HMA Pavement Thickness Design

HMA Pavement Thickness Design

Two primary parameters used in the thickness design of HMA pavements are subgrade soil properties and design traffic characterization. Each of these parameters has changed significantly over the past 50 years.

Subgrade Characterization

The characteristics of the subgrade have long been recognized as a critical input to HMA pavement design. Nationwide, in the 1950s and 1960s, soil characteristics were evaluated using soil classification systems (gradation and Atterberg limits), California bearing ration (CBR), R- value, and the triaxial strength tests. In the North Central region, in the early 1950s, states were primarily using the CBR, with one state using the triaxial test, one state using a cone test, one using a shear test, and four relying upon soil classification methods ( Figure 7 ). At this time, all states were routinely conducting soil classifications, using either the Highway Research Board (HRB) method, the Public Roads Administration method, or other pedological methods (39).

Cone Test Shear Test Soil Class

Subgrade Soil Characterization

FIGURE 7 Subgrade soil characterization methods used by the North Central states in the mid-1950s (39).

Smith and Skok 75

In the early 1970s, little had changed in the way that these highway agencies characterized their subgrade soils. However, beginning with the release of the 1986 AASHTO Pavement Design Guide, many highway agencies began moving toward the use of resilient modulus to characterize their soils. Although resilient modulus had been used before this time, the inclusion of resilient modulus as an integral part of the 1986 AASHTO pavement design procedure strongly encouraged this move. Moreover, the resilient modulus was recognized as being an improved method of characterizing the subgrade soil.

Traffic

In the period of the late 1950s and early 1960s (before the results of the AASHO Road Test were published), state highway agencies evaluated traffic in a number of different ways, including

• Design, maximum, or equivalent wheel load; • Traffic volume, in terms of either total traffic per day or number of commercial trucks

per day; and • Traffic index, an empirical approximation of the accumulated effect of wheel loads and repetitions.

With the release of the AASHO Interim Guide in 1962, a methodology was provided for converting mixed traffic into a single overall traffic loading indicator, the 18-kip ESAL. In this way, the damage caused by different axle types, configurations, and loadings are all expressed in terms of the equivalent damage caused by a standard 18-kip single-axle. The ESAL loadings are calculated using axle load equivalencies derived from the AASHO Road Test. By the early 1970s, eight of the 13 states in the North Central region had adopted the AASHO load equivalencies (21).

The ESAL factor continues to serve as the basis for many HMA pavement design procedures, but the recent movement toward M-E design procedures is advancing the use of load spectra data. Load spectra are the summation of axle load magnitudes and repetitions expected over the design life of the pavement; in the M-E design process, the load spectra data are used to compute the individual and accumulated effects of axle loads and types on pavement performance.

Design Procedures

A wide variety of design procedures have been used in the design of HMA pavements, including those based on theoretical and semitheoretical considerations, those based on empirical procedures, and those based on soil classifications. In the mid-1950s, a number of different HMA pavement design procedures were in use, as shown in Figure 8 (40). In the North Central region, CBR and Group Index (soil classification method) were commonly used.

After the release of the AASHO Interim Guide in 1962, highway agencies began using the new procedure and comparing and contrasting it with designs produced by their current design procedures. By the early 1970s, more than 60% of all state highway agencies had adopted the AASHO Interim Guide nationally, and within the North Central region nine of the 13 highway agencies were using it directly in their pavement design work (21). By the late 1990s,

76 Pavement Lessons Learned from the AASHO Road Test and Performance of the Interstate Highway System

FIGURE 8 HMA pavement design procedures used in late 1950s (40).

more than half of the highway agencies were using the 1993 AASHTO Design Guide for HMA pavement design, including eight states in the North Central region (23).