EFFECTS OF MIXING SPEED ON AIR CONTENT
The results are presented of two series of tests which indicate that mixing speeds up to 18 rpm and even higher are feasible. Generally, uniformity of mixing was improved and in no case was uniformity poorer for concretes mixed at 18 rpm than at more conventional speeds of 10 to 12 rpm. In the first series, batches were mixed at 12, 17, 22 and 27.5 rpm. A total of 12 eight cubic yard batches were mixed in a single 8 cubic yard truck mixer, each for a total of 100 revolutions. Ingredients were ribboned simultaneously into the drum. The second series consisted of a study of 23 different loading procedures. The concretes were mixed at either 12 or 18 rpm for either 50 or 150 revolutions mixing. A total of 92 batches were tested but only in certain instances were all four combinations rpm and revolutions mixed. In both series of tests, the concretes contained 5 sacks of cement per cubic yard. The aggregates used were a 1-inch maximum size gravel and a natural quartz sand. The target slump was 3 to 4 inches, but ranged from 1 to 6 inches. The target air content was 6 percent. The dosage of Neutralized vinsol resin admixture was adjusted to compensate for normal variations in materials, temperature and other factors. The admixture was batched either on the sand or in the water from the meter in the plant. The method for batching is known to have important effects on the resulting air content, although the particular methods chosen here did not seem to have an important effect on the resulting air content. Figures are presented, and tables summarize the results. It was concluded on the results of the study that mixing at 18 rpm will produce equal or slightly higher air content than mixing at 12 rpm. The method of loading the ingredients into a mixer has an important effect on the amount of air obtained from a fixed amount of admixture. Revolutions within the limits of 50 and 150 have relatively little effect on air content. In these tests, the air content was quite stable and over 80 percent of the initial air content was retained up to 2 hours after batching. About 400 total revolutions of mixing and agitating were accumulated in this period.
National Ready Mixed Concrete Association900 Spring Street
Silver Spring, MD United States 20910
- Gaynor, R D
- Mullarky, J I
- Publication Date: 0
- Features: Figures; Tables;
- Pagination: 8 p.
- TRT Terms: Air content; Cement; Concrete; Loads; Mixers; Mixing; Quartz; Slump test; Speed; Testing
- Uncontrolled Terms: Batching; Loading; Slumps; Vinsol resin
- Subject Areas: Construction; Highways; Materials;
- Accession Number: 00262510
- Record Type: Publication
- Files: TRIS
- Created Date: Nov 20 1974 12:00AM