Determination of Suitable Imaging Techniques for the Investigation of the Bonding Zones of Asphalt Layers

Abstract

The material behavior of asphalt depends on its composition of aggregate, bitumen, and air voids. Asphalt pavements consist of multiple layers, making the interaction of the materials at the layer boundary important so that any stresses that occur can be relieved. The material behavior at the layer boundary is not yet understood in detail, as further methods of analysis are lacking in addition to mechanical methods. For this reason, the layer boundary of asphalt structures was analyzed using imaging methods. The aim of this research was to find an imaging method that allows a detailed analysis of the bonding zone of asphalt layers. Two different imaging techniques were used for this purpose. One is a 2-D imaging technique (asphalt petrology) and the other is a 3-D imaging technique (high-resolution computed tomography). Image analysis is a widely used technique in materials science that allows to analyze the material behavior and their composition. In this research, attention was paid to the analysis of the position of the bitumen emulsion, because the contained bitumen is supposed to bond the layers together. It was found that the application of 2-D imaging (asphalt petrology) lacked the precision for a detailed analysis of the individual materials at the layer boundary. With high-resolution computed tomography, a detailed view is possible to visualize the individual materials at the layer boundary in 3D. However, it is difficult to differentiate the materials because there are no gradations in the gray values due to the identical densities. However, it is possible to differentiate between the bitumen from the asphalt and from the emulsion if a high-density tracer is added to the bitumen emulsion for the CT studies. The results of the investigations are presented in this article.

Keywords: X-ray computer tomography; asphalt; asphalt petrology; bitumen emulsion; layers bond.

Figure 1

Visualization of the transition zones and pore space of different concretes using CT [22].

Figure 2

Grinding process of asphalt petrology: (1) water cooled grinder (2) different grinding wheels (3) specimen before and after grinding process (4) surface of specimen after grinding process (epoxy resin is only left in the voids). [17].

Figure 3

Identification of air voids using pixel-based segmentation.

Figure 4

Schematic structure of a µ-CT measurement [22].

Figure 5

Measuring unit of the µ-CT Xradia 520 versa [22].

Figure 6

Analysis of voids over depth using asphalt petrology (red arrows: void analysis; green arrows: location of aggregate).

Figure 7

CT-Scan of an asphalt composite sample with a resolution of 42.9 µm: (a) 3-D reconstruction of the sample which provides an overview of the measurement; (b) model of the entrapped air; (c) sectional view into the sample.

Figure 8

Tracers with different grain sizes.

Figure 9

Particle size distribution of barite powder and barite sand.

Figure 10

Schematic drawing of the material preparation.

Figure 11

Cross-section through the CT scan of the bitumen samples with 10 wt%, 15 wt%, and 20 wt% barite powder (left) and evaluation of the gray scale histograms for the different dosages (right).

Figure 12

Cross-section through the CT scan of the bitumen samples without and with 10 wt% and 20 wt% barium sulfate sand (left) and evaluation of the gray scale histograms for the different dosages (right).

Figure 13

Sectional view through the bitumen emulsion with 10 wt% coarse barite sand.

Figure 14

DSR results of addition of barite powder.

Figure 15

DSR results of addition of barite sand.

Figure 16

Correlation between phase angle and gray value—barite sand.

Figure 17

Correlation between phase angle and gray value—barite powder.

Figure 18

Correlation between complex shear module and gray value—barite powder.