Belt scales with weigh idlers and variable-speed drives in the weigh bridge for this portable roller-compacted concrete plant adjust the production rate to precisely blend aggregate.
Astec officially entered the concrete business in December 2008 with formation of its Concrete Products Group around modular applications of its twin-shaft mixer, a plant capable of producing low-slump and conventional concrete in a continuous process.
Astec's concrete heritage is surprisingly deep. In the 1970s, Barber Greene adapted some of its twin-shaft asphalt mixers for customers specifically to mix cement-treated base (CTB). Those Barber Greene operations were purchased by Astec. In the early 1980s, customers asked the group to develop a similar plant around a twin-shaft mixer to produce the stiff permeable-concrete mix for the Disney's Epcot-Center parking lot in the Orlando swamplands. Shortly afterward, the same plant was used to produce the concrete that paved The Port of Los Angeles.
Astec's version of the concrete plant produced in a continuous process.
"From that point forward, Astec continued to build on order that CTB plant to meet customers' needs," says Jim Johnson, director of sales for Astec's Concrete Products Group. He says the twin-shaft mixer became the machine of choice for mixing stiff roller-compacted concrete.
As world competition for concrete recently has driven up the material's price, contractors began to ask Astec for a twin-shaft, continuous-process mixer that can make conventional concrete as well as stiffer roller-compacted and permeable concrete mixes.
"They can't afford to have a tilt-drum mixer for conventional concrete and a twin-shaft mixer for roller compacted concrete," says Johnson.
Astec's modular systems design offers the mixer, weigh bridge, self-erecting silo, aggregate bins, screen decks and other pieces to configure complete plants for Portland cement concrete, cement treated base, or roller compacted concrete.
The continuous process makes use of Astec's experience with using belt scales and variable-frequency conveyor drives and control systems. The smaller twin-shaft mixer only holds 2 to 4 yards, compared to traditional batch plants that can mix up to 10 yards at a time. If the plant must be shut down, there is less waste in the mixer.
Johnson claims the twin-shaft mixer incorporates water and cement in high-slump mixes thoroughly enough that the amount of cement in the mix can be reduced without fear of falling short of compressive-strength specifications.
Single-mixer plants are available with capacities from 80 to 350 cubic yards, and dual-mixer arrangements are available. Portable plants transport in three or four loads.
Continuous-blending dry concrete plants, aggregates from individual bins are placed on the fully shrouded conveyor in layers, allowing dry cement to be spread in between. At discharge, a water distribution system delivers a continuous curtain of water, sprayed around the falling aggregates to wet the materials uniformly.
