Pavements are an essential part of our life. We use them as roads, runways, parking lots, and driveways. Pavements are engineered structures and are important for our everyday life, commerce and trade, and defense. Surface transportation is the most widely used mode of transportation in the world, and a country’s development is often measured in terms of its total paved road mileage. The construction of roads is and will continue to be a major industry in developing countries, and as the infrastructure matures, it will be a major industry in developed countries as well.
Like any other engineered structure, pavements are expected to be adequately strong and durable for their design life. They are expected to function properly by providing a smooth traveling surface for the traffic under various conditions of the environment. In order to ensure this, pavements must be designed, constructed, maintained, and managed properly.
Pavements can be broadly classified into asphalt (or ﬂexible) and concrete (or rigid) pavements. Pavements consist of different layers, more so in the case of asphalt pavements than concrete ones. From the bottom up, these layers are known as the subgrade, subbase, base, and binder and/or surface. There are certain pavements with asphalt surface layers on top of the concrete layers.
In the United States, there are about 4 million miles of roads, of which approximately 2.5 million are paved. The federal government and state departments of transportation (DOTs) spend a significant portion of their budget on maintaining and managing existing pavements and rehabilitating old pavements. More than 90% of commodities are transported on highways in the United States. Roads in poor condition end up costing the DOTs a lot of money for repairs, as well as to the users for repairing damaged vehicles. These roads are also unsafe for travel—for example, more than 30% of traffic fatalities in the state of Massachusetts in the United States have been reported to be due to poor road conditions.
The most important function of the pavement is to withstand the load applied from a vehicle such as a truck or an aircraft, without deforming excessively. The layered structure of the pavement is meant for ensuring that the load is spread out below the tire, such that the resultant stress at the bottom layer of the pavement, the subgrade, is low enough not to cause damage. The most signifcant load applied to a pavement surface comes from a truck or an aircraft tire. The approach in a ﬂexible pavement is to spread the load in such a way that the stress at the subgrade soil level is small enough so that it can sustain the stress without any major deformation. When the existing soil is not stiff enough to support the relatively small stress, then there is a need to improve the soil. There is also a need to improve the soil if it is susceptible to moisture. Such a problem can be solved by treating the soil with an additive, such as lime and a Portland cement.
Since pavements are exposed to the environment, a very important factor in the design of pavements is the consideration of water, which could be coming from rain/snow (surface water) and/or from the ground (ground/subsurface water). Since water can be detrimental to a pavement, a basic necessity of designing a proper pavement is to provide adequate drainage for both surface and subsurface water. Standing water on a pavement can cause hydroplaning, skidding, and accidents. There is a need to make sure that water from precipitation is drained away quickly and effectively and that there is no depression on the roads to collect water. Water present in frost-susceptible soils in the subgrade can freeze, causing heaving and failure of the pavement. Therefore, frost-susceptible materials should be avoided. If this is not possible, then the pavement structure above the subgrade should be thick enough to prevent the freezing front from reaching the frost-susceptible soil.
Similarly, as one expects some water to make its way through random cracks and joints, proper subsurface drainage must be provided, and the material within the pavement structure should be made resistant to the actions of water—otherwise the aggregates, for example, would be washed away due to repeated traffic-induced pressure or freeze-thaw pressures.
In most cases, it is not possible to completely avoid water ﬂowing inside a pavement. Such ingress of water can physically remove materials from inside a pavement structure and also freeze and cause deformations in the pavement. To prevent this, the pavement material must be selected properly and, if needed, modified.
Generally, the layers in a pavement improve in quality as one goes up from the bottom to the surface layer. The surface layer, which can be asphalt or concrete, is the most expensive and stiff/durable layer in the entire pavement structure. Components of this layer are mostly naturally occurring materials, for example, asphalt binder is a by-product of the petroleum distillation process and aggregates are obtained from rock quarries or riverbeds. These materials are combined and used in different proportions to produce the final material that is used in the pavement. For example, asphalt binder is mixed with aggregates to produce hot mix asphalt (HMA) for asphalt pavements, while Portland cement is mixed with aggregates in Portland cement concrete (PCC) pavements. In both cases, the mixing must be conducted in the correct proportions to ensure adequate quality of the mixture. It is important to find out whether the resultant mix has the adequate strength and stiffness through testing. Such testing is generally conducted in the laboratory during the mix design process. During this process, loading and effect of environmental conditions can be simulated in the laboratory. If the responses to this testing do not meet our expectations, then the mix needs to be redesigned and/or the materials need to be reselected. These “expectations” are specifications that have been developed on the basis of experience and research. State DOTs or agencies can use their own specifications and/or use specifications from organizations such as the American Association of State Highway and Transportation Officials (AASHTO) or the Federal Aviation Administration (FAA). Similarly, testing is also conducted according to standards, which again can be from state DOTs or the AASHTO. Whether or not the material can be placed and compacted is also determined during the mix design process.
In most cases, pavement engineers are restricted to using locally available materials, with or without some modifications, because of economic and practical reasons. With these available materials, it is important to determine what thickness of each layer, and hence the entire pavement, is required to carry the loads under different environmental conditions without any problem. This step, known as the structural design, makes sure that the pavement structure as a whole can withstand traffic for its design life, even though the traffic might increase and the properties of the layer might change cyclically and/or progressively during its design life.
Generally, several layers are present in an asphalt pavement. From the bottom up, the layers are known as the subgrade, subbase, base, and binder and/or surface. Generally, the bottommost layer is soil; the subbase and/or base layers can be granular soil, or aggregates or asphalt-aggregate mixtures (mixes); and the binder and surface are asphalt mixes. While designing, adequate thickness to each layer is assigned, so as to obtain the desirable properties in the most cost-effective way. Concrete pavements may not have as many layers, and in many cases the concrete slab rests on a stabilized subgrade, which consists of soils modified with some additives.