Designing Longevity into Concrete

By Eric Ferrebee, Contributing Author

While concrete is one of the oldest construction materials, experts have continued to develop techniques for its design and optimization. As a result, modern concrete pavement construction practices can minimize early-age distresses and reduce cost, time investments and environmental impacts. 

Research-based advancements in design, construction and materials technology are improving processes for new construction, maintenance and rehabilitation.

Concrete’s inherent durability, combined with emerging technologies and practices, has led practitioners to become more ambitious over the years in terms of what can be achieved with a concrete pavement’s design life. 

A Federal Highway Administration (FHWA) Concrete Pavement Technology Program (CPTP) Tech Brief states, “In the past, concrete pavements were routinely designed and constructed to provide low-maintenance service lives of 20 to 25 years…More recently, there has been a movement toward construction of pavements with a longer initial service life—40 or more years.” 

A 20-year design life was standard when the interstate highway system was originally constructed. Many of those concrete pavements met and exceeded those initial design lives. Now agencies are recognizing that longer life concrete pavements are not only feasible, but more cost-effective. 

Successful achievement of a concrete pavement’s full 40-plus-year service life depends upon good structural design, the use of durable materials and following best practices in the construction process. It is also important to provide timely maintenance and preservation activities throughout the pavement’s service life. 

This typically means using concrete pavement preservation techniques such as full- or partial-depth repair, diamond grinding, joint and crack sealing and minor surface treatments, all conducted at the proper time. 

Concrete overlays are another option for rehabilitating pavement, and it can be used on existing concrete, composite or asphalt pavements. Like new concrete pavements, these can be designed for long-life with many achieving similar lives to new, full-depth concrete pavements. 

Structural Design

Best practices begin with the pavement’s foundation, which consists of the subgrade, or ground surface, and the subbase and base layers. Together, these layers need to provide uniform support, have volumetric stability with minimal erodibility and provide proper drainage. 

Subbase and base layers should be constructed of non-erodible materials and should facilitate good drainage, since slab cracking and pumping can result from poor drainage. 

One common misconception is that stiff and stabilized base layers are always needed for pavement foundations. Since concrete is a rigid surface, it distributes the load over a larger area than an asphalt pavement, meaning the excess stiffness of the foundation layers is typically not required and may not result in the most economical pavement section. 

Modern design tools can be used to evaluate the performance impact of various foundation layers, and they should be used in conjunction with their expected cost to determine what is the best solution. In many instances, a dense graded aggregate base can provide the most economical section, having adequate drainage, support and stability. 

In some instances, stiff, stabilized bases may provide performance benefits, but the associated costs should be weighed against the predicted performance. 

Historically, increasing slab thicknesses has been the go-to solution to extend the life of a concrete pavement. Doing so not only enhances the structural capacity of a pavement but also will accommodate the minor loss of thickness associated with future diamond grinding preservation activities. 

In fact, an old industry rule of thumb was, “one extra inch of concrete thickness will approximately double the load carrying capacity of a concrete pavement.” This can be easily evaluated even with old design methods such as AASHTO’s 1993 pavement design procedure. 

The American Concrete Pavement Association (ACPA) showed this with a number of examples for streets and highway pavements where the AASHTO 93 design guide was utilized to evaluate the expected traffic (in the form of equivalent single axle loads or ESALs) for various thicknesses of concrete pavement. 

When scenarios are run using, for instance, interstate highway design thicknesses of first 10 and then 11 inches, an 85% increase in load carrying capacity is associated with the 11-inch thickness. Similarly, going from an 8-inch to a 9-inch-thick concrete pavement results in an increase of 105%.

State departments of transportation that have experimented with designing for extended life of their concrete pavements have typically defaulted to utilizing increased slab thicknesses in their designs. This strategy has not only been shown to work with the older AASHTO 93 design procedure, but also with modern design methods.

The strategy of adding some minimal thickness of concrete effectively reduces the applied stresses in the concrete from the added years of traffic loadings. This additional thickness will have a minor added cost, but the extra years of service life can result in reduced life-cycle costs. 

Optimizing Designs

While increasing the concrete thickness has been the typical strategy for achieving long-life concrete pavements, it is not the only method for optimizing concrete pavement designs. Additional strategies for optimization focus on features of a concrete pavement such as the geometry of slabs, the joints and load transfer systems and the mixtures themselves. 

Increasing concrete thickness is an effective strategy to achieve long-life because it helps reduce stresses from the applied traffic loads. The other concrete dimensions (slab length and width) can also be harnessed to reduce stresses. 

Over the years, typical concrete slab length or joint spacing has been reduced from 20 feet or greater down to a modern practice of 15 feet (for pavement thicknesses approximately 8-9 inches or greater). This effectively reduces stresses in the slabs and reduces the risk of cracking and spalling. 

Decreasing the joint spacing to less than 15 feet can help further limit curling and warping and minimize the number of times a slab is loaded simultaneously on both sides of the slab by trucks traversing the pavement (i.e. cutting the joints just short of the typical axle spacing of a semi-truck). Evaluating the impact of joint spacing on design can only be achieved with modern design tools. 

In addition to slab length, slab width can also help reduce stresses and result in optimized designs. Widening a slab from a conventional 12 feet to 13 feet and maintaining the traffic striping at 12 feet will shift traffic further off of the free edge of the pavement and reduce the applied edge stresses. 

Similar effects can be achieved by using tied concrete shoulders. In the past, some agencies have tried utilizing slab widths of 14 feet, but in some instances, this has resulted in longitudinal cracking. This indicates that there is a limit to the practicality of optimizing the slab width. 

The primary consideration for optimizing joints spacing and slab geometry is to reduce stresses in the concrete slab. This minimizes the risk of cracking, similar to the effect of adding thickness. However, optimized slab geometry can also have a significant impact on expected slab faulting. Proper joint geometry and load transfer at the joints can reduce cracking and faulting, resulting in a smooth concrete pavement optimized for long-life. 

It has become standard practice to include dowel bars, sized and located appropriately, at transverse joints. Corrosion-resistant bars will optimize the life of the pavement by providing load transfer throughout the service life. Dowel bar specifications have been evolving to consider newer materials that are designed to deliver long-term performance. 

While design tools do not currently consider the impact of dowel materials, they can evaluate the effect of dowel size on expected faulting and roughness performance. 

Design optimization allows engineers to evaluate various design features and see their effect on performance, cost and environmental impact. Some of the strategies mentioned may result in increased maintenance costs. 

For example, shortening the joint spacing does result in more joints and therefore would require slightly increased construction costs and efforts to maintain joint sealants throughout the pavement’s life. 

The Concrete Sustainability Hub at the Massachusetts Institute of Technology has recently developed a simplified pavement life-cycle assessment and life-cycle cost analysis tool that can assist in these types of evaluations. 

Durable Concrete Mixtures

A durable, long-life concrete pavement relies on having a proper concrete mixture. In general, that means a mixture optimized in cementitious materials that contains high-quality aggregates and has a good air-void system. 

The mix should have good volume stability under temperature and moisture variations. The concrete pavement industry has spent the last 15 years developing the Performance Engineered Mixtures (PEM) process, helping to develop and achieve durable concrete pavements that enable materials to realize the desired long-life. 

A low water-to-cementitious materials ratio (w/cm) is also desirable. A w/cm of 0.42 or lower is common for pavements in a moist environment that will be subjected to freeze-thaw cycles, but some states are achieving long-life pavements with lower w/cms. 

For example, Minnesota’s standards for high-performance concrete pavements include w/cms of 0.40 or less. 

Supplementary cementitious materials (SCMs) can be used to enhance a concrete pavement’s characteristics. Fly ash and slag are among the most common SCMs and are used to replace part of the cement in a mixture. These SCMs can increase strength and durability as well as improve workability. They can also mitigate alkali-silica reactivity, improve sulfate resistance, and reduce bleeding and drying shrinkage.

It’s worth noting that while concrete strength requirements are often a focus of attention when designing and installing a concrete pavement, overdesign in this respect should be avoided. 

Designing a concrete mixture with durability and workability in mind, not just increasing strength, is optimal for the long life of a pavement.

Adhering to best practices during placing and finishing represents the final phase in creating a long-life pavement. Concrete should ideally be placed during non-peak temperatures and be properly cured. Proper quality control measures and inspection efforts, along with modern specifications, are essential to achieving concrete pavements that are built to be smooth and can achieve long-life. 

By focusing on optimization and durability from design through construction and eventual maintenance activities, pavements can not only meet but exceed service life expectations. As the industry continues to evolve, embracing advancements and adhering to best practices will ensure that infrastructure investments yield sustainable, cost-effective, and resilient outcomes for generations to come.

Eric Ferrebee, P.E., is the senior director of technical services at the American Concrete Pavement Association.