Silos do not have compartments, so they either come preloaded with the proper concrete mix or the user dumps bagged mix into them. Silos are typically loaned by the company providing the concrete mix. Bagged concrete mix can also be placed directly into the mixer without using a silo. To operate a silo mixer, the blades are engaged and the operator puts about half of the required water into the mixer before dumping in the concrete mix. More water is then added to get the right consistency. The process takes at most a few minutes before the concrete is ready to place.
Stone Equipment's silo mixers are hydraulic and come in 12- and 20-cu.-ft. capacities. These mixers have an offset or high dump, so the mixer can discharge to a pump, wheelbarrow or material placement system at heights up to 56 in.
Easy to learn and operate
Learning to make concrete with these mixers is fairly simple. "Normally when we sell a unit, we send a technician to spend a day or two to train them," Grant says. The operator needs to be able to recognize what good concrete looks like, although the only way to be certain the mix is right is to do lab-based strength tests.
Global uses a similar procedure, talking with customers in advance to find out what type of mixes they use so they have those mix designs prepared on the computer. The trainer calibrates the machine to determine the gate settings for the desired mix design, which takes about 90 minutes, and then does a yield test on 1 yd. of new concrete.
"We suggest that every new operator do a strength test so he has it on record," Horning says. The trainer then observes the customer doing a real job to be sure they are doing it correctly and in a manner appropriate for the conditions of the materials.
Stone supplies operator manuals in addition to having technical service and customer service departments. Its products are sold through authorized dealers who provide direct training to their customers.
Portable ready-mix batch plants are another alternative to waiting for ready-mix deliveries, but they don't save as much money as volumetric mixers. "A mobile batch mixer makes concrete and delivers it, while a ready-mix batch plant requires both an operator and a driver, so a mobile batch mixer saves at least two jobs," points out Grant. He estimates that producing concrete with a mobile batch mixer costs about half the retail price of ready mix concrete in the local area.
"We at Global have always thought that rather than being competitive with regular drum trucks, our mixers should be complementary," Horning points out. "The drum truck is designed to dump 10 yds. as quickly as possible and they do a great job. The mobile mixer is designed for versatility and to address short loads. Generally speaking, a ready-mix contractor who does 20 percent of his business in short loads will make money with a mobile mixer."
"Where they save money is really in the time and the cost of the ready mix service," says Varel. "If you're going to use a whole truck, hands down, that's probably the way to go. But if you need a partial truck or have a specific time frame for doing the work, one of these mixers is better."
Rent or buy?
Mixers are often sold to customers other than contractors who then rent them out. Determining whether it is more cost-effective to rent or purchase a mobile mixer depends on how much it will be used.
"If a contractor is processing at least 3,000 yds. of concrete a year, the investment in one of these becomes very attractive," Horning says. He recommends that before a contractor buys a mobile mixer, they should make sure of the availability of materials and research what the cost of those materials will be compared to buying premade ready-mix.
Using volumetric mixers saves contractors money because they don't pay workers to wait for concrete and they can buy cement and other components in bulk, notes Grant.
"With volumetric mixers, contractors can control their own business schedule," Horning says. "They don't have to wait around. They can start working at any time. The machine is very versatile. It has a simplicity of operation. The concrete can be accurately metered. If you're comparing it to a large batch plant, it has a lower initial investment cost. Contractors will buy these things either to start a retail business for 3,000 yds. and up annually or if they are doing a lot of concrete work themselves. If they want to control their own destiny and put more money in their pockets, these mixers will make sense for them."