Rapid International Ltd

The Rapid Continuous Mixing Plant was recently involved in the mixing of Roller Compacted Concrete (RCC) for a new type of road surface on Highway 78 in Aiken, South Carolina, USA.

As the current road surface had deteriorated to the point where a thin resurface treatment was not a viable option, 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 ; the Department of Transport chose to diamond grind the RCC surface rather than cover it with a thin asphalt or conventional concrete surface.

Satterfield Construction Inc. of Greenwood, S.C. was the general contractor for the project and they performed milling and hauling of asphalt. The Morgan Corporation of Spartanburg, S.C. (RCC subcontractor) who owns the Rapid Continuous Mixing Plant prepared the subgrade and performed the RCC work.

Adding High-Speed Roads to List of RCC Pavement Applications
This method was chosen based on minimizing traffic disruption, long-term durability, and cost of construction. 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 in the U.S, 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 4-lane, 1-mile long route of failed asphalt pavement that required rehabilitation or replacement. The road has a speed limit of 45 mph 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.

Rapid International’s Mobile Continuous Mixing Plant
The benefit of using the Rapid Mobile Continuous Mixing Plant for this project is that the plant is totally mobile and completely self-contained with its own power source.  It is fitted with a self erecting system, using hydraulics, which means that the plant can change from travel mode into a fully operational within a few hours. The plant is manufactured by Rapid International Ltd which is based in Northern Ireland. The Rapidmix 400 & 600 are both sold via a dealer network in the US which is overseen by Rapid Intl USA.

The plant can mix up to 600 Tonnes per hour depending on application and feed rates are fully adjustable for the aggregate, cement and water systems. For the purposes of RCC applications, the plant has the mixing efficiency needed to evenly disperse the relatively small amount of water used.

The Morgan Corporation set up the Continuous Mixing Plant close to site. Dump trucks then 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 concrete 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 2 passes in static mode followed by 2 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.

Diamond Grinding to Improve Surface Smoothness

To improve the riding surface smoothness of the RCC diamond grinding was used. 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 mph or higher, a smooth road has an IRI 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.

Potential of RCC
This project marks the first successful completion of diamond grinding RCC pavement for a major road in the United States. According to Andrew Johnson, Ph.D., P.E., State Pavement Engineer with SCDOT, "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."

To find out more about the Rapid Continuous mixing plant and watch videos of it in action, click here