Chapter 1: Introduction

This chapter presents an overview of pavements and how a pavement preservation strategy that implements preventive maintenance treatments can be used to preserve the condition of a highway system and extend its service life. A discussion of pavement distresses identifies the potential causes of the distress types. Also discussed are the various treatment types used in Preventive Maintenance, including the selection of the most appropriate treatment. These topics are discussed in greater detail in Chapters 2-10.

1.0 Pavement Structure & Design

1.1 What is a Pavement?

A pavement is a layer or layers of compacted material designed to withstand the stresses applied by vehicle wheel loads and provide a smooth riding surface. Pavements are engineered structures by which stresses applied from moving wheel loads are transferred to the native soil (subgrade). Figures 1 and 2 illustrate the ways in which stress is applied to the roadway and how a pavement’s structural section can reduce the stress applied to the subgrade.

1.2 Flexible & Rigid Pavements

Pavement can be considered as the entire road structure. This chapter is concerned primarily with the surface of the pavement; however, structural aspects of the remaining parts also contribute greatly to the performance of any surfacing and must be understood.

The two main types of pavement in common use are flexible and rigid pavements. The difference between the two is the manner in which they distribute the applied load to the subgrade.

A rigid pavement -- Portland cement concrete (PCC) – due to its high rigidity and high stiffness (modulus of elasticity), tends to distribute the loads over a wide area of subgrade, resulting in the PCC slab carrying the majority of the load. Load distribution is considered the major factor in the performance of rigid pavements. This characteristic allows minor variations in the subgrade strength to have little effect on the structural capacity of the rigid pavement. However, severe cracking problems can arise if the subgrade cannot support the slab.

By contrast, flexible pavements consist of layers of granular materials and/or asphalt bound materials (such as hot mix asphalt or asphalt surfacing) with lower rigidity and stiffness compared with rigid pavements. Such pavements generate their load-bearing capacities based largely on the load distribution characteristics of the individual layers. The strength of a flexible pavement is built up with thick layers. These layers distribute the applied loads over the subgrade. As a result, the design thickness of the pavement is influenced by the load distribution mechanism and the strength of the subgrade. For these reasons, the material properties comprising each layer, its thicknesses, the subgrade strength, and the loading level are critical design parameters.

Pavement design is based on the structural analysis of multi-layered pavements subjected to traffic loading. A key to predicting performance is accurately modeling the stress and strain responses of the pavement and accounting for subgrade and environmental conditions affecting material properties.

1.3 Pavement Structure

A pavement’s structure can be broken down into three main components, each of which plays an important role in the overall performance of a pavement. The components are:

• Foundation
• Base
• Surfacing

Figure 3 shows typical pavement cross sections for both types of pavements.

1.3.1 Foundation

The foundation is comprised of the subgrade and, in some cases, the subbase. The foundation carries the loads created by construction traffic. Structurally, it is the final layer to which stress is transferred and is characterized by its compressive strength or bearing capacity. Because the foundation is generally the weakest layer, stresses must be spread over as wide an area as possible.

1.3.2 Base Layer

The base layer is a main structural element of the design and can consist of several layers. It is required to spread the wheel load so that the foundation is not over-stressed. Its stiffness and its fatigue resistance (if stabilized) characterize its behavior in the pavement structure.

The base layer can consist of compacted high-grade aggregate, lean concrete, Portland cement concrete, or dense graded asphalt. In some areas where drainage is poor or traffic is very heavy, a large stone mix with high voids can be used in the bottom part of this course. In situations where the subgrade is very weak, a binder-rich layer may be used at the bottom of this course.

1.3.3 Surfacing Layer

The surfacing layer ensures adequate skid resistance, provides a smooth riding surface, and acts as a protective layer for the underlying materials. It may or may not be structurally significant. Surfacings range from surface seals to thin hot-mix overlays. If the surfacing is greater than 40mm (1.5 in) thick, it will contribute to the structural integrity of the pavement and must be considered in the design. The surface layer is where most rutting and other defects occur. With this in mind, the design of this layer is very important.

Surface layers are characterized by their stiffness, creep resistance, moisture resistance, resistance to low-temperature cracking, fatigue resistance, and skid resistance. Figure 4 illustrates the main elements of the pavement structure with respect to the surfacing layer.