Pavement Structure

HMA pavements are flexible pavements. Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading. A flexible pavement structure is typically composed of several layers of material each of which receives the loads from the above layer, spreads them out, then passes them on to the layer below. Thus, the further down in the pavement structure a particular layer is, the less load (in terms of force per area) it must carry (see Figure 1).

Basic Structural Elements

Material layers are usually arranged within a pavement structure 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. A typical flexible pavement structure (see Figure 2) consists of:

• Surface Course. The layer in contact with traffic loads. It provides characteristics such as friction, smoothness, noise control, rut resistance and drainage. In addition, it prevents entrance of surface water into the underlying base, subbase and subgrade (NAPA, 2001¹). This top structural layer of material is sometimes subdivided into two layers: the wearing course (top) and binder course (bottom). Surface courses are most often constructed out of HMA.
• Base Course. The layer immediately beneath the surface course. It provides additional load distribution and contributes to drainage. Base courses are usually constructed out of crushed aggregate or HMA.
• Subbase Course. The layer between the base course and subgrade. It functions primarily as structural support but it can also minimize the intrusion of fines from the subgrade into the pavement structure and improve drainage. The subbase generally consists of lower quality materials than the base course but better than the subgrade soils. A subbase course is not always needed or used. Subbase courses are generally constructed out of crushed aggregate or engineered fill.

Unbound Aggregate Base

Aggregate base and subbase layers are used under both asphalt and concrete pavements. An aggregate base stabilizes the structure over soft subgrade soils and provides a working platform, so that construction equipment does not rut or weaken the subgrade. In asphalt pavements it is a structural layer that helps distribute load through the depth of the pavement; under concrete its main role is to give the slabs uniform support. Because unbound aggregate lets water pass between particles, it can also serve for drainage, and in cold climates it helps insulate the pavement from freeze-thaw cycles.

Materials

Natural aggregates come from quarries and pits, and the types available depend on the geology of the region (see Aggregate). Where local sources are scarce or of poor quality, recycled and manufactured materials can substitute: reclaimed asphalt pavement (RAP) and recycled concrete can make high-quality base layers, and industrial byproducts such as furnace slag and waste glass are also feasible.

Properties

Important properties include gradation, resistance to degradation, and particle shape, texture and angularity. Uncrushed aggregate should be used with care: rounded particles slide and reorient more easily under load, which reduces strength and can lead to deformation and rutting. Many agencies (as of 2013, roughly half of U.S. states) specify crushed aggregate for unbound base layers. Crushing also increases the fines, so gradation must be checked and maintained.

Density, moisture content and drainage also matter, and the base must be compacted to the required density before the surface layers are placed (see Embankment and Base Compaction Control). A dense-graded base, with particles of all sizes packed tightly, generally gives the most structural support, while open-graded or gap-graded bases with fewer small and intermediate particles drain better. Some designs therefore use more than one base or subbase layer with different characteristics.

Tests

• Gradation: sieve analysis on washed material, since dry sieving can give erroneous results when fines cling to larger particles.
• Shape and texture: coarse aggregate angularity and flat and elongated particles.
• Toughness and abrasion resistance: Los Angeles abrasion and Micro-Deval.
• Durability: sodium sulfate or magnesium sulfate soundness.
• Strength: the triaxial test gives the most realistic estimate of how the aggregate behaves under load but requires training and equipment; some agencies use simpler empirical tests such as the California bearing ratio instead.

@misc{pavint-pavement-structure,
  author = {{Pavement Interactive}},
  title = {Pavement Structure},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/design/structural-design/pavement-structure/}},
  note = {Accessed [date]}
}