Ballast, base and asphalt surface course: what they are and what they do

Ballast, base and surface course are three stone-material layers with different roles in a pavement's structure. Ballast supports the subgrade or serves as the running surface on unpaved roads; base and sub-base provide structural support under the surface course; and the surface course is the top layer that carries traffic. Standard N-CMT-4-02-001/21, issued by Mexico's Secretaría de Infraestructura, Comunicaciones y Transportes (SICT), has regulated sub-base and base materials since October 2021.

Key facts

• A pavement is structured, from bottom to top, into subgrade, sub-base, base and surface course.
• SICT's N-CMT-4-02-001/21 has regulated sub-base and base materials since October 6, 2021.
• Ballast serves as a support layer or as the running surface on unpaved roads.
• The surface course is the only layer that carries vehicle traffic directly.
• Triturados Gramol produces ballast, base and surface-course material at its Coahuila and San Luis Potosí plants.

What is a pavement's structure?

A pavement is built in layers, each with its own material and function. From bottom to top: first the subgrade, the natural ground, compacted and leveled; above it, the sub-base, a stone-material layer that spreads the load down into the ground; above that, the base, another stone-material layer, of higher quality than the sub-base, giving immediate support to the final layer; and at the top, the surface course, which carries traffic directly.

Sub-base and base materials are regulated by SICT's N-CMT-4-02-001/21, in force since October 6, 2021, which sets the requirements each stone material must meet depending on the layer it goes into.

What is ballast and what is it used for?

Ballast is a coarse-graded stone material, with particles of different sizes that give it stability once compacted. It is used mainly in two ways: as a support layer between the subgrade and the upper layers of a pavement, or directly as the running surface on rural or unpaved roads that have no asphalt surface course.

Its role is to provide a firm, drivable base with a lower-cost material than the upper layers, suited to light-traffic roads or as an interim step before paving. Triturados Gramol produces it as part of its stone-aggregate line, alongside gravel, sand, base and surface-course material.

On urban projects, ballast is also used as fill and leveling material before other layers are laid, especially on stretches where the subgrade needs its level corrected. Its coarse, varied grading lets it compact into a stable layer even with light equipment, unlike a material of a single particle size.

What is the difference between base and sub-base?

Base and sub-base are both compacted stone-material layers under the surface course, but they serve different roles in a pavement's structure. The sub-base sits lower, in contact with the subgrade, and transmits and spreads the load into the native ground; it usually allows intermediate-quality material. The base sits just under the surface course, receives the load traffic transmits before it reaches the sub-base, and requires greater load-bearing capacity and finer grading.

SICT's N-CMT-4-02-001/21 sets the acceptable materials for both layers, with different requirements for each based on its position in the pavement structure.

What is an asphalt surface course and what does it contain?

The surface course is a pavement's top layer, the one in direct contact with vehicles. It is built by combining stone aggregate — gravel and sand graded per NMX-C-111-ONNCCE-2018 — with a binder, which in an asphalt surface course is asphalt. There is also a hydraulic-concrete surface course, which uses cement instead of asphalt as the binder, with a different construction process.

The surface course's quality depends on both the binder and the stone aggregate used: aggregate graded outside the standard compromises the surface course's service life, regardless of how good the asphalt or the cement is.

When an existing road shows surface damage — potholes, cracking or loss of texture — the work is usually limited to renewing just the surface course, without touching the base or sub-base if they are still in good condition. That is one reason it helps to understand the full pavement structure before deciding what material to order for a repair: ordering surface-course material when the problem is in the base does not fix the underlying failure.

How does Grupo Server help?

Triturados Gramol, a Grupo Server company founded in 2003, produces ballast, base and surface-course material —along with gravel and sand— with two operating plants, one in Coahuila and one in San Luis Potosí. Server Construcciones, the group's founding company since 1966, carries out paving work with those materials.

You can review Triturados Gramol's profile, Server Construcciones' profile, or write in through the contact form to specify materials for a paving project.