Figure 3.13 Illustration of core and ring virtual cutting technique
3.6 Applications of X-Ray Images on Asphalt Concrete
The applications of X-ray CT in the asphalt mixtures investigated a wide range of interesting aspects. This section seeks to cover related advancement in utilising X-ray CT to determine the microstructure of manufactured specimens and the influence on the mechanical performance.
Masad et al. (1999a) investigated the evolution of the internal structure during Superpave Gyratory compaction (SGC) at different levels of compaction (Figure 3.14). It was found that up to a certain limit of compaction, the orientation of aggregates towards the horizontal direction increased after which it started to decrease. With higher number of gyrations
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the specimens tended to reduce the air voids content, and led the middle part to become more compacted. On the contrary, Lo Presti et al. (2014) found that some SGC specimens had more air voids in the middle than the bottom and top parts. It was concluded that the response to SGC depends significantly on the height and diameter of the specimen that determine the amount of transmitted energy. In general, increasing specimen height results in heterogeneous air voids distribution within the height.
Figure 3.14 Effect of the number of gyration on the void content and aggregate orientation in the SGC (Tashman et al., 2007)
Hassan et al. (2012) conducted a comparison between the effect of different compaction methods (gyratory, vibratory and slab rollers) on the aggregate structure. Specimens were sectioned vertically to inner (core) and outer (ring) parts, and horizontally to top and bottom sections through imaging techniques. Coarse aggregates were found to be more concentrated in the bottom part of all specimens due to gravity. For gyratory and vibratory prepared specimens, the inner section showed slightly more content of coarse aggregates than outer section. In the roller compactor, finer aggregates were found to be more concentrated in the outer section of the specimen. This was
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attributed to the cutting process in the prepared slab which tended to reduce the size of aggregates in the outer section of the specimen. In terms of orientation, for all types of compaction method, aggregates closer to the centre of the specimen were found randomly distributed while aggregates near the edge preferred to form a circumferential alignment.
Generally, air voids are considered the primary parameter of damage and the main factor to investigate permeability of the material. Tracking the water in the images can be utilised to study the permeability, degree of saturation, and hence moisture damage (Arambula et al., 2007, Khan et al., 2010).
Additionally, several studies investigated the air voids characteristics as a parameter to study the deteriorating of the resistance of the pavement. Air voids reduce the effective load transmitting area and hence weaken the material and accelerates the deterioration (Abdul Hassan et al., 2014).
Damage evolution in specimens loaded for mechanical testing can be investigated by comparing the change in the characteristic parameters before and after the test. Khan (2010) found a non-uniform increase in air voids (damage) throughout the specimens in a strain rate test. In addition, specimens tested at high temperature and low strain rate exhibited a greater increase in air voids.
Tashman (2003) investigated damage evolution in specimens loaded in triaxial compression at several pre-identified strains. The study revealed that damage is a localized phenomenon in which a critical section exists within the specimen that leads to failure. The critical section develops larger crack growth higher than the rest of the specimen. Comparing the increase of air
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voids between the top, middle, and bottom sections of the tested specimen (Figure 3.15) exhibited significant cracking, dilation, and minor structural change respectively. This emphasises the need for homogenous material manufacturing through laboratory compaction.
Figure 3.15 Change in the air voids content at different strain levels of the top, middle, and bottom sections of specimens (Tashman, 2003)
The effect of rubber addition on the air voids properties of asphalt mixture was studied by (Hassan, 2012). In comparison to the control mixture (0%
rubber), the 2% modified mixture displayed a non-uniform air voids distribution along the height. When the rubber content was increased to 3%
in the mixture, the air void distribution displayed less variation between the top, middle, and bottom sections. In addition, increasing the rubber content from 2 to 3% developed 70% increase in the number of air voids, while the control mixture had the highest number as well as the most circular shaped air voids. After applying constant strain rate tests up to approximately the highest stress level at 400C, the 3% and 0% rubberised mixtures exhibited more damage in the middle than the top sections of the specimen. This highlights the importance of characterising the air voids properties in local sections rather than considering the total air voids content as the middle region had the lowest air voids content before testing.
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Monotonic constant strain and triaxial repeated creep and recovery tests were applied on asphalt mixture specimens to an identical final strain level by Saadeh (2005). Comparing the pair of X-ray images before and after testing, it was observed that with the same observed macroscopic strain level, the different loading conditions and stress levels resulted in different microstructural distribution. In addition, specimens in triaxial testing showed less dilation than constant strain tested specimens.