What is a Base Course in Road Construction?

The base course in road construction is the load-carrying part of the road surface that carries the weight of the traffic to its foundation. It’s above the sub-base but below the binder course. Whilst base course is a commonly used term, the more widely accepted term in UK surfacing is now simply “base”.

In this blog, we’ll explain what the base course is, the materials used for it, and why its role matters so much.

Base course in summary:

What a base course does

A base course is the main structural layer of the road. Its job is to take the concentrated loads from traffic above and spread them over a wide area so that the stress reaching the sub-base stays within what it can safely carry.

By doing so, it:

• Supports the binder course
• Limits deformation to the road surface
• Protects the foundation from stresses that could cause settlement or failure
• Maintains pavement regularity
• Distributes traffic loads

A strong, sturdy base course enables a road to carry heavy, repeated traffic for its lifetime whilst also making it easier and cheaper to remedy issues on the layers higher up.

What is the difference between a base, sub-base and binder course?

Below, we explain the key differences between the roles, position, and materials of a base, sub-base and binder course. Our dive into the layers of road construction explains it in more detail, but our table below outlines the key things you need to know:

What is a base course made of?

The base course in road construction is often made of asphalt, but in some cases, it may be cement-bound; this depends on where it is required. The loads a surface carries play a considerable role, so the base course on a walkway, for example, might be made of something different to the base course found on a driveway or a road.

Commonly used materials for a base course in UK surface construction include:

• Asphalt concrete: The dense, bound asphalt used for most modern road bases.
• Dense bitumen macadam: A long-established dense asphalt base and binder material, now classified within the asphalt concrete family, though widely still specified under its own name.
• Cement-bound material: Hydraulically bound, used under concrete or in heavy-duty designs. Also referred to as CBM.
• Recycled or reclaimed asphalt: Also known as RAP, used as a proportion within the base mix.
• High modulus base: Referred to as HMB, this is a stiffer asphalt base for heavy traffic areas and commercial loading locations.

How thick is a base course?

There isn’t a one-size-fits-all size for a base course, as thickness depends on how the surface is used. The thickness must retain heat for compaction but not be too thin, as it may not compact as well as it should.

The desired thickness depends on:

• The traffic the surface encounters
• The strength of the formation
• The sub-base
• Drainage
• Asphalt stiffness
• Construction tolerance
• Planned lifespan of the road

How is the base course laid?

A base course is laid in a careful, multi-step process so a strong, load-bearing layer can be placed beneath the pavement or road surface. Before the base course is laid, a surfacing specialist like Tidey & Webb will conduct several checks. Our list below outlines how a base course is laid and then compacted:

• The foundation (sub-base and formation) is inspected and accepted as suitable.
• Levels are checked, as is surface regularity, and drainage is sufficient before any asphalt is laid.
• Delivery, paving and rolling is coordinated so materials don’t sit and lose workability.
• The base course is laid to the design thickness in a controlled continuous run, avoiding aggregate segregation and maintaining paver speed.
• The course is kept within the correct depth window. It should remain thick enough to hold heat for compaction but not so thick that it cannot be compacted to its full depth. If the base is too thin, it cools too quickly and won’t close up.
• Maintain edge stability and set out the joint positions.
• Begin compaction while asphalt remains within a suitable temperature range. A consistent rolling pattern should enable the base course to achieve the correct density without displacing the mixture.
• The finished base is protected with a bond or tack coat before the binder course is added.

How can you tell if a base course is failing?

A base course can fail for many reasons, and it is not always easy to detect. A visible problem on the surface layer isn’t the clearest indication of which layer is damaged or failing. You may be able to detect issues with the base course if you notice:

• Settlement
• Water pumping
• Surface movement
• Edge collapse
• Rutting
• Widespread cracking

For a complete resolution, you may need our team; we can establish where the fault lies and how extensive the repairs may be.

Can you repair or reuse an existing base course?

You can repair or reuse a base course but only in certain circumstances.  The most important thing is to know just how deep the fault lies. This helps you tackle the issue the first time rather than repair it again a few months later.

Broadly, there are three outcomes:

• Reuse the base as it is: If the damage is only present at the surface or binder course, and the base is still in good condition, draining as it should and remains well-compacted, the layers above can be removed and laid again without disturbing the base.
• Strengthen or partially replace it: If the weakness is only in one place, the affected area can be removed, reinstated, and re-compacted before the upper courses are added.
• Reconstruct it: Where there are widespread issues, it’s evident that there are faults in the foundation beneath the base, or you see repeated failures in the same place; it often means that the base, and perhaps the layer below, needs to be removed and rebuilt. Patching over it only means you’ll revisit it sooner rather than later.

Whichever route suits you, always aim to fix the cause and not just the symptom. Our surfacing specialists can trace the issue, normally finding the cause lies in poor foundations, inadequate materials or low-quality compaction.

Reusing a base that is in good condition saves considerable cost and disruption but should only be considered after a professional diagnosis.

Why the base course matters for commercial surfacing

The base course plays an important role in commercial surfacing. Car parks, business parks, industrial sites and more all carry heavy and consistent traffic. A weak base course, not designed to withstand such heavy loads, can soon falter, leading to repeated repairs until the issue is addressed properly.

Contact the surfacing specialists

If a car park, access road or other surface you are responsible for is failing and you have concerns about the base course or other layers, speak to our team. At Tidey & Webb, we use our years of experience, highly trained specialists, and the latest technology to diagnose and repair or replace faulty surfaces. Based in West Sussex, but serving Sussex, Surrey, Hampshire and Kent, we are surfacing and civil engineering experts for the south. Contact us today to get a free quote or book a site visit.