By Eric Ferrebee, Contributing Author
Roundabouts, which provide safer and more efficient crossovers compared to standard intersections, are being built more often in contemporary traffic management systems. Their circular design allows for continuous traffic movement, reducing wait times and the chance of severe mishaps.
Roundabouts can decrease accident fatalities by up to 90% and injury-related crashes by 76%, with a 35% reduction in all crashes, according to Federal Highway Administration (FHWA). Moreover, the slower travel speeds are generally safer for pedestrians. Roundabouts also augment traffic capacity and negate the requirement for traffic signal maintenance, making them an attractive choice for urban planning and civil engineering professionals.
However, roundabouts bring special challenges for pavement engineers and construction crews.
Given a roundabout’s continuous vehicular movement, designers and engineers need long-term, low-maintenance solutions that will require minimal future traffic disruptions for preservation and rehabilitation activities.
Concrete pavements provide a durable, long-life solution that requires minimal maintenance for roundabouts. Since concrete is inherently strong with a durable surface, it also is not prone to rutting and shoving, which can be issues experienced by asphalt pavements under repeated turning movements of heavy traffic. All these conditions make concrete the logical choice for constructing roundabouts.
The Role of Joints
Jointing is one of the main considerations for concrete pavement construction. Appropriate joint design is crucial to manage the placement of cracks and to assure the long-lasting performance of any concrete pavement.
On a typical concrete roadway, the pavement will have a rectilinear jointing pattern. However, rectilinear jointing can pose problems if used in roundabouts, due to their circularity and non-standard geometries. Rectilinear jointing in roundabouts can lead to irregularly shaped slabs, improper load transfer and increased wear and tear.
To tackle these challenges, engineers and contractors must establish a jointing strategy at the start of the design process.
The "isolated circle" approach and the "pave-through" method are two strategies that are usually employed. In the isolated circle technique, the roundabout's central circle is separated from the incoming roads. The pave-through method involves extending one of the incoming roadways' pavements through the roundabout, isolating the remaining legs.
Each method aims to ensure that the joints in the circular segment radiate from the center, while the joints in the legs run perpendicular to the pavement edges. These two methods have step-by-step guidance on joint layout that can be found on Wikipave.org.
While the isolated circle and pave-through methods are two of the most common jointing plans for concrete roundabouts, other solutions have also been used. The pinwheel method has been utilized in several states, as its joint pattern provides visual cues that help guide drivers out of the circular roadway.
Some states also have used continuously reinforced concrete pavement for roundabouts, eliminating the use of transverse joints and instead maintaining tight cracks through significant steel reinforcement. Other strategies also have been experimented with, including precast concrete roundabouts and even fiber-reinforced concrete roundabouts.
Pavement Thickness
Pavement thickness is an important consideration when building roundabouts, as it directly influences the durability and performance of the roadway. For most residential, collector and business roadways, the pavement thickness at roundabout intersections typically matches the thickest of the two approaching roadways.
However, intersections that handle high truck traffic from intersecting roadways may need additional thickness to effectively accommodate the increased load and prevent premature wear or damage. Using pavement design tools can help determine the appropriate thickness.
Engineers and planners can use tools like PavementDesigner.org to help them determine the appropriate thickness for a specific roadway. This resource calculates the required thickness based on factors such as the combined traffic through an intersection, soil type and concrete material properties, while providing design guidance supporting a well-informed decision that will increase the longevity and safety of the pavement.
Proper pavement design is paramount for maintaining the integrity of roundabouts, ultimately leading to safer driving conditions and improved traffic flow.
Mix Design Specifications
Although the materials and mix design specifications for roundabouts generally match those used in other concrete roadways, some modifications might be necessary to accommodate a variety of roundabout construction methods.
These modifications can depend on paving techniques and concrete delivery methods, such as slipform paving, fixed form with screed, hand-finished techniques and even pumping concrete.
Each method presents its unique benefits and challenges. Ensuring the right mix design is crucial, not just for attaining the desired durability and performance, but also for accommodating factors like traffic loads, environmental conditions and the structure's longevity.
A well-designed mix will enhance the roundabout's constructability and durability while providing a smooth driving experience.
Managing Traffic
In any location—but particularly in urban or congested ones—keeping traffic moving during construction is paramount. Developing a phased plan to manage traffic during a roundabout’s construction can help minimize disruptions, especially because roundabouts may require more construction phases than standard concrete paving projects.
Although complete or partial closures are most effective from a time and cost perspective, maintaining traffic flow throughout construction can be done with proper phasing and planning. A common tactic is to create a temporary bypass to allow the uninterrupted construction of the roundabout's circular roadway.
The Future of Concrete Roundabouts
The development of novel concrete roundabouts has arisen as a solution for managing increasing urban traffic congestion and enhancing road safety.
These innovative designs go beyond conventional roundabouts, incorporating features that improve traffic flow and pedestrian safety. One development is the use of turbo roundabouts to help further improve traffic flow and increase safety.
Another substantial innovation in concrete roundabouts is the inclusion of sustainable materials and practices. Using optimized mix designs with low-carbon materials not only diminishes the environmental impact but also delivers comparable strength and durability.
Utilizing local and recycled materials can also help improve costs and reduce environmental impacts. Such eco-friendly features have gained importance in urban planning as cities strive to balance infrastructure needs with environmental stewardship.
Other design approaches, such as mini roundabouts or oversized roundabouts with dedicated bicycle lanes, cater to the diverse needs of road users. Mini roundabouts, designed for smaller intersections, require less space yet still improve traffic flow and safety. Larger roundabouts equipped with separate lanes for cyclists encourage safer coexistence on the road.
By prioritizing all modes of transportation, these designs improve accessibility and promote active transportation.
Roundabouts provide numerous benefits, including safer roads, more efficient traffic movement and lower maintenance costs. These benefits make the perceived challenges in their construction well worth it.
By using technology and adhering to best practices in joint design and mix design, engineers and contractors can successfully construct concrete roundabouts that stand the test of time.
The American Concrete Pavement Association (ACPA) serves as a valuable resource for those participating in the design, construction and preservation of concrete pavements, advocating the use of concrete in roundabouts and other heavy-traffic areas.
Improving Infrastructure in Newport, Ky.
To accommodate increased commercial and residential development, the Kentucky Transportation Cabinet (KYTC) needed to improve the KY 9 corridor, including installing two high-traffic roundabouts on a complex riverfront site.
Kentucky chose concrete for the Newport roundabout because of its ability to handle the complex geometry and elevation transitions required. Concrete’s durability and minimal maintenance needs made it an ideal choice for this critical infrastructure project.
- Enhanced Safety: The concrete roundabouts feature 10-foot lane widths and improved joint design, offering superior safety for motorists and pedestrians.
- Reduced Maintenance: Unlike asphalt, which requires resurfacing every 5-10 years, concrete needs only minor preservation techniques, minimizing traffic disruptions and emissions from delays.
- Award-Winning: The KY 9 project was recognized for its excellence, winning the 2019 American Concrete Paving Association Excellence in Concrete Paving Award and the 2021 ACEC Kentucky Grand Conceptor Award.
Location: Newport, Kentucky.
Contractor: Prus Construction.
Owner: Kentucky Transportation Cabinet (KYTC) District 6.
Engineer: Gresham Smith.
Eric Ferrebee, P.E., is a senior director of technical services at the American Concrete Pavement Association.
