A typical flexible pavement is constructed as shown in figure 3-16, which also defines the parts or layers of pavement. All layers shown in the figure are not present in every flexible pavement. For example, a two-layer structure consists of a compacted subgrade and a base course only. Figure 3-17 shows a typical flexible pavement using stabilized layers. (The word pavement, when used by itself, refers only to the leveling, binder, and surface course, whereas flexible pavement refers to the entire pavement structure from the subgrade up.) The use of flexible pavements on airfields must be limited to paved areas not subjected to detrimental effects of jet fuel spillage and jet blast. In fact, their use is prohibited in areas where these effects are severe. Flexible pavements are generally satisfactory for runway interiors, taxiways, shoulders, and overruns. Rigid pavements or special types of flexible pavement, such as tar rubber, should be specified in certain critical operational areas.

Select materials will normally be locally available coarse-grained soils, although fine-grained soils maybe used in certain cases. Lime rock, coral, shell, ashes, cinders, caliche, disintegrated granite, and other such materials should be considered when they are economical.

Subbase materials may consist of naturally occurring coarse-grained soils or blended and processed soils. Materials, such as lime rock, coral, shell, ashes, cinders, caliche, and disintegrated granite, maybe used as subbases when they meet area specifications or project specifications. Materials stabilized with commercial admixes may be economical as subbases in certain instances. Portland cement, cutback asphalt, emulsified asphalt, and tar are commonly used for this purpose.

Processed materials are prepared by crushing and screening rock, gravel, or slag. A properly graded crushed-rock base produced from sound, durable rock particles makes the highest quality of any base material. Crushed rock may be produced from almost any type of rock that is hard enough to require drilling, blasting, and crushing. Existing quarries, ledge rock, cobbles and gravel, talus deposits, coarse mine tailings, and similar hard, durable rock fragments are the usual sources of processed materials. Materials that crumble on exposure to air or water should not be used. Nor should processed materials be used when gravel or sand-clay is available, except when studies show that the use of processed materials will save time and effort when they are made necessary by project requirements. Bases made from processed materials can be divided into three general types-stabilized, coarse graded, and macadam. A stabilized base is one in which all material ranging from coarse to fine is intimately mixed either before or as the material is laid into place. A coarse-graded base is composed of crushed rock, gravel, or slag. This base may be used to advantage when it is necessary to produce crushed rock, gravel, or slag on site or when commercial aggregates are available. A macadam base is one where a coarse, crushed aggregate is placed in a relatively thin layer and rolled into place; then fine aggregate or screenings are placed on the surface of the coarse-aggregate layer and rolled and broomed into the coarse rock until it is thoroughly keyed in place. Water may be used in the compacting and keying process. When water is used, the base is a water-bound macadam. The crushed rock used for macadam bases should consist of clean, angular, durable particles free of clay, organic matter, and other objectional material or coating. Any hard, durable crushed aggregate can be used, provided the coarse aggregate is primarily one size and the fine aggregate will key into the coarse aggregate.

In a theater of operations where deposits of natural sand and gravel and sources of crushed rock are not available, base courses are developed from materials that normally would not be considered. These include coral, caliche, tuff, rubble, lime rock, shells, cinders, iron ore, and other select materials. Some of these are primarily soft rock and are crushed or degraded under construction traffic to produce composite base materials. Others develop a cementing action, which results in a satisfactory base. The following text describes the characteristics and usage of some of these materials:

1. CORAL. Uncompacted and poorly drained coral often results in an excessive moisture content and loss of stability. The bonding properties of coral, which are its greatest asset as a construction material, vary with the amount of volcanic impurities, the proportion of fine and coarse material, age, length of exposure to the elements, climate, traffic, sprinkling, and method of compaction. Proper moisture control, drainage, and compaction are essential to obtain satisfactory results.

2 CALICHE. A variable material that consists of sand, silt, or even gravel, that when saturated with water, compacted, and allowed to settle, can be made into high-quality base courses, especially caliches that are cemented with lime, iron oxide, or salt. Caliches vary, however, in content (limestone, silt, and clay) and in degree of cementation; therefore, it is important that caliche of good uniform quality be obtained from deposits and that it be compacted at optimum moisture.

3. TUFF. A porous rock usually stratified, formed by consolidation of volcanic ashes, dust, and so forth, and other cementitious materials of volcanic origin, may be used for base courses. Tuff bases are constructed the same as other base courses except that after the tuff is dumped and spread, the oversize pieces are broken and the base compacted with sheepsfoot rollers. The surface is then graded, compacted, and finished.

4. RUBBLE. It may be advantageous to use the debris or rubble of destroyed buildings in constructing base courses. If so, jagged pieces of metal and similar objects are removed.