How Cement Mixers Work

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.