Will 2018 be the year the United States launches major infrastructure projects? The parlous state of the nation’s roads and bridges, from those maintained by local authorities to the federal interstate highway system, often takes precedence in discussions about potential infrastructure projects.

Road design and construction techniques and materials have evolved over the decades, increasing safety and durability while reducing construction and maintenance costs and environmental damage. Whether funding is applied to new construction or repair, highway engineers can apply these tools to extend the working life of this essential and complex infrastructure network.

Geometric Design of Roadways

Starting in 1937, the American Association of State Highway and Transportation Officials (AASHTO) promulgated best practices and policies for highway engineering. The association published its first guidance on geometric highway design – “A Policy on Geometric Design of Rural Highways” – in 1954, adding titles specific to different road types in AASHTO’s functional classification. Design criteria for all types of roads now appear in AASHTO’s Green Book; a new edition was published in September 2018. Federal highway regulations and standards for construction, reconstruction, rehabilitation and other activities comprise many of AASHTO’s recommendations.

Geometric design provides a standard, dimensionally based process, using design dimensions available in standard design tables, charts and equations. Starting with a set of specifications – the road’s route, usage,Source: Jo Naylor / CC BY 2.0 expected traffic, weather conditions and many others – highway engineers develop a detailed set of design drawings and material specifications.

This process encompasses three key components:

• Horizontal alignment refers to a road’s straight sections, called tangents, and the horizontal curves that connect them.
• Vertical alignment refers to vertical straight lines and the curves that join them, reflecting the road site’s physical gradients.
• Cross section consists of elements that affect pavement life and riding quality. Camber, curbs and other elements like bicycle and pedestrian paths are cross-section elements.

The National Academies of Sciences, Engineering and Medicine's more recent (2016) book, “A Performance-Based Highway Geometric Design Process,” reviews highway design history and suggests a revised, more expansive geometric design process focused on improving transportation performance. Existing design processes omit variables, like non-geometric elements (such as traffic control and speed regulation), political and financial considerations and the potential impact of future systems (such as self-driving vehicles). Updated design processes should produce roadways that take new technology and uses into account.

(Check out Paving Machines on IEEE GlobalSpec.)

Roadbed Preparation

The ancient Romans are justly praised for their roadbuilding acumen. Contemporary roadbeds use an adaptation of Roman methods, creating layers underneath the final paving to increase road durability. The materials used for these layers vary depending on the choice of final pavement – concrete or hot mix asphalt (HMA) – the geology of the course of the road and financial constraints, among other factors.

Roadbeds for both flexible pavement (HMA) and rigid pavement (Portland cement concrete) roads require three layers: prepared subgrade, base or subbase and the final pavement material. HMA roadbeds sometimes include both subbase and base to provide additional load distribution. Rigid pavement omits this layer.

The subgrade – the existing soil – provides part of the road’s structural support. Preparation depends on ground structure. Sometimes soil compaction is adequate; sometimes road builders amend the soil with lime, cement or polymers to achieve proper stability.

The subbase, when used in HMA roads, serves several functions: preventing fine subbase particles from migrating to pavement from the subgrade; improving drainage; minimizing frost damage; and providing a surface where construction machines can park and travel. Materials used in the four- to 16-inch thick subbase are usually aggregates of local materials.

The roadbed base provides load distribution and additional drainage. Durable aggregates, either stabilized or unstabilized, are commonly used.

The surface layer – the pavement – is in contact with traffic and therefore is the stiffest part of the road. Asphalt pavement is mixed to optimize performance for expected road conditions and use, and is laid down with an asphalt paver. Concrete roadways can be laid either slipform using a machine that spreads, shapes and float-finishes the concrete in one operation, or fixed-form, where concrete is extruded into fixed metal forms that are removed when the concrete is hard enough to hold its shape.

(Check out Soil and Asphalt Compactors on IEEE GlobalSpec.)

Road Surfacing Materials

Hot-mix asphalt and Portland cement concrete have been the standard surfacing materials since the turn of the 20th century, preceded in the 1800s by bituminous pavements such as tar macadam and sheet asphalt. HMA is the predominant choice for road surfacing due to its lower construction costs and suitability for many types of roads. Rigid concrete pavement lasts longer – 20 to 30 years in general – and is preferred for airport runways and high-use highways.

Hot-mix Asphalt

Referring to HMA as asphalt is something of a misnomer. Asphalt is the sticky, brownish-black hydrocarbon used to bind aggregates together into asphalt concrete, which is generally just called asphalt. Asphalt Stratum of materials used in modern road construction for a heavily traveled highway. Source: Ohio State Fair / CC BY-SA 3.0concrete has evolved over the last century to provide more durable and safe road surfaces.

Since its introduction in 1993, the asphalt industry and highway agencies have adopted the Superpave asphalt mix design system. Superpave is one outcome of a federally sponsored highway research effort begun in 1987 to improve performance, durability and safety of U.S. roads.

The system consists of three components:

• An asphalt binder specification: Binders affect stiffness, strength, durability, fatigue life, raveling, rutting and moisture damage of the final mix. Specifications include viscosity, elasticity and durability. Binder modifiers alter final mix properties as needed for each paving job.
• A design and analysis system based on the volumetric properties of the asphalt mix. The relative mass and volume of binder and aggregates affect the product’s performance; the system also considers voids in the compacted mix.
• Mix analysis tests and performance prediction models.

The Superpave design process is guided by extensive tables of component properties and protocols intended to fine-tune the final paving mixture into the optimal product for a given situation.

Modern asphalt concrete can be designed with an assortment of properties in addition to those affecting durability and safety. Quiet pavement can reduce road noise by as much as seven decibels. Porous pavement allows water to seep into the ground below rather than running off into a storm drain. Perpetual pavement has three asphalt layers; only the top layer needs to be replaced when it wears off. These roads could last up to 50 years, longer than concrete roads, greatly reducing maintenance and replacement costs. Follow this link to an excellent discussion of innovations in asphalt paving materials.

(Find Asphalt and Road Surfacing Materials suppliers here.)

Portland Cement Concrete

Rigid road surfaces are nearly always some form of Portland cement concrete mixed with aggregates, sand and water. Less-expensive ingredients, such as fly ash, are sometimes incorporated to replace some of the cement. Portland cement concrete surfaces are more expensive to build than asphalt concrete but typically last 20 to 30 years.

Concrete roads are classified according to the kind of joint used and presence of reinforcing steel.

• Jointed plain concrete pavement (JPCP) has no reinforcing steel and uses contraction joints to prevent cracking.
• Jointed reinforced concrete pavement (JRCP) which has both steel reinforcements and contraction joints.
• Continually reinforced concrete pavement (CRCP) does not require contraction joints due to the inclusion of continuous reinforcing steel.

Whether or not the pavement has steel reinforcement, metal dowel bars and tie bars connect concrete slabs and provide load transfer, which in turn helps increase road surface longevity.

When a concrete road is rebuilt or replaced, the existing road surface can be effectively recycled. Breaking up old concrete into pieces of various sizes provides aggregate for roadbed fill or to reuse in a new batch of concrete, whether asphalt or Portland cement concrete. The recycled material also serves as fill or a component of an embankment.

Research from the University of Michigan could lower the cost of concrete pavement and provide longer-lasting road surfaces. The university has developed a formula for high-quality, ultra-high-performance concrete (UHPC) that it is making available at a cost much lower cost than commercial brands – 70 percent lower. Although the new recipe costs more than regular concrete, the extended wear life and lower maintenance could make up for the difference.

(Find Hydraulic and Portland Cement suppliers here.)

For additional information, check the websites and the book listed below.

www.pavementinteractive.org

www.asphaltpavement.org

www.cement.org

Athanassios Nikolaides: Handbook of Highway Engineering. CRC Press, 2017