# What Is Granular Base Equivalency?

GBE, or granular base equivalency, is a method of measuring the thickness of the pavement. It takes into account the various layers of the pavement, which have different materials and properties by using coefficients to convert the thickness of each layer into a standardized unit of measurement.

To calculate the GBE the depth of each layer is multiplied by its corresponding granular equivalency factor and the results are added together to give the total granular base equivalency. This measurement is frequently used in the design and performance evaluation of pavements.

Pavement performance modeling is a field of study that investigates how pavement degrades over time and how to predict its future condition.

There are various methods for assessing the health of pavement, including indices like the Pavement Condition Index (PCI), International Roughness Index (IRI), and Present Serviceability Index (PSI), as well as more specific measures like rutting and cracking.

There are several approaches to pavement performance modeling, including mechanistic models, mechanistic-empirical models, survival curves, Markov models, and machine learning algorithms. The most common method is the International Roughness Index (IRI).

## Equivalent Granular Base Moduli: Prediction

A method has been developed to accurately determine the equivalent one-layer modulus for unbound aggregate material in flexible highway pavement, which takes into account the nonlinear behavior of the granular material.

This method is based on predictive regression equations derived from multi-layer elastic theory computer simulations.

The equivalent granular modulus is influenced by various pavement layer factors that affect the bulk stress in the granular layer, such as surface and base thickness, surface and subgrade modulus, and base course quality as represented by the K1 value in the nonlinear modulus expression MR = K1- 48300^0.5. The method also includes nomographic solutions and has been found to be in relatively good agreement with methods developed by the U.S. Army Corps of Engineers, Kentucky Highway, and Shell Oil.

As it takes into account important factors that affect the nonlinear response of unbound granular materials, this technique is recommended for use in highway pavement design and analysis using theoretical layered approaches.

## Example

In this example, a road from the Long-Term Pavement Performance (LTPP) database consists of several layers: subbase, base and three layers of hot mixed asphalt concrete. The Long-Term Pavement Performance Program (LTPP) is a research initiative supported by the Federal Highway Administration (FHWA) that collects and analyzes pavement data in the United States and Canada.

The LTPP has the largest road performance database currently available. The thickness of each layer is listed in millimeters.

Using a conversion coefficient, the equivalent thickness or Gross Base and Subgrade (GBE) for each layer can be calculated. The total GBE for this road is 805.7 millimeters.

 Layer Thickness Conversion coefficient Equivalent thickness or GBE Hot mixed asphalt 30.5 2 61.0 Hot mixed asphalt 58.5 2 117.0 Hot mixed asphalt 38.1 2 76.2 Base 144.8 1 144.8 Subbase 607.1 0.67 406.7 Total 805.7

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