Benefits of Concrete Streets

Concrete pavements can be designed to serve adequately every class of street from a heavily traveled arterial or industrial to a lightly traveled residential.

A comprehensive and highly successful study of pavement performance was conducted by the American Association of State Highway and Transportation Officials at the AASHO Road Test near Ottawa, Illinois, in 1955-60. The test established firm relationships between repetitions of loads and pavement performance.

Performance of a pavement is a measure of how well and how long it serves traffic. It is measured in terms of a serviceability index. At the AASHO Road Test a rating team assigned ratings of 0, very poor, to 5, very good (Fig. 3 below). When a pavement's serviceability index dropped to 1.5 it was considered failed and taken out of test. In the charts that follow (Figs. 4, 5, and 6), the serviceability index at the end of the test (1.1 million load repetitions) are shown. Or if the pavement failed, the number (in 1,000's) of load repetitions to failure.

Of special significance to municipal engineers and officials are the data relating to the lighter pavement designs. Typical examples of the high serviceability delivered by concrete sections are shown on the charts. The thinnest concrete design in more than one test was 3½-in. concrete with no subbase. Not only did this thin design show a final serviceability of "good" under 6,000-lb loads, but it also withstood more than 100,000 loads of 12,000 lb. (See Fig. 4 and Fig. 5.)

Fig. 3. Serviceability index & rating

When pavement thickness was increased to 5 in. with no subbase, a "very good" rating was maintained for the entire test period for the 6,000-lb loads (see Fig. 4). On another similar 5-in. section more than a quarter million 18,000-lb loads were carried (see Fig. 6).

The 6½-in. concrete pavement with no subbase sustained 12,000-lb loads for the full 1,114,000 load applications of the test. Final serviceability again was "good" (see Fig. 5). Even under deliberate overloading of a 22,400-lb single-axle load, the 6½-in. concrete on no subbase went out of test only after more than 300,000 load applications (not shown). These data show that concrete pavement designed for street traffic can withstand the usual overloads without distress—offering conclusive evidence to support the choice of concrete for city streets.

In addition to providing for structural design analysis, concrete streets are immune to spring breakups so common to asphalt streets. The beam strength inherent in every slab of concrete distributes traffic loads over wide areas of the subgrade and thus permits passage of heavy loads without disruption of the underlying subgrade.

With traffic accidents claiming many lives each year, safety is a vital element in planning and building city streets. Highway engineers, safety officials, and the motoring public are all looking for every possible means of reducing this toll. Safety authorities have long agreed that there are three essential elements in any traffic safety program—education, enforcement, and engineering. While engineering can't build an accident-proof highway, it can provide greater safety through the use of concrete pavements. Concrete's gritty texture maintains its skid-resistant properties throughout its life—wet or dry.

Night driving is always hazardous, but on city streets it is particularly so because of many pedestrians, bicyclists, and parked cars. It is most desirable for a residential street to have high light reflectivity. The added safety factor of concrete's extra visibility is hard to measure, but if it contributes to the saving of even one life, it would be of immense value. In addition to promoting greater driving safety, bright, well-lighted streets are a deterrent to crime, as law officials have frequently observed.