Concrete played a major role in the construction of the U.S. Interstate Highway System during the past 60 years. The national focus has shifted from building new highways to maintaining and repairing the existing highway network. Recent advances in concrete technology enable highway contractors to rehabilitate the nation's 160,000 mile national highway system to extend its useful life with minimal disruption of traffic.
The national highway system, which includes the nearly 45,000 mile interstate system, carries 40 percent of the nation's total traffic, including 70 percent of the commercial traffic and 90 percent of the tourist traffic, according to the Federal Highway Administration. About 60 percent of the interstate system is concrete, especially in urban areas where FHWA anticipates heavy traffic loads. Concrete was selected, in part, because of its durability.
Concrete can support heavy loads, such as truck traffic, with less deformation than asphalt. Although the initial cost of concrete used to be higher than for asphalt, today concrete has become the least expensive alternative for new construction on a first-cost basis in addition to maintenance costs being generally lower. In addition, concrete generally has a useful life of twice that of asphalt. Concrete commonly serves 20 to 30 years without needing major repair, while asphalt typically lasts only eight to 12 years before resurfacing or significant repair is required.
History of Concrete Highways
The first concrete highway constructed in the United States was a 24-mile long, 9-foot wide, 5-inch thick strip of concrete pavement built near Pine Bluff, Arkansas, in 1913—five years after the introduction of the Model T Ford. By 1914, concrete had been used to pave 2,348 miles of roadway. Highway construction received a significant push forward two years later when President Woodrow Wilson signed the first Federal-Aid Highway Act directing the federal government to help states finance road building. In 1919, Oregon became the first state to level a fuel tax on gasoline to finance road construction. Today this is still the primary method of financing road building and maintenance. The Pennsylvania Turnpike, built on a railroad right-of-way during the 1930s, was the first major intercity turnpike or toll road in the United States and was constructed of concrete.
Significant technical and design developments during the 1930s and 1940s made concrete paving faster, less expensive, and more durable. Road designers stopped requiring contractors to build roads that were thicker at the edges—concrete highways were generally six inches thick at the middle and eight- or nine-inches thick at the edges—and permitted construction with a uniform concrete depth, saving time and money. Designers began to require that sub bases of gravel, crushed stone, or slag be placed beneath concrete highways in the late 1930s, when an increase in heavy truck traffic caused pumping, a phenomenon in which a concrete slab loses support and cracks as wet clay and soil particles underneath shift and are pumped from beneath the slab at its edges.
In the 1940s, some highway departments began to use soil-cement as a subgrade for highways. At this time, contractors also changed their method of creating pavement joints. Rather than forming the joints when the concrete was fully plastic by lumping it up to either side of the joint, contractors began sawing the concrete once it was partially hardened to create a smoother joint. This change in procedure helped create more even highway surfaces, and eliminated the familiar "bump, bump" drivers feel at some aging slab joints.
At this time, concrete pavement also exhibited problems with scaling, the flaking or peeling away of the surface, which studies determined to be the result of freeze-thaw cycles, accelerated through the use of deicing salts. Studies showed that the introduction of tiny air bubbles in the concrete mix could reduce the problem. This led to the development of air-entrained concrete, now used in virtually all U.S. road building. The invention of the slip-form paver in 1949 was another milestone in the development of concrete paving technology, as it allowed road crews to place wide sections of concrete continuously and therefore far more efficiently than before. Slip-forming is now used for highway paving projects in almost every state.
Many consider the construction of the interstate highway system, during the 1960s and 1970s, to be a heyday for concrete paving, and road building in general. But even as thousands of miles of concrete highway were formed, research and development continued, improving methods of placing and maintaining concrete. In 1976, the U.S. Congress recognized the need to specifically finance maintenance of the highway system, and approved federal funding for the 3R Program: restoration, rehabilitation, and resurfacing.
New Construction Techniques
Several relatively new techniques make it possible for concrete contractors to rehabilitate and resurface highways efficiently with minimum traffic interruption. Among these is fast-track concrete pavement technology, in which high-early-strength concrete is used to allow reconstructed roads to open more quickly. While conventional concrete mixes might require a curing time from five to 14 days, fast-track concrete can meet roadway opening strengths in 12 hours or less. Although combinations of ingredients vary, high-early-strength concrete commonly includes a higher proportion of the standard Type I cement to water or contains high-early-strength cement, known as Type III cement. Type III cement is virtually identical to Type I, except that Type III cement particles are ground much smaller. The smaller cement particles increase the surface area, allowing more cement contact with the water in the concrete mix, meaning faster hydration is achieved. Generally, fast-track concrete provides good durability because most of these concretes are air entrained and have a relatively low water content—factors that improve strength and decrease the chloride or salt permeability that damages steel reinforcement and contributes to deterioration.
Another relatively new technique that promises to improve highway smoothness and longevity is dowel retrofitting of existing concrete pavement that has undowelled slab joints. Since 1980, the slab joints of most new concrete highway pavements in areas where heavy loading is anticipated have been dowelled with 18-inch long smooth steel bars. The dowels bridge the joint sawed between the pavement slabs and help transfer traffic loads from one concrete slab to the next. The retrofitting technique involves cutting slots across the pavement joints, inserting the bars, patching the slots with fast-track concrete mixes, and then diamond-grinding the road to obtain a smooth surface. Department of Transportation officials in Washington—the first state to undertake dowel retrofitting on a large scale—expect to extend the life of some of the state's 30-year-old concrete highways by 10 to 15 years using the new technique.