SAPEM Chapter 2 - Sanral

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South African Pavement Engineering Manual Chapter 2: Pavement Composition and Behaviour Nature of Materials. This involves the nature of individual component materials that are used to make up the pavement layers. The pavement engineer needs to have an in-depth understanding of the nature of all the materials that are incorporated in a layer, i.e., mineral aggregates, binders, moisture, additives and how these materials interact and change over time. The nature of materials is discussed further in Section 5 and Chapter 10, Section 3.5.3. Material Response to Loading. The pavement engineer needs to understand how all the various pavement layers respond to loading and the stresses that occur within the layers. To understand these interactions, it is first necessary to have a good understanding of the concepts of stress and strain, i.e., mechanics of materials, and typical pavement material models. These concepts are discussed in Section 9, Mechanics of Materials and Section 10, Material Science. Pavement Behaviour. Pavement behaviour involves understanding the behaviour of the composite system after repeated loads and how the pavement material properties change over time. Structural behaviour is a measure of the rate of change in key structural characteristics, such as deflection, with time. Structural behaviour depends on material properties such as stiffness, resilient modulus, tensile strength, compressive strength and shear strength, amongst others, as well as the interaction between different layers. Pavement behaviour is discussed in more detail in Section 9.3 of this chapter, and in Chapter 10, Section 3.5. Functional Performance. The pavement engineer needs to understand how different distress patterns translate into functional problems and affect road users over time, and vice versa. Distress types, and their effect on road users, are discussed in Section 6.1 with further discussion on functional performance in Section 8. Road User Needs. At the top of the technical pyramid are the requirements of the road user, set for the pavement. Safety and comfort are amongst the highest priorities, although the environment and economics are also considered. Section 3: Basic Principles of Roads Page 9
South African Pavement Engineering Manual Chapter 2: Pavement Composition and Behaviour 4. PAVEMENT STRUCTURE The pavement structure is the combination of layers and subgrade that carries the traffic loads. A typical pavement layered structure and names for each layer used in contemporary road construction, Figure 8, is not that far removed from the Roman approach, it is just more refined. The pavement engineer needs to understand the particular behaviour of each layer and how this influences the layer’s ability to fulfil its purpose. For example, the base layer’s ability to spread loads onto the underlying layer and support the surface is of particular importance. Concrete (15 to 35 cm) surfacing and base Subbase usually cemented Selected subgrade Subgrade Flexible Pavements Figure 8. Typical Pavement Structures Rigid Pavements The purposes of the various layers in the pavement are described below: Surfacing: This is a functional wearing course that provides waterproofing, skid resistance, noise-damping, durability against the elements, visibility and drainage. For surfaced roads, the upper layer is bound, consisting of spray seals, asphalt or concrete. Base: This is a load spreading layer that is the most important structural component of the pavement. The layer must provide the required support for the surfacing and distribute the very high tyre pressures and wheel loads uniformly over the underlying layers and subgrade. The base comprises bound material, e.g., asphalt, concrete or stabilised, or it can be unbound, e.g., crushed stone or gravel base. Subbase: This layer provides support for the base as well as a platform upon which to construct a structural base layer of high integrity. It also protects the underlying selected subgrade layer by further spreading the load. Selected subgrade: These layers are primarily capping for the subgrade to provide a workable platform to construct the imported pavement layers. At the same time, these layers provide depth of cover over the subgrade to reduce the stresses in the subgrade to acceptable levels. Subgrade: This is the existing material upon which the pavement must be constructed. It can be modified with stabilisers to reduce plasticity, ripped and recompacted to achieve uniform support, or undercut and replaced, depending on its quality. Typically, the higher up the layer is in the pavement structure, the more expensive the material to obtain or manufacture. The asphalt surfacing in a pavement layer is generally the most expensive layer in the pavement structure. It is also typical for the stiffer pavement layers to be at the top of the pavement structure. The exception to this is “inverted” flexible pavements, where the subbase layer is cement stabilised and the base layer is a good quality granular layer. These pavements are widely used in South Africa. 4.1 Pavement Design Pavement design is the process to decide on the pavement structure, in terms of the type of structure, the materials to be used and the layer thicknesses. The basic objective of pavement design is to combine materials of sufficient strength in a layered system of to provide the desired functional and structural service levels over the design period, subject to the applicable traffic demand and particular environment. The functional and structural service levels, the rates at which these service levels deteriorate, the cost associated with the provision and maintenance of these service levels and the savings by the road users resulting from improved service levels determine the economic viability of a design. Although the final design decision is dictated by the most economically viable design, the process of ensuring that a pavement with adequate strength is provided is critical. The philosophy, principles and details of several different design methods applicable to new pavements and pavement requiring rehabilitation are included in Chapter 10. Section 4: Pavement Structure Page 10
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SAPEM Chapter 2 - Sanral

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