Asphalt Paving Layers and Road Base for Internal Roads

An internal road is built in layers: compacted subgrade, subbase, aggregate base, prime coat, asphalt binder course, tack coat and wearing course. Each layer is tested before the next goes on. Thickness and material class come from the pavement design and the owning authority's specification, not from the contractor.

Subgrade: the layer everyone forgets

The subgrade is the natural ground or the filled and compacted ground under the road. Its strength decides how thick everything above it has to be. Topsoil and soft or organic material come off, the exposed surface is levelled and compacted, and the geotechnical report tells you what to expect. Sabkha and high groundwater on the Eastern and Red Sea coasts, and basalt around Madinah, change the method before a single layer is placed. Compaction is checked against the modified Proctor test, ASTM D1557, with field density tests on the compacted surface.

Subbase and base: the load-spreading layers

The subbase and aggregate base spread the wheel load over the subgrade. Each is placed in layers, watered to the right moisture, rolled to the density the specification sets and tested. Typical test records include gradation, modified Proctor, field density and, for the CBR strength of the material, ASTM D1883. The material class and the thickness of each layer come from the pavement design. Our road base and subbase works page lists what a package includes, and the records that go with each layer.

Levels matter here. The base is the last chance to correct falls before asphalt locks them in, so a survey of levels before paving is worth the half-day it takes.

Prime coat and tack coat

Two thin coats do two different jobs. A prime coat is sprayed on the finished base before the first asphalt layer. It seals the surface and binds the loose top of the aggregate. A tack coat is sprayed between asphalt layers, and on kerbs and manholes before paving, so the layers bond into one. Skipping a tack coat is a classic cause of slippage and cracking later.

Binder course and wearing course

The binder course is the lower asphalt layer and gives structural strength. The wearing course is the top layer that carries tyres, resists rutting and sheds water. Both come from a hot mix plant, are hauled to site, laid by paver and compacted with rollers while hot. Mix design follows Marshall (ASTM D6927) or Superpave as specified, with the binder grade chosen for Saudi pavement temperatures and performance grading per AASHTO M 320. A mix design submission goes to the engineer before paving starts.

After paving, cores are taken to check thickness and density against the specification. See asphalt paving works for the scope and documents.

Laying in Saudi heat

Hot mix must reach the paver and the rollers while it is still hot enough to compact. In summer the ambient and surface temperatures help with that, but long hauls and idle trucks still cool a load. Agree with the engineer the mix temperature at delivery, the laying window and the rolling pattern, and keep a record of each load's temperature.

What changes the price

Asphalt is usually priced per square metre at a stated thickness or per tonne laid. These items move the number:

• road width and total area, and the number of layers
• the condition and level accuracy of the base
• distance from the hot mix plant to site
• small or fragmented areas, which cost more per square metre than a long open road
• night work or phased access, where the site stays in use

We don't publish a rate. It depends on those factors, and the offer states what it covers.

Car parks and internal roads on real sites

Hospitals, warehouses, compounds and commercial sites each load their roads differently. A hospital has ambulances and delivery trucks. A warehouse has heavy trucks that turn in tight radii and park at docks, which is where pavements fail first. Ask the designer which areas carry heavy traffic, and make sure the build-up reflects it. Coordinate falls to gullies, lighting pole bases, utility crossings and shade structures before paving, not after. The car parks and internal access roads page and the interlocking pavers, kerbs and sidewalks page cover the finishing elements.

What to send with your enquiry

Send the pavement design or typical section, the area or length and width by road type, the geotechnical report, the specification clauses for materials and tests, the levels drawing and the location of the nearest plant. Say whether the site stays in use during works. The roads and paving service covers the full range.

FAQ

What are the layers of an asphalt road?

Subgrade, subbase, aggregate base, prime coat, binder course, tack coat and wearing course, in that order from the ground up.

What is the difference between a prime coat and a tack coat?

A prime coat seals and binds the granular base before the first asphalt layer. A tack coat bonds one asphalt layer to the next.

Which tests are done on the base layers?

Gradation, modified Proctor to ASTM D1557, field density and CBR to ASTM D1883, as the specification requires.

How is asphalt paving priced?

Per square metre at a stated thickness or per tonne laid. Width, layers, base condition, plant distance and access all change the number, and the offer states its basis.