|Superiority of Hydromix® Concrete|
This is the study premise:
Their plan involved 3 phases:
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:
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)
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.