As infrastructure managers look for ways to economize materials for pavement and water projects, an increasingly cost-effective choice is roller-compacted concrete (RCC).
The key to this concrete's growing popularity is the ability to place large quantities of it in a short time. And with simple, fast construction methods, pavements and dams can be put into service quickly.
As the price of crude oil drives up asphalt prices, the cost of the more durable and higher-strength RCC for pavement projects has undercut asphalt's advantage, even on a first-cost basis. For water projects, dams made with this concrete are being chosen over earthen embankments because of the speed of construction and the volume of materials needed. For the same height and crest length, an RCC dam requires as little as 1/8 the volume of an earthen embankment.
Recently, it has also seen use as a high-strength base course for city streets with an asphalt top course.
Fewer Steps To Completion
For pavements, an RCC mix has a low water/cement ratio (around 0.35, although it is more typically specified as a water percentage of 6% to 8%) and contains well-graded aggregates (usually no larger than ¾-inch). This produces a stable, zero-slump concrete that doesn't require forms, dowels, reinforcement, or finishing. When placed, it's dry enough that final consolidation can be achieved through the use of compaction rollers. And with such a dry mix, shrinkage (and therefore cracking) is minimal.
Mixes for dams and dam rehabilitation are similar except that well-graded aggregates containing particles as large as 2 inches are used and the water/cement ratio is significantly higher.
RCC is typically mixed in a pugmill, a high-energy mixer that ensures complete mixing with the very low water content. Hauling to the jobsite is usually done with dump trucks. For pavements, it is placed using graders or paving machines. For dam construction, it is spread into horizontal layers with dozers. The stiff mix allows compaction equipment immediate access.
Adequate compaction is critical to a successful project. High-density pavers such as the ABG Titan from Ingersoll-Rand and Vogele America Inc.'s Super 2100 use tamping and pressure bars to provide more than 90% compaction directly from the back of the paver. Vibratory drum rollers typically are required to achieve the final specified compaction. For a smoother surface, rubber-tire rollers can also be used. Four to six passes should achieve the required 98% of the maximum Modified Proctor density.
Due to its low water content, curing is especially important to ensure the moisture needed for cement hydration. This is accomplished by applying a membrane-type curing compound or misting the surface with water. Curing compounds are not, however, used in dam applications because they may form “bond-breaker” films at the interface between consecutive lifts and prevent proper bond at lift joints.
Composite pavements, which combine a concrete base with an asphalt topping, have been in use for years. The newest method is to use RCC instead of conventional concrete for the base.
Columbus, Ohio, has placed more than 100 street projects with this technique (see the January 2005 issue of PUBLIC WORKS for more on these projects). As a base course it provides the same high-strength pavement as conventional concrete, but more quickly and at a lower cost. By 2020, 80% of our nation's dams will be at least 50 years old, requiring replacement or major rehabilitation. RCC can provide overtopping protection for concrete or earth-fill dams and spillways. This allows water authorities to leave the existing material in place, and armor it with a layer of RCC.
RCC was the material of choice to buttress Loch Raven Dam, an almost century-old concrete structure in Maryland. Initiated in 2002, the project brought the dam into compliance with safety requirements and significantly increased its spillway capacity.
While RCC doesn't provide a surface profile suitable for high-speed roads, it's an excellent choice for shoulders. The Georgia DOThas built shoulders along 35 lane-miles of I-285 in Atlanta.