Imagine a concrete mix for pavements delivered to the job site like hot mix asphalt, placed by asphalt pavers, rolled and compacted like hot mix asphalt,
Imagine a concrete mix for pavements delivered to the job site like hot mix asphalt, placed by asphalt pavers, rolled and compacted like hot mix asphalt, accommodating loads almost immediately like hot mix asphalt, and Û due to skyrocketing liquid asphalt prices Û at a cost comparable to hot mix asphalt. That’s what state and local road officials, road contractors, and Concrete Products magazine observed in Nashville at a roller compacted concrete (RCC) workshop and demonstration, sponsored by the Southeast Cement Association, Portland Cement Association, and American Concrete Pavement Association – Southeast Chapter.
Roller Compacted Concrete Pavement: Design and Construction Seminar and Demo, held in May, featured a high-level panel describing roller compacted concrete properties, current mix designs, field applications, and paving and compaction equipment needed to successfully place RCC. A field installation at a Tennessee DOT equipment yard rounded out the workshop.
RCC mix for the event was donated by two Nashville ready mixed producers: IMI/Irving Materials, Inc., and Metro Ready Mix Concrete, Inc. Mix designs were tested at Middle Tennessee State University with plant alterations by the suppliers. The May workshop will be followed by a similar event in Atlanta on Nov. 6. More information is available at www.secement.org/rcc_seminar.htm for details.
GAINING ON THE COMPETITION
Roller compacted concrete is a stiff, zero-slump portland cement concrete mix. With the escalation in asphalt prices eliminating hot mix asphalt’s long-standing price advantage versus concrete, some in the concrete industry now say that a more durable product Û RCC pavements Û can be placed at about the same cost, with the same machines used in asphalt paving, for less critical pavement applications such as rural or local streets, highway shoulders, and parking areas. Documents detailing RCC applications, mix design, production and placement can be downloaded at http://www.secement.org/rcc.htm. Additional information is available at http://www.cement.org/pavements/pv_rcc.asp.
Since 2004, RCC’s binder price has not risen as much as liquid asphalt. Cement and concrete industry experts note that RCC has a gradation similar to asphalt pavement, is placed with an asphalt paver, and then typically compacted with vibratory and rubber-tired rollers. Together, they emphasize, these qualities help RCC give asphalt pavements a run for their money wherever rougher RCC pavements can be used.
Through April 2007, hot mix asphalt prices have risen, while concrete has stayed relatively stable, occasional cement shortages notwithstanding. Associated General Contractors of America Chief Economist Ken Simonson reported in AGC’s March 2007 Construction Inflation Alert that the Producer Price Index (PPI) for liquid asphalt at the refinery Û as tracked by the U.S. Bureau of Labor Statistics Û had risen by February 2007 nearly 37 percent over the preceding 12 months, compared to nearly 6 percent for cement. Simonson noted the PPI for asphalt paving mixes had increased just over 27 percent in the 12 months ending December 2006, while ready mixed had risen nearly 10 percent.
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, the current calm is likely 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 percent rate, with wages rising at a 5 percent pace.
In the Pacific Northwest, 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. By contrast, the average unit bid price for a yard of concrete pavement was $246.34, down $20.33 from the average of $266.67 in the second quarter 2006. The trend of sharply rising liquid asphalt costs is chief among factors the cement industry seeks to exploit as it promotes roller compacted concrete.
An important element of RCC is its cost, affirmed Nashville panelist Will Gray, project manager, A.G. Peltz Group, LLC, a Birmingham, Ala.-based contractor specializing in RCC placements. It’s cheaper than most traditional concrete mixes and structures. For residential streets, a 20 percent cost reduction for RCC pavements is typical, compared to conventional portland cement concrete, said panelist David Luhr, Ph.D., P.E., program manager – Soil Cement/RCC for the Portland Cement Association.
Yet, that differential can vary a great deal, Luhr told Concrete Products. A general principle to keep in mind is that RCC uses an asphalt construction process and asphalt aggregate, so the main difference between RCC and asphalt pavements is the cost of asphalt and portland cement, and how the price of these binders fluctuates on the open market. Today, with the higher cost of liquid asphalt, 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.
With the cost of asphalt these days, we definitely see an opportunity to promote RCC for pavements, said Barry Wilder, pavement applications director for Tennessee, Southeast Cement Association (SCA), a workshop-sponsoring organization. That’s one of the reasons behind our workshops.
While RCC contains the same basic ingredients as conventional concrete, i.e., 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. Unlike conventional concrete mixes, RCC is not air-entrained; and, it has lower water and paste contents, but larger fine-aggregate content than standard concrete. It contains a nominal maximum-size aggregate (NMSA) aggregate of Ê- or ?-in. (see chart on page PC40).
Retarders or water reducers can be used to increase working time. Given the consistency of the mix, superplasticizers are commonly used in dry batch production to improve workability. Introduction of air-entraining admixtures is an option, but RCC is inherently freeze/thaw-resistant, even without air entrainment. Fibers are not often used.
According to the Portland Cement Association (PCA), RCC is sufficiently stiff to compact by vibratory rollers and does not require many of the features or techniques associated with conventional concrete: forms, finishing, dowels, or reinforcing steel. 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.
While RCC conventionally has been used for parking areas and equipment yards for container ports, rail terminals, truck terminals, and industrial yards where pavements are subjected to large forklift trucks, straddle carriers, log stackers, and mobile cranes, it has not been commonly used for roadways. The concrete industry now seeks to popularize RCC for low-volume roads in urban areas, highway shoulders, and urban parking areas.
To date, RCC has been used on roads in and around Chattanooga and Knoxville, Tenn., in Alliance, Neb., and in Columbus, Ohio. In Georgia, RCC was installed on 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, S.C., with the assistance of PCA, a 1000-ft. RCC installation was completed in March 2002 on Powell Pond Road for demonstration purposes. Mix supplied by Lafarge was placed by C. Ray Miles Construction Co. to a 6-in. depth. The performance of the section is being monitored.
RCC owes much of its economy to high-volume, high-speed construction methods, reports PCA. It is blended at or near the construction site in continuous-mixing, high-output pug mills, whose capacity is sufficient to evenly disperse the relatively small amount of water used.
Conventional dump trucks transport the RCC and discharge it into an asphalt paver that places the material in layers up to 10 inches thick. The material is dry enough to support a vibratory roller immediately after placement, yet wet enough to permit adequate distribution of cement paste.
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, as it makes moisture available for hydration Û 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 in moisture.
Cured RCC pavements exhibit high flexural strength (500 to 1,000 psi) and high compressive strength (4,000 to 10,000 psi). Once cured, specs typically 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 demonstrated in seven days. Tests performed on other projects, SCA notes, indicate that RCC flexural strength is equivalent to traditional concrete pavement.
Because the equipment used to place RCC is large, construction site geometry is significant, said A.G. Peltz Group’s Gray. It’s hard to get that large equipment in and out of small areas, he explained. We will not do a lot of twisting and turning. If your job has a lot of 5-ft. 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.
Moreover, Gray urged contractors to choose appropriate clients for RCC, as its final appearance likely would not be acceptable for a resort, but would be suitable for a robust industrial application. RCC is durable and has a good service life, he affirmed. It’s not the prettiest thing you’ve ever seen, but a lot of customers use it, because it’s fast Û we can lay a lot of material in a small amount of time Û 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 many of the same ingredients as conventional concrete, Gray reported. It takes added water just to get to a slumped state, he continued. We won’t place it around reinforcing steel, since we can’t consolidate that real stiff mix around it. There’s no finishing with trowels and bull floats. Instead, we finish 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. Our mixes contain 400 to 600 lb. of binder per cubic yard, and the binder content will be tied in with the aggregate used, Gray said. Different aggregates Û and different applications Û 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, he observed. A lot of RCC’s characteristics depend on aggregate, including strength, workability, and surface appearance.
In addition to the 400 to 600 lb. of binder per yard, Peltz’s RCC mixes include 3,400 to 3,700 lb. 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 noted. The water content is dictated by the optimum moisture for compaction.
Test sections are imperative for high-profile jobs, Gray emphasized during the workshop. In my opinion, if you are going to do a large-scale job, you’d better do a test section, Gray asserted. 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 ensure all the lab data is going to work out.
Most pavements will require density of 98 percent of modified Proctor, Gray said. If you don’t get density in this rigid pavement situation, you will have problems, he predicted. 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.
MIX DESIGN VARIABILITY
Within certain parameters, RCC mix design may vary widely, according to panelist David Luhr of PCA. All RCC is not created equal, he stated. 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.
Slab smoothness sometimes is defined in RCC specs, either as a deviation from a straightedge, such as ∫-in. over 10 ft., or by application of the IRI [International Roughness Index]. RCC is not as smooth as conventional concrete, Luhr informed workshop attendees. 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.
Durability and uniformity also are major considerations in designing the RCC mix. All of these factors 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 [mix] is not necessarily right for every application. Instead, we look at what is appropriate for a specific owner and what we’re building it for.
For big projects, noted Peltz’s Gray, RCC often will be mixed on site using a classic pug mill, which can produce 200 to 400 yd. per hour. The problem with pug mills, however, is that they are expensive to bring on site, he pointed out, and, if you don’t have a big job, there’s no way to overcome their mobilization costs.
Like any other granular material, RCC is transported to the job site in dump trucks. Following material delivery, European-sourced asphalt pavers and compactors are suitable for placing RCC, advised Brodie Hutchins, general manager, Vàgele America Inc. The machine’s highly rugged construction is a primary advantage, but more crucial is the ability of a European paver to get high levels of compaction at the screed.
There are major differences between American and European paver tractors, 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 a more ÎboxyÌ design, Hutchins observed. With European pavers, we don’t pave as though we are placing an American-style asphalt overlay, he explained. We are talking deeper depths and wider widths, which works out to a slower pace.
A substantial difference is evident also in the weights, Hutchins added. An average U.S.-style paver with screed weighs around 45,000 lb., compared to a European-style machine with a screed in the neighborhood of 60,000 lb., he said. American machines have a basic width of 10 ft. for highway jobs, whereas most of the European machines will be 8 ft. wide. They look much bigger, but actually, they are narrower in width.
In the U.S., most screeds are front- and rear-mounted, i.e., screed extensions may be mounted on either the front or rear of the main screed. But in Europe, everything is rear-mounted or fixed, Hutchins reported. Consequently, 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., energy in the screed is primarily vibratory, using shaft-driven integral vibrators. A European-style high-compaction screed employs other compaction mechanisms, such as tamper or tampers, pressure bars, and vibration, Hutchins continued. A typical 10-ft. screed for a U.S.-style paver will weigh 7,000 to 8,000 lb., whereas in Europe, it will be a 10,000-lb. screed. The tampers and eccentric shafts, plus other components, in the European screed add to the weight.
In addition to RCC, or Îpaver compacted concreteÌ (PCC), European-sourced pavers can effectively 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.
Using a paver for base or other heavy material eliminates a lot of the guesswork and repetitive actions involved in 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 elaborated during the workshop. Or, you can place it to grade with accuracy and compaction, depending on the material Û and, you don’t have to clean up the edges.
Densities behind the tamping screed typically will be in the low 90s, Hutchins noted. On one placement, we got 99 percent density in a 10-in.-deep slab, he recounted. High screed compaction reduces the roll down and the number of machines required for 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 demonstrated during the workshop at the Tennessee Department of Transportation regional office was a Vàgele Super 2100-2 with extendable TP2 screed, plus single tamper bar and two pressure bars on the back. It placed 6-in.-deep RCC at variable widths, and all compaction was performed at the screed. A Hamm HD 110 HV roller, without vibration, was used for finishing only.
|Sieve Size||Percent Passing by Weight|
|1-in (25 mm)||100||100|
|Ê-in. (19 mm)||90||100|
|_-in. (12.5 mm)||70||90|
|?-in. (9.5 mm)||60||85|
|No. 4 (4.75 mm)||40||60|
|No. 16 (1.18 mm)||20||40|
|No. 100 (150 m)||6||18|
|No. 200 (75 m)||2||8|
|From Production of Roller Compacted Concrete, PCA, Pub. #IS332, downloadable at www.secement.org|