SPA Application: Characterization of Materials

Many current procedures for the structural design of pavements require an accurate determination of layer moduli. To implement the 2002 Mechanistic Pavement Design, investigating the feasibility of replacing existing methods with more mechanistic approaches is desirable.

Nondestructive testing techniques are widely used to determine moduli of pavement materials, to determine the critical strains and thus estimate the remaining life of a pavement system. Seismic methods provide fundamentally correct linear elastic moduli of different layers which in many instances are more reliable than other nondestructive testing methods. Since seismic moduli are low strain moduli, load-induced nonlinear and time dependent behaviors of pavement materials have to be taken into consideration.

Under a recent project funded by the Texas Department of Transportation a theoretically sound algorithm that uses

1. seismic moduli,
2. well substantiated nonlinear relationships and
3. time-dependent models

was designed that provides the necessary moduli for pavement design and analysis. For the asphalt layer, the inputs to the system are the seismic modulus and parameters for time dependent models such as those advocated by the 2002 Mechanistic Pavement Design based on complex modulus. For the base and subgrade layers, the seismic moduli and the nonlinear parameters of each layer are required.

Case Studies for the Characterization of Materials

Design Modulus Values Using Seismic Data Collection
by
Soheil Nazarian, Imad Abdallah, Deren Yuan and Liqun Ke
University of Texas at El Paso
Center for Highway Materials Research

Executive Summary

In many current procedures for structural design of pavements, an accurate determination of layer moduli is required. TxDOT has acquired the state-of-the-art equipment to perform laboratory and field modulus tests. With the onset of the movement toward the AAHSTO 2002 Mechanistic Pavement Design, investigating the feasibility of supplanting the existing methods with more mechanistic approaches would be desirable. In any mechanistic pavement design procedure, accurate determination of moduli under load conditions similar to wheel loads is necessary.

Seismic methods provide moduli of different layers, which in many instances, may have distinct advantages over other methods used in the state of practice. Especially, seismic moduli are fundamentally-correct material properties, which can often be measured equally easily in the laboratory and in the field. Understanding the potential, TxDOT has invested in the development and practical use of seismic methods. As a reuslt, the Seismic Pavement Analyzer (SPA) and the Portable Seismic Pavement Analyzer (PSPA) are available to TxDOT to measure seismic moduli practically and economically. Unfortunately, a formal design methodology that uses seismic moduli is not available. One reason for this matter is that the seismic moduli are measured at strain and stress levels that are smaller that those imposed by traffic.

In this report, a comprehensive literature search that covers the different aspects of pavement design with seismic moduli is presented. Based on the experience of the researchers and practioners in nondestructive testing, pavement design, geotechnical engineering, seismology and earthquake engineering, a comprehensive conceptual design methodology has also been suggested. The comprehensive methodology has been analyzed and simplified so that it can be practical enough for use by TxDOT. The research issues and unanswered questions are comprehensively detailed. Several case studies are included to show the significance of the concept and the feasibility of it.

Based on this study, the use of seismic moduli in a mechanistic pavement design methodology is reasonable and feasible. To implement a fully-mechanistic design procedure or to develop performance-based specifications, seismic moduli may be a better alternative. This statement is substantiated by the fact that the state of stress and strain are much better understood and defined under seismic tests. Adjusting seismic moduli for the state of stress that the pavement is experiencing under the actual wheel load may be simpler than to understand fully the stress regimes developed during the FWD tests. Many years of research in geotechnical earthquake engineering that dealt with a similar problem has proven this.

Several issues have to be resolved before seismic moduli can be used for design. The most important issue to be addressed is to define a balance between the sophistication in the field tests, with the number and the nature of laboratoy tests, with the design algorithm.

Specifically, the following items have to be considered:

1. The simpest computer algorithm that can provide the capability of determining nonlinear properties of different layer should be identified.
2. The most appropriate model to characterize the base and subgrade should be established.
3. The possibility and the negative consequences of cataloging or estimating some of the parameters used in the material models should be explored.
4. The validity of the entire system, consisting of the model used for estimating stresses and strains within a pavement section, and the nonlinear models used to determine moduli should be determined.
5. The proposed system should be fine-tuned until an acceptable compromise between the accuracy of the results, reasonableness of the laboratory and field tests, and ease of use of computer models are struck.

DETERMINING DESIGN MODULUS VALUES WITH SEISMIC DATA

Imad N. Abdallah
Soheil Nazarian
Deren Yuan

Center for Highway Material Research
The Universit y of Texas at El Paso
500 E. University Avenue
El Paso, Texas 79768

ABSTRACT

In many current procedures for structural design of pavements, an accurate determination of layer moduli is required. With the onset of the movement toward the AASHTO 2002 Mechanistic Pavement Design, investigating the feasibility of supplanting the existing methods with more mechanistic approaches would be desirable. Nondestructive testing techniques are widely used to determine moduli of pavement materials, to calculate the critical strains and, thus, to estimate the remaining lives of pavement systems. A relatively new nondestructive testing device is the Seismic Pavement Analyzer (SPA) whose operating principle is based on generating and detecting seismic waves in a layered medium. Seismic methods provide fundamentally-correct linear-elastic moduli of different layers. However, seismic moduli are low-strain moduli and cannot be directly used in pavement analysis and design. To incorporate in pavement design and analysis, seismic moduli of different layers have to be adjusted to represent moduli at the state of stress and strain-rate that are close to those applied by truck traffic. To do so, the nonlinear behavior of base and subgrade and the viscoelastic behaviors of AC layers should be considered. The nonlinear and viscoelastic parameters, which vary widely for different types of materials,can be preferably obtained from laboratory testing. In this paper the process and results of using seismic data and laboratory tests to determine design moduli are presented. A process where the FWD measurements were used to validate the algorithm is also included. The methodology seems quite feasible.

Geomedia SPA Application: Characterization of Materials