Pavement Design

In highway engineering terms, a pavement is any paved surface and can include carriageways, footways and any other paved area. This article outlines different types of paved surfaces and the principles of their design.

Types of Carriageway Pavement

Flexible

In its simplest terms, a flexible pavement comprises layers of gravel bound together with bitumen, laid on top of a rolled gravel sub base. It is flexible because the bitumen is elastic in nature, allowing the pavement to expand and contract slightly with changes in temperature and as a result of traffic loading.

A flexible carriageway is typically made up of the following layers (from top to bottom). Note that other materials and forms of construction are available depending on specific circumstances:

Surface Course – 30mm to 50mm of asphalt comprising an aggregate with relatively high skid resistance. This course protects the main fabric of the road from tyre wear and provides skid resistance. Historically, Hot Rolled Asphalt (HRA) was used which comprised an asphalt layer with stone chips rolled into the surface to provide skid resistance. These days, Thin Surface Course Systems such as Stone Mastic Asphalt are more prevalent, where the stone within the asphalt layer provides skid resistance.

Binder Course – 50mm to 80mm of asphalt.

Base – 100mm to 300mm of bituminous material which provides the main structural integrity of the carriageway and resists deformations due to heavy vehicles.

Sub-base – the road foundation comprising a layer not less than 150mm thick of granular material (usually Type 1 as defined in the Specification for Highway Works).

Rigid Construction

Here the road is made up from layers of cementitious material, forming large rigid slabs. Movement is allowed for by regular expansion and contraction joints. There are two basic kinds of concrete pavement. Jointed Reinforced Concrete (JRC) Pavement comprises a series of bays of concrete between 150mm and 250mm thick, each reinforced. The bays are dowelled together to prevent differential settlement but to allow horizontal movement. A typical bay length would be between 5m and 10m in length. A Continuously Reinforced Concrete Pavement comprises much longer bays which make use of much heavier reinforcement to resist expansion and contraction. As there are fewer joints, the ride quality is generally better than JRC. The reinforced concrete pavement lies on a cement bound sub-base (sometimes referred to as Hydraulically Bound Material) which provides a more rigid subgrade. Composite Construction As its name suggests, this pavement comprises a flexible surface and binder course overlaying a concrete base. The base can be Jointed Reinforced Concrete or Continuously Reinforced Concrete Base (CRCB), which is essentially the same as CRCP. To prevent the joints in the concrete base propagating through the asphalt overlay, various reinforcement grids can be laid across the joints. This prolongs the life of the pavement.

Principles of Pavement Design

In essence there are two main criteria that need to be considered when designing a pavement. The first is the expected vehicle loading and the second is the strength of the underlying ground.

Vehicle Loading

Vehicle loadings are calculated based on the expected traffic flow that will use the road over its design life, typically 40 years for a new build carriageway. It is only the larger vehicles that are considered – the damage caused by hundreds of cars is negligible compared with the damage caused by one articulated lorry. From the Annual Average Daily Traffic Flow, the number of large vehicles per day is extracted, based on traffic surveys). These are then converted into a number of ‘standard axles’ – a maximum legal articulated lorry is 6 **CHECK** standard axles, whereas a bus is 2 standard axles. These can be summed and then multiplied up over the design life of the road, factoring in expected traffic growth, until a design traffic flow is obtained. Over the lifetime of the road, the design flow is stated in terms of the number of million standard axles (msa). These flows can be used within the design standards to identify pavement layer materials and thicknesses to carry the design traffic.

Underlying Ground

The design manual for roads and bridges allows for a number of ways of establishing the treatment required for the existing ground, including a simple method which tends to over design the foundation, and an analytical method which gives a less conservative result, and is therefore better applied to larger projects where a 20mm reduction in pavement thickness can amount to a large cost saving. The standards refer to the foundation modulus which is a measure of the stiffness of the underlying ground. The stiffer the foundation, the less thick the carriageway construction needs to be to carry the design traffic. The purpose of the foundation is to stiffen the underlying ground to the point where it is stiff enough to support the carriageway construction. The foundation comprises the sub-base plus any other ground improvement needed to stiffen the underlying soil. This is known as the capping layer. A typical capping layer might be 300mm of granular material. Other alternatives include lime and / or cement stabilisation, or a thickened sub base layer.