For more than 30 years, contractors have benefited from the availability and quality of very specialized concrete mixes volumetric concrete producers (VCP) provide. From important projects like overnight tear-outs and replacement projects with fast-setting concrete to bridge deck construction using latex-concrete, VCPs have carved out a profitable niche in the concrete construction industry.
Now, there's another, perhaps even greater, opportunity for VCPs. Pervious concrete is becoming not only the fastest-growing concrete specialty product for the industry. It could be the next opportunity for volumetric producers to expand.
The theory behind pervious concrete is very different than traditional concrete mixtures. Unlike other structural concrete, pervious is free-draining when hardened. Pervious concrete has an air-void content ranging from 15% to 30%. The concrete is so porous, that water drains through to the sub-grade at a rate of about three gallons per hour.
To achieve this, pervious concrete has a different mix design. The material usually uses a single graded coarse aggregate (at about 2400 to 2800 pounds per yard); cement (at about 375 to 650 pounds per yard); and a water/cement ratio of 0.27 to 0.38.
The final product is a very harsh mix that requires close monitoring. The key to durability is thoroughly coating each coarse aggregate particle with enough cement. Most ready-mix concrete producers try to limit the number of mixing revolutions to avoid balling or degradation. It's also important to check the cement percentage. Adding more cement than necessary to coat the aggregate surface may result in cement balls.
The unique qualities of this mix could make it difficult to deliver in traditional drum mixers. Issues include:
- Pervious concrete tends to be very dry and sticky. It can affix itself to the fins of a mixer, slowing production rates and often leading to segregation.
- To facilitate discharge of pervious concrete, mixer drivers often must add water at the jobsite, altering the desired mix design.
- Cement-rich pervious concrete has a short workable placement period.
Most ready-mix concrete producers use a mid-range, water-reducing admixture to help solve these problems.
But some volumetric concrete producers have found a better way.
The California experience
In his technical paper, “Pervious Concrete, the California Experience,” the California Nevada Cement Promotion Council's Andy Youngs credits volumetric concrete producers for promoting what has become a more than a 1.3 million-square-foot market for pervious pavement in Northern California.
“The use of volumetric mobile mixers for pervious concrete first debuted in the Lake Tahoe region, where the ready-mix firms felt that pervious was too new and too small a market for them to deal with,” wrote Youngs, the promotion council's technical consultant.
Providing quality concrete to meet tight specifications is common for Precision Concrete Materials, a Sacramento, Calif.-based volumetric producer. Chris Pisano's team currently operates 10 volumetric mixers.
In the last two years, Pisano has expanded into delivering pervious concrete. According to Robert Wallace, executive director of the Concrete Promotion Council of Northern California, Precision's volumetric mixers have provided concrete for two high-profile pervious concrete projects.
One important key to Precision's success in the pervious arena has been its investment in training. The producer has participated in the National Ready Mixed Concrete Association's (NRMCA) Certified Pervious Concrete Technicians' program.
Modeled after the American Concrete Institute's Flatwork Finisher Certification, NRMCA's program includes classroom training and pervious concrete placement demonstrations. Precision's employees participating in the program review their basic concrete technology and how to use pervious concrete materials with proper mix proportioning. They also receive an overview of the general design principles of pervious concrete pavements and pervious concrete construction techniques.
Another producer that has experimented with pervious concrete mixes is Rick Nations of Nation's Mini-Mix in Eugene, Ore. Nation's has been working with Michiels International, of Kenmore, Wash. Michiels distributes an admixture that is used in batching the PercoCrete brand of pervious concrete.
Michiels believes volumetric mixers can be effective in batching their product. “As the pervious market grows, we expect more volumetric concrete producers to adopt turnkey products like ours and capture a significant share of the market,” says Michiels.
Trailer for Pervious Deliveries
Producers batching pervious concrete in central mix plants should consider using the recently introduced gooseneck, live-bottom delivery trailer, designed exclusively for fresh concrete. A special pneumatic gate and a heat- and oil-resistant conveyor belt restrict leakage. The entire 40-ton payload can be unloaded in about one minute. An internal wash-out system keeps the trailer and unloading area clean from carryback.
For more information, telephone Flowboy at 319-363-8281 or visit www.flowboy.com.