable for construction, combining heavy-duty machinery, powerful engines, and durable design to transport concrete. These trucks typically have engines ranging from 250 to 300 horsepower.
- The mixer itself, frequently a volumetric plant, is custom-built to suit the truck’s engine and chassis, providing a flexible solution to meet specific business needs.
- A cement mixer’s operation heavily depends on its axles and braking system, which are essential for managing the substantial weight of the load.
While most construction machinery is easy to understand—cranes lift, dump trucks load and unload, bulldozers push, and graders level—the cement mixer stands out as the one machine that’s often misunderstood. It's adored by children, despised by impatient drivers, and puzzling to anyone who hasn't had the experience of operating one of these massive, 30,000-pound (13,608-kilogram) giants.
Concrete has existed in various forms long before the Romans constructed the Appian Way, but the transit mixer is a product of the 20th century. Regardless of its modern origins, it's clear that the mixer is here to stay.
The confusion begins with terminology. What many refer to as a cement mixer is actually a concrete mixer in the construction world. These mixers come in a wide range of types, sizes, and configurations, each designed to handle the various tasks required of them. This versatility means the machine adapts and transforms as the demands of the concrete industry evolve.
In this article, we will explore the main types of mixers, from the classic drum-shaped ready-mix transit mixer to the increasingly popular volumetric mixer, which functions as a mobile concrete plant. Understanding how cement mixers operate and the reasons behind their design reveals a fascinating mix of both traditional and modern technologies. After reading this, you'll view cement mixers in a whole new light.
Before diving in, let’s first clarify the distinction between cement and concrete. To put it in baking terms, the difference between concrete and cement is akin to the difference between flour and a loaf of bread. Concrete is a broad term that refers to a mixture of aggregates — typically stone or gravel, water, and cement. Modern cement is a complex mix of finely ground minerals, commonly referred to as ‘portland’ cement. Concrete is created by combining these three ingredients in a mixer, whether it’s stationary or mobile, with water being absorbed by the cement to bind the aggregates together and form concrete.
History of Mixers
In 1916, Stephen Stepanian invented and filed for a patent on the first motorized transit mixer, aiming to replace the cumbersome horse-drawn mixers of the time. These old mixers used wooden paddles to churn the mixture as the wheels turned, but they were slow and inefficient. However, the same could be said about engines and trucks back then. By the 1940s, engine and truck-frame technology had improved enough to build a rugged vehicle capable of hauling tons of wet, or unset, concrete. During the post-World War II construction boom, mixer trucks truly came into their own.
Today's large drum mixers on the road are not far from Stepanian's original vision for an improved concrete hauler. Modern mobile transit mixers combine an engine, a truck frame, and a rotating mixer in a versatile setup. These mixers are much larger than the ones used on construction sites but work similarly. A powerful motor independent from the engine rotates the drum mounted on the truck, and blades or a screw powered by the motor keep the aggregate, water, and cement moving. This motion prevents the premixed concrete from setting, though the delivery time is always a race against the clock. Most cement manufacturers recommend that the time between mixing and pouring doesn't exceed 90 minutes, ideally even less than an hour.
As technology has progressed, the basic design of transit mixers has evolved. While many still use rotating drums, modern mixers don't just pick up wet cement and transport it. A few mixers still follow this process, mostly for road projects where the concrete needs to be poured immediately.
Most transit mixers are equipped with a separate water tank. Throughout the trip, the spinning drum continues to mix the dry ingredients — aggregate and cement. As the truck nears the job site, water is added to create fresh concrete for delivery.
This method is known as "batch" delivery of ready-mixed concrete, where ingredients are mixed off-site and transported to the job site. Although advancements in technology now allow for mixing concrete directly at the job site, transit mixers remain the primary workhorse in the industry.
Volumetric and metered mixers are increasingly prevalent, essentially functioning as on-site custom concrete plants. These trucks feature individual compartments for holding aggregate, cement, and water, all managed by a computer system connected to augers and pumps. At the job site, customers can request a specific type of concrete (there are more than a dozen varieties) to be mixed right on the truck.
Volumetric and metered mixers are commonly used in high-rise construction, often paired with pump trucks to deliver concrete as high as 15 stories or more above the ground.
Concrete, a mixture of binder, aggregate, and water, dates back to the Egyptian pharaohs' time when mortar made from water, sand, and lime was used to build sections of the pyramids. The Romans, too, employed a form of concrete for building their aqueducts, the Coliseum, and other significant structures. In the 1800s, scientists began refining hydraulic cement, which remains in use in construction today. The term Portland cement comes from the Isle of Portland, off the English coast, where the mineral components used in modern concrete were first isolated. By 1908, Thomas Edison began experimenting with precast concrete houses in Union, NJ. Many of these homes still stand and are in use today. Additionally, the Hoover and Grand Coulee dams, built in 1936, continue to be monumental feats of engineering.
Concrete's Toll on the Mixer
Concrete comes in a vast range of types, but one thing remains true – it’s heavy. A large batch of concrete can weigh over 30,000 pounds (13,608 kilograms), excluding the truck's own weight, which ranges from 10,000 to 30,000 pounds (4,536 to 13,608 kilograms). To carry this immense weight, the truck must be powerful, and to navigate the challenging terrain of a construction site, the truck needs to be durable.
Concrete trucks consist of three primary components: the engine, the frame, and the mixer. Typically, the truck's engine and frame are provided by the manufacturer, and these can come with features such as a sleeper cab or advanced computer navigation systems. The mixer, often referred to as a volumetric plant, is added separately at a later stage. This modular approach allows companies to create custom-built trucks tailored to their specific needs, with costs ranging from $30,000 to over $100,000. For instance, some businesses may opt for a heavier engine paired with a lighter drum, which can later be repurposed as a trash hauler with some modifications.
Truck engines generally produce between 250 and 300 horsepower, depending on their intended use, although some manufacturers offer engines with over 400 horsepower. Horsepower measures an engine's power output, or its 'oomph.' Diesel engines, such as those made by Cummins or Caterpillar, are most commonly used. Diesel engines generate more torque at lower RPMs than gasoline engines, making them ideal for heavy-duty applications like hauling and towing. These engines are also known for their durability, often lasting over a million miles (1,609,000 kilometers) with proper maintenance, thanks to their robust construction.
In contrast to gasoline engines, diesel engines rely on compression ignition, requiring a sturdier engine block to handle the immense forces inside them. This compression ignition also means that diesel engines operate with a higher compression ratio, resulting in greater power output. The generated power is then converted into torque through specialized gearing in the transmission. Concrete mixers typically feature between 7 to 18 gears, which can be either manual or automatic, along with differentials to help transfer the torque efficiently.
Concrete trucks typically generate between 1,000 and 3,000 foot-pounds of torque. To put this into perspective, while a concrete truck may not win a quarter-mile race against a sports car, it can easily smash through a concrete barrier without a noticeable drop in engine RPM and keep moving forward.
Gasoline engines produce torque at higher RPMs compared to diesel engines. Anyone who's towed a boat or trailer with a gas-powered vehicle has likely had to press the accelerator harder to climb a hill. In contrast, diesel engines excel at delivering torque at lower RPMs, meaning that easing off the accelerator while going uphill actually helps generate more torque.
Even the most powerful trucks with high torque cannot effortlessly manage a 60,000-pound load. This is where the truck's axles come into play, serving a critical role beyond just keeping the wheels and tires in place.
Many modern trucks come with live axles, which are typically non-drive axles fitted with air brakes. These brakes aid in steering the truck. Most trucks have at least three axles behind the cab, with some larger volumetric mixers featuring up to six. Certain axles can be raised or lowered to better distribute the weight of the truck and its load.
The type of truck is often determined by state-specific bridge laws. Every state imposes restrictions on the weight of construction vehicles when crossing bridges. Contractors who violate these laws face fines and penalties, with each state having its own regulations, including the need for a bridge axle – an additional axle at the rear of the truck that helps distribute weight while crossing.
The rumbling and grumbling of mixer trucks, along with the near-magic of pouring cement, has made them a staple in several children's shows and toys. A standout example is Tumbler, a yellow and green truck from the "Bob the Builder" series. Patrick, another transit mixer, made a brief cameo in the "Thomas and Friends" videos, based on the Thomas the Tank Engine character. Patrick claimed to be the most important tool on the construction site. In these stories, the machines learn to collaborate to accomplish the task, proving that no one piece of equipment is more important than the others.
Different Types of Cement Mixers
While all mixers rely on power, torque, and weight distribution to transport the concrete to the site, the real difference in truck designs is seen when unloading the concrete from the truck.
The majority of older batch model trucks in the mixer fleet use a simple tip-and-pour mechanism to discharge the concrete. A chute is connected to a port, allowing the concrete to flow (or pour, depending on consistency) from the mixer to the job site. The truck driver typically operates the machinery and directs the chute. Many transit mixers are attached to a hydraulic lift bed, which can tilt the drum, similar to a dump truck, when required.
Many newer volumetric mixers and transit mixers now employ pumps to transfer concrete from the truck to the work site. These pumps, typically reciprocating piston pumps, are often mounted at the front or rear of the truck. A front-mounted pump allows the driver to navigate to a specific section of the site and control the flow of concrete from within the cab. The controls can range from mechanical to electro-mechanical, hydraulic, or entirely electronic. More advanced trucks are integrating on-board computers to oversee pumps and other mixer components.
Before the concrete is poured or pumped, various simpler machines work together to prevent the concrete from setting prematurely and even mix it on-site. In older mixers, paddles were used to stir the concrete and prevent it from "settling out" or separating into its individual components. This method has largely been replaced by augers and fins. Inside traditional batch mixers, a series of concentric fins with a corkscrew design help move the wet concrete toward the back of the mixer. Upon arrival at the site, the driver reverses the drum’s spin to push the concrete out via the chute. Then, gravity takes over to finish the job.
Volumetric mixers rely on augers to move the concrete, similar to the blades used in batch mixers but smaller in size. Inside the mixer, the operator inputs data, and multiple augers blend aggregate and cement. Water is added to the mixture, and larger augers continue to mix the components thoroughly.
The ancient Greek polymath, Archimedes, is often credited for inventing a pump that utilized a large helical screw rotating within a closed tube to elevate water from a source to the desired location. This screw mechanism operates as a form of inclined plane, which is one of the basic machines, spiraled around a central axis. This invention paved the way for the development of augers for moving dry materials, and the spiral fins used in corkscrews to direct liquids. These technologies are now commonly applied in industries like concrete mixing at plants and in transit mixers.
The Struggles in Maintaining Cement Mixers
Concrete's thick consistency and quick setting nature often leave behind residue after each use. To remove it, workers must climb into the drum with a jackhammer for cleaning.
iStockphoto/Thinkstock"Breaking up is hard to do" is more than just a song lyric for owners of concrete trucks. The reality is that even after the last bit of concrete exits the drum or pump, the truck is not completely empty. A small amount of concrete always remains, especially in drum mixers. Efforts to clean the truck, such as rinsing the drum with water while it rotates or using a pressure washer, still leave some residue behind. Over time, this residual concrete builds up, reducing the truck's capacity and causing more issues with each subsequent load.
Various companies provide solutions like chemical treatments, acid baths, and high-pressure washes for cleaning out the drums. However, when the concrete buildup becomes too much, mechanical removal is the only remaining option. Many smaller companies deal with this by sending in workers with jackhammers to break up the hardened concrete. This niche job gained wider recognition through an episode of the Discovery Channel's "Dirty Jobs" series, featuring host Mike Rowe working with Jim's Chipping Service, a company that specializes in removing the hardened material. These highly specialized trucks play a critical role in the construction industry and are unlikely to disappear anytime soon. But how will cement mixers evolve in the future? Keep reading to find out.
These highly specialized vehicles play a critical role in the construction industry, and they are unlikely to be phased out anytime soon. But what does the future hold for cement mixers? Keep reading to discover more.
The Future of Cement Mixers
The future of the concrete mixer remains uncertain. Like many sectors, the concrete industry is transitioning to more sustainable practices. Manufacturers of volumetric concrete mixers claim their vehicles are more eco-friendly, as they mix materials on-site—eliminating the need for trucks to run their engines to keep the concrete from hardening. This not only conserves fuel but also ensures that only the necessary amount of concrete is mixed, reducing material waste and preventing the dumping of excess concrete [source: Modern Contractor Solutions].
Peterbilt, a truck manufacturer, is exploring the use of compressed air "push" systems for trucks. In this system, the truck is powered up with compressed air, and once it reaches a certain speed, the diesel engine activates. Since it takes more fuel to overcome inertia when starting an engine, using relatively inexpensive compressed air to get the truck moving offers potential fuel savings in the long run. Currently, this technology is primarily being developed for inner-city delivery vehicles rather than larger ones like cement mixers.
IVECO Trucks in Australia is focusing on perfecting a compressed natural gas (CNG) engine designed for heavy-duty trucks. While these CNG trucks offer similar mileage to their diesel counterparts, they emit 40% less carbon dioxide, and the natural gas used is cheaper than diesel fuel [source: IVECO]. Like many new engine systems, CNG technology is paired with advanced electronics to optimize fuel efficiency and ensure maximum performance from each tank of fuel.
