Advice for Portable Concrete Mixing Plants
When planning to produce concrete for a paving project, there is one key thought: How can I get the most yards of concrete out of my batch plant? Or, what can I do to keep the paver moving faster?
But long before the first load drops into the dump track, plant managers must consider many details. These might seem minor, but you’ll find your production capacity limited by overlooking these important considerations.
Frank Kozeliski knows a thing or two about concrete plants. He regularly erected and ran portable plants for many jobs, including road paving, when he owned Gallup Sand & Gravel in Gallup, N.M. Kozeliski remains a concrete industry consultant and serves on many ACI committees. He offers advice on setting up and running portable plants.
Know the rules
There is a lot of investigative work to do before submitting your bid. It’s important to read through the bid documents to check who is responsible for obtaining all of the necessary air quality, water discharge, and washout permits for cleaning trucks and other equipment. This is extremely important if you are planning on setting up the plant on a site that is not included in the primary work area. In some jurisdictions, this process could take up to one year.
Check out load restrictions on the access roads that will lead into the plant area. Normally, your hauler or aggregate supplier can be a huge help in determining the best hauling pattern. Also, review local utility right-of-ways.
If you plan to work at night or on weekends, make sure there are no local noise restriction ordinances. This is especially true if you plan on refilling stockpiles or paving. Be wary of neighbors who will not want noisy trucks nearby. These people can be fickle. After Gallup set up one portable plant, Kozeliski recalls having to satisfy the concerns of one nearby homeowner who claimed the noise was preventing his chickens from laying eggs.
Make sure you arrange for adequate supply and rate of flow of water. Most experts recommend that you need to plan for 50 gallons of water for every yard of concrete. But it’s not only the quantity, it’s how fast you can replenish your holding tanks. Don’t assume water is nearby and available at the pressure and flow rate you will need at peak operations. Gallup managers often installed an additional water tank near the plant. And unless you are connected to a municipal suply, you should consider using a water strainer, which is necessary to keep rocks or rust from damaging the plant’s water meter.
Adequate electric service is always an important consideration. For short runs, securing line service is too expensive. Kozeliski preferred operating his portable plants with portable generators. “We would just bring in our own generator to eliminate obtaining permits for additional service,” he says. Given the cost of fuel, you may want to secure a smaller auxiliary generator set to operate at night for site lighting.
And no plan is complete with concerns about stockpile space. Make sure you have enough space as to stockpile at least a two-day supply of aggregates and cement. Also be ready to spend some time and money to create a sturdy and well-sloped subgrade on which to store materials. For instance, putting sand on top of a clay-like surface will result in water infiltrating the sand through capillary action. “We learned that the hard way,” Kozeliski says. “We used a cement-treated base and compacted it.”
Make sure to include security as part of your prebid planning. Don’t leave a portable plant exposed and out in the open. Install a fence around it to keep animals away and to prevent children from playing on the stockpiles.
Feeding the plant
When operating a fast portable plant, it is imperative “to get the materials to the plant quick enough so you can get the material into the batcher,” Kozeliski says. “When operating at 300- or 400-yard-per-hour production rates for a full shift, you have a logistic challenge of getting materials, such as cement and fly ash, into and through the plant.”
Start with cement. Make sure cement pigs, or storage tanks, have enough storage capacity to operate the full shift. These are pressurized units, so develop a plan with your bulk hauler on how best to keep them refilled. Off-hour deliveries can ensure that outflow rate is consistent. Try to place the units close to the batcher to minimize air pressure or line loss so that the fill rate is correct.
Today’s portable concrete paving plants are designed to be operated at a consistent rate. “With the new computers and sensors, plants run best when feed rates are consistent,” he says. “If the computer senses the material feed rate into the weigh hopper is constant, you get really good accuracy on the batch tolerance.”
Even though most plants produce only one mix design at a time, Kozeliski suggests erecting three- or four-bin aggregate capacity plants. He says the setup may help you use combined aggregate gradation. The additional aggregates can help reduce mix variation and reduce the amount of cement used for the mix.
“For high-capacity plants, you probably want a conveyor with a feed hopper for each overhead bin,” he says. “So you just keep filling these bins up, and you have automatic sensors. As the material drops down into the overhead bins, the conveyor automatically turns on and fills it back up.”
Finally, when the plant is removed from the jobsite, “clean everything, level it off, and plant some grass seed,” Kozeliski suggests. “Make it all go back to nature, so it looks nice and all of the neighbors are happy.”
Consider donating any leftover material to the local community. This will create goodwill and make it easier if you must return for another job in the same area.