Central mixed PCC is usually proportioned by batching the separate ingredients into a central plant mixer (see Figures 1 through 4) where they are completely mixed before discharge into a transport vehicle. Transport vehicles can be concrete mixing trucks or conventional end and bottom dump trucks depending upon travel distance and other requirements. About 20 percent of the concrete plants in the U.S. use a central mixer (NRMCA, 2002). Central mixing plants can either be permanent or mobile (see Figure 5) and offer the following advantages (NRMCA, 2002):
- High production volume. A high production volume would be on the order of 3000 m3/day (3950 yd3/day). Most central mix drums can mix about 9 m3 (12 yd3) in a single batch and produce fully mixed PCC in excess of 150 m3/hr (200 yd3/hr). Mixing times are on the order of 30 to 90 seconds (ACPA, 1995).
- Improved quality control. Since mixing is controlled by a central facility and not by individual truck, PCC quality can be more closely monitored and controlled.
- Reduced wear on truck mixer drums. If used, concrete mixing trucks are used as an agitating haul unit rather than a mixing unit. Agitating rotation speeds are much slower than mixing speeds and thus, produce less wear on drum components.
Footnotes (↵ returns to text)
- National Ready Mixed Concrete Association (NRMCA). (2002). NRMCA web site, Concrete Basics home page. National Ready Mixed Concrete Association. Silver Spring, MD. http://www.nrmca.org. Accessed 11 February 2002.↵
- American Concrete Pavement Association (ACPA). (1995). Construction of Portland Cement Concrete Pavements. National Highway Institute Course No. 13133. AASHTO/FHWA/Industry joint training. Federal Highway Administration, Department of Transportation. Washington, D.C.↵