A systems approach to design considers numerous variables to extend life and reduce costs — By Dan Dawood, P.E.
The cold climate found in regions such as Canada, the northern U.S., and northern Europe poses many challenges to engineering a long-lasting, high-quality pavement. The cold can cause concrete and asphalt pavements to crack prematurely and develop many other distresses, leading to more maintenance needs and increased cost. Numerous variables are involved in engineering pavement projects in cold climates, including structural design, material selection, construction techniques, and the timing of maintenance interventions.
Transtec has found it best to approach projects like these from a systems perspective, emphasizing the interdependency of all the variables involved. Rather than focusing on one aspect, or assuming similar pavements, materials, and construction techniques result in the same fashion in all areas, Transtec understands the relationship between pavement performance and material characteristics under various climatic conditions.
Optimizing a concrete pavement in Winnipeg
Transtec provided pavement design and life cycle advisement for the 7.6-kilometer Southwest Rapid Transitway (Stage 2) public-private partnership (P3) project in Winnipeg, Manitoba. The pavement structure for the transitway required the structural capacity to carry heavy bus traffic.
The cold climate in Winnipeg causes subgrade soils to thaw only slightly during summer months, which leads to pavement profile deformation and other distresses. Typical assessments may overestimate the maintenance required, leading to higher costs. Transtec’s expertise in innovative pavement design means providing recommendations to improve the pavement’s life and decrease the cost for pavement design, construction, and maintenance.
Transtec evaluated northern U.S. states and other Canadian provinces to assist the concessionaire in finding competitive pricing for operations and maintenance strategies, such as diamond grinding. Transtec also recommended enhanced joint and drainage design to mitigate the effects of low temperatures within the base foundation structure.
In addition, Transtec provided recommendations and a separate verification process to ensure an optimized pavement design. AASHTOWare Pavement ME Design, a mechanistic analysis, was used to assist in verifying the proposed operations and maintenance plan. Transtec analyzed the designer’s predicted maintenance and operational needs by concurrently running similar pavement designs to observe their rehabilitation models in the software.
The completed transitway will connect Winnipeg’s downtown with the Southwest sector and the University of Manitoba and will allow city transit to bypass heavy traffic on Pembina Highway. The Southwest Rapid Transitway is one of the City of Winnipeg’s high-priority projects and is part of the Transportation Master Plan to address projected population growth. Construction began in late summer 2016, with expected completion in late 2019.
Designing asphalt pavements to withstand the elements
On asphalt pavements in cold climates, Transtec focuses on binder selection, especially the low-temperature properties of the binder. High-quality crude oil is often the key to improved low-temperature properties. Binders can be further modified with polymers to ensure even higher performance. Aggregates used in the asphalt mixture can also be engineered to resist rutting and cracking. This begins with assessing the source quarry or pit, crushing methods used, and the final size and shape of the aggregate particles.
Through field reviews of existing pavements in cold climates and a study of the methods typically used to construct pavements there, Transtec found that common distresses include thermal cracking and deterioration from numerous construction-related defects. This type of evaluation can help reveal materials and construction methods that can be used mitigate premature deterioration.
Design-build and P3 projects commonly specify rigorous pavement performance requirements, including cracking, rutting, and ride quality. To meet these requirements when constructing pavements in cold climates, Transtec recommends designing and constructing to a higher standard than the minimums usually specified. It is often better to construct pavements smoother than typically done for other projects in a similar area for the pavements to remain smooth during their lives. To achieve this, construction methods that improve smoothness must be implemented.
Engineering a smoother pavement improves its long-term performance. The time it will take to reach the pavement’s performance limit is extended, which means there will be fewer interventions or overlays, reducing the impact to the public traveling on the road. Fewer interventions also decreases the pavement’s life cycle cost, which saves the taxpayer money. Decreasing costs and reducing impact to the public is a common challenge for projects in cold climates, and particularly for urban highways that carry significant levels of traffic.
Building large-scale projects under stringent performance requirements in cold climates makes it difficult to know exactly how the pavement will perform. Transtec engineers’ expertise and experience in all aspects of pavement engineering can help clients with more dimensions of a project, providing guidance for the developers to ensure a quality, long-lasting pavement.
Watch a video about pavement design on the Southwest Rapid Transitway at https://youtu.be/ARzasxsYlWA.
Dan Dawood, P.E., has been the director of business development and engineering at The Transtec Group since November 2008. He leads the Alternative Delivery Projects team for Transtec. Prior to joining Transtec, Dawood served as the chief pavement engineer for 16 years with the Pennsylvania Department of Transportation, where he worked for 25 years. The Transtec Group (www.thetranstecgroup.com) is a pavement engineering firm that specializes in pavement design, pavement research, pavement testing, pavement construction support, and creation of new pavement software and hardware tools.