Title: Effect of Grinding in the Large Mixers on Aggregate Grading at Hiwassee Dam
Author(s): W. R. Waugh
Publication: Journal Proceedings
Appears on pages(s): 9-20
Preliminary investigations revealed that changes in grading due to grinding in the mixers would be a major consideration in the concrete control. Crushing tests gave the first indication of the instability of graywacke and laboratory mixing tests were made to determine the relative grinding characteristics of graywacke and dolomite. Preliminary tests served only to confirm the exis-tence of the problem, and field investigations were begun concurrently with concrete placing to deter-mine amount and character of grinding taking place in the large field mixers. Grab samples were analyzed periodically as a basis for adjusting ingoing grading to compensate for changes in grading due to grinding to get a desired aggregate grading in the mixed concrete. Later full batches were analyzed as an additional check on changes in grading due to grinding. Split batch charging of the face concrete was adopted after job trials indicated that the amount of grinding could be reduced by withholding coarse rock and cobbles from the mixers until one half the mixing time had elapsed. Variable grinding during mixing made production of concrete of uniform quality more difficult. Several conclusions in regard to grinding are given at the end of the paper.