Modern volumetric mixer equipment is making inroads in concrete delivery and placement. Truck or trailer-mounted volumetric mixers afford concrete production at the job site in any amount with minimal waste.

Volumetric mixers have better capability to introduce admixtures or color to concrete than ever before. Furthermore, current models benefit from new fabricating materials, better controls and, at least in the U.S., three decades of user and manufacturer experience. Consequently, they are even better suited for providing today's specialty, value-added concretes - such as colored or special admixture-modified mixes - in smaller, but higher-markup, quantities. A variety of applications including color patios and plazas; high-strength, high-durability bridge decks; fast-setting pavement patches; or even intersection whitetoppings to compete with hot mix asphalt spurs specialty-concrete demand, opening the door for volumetric-mixer operators.

Versatile by nature Versatility is the hallmark of the volumetric mixer. It can provide fresh concrete at any slump, or even multiple slumps in one delivery. Mix designs can be changed on-site or for the next delivery that morning. Such changes can even be made on the fly during delivery by an experienced operator.

Because the concrete is mixed fresh, with volumetric mixers there's no waiting on the customer's part for the late arrival of the next mixer truck due to traffic congestion - while holding up construction and wasting manpower and wages. "The volumetric mixer has major advantages," says Brooks Strong, president of Volumetric Mixers by Strong Inc., Houston. "You can load it tonight and pour it tomorrow. You can load and drive 100 miles and pour. You can pour in six different locations."

"An awakening is happening in this industry," adds Gary Ruble, president and CEO of Cemen Tech Inc., Indianola, Iowa. "Customers are realizing this is a concrete-producing machine, and it's not just for conventional applications. Its labor-saving attributes for the precast industry are likewise tremendous."

"The market as a whole has blossomed, especially over the past three to four years," notes David Ankeny, chief operating officer of Elkin Manufacturing Inc., Indiana, Pa. "There seems to be more of an admission that there is a niche in the concrete market for mobiles. The admixture laboratories are refining their products and as they improve, it seems as though there is a trend for mobile units to do more specialized mixing."

Volumetric mixers contain all the materials needed to produce concrete - coarse aggregate, sand, water, cement, pigments and/or admixtures - in a mobile, self-contained package. Because the components of concrete are kept separate until needed, a volumetric mixer truck can be loaded the night before and driven out on the job at dawn the next day, or kept charged for emergency calls.

"These units have the ability to deliver concrete anytime, anywhere," says John Kurtz, president of Zimmerman Industries Inc., Ephrata, Pa. "You can load the truck and drive halfway across the country to deliver concrete, which you can't do with a barrel mixer. And volumetric mixers can pour small quantities of concrete economically and profitably.

"It's not economically feasible for a ready-mix producer to crank up a batch plant in the middle of the night, put two yards of concrete in a barrel mixer, and run it across town. But with a volumetric mixer you can make such a delivery."

Kurtz says that five cubic yards or less is an optimum delivery size for volumetric mixers, assuming the pour is not in an isolated area. And according to the National Ready Mixed Concrete Association, he adds, loads of 5 yards or less account for 30 percent of all ready-mix delivered.

Functional characteristics Volumetric mixers are so functional, manufacturers contend, that they can serve as highly mobile mini-batch plants in a stationary mode, feeding concrete pumps in difficult locations. Unlike conventional batch mixers, they proportion by dry material or liquid volume, not weight.

Improvements in volumetric mixers include much more precise controls. "When the volumetric mixer first came out, it was just a simple way of mixing concrete on site," Kurtz explains. "Not a whole lot of thought was given to the accuracy of material feed."

"But over the years, as volumetric mixing has become more popular, in particular doing work for state DOTs, there was a requirement to become more accurate. The equipment has become more accurate in producing very specific mix designs, and models have become more operator-friendly." Elkin is among manufacturers striving to simplify operation of its volumetric mixers, he adds: "We want to make the equipment as easy as possible to run."

Small jobs defining benefit While they are more appropriate than ready- mix trucks in some instances for large pours, especially on remote locations or serving a concrete pump near which there is room for raw material stockpiles and a silo, small jobs have been the defining benefit of the volumetric mixer.

"A good percentage of our owners are doing short load business," Cemen Tech's Ruble says. "It's profitable because you can make multiple stops without reload, you don't have wasted product since the concrete is not setting in transit, and you can do things that you otherwise could not do with a drum truck."

"Say you're pouring a residential driveway," adds Brooks Strong. "a volumetric or barrel mixer will handle the pour equally well. But when you pour the approach to the driveway it may have a pronounced slope, and you will want a higher slump. With the volumetric mixer, all it takes is a turn of the handle to reduce the volume of water to the mix, producing a much lower-slump concrete." And a hand-packed pour for the adjacent curb will require a still-stiffer mix, he notes, which the volumetric mixer operator can adjust for on the fly.

"The operator may go to the first pour of the day and deliver two yards of 2,500 psi mix," Kurtz says. "But he will go directly to his next delivery, perhaps a sidewalk repair or patio, which will require a 3,500 psi mix. It is a great advantage to be able to change the mix from jobsite to jobsite." Changes are made by the operator adjusting material feed gates, he adds.

One of Cemen Tech's customers is a firm which erects light standards. "The company casts bases for parking lot lights, and the owner swears by the machine because he can produce a small batch, pull across the lot, and produce another quarter-yard," says Ruble. "If he had a drum truck delivering each quarter-yard it would cost him a lot of money."

Long-strip urban sidewalk pours over several blocks that require hand finishing also may be better suited for volumetric mixers, Strong notes. Toward the end of the pour, concrete still in the ready-mix barrel is setting up at the same rate as the first pour. "With the volumetric mixer, you are placing as fresh a concrete at the end of the pour as you did at the start," he adds.

This advantage applies to any placement requiring an extended length of time to utilize concrete - such as a fence contractor pouring post foundations. "The fence contractor can't afford to have any other kind of truck out there," Strong says.

Often, Zimmerman's Kurtz adds, the short yardage deliveries best suited for volumetric mixers are received by small contractors, who may not be able to finish slabs as quickly as large contractors. Once again, the volumetric mixer is well-suited for the application as the concrete isn't mixed until finishers are ready.

Color suitability In addition to small pours, these mixers are especially geared for colored concrete. The ability to mix the color at the jobsite and to the customer's approval will make volumetric mixers more desirable for this application than conventional barrel trucks.

"The product is very well suited for color, because you can adjust it on the fly," Ruble contends. "The ability to match color precisely is extremely important." He describes a customer's application in which colored wall panels are cast day after day. The ability to precisely match color is a big plusfor the customer. "If you're doing regular plant batching, you will have a much tougher time," Ruble says.

Another application of color is matching sidewalks and driveways in Tucson, Ariz., he adds, where ready-mix suppliers are hard pressed to satisfy neighbors' wishes of exact matches. But volumetric mixers serve the need, Ruble notes.

"A lot of ready mix companies don't want color in their drum because it's a pain in the neck to get it all out," Strong says. "The next guy isn't going to want colored concrete. Equipment now available for volumetric mixers will do an outstanding job with colored mixes."

All things being equal, a volumetric mixer will better serve production of smaller-sized loads of colored concrete, while ruling out production shutdown of a larger plant to empty, clean or recharge bins. Changes to different colors are easily effected on site. The operator also can precisely fine tune use of costly pigment to save money. The working end of the volumetric mixer is easily and thoroughly cleaned prior to the next delivery, ensuring purity of the next concrete order.

All manufacturers will offer a system for introducing admixtures to the concrete mix. For example, at least 80 percent of the units Zimmerman sells include a fiber feeder for a customer to supply value-added fibrous concrete, Kurtz says. For customers not wanting fibrous mixes, the feeder is turned off.

Learning curve For the uninitiated concrete supplier - and the contractor and customer - the volumetric mixer poses a slight learning curve in both delivery of material and finishing on the job site. Mostly, the process requires "unlearning" aspects of barrel truck delivery.

"Crews are accustomed to getting a whole load of concrete all at once in a very short period of time," observes Strong. "They have their methods and routines worked on that basis, and tend to be resistant to change. A lot of people are not aware of the new features, lower maintenance and higher production rates that newer volumetric mixers offer."

Moreover, the concrete coming out of a volumetric mixer can appear different than material from a barrel mixer. Because cement hydration begins only as the concrete leaves a volumetric unit, the mix has an apparently "longer" set time than finishers are accustomed to. "For the finisher, it will take longer because hydration has just begun," Zimmerman's Kurtz affirms.

Cemen Tech's Ruble says his firm is advising customers to consider using accelerating admixtures for that very reason. "Operators used to say that concrete coming out of the mixer was difficult to handle," he notes. "We don't hear that any more."

Also, the concrete will appear looser and have a higher slump than product from a barrel mixer. This is attributed to the fact that aggregates have yet to absorb a certain amount of free water. Hence, concrete begins to lose slump after a few minutes.

Kurtz argues that, all things being equal, the concrete out of a volumetric mixer will have a higher psi than barrel-mixed concrete because the hydration bonds formed in a rotating mixer are not continuously broken in an attempt to keep the concrete flowable.

The learning curve applies to drivers as well, because the driver is also the operator. A moderate amount of training is required, which tends to preclude the use of contract drivers. But in the future, refined, computerized control assemblies will further automate on-site mix production, easing the demands on the operator and documenting exactly what was sold to the customer.

The learning curve likewise applies to industry material suppliers. One manufacturer believes that admixture companies are finding that the volumetric mixer auger, in mixing smaller quantities of materials at a given point in time, provides better control of the final mix. This applies to colored concretes as well as admixtures, the manufacturer adds.

Selling against big mixers Volumetric mixers first appeared in the U.S. in the mid-1960s. But the process and the mixer pre-date the 60s: The Panama Canal was constructed using volumetric proportioning, according to Cemen Tech. The first patent for a volumetric mixer in the U.S. was granted to Harold Zimmerman in 1960, Kurtz notes.

Volumetric mixers traditionally have been identified with smaller deliveries, but at least one manufacturer is taking aim at selling equipment in the high-volume market currently served by barrel mixers.

"In most markets, volumetric mixers primarily are used for the smaller deliveries of concrete that ready-mix companies are not getting to," Strong says. "But in other markets that have been using volumetrics for a number of years, there are people who prefer them for many different types of work. These include customers who demand the advantages of fresh mixes."

"We're getting ready-mix companies coming back and buying volumetric units," Ruble adds. "It's a credit to better machines and better admixtures. You can tailor a continuous mix-delivered product to where it can match a ready-mix order. For the producer, it gives a way to handle short loads and call backs."

Their long-term competitiveness against barrel mixers will depend on their functional attributes, Kurtz notes. His firm has sold volumetric mixers to intensive users of drum mixers, but for specific reasons. "Their application must lend itself more to volumetric mixers than to drum mixers," he says. "Customers will start out in drum mixers, see shortcomings for certain types of orders and sites, and look for alternatives."

Beyond site delivery, larger, "stationary" volumetric mixers work well in precast plants, as they are wheeled to different casting locations. Cemen Tech, for one, is finding precast plants to be a strong market sector.

Entrepreneurs' tool Another element that may have suppressed acceptance of volumetric mixers by the ready-mix community was the fact that an entrepreneur could quickly get into the concrete business and compete.

As a result, 30 years ago major ready-mix producers did not want to lend credibility to volumetric mixers. "They didn't like it one bit when people started popping up everywhere with a mixer or two, and grabbing a piece of the work," Strong says.

Now, in Texas, mainline producers are getting volumetric mixers of their own. In the Dallas-Fort Worth market, at least one ready-mix company is adding volumetric mixers to its fleet.

Despite their being marketed for smaller jobs, in some cases volumetric mixers are more efficient than barrel trucks for large jobs. "When they are set up as a small batch plant serving a concrete pump, with the mixer loading as pumping commences, you can have a continuous mixing operation underway, producing 500 to 600 yards per day," Strong notes. Such an arrangement can replace up to a dozen ready-mix trucks in a day depending on plant distance, he says.

The elimination of nonproductive truck movements before and after a pump job is one big advantage, he adds. "That pulling-in and pulling-out is a large percentage of the total time that you're making concrete. But volumetric mixers hang the mixing auger over the pump, and as long as you can keep the mixer and silo going, it never stops." For these reasons, early model volumetric mixers were dubbed "batch plants on wheels."