U.S. Highway 78 in Aiken, South Carolina, had deteriorated to the point where a thin resurface treatment was not a viable option. Looking for a solution to replace the failed asphalt roadway, the South Carolina Department of Transportation (SCDOT) explored multiple options, primarily focusing on minimizing traffic disruption, long-term durability, and cost of construction. The repair method chosen consisted of milling out the distressed asphalt and replacing it with 10 inches of roller-compacted concrete (RCC). To provide the desired ride quality for high speed traffic SCDOT chose to diamond grind the RCC surface rather than cover it with a thin asphalt or conventional concrete surface.
RCC pavement on US-78 in Aiken, South Carolina, was opened for traffic one to two days after placement
When it was successfully completed in August 2009, the diamond ground RCC pavement paved the way to add high-speed roads to the list of RCC pavement applications. For the past three decades, RCC pavements have been mainly used where strength, speed of construction, and economy are the primary needs. RCC became the pavement of choice for many intermodal and military facilities; parking, storage, and staging areas; intersections; highway shoulders; and low-speed roads. However, this recent application could certainly open doors to the use of unsurfaced RCC for high-speed roads as well.
The project consisted of a four-lane, one-mile long route of failed asphalt pavement that required rehabilitation or replacement. The road has a speed limit of 45 miles per hour and an average daily traffic count of 6,000 vehicles. As previously mentioned a key criterion was the minimization of traffic disruption. Traffic control measures were implemented to maintain at least one lane in each direction open for traffic at all times.
Satterfield Construction, Inc. of Greenwood, South Carolina, was the general contractor for the project. Satterfield performed milling and hauling of asphalt and Morgan Corp of Spartanburg, South Carolina (RCC subcontractor) prepared the subgrade and performed the RCC work. To prevent damage to the subgrade from rain and for safety reasons, construction procedures were sequenced to ensure that all milled areas were repaved during the same work shift.
The roller-concrete cement was mixed using a Rapidmix 600C pugmill mixer set up near the jobsite. Dump trucks transported the material to the pavement site and discharged the RCC into a Gomaco RTP-500 Material Transfer Device equipped with a 35-foot long swivel placing conveyor. The RCC was then conveyed to an ABG high-density paving machine (IR ABG Titan 7820) that placed the RCC in a single 10-inch lift. A 12-ton dual steel drum vibratory roller and a rubber-tire roller were used to compact the RCC to the required density. The rolling pattern with the steel drum roller generally consisted of two passes in static mode followed by two passes in vibratory mode and finished with one more pass in static mode. The rubber-tire roller provided final compaction and tightening of the surface texture.
Following compaction, concrete curing compound was sprayed on the RCC surface to prevent excessive moisture evaporation and provide adequate moisture for cement hydration especially at the surface. One to three hours after final compaction, an early-entry saw was used to cut transverse control joints spaced at 20 feet apart. To minimize disruption, the pavement was re-opened for traffic generally within 24 hours from RCC placement. Project construction began on July 16 and was completed by August 1, 2009.
RCC Mix Design
Froehling & Robertson, Inc. of Greenville, South Carolina, performed the laboratory mix design and quality control testing during construction. RCC aggregates consisted of crushed granite graded aggregate base from Aggregate USA’s Dogwood Quarry in Grovetown, Georgia. The aggregate grain size distribution is listed in Table 1.
The roller-compacted concrete (RCC) mix contained portland cement Type I/II from Giant Cement, Holly Hill, South Carolina. A 4,000-pounds-per-square-inch (psi) compressive strength at 28 days was specified. An RCC mix with a water-cement ratio of 0.41 (based on saturated surface dry condition of aggregates) was selected for the project. Compressive strength of field molded cylinders averaged 4,240 psi at three days; 4,610 psi at seven days; and 5,250 psi at 28 days.
Diamond Grinding to Improve Surface Smoothness
To improve the riding surface smoothness of the roller-compacted concrete the SCDOT chose to use diamond grinding. Prior to grinding, the newly constructed RCC pavement had an average International Roughness Index (IRI) of 200 inches per mile. Where soft subgrade was encountered, slightly wetter than optimum RCC was needed to achieve the required density. Compacting wetter RCC mixture against a weak compressible subgrade contributed to higher IRI values. Where RCC was placed and compacted against stiff subgrade, the IRI ranged from 100 to 120 inches per mile before grinding.
For typical roads with speed limits of 45 miles per hour or higher, a smooth road has International Roughness Index of 85 inches per mile or less. Approximately one week after placement, a milling machine was used to remove higher surface spots of RCC. This was followed by diamond grinding to provide the final smooth surface. Penhall Company was the grinding subcontractor. After grinding, the pavement IRI was determined to be within the range of 50 to 60 inches per mile.
This project marks the first successful completion of diamond grinding roller-compacted concrete pavement for a major road in the United States.
Andrew Johnson, Ph.D., P.E., state pavement engineer with SCDOT said, "RCC has a great deal of potential as an economical, rapid, and durable paving material. However, it has not provided the ride and surface texture the public expects for high volume, high speed traffic. By using grinding to achieve a smooth, quiet surface, we can use RCC in a much wider variety of situations without the added cost of a hot mix asphalt surface layer."