Roller-compacted concrete (RCC) is one of those products that’s so old it’s new. The U.S. Army Corps of Engineers researched RCC in the early 1970s for dam construction. It was used to build a log-sorting yard on Vancouver Island in British Columbia in 1976. The Willow Creek Dam in Oregon was built in 1982 and Concrete International dedicated its entire February 1987 issue to RCC, describing it as a concrete for the future. It’s been used for airport runway aprons, secondary roadways, highway shoulders, residential streets, intermodal yard construction, parking areas, and dams. Many RCC dams have been built in Asia.
Until recently, though, its use has been sporadic throughout the U.S. and has received little attention. This could all change, however, because of more refined mix designs and better placing equipment.
All about consistency and density
Wayne Adaska, director of pavements for the Portland Cement Association (PCA), Skokie, Ill., defines RCC as a no-slump concrete compacted by vibratory rollers. No forms, finishing, or reinforcing steel is used and production can be very high compared to traditional concrete.
RCC is a very dry concrete. If you take a handful of the mix, squeeze it, and then open your hand again, it forms a cohesive ball and only fine particles are left on your hand. Jerry Larson, executive director of the Indiana Ready Mixed Concrete Association, Indianapolis, says using water-cement (w/c) ratios (which can be 0.35 to 0.45) to describe the amount of water in RCC isn’t as helpful as reporting the total water compared to the total weight of a mix. This typically runs at 5% to 6%. “The goal is to have concrete dry enough to support the weight of a rolling compactor, and wet enough to be cohesive,” he says.
Because it’s so dry, it’s delivered to jobsites in a dump truck rather than a ready-mix truck, and when it’s used for paving, it’s placed with what amounts to an asphalt paver. The most effective are high-density pavers with tamping bars that can compact the concrete up to 95% density directly behind the paver. Steel drum vibratory compactors follow immediately behind the paver to complete the compaction process. No concrete finishers are needed since there are no finishing operations, which contributes to increased production. The process is complete after compaction.
To achieve high strength, durability, and resistance to freeze/thaw cycling, RCC mixes must achieve at least 98% of maximum density. Given the high density and low total water, it’s not difficult to achieve compressive strengths of 5000 to 7000 psi (even 9000 psi isn’t unusual), with flexural strengths between 650 and 800 psi. The high density and low paste content also contributes to low permeability and reduced shrinkage.
The consistency or slump of RCC is different than conventional concrete. RCC mixes are typically non-air entrained, have lower cement content, higher fines content, and smaller top-aggregate sizes. Due to the lack of paste in RCC, the aggregate gradation becomes extremely important. Special attention is given to using well-graded or combined aggregate gradations to help minimize segregation and achieve the required density. For a tighter, better-appearing surface, maximum size aggregate is between 1/2 and 3/4 inches. Because of the extremely low moisture, a small amount of evaporation can cause problems, so trucks carrying RCC to a jobsite should be covered. RCC surfaces can dry out quickly so curing should be applied immediately after final compaction.
Paving with RCC
There are many differences between RCC and conventional concrete used for paving. Adaska says typical concrete for slipform highway pavement has a slump of 1 to 2 inches; whereas RCC has zero slump. RCC looks like a concrete mix without water added and, unlike standard ready-mixed concrete, there is no opportunity to adjust water at the jobsite.
Mixing RCC is best achieved using a central mixing plant with a twin-shaft mixer, or a pug mill, and delivered to the site in dump trucks.
You can’t achieve the required density of RCC on a poor subgrade—if the subgrade is soft, you can’t get the necessary compaction. The base must be dry, flat, and compacted to the same level as the concrete. RCC typically is installed by pavement thickness so the profile of the subgrade becomes the profile of the pavement surface. Gomaco, Ida Grove, Iowa, however, manufactures both string-guided and string-less equipment for placing RCC that result in variable thicknesses and flatter surfaces.
The best way to place RCC pavements is with a high-density paver with tamping bars that compact the concrete as it leaves the machine, resulting in a flatter surface. This is followed by a couple of passes with a vibratory rolling compactor to achieve densities that are 98% of maximum or higher. If a vehicle or skidloader is driven on the pavement immediately after completing the compaction phase, it barely leaves a tire print.
RCC for dams
RCC is ideal for mass concrete placements, such as for dams, because of its high strength at very low cement contents, speed of construction, and low heat generation. One recent project was built on the Illinois-Indiana border for the Metropolitan Water Reclamation District of Greater Chicago in the Thornton Quarry. When the city’s Deep Tunnel fills with storm water, part of the quarry will serve as a reservoir to contain combined sewer overflows until wastewater treatment plants can catch up with the flow. This arched dam is 84 feet wide and 130 feet long at its base and 36 feet wide and 240 feet long at the top, standing 112 feet tall.
The joint-venture general contractors for the project are F.H. Paschen and Cabo Construction Corp., both in Chicago. Tim Bea, Paschen’s project manager, says preparation for the site began in 2010 when crews removed all loose, weathered, broken, and fractured rock from the quarry walls. A final scale removal was performed shortly before RCC placement began, with daily cleaning of wall surfaces using a pressure washer.
Bea says Cabo Construction rented a batch plant with a twin-shaft mixer from Ozinga and bought aggregate from the quarry. Cabo’s personnel batched and placed the concrete. Batch plant personnel started work each day two hours prior to placement to run moisture tests on the aggregate piles, batch a sample load, and perform consistency tests.
The RCC was delivered by conveyor and dumped in piles. A laser-guided bulldozer distributed the material throughout the area of the placement in 1-foot lifts. Rolling vibratory compactors, monitored by nuclear density testing, consolidated the concrete to an average 96.5% minimum of the theoretical air-free density. When you walk on the RCC after compaction, it feels spongy underfoot—very different than RCC pavement.
As the RCC was placed, workers filled small pockets around the perimeter of the placement with a loose grout mix. After the RCC was placed over the grout, a gang-mount vibrator system attached to a mini-excavator vibrated the grout up through the RCC. This process created the look of more conventional concrete on the form surfaces and is referred to as grout enriched vibratable concrete (GEVR). This also aided in achieving a good bond between the dam and the rock abutments.
The Joliet Intermodal Project
When CenterPoint Properties, Oakbrook, Ill., decided to pave 27 acres (requiring 27,000 cubic yards of concrete) for an intermodal container storage facility—a place where containers are staged for rail to truck transloading—they specified RCC pavement. Dan Darden, vice president of construction for Logistics Concrete, Thornton, Ill., says the specification required using a high-density paver, so they bought a Vogele paver. To spread the cost of this purchase, they decided to market RCC and get fully involved in this work. Darden says they worked out their routine after the first few placements and were able to produce very flat pavement. By rolling the pavement without the vibratory action turned on, they achieved the required density and kept the pavement flat. Workers placed a curing compound immediately after the rolling operation was completed. Sawed control joints followed about 12 hours later to create 30-foot square panels.
The open texture of RCC pavement looks more like asphalt than concrete. By reducing the top size of aggregates and increasing the amount of fine aggregate, a finer finish is produced. Justin Ozinga, executive vice president of the Illinois division for Ozinga Brothers Ready-Mix Concrete, says learning how to make good RCC is a challenge. “You need a tight aggregate gradation to attain high density and that involves very close cooperation with your aggregate supplier.” Controlling segregation by careful management of aggregate piles and loading fresh concrete into trucks is critical. Aggregate pile management must produce uniform moisture content. To work out the details, Ozinga placed several trial batches before the project started.
In 2003 the PCA Southeast Region, Atlanta, mounted a major marketing effort to get RCC specified for highway shoulders. A 34-mile asphalt replacement project resulted, followed by several more DOT projects in the southeastern states over the next five years. This marketing continues to generate projects.
For several years the Village of Streamwood, a Chicago suburb, has replaced residential asphalt streets with RCC using their own staff and placing equipment. Using a standard asphalt paver, city workers top the RCC with a 1-inch-thick asphalt layer to provide a smooth riding surface. The cost is competitive with asphalt paving but the city thinks the long life of RCC is where the real money will be saved.
Another RCC marketing effort, by Prairie Materials, Bridgeview, Ill., is to the City of Chicago for residential pavement. Chicago’s water department recently removed an asphalt pavement on a residential street to install new water lines and replaced it with RCC using a standard asphalt paver and a 1-inch asphalt topping. The RCC pavement was opened to traffic on the same day as the placement. If the RCC performs well in this severe freeze/thaw environment, the city will include it in its specifications.
The market potential for RCC is considerable. If residential streets in the U.S. switched from asphalt to RCC, there would be enormous opportunities for concrete. Many concrete associations and companies are marketing RCC; hopefully increased awareness will encourage specifiers to include it on projects. RCC has many good qualities: its density makes it nearly impermeable, its high compressive and flexural strength, fast-track production and early trafficking, low mass-concrete temperatures, high durability, and low cost. All it needs is more acceptance and exposure.
Nuclear density testing follows the paver to indicate to the placing crew the density being achieved.
Curing RCC is especially important given the low amount of water in the mix.