Flexible pavements are generally satisfactory forrunway interiors, taxiways, shoulders, and overruns.Rigid pavements or special types of flexible pavement,such as tar rubber, should be specified in certain criticaloperational areas.MATERIALSSelect materials will normally be locally availablecoarse-grained soils, although fine-grained soils maybeused in certain cases. Lime rock, coral, shell, ashes,cinders, caliche, disintegrated granite, and other suchmaterials should be considered when they areeconomical.SubbaseSubbase materials may consist of naturallyoccurring coarse-grained soils or blended and processedsoils. Materials, such as lime rock, coral, shell, ashes,cinders, caliche, and disintegrated granite, maybe usedas subbases when they meet area specifications orproject specifications. Materials stabilized withcommercial admixes may be economical as subbases incertain instances. Portland cement, cutback asphalt,emulsified asphalt, and tar are commonly used for thispurpose.Base CourseA wide variety of gravels, sands, gravelly and sandysoils, and other natural materials such as lime rock,corals, shells, and some caliches can be used alone orblended to provide satisfactory base courses. In someinstances, natural materials will require crushing orremoval of the oversize fraction to maintain gradationlimits. Other natural materials may be controlled bymixing crushed and pit-run materials to form asatisfactory base course material.Many natural deposits of sandy and gravellymaterials also make satisfactory base materials. Graveldeposits vary widely in the relative proportions ofcoarse and fine material and in the character of the rockfragments. Satisfactory base materials often can beproduced by blending materials from two or moredeposits. Abase course made from sandy and gravel] ymaterial has a high-bearing value and can be used tosupport heavy loads. However, uncrushed, cleanwashed gravel is not satisfactory for a base coursebecause the fine material, which acts as the binder andfills the void between coarser aggregate, has beenwashed away.Sand and clay in a natural mixture maybe found inalluvial deposits varying in thickness from 1 to 20 feet.Often there are great variations in the proportions ofsand and clay from the top to the bottom of a pit.Deposits of partially disintegrated rock consisting offragments of rock, clay, and mica flakes should not beconfused with sand-clay soil. Mistaking such materialfor sand-clay is often a cause of base course failurebecause of reduced stability caused by the mica content.With proper proportioning and construction methods,satisfactory results can be obtained with sand-clay soil.It is excellent in construction where a higher type ofsurface is to be added later.Processed materials are prepared by crushing andscreening rock, gravel, or slag. A properly gradedcrushed-rock base produced from sound, durable rockparticles makes the highest quality of any base material.Crushed rock may be produced from almost any type ofrock that is hard enough to require drilling, blasting, andcrushing. Existing quarries, ledge rock, cobbles andgravel, talus deposits, coarse mine tailings, and similarhard, durable rock fragments are the usual sources ofprocessed materials. Materials that crumble on exposureto air or water should not be used. Nor should processedmaterials be used when gravel or sand-clay is available,except when studies show that the use of processedmaterials will save time and effort when they are madenecessary by project requirements. Bases made fromprocessed materials can be divided into three generaltypes-stabilized, coarse graded, and macadam. Astabilized base is one in which all material ranging fromcoarse to fine is intimately mixed either before or as thematerial is laid into place. A coarse-graded base iscomposed of crushed rock, gravel, or slag. This basemay be used to advantage when it is necessary toproduce crushed rock, gravel, or slag on site or whencommercial aggregates are available. A macadam baseis one where a coarse, crushed aggregate is placed in arelatively thin layer and rolled into place; then fineaggregate or screenings are placed on the surface of thecoarse-aggregate layer and rolled and broomed into thecoarse rock until it is thoroughly keyed in place. Watermay 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 shouldconsist of clean, angular, durable particles free of clay,organic matter, and other objectional material orcoating. Any hard, durable crushed aggregate can beused, provided the coarse aggregate is primarily one sizeand the fine aggregate will key into the coarse aggregate.Other MaterialsIn a theater of operations where deposits of naturalsand and gravel and sources of crushed rock are notavailable, base courses are developed from materialsthat normally would not be considered. These include3-18
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