Understanding the AASHTO 1993 Pavement Design Guide

Empirical equations are the cornerstone of pavement design, linking observable pavement characteristics to performance outcomes. The 1993 AASHTO Guide for Design of Pavement Structures presents a foundational empirical equation for flexible pavement design that remains widely utilized. This article delves into this essential equation, exploring its components, assumptions, inputs, and outputs, providing a comprehensive understanding for engineers and pavement professionals.

The basic empirical equation from the 1993 AASHTO Guide for flexible pavements is expressed as:

Log10(W18) = ZR * So + 9.36 * Log10(SN + 1) - 0.20 + (Log10(ΔPSI/(4.2 - 1.5)))/(1 + 1.6*10-7/(SN+1)^5.19) + 2.32 * Log10(MR) - 8.07

Where the variables are defined as follows:

Variable	Description	Unit
W18	Predicted number of 80 kN (18,000 lb.) ESALs
ZR	Standard normal deviate
So	Combined standard error of the traffic prediction and performance prediction
SN	Structural Number (an index of total pavement thickness)	inches
ΔPSI	Difference between initial design serviceability index (po) and terminal serviceability index (pt)
MR	Subgrade resilient modulus	psi

The Structural Number (SN) itself is calculated from the pavement layer thicknesses and material properties:

SN = a1D1 + a2D2m2 + a3D3m3+…+ aiDi mi

Where:

• ai = ith layer coefficient
• Di = ith layer thickness (inches)
• mi = ith layer drainage coefficient

This equation, while not the only empirical model available, is crucial for grasping the fundamental principles of empirical pavement design. It highlights the factors considered in such models and how empirical observations are integrated into pavement design. The following sections will further explore the specific assumptions, inputs, and outputs associated with the 1993 AASHTO Guide flexible pavement empirical design equation.

Assumptions of the 1993 AASHTO Design Guide

AASHO Road Test Loop 2

The 1993 AASHTO pavement design equation is rooted in the AASHO Road Test, conducted in the late 1950s. Equations derived from this test linked serviceability loss, traffic loading, and pavement thickness under specific conditions. Consequently, these equations carry inherent limitations when applied outside of those conditions:

• Material Specificity: The equations were developed using materials and roadbed soil unique to the AASHO Road Test site. Applying them to pavements constructed with significantly different materials requires careful consideration and potential adjustments.
• Environmental Context: The equations are based on the environmental conditions prevalent at the AASHO Road Test location. Extrapolating these equations to vastly different climates without accounting for environmental factors like freeze-thaw cycles or extreme temperatures can lead to inaccuracies.
• Accelerated Testing Period: The AASHO Road Test was an accelerated two-year experiment, not a typical 20+ year pavement lifespan. Long-term environmental effects and material aging are difficult to accurately predict based on such short-term data.
• Uniform Traffic Loads: The AASHO Road Test utilized vehicles with consistent axle loads and configurations, unlike the mixed traffic found on real-world roadways. This necessitates the use of Equivalent Single Axle Loads (ESALs) to represent the cumulative damage of diverse traffic.

To bridge the gap between the controlled AASHO Road Test conditions and real-world pavement design, several key assumptions are made when using the 1993 AASHTO Guide equations:

• Abstract Subgrade Support: The concept of subgrade support from the AASHO Road Test is generalized to other subgrade soils through an abstract soil support scale, often represented by the resilient modulus (MR).
• ESALs for Mixed Traffic: Mixed traffic loading can be effectively represented by Equivalent Single Axle Loads (ESALs), allowing for the application of the equations to diverse traffic scenarios.
• Layer Coefficients for Material Characterization: Different pavement materials (surface, base, subbase) can be accounted for by assigning appropriate layer coefficients (a) that reflect their relative structural strength.
• Extrapolation to Longer Design Periods: The accelerated testing results can be reasonably extrapolated to longer design periods, although this assumption introduces uncertainty and should be considered with caution.

Understanding these limitations and assumptions is crucial when utilizing the 1993 AASHTO Guide empirical equation. Applying the equation without acknowledging its underlying assumptions can lead to inaccurate and potentially flawed pavement designs. It is essential to recognize the context within which the equation was developed and exercise engineering judgment when applying it to new situations.

Inputs for the 1993 AASHTO Pavement Design Equation

The 1993 AASHTO Guide equation necessitates several input parameters related to traffic loading, pavement structure, and subgrade support. These inputs are critical for accurate pavement design and include:

• Reliability: Reliability is a crucial concept in pavement design, representing the probability that a pavement will perform satisfactorily throughout its design life, considering variations in construction, environment, and materials. The reliability level is incorporated into the equation through the standard normal deviate (ZR) and the combined standard error (So). Higher reliability values lead to thicker, more robust pavement designs.

• Pavement Structure (Structural Number – SN): The Structural Number (SN) is a key index representing the overall structural capacity of the pavement. It is a function of the thicknesses and structural coefficients of the pavement layers. A higher Structural Number indicates a stronger pavement structure. The SN is calculated using layer coefficients (a) for each material and drainage coefficients (m) for unbound layers. Drainage coefficients account for the potential loss of strength due to moisture. Values typically range from 1.0 (no drainage issues) down to 0.4 (poor drainage, frequent saturation). It’s important to note that while drainage coefficients can adjust layer thickness, addressing fundamental drainage problems through proper drainage design is often more effective than simply increasing layer thickness. In many cases, neglecting the drainage coefficient (setting m=1.0) is a conservative approach to avoid over-reliance on drainage factors.

• Serviceability: Serviceability reflects the pavement’s ride quality and condition as perceived by users. The AASHTO design guide uses the Present Serviceability Index (PSI) to quantify serviceability on a scale of 0 to 5, with 5 being a perfectly smooth pavement and 0 representing an impassable pavement. The design equation uses the change in PSI (ΔPSI), which is the difference between the initial serviceability index (po) and the terminal serviceability index (pt).

  • Initial Serviceability (po): This is the PSI immediately after construction. Typical values range from 4.0 to 5.0, depending on construction quality and smoothness.
  • Terminal Serviceability (pt): This is the minimum acceptable PSI at the end of the pavement’s design life. Terminal serviceability values typically range from 1.5 to 3.0, depending on the road’s functional classification and traffic volume (e.g., higher values for interstates, lower for residential roads).

• Subgrade Resilient Modulus (MR): The subgrade resilient modulus (MR) represents the stiffness of the subgrade soil and its ability to support the pavement structure. It is a measure of the subgrade’s elastic response to repeated traffic loads. A higher MR value indicates a stiffer, stronger subgrade, requiring a thinner pavement structure. MR is typically determined through laboratory testing or estimated using correlations with other soil properties.

Outputs of the 1993 AASHTO Pavement Design Equation

The 1993 AASHTO Guide equation can be manipulated to solve for various outputs, provided all other variables are known. Typically, the equation is solved to determine either the allowable traffic loading (ESALs) or the required Structural Number (SN), which then translates to pavement layer thicknesses.

For accurate design, the flexible pavement equation should ideally be solved iteratively in conjunction with the flexible pavement ESAL equation. This iterative process accounts for the interdependence of Structural Number and ESALs:

1. Structural Number and ESAL Capacity: The Structural Number (SN) directly influences the total ESALs a pavement can withstand. This relationship is explicitly defined in the design equation.
2. Structural Number and Load Equivalency: The SN also affects the calculation of ESALs for different axle loads. A stronger pavement (higher SN) can tolerate more repetitions of heavier loads.

The iterative design process typically involves these steps:

1. Gather inputs for the flexible pavement design equation (ZR, So, ΔPSI, MR).
2. Gather inputs for the flexible pavement ESAL equation (Lx, L2x, G).
3. Assume an initial Structural Number (SN).
4. Calculate load equivalency factors for each load type using the ESAL equation with the assumed SN.
5. Estimate traffic volume for each load type over the design life and calculate total ESALs.
6. Use the assumed SN in the design equation to calculate the ESAL capacity of the pavement.
7. Compare the ESAL values from steps 5 and 6. If they are within a reasonable tolerance (e.g., 5%), the assumed SN is acceptable. Otherwise, adjust the SN and repeat steps 4-7.

In practice, a simplified approach is often used where the design equation is solved independently of the ESAL equation. This involves assuming an ESAL value independent of the Structural Number. While technically not entirely accurate, this simplified method generally yields reasonably accurate pavement thickness designs. The simplified design process is as follows:

1. Assume a Structural Number (SN) for initial ESAL calculations.
2. Determine load equivalency factors (LEFs) for typical load types using the ESAL equation and the assumed SN.
3. Estimate traffic volume for each load type and calculate total ESALs.
4. Gather inputs for the flexible pavement design equation (ZR, So, ΔPSI, MR).
5. Solve the design equation for the required Structural Number (SN).
6. Verify that the calculated SN is reasonably close to the SN assumed in step 1. This check is sometimes overlooked, but it’s a good practice.

1993 AASHTO Design Guide Utility

For practical application of the 1993 AASHTO Guide basic design equation for flexible pavements, design utilities and calculators are invaluable. These tools automate the calculations and provide insights into variable descriptions, typical values, and equation considerations.

1993 AASHTO Flexible Pavement Structural Design Calculator

By understanding the principles and applications of the 1993 AASHTO pavement design guide, engineers can effectively design flexible pavements that meet performance requirements and ensure long-lasting infrastructure.