1. Introduction
Asphalt pavement is widely used for its advantages such as comfortable driving, low noises, small vibrations and convenient maintenance [
1]. Meanwhile, asphalt pavement is prone to multiple types of distresses due to the effects of traffic and the environment on the materials and structure [
2,
3]. As a result, the service life and maintenance cost of asphalt pavement are negatively affected and require timely prediction and management [
4]. Traditional methods measuring the properties of asphalt layer materials and asphalt mixtures utilize large scale instruments in the laboratory. Their setup and loading are complicated and time-consuming, which cannot satisfy the demand for efficient damage detection and maintenance decision making. Furthermore, these methods have limitations including destruction in order to service asphalt pavements, wastage of manpower and materials, etc. [
5] Therefore, it is necessary to develop and improve nondestructive testing (NDT) technology in the structural characterization and evaluation of pavement materials.
Currently, several NDT techniques are applied in the laboratory and field for asphalt mixtures and structures, among which the acoustic system is a widely used method. Acoustic pulses are generated at one position of the test sample by a transmitter device and then are received at the same or another position by a receiver senor. The system can be divided into sonic and ultrasonic, and the frequency of the mechanical vibration or acoustic pulse in the ultrasonic system can be higher than 20 kHz [
6]. The pulse propagates through the test sample in the forms of three mechanical waves: a longitudinal wave, transverse wave and Rayleigh wave [
7]. Defects between locations of the transmitter and receiver are reflected on the characteristics of received signals. As they come directly from the sample, properties reflecting the composition of the sample can be obtained as well. Simply speaking, material information is characterized from its effects on the external waves. An acoustic wave also occurs within the material as its internal structure changes due to aging, cracking, plastic deformation, etc. [
8,
9,
10]. This is essentially caused by the release of localized stress energy [
9]. Therefore, the development of material deterioration can be monitored by the continuous recording and analysis of received signals.
Accordingly, applications of ultrasonic testing in asphalt mixtures can be categorized into defect detection and property measurement. Maillard et al. [
11] characterized the fracture and healing of asphalt binders in fracture tests by analyzing the signal evolution. The signal amplitude and intense alternation were utilized as indicators. Chang [
12] monitored microdamage in asphalt thin films and microfractures in asphalt binders and asphalt/aggregate interfaces from received acoustic emission (AE) signals in mechanical tests. The occurrence, source and intensity of AE signals revealed the material state. Cheng et al. [
13] conducted water–temperature–radiation (WTR) cycle tests on asphalt mixtures and evaluated the damage state of samples using the ultrasonic detection method. Damage degree was assumed to be reflected on the ultrasonic waveform, spectrum and velocity. Birgisson et al. [
14], Cui et al. [
15] and Dovom et al. [
16] characterized the moisture damage of asphalt mixtures in the ultrasonic wave velocity test. Mallick et al. [
17] detected the moisture-induced damage of fresh/recycled composite asphalt mixtures in the ultrasonic pulse velocity test. In these studies, moisture damage was believed to affect the material integrity, resulting in the wave velocity change. Pan et al. [
18] and Qiu et al. [
9,
10] partitioned and characterized the continuous damage process of asphalt mixture samples in bending tests using the ultrasonic wave method. The first research team utilized wave velocity as the indicator, while the second research team calculated the AE energy and Felicity ratio from time histories of the load and AE signal. Meng et al. [
19] quantified the freeze–thaw damage of asphalt mixtures in the Rayleigh wave test. Rayleigh wave velocity was utilized to describe the freeze–thaw damage along with natural frequency and damping ratio.
Representative studies on property measurement using ultrasonic testing are summarized in
Table 1. Their main target is to develop a rapid testing method to replace traditional ones considering the operational time of ultrasonic testing. Different models and assumptions can be applied for the tested material according to the purpose of the test. For example, a traditional compression–tension test measures the dynamic modulus of viscoelastic materials. The material properties of asphalt mixtures obtained from the corresponding ultrasonic test are supposed to be frequency-dependent. However, for the properties applied in the deterioration determination or the quality assurance and control (QA/QC), simple and fast testing and analysis procedures showing the property variations with time and sample location are more important. Therefore, these properties can be simplified. For example, if a fixed frequency is applied throughout the study, single modulus values can be used to indicate the stiffness of asphalt mixtures of different types and conditions.
In this study, the second research direction was followed, in which asphalt mixture properties were efficiently determined using ultrasonic testing technology. Relationships between the ultrasonic wave velocity and density, air void content and dynamic modulus were established. In particular, master curve models of the dynamic modulus and phase angle were constructed, and the dynamic modulus measured in a conventional laboratory test and predicted by an analytical model were compared. The main reason is that current inspections and evaluations of in-service pavements are carried out intermittently. Constructed relationships between the physical, mechanical and acoustic properties can provide evidence for a potential transfer from acoustic information received from the field to material information [
20]. Eventually, material deterioration can be revealed by the material information obtained at different stages of the pavement’s service life.
The rest of this paper is structured as follows:
Section 2 describes the material selection and sample preparation for traditional material property measurements and ultrasonic wave tests;
Section 3 introduces both traditional and ultrasonic testing methods on prepared samples;
Section 4 presents the relationship construction for the ultrasonic wave velocity and material properties;
Section 5 summarizes the conclusions from this study and recommendations for future research.
Table 1.
Representative methods for material properties using ultrasonic testing technologies.
Table 1.
Representative methods for material properties using ultrasonic testing technologies.
Material Property | Author | Ultrasonic Testing Description | Analytical Method | Findings/Contributions |
---|
Bulk Specific Gravity | Sztukiewicz [21] | Measure the propagation time/velocity of longitudinal ultrasonic wave at a constant frequency (500 kHz). | Build a linear relationship between the bulk specific gravity () and wave propagation velocity () as | |
Air Void Content | Zargar et al. [22] | Measure the ultrasonic velocity of compressive wave at a constant temperature (25 °C) and frequency (54 kHz). | Build a linear relationship between the air void content (AV) and ultrasonic velocity (UV) at the sample center as | An increase in the air void content resulted in a decrease in the ultrasonic velocity. Model parameters a and b were calibrated for asphalt mixtures with different binder types. Air void content variation with sample location was captured by the ultrasonic wave transmission techniques.
|
Lame’s Constants [23] | Birgisson et al. [14] | Produce compression (P-wave) and shear (S-wave) ultrasonic waves and measure their velocities and . | Apply the theory of elastodynamics and assume the asphalt mixture sample a homogenous and isotropic solid. Determine Lame’s constants from their relations with wave velocities and other material properties as
where E is Young’s modulus; ν and ρ are Poisson’s ratio and material density, respectively
| |
Tigdemir et al. [24] | |
Norambuena-Contreras et al. [25] | |
Di Benedetto et al. [26] | Extend the assumption to linear isotropic viscoelastic materials as where magnitude of the dynamic modulus
and phase angle φ are both frequency dependent Calibrate an analytical model (2S2P1D) for the dynamic modulus of asphalt mixtures from the compression–tension test. Calculate the dynamic modulus using the analytical model at the temperature and frequency of the ultrasonic wave test.
| The time–temperature principle (TTSP) was validated for high frequencies by comparing the dynamic modulus from two testing methods; The ultrasonic wave test detected the anisotropy of the asphalt mixture sample and discrepancies between different compaction methods.
|
Mounier et al. [27] | Calculated Poisson’s ratio using and had different values. It was explained by the fact that the Poisson’s ratio is frequency dependent and wave velocities at different sample directions should be different. The ultrasonic wave test overestimated the dynamic modulus.
|
Larcher et al. [28] | |
2. Material Selection and Sample Preparation
Four different asphalt mixtures were prepared in this study including two dense graded asphalt mixtures with maximum aggregate sizes of 13 mm (AC-13) and 20 mm (AC-20), one semi-open graded gravel mixture with 20-mm maximum aggregate (AM-20) and one stone mastic asphalt mixture with 13-mm maximum aggregate (SMA-13). In AC-13 and SMA-13, basalt was used as the coarse aggregate and limestone was used as the fine aggregate, while in AC-20 and AM-20, limestone was used as both coarse and fine aggregates. The gradation design of mineral aggregates in asphalt mixture samples followed Technical Specifications for Construction of Highway Asphalt Pavements (JTG F40-2004) [
29] and results are presented in
Table 2. A styrene-butadiene-styrene (SBS) modified asphalt binder was used in all samples to hold aggregates together. Designed asphalt binder contents in AC-13, AC-20, SMA-13 and AM-20 mixtures were 5.0%, 4.4%, 5.9% and 3.8%, respectively. Laboratory tests on the binder followed Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011) [
30] and basic properties are presented in
Table 2. Twelve (four mixture types × three duplicates) cylindrical asphalt mixture samples with 100 mm in diameter and 150 mm in height were prepared from compaction using the standard gyratory compaction method, then cut and cored [
31]. They were subsequently used in laboratory tests on physical, mechanical and acoustic properties. Samples preparation is presented in
Figure 1.
5. Conclusions and Future Work
This study aims to build relationships between acoustic, physical and mechanical properties of asphalt mixtures. Traditional material property tests and ultrasonic wave tests were conducted on 12 asphalt mixtures of four different types. Regression models were established between wave velocity and material physical properties. A theoretical model was utilized to process the ultrasonic wave test results to predict the dynamic modulus specified in the traditional test. Main conclusions drawn in this study are summarized as follows:
Linear function can describe the positive correlation between ultrasonic wave velocity and bulk specific gravity. The R2 of the fitting function ranges between 0.60–0.70. This correlation is negatively affected by the increase in the test frequency.
Linear function can describe the negative correlation between ultrasonic wave velocity and air void content. The R2 of the fitting function ranges between 0.75–0.85.
The dynamic modulus of asphalt mixtures can be predicted from a theoretical model for wave velocity in a linear isotropic viscoelastic material. With density, Poisson’s ratio, phase angle and ultrasonic wave velocity, a similar dynamic modulus can be obtained as from the laboratory dynamic modulus test.
Results in this study indicate that ultrasonic testing can serve as a rapid tool to obtain the physical and mechanical properties of asphalt mixtures. For future research, more numbers and types of asphalt binders, aggregates and properties should be considered for a comprehensive investigation of the relationships between the physical, mechanical and acoustic properties of asphalt mixtures. Moreover, samples with different damage types and degrees can be characterized using ultrasonic testing to extend its applications in the laboratory and field.