Paving with roller compacted concrete: RCC topped with asphalt quickly provides a durable street
“This is the finest product for city streets to come along in years,” said Marty Savko of Nickolas Savko & Sons, Columbus, Ohio. “I first read about roller compacted concrete (RCC) pavement in 1995 and it sounded interesting so I did some research then sent our engineers to some jobs to learn more. The more we learned, the better it sounded, so I convinced the city to try it. Since 2001, over 100 street projects have been constructed with RCC pavements in central Ohio. Now we are producing 100,000 cubic yards of RCC a year.”
Savko’s first RCC street project was in Gahanna, Ohio. “Here in Gahanna, we like RCC for residential street rebuilds,” said city engineer Karl Weatherholt. “We’re able to get residents back on the street much faster than with an ordinary concrete base.”
What is RCC?
RCC was first developed as a paving material in 1976. Some Canadian builders had the idea of doubling the amount of cement in a soil cement mix to stabilize the surface at a log sorting yard. They got much more than they had expected–it was strong and durable, and it went down quickly and easily. A 1998 Portland Cement Association (PCA) study of this log sorting yard in British Columbia found quite a bit of cracking–not surprising since this was very early in the development of the construction method. Nonetheless, the yard continues in full service today and “the operators report that they are very satisfied with the performance of the RCC pavement.”
RCC is a very dry (zero-slump) concrete mix with 3/4 inch maximum size aggregate and overall well-graded aggregate so that it remains stable under the action of a vibratory roller. The RCC is placed using dump trucks and a modified asphalt paver, then rolled to get the needed compaction. The resulting surface looks a little rough, but it gains strength rapidly and the very low water-cement ratio soon far surpasses conventional concrete with an equivalent amount of cement. Since it is not as smooth as finished concrete, RCC is typically used as the surface course primarily for industrial areas, parking lots, or low-speed applications (less than 35 mph). For example, the Georgia DOT is currently constructing RCC shoulders on 15 miles of I-285 in Atlanta.
In central Ohio, composite roadways (concrete base with asphalt overlay) have been the standard for city streets for many years. The higher strength concrete base course is more durable and prevents the rutting that might occur with a full-depth asphalt base course. A study of city streets in Columbus by Michael Darter of ERES, a Champaign, Ilk-based engineering firm, concluded that the concrete base provided a load-carrying capacity about four times greater than full-depth asphalt.
Savko’s brainstorm was to replace the conventional concrete with RCC. In this application in central Ohio, RCC is normally placed in a single 6-inchthick layer with a 1 1/2-inch-thick asphalt topping. Occasionally, designers specify it at up to 9 inches thick, although Savko believes that this is over-designed because the RCC is so much stronger than engineers believe it will be. It can easily achieve 7000-psi compressive strength and 700-psi flexural strength. David Luhr, a PCA engineer and program manager for RCC said, “The higher strength of the RCC allows for the design of a thinner pavement section and will provide longer pavement life.”
Using RCC as the base course for city streets with a thin asphalt overlay is quickly gaining popularity in Ohio and elsewhere. For one thing, since RCC is basically a wet, compacted gravel, light traffic can go onto the new pavement almost immediately–within 10 feet behind the paver in extreme cases. This allows streets to be reopened much more quickly than with conventional concrete–usually about 2 hours after the RCC goes down. Residents can get back home the same evening after paving. Emergency vehicles can get return almost immediately if necessary. Truck traffic, however, should be kept off the pavement for a few days to allow the RCC to gain strength.
As a base course, RCC is much stronger than asphalt, and cheaper and faster going down than conventional concrete. “In one of the Columbus projects,” said Luhr, “the city required the contractor to obtain 400psi flexural strength before they could switch traffic back over to the completed RCC. The contractor was able to obtain that in about 24 hours. If the design calls for an asphalt or concrete surface over the RCC, the surfacing operation can begin within a few hours after placing the RCC.”
Because of the very low water content, RCC mixing is not as productive using a central mix plant–so a pug mill is often used, a very high-energy mixing device. Savko bought his own pug mill. Coming out of the mixer, the material looks very much like wet gravel, and it is then transported to the construction site in dump trucks. “The important thing is to control the moisture content,” said Savko. “We keep it at 8%. Even 1% over that is too much–the roller starts leaving marks.”
On a city street repair job, Savko mills out the old asphalt to the required depth of RCC plus asphalt topping. On many jobs, they take out the curbs as well. They then install new concrete curbs with a slipform curb machine. This also establishes grade elevation. The RCC can be laid with a standard asphalt paver, but then it requires more rolling to get it to the required compaction. Savko prefers to lay RCC with an ABG Titan paver with a dual tamping screed that can achieve compaction of nearly 90% right out of the paver (Vogele also makes a high-density paver). These pavers tamp at 1500 times per minute and are available in varying widths.
Savko then uses steel drum vibratory rollers to compact the RCC to achieve the specified density of 150 pounds/cubic foot. “We can usually get the required density with six to eight passes of our double drum Ingersoll-Rand DD90 vibratory roller;” said Savko. City inspectors follow behind, testing the density with nuclear gages to assure the proper compaction. There is no further finishing on an RCC surface.
Workers spray the surface of the RCC to keep it from drying out during its initial curing. They also spray the edges of the RCC course until the adjacent lane is paved to prevent a longitudinal cold joint. Within a few hours, transverse control joints are sawed at about 30-foot spacing (versus 20 feet for conventional concrete). No steel is used in the pavement–experience has shown excellent aggregate interlock at joints making dowels unnecessary. Soon after the joints are cut, the thin (1 1/2-inch-thick) asphalt layer is placed with an asphalt paver and rolled.
On a project in Calgary, Alberta, a major intersection was completely reconstructed over a 48-hour weekend by Standard General Construction Co., Calgary. More than 6000 square meters were replaced under full traffic conditions in one weekend, with one-half of the intersection closed at any one time. As a test, half of the intersection was paved with 150 mm of RCC and 15 mm of polymer-modified asphalt; the other half had 150 mm of RCC and 35 mm of conventional asphalt. The existing asphalt was milled out and the surface was swept clean. Standard General then placed RCC with an ABG Titan paver and compacted with a 16-ton Dynapac dual steel drum vibratory roller.
Final rolling was completed with a rubber tire packer to get a smoother surface. Workers then fogged the RCC until RCC placement on that half of the intersection was completed when a tack coat was sprayed on to seal the moisture in and prepare for the asphalt placement. Once asphalt was placed and had cooled, the intersection was reopened on schedule.
A study in 1986 seemed to suggest that there could be some susceptibility for RCC pavements to be damaged by freeze-thaw action. The U.S. Army Corps of Engineers performed a study in 1990 that found no damage at all to test sections of RCC from freezing and thawing and that concluded that “RCC is a suitable construction material for pavements in cold regions.” But, since it is difficult to get entrained air into RCC, the fear of surface scaling persists. When covered with an asphalt layer, this is no longer a problem, and no damage to composite pavements from freeze-thaw has been found. PCA research on numerous projects in North America found excellent freeze-thaw durability on unsurfaced RCC pavements.
The city of Columbus commissioned a study (by Resource International, Weterville, Ohio) of composite pavement with an RCC base. This report concluded that the RCC provided performance equivalent to conventional concrete and that both should “provide more than 30 years of service life under residential traffic conditions.”
Savko is so convinced that this pavement is durable that he provides a 5year unconditional warranty. “We offer this with no questions asked to anyone we place RCC for.” So far, no one has taken him up on this, because all of the RCC composite pavements he’s placed are performing perfectly. “Our RCC jobs have been cored more than any other jobs in the state,” said Savko. “They keep looking for something wrong but haven’t been able to find it.”
One might expect to pay a premium for RCC with all of its advantages, but in Columbus it is actually $2 per square yard less than conventional concrete. In many areas, RCC is cost-competitive with asphalt pavement (for an equal thickness of concrete).
More information on RCC pavements is available from PCA (www.cement .org/pavements) and from ACI (ACI 325.10R-95, State-of-the-Art Report on Roller Compacted Concrete Pavements).
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