Personnel Qualifications: The document reflects the committee's desire that operators receive manufacturer-sponsored training or at least four weeks of on the job training by qualified personnel. This recommendation is important because the operator must understand how to adjust the fresh concrete's yield and quality through the proper use of the equipment, rather than by just adding water. The committee suggested in the document that the operator successfully completes at least the ACI Concrete Field Testing Technician Grade I Certification.
Applications: Four applications have been added to the new guide. These are the production of latex-modified overlays, controlled, low-strength fill, fiber-reinforced concrete, and military applications.
Latex-modified overlays are mixes that incorporate an emulsion that replaces most of the mixing water. This concrete mixture is used to overlay bridge decks and parking garage decks, as the final hardened product creates an impermeable layer that practically prevents deicing salts from penetrating the deck and reacting with reenforcing steel. The VMCM designed for this application is factory-equipped with a second reservoir and associated pumping and metering equipment used to introduce the latex emulsion.
Controlled, low-strength fill is a mixed material that is used for fill. Properly equipped VMCMs are able to produce normal-strength concrete as well as low-strength fill, even from the same load of raw materials.
Manufacturers have developed fiber feeders and fiber choppers which continuously feed a controlled amount of fibers into the mix. The operator can control the fiber rate, as well as whether the concrete contains fiber or not. The flexibility of VMCM mixers allows different concretes to be produced from the same load of raw materials.
The U.S. military is the largest user of VMCM equipment. The equipment's compact size and ease of operation allows quality, in-theater concrete production to rapidly commence. The equipment's size makes it easy to transport by air. The development of rapid-hardening and rapid-strength concretes makes VMCM mixers the logical choice to produce these concretes. The military, especially the Air Force, requires these types of concrete for rapid repairs of runways.
Calibration: The guide recommends that the minimum calibration interval to be six months or 2500 yards of production, whichever happens first. The committee felt it was important to recommend that any time there is a change of raw material, the producer should perform a calibration. The new calibration form would help this process, as it is easier to use and more generic than the previous form included as an attachment.
Pre-production Testing: The document also contains the committee's recommendation that, as with any concrete production system, mix designs used in VMCMs should be tested to ensure material compatibility. Proper testing will also ensure the raw materials selected are compatible with the very short (15 seconds) vigorous mix of VMCM equipment. The committee warns that some cements and admixtures require a longer mix time and thus may not be suitable for VMCM production.
The committee also stated in the document that it believes one of the more important pre-production tests to be performed with VMCM equipment is a volumetric yield. It suggests that the producer construct a box with a volume of exactly ¼ yard. The test is conducted by setting the controls on the VMCM unit to produce exactly ¼ yard of fresh concrete. The material is directly deposited into the yield box. This process ensures the mix design is correct and that the VMCM unit is performing to specification.
The committee pointed out that this quick test can be easily performed during production. And after the yield measurement, the fresh concrete can be incorporated into the project.