In certain situations where high base stiffness is desired, base courses can be constructed using a variety of HMA mixes. Aggregates can be either stabilized or unstabilized. Base courses are most typically constructed from durable aggregates (see Figure 4) that will not be damaged by moisture or frost action. Base courses are usually constructed out of: It provides additional load distribution and contributes to drainage and frost resistance. The base course is immediately beneath the surface course. This layer provides the bulk of the HMA structure. This way, the wearing course can be rehabilitated before distress propagates into the underlying intermediate/binder course. A properly designed (and funded) preservation program should be able to identify pavement surface distress while it is still confined to the wearing course. It is meant to take the brunt of traffic wear and can be removed and replaced as it becomes worn. This is the layer in direct contact with traffic loads. This top structural layer of material is sometimes subdivided into two layers: In addition, it serves to prevent the entrance of excessive quantities of surface water into the underlying base, subbase and subgrade ( NAPA, 2001 ). It provides characteristics such as friction, smoothness, noise control, rut and shoving resistance and drainage. The surface course is the layer in contact with traffic loads and normally contains the highest quality materials. Various flexible pavement cores from Washington State. The scale at the right edge of the photo is in inches. The signs on top of the pictured cores indicate the State Route (SR) and the Mile Post (MP) where the core was taken. Basic flexible pavement structure.Īs seen in Figure 3, a flexible pavement structure can vary greatly in thickness. A typical structural design results in a series of layers that gradually decrease in material quality with depth. The underlying layers are less stiff but are still important to pavement strength as well as drainage and frost protection. The surface course (typically an HMA layer) is the stiffest (as measured by resilient modulus) and contributes the most to pavement strength. Each of these layers contributes to structural support and drainage. A subbase is not always needed.Ī typical flexible pavement structure (Figure 2) consists of the surface course and the underlying base and subbase courses. This is the layer (or layers) under the base layer. This is the layer directly below the surface course and generally consists of aggregate (either stabilized or unstabilized) or HMA. It may be composed of one or several different HMA sublayers. This is the top layer and the layer that comes in contact with traffic. This section describes the typical flexible pavement structure consisting of: In flexible pavements, material layers are usually arranged in order of descending load bearing capacity with the highest load bearing capacity material (and most expensive) on the top and the lowest load bearing capacity material (and least expensive) on the bottom. Other pavements that are surfaced with asphalt materials, such as bituminous surface treatments are also classified as flexible pavements. Thus, the further down in the pavement structure a particular layer is, the less load (in terms of force per area) it must carry (Figure 1). Each layer receives the loads from the above layer, spreads them out, then passes on these loads to the next layer below. A flexible pavement structure is typically composed of several layers of material. HMA pavements are classified as “flexible” pavements because the total pavement structure deflects, or flexes, under loading.
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