Peaking prices for liquid asphalt embolden cement and concrete groups to promote RCC pavements placed with asphalt pavers. 

The war between hot-mix asphalt and portland cement concrete has taken a new turn, with a bold push by cement and concrete interests to promote roller compacted concrete on a cost basis against asphalt.

Contractor Nickolas Savko & Sons places RCC in Columbus, Ohio as a superstrong base for asphalt overlay.

Roller compacted concrete is a mix containing a binder whose price, since 2004, has not run up as much as liquid asphalt. RCC has a gradation similar to an asphalt pavement, is placed with an asphalt paver, and then typically compacted with vibratory and rubber-tired rollers, the cement and concrete industry state. Together, these qualities help RCC give asphalt pavements a run for their money wherever rougher RCC pavements can be used.

At a May RCC workshop and demonstration in Nashville, a test cylinder is prepared by the onsite lab.

Roller compacted concrete is a stiff, zero-slump portland cement concrete mix. With a runup in asphalt prices removing hot-mix asphalt’s conventional price advantage versus concrete, the concrete industry now says that a more durable product — RCC pavements — can be placed at the same cost by the same machines as asphalt pavements, but for less critical pavement applications, such as rural or local streets, highway shoulders, and parking areas.

Rising asphalt prices aid RCC

Through April 2007, asphalt prices have risen, while concrete has stayed relatively stable, occasional cement shortages notwithstanding.

Above, a Hamm HD 120 HV compacts RCC at a May workshop and demo in Nashville;


Above, at the demo, a Vögele Super 2100-2 with high-density screed places RCC 6 inches deep at a Tennessee DOT facility.

In the March 2007, Construction Inflation Alert of the Associated General Contractors, AGC chief economist Ken Simonson noted that the Producer Price Index — as tracked by the U.S. Bureau of Labor Statistics — for liquid asphalt at the refinery by February 2007 had risen nearly 37% over the preceding 12 months, compared to nearly 6% for cement. Simonson noted the PPI for asphalt paving mixes had risen just over 27% in the 12 months ending December 2006, while ready mixed concrete had risen nearly 10%.

“From early 2004 to mid-2006, the construction industry was plagued by runaway materials cost increases,” Simonson said in March 2007. “Many of these price increases have slowed or even reversed course, modestly, in recent months. Unfortunately, it seems likely that the current calm is only a lull between storms and not a return to the inflation-free period of 2001-2003. By the end of 2007, materials costs could be rising again at a 6 to 8% rate, with wages rising at a 5% pace.”

In the Pacific Northwest, the Washington State DOT reported a first quarter 2007 average bid price of $59.34 per ton of HMA, up $4.76 per ton from the last quarter of 2006, with a high average per ton of $65.26 in western Washington, while the average unit bid price for a cubic yard of concrete pavement was $246.34, down $20.33 from the average of $266.67 in the second quarter 2006.

It is this environment of sharply rising liquid asphalt costs that the cement industry seeks to exploit as it promotes roller compacted concrete.

“An important element of RCC is its cost,” said Will Gray, project manager, A.G. Peltz Group, LLC, a Birmingham, Alabama-based contractor specializing in RCC placements. “It’s cheaper than most traditional concrete mixes and structures.”

For residential streets, there is a 20% lower cost for RCC pavements compared to conventional portland cement concrete, said David Luhr, Ph.D., P.E., program manager, soil cement/RCC, for the Portland Cement Association.

“That differential can vary a great deal,” Luhr told Better Roads. “One of the general things to keep in mind is that RCC uses an asphalt construction process and asphalt aggregate, and the main difference between pavements built with RCC and asphalt is the cost of asphalt and portland cement, and how the price of these binders fluctuate on the open market. Today, with the higher cost of liquid asphalt cement, in some places, it can be cheaper to build pavements with RCC than with asphalt. And even if you are pretty close to the price of an asphalt pavement, you will have the additional advantages of RCC’s higher strength, longer life, and lower maintenance costs.”

Both Gray and Luhr were speakers at a May workshop and demonstration on RCC held in Nashville. “We definitely see an opportunity to promote RCC for pavements with the cost of asphalt these days,” said Barry Wilder, pavement applications director for Tennessee, Southeast Cement Association, a workshop sponsoring organization. “That’s one of the reasons behind our workshops.”

About roller compacted concrete

While RCC contains the same basic ingredients as conventional concrete — cement, aggregates, and water — it’s mixed in a drier form and can be placed with conventional asphalt pavers, or pavers with intensive compaction components at the screed.


At a May 2007 Nashville demo, technicians check compaction of RCC using a nuclear density gauge.

Unlike conventional concrete mixes, RCC is not air entrained, and has a lower water content, lower paste content, but larger fine aggregate content than conventional concrete. It contains a nominal maximum size aggregate of 0.75 or 0.625-inches.

Admixtures are used in RCC. Retarders or water reducers can be used to increase working time. Given the dryness of the mix, superplasticizers are commonly used in dry batch production to improve workability. Addition of air-entraining admixtures is possible, but RCC already is extremely freeze/thaw resistant, even without air entrainment. Fibers are not used often.

RCC is stiff enough to compact by vibratory rollers, and does not require many of the features we associate with concrete: forms, finishing, dowels, or reinforcing steel, reports the Portland Cement Association, a workshop sponsor. RCC is compacted immediately after placement, in a continuous process performed until the finished pavement meets density requirements. The pavement is then cured, and in many cases, joints are sawed to control cracking.

 
Above, in Columbus, Nickolas Savko & Sons places RCC with two pavers operating in tandem, resulting in a fresh joint between lifts; right, RCC is placed on an urban artery.

While RCC conventionally has been used for parking areas and equipment yards for container ports, rail terminals, truck terminals, and industrial yards where the pavements are subjected to large forklift trucks, straddle carriers, log stackers, and mobile cranes, it has not commonly been used for roadways. The concrete industry now seeks to popularize RCC for low-volume roads in urban areas, for highway shoulders, and urban parking areas.

RCC has been used on roads in and around Chattanooga and Knoxville, Tennessee, in Alliance,

Nebraska, and in Columbus, Ohio. In Georgia, RCC was used as shoulder on the 35 lane-miles of shoulder along I-285, the Atlanta Perimeter/Beltway, marking the first time the material was used on a major highway reconstruction job.

In Aiken County, South Carolina, with the assistance of the PCA, a 1,000-foot demonstration section of RCC was placed on Powell Pond Road in March 2002. The material was produced by Lafarge and placed by C. Ray Miles Construction Company to a depth of 6 inches. The performance of this section is being monitored.

RCC owes much of its economy to high-volume, high-speed construction methods, reports the PCA. RCC is blended in continuous-mixing pug mills at or near the construction site. These high-output pug mills have the mixing efficiency needed to evenly disperse the relatively small amount of water used.

Conventional dump trucks transport the RCC and discharge it into an asphalt paver, which places the material in layers up to 10-inches thick. The material will be dry enough to support a vibratory roller immediately after placement, yet wet enough to permit adequate distribution of cement paste.

A.G. Peltz places RCC right next to new portland cement concrete curb.

Compaction is the most important stage of construction, providing density, strength, relative smoothness, and surface texture. Use of an asphalt paver with compacting or tamping screed — where most of the compaction will take place — can eliminate all but one roller on a project.

Curing ensures a strong and durable pavement. Curing makes moisture available for hydration, the chemical process that causes concrete to harden and gain strength. A water cure sprays the pavement to keep it wet, while a spray membrane also can be used to seal moisture inside.

Cured RCC pavements exhibit high flexural strength, from 500 to 1,000 pounds per square inch, and high compressive strength, 4,000 to 10,000 psi. Once cured, specs require RCC to develop a compressive strength of at least 5,000 psi in 28 days.

In South Carolina, strengths of over 3,000 psi were developed in three days and strengths over 5,000 psi were developed in seven days. Tests performed on other RCC projects indicate that the flexural strength of RCC is equivalent to traditional paving concrete, reported the SCA.

Workshops promote RCC pavements

The case for roller compacted concrete pavement was laid out in late spring at a workshop on RCC held in Nashville. Roller Compacted Concrete Pavement: Design and Construction Seminar and Demo was held May 24 and featured a blue-ribbon panel describing today’s roller compacted concrete, mix designs, field applications, and paving and compaction equipment needed to successfully place RCC. A field installation at an equipment yard of the Tennessee DOT rounded out the workshop.

At a May demo in Nashville, RCC can be walked on as soon as it’s  placed.

The workshop was co-sponsored by SCA, the PCA, and the American Concrete Pavement Association, Southeast Chapter.

The May workshop will be followed by a similar event in Atlanta Tuesday, November 6; see For More Information for a link to the event’s Web site.

Construction site geometry is important because the equipment used to place RCC is large, said Will Gray, A.G. Peltz Group, LLC, a contractor active in RCC placements and development, at the Nashville workshop.

“It’s hard to get that large equipment in and out of small areas,” Gray said. “We will not do a lot of twisting and turning. If your job has a lot of 5-foot radii it won’t be good for RCC. You need large, square areas and the right-size job to get the economies of scale that make RCC attractive.”

On I-285 in 2005, contractor A.G. Peltz places RCC for 10-foot-wide shoulders using an asphalt paver with 10- to 12-ton steel drum and pneumatic-tire rollers.

Also, Gray urged contractors to choose the right clients for RCC, adding its final appearance would not be acceptable to a resort, but would be for a robust industrial application. “RCC is durable and has a good service life,” Gray said. “It’s not the prettiest thing you’ve ever seen, but a lot of customers use it, because it’s fast — we can lay a lot of material in a small amount of time — and it has good early-strength gain characteristics, so they can put the pavement to use rather quickly.”

In an unconsolidated state, RCC is a no-slump material with a lot of the same materials as conventional concrete, Gray said. “It takes water added just to get to a slumped state,” he said. “We won’t place it around reinforcing steel, and we can’t consolidate that real stiff mix around it. There’s no finishing with trowels and bull floats. Instead, we are finishing it with a roller, attaining final compaction externally with the roller instead of internal vibration with hand-held vibrators.”

Type I or Type II cement will be used, sometimes with fly ash added. “Our mixes will be between 400 to 600 pounds of binder per cubic yard, and the binder content will be tied in with the aggregate used,” Gray said. “Different aggregates — and different applications — will require different binders. Like other concrete mixes, there are many parameters to consider.”

Aggregate will be uncomplicated, just a well-graded base stone grading will do, Gray added. “Getting the right aggregate is one of the most important things you will do to get an RCC project,” Gray said. “A lot of RCC’s characteristics depend on aggregate, including strength, workability, and surface appearance.”

In addition to the 400 to 600 pounds of binder per cubic yard, Peltz’ RCC mixes will have 3,400 to 3,700 pounds of well-graded aggregate, and just 20 to 30 gallons of water, depending on the binder content and aggregate. “The water:cement ratio will be between 0.3 and 0.45,” Gray said. “The water content is dictated by the optimum moisture for compaction.”

Test sections are imperative for high-profile jobs, Gray told the workshop. “In my opinion, if you are going to do a large-scale job, you’d better do a test section,” Gray said. “If you don’t do a test section, all that stuff you did in the lab may go out the window. Labs can do a lot in a controlled environment, including potential strengths and densities; but once in the field, that’s not always possible. A full-width field test section will make sure all the lab data is going to work out.”

Most of the pavements will require density of 98% of modified Proctor, Gray said. “If you don’t get density in this rigid pavement situation,” he said, “you will have problems. Stress starts at the bottom, and if you don’t have density at the bottom of the slab, a crack will form, and that crack will propagate to the top.”

Subgrade conditions are important for a successful RCC placement, Gray added. “If it’s good enough for an asphalt or concrete placement, it’s good enough for RCC,” he said. “If you have a very dry subgrade, try to moisten it, because you’re already dealing with a dry mix, and you don’t want to lose a lot of moisture out of the bottom.”

Different flavors of RCC

Within those parameters, the mix design of RCC will vary widely, said another panelist.

“All RCC is not created equal,” said PCA’s Luhr. “It’s all up to the owner and his requirements. There is not one flavor of RCC; there are many different ways of building it and types of final product.”

Smoothness of slab is sometimes seen in RCC specs, either seen as a deviation from a straightedge, such as 0.25 inch over 10 feet, or application of the IRI, International Roughness Index.

“RCC is not as smooth as conventional concrete,” Luhr told the workshop, “so if we are building an interstate highway, the surface will not be RCC, as we cannot attain the same level of smoothness as with conventional concrete.”

The durability and uniformity of the RCC also are major considerations in the mix. “All of these considerations are dependent on the owner and the function of the final product,” Luhr said. “We could ask for a high, moderate, or low level of any one of these attributes. One is not necessarily right for every application. Instead, we look at what is right for a specific owner and what we’re building it for.”

Equipment for RCC

For big projects, Peltz’ Gray said, RCC often will be mixed onsite using a classic pug mill, which can produce 200 to 400 cubic yards an hour. “The problem with pug mills is that they are expensive to bring onsite, and if you don’t have a big job, there’s no way to overcome their mobilization costs,” Gray said.

RCC, like any other granular material, is taken to the jobsite in dump trucks. Once there, European-sourced pavers and compactors intended to place asphalt are ideal for placing RCC, also, said Brodie Hutchins, general manager, Vögele America, at the May workshop. The more-rugged construction of the paver is one reason, but a more important one is the ability of a European paver to get high levels of compaction at the screed.


“There are major differences between the tractors of American and European pavers, and most importantly, the screed,” Hutchins said. “Much of the difference lies in the compaction available at the screed.”

European pavers are substantially taller and of more of a boxy design, Hutchins said. “With European pavers, we don’t pave as though we are placing an American-style asphalt overlay,” he said. “We are talking deeper depths, and wider widths, which works out to a slower pace.”

There is a substantial difference in the weights, too, Hutchins said. “An average U.S.-style paver with screed is around 45,000 pounds, versus a European-style machine with a screed, in the neighborhood of 60,000 pounds,” he said. “American machines have a basic width of 10 feet for highway jobs, whereas most of the machines from Europe will be 8-foot machines. They look much bigger but actually they are narrower in width.”

In the U.S., most of the screeds are front- and rear-mounted screeds, meaning the screed extensions may be mounted on either the front or the rear of the main screed. “But in Europe, everything is rear mounted, or fixed,” Hutchins said. “That means their extensions are bolted onto the main screed, so you don’t have the ability to go in and out unless you have a rear-mounted screed with hydraulic extensions.”

And in the U.S., the energy in the screed is primarily vibratory, using shaft-driven integral vibrators. “On a European-style high-compaction screed, there are other compaction mechanisms, such as tamper or tampers, pressure bars, and vibration,” Hutchins said. “A typical 10-foot screed for a U.S.-style paver will weigh 7,000 to 8,000 pounds, where in Europe, it will be a 10,000-pound screed. The tampers and eccentric shafts, and other components, in the European screed add to the weight.”

In addition to RCC, or paver compacted concrete, European-sourced pavers with these attributes will place cement-treated base, crusher-run aggregate, soil cement, drainable concrete, and asphalt-treated base. “Some contractors even will put in stone base using these pavers,” Hutchins said.

Use of a paver for base or other heavy material eliminates a lot of the guesswork and repetitive actions caused by conventional placement, Hutchins added. “Count the number of machines required for conventional base work. You will have dumping, dozing, blading, rolling, do it again until you get it right, and then throw away the excess,” he told the workshop. “Or you can place it to grade with accuracy and compaction, depending on the material. And you don’t have to clean up the edges.”

Typical densities behind the tamping screed will generally be in the low 90s, Hutchins said. “On one placement, we got 99% density in 10-inch-deep slab,” he said. “High screed compaction reduces the roll down and the number of machines you need to have on a project. Your yield loss is virtually eliminated, and you have better production rates with fewer machines. You put RCC down one time, and roll it for finishing purposes, without the marks of dozers or multiple passes with rollers.”

The paver used in the workshop demonstration at the Tennessee Department of Transportation regional office was a Vögele Super 2100-2 with extendable TP2 screed, with single tamper bar and two pressure bars on the back. The Super 2100-2 was placing RCC 6-inches deep, at variable widths, and all compaction was being obtained at the screed. A Hamm HD 110 HV roller was being used for finish only, without vibration.

PCA concurs on pavers

In its literature, the PCA concurs with Hutchins’ theme, maintaining that Eurostyle asphalt pavers make it possible to place RCC pavements without the need for much additional compactive effort. “Newly developed high-power compaction screeds for the drier, stiff concrete mixes typically used in RCC have been used with some success in Europe,” PCA says. “Eliminating the need for consolidation using rollers makes this paver compacted concrete extremely attractive.”

PCA quotes Siegfried Riffel, responsible for project management for highway construction with Heidelberg Cement in Germany, as saying these European pavers place, then compact, the zero-slump concrete mix from the surface with tamping, vibrating, and pressing compacting systems, achieving the minimum specified density.

Densities equal to or greater than the required 98% of a modified Proctor test have been obtained directly out of the back of European pavers, PCA says, resulting in a working width, paving depth, transverse road profile, surface accuracy, and surface texture of the highest possible quality achievable for roadway pavements.

“PCC placed with these new high-density pavers offers many technical and economic advantages,” PCA says. “It is, for example, possible to achieve high quality in terms of strength, durability, and surface finish at relatively low device and personnel costs. The fully mechanical compacting process makes it possible to walk on the surface or lay the second layer with the road paver directly after the high-power compaction screed. Depending on the desired thickness and width of the installation, the concrete can be laid very quickly, from 60 up to 120 meters (approximately 200 to 400 feet) per hour.”