Figure 16-5.—Pavement deflection results in tensile and compressive stresses in pavement structure.asphalt pavements is similar to the problem of designingany other complex engineering structure. When asphaltpavement was first being introduced, determining theproper thickness was a matter of guesswork, rule ofthumb, and opinion, based on experience. Almost thesame situation once prevailed in determining thedimensions of masonry arches and iron and steelstructures. However, these early techniques have longsince yielded to engineering analysis. Similarly, basedon comprehensive analysis of vast volumes ofaccumulated data, the structural design of asphaltpavements has now been developed into a reliableengineering procedure.There is no standard thickness for a pavement.Required total thickness is determined by engineeringdesign procedure. Factors considered in the procedureare as follows:1. Traffic to be served initially and over the designservice life of the pavement2. Strength and other pertinent properties of theprepared subgrade3. Strength and other influencing characteristics ofthe materials available or chosen for the layers (orcourses) in the total asphalt pavement structure4. Any special factors peculiar to the road beingdesignedStage ConstructionBecause weight and traffic volume normallyincrease, pavement originally built thick enough tohandle immediate traffic volumes may not be thickenough and strong enough to handle future needs. Withasphalt pavement, this problem can be meteconomically by first building the thickness required,then adding, when needed, layers of asphalt to increasetotal pavement thickness. This procedure is called stageconstruction.It avoids excessive investment in thebeginning; and when a new layer of asphalt is added, thewearing surface is equal to or better than the original.Subgrade EvaluationSeveral methods for evaluating or estimating thestrength and supporting power of a subgrade are in usetoday, including the following:1. Loading tests in the field on the subgrade itself.For example, the plate bearing test uses large, circularplates, loaded to produce critical amounts ofdeformation on the subgrade in place.2. Loading tests in a laboratory using repre-sentative samples of the subgrade soil. A test commonlyused by the Seabees is the California bearing ratio(CBR) test, which is sometimes used on the subgrade inplace in the field.3. Evaluations, based on classification of soil byidentifying and testing the constituent particles of thesoil.16-5
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