Material and Design of Highway Construction

The construction of the modern highway is achieved through a detailed survey process and significant sub-grade preparation. The methods used for designing and constructing roadways have become more and more sophisticated over time. Skill levels vary from project to project and depend on factors such as new construction versus reconstruction and urban versus rural projects.

During the process of Highway Construction, it is to decide which material and design to use.

Methods and Procedures

There are two basic elements to highway construction: technical elements and commercial elements. Technological elements involve the materials used for the road work, the quality of these materials, installation techniques and traffic patterns. Commercial elements involve contracts, environmental impacts, local and political aspects and concerns of the public.

In general, road construction is initiated at the lowest point of the work site and moves upward. The work areas are surveyed to determine the equipment required for excavating and grading the site, the vehicles needed to transport material to and from the site, the type of materials to be excavated, de-watering methods, stabilization requirements for excavating procedures and sufficient water for compaction and dust control.

The Sub-base

The sub-base has a layer of materials that is sandwiched between the sub-grade and the base course of the roadway. The sub-base can range from four to 16 inches in thickness and is able to withstand the weight requirements of the pavement along with the expected traffic load. Sub-base materials tend to be either gravel or crushed stone. They can also be comprised of subgrade soil stabilized with cement, fly ash or lime.

More recent roads may be constructed with a permeable sub-base, which allows surface water to drain away rather than pool at or near the surface. Permeable sub-base courses also prevent water below the surface from rising to the level of the pavement. If local material prices are high or are not easily accessible, highway engineers often increase the loadbearing capacity of the underlying by using Portland cement, foamed asphalt or a polymer soil stabilizer. A sub-base is often omitted when foot traffic alone is expected but is always present for surfaces traversed by vehicles.

Sub-base Materials and Thickness

The quality of sub-base materials is extremely important for the life of the road and can often outlast the life of the outer layer. If the sub-base is in good condition, a new layer can be applied directly to the top without replacing the sub-base. Sub-bases are either made with unbound granular materials or cement-bound materials. Unbound granular materials are usually crushed stone, slag, concrete or slate combined with a fine or coarse material as needed to meet grading standards. In general, natural granular material does not require crushing. The gravel must be free of dirt, organic matter, shale and other substances that could affect the integrity of the material.

The quality of sub-base materials is extremely important for the life of the road and can often outlast the life of the outer layer.

Cement-bound materials are available in multiple configurations. Solid concrete is used for roadways bearing heavy loads. The thickness requirements for mass concrete roads range anywhere from four to six inches and may be reinforced with steel mesh or polymer fibers. Other cement-bound sub-base materials include soil cement and “lean mix” concrete. Lean concrete has a cement content of less than 10 percent of the total contents of aggregate, sand, fillers and cement combined.

The thickness of a sub-base ranges from three to four inches for garden paths and four to six inches for driveways or paths that receive heavy foot traffic. Heavily-used roads tend to have sub-base depth of six to nine inches, while highways can reach depths of 16 inches or more. With modern milling machines, low-quality sub-base materials such as large pieces of rock and concrete can be crushed and combined with binders to give it strength and stability.

The Base Course and the Wearing Course

The base course is the layer directly above the sub-base. If there is no sub-base, the base course is built directly on top of the sub-grade. The thickness of the base course can range from four to six inches and generally consists of crushed rock or concrete.

The base course is uniformly spread and compacted to provide a stable foundation for additional aggregate layer, the binder course and the wearing course. Base course aggregates can be either crushed stone or crushed gravel. Crushed stone aggregate bases must be free of dirt and containing no more than eight percent of non-uniform pieces. Crushed gravel base courses are made from hard rock crushed to specific sizes. They, too, must be free from non-uniform pieces of rock, dirt and any other potentially compromising material.

The wearing course is the topmost layer on a roadway. In rigid pavements, the wearing course is a concrete slab made with Portland cement. In flexible pavements, the wearing course is made with asphalt concrete or another flexible paving material. The term “surface course” is sometimes used interchangeably with wearing course but is technically the term for a thin surface layer such as the chip seal used on lightly traveled roads as a low-cost resurface.

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