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 arc sawed at about 30-fool 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 (1½ 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.DURABILITY CONFIRMED
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. But 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 equivalent performance 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 five-year 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.”
With all of its advantages, one might expect to pay a premium for RCC, 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 the American Concrete Institute (ACI 325.10R-95, State-of-the-Art Report on Roller Compacted Concrete Pavements).