a concrete mixer is the laison between the concrete batch plant, and the customer where the pour is taking place. you have 90 minutes to get it there.

The concrete mixer is one of those machines that seems simple and ubiquitous, and yet it is a marvel. The machine is the go between from the batch plant where it is mixed, to the customer who will pour the slab and let it set up. The machine “suspends” the setup of the concrete by continually mixing the material in the drum located behind the driver. On paper, this concept makes sense, and seems simple. In the real world, the weight of the concrete, along with an additional 300 gallons of water (1,300 liters) to keep everybody happy, the combined weight of concrete and water can approach 40,000 lbs. (18,000 kg), with a combined weight of truck and material that can reach 72,000 pounds (32,600 kg).

The constant rotation of the drum while after the material has been accepted, along with adding water at the appropriate time is what keeps the concrete from setting up. 90 minutes is considered the maximum for travel time, and anything less than that is preferred. Once the truck appears at the required location, the material is fed through the discharge chute for final placement. The mixing drum, which was spinning to keep the materials from setting up, is reversed, and the concrete is pushed to eject through the chute.

In the United States, the size of concrete mixers is usually limited by bridge weights. There are states, counties and municipalities that limit what weights are allowed on specific bridges in specific areas. The suspended axles that you see in the photos are referred to as bridge axles, that help distribute the weights over the truck. Most newer mixers have live axles with air brakes to handle the massive weight that these vehicles carry. The frames and tires also address the heavy duty nature of transported materials, and the sites where they discharge.

Just to be clear, most mixers do not pick up a load of wet mixed” cement, and drop it off at the jobsite - this may happen, but the jobsite has to be very close to the spot where the material is needed. It is more common for the truck to be loaded with dry mix, spin the dry ingredients while transporting, and then add the water that is in the truck at the appropriate time during the transport trip, creating a batch ready to pour when the truck is at the jobsite.