Roads are common assets that project owners must maintain for decades. A well-designed and well-supervised road needs almost no repair over the life of the project, while a road built by "pouring concrete on fill without any testing" often cracks or settles within the first few years, and repairing a road already in service always costs more than doing it right from the start. This article is written for project owners, contractors and engineers who must make decisions about roads in projects, housing estates and factories.
What Is a Reinforced Concrete Road?
A reinforced concrete road is a reinforced concrete pavement whose surface is a concrete slab reinforced with wire mesh or rebar, placed on a compacted subbase and subgrade. It is classed as a rigid pavement because the stiff concrete slab spreads wheel loads over a wide area before transferring them to the soil, unlike an asphalt (flexible) pavement, whose flexible surface relies on thicker pavement layers to spread the load. The advantages of concrete roads are long service life, good resistance to trucks and standing water, and low maintenance, which is why they are standard for housing estates, factories and general projects. The drawbacks are a higher construction cost per square meter, the need for joints, and more difficult spot repairs than asphalt roads.
How Many Layers Does a Concrete Road Have?
A reinforced concrete road typically has 3 main layers: natural or compacted subgrade, subbase, and the reinforced concrete surface. Each layer passes the wheel loads down to the soil in turn. A road is only truly strong when every layer is strong, not just when the concrete is thick.
- Subgrade: natural soil or fill compacted to form the foundation for everything above. Its strength is measured by the CBR value, the main variable in setting pavement layer thickness. New fill must be compacted in layers, with density tested on every layer.
- Subbase: selected material such as coarse sand, laterite or compacted crushed rock, typically about 15 cm thick. It spreads load, provides a level platform and cuts off seepage paths beneath the pavement.
- Reinforced concrete surface: a concrete slab reinforced with wire mesh or rebar, about 15–25 cm thick depending on traffic, with joints designed to control cracking.
Some projects add a base layer for heavy-traffic roads, or lay a plastic moisture barrier under the concrete to stop cement paste draining into the sand layer. All of this depends on each project designer's specifications.
How Thick Should the Concrete Surface Be?
A typical concrete surface thickness is 15 cm for housing-estate roads used mostly by passenger cars, increasing to 18–25 cm where trucks run regularly. The table below gives reference thickness ranges common in project roads, as a starting point for discussion with the designer.
← Scroll to see the full table →
| Use | Concrete surface (cm) | Remarks |
|---|---|---|
| House driveway / car parking area | 12–15 | Light vehicles, no trucks |
| Housing-estate side street / subdivision road | 15 | Mainly passenger cars, some delivery vehicles |
| Main project road | 18–20 | Regular medium trucks, e.g. 6-wheel trucks |
| Factory access road / container yard | 23–25 | Heavy trucks and trailers; should be designed case by case |
These figures are general reference ranges used with a compacted subbase about 15 cm thick, not fixed requirements. The correct thickness for each project must be calculated by an engineer from actual traffic, vehicle axle loads and subgrade strength (CBR), because very soft soil may first need ground improvement or additional pavement layers, not just thicker concrete.
What Standards Are Used for Project Road Design?
Reinforced concrete roads in private projects in Thailand commonly refer to the highway standards of the Department of Highways and the Department of Rural Roads, together with ASTM test standards. Roads in land subdivision projects must also comply with land allocation requirements. The relevant groups of standards are summarized below.
- Highway agency standards: material and construction specifications for pavement layers, such as subbase material properties, compaction density criteria and concrete paving methods, which private projects commonly adapt
- Land allocation requirements (under the Land Allocation Act B.E. 2543 (2000)): set minimum right-of-way and carriageway widths for subdivision roads according to the number of plots, with details following the requirements of each province's Land Allocation Committee
- ASTM test standards: used for quality control of soil and pavement layers, e.g. ASTM D1883 (laboratory CBR), ASTM D1556 (field density by sand cone), ASTM D6951 (DCP) and ASTM D1195/D1196 (plate bearing)
- Designer's concrete specifications: concrete compressive strength, rebar type and size, joint spacing and dowel details, specified in each project's construction drawings
For an overview of a project's complete civil design (structures, roads, drainage), read Guide to RC structural and civil engineering design or see the scope of SPN's civil engineering design services
Why Test the Soil Before and During Road Construction?
Because almost all damage to concrete roads starts in the pavement layers beneath, not in the concrete itself. Soil testing therefore happens at 2 stages: before design (to obtain soil strength for setting pavement layer thickness) and during construction (to confirm that compaction actually meets the specification). The tests most often used for roads are:
- CBR Test (ASTM D1883): measures the relative bearing strength of the subgrade in the laboratory and is the starting input for pavement thickness design in almost every method
- Field Density Test (ASTM D1556): checks field compaction density by the sand cone method on every completed compacted layer, to confirm the specified percentage compaction (typically about 95% or more of maximum dry density). Read more in Field Density Test for compacted fill
- DCP Test (ASTM D6951): a portable lightweight dynamic cone penetrometer that quickly assesses pavement layer strength at many points, well suited to checking uniformity along a whole road. Read more at What is the DCP test?
- Plate Bearing Test (ASTM D1195/D1196): loads a steel plate on the pavement layer and measures the actual settlement, used to confirm the bearing capacity of pavement layers or heavy-vehicle yards. Read more at Plate Bearing Test for soil bearing capacity
For roads on soft soil or deep fill, an additional soil investigation should be carried out to check soft layers and assess long-term settlement before design. See SPN's soil investigation and testing services
Why Do Joints and Drainage Matter?
Concrete shrinks naturally; if joints are not cut so it cracks along set lines, the road will crack on its own. The main joint in a reinforced concrete road is the contraction joint, saw-cut within the right time window after pouring to a depth of about one-third to one-quarter of the slab thickness. Joint spacing is related to thickness: the thinner the slab, the closer the joints. There are also construction joints where each day's pour stops, and expansion joints where the slab meets other structures. Heavy-traffic roads commonly use dowel bars at joints to prevent adjacent slabs from faulting.
Drainage is the most often overlooked factor in road life. The road surface must have a central crown with a cross-fall of about 1.5–2% to shed water into the drains on both sides. If standing water seeps under the slab, the pavement layers soften and are pumped out through the joints as vehicles pass (pumping), until voids form under the slab and it eventually cracks. Road design must therefore always go hand in hand with the project's drainage system design
The thicknesses and dimensions in this article are conceptual reference ranges, not construction drawings and not design advice for any particular project. Each road must be designed by an engineer from actual traffic data and site soil test results, and the requirements of the relevant authorities should be checked before construction.