1
Introduction

Learning Objectives

This chapter introduces the types of roadway pavements and their basic structural characteristics. Furthermore, it demonstrates the size and importance of the pavement infrastructure in the overall economic activity of the United States.

1.1 Pavement Types

There are three general types of roadway pavements, namely flexible, rigid, and composite. Flexible pavements typically consist of asphalt concrete (AC) placed over aggregate base/subbase layers supported by the compacted soil, referred to as the subgrade. Note that some asphalt-paved surfaces are too thin to be considered flexible pavements, such as bituminous surface treatments (BST) or AC layers thinner than 1.5?cm placed over the subgrade. Rigid pavements typically consist of a portland concrete (PC) slab placed over the subgrade with or without a middle aggregate base layer. Composite or semi-rigid pavements are typically the result of pavement rehabilitation, whereby PC is placed over damaged AC, or vice versa.

The terms flexible and rigid relate to the way AC and PC pavements, respectively, transmit stress and deflection to the underlying layers. An ideal flexible layer transmits uniform stresses and nonuniform deflections, while the opposite is true for a rigid layer. In practice, the stress and deflection distributions in AC and PC concrete pavements depend on the relative stiffness of these layers with respect to those of the underlying granular layers. This ratio is much lower for AC than for PC, which justifies their generic designation as flexible and rigid, respectively. As described in later chapters, the structural behavior of these two pavement types differs significantly and affects the way they are analyzed and designed.

Figure 1.1 shows a typical cross section of a flexible pavement. The AC layer consists of one or more sublayers or lifts, the surface one being referred to as the friction course and the lower one as the leveling course. The base and subbase aggregate layers are placed on top of the subgrade. The base layers can be unbound (i.e., non-cemented) or bound (i.e., stabilized using an asphalt binder or portland cement). High-plasticity clay subgrades are often improved by lime stabilization. Tack coats are used to provide adhesion between layers. A fabric or other geotextile placed between the base and the subgrade acts as a filter, preventing the migration of fines between them and maintaining their integrity.

Figure 1.1 Typical Section of an Asphalt Concrete Pavement

Typically, the AC layer is designed with no interconnected voids (i.e., air voids 4-8% by volume of mix) and hence relies on surface runoff for precipitation drainage. Alternatively, ACs with interconnected voids (i.e., air voids in the 15-20% range by volume of mix) allow drainage through the surface. They are referred to as permeable friction courses (PFC). This design requires a lower impermeable AC layer to prevent water from penetrating the base layer. Water runoff can be removed from the edge of the pavement through either ditches or drainage pipes.

Figure 1.2 shows a typical section of a rigid pavement. The PC layer is placed either directly on top of the subgrade or on top of a granular base layer.

Figure 1.2 Typical Section of a Portland Concrete Pavement

Unreinforced rigid pavements are referred to as jointed plain concrete pavements (JPCPs). JPCPs require transverse joints at prescribed intervals to accommodate thermal and shrinkage tensile stresses (Figure 1.3a). They are constructed by cutting transverse surface grooves using a rotary saw before the concrete is fully cured. These joints, in addition to relieving thermal and shrinkage stresses, provide some vertical load transfer between adjacent slabs through aggregate interlock.

Figure 1.3 Typical Configuration of JPCPs (a), JDRCPs (b), and CRCPs (c)

Vertical load transfer is important for preventing the buildup of pore water pressure under the joint which could lead to fine aggregate loss (i.e., flushing) and downstream slab settlement (i.e., faulting). A better vertical load transfer mechanism in jointed rigid pavements is through dowel bars (Figure 1.3b). These pavements are referred to as jointed dowel reinforced concrete pavements (JDRCPs). The dowel bars are smooth, have a collapsible end cap, and are epoxy-coated to prevent rusting. They provide vertical load transfer while allowing expansion/contraction of the slabs without generating stresses in the concrete. Reducing vehicle excitation dictates the randomization of joint spacing (e.g., 2.1, 2.7, 3.3, and 4.5?m), as well as their skewed arrangement with respect to the longitudinal axis of the pavement1.

Another rigid pavement design consists of continuously reinforced PC (CRCP) (Figure 1.3c). The reinforcement consists of continuous, deformed steel bars placed near the neutral axis of the slabs. CRCPs develop transverse hairline cracks due to thermal and shrinkage stresses, but the reinforcement keeps the cracks from opening and maintains the structural integrity of the slabs.

Several alternative joint types are used in PCs, including construction joints necessary where work is interrupted (Figure 1.4a) and expansion joints necessary where concrete pavements come against other rigid structures, such as bridge abutments (Figure 1.4b). The successful design and construction of joints and their reinforcement contribute significantly to the performance of concrete pavements.

Figure 1.4 Special Function Portland Concrete Pavement Joints; Construction Joints (a) and Expansion Joints (b)

1.2 Pavement Infrastructure Overview

The staggering size of the roadway pavement infrastructure in the United States can be appreciated by the inventory data shown in Table 1.12. This table shows total roadway centerline-km length by functional class, namely interstate, arterials, collectors, and local roadways in rural and urban areas. These functional class designations relate to the geometric standards of roadways, as well as the combination of access and mobility they afford. Their total length is 4.7 million and 2 million centerline-km in rural and urban areas, respectively. Their overall length is 6.7 million centerline-km, or approximately 18 times the distance to the moon. The interstate highway system has a length of 78,511 centerline-km. This combined with an additional 275,931 centerline-km of other freeways comprise the National Highway System (NHS), as designated by the 1995 Public Law 104-593. The NHS represents about 4% of the total roadway pavement length but carries over 44% of the vehicle-km traveled4. Managing this vast infrastructure investment is a very challenging task.

1.3 Significance of Pavement Infrastructure to the National Economy

Transportation plays a very important role in the Nation's economic activity. Approximately 19% of the average household expenditures are directly related to transportation (Figure 1.5). In 2014, 82.4% of the person-kilometers traveled in the United States used the roadway system (Figure 1.6), while in 2019, 64.6% of total freight transported was carried by truck (Figure 1.7).

The vehicle-miles traveled (VMT) annually is a very good indicator of the health of the economy, as suggested by its strong interrelation with the annual gross domestic product (GDP), that is the annual sum of goods and services transacted nationwide (Figure 1.8). These simple facts demonstrate the important role roadway infrastructure plays in the Nation's economic activity.

1.4 Funding Pavements

The value of the roadway infrastructure is in the trillions of dollars. The ongoing annual capital outlays for roadway preservation, capacity expansion, and new route construction are in the tens of billions. In 2017, the total capital and maintenance expenditures for the roadway infrastructure in the United States amounted to $105.4 billion and $51.4 billion, respectively. These amounts include expenditures for pavements and bridges but exclude administrative, law enforcement/safety, and interest-related expenditures7. Figure 1.9 shows an increasing trend in capital outlays for highway construction over the last 20?years with an average growth rate of 3.03% annually (i.e., dollars not adjusted for inflation). These figures demonstrate the extent of public investment in highway pavements and the financial commitment required to maintain them.

Table 1.1 Length Inventory (Centerline-km) of US Roadway Pavements

(Source: Ref. 2 / Federal Highway Administration / public domain)