NON-DESTRUCTIVE CHARACTERIZATION OF ASPHALT CONCRETE MIXTURES THROUGH RESONANT COLUMN TESTING

ABSTRACT

 

Asphalt concrete is a multiphase viscoelastic composite material; hence its mechanical properties change with frequency of the applied loads and the temperature. In many of the design tools such as the Mechanistic Empirical Pavement Design Guide (MEPDG), changes in dynamic modulus as a function of the loading frequency is the primary input. There are very limited testing devices that can directly measure the shear modulus of asphalt concrete. Most of the available devices for asphalt concrete testing are limited to a maximum loading frequency of 33 Hz. However, devices that are capable of exerting loads at higher frequencies are generally limited with respect to the maximum attainable strains. Treating an asphaltic mix as a thermo-rheologically simple material makes it possible to extrapolate its viscoelastic behavior from the low frequency to the high frequency ends through a modulus master-curve. However, such extrapolations are not always accurate and can result in significant errors in pavement design and performance prediction. In this dissertation, a conventional soils testing Resonant Column (RC) apparatus is retrofitted to test stiffer materials such as asphalt concrete at a range of temperatures and strains. Through the proposed RC test, the frequency dependent modulus of asphalt concrete at frequencies considerably higher than those attainable by the available devices, i.e. up to 300 Hz, and at strain levels comparable to the conventional modulus tests (about 100µe) were measured. A set of dynamic modulus (DM) tests were performed on both the full-size and the proposed small-size specimens. It was confirmed that the small specimens exhibit the same properties as the full-size specimens. Non-destructiveness of the RC testing of asphalt concrete was also confirmed by investigating the repeatability of RC tests at each temperature, the reproducibility of the same results after one complete set of temperature-frequency sweep tests, and the modulus changes from Impact Resonance (IR) tests. Thus, the same specimen can be used for conducting multiple tests at different temperatures.

 

Finally, RC technique was successfully used to characterize a wide range of asphalt concrete technologies including: conventional Hot Mix Asphalt (HMA), Warm Mix Asphalt

(WMA), high Reclaimed Asphalt Pavements (RAP) content, and high Recycled Asphalt Shingles (RAS) mixes. Results indicate that the conventional modulus master curve underpredicts the actual modulus at higher frequencies. Therefore, the proposed power-form modulus-frequency functions are recommended in this dissertation to be used along with a variable Poisson’s ratio to achieve a better prediction, when an indirect calculation of shear modulus of asphalt concrete is needed. It was concluded that RC test can non-destructively characterize different asphalt concrete technologies at a wide range of temperatures. The RC test provides useful insights about the high frequency modulus and damping of asphalt concrete mixes.

 

 

    

TABLE OF CONTENTS

List of Figures  …………………………………………………………………………………………………………. vii

List of Tables …………………………………………………………………………………………………………… x

Acknowledgements …………………………………………………………………………………………………… xi

Chapter 1 INTRODUCTION AND OBJECTIVES ………………………………………………………. 1

Definition of the Problem …………………………………………………………………………………… 1

Equipment Limitations at Higher Frequencies ……………………………………………….. 1

Need for Direct Measurement of Shear Modulus ……………………………………………. 2

Asphalt Concrete Attenuation Quantification …………………………………………………. 3

Need for Small Scale Specimen Test …………………………………………………………….. 3

Evaluation of Non-destructiveness of RC ………………………………………………………. 4

Research Goal …………………………………………………………………………………………………… 4

Organization of this Thesis …………………………………………………………………………………. 5

Chapter 2 RESONANT COLUMN TESTING AND MODIFICATIONS ……………………….. 7

Introduction ………………………………………………………………………………………………………. 7

Literature Review of Research …………………………………………………………………………….. 8      Modal Analysis Assumptions ………………………………………………………………………. 10

Torsional Mode of Excitation ………………………………………………………………………. 11

Flexural Mode of Excitation ………………………………………………………………………… 14

RC Testing and Asphalt Concrete …………………………………………………………………. 15

Retrofitting a Conventional RCA ………………………………………………………………………… 16

RC Calibration and Modification of the Mass Polar Moment of Inertia …………….. 16

Coupling Between the Specimen and Drive System ………………………………………… 20

Closed Loop Temperature Controlled Chamber ……………………………………………… 22

Assessment of the modal analysis assumption ………………………………………………… 25

Summary and Conclusions ………………………………………………………………………………….. 28

Chapter 3 NON-DESTRUCTIVENESS OF ASPHALT CONCRETE RC TESTING……….. 30

Introduction ………………………………………………………………………………………………………. 30

Experimental Program and Studies ………………………………………………………………………. 32

Sample Preparation …………………………………………………………………………………….. 32

Impact Resonance Test ……………………………………………………………………………….. 33

Resonant Column Tests ……………………………………………………………………………….. 35

Shear Modulus and Damping from RC Tests ………………………………………………………… 36

Repeatability of RC Test …………………………………………………………………………………….. 39

Investigating Modulus Changes Using IR Tests …………………………………………………….. 40

Summary and Conclusions ………………………………………………………………………………….. 43

Chapter 4 SIGNIFICANCE OF SMALL SCALE SPECIMEN ………………………………………. 45

Introduction ………………………………………………………………………………………………………. 45

Mix Types and Sample Preparation ……………………………………………………………………… 47

Small versus Full Size DM Tests …………………………………………………………………………. 48

DM Test Procedure and Master Curve Development ………………………………………. 48

Comparison of Dynamic Moduli ………………………………………………………………….. 50     Comparison of Phase Angles ……………………………………………………………………….. 51

Correlation Between RC and DM Results …………………………………………………………….. 52

Summary and Conclusions ………………………………………………………………………………….. 58

Chapter 5 DAMPING QUANTIFICATION FOR ASPHALT CONCRETE ……………………. 60

Theoretical Background ……………………………………………………………………………………… 60

Forced and Free SDOF Vibration …………………………………………………………………. 61

Selected Measures of Damping ……………………………………………………………………………. 64

Phase Angle ………………………………………………………………………………………………………. 64

Loss Factor h ……………………………………………………………………………………………… 65

Quality Factor …………………………………………………………………………………………….. 66

Logarithmic Decrement ………………………………………………………………………………. 70

Damping Quantification for Asphalt Concrete ………………………………………………………. 71

Application of the Half-Power Bandwidth Method …………………………………………. 72

RC Quality Factor Results and DM Phase Angle ……………………………………………. 73

Number of Cycles and Log Decrement Calculation ………………………………………… 74

Observed Linear and Non-Linear Decay Responses of Decay ………………………….. 82

Summary and Conclusions ………………………………………………………………………………….. 85

Chapter 6 CASE STUDIES OF RC APPLICATION TO ASPHALT CONCRETE

CHARACTERIZATION ……………………………………………………………………………………. 88

Introduction ………………………………………………………………………………………………………. 88

Case Study of WMA Technologies ……………………………………………………………………… 89

Introduction and Background of Research ……………………………………………………… 89

Material Used and Sample Preparation ………………………………………………………….. 90

Resonant Column Testing of WMA Mixes ……………………………………………………. 93

Test Results and Discussions ……………………………………………………………………….. 94

Design and Evaluation of High RAS/RAP Mixes through RC ………………………………… 100

Introduction and Research Background …………………………………………………………. 100

Materials Tested and Sample Preparation ………………………………………………………. 102

Conventional HMA and RAP Mix design ……………………………………………………… 103

RAS Characterization and Design ………………………………………………………………… 107

Sample Preparation for Mechanical Testing …………………………………………………… 109

RC Testing of RAP/RAS Mixes …………………………………………………………………… 110

Comparison of Shear Modulus ……………………………………………………………………… 110

Comparison of Damping Ratios ……………………………………………………………………. 113

Summary and Conclusions ………………………………………………………………………………….. 114

Chapter 7 CONCLUSIONS AND SUGGESTIONS ……………………………………………………… 116

Conclusions ………………………………………………………………………………………………………. 116

Suggestions for Future Work ………………………………………………………………………………. 120

Bibliography ……………………………………………………………………………………………………… 122

Appendix A  Details of Calculations for Mass Polar Moment of Inertia after

Retrofitting of the Resonant Column Tester through Adding Pieces to

Increase the Mass ……………………………………………………………………………………….. 131

Appendix B  Damping Quantification of Asphalt Concrete Using RC ……………………… 134

Chapter 1  

 

INTRODUCTION AND OBJECTIVES

Definition of the Problem

Equipment Limitations at Higher Frequencies

Proper material characterization provides essential inputs for pavement design and performance prediction models. Asphalt concrete, as the main component of flexible pavements, is a Visco-Elasto-Plastic (VEP) material. Thus, its mechanical properties are strongly dependent on temperature and frequency of the applied load and cannot be determined at a single set of temperature-frequency. It is neither practical nor justifiable to perform mechanical tests at each and every loading frequency and temperature to track the viscoelastic behavior of mixes. Therefore, it has become common practice to determine engineering properties of asphaltic mixes at a limited set of temperature-frequency combinations and extrapolate the material response through a modulus master curve for a wider range of frequencies. Generally, most of the available asphalt concrete testing devices are limited to a maximum loading frequency of 33 Hz. Change of dynamic modulus as a function of loading frequency is the input to many design tools such as Mechanistic Empirical Pavement Design Guide (MEPDG). This function is currently determined based on the experimental measurements in the range of 0.1Hz to 25Hz and is then extrapolated through a modulus master curve for frequencies beyond those attainable in the laboratory. Therefore, complementary evaluation of mixes at frequencies higher than those that are attainable through conventional tests can provide a unique insight into the nature of various asphaltic mixes.

This is especially advantageous in studying the emerging technologies such as Warm Mix Asphalt (WMA), mixes containing Reclaimed Asphalt Pavements (RAP), and mixes containing Recycled Asphalt Shingles (RAS). Such technologies have mechanical properties that are generally different than the conventional Hot Mix Asphalt (HMA) mixes. With regards to the frequency and temperature dependency of asphalt concrete properties, the first hypothesis of this research was that modified RC testing of asphalt concrete specimens can yield more reliable results at higher frequencies as compared to the extrapolated results from conventional modulus master curve and DM test.

Need for Direct Measurement of Shear Modulus

Most of the available test methods measure Young’s modulus of elasticity and the associated shear modulus would be calculated by presuming a value for Poisson’s ratio. There are very limited tools for direct measurement of the shear modulus of asphalt concrete mixes. Therefore, established theoretical relationships between Young’s modulus of elasticity and shear modulus of elasticity are used. Such relationships generally use Poisson’s ratio to calculate shear modulus of asphalt concrete based on Young’s modulus. Thus, accounting for the variation of Poisson’s ratio with respect to the temperature and the modulus can be advantageous. Second hypothesis of this research was that indirect calculation of shear modulus by presuming a constant Poisson’s ratio results in a significant deviation from the actual shear modulus of asphalt concrete mixes. A proper test is needed to measure shear modulus of asphalt concrete specimens directly. Resonant Column (RC) test in soils is a tool to study the dynamic properties such as maximum shear modulus and torsional damping of soils. Thus, it was hypothesized that with some modifications this technique can be applied to study shear properties of asphalt concrete mixes.

Asphalt Concrete Attenuation Quantification

Attenuation plays an important role in asphalt concrete performance and can reveal useful information about energy dissipation characteristics, which can contribute to its fatigue resistance. Different methods exist for laboratory measurement of materials damping properties and the results can be interpreted in various ways. Phase angle from conventional dynamic modulus testing is the most commonly used measure of damping for asphalt concrete mixes. The method selected for attenuation quantification is often dependent on the measurement procedure or the available apparatus. In RC testing of soils, the most common method of damping quantification is studying the decay of the free damped vibration response through logarithmic decrement method. ASTM states that in case of soils testing, a maximum number of 10 consecutive cycles should be used to obtain reliable results from logarithmic decay method. Previous research on soils suggests using up to five cycles of free oscillation for such application. When it comes to RC testing of asphalt concrete, information is especially scarce. Thus, responses of a wide range of asphaltic mixtures in both forced and free vibrations were investigated in this research. The results were later used to determine the proper number of cycles to be used for calculating the damping ratio of asphalt concrete specimens.

Need for Small Scale Specimen Test

The proposed RC procedure for testing asphalt concrete uses a cylindrical specimen of smaller dimensions as compared to the standard specimen size of conventional dynamic modulus test (60 mm diameter by 120 mm height versus 100 mm diameter by 150 mm height). Thus, the correlation between the dynamic modulus results from small size and standard size specimens must be established to ensure the usability of the mechanical properties measured through the proposed RC test. Further, the results from RC testing of asphalt concrete specimens were compared with the moduli from master curve to compare the performance of the modulus master curve extrapolations using the small scale specimens.

Evaluation of Non-destructiveness of RC

It was hypothesized that RC testing of asphalt concrete is a non-destructive technique and, hence, the same specimen can be used for performing multiple tests at different temperatures and at different strain levels. Although RC testing is believed to be non-destructive in case of most soils, this hypothesis must be tested since the specimen would be subjected to elevated temperatures during a set of RC tests. This topic was investigated by means of conducting a set of before and after IR modulus measurements on the same specimens that were subjected to RC. Repeatability of the RC tests at each temperature, and reproducibility of the same results after one complete set of temperature-frequency sweep tests were studied to assert the non-destructiveness of the RC testing of asphalt concrete specimens.

Research Goal

The goal of this research is to study the mechanical properties of conventional and unconventional asphalt concrete mixtures at loading frequencies higher than those that are currently attainable through laboratory testing protocols for asphalt concrete. This includes direct measurement of shear modulus and damping of the mixes at a wide range of temperatures. The selected experimental methodology involved studies in the laboratory with a resonant column apparatus retrofitted specifically for this purpose.

Organization of this Thesis

Chapter two presents theory of resonant column testing, followed by details of retrofitting a conventional Resonant Column Apparatus (RCA) for testing asphalt concrete samples. Details of device calibration for both the original and the retrofitted configurations of the RCA are discussed in this chapter. Major assumptions that are commonly made towards deriving the closed-form solution for a cylindrical specimen in RC testing are summarized and studied to make sure that they are not violated after modifying the conventional RCA. Three major issues for testing asphalt concrete through a conventional RC, namely temperature equilibrium, specimen-device coupling, and natural frequency of the device, are discussed and addressed here.

Chapter three presents the results of an experimental study conducted to evaluate the nondestructiveness of RC testing of asphalt concrete specimens, especially, due to subjecting the specimens to multiple cycles of loading and frequency-temperature sweep tests. Impact Resonance (IR) test was used to perform a before-after study on the moduli of asphalt concrete specimens subjected to RC tests. Repeatability of RC test on asphalt concrete is also investigated in this chapter.

Chapter four summarizes the experimental studies conducted to assess the significance of specimen size used in the proposed RC test as compared to the standard specimen size in Dynamic Modulus (DM) testing. The correlation between the moduli measured from RC test and those obtained from conventional dynamic modulus master curve are investigated in this chapter. The importance of using a variable Poisson’s ratio for asphalt concrete is also studied.

Chapter five presents a review of the viscoelastic damping and its measures, followed by the available methods and experimental techniques for attenuation quantification. The review centers on asphalt concrete damping. The logarithmic decrement method, half-power bandwidth quality factor, and phase angle are investigated in this chapter. A suggestion is made on the optimum number of cycles to be used when applying the logarithmic decrement method to calculate damping ratio of asphalt concrete mixes in RC test.

Chapter six centers on the application of the proposed RC testing procedure in characterization of a wide range of asphalt concrete technologies, including a conventional hot mix asphalt (HMA) as control, Sasobit, Evothrem and Foaming as three WMA technologies, and mixes containing high amounts of RAP and RAS. Details of material characterization and mix design are also presented in this chapter. A new parameter for temperature sensitivity of asphalt concrete mixes is presented and discussed in this chapter.

Chapter seven presents a summary of the most important conclusions drawn based on this dissertation and suggestions for the future work.

NON-DESTRUCTIVE CHARACTERIZATION OF ASPHALT CONCRETE MIXTURES THROUGH RESONANT COLUMN TESTING

Sharing is caring!