A rigid pavement system is often created with a PCC surface course on top of either the subgrade or an underlying base course. Because of its relative stiffness, the pavement structure distributes loads across a large area while requiring just one or two structural layers (see Figure 1). PCC can be made from several different materials including cements, concretes, and asphalt mixtures.
The PCC layer is the thickest part of the pavement structure, with thicknesses ranging from 3 inches to 2 feet or more. It provides protection for the underlying material and serves as a load distributor by transferring load between itself and the subgrade. The term "rigid" implies that it will not break down under traffic load conditions. However, prolonged exposure to heat or moisture may cause the cement in the upper layer to lose some of its strength, so it is important to include some type of protective surfacing beneath this layer to prevent vehicles from hitting hot spots on the road.
Load distribution in PCC roads is primarily achieved through the use of variable density throughout the life of the roadway. At initial construction, the density of the concrete is high in areas likely to see heavy traffic flow to ensure adequate depth of concrete for later repairs or replacements. As the concrete ages, the volume of voids within the mixture increases due to the setting of chemical reactions within the binder.
There are two types of pavement that are commonly used for this purpose: flexible pavements and stiff pavements. This chapter discusses pavement kinds, layers, and functions, as well as pavement failures.
Flexible pavements are those that can be molded to fit the contours of the surface on which they lie. They usually contain a core material, such as gravel or rock, between two layers of asphalt. The core provides some degree of flexibility to the pavement while the outer layers provide traction and wear resistance.
Stiff pavements are designed with a flat surface across their widths. They are typically made from concrete or steel. The primary function of these surfaces is to bear traffic loadings. They may also have other purposes, such as water drainage or energy conservation through insulation. These other functions are generally provided by additional materials or designs incorporated into the pavement structure.
Pavement layers are the different materials that make up a pavement. Layers are classified according to physical characteristics (flexibility, thickness), functional properties (bear weight, resist damage), or both.
Flexible pavements get their name from the fact that the whole pavement structure deflects, or flexes, when subjected to weight. Typically, a flexible pavement construction is made up of numerous layers of materials. Each layer takes loads from the layer above, distributes them, and passes them on to the layer below. The end result is a smooth surface that can handle high traffic volumes without buckling.
There are two types of flexible pavements: those that are designed in place and constructed while they are in use, and temporary ones that are placed in the road bed and removed after use. Flexible pavements are used where damage to the road surface can happen due to heavy vehicles, such as trucks. These include urban roads, highways, and bridges that carry large amounts of traffic. The reflective nature of the pavement allows drivers to see better at night, when traditional signs cannot be seen.
Flexible pavements are also used in areas where regular asphalt or concrete pavements are not suitable because of environmental concerns (such as where there is arsenic contamination in the soil) or cost considerations. One type of flexible pavement that has been developed over the past decade or so is polymer-modified asphalt (PMA). This type of pavement uses small particles of polymers that are mixed with normal asphalt and then heated to create a stronger, more durable material that can support heavier vehicles. It is estimated that PMAs can last 10 to 20 years before they need to be replaced.
Rigid pavements are divided into three types:
The purpose of structural design is to establish the number, material composition, and thickness of the various layers necessary to handle a certain loading regime inside a pavement structure. The surface course as well as any underlying base or subbase strata are included. The designer must also take into account any special requirements for drainage or other functionalities that may be needed for the particular project.
There are several different materials used in pavement design: macadam, asphalt, concrete, and natural stone. Each has advantages and disadvantages that should be considered when selecting a material for your project. It is important to understand how a pavement will be maintained or repaired so that proper decisions can be made regarding its longevity. For example, if you plan to use hot-mix asphalt for your surface course, it is important to determine whether the project will require periodic resurfacing. Resurfacing may be required because of damage caused by vehicles driving on the surface or due to weathering. If this occurs too often, you might want to consider using concrete instead.
In addition to the type of material used, there are several other factors that may influence the selection process including budget, location, functionality needs, etc.
Structural designs for roads are developed using methods equivalent to those used for buildings. However, since roads must be able to withstand greater loads than houses, they require stronger foundations and larger cross-sections of material.
Those whose surfaces are made of bituminous (or asphaltic) materials. These pavements are referred to be "flexible" because the entire pavement structure "bends" or "deflects" as a result of traffic loads. A flexible pavement construction is often made up of numerous layers of materials that may flex. The top layer, which receives the most abuse from vehicles, trucks, and trailers, is usually made of a plastic material. This surface wears out first and must be replaced frequently. Below it is laid a base course, which is a thick layer of compacted rock or gravel used to give the pavement strength and durability. Next comes an intermediate course, which is made of stone chips or sand mixed with the bitumen and placed between the base course and the road's subgrade. This layer acts as a cushion so that any unevenness in the base course will not be felt by vehicles traveling over it. Finally, there is a thin topcoat applied to protect the stone from weathering and extend its life.
Asphalt pavement is one of the most widely used types of highway surfacing materials in the world. It is flexible, durable, and resistant to heat and water. Asphalt mixes used for pavement production contain mineral fillers such as limestone, clay, and sand, which contribute to the pavement's strength and elasticity. Paving contractors typically use hot-mixed asphalt during construction of new roads to ensure proper adhesion between layers.
Seal coat, surface course, tack coat, binder course, prime coat, base course, sub-base course, compacted sub-grade, and natural sub-grade are typical layers of a standard flexible pavement (Figure 2). The seal coat is applied to all surfaces except the sub-base, which does not receive a coating. The function of the seal coat is to provide adhesion when used with a suitable adhesive, such as tar, asphalt, or hot-melt; to prevent water from reaching the underlying material; and to protect the top layer of the road against damage due to traffic and weather conditions.
The surface course consists of one or more layers of fine graded sand and/or stone that are placed on top of the seal coat to give the finished product a smooth surface. This layer provides traction for vehicles driving over it, prevents erosion of the soil beneath it, and adds color to the final product. The tack coat is used to bind the sand and/or stone together to ensure that they don't move during placement on the site and to help them adhere to the seal coat underneath.
The binder course consists of an aggregate (gravel, crushed rock, etc.) mixed with bitumen or other binding material. This mixture is spread over the surface course to form a flat, even layer.