# Pavement Structure HMA pavements are flexible pavements. Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading. - Canonical URL: https://pavementinteractive.org/reference-desk/design/structural-design/pavement-structure/ - Last updated: 2026-11-01 HMA pavements are [flexible pavements](https://pavementinteractive.org/glossary/flexible-pavement/). [Flexible pavements](https://pavementinteractive.org/glossary/flexible-pavement-2/) 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](https://pavementinteractive.org/glossary/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](https://pavementinteractive.org/reference-desk/design/structural-design/pavement-structure/), [subbase](https://pavementinteractive.org/reference-desk/pavement-types-and-history/pavement-types/hma-pavement/) and [subgrade](https://pavementinteractive.org/reference-desk/design/design-parameters/subgrade/) (NAPA, 2001[^1]). This top structural layer of material is sometimes subdivided into two layers: the [wearing course](https://pavementinteractive.org/glossary/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. ![Basic Flexible Pavement Structure](https://pavementinteractive.org/wp-content/uploads/2008/07/Hma1.jpg) *Figure 1.* Basic Flexible Pavement Structure ## 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](https://pavementinteractive.org/reference-desk/materials/aggregate/introduction/)). 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](https://pavementinteractive.org/reference-desk/materials/aggregate/gradation-and-size/), 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](https://pavementinteractive.org/glossary/surface-layer/) are placed (see [Embankment and Base Compaction Control](https://pavementinteractive.org/reference-desk/construction/site-preparation/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](https://pavementinteractive.org/reference-desk/testing/aggregate-tests/gradation-test/) on washed material, since dry sieving can give erroneous results when fines cling to larger particles. - **Shape and texture:** [coarse aggregate angularity](https://pavementinteractive.org/reference-desk/testing/aggregate-tests/coarse-aggregate-angularity/) and [flat and elongated particles](https://pavementinteractive.org/reference-desk/testing/aggregate-tests/flat-and-elongated-particles/). - **Toughness and abrasion resistance:** [Los Angeles abrasion](https://pavementinteractive.org/reference-desk/testing/aggregate-tests/los-angeles-abrasion/) and Micro-Deval. - **Durability:** sodium sulfate or magnesium sulfate [soundness](https://pavementinteractive.org/reference-desk/testing/aggregate-tests/sulfate-soundness-test/). - **Strength:** the [triaxial test](https://pavementinteractive.org/reference-desk/testing/aggregate-tests/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](https://pavementinteractive.org/reference-desk/design/design-parameters/california-bearing-ratio/) instead. ## Sources [^1]: NAPA (2001). HMA Pavement Mix Type Selection Guide. IS 128. National Asphalt Pavement Association and Federal Highway Administration. Lanham, MD and Washington D.C. respectively.