Stephen Stepanian developed and patented the first motorized transport mixer in 1916 to replace the horse-drawn concrete mixers in use at the time. As the wheel turned, wooden paddles stirred the mixture, but this design was of limited use—it was cumbersome and slow. However, the same can be said for engines and trucks during this period. But by the 1940s, engines and truck frame structures met the need for rugged vehicles capable of hauling thousands of pounds of wet or unset concrete. With the post-World War II construction boom in full swing, the mixer truck came into being.
The large drum mixers you see on the road today haven’t changed much from Stepanian’s vision of a better concrete hauler. Mobile transport mixers are a mix-and-match choice of engines, truck frames, and rotary mixers. Mixers are similar to small mixers found on construction sites, but on a larger scale. A large motor, separate from the engine, spins a drum on the car body, and a series of blades or screws driven by the same motor keep the aggregate, water and cement in constant motion. This prevents the ready-mix concrete from setting, even though the clock is constantly ticking to deliver loads to construction sites, road sections or parking lots. Most cement manufacturers recommend keeping the time between mixing and pouring to a maximum of 90 minutes. It would be better to have it on site in less than an hour.
As technology changes, so does basic mixer design. While many transport mixers still have rotating drums, most don't simply pick up a load of wet cement and transport it. The few that still travel mainly to road stations where the mixture can be poured immediately.
Most transport mixers have a separate water tank on the truck. The rotating drum keeps the dry ingredients, aggregate and cement mixed during most of the stroke. When drivers are within a few miles of the site, water is added to create fresh concrete for delivery.
This is considered a "batch" delivery of ready-mix concrete, where the ingredients are mixed off-site and transported to where they are needed. Advances in technology have made it possible to mix concrete on the job site, although transport mixers are still the workhorses of the field.
Volumetric and metering mixers are becoming increasingly common. Both types are essentially custom-built concrete plants on site. Separate storage tanks for aggregate, cement and water are loaded onto a truck, and computers are connected to augers and pumps. On site, customers can order a specific type of concrete (there are more than a dozen) that can be mixed by truck.
Volumetric and metering mixers are commonly used in high-rise building construction and can be used with pump trucks to transport concrete more than 15 floors above the ground.
A Concrete Past
Some form of concrete—a mixture of binders, aggregates, and water—has been around since the days of the Pharaohs in Egypt, when water, sand, and Lime was mixed and used as mortar for the building parts of the pyramid. The Romans also used a type of concrete when building aqueducts, stadiums, and other major buildings. Scientists in the 1800s began experimenting and perfecting hydraulic cement, which is what is still used in the construction industry today. Portland cement takes its name from the Isle of Portland off the coast of England, where deposits of the mineral components used in modern concrete were first discovered and chemically separated. By 1908, Thomas Edison was experimenting with building precast concrete homes in Union City, New Jersey, and most of these homes were still in use. Then there are the famous Hoover and Grand Coulee Dams, built in 1936 and standing among the wonders of the world.