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In September 2007, in a 98 page report titled "Improving Portland Cement Concrete Mix Consistency and Production Rate through Two-Stage Mixing", an Iowa State University research team presents their findings concerning the production of concrete readymix. In this report they detail their studies on the effects of high shear mixing of cementitious powders to make slurry and a process they call "two stage" mixing of concrete, which is nothing more than adding the premixed slurry (stage 1) to the course and fine aggregates (stage 2). For one of their field tests they selected a Hydromix® equipped plant to meet the requirements of their tests - high shear slurry production prior to adding that slurry to the aggregates.

All quotations on this page come directly from that report. If you wish to read the full report and these links are no longer valid, please contact Hydromix®.
-- Iowa State University research site -- The full report

This is the study premise:

"A two-stage process for concrete involves mixing a slurry of cementitious materials and water, then adding the slurry to course and fine aggregate to form concrete. Research has indicated that this process might facilitate dispersion of cementitious materils and improve cement hydration, the characteristics of the interfacial transition zone (ITZ) between aggregate and paste, and concrete homogeneity.
The goal of the study was to find optimal mixing procedures for production of a homogeneous and workable mixture and quality concrete using a two-stage mixing operation."

Their plan involved 3 phases:

Phase 1 Overview and conclusions -- In the phase 1 study, two mixers were used in the laboratory, a low shear and a high shear mixer, both with a high and low speed. Slurry was made using a variety of powders including all portland cement, portland cement blended with fly ash, or with slag, or with both. Each of these comprised individual batches, one mixed in each mixer, one at low and the other at high speed. Measurements were taken from each batch of fresh slurry for heat of hydration, maturity and rheology. Compressive strength, degree of hydration and scanning electron microscope imaging tests were also conducted on the cured specimens.
Conclusion summary: "Results from the paste study show that concretes produced by two-stage mixing with high shear slurry mixing will provide more workable concrete with a higher degree of hydration and improved compressive strengths, especially at early ages."
"The degree of hydration tests indicate that high shear mixing always produces a slightly greater degree of hydration that normal mixing."

"High shear mixing generally produces pastes with slightly improved compressive strength when compared to normal mixing. In this study, the improvement was greater at early ages of three and seven days."
"SEM (scanning electron microscope) image analysis shows that high shear mixing produces a smaller percent of unhydrated cement particles that increases with increased mixing time, confirming the degree of hydration test results."
(emphasis added)

Phase 2 Overview and conclusions -- The authors of the report cite several other studies as well as ASTM standards for concrete mixing procedures as the basis for defining concrete quality. The goal to produce quality concrete is to achieve homogeneity of the final product. To do so requires that the process match the mixer type. When the mixer is a concrete truck, "non-uniformity in truck-mixed concrete is caused by agglomerations of concrete materials inside the mixer, including head packs and cement balls." The design of mixer truck barrels is noted to be one of the least efficient of the available types, which supports the common acceptance that more efficient designs like central mixer drums produce ready mixed concrete superior to dry batch plants.
The authors continue by citing research previously done concerning various two-stage mixing processes. One such study concluded "the microstructure and the paste/aggregate bond were improved by limiting the amount of direct water contact with the aggregate during mixing, and 28-day compressive strengths of samples prepared by the slurry premixing method were greater" than the previously accepted industry standard for reducing the above mentioned problems with dry batching into truck mixers.

Conclusion summary: "Compressive strength results show that the two-stage mixing process generally produces a 10 percent increase in strength at 28 days, and tensile strength results show equal or 10 percent greater strengths when using two-stage mixing compared to" their basis method.
They also note that the increase in hydration resulting from this type of process suggests to not add air entrainment to the slurry mixing process, but afterward when the aggregates are added. "Due to the inability of the fine cementitious particles to entrain air, air entrainment is less effective in the two-stage process when the AEA is added in the slurry." (Field tests show adequate air entrainment when AEA is added to the final mix.)
Their two-stage tests involved mixing batches in the lab for 30 seconds, and for 60 seconds. Unexpectedly, their results favored the lesser time. "Two-stage mixing at 30 seconds is recommended as the optimal mixing process." Which concludes that longer mixing times for slurry production are not beneficial.
(emphasis added)

Phase 3 Overview and conclusions -- "The laboratory phases established that improved strength and adequate fresh and hardened concrete properties can be obtained using a two-stage mixing process. While laboratory studies can be important indicators of concrete performance, it is necessary to conduct field studies to determine whether the improved performance can be obtained in the field."
The research team used the following mixers in their field studies:

Project One The two day study compared a Hydromix® equipped previously dry batch plant to a conventional drum type wet batch ready mix plant. One batch design was used between the two plants. The materials used were Type 1 portland cement, Class C fly ash, air entrainment agent and water reducer. Natural river sand and crushed limestone were used for fine and course aggregate.
Immediately after mixing, fresh concrete properties of slump and air entrainment were measured. Then samples were cured using industry standard procedures and compressive strength was measured at 3, 7 and 28 days; with split tensile strength tested at 56 days.

Conclusion summary: "The compressive strength data indicate that the Hydromix® mixing process produces slightly higher compressive strength at all ages of testing. The data indicate that the Hydromix® mixing method produced slightly stronger concrete at early ages. Using this knowledge, an engineer who is concerned with early-age strength development rather than the 28-day strength could specify Hydromix® concrete to better meet specifications."
"The results indicate that the Hydromix® mixing method produces a concrete with about 15% higher tensile strength. These results indicate that the Hydormix mixing method produces a more homogenous concrete, illustrated by a smaller standard deviation when compared to the central plant concrete".
"The Hydromix® mixing method produced a concrete significantly more resistant to chloride ingress." (Emphasis added)

Project Two The two day study compared a unique slurry pre-mixer and a convetional wet batch plant. One batch design was used between the two plants. The slurry pre-mixer added water to cementitious powders in a horizontal mixing chamber with stationary rods at the top of the chamber, and rotating rods ("fingers") located on the bottom of the chamber. The slurry was then charged with the aggregate into a ready mix truck. Immediately after mixing, fresh concrete properties of slump and air entrainment were measured. Samples were cured using industry standard procedures and compressive strength was measured at 3, 7 and 28 days; with split tensile strength tested at 56 days.
Conclusion summary: "The compressive strength data indicates that the central plant mixing process produces slightly higher compressive strengths at early ages, with the 28-day strengths being equal."
Tensile strength and chloride permeability were essentially equal between these two processes.

Conclusion: Quick, high shear mixing of cementitious materials into a homogenous slurry before adding to dry aggregates in a two-stage process makes stronger, more workable ready mixed concrete. Of the equipment tested in this study, the Hydromix® slurry mixing system is superior to the other systems, including central drum mixers which is equal or better than the other slurry mixing process. The Hydromix® system consistently produces greater compression and flexural strengths, more homogenous and more easily workable ready mix than the competition. And we do this at a considerably lower cost.