## 3 Results and Discussion
### Indirect Tensile Strength (ITS) Test  
The ITS test was conducted on modified asphalt mixtures at unaged, STA and LTA conditions. Figure 10 shows the strength of asphalt mixtures at failure increasing with ageing. This result was expected, based on previous studies about the effect of ageing on asphalt mixture performance [20; 24; 25]. The ITS for all aged samples increased. The ITS for unaged 2 NCMB and 4 NCMB showed an improvement compared with 0 NCMB. ITS increased 13% and 8% for 2 NCMB and 4 NCMB, respectively. The increase in viscosity of modified bitumen, due to adding NC particles, was reflected in an increase in the strength of the mixtures. However, the same trend was observed in mixtures after STA.  
Figure 8: The ratio of the carbonyl band.  
After the application of STA on the mixtures based on their own mixing and compaction temperature, the ITS test was conducted at 20 \({}^{\circ}\)C. The results showed an improvement for all mixtures, as can be observed in Figure 9. This is a good result, which highlights the importance of STA in the paving stage.  
The ITS result showed a considerable increase for both 2 NCMB and 4 NCMB, compared with 0 NCMB. The increase was recorded as 7% and 13% for 2 NCMB and 4 NCMB, respectively. During production and compaction in the field, which is simulated by STA in the lab, the loss of volatile components and oxidation of the bitumen occurred. Oxidation and volatile loss (which first results from evaporation of volatile components, and then by penetration of the oil component of the bitumen into aggregate particles) leads to an increase in mixture hardness. Increasing the mixture hardness, specifically in STA, plays a  
Figure 10: Indirect tensile strength (ITS) results.  
Figure 9: The ratio of the sulfoxide band.  
significant role in improving the load-bearing capacity and resistance to permanent deformation (rutting) of the asphalt mixture in its early service life; a stiffer mixture is produced, leading to a reduction in the mixture flexibility [1; 10; 45]. NC particles improve the strength of the mixture for STA, and it is believed to play an important role in LTA. The result obtained is in good agreement with previous studies, where nano-clay increased the viscosity of the modified binder and subsequently improved the strength of the mixtures [36].  
The ITS was reduced for 0 NCMB after LTA, while the increase in ITS for 2 NCMB and 4 NCMB extended into LTA. The ITS was reduced from 1.41 MPa for STA to 1.36 MPa for LTA for 0 NCMB. The progressive oxidation during LTA leads to an increase in binder viscosity, which results in a strength reduction of the 0 NCMB mixture. The increase in viscosity is related to the increase in asphaltene content; the flexibility is reduced, making the bitumen more brittle and hence more easily broken. The ITS increased from 1.51 MPa for STA to 1.61 MPa for LTA for 2 NCMB, while the ITS for 4 NCMB increased from 1.59 MPa for STA to 1.68 MPa for LTA. This indicates improvement of the mixture strength after ageing as a result of the addition of NC particles. The ITS values of the 2 NCMB and 4 NCMB mixtures indicate improvement of the cohesion of the aged bitumen in the mixture, which could reflect improved resistance against cracking. The improvement in ageing resistance for the mixtures using halloysite nanotube modified bitumen is in good agreement with previous studies done by [PERSON] et al. [20] and [PERSON] et al. [44].