Section 3: Pavement Types

Flexible Pavement

A flexible pavement structure is typically composed of several layers of material with better quality materials on top where the intensity of stress from traffic loads is high and lower quality materials at the bottom where the stress intensity is low. Flexible pavements can be analyzed as a multilayer system under loading.

A typical flexible pavement structure consists of the surface course and underlying base and subbase courses. Each of these layers contributes to structural support and drainage. When hot mix asphalt (HMA) is used as the surface course, it is the stiffest (as measured by resilient modulus) and may contribute the most (depending upon thickness) to pavement strength. The underlying layers are less stiff but are still important to pavement strength as well as drainage and frost protection. When a seal coat is used as the surface course, the base generally is the layer that contributes most to the structural stiffness. A typical structural design results in a series of layers that gradually decrease in material quality with depth. Figure2-1 shows a typical section for a flexible pavement.

Figure 2-1. Typical section for a flexible pavement.

Perpetual Pavement

Perpetual pavement is a term used to describe a long-life structural design, using premium HMA mixtures, appropriate construction techniques and occasional maintenance to renew the surface. Close attention must be paid to proper construction techniques to avoid problems with permeability, trapping moisture, segregation with depth, and variability of density with depth. A perpetual pavement can last 30 yr. or more if properly maintained.

The pavement is designed to withstand an almost infinite number of axle loads without structural deterioration due to either classical bottom-up fatigue cracking, rutting of the HMA layers, or rutting of the subgrade by limiting the level of load-induced strain at the bottom of the HMA layers and top of the subgrade, and by using deformation resistant HMA mixtures. Figure2-2 shows a generalized perpetual pavement design. Detailed construction considerations are available through this link.

Figure 2-2. Generalized perpetual pavement design.

Rigid Pavement

A rigid pavement structure is composed of a hydraulic cement concrete surface course, and underlying base and subbase courses (if used). Another term commonly used is Portland cement concrete (PCC) pavement, although with today’s pozzolanic additives, cements may no longer be technically classified as “Portland.”

The surface course (concrete slab) is the stiffest and provides the majority of strength. The base or subbase layers are orders of magnitude less stiff than the PCC surface but still make important contributions to pavement drainage, frost protection and provide a working platform for construction equipment.

Rigid pavements are substantially ‘stiffer’ than flexible pavements due to the high modulus of elasticity of the PCC material resulting in very low deflections under loading. The rigid pavements can be analyzed by the plate theory. Rigid pavements can have reinforcing steel, which is generally used to handle thermal stresses to reduce or eliminate joints and maintain tight crack widths. Figure2-3 shows a typical section for a rigid pavement.

Figure 2-3. Typical section for a rigid pavement.

Continuously Reinforced Concrete Pavement (CRCP)

CRCP provides joint-free design. The formation of transverse cracks at relatively close intervals is a distinctive characteristic of CRCP. These cracks are held tightly by the reinforcement and should be of no concern as long as the cracks are uniformly spaced, do not spall excessively, and a uniform non-erosive base is provided. Figure2-4. shows a typical section of CRCP.

Figure 2-4. Continuously Reinforced Concrete Pavement.

Concrete Pavement Contraction Design (CPCD)

CPCD uses contraction joints to control cracking and does not use any reinforcing steel. An alternative designation used by the industry is jointed concrete pavement (JCP). Transverse joint spacing is selected such that temperature and moisture stresses do not produce intermediate cracking between joints. Nationally, this results in a spacing no longer than 20 ft. The standard spacing in Texas is 15.0 ft.

Dowel bars are typically used at transverse joints to assist in load transfer. Tie bars are typically used at longitudinal joints. Figure2-5 shows a typical section of CPCD.

Figure 2-5. Concrete Pavement Contraction Design (CPCD).

Jointed Reinforced Concrete Pavement (JRCP)

JRCP uses contraction joints and reinforcing steel to control cracking. Transverse joint spacing is longer than that for concrete pavement contraction design (CPCD) and, in Texas, it typically ranges from 30 ft. to 60 ft. This rigid pavement design option is no longer endorsed by the department because of past difficulties in selecting effective rehabilitation strategies. However, there are several remaining sections in service. Figure2-6 shows a typical section of jointed reinforced concrete pavement.

Figure 2-6. Jointed Reinforced Concrete Pavement (JRCP).

Post-tensioned Concrete Pavements

Post-tensioned concrete pavements remain in the experimental stage and their design is primarily based on experience and engineering judgment. Post-tensioned concrete has been used more frequently for airport pavements than for highway pavements because the difference in thickness results in greater savings for airport pavements than for highway pavements.

Composite Pavement

A composite pavement is composed of both hot mix asphalt (HMA) and hydraulic cement concrete. Typically, composite pavements are asphalt overlays on top of concrete pavements. The HMA overlay may have been placed as the final stage of initial construction, or as part of a rehabilitation or safety treatment. Composite pavement behavior under traffic loading is essentially the same as that of a rigid pavement.

Rigid and Flexible Pavement Characteristics

The primary structural difference between a rigid and flexible pavement is the manner in which each type of pavement distributes traffic loads over the subgrade. A rigid pavement has a very high stiffness and distributes loads over a relatively wide area of subgrade – a major portion of the structural capacity is contributed by the slab itself.

The load carrying capacity of a true flexible pavement is derived from the load-distributing characteristics of a layered system (Yoder and Witczak, 1975). Figure2-7 shows load distribution for a typical flexible pavement and a typical rigid pavement.

Figure 2-7. Typical stress distribution under a rigid and a flexible pavement.

Section 7: Information Needed for Pavement Design

Overview

Specific and accurate information is needed and critical for effective decisions regarding pavement design and rehabilitation. The information will also be included in the pavement design report. This section discusses the major requirements critical to a pavement design:

  • traffic loads
  • serviceability index
  • reliability (confidence level)
  • material characterization
  • drainage characteristics and
  • evaluating existing pavement conditions.

Traffic Loads

One of the primary functions of a pavement is load distribution. Therefore, in order to adequately design a pavement, representative loading characteristics must be surmised about the expected traffic it will encounter. Loads, the vehicle forces exerted on the pavement (e.g., by trucks, heavy machinery, airplanes), can be characterized by the following parameters:

  • tire loads
  • axle and tire configurations
  • typical axle load limits
  • repetitions of axle loads
  • traffic distribution (by direction and lane)
  • traffic projections.

Traffic loads, along with environment, damage pavement over time. The simplest pavement structural model asserts that each individual load inflicts a certain amount of unrecoverable damage. This damage is cumulative over the life of the pavement and when it reaches some maximum value, the pavement is considered to have reached the end of its useful service life.

Pavement structural design requires a quantification of all expected loads a pavement will encounter over its design life. This quantification is usually done in one of two ways:

  1. Equivalent single axle loads (ESALs). This approach converts axle configurations and axle loads of various magnitudes and repetitions (‘mixed traffic’) to an equivalent number of ‘standard’ or ‘equivalent’ loads.
  2. Load spectra. This approach characterizes loads directly by number of axles, configuration, and load. It does not involve conversion to equivalent values. Structural design calculations using load spectra are generally more complex than those using ESALs since the impact of each specific axle load is evaluated.

Both approaches use the same type and quality of data, but the load spectra approach has the potential to be more accurate in its load characterization.

Tire Loads

Tire loads are the fundamental loads at the actual tire-pavement contact points and are generally assumed to be equal for all tires on any given axle. For most pavement analyses, it is assumed that the tire load is uniformly applied over a circular area. Also, it is generally assumed that tire inflation and contact pressures are the same (this is not exactly true, but adequate for approximations). The following equation relates the radius of tire contact to tire inflation pressure and the total tire load:

Where:

  • a = radius of tire contact
  • P = total load on the tire
  • p = tire inflation pressure.

Axle and Tire Configurations

While the tire contact pressure and area is of vital concern in pavement performance, the number of contact points per vehicle and their spacing is also critical. As tire loads get closer together their influence areas on the pavement begin to overlap. At this point, the design characteristic of concern is no longer the single isolated tire load, but the combined effect of all the interacting tire loads.

Therefore, axle and tire arrangements are quite important. Tire-axle combinations (see Figure2-11) are typically described as:

  • single axle – single tire (truck steering axles, etc.)
  • single axle – dual tires
  • tandem axle – single tires
  • tandem axle - dual tires

Figure 2-11. Tire axle configurations.

Other axle configurations exist (tridem [or three axle], quad [or four axle]), but generally represent a small fraction of the entire population.

Typical Axle Load Limits

Federal and state laws establish maximum axle and gross vehicle weights to limit pavement damage. The range of weight limits in the U.S. varies, based on federal and state laws.

Table 2-4: Axle Load Limits*
Axle** / Limits (lb.)
Single Axle / 20,000
Tandem Axle / 34,000
Gross Vehicle Weight / 80,000
*Based on various federal and state laws.
Table 2-4

**Limits for tridem and quad axles are generally governed by the bridge formula:

Where:

  • W = load limit for the axle group
  • L = distance in feet between the extreme axles within the group
  • N = number of axles in the group.

Repetitions of Axle Loads

Although it is not difficult to determine the wheel and axle loads for an individual vehicle, it becomes complicated to determine the number and types of wheel/axle loads a particular pavement will be subject to over its entire design life. However, it is not the wheel load, but the damage to the pavement caused by the wheel load that is the primary concern.

There are two basic methods for characterizing axle load repetitions:

  1. Equivalent single axle load (ESAL). Based on AASHO Road Test results, the most common approach is to convert axle configuration and axle loads of various magnitudes and repetitions (‘mixed traffic’) to an equivalent number of “standard” or “equivalent” loads. The most commonly used equivalent load in the U.S. is the 80 kN (18,000 1b.) equivalent single axle load (normally designated ESAL).
  2. Load spectra. The proposed M-E Guide for the Design of New and Rehabilitated Pavement Structures (NCHRP 1-37A) essentially does away with the ESAL and determines loading effects directly from axle configurations and loads. This is a more precise characterization of traffic but relies on the same input data used to calculate ESALs.

A typical load spectrum input would be in a form of a table that shows the relative axle load frequencies for each common axle combination (i.e., single axle, tandem axle, tridem axle, quad axle) over a given time period. Often, load spectra data can be obtained from weigh-in-motion stations.

Traffic Distribution

Along with load type and repetitions, the load distributions across a particular pavement must be estimated. For instance, on a six-lane interstate highway (3 lanes in each direction) the total number of loads is probably not distributed exactly equally in both directions. Often one direction carries more loads than the other. Within that one direction, not all lanes carry the same loading. Typically, the outermost carries the most trucks and is subjected to the heaviest loading.

As a result, pavement structural design should account for these types of unequal load distribution. This is usually accounted for by selecting a “design lane” for a particular pavement. The loads expected in the design lane are either a) directly counted or b) calculated from the cumulative two-direction loads by applying factors for directional distribution and lane distribution.

The 1993 AASHTO Guide offers the following basic equation:

Where:

  • w18 = traffic (or loads) in the design lane
  • DD = directional distribution factor, expressed as a ratio, that accounts for the distribution of loads by direction (i.e., east-west, north-south).
  • Example: One direction may carry a majority of the heavy truck loads; that direction would be designed differently or, at a minimum, control the structural design. Generally taken as 0.5 (50%) for most roadways unless more detailed information is known.
  • DL = lane distribution factor, expressed as a ratio, accounts for the distribution of loads when two or more lanes are available in one direction
  • = the cumulative two-directional 18-kp ESAL units predicted for a specific section of highway during the analysis period.

For instance, on most interstate routes, the outside lane carries a majority of the heavy truck traffic.

The Transportation Planning and Programming Division (TPP) posts the directional distribution in the Traffic Analysis for Highway Design report, but this distribution is related to peak ADT distributions (the 30th highest hourly volume) that affect level of service for geometric design versus loading for structural design.

The assumption made in the Traffic Analysis for Highway Design report is traffic loading is equivalent in both directions. If the designer anticipates the truck directional distribution to be different from 50/50 or loads to be significantly greater in one direction, then this concern should be indicated in the request submitted to TPP for project level traffic data.

Recommended lane distribution factors for both flexible and rigid pavement design are:

  • one or two lanes in one direction, use 100% one-direction ESALs
  • three lanes in one direction, use 70% of one-direction ESALs
  • four or more lanes one direction, use 60% of one-direction ESALs.

Traffic Projections

TPP provides traffic projections (“Single Source Traffic Data Operating Procedures” from the Transportation Planning Policy Manual, Chapter 3, Section 4). The designer must request a 20-yr. traffic projection for flexible pavements and 30-yr. traffic projection for rigid pavements from the Traffic Section of TPP. Requests for traffic projections should be coordinated with the district director of Transportation Planning and Development (TPD).

Traffic loading is only rendered in terms of ESALs (versus axle load spectra). This is the standard method of evaluating loads for highways designed by the department. ESALs are evaluated for flexible and rigid pavements differently as a result of empirical relationships developed following the AASHO Road Test. Estimates for each type of pavement will be different. ESAL estimates will also vary slightly based on the overall pavement structure (also a result of empirical relationships).

The default structure used by the Traffic Section of TPP for traffic loading estimates is an 8 in. rigid slab or a flexible pavement of structural number (SN) 3. These structural classes are for highways with light traffic. If the highway has moderate to heavy traffic, provide TPP with the closest estimate for the appropriate structural class in terms of SN (flexible pavement) or thickness in inches (rigid slab). For more information, refer to “AASHTO Design Procedure.”

The pavement designer can help ensure accurate traffic projections are provided by documenting local conditions and planned economic or civic development that may affect future traffic loads and volumes.

TPP should be notified of special traffic situations when traffic data is requested. Examples of special situations:

  • a street that is or will be a major arterial route for city or school buses
  • a roadway that will carry truck traffic to and from heavily used distribution or freight centers
  • a highway that will experience an increase in traffic due to a connecting major high-traffic highway that will be constructed in the near future
  • a roadway that will experience a decrease in traffic due to the future opening of a parallel roadway facility.

For more information on traffic projection, refer to “1430: Obtain traffic data” in the Project Development Process Manual.

Serviceability Index

Serviceability is a concept derived during the AASHO Road Test. This concept is related to the primary function of a pavement structure: to provide the traveling public with a smooth, comfortable, and safe ride. A scale ranging from 0 to 5 is used to evaluate a pavement’s present serviceability index (PSI); pavement with a rating of zero is impassible and a rating of 5.0 would be perfectly smooth. Figure2-12 illustrates the concept of serviceability index.

All pavements, when newly constructed or rehabilitated, are expected to begin at a high level of serviceability with a decrease in serviceability over time and traffic loading as the pavement becomes more distressed and rough.