The subgread ultimately carries all traffic loads. Therefore¸ the structural function of a pavement is to support a wheel load on the pavement surface and transfer and spread that load to the subgrade without exceeding either the strength of the subgrade or the internal strength of the pavement itself. Figure 1.5 shows wheel load¸ W¸ being transmitted to the pavement surface through the tire at an approximately uniform vertical pressure¸ P₀. The pavement then spreads the wheel load to the subgrade so that the maximum pressure on the subgrade is only P₁. By proper selection of pavement materials and with adequate pavement thickness¸ P₁ will be small enough to be easily supported by the subgrade. 1.Surface Course Asphalt pavement is a general term applied to any pavement that has a surface constructed with asphalt. Normally¸ it consists of a surface course (layer) of mineral aggregate coated and cemented with asphalt¸ and one or more supporting courses¸ which may be the following: *Asphalt base¸ consisting of asphalt-aggregate mixtures *Crushed stone (rock)¸ slag¸ or gravel * Portland cement concrete An asphalt pavement structure consists of all asphalt-bound layers and unbound (gravel or crushed stone) layers above the prepared subgrade. The upper or top layer consists of an asphalt surface. This surface can be a chip seal (for low-volume roads) or a high quality HMA for higher-volume roads and parking lots. The surface of an asphalt pavement exposed to vehicular traffic must be tough to resist distortion and provide a smooth, skid-resistant riding surface. It must be waterproof and sloped to shed surface water to the roadside, thus protecting the entire asphalt pavement structure and the subgrade from the weakening effects of moisture. It must resist wear caused by traffic and still retain necessary anti-skid properties. It must also be bonded to the layer or course beneath it. 2. Base and Subbase Layers Although a variety of bases and subbases may be used in asphalt pavement structures, they generally consist of compacted granular materials (such as crushed rock, slag, gravel, sand, or a combination of such) or stabilized soil. One of the main advantages of asphalt pavements is the economy achieved by using locally available materials. Generally, it is preferable to treat the granular material used in bases. The most common treatment is mix emulsified, cut-back, or expanded asphalt with the granular material, thus producing an asphalt-bound base. When the entire pavement structure above the subgrade consists of asphalt mixtures, it is called a full-depth asphalt pavement. This is generally considered the most cost-effective, dependable type of pavement for today´s traffic. The base and subbase are structural elements of the pavement. In conjunction with the overlying asphalt surface, their purpose is to distribute traffic wheel loads over the subgrade or. To perform this function, bases and subbases must be built with necessary internal strength properties. In this respect, full-depth asphalt pavements have a special advantage over pavements with granular bases wheel load, W, slightly deflects the pavement structure, causing both tensile and compressive stresses within the pavement. Asphalt pavement layers have both tensile and compressive strength to resist these internal stresses. Untreated granular bases have no tensile strength. Therefore, asphalt bases spread the wheel load over broader areas than do untreated granular bases. As a result, less total pavement structure thickness is required for an asphalt base. This applies to all loading conditions. 3.Determining Required Pavement Thickness A significant advance in highway engineering is the realization and demonstration that the structural design of asphalt pavements is similar to the problem of designing any other complex engineering structure. When asphalt pavement was first introduced, determining the proper thickness was a matter of empiricism and opinion based on experience. Similarly, the same situation once prevailed in determining the dimensions of masonry arches and of iron and steel structures. However, these early techniques have long since yielded to engineering analysis. Based on comprehensive analysis of vast volumes of accumulated data, the structural design of asphalt pavement has now evolved into a reliable engineering procedure. Research, aimed at further refinements and a fully mechanistic design procedure, is continuing. There is no standard thickness for a pavement. Required total thickness is determined by engineering design procedures. Factors considered in the procedures are the following: * Traffic to be served initially and over the design service life of the pavement. * Strength and other pertinent properties of the prepared subgrade * Strength and other influencing characteristics of the materials available or chosen for the layers or courses in the total asphalt pavement structure *Any special factors peculiar to the road being designed, such as environmental factors 4.Stage construction Because weigh and traffic volume will normally increase each year, pavement originally built thick enough to handle immediate traffic volumes may not be thick enough and strong enough to handle future needs. With asphalt pavement, this problem can be economically met by first building the thickness required to cope with initial and short-term loading and then adding, when needed, additional layers of asphalt to increase total pavement thickness. This procedure is called stage construction. It avoids extensive investment in the beginning; when a new asphalt pavement layer is added, the wearing surface is equal to or better than the original. When road-building funds are limited, a strong all-weather asphalt base can be placed and opened to traffic for a year or more. When additional construction founds are available, the asphalt surface course can be added. Similarly, in new housing developments, asphalt base can be placed on all new streets to serve until the development is finished and heav construction traffic ceases. The asphalt surface is then added after first repairing any damage or settlement suffered by the base while supporting the construction traffic. The streets are then at full strength and in an attractive condition appropriate to a new living area. Many examples of planned stage construction are evident in the interstate highway system. Asphalt pavements have been built with sufficient thickness to serve for the first few years, with founds programmed to add strengthening thickness when required by increases in traffic.