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.
And the possibility of higher-speed roads is not out of the question. Wayne Adaska, director of public works at the Portland Cement Association, says that as construction techniques improve and top size aggregates are reduced from the typical ¾ inch to as small as ½ inch, RCC pavement surfaces have become smooth enough for medium-speed traffic.
RCC is also used for pavements at composting and recycling facilities. The hard surface keeps recycled materials from being contaminated by gravel and soil, and it stands up to the heavy loads and punishment of the equipment.
American Iron and Metal needed a durable platform for a new metals recycling facility in Levis, Quebec. The company chose RCC for its low permeability, and was able to place the pavement and put the yard back into service within 72 hours.
— Palmer is former editor in chief of PUBLIC WORKS. Newman has worked in the construction industry for more than 30 years as a builder and inspector.
For more information on roller-compacted concrete and its application to public works projects, visit the "Article Links" page.
All the Right Reasons
Eleven ways to justify specifying roller-compacted concrete for a major road or water project
Low cement content: RCC for pavement applications uses 400 to 600 pounds of cement (or a combination of cement plus fly ash or silica fume) per cubic yard—similar to conventional concrete. However, at these cement contents with low water-cement ratios, compressive strengths of up to 10,000 psi can be achieved—much higher than conventional concrete at the same cement content. For gravity dams, less cement is required to meet the design strength, generally in the range of 200 to 400 pounds/cubic yard.
- Marginal aggregates: Marginal materials and industrial byproducts can be used successfully, especially for dam projects, as long as the fines content is controlled. Onsite aggregate is often used in dam projects.
- No reinforcement: The elimination of steel reinforcement represents a significant savings in materials and labor.
- Simple construction: It's easy to place, and contraction joints in pavements are unnecessary—unless crack control is important to improve appearance. Joints are required, however, in gravity dams to control thermal and shrinkage cracking. Finishing is also not needed, leading to rapid construction and a substantial reduction in labor.
- High compressive strength: RCC designed for pavements typically yields concrete with up to 10,000 psi, giving pavements high-impact resistance and the ability to handle heavy loads. For most gravity dams, high strength is not needed, leading to low cement contents.
- High flexural strength: Flexural strength of up to 1000 psi helps pavements span soft spots in the subbase and provide longer fatigue life.
- Durability: High density and low permeability reduces water absorption, making it more durable and resistant to freeze-thaw damage. Some of these pavements have been in place in eastern Canada for nearly 30 years.
- Cost: The overall cost beats conventional concrete, and it competes head-to-head with asphalt pavements and earthen-filled dams, providing a better product at an equal or lower cost.
- Life-cycle cost: The combination of strength and durability leads to less expense for maintenance and repair, and provides longer life cycles, thus lower life-cycle costs.
- Return to service: These pavements can be used by light traffic much sooner than other pavements, sometimes within a matter of hours. For neighborhood streets, this means residents can usually drive home the same day the pavement is laid. For dam construction, as soon as a lift is placed, machinery can immediately begin work on the next lift.
- Born green and still green: The invention of RCC was in response to an environmental concern when the Canadian logging industry came under pressure to clean up the waterways they were using for log transport. The solution was to build haul roads that would stand up to the extreme conditions. RCC combines that ability with easy recyclability, and a lighter color that reduces lighting needs and heat island effect.