Shrink Mixed PCC

Shrink mixed PCC is partially mixed in a plant mixer before discharging into a truck mixer.  The short mixing period in the plant mixer reduces the bulk volume of the overall ingredients by allowing fine aggregate, portland cement and water to fill the large void space in bulk coarse aggregate.  Typically, 1 m3 (1.3 yd3) of fully mixed PCC requires about 1.58 m3 (2.07 yd3) of individual ingredients (ACPA, 1995[1]).  Thus, with shrink mixing, more PCC can be loaded into each truck mixer.   Many central mixing plants use a stationary plant-mounted mixer to shrink mix PCC before charging their truck mixers.  The amount of mixing that is needed in the truck mixer varies in these applications and should be determined via mixer uniformity tests. Generally, about thirty turns in the truck drum, or about two minutes at mixing speed, is sufficient to completely mix shrink-mixed concrete (NRMCA, 2002[2]).

Footnotes    (↵ returns to text)
  1. 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.
  2. National Ready Mixed Concrete Association (NRMCA).  (2002).  NRMCA web site, Concrete Basics home page.  National Ready Mixed Concrete Association.  Silver Spring, MD.  Accessed 11 February 2002.