Most precast producers have tried to use self-consolidating concrete (SCC). They recognize that SCC's fast-flowing, yet segregation-free, rheology can lead to faster placement and defect-free surfaces.
Even so, SCC has one operational drawback. Transporting this liquid-like material from mixer to form can be a challenge. Most precast operations were engineered to handle fresh concrete with about a 4-inch slump. Plant foremen are spending countless hours searching for methods to convert overhead crane and mobile delivery systems originally designed for a stiffer mix to carry material with a 23-inch spread.
That's why a new SCC delivery system unveiled for worldwide release at bauma earlier this year could be of interest. Autocor offers a spill-free transportation solution to a precast plant's automation puzzle when using SCC on a large scale.
This self-contained placement system was originally designed about two years ago for a large construction project in northern Germany. The producer had to precast multiple sections of the tower bases that, when combined together, would support wind vanes. Since the forms were rather tall, the producer wanted an SCC placement method that avoided the costs of establishing an overhead crane or delivery bucket system.
Putzmeister engineers solved the placement problem by combining several well-proven components from their pump and placement toolbox into a single production unit. Putzmeister's Autocor uses a rotor pump, a 23-foot agitator drum, and an optional 65-foot overhead boom to efficiently place SCC at 78 cubic yards per hour.
They installed their control system for the concrete placing boom. With it, the pouring crew was outfitted with a placement control system equipped with the safety and production features found on large mobile concrete pumps. The automation allows the pouring crew to focus on the filling process. The system is easy to learn to operate, as the laborer controls all movement with a joystick remote control.
This type of automated placement system offers producers several ways to increase productivity and quality when using SCC.
The first and perhaps greatest benefit is the unit provides a storage buffer of fresh material between the mixer and the pour. The drum mixer rotates slowly and agitates just enough to help keep the SCC in its proper rheological form. This extends storage life. The mixing action also can help blend in different batches of the same mix, eliminating problems associated with leftovers.
Another important benefit is the holding hopper can maximize cranes or mobile transport usage. Deliveries can be scheduled for full loads, rather than short make-up loads. Mixers can also be charged at full-load capacities each cycle time.
The pouring machine eliminates a good deal of process waiting time, increasing crane utilization. Cranes no longer need to hover over forms waiting for the bucket to empty. Crews can establish their own pour rates regardless of form stripping activity.
Autocor also reportedly has less material waste. First, the agitator hopper minimizes bucket and transport cleanout. With mixers batching at an optimum capacity, mix quality is improved. And since delivery trips can be reduced, spills are less likely.