Concrete produced by volumetric mixers is often eyed with suspicion because of the short duration that ingredients are in the mixer before discharge.
But is this actually indicative of inferior quality? And is the mixture actually mixed less thoroughly than by drum mixers?
Visual Inspection
As a fellow who spent his career in the ready-mix field, I too held the view that volumetric mixers could not stand up to the scrutiny that ready-mix concrete is subject to, solely because of this issue. But after a year and a half as an owner-operator in the volumetric industry, I've closely monitored the product and never witnessed anything resembling unmixed concrete come down a chute.
Like most folks in the concrete industry, I have however seen many a "meatball" tumble down the chute of a ready-mix truck - from "dry batch" operations where the truck serves as the sole means of mixing, as well as "wet batch" plants where the mixture is produced at a central facility and the truck provides only for transportation and agitation of the product.
The avoidance of "meatballs" in ready-mix is achieved primarily by introducing the ingredients (water, coarse/fine aggregates, cement, and admixtures) proportionately to the mixer in the proper sequence. Of course, moisture content in the aggregates plays an important and related role as well.
In volumetric mixers, all of the ingredients are introduced to a hopper in the correct proportions, simultaneously. From here, they continuously feed the mixing auger which also serves to discharge the material.
Bottom line: if you're on a job with volumetric mixers and you've brought your fork and spaghetti hoping for meatballs, you'll be sadly disappointed.
Mixing Time
The misconception about volumetric mixing being an inferior method is largely associated with the fact the ingredients are in the mixer a matter of seconds before discharge. This is contrasted with ready-mix where concrete is to be mixed (per ASTM C94) for a minimum of 70 revolutions at mixing speed (~14 revolutions per minute). This equals approximately 5 minutes. This would be deemed adequate for a full 10 CY load.
To treat this matter fairly, we'll need to do a math exercise. Let's say you have roughly 0.25 CY of material in the mixing auger and it is in there for 10 seconds prior to discharge. The quantity you have mixed in this time represents 1/40 of the volume of a full drum mixer. Multiply the 10 seconds by 40 and you get 6 minutes, 40 seconds of total mixing for 10 CY using a volumetric mixer. So an apples-to-apples comparison reveals volumetric mixers actually treats the concrete more thoroughly as far as time goes. Because such a small amount of material is in the mixer at any time, it is arguably equal or greater in mix uniformity.
Any Valid Concerns?
So is there any merit to concerns about mixing by this method? Because the mix ingredients are fed to the auger and the mixture is discharged from the auger simultaneously, any variation in the mix constituents (i.e. aggregate gradation or moisture) is seen in real-time. This means there is the potential to see greater variation in slump, for instance, using a volumetric mixer than a drum mixer.
The upshot to witnessing changes in real-time is the end user is afforded greater control of their product. This feature means you can either actively compensate for changes in the materials or change the mix altogether to suit your needs.
The Verdict
In summary, a competent operator is key to producing a consistent, high-quality mix. As an advocate for the volumetric industry, I would argue the advantages of superior product control far outweigh the added responsibility of managing your raw ingredients.
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