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Water ingress into the flexible pavements causes failure due to the degradation of the adhesion between bitumen and aggregates. The adhesion loss results in durability decrease of Hot Mix Asphalt (HMA), which consequently leads to early damages of asphalt pavements involving stripping, raveling, and hydraulic scour. The moisture presence can accelerate growth of common distresses such as permanent deformation along wheel path, fatigue cracks, and potholes.
Moisture damage resistance of asphalt mixtures can be studied using field tests, experimental laboratory testing or analytical methods. Modified Lottman testing method is a widely-used experimental procedure that has been adopted by SuperPave mix design and enables pavement engineers to check whether or not a mixture is moisture damage resistance. However, one of the main drawbacks of this test is lack of attention to the failure mechanisms due to no measurement of the main characteristics of the asphalt mixes. Tensile Strength Ratio (TSR) is the proposed index for the moisture susceptibility assessment. It does not provide much information with regard to the several different failure mechanisms of asphalt mixtures. Thus, the reasons for high or poor resistance of a mixture to moisture remain unknown until several redesigns of asphalt mixture reveal them. It can be inferred that there is a need for a method with ability to quantify the adhesion of bitumen-aggregate-water system based on the materials’ basic characteristics. As a result of a better understanding of the adhesion mechanisms between the bitumen and the aggregate surface, effective detections for the high moisture susceptible asphalt mixtures can be made and, once the deficiencies are identified, they can be tackled precisely. Surface free energy (SFE) method is one of the sophisticated methods for adhesive and cohesive parameters measurement. It employs the fundamental properties of asphaltic materials that are influential in their strength against loss of adhesion and cohesion. Adhesion of asphalt binder and aggregate as well as cohesion of asphalt binder in dry and wet conditions are considered the most important parameters of SFE method. Cohesion and adhesion in the aggregate-asphalt binder system are due to acid-base forces and Lifshitz-van derWaals intermolecular forces. According to the thermodynamic theories, the formation of crack and rupture on the contact surface of aggregate and binder as well as binder itself are related to thermodynamic changes in SFE of adhesion and cohesion, respectively. Therefore, to compute the work of adhesion and cohesions, it is essential to know the SFE components of both binder and aggregate.
In recent years, application of materials with nano-structure in various engineering fields has been growing dramatically. Carbon Nanotubes (CNT) is one of the promising nano-structure additives in pavement engineering that is able to improve rheological properties of binder and the performance of asphalt mixes.
In this study, the effect of CNT on moisture susceptibility of asphalt mixtures was investigated. Two types of aggregates, limestone and granite, were mixed with modified base bitumen of 60/70 penetration grade by CNT additive at three different concentrations and, in order to assess their moisture damage resistance, mechanical methods with thermodynamic concepts were used. Within this research, the concept of surface free energy (SFE) method helped to better understand the micro-mechanism of the effects of CNT on moisture damage of HMA.
Cheng et al. proposed two models for the assessment of major moisture failure mechanisms. SFE concepts were used in the models and the results of them proved excellent agreement with mechanical experiments. Hefer pursued the optimization of techniques to characterize SFE, as well as the consideration and evaluation of additional factors that influence adhesion in the presence of water. They presented a synthesis of theories and mechanisms of asphalt binder–aggregate adhesion, and existing and potential techniques for SFE characterization were reviewed to establish firm background knowledge on this subject. Zollinger used SFE measurements and dynamic mechanical analysis (DMA) for moisture damage susceptibility assessment. Kim et al. quantified the level of adhesive fracture using the percentage of the surface area of aggregate exposed to the moisture as a significant index. The universal gas adsorption and the Wilhelmy plate were utilized to measure the surface free energies of aggregate and asphalt, respectively, which then was used to determine the index. Howson et al. proposed remedies for moisture damage resistance improvement based on a database of SFE measurements. The remedies include modification with polymers, antistrip agents, lime, other additives, change of materials in extreme cases.
With regard to CNT, numerous studies have been conducted. Ziari et al. showed that adding sufficiently high dosage of CNT can lead to a significant improvement in fatigue and rutting resistance, complex modulus, and physical properties of asphalt binders. Xiao et al. [16, 17] found that CNT at high dosages increases the resistance against permanent deformations of short-term aged binders. Amirkhanian et al. proved the positive effect of CNT at a sufficiently high percentage (1%) on fatigue rutting resistance of both asphalt binders and mixtures. Santagata et al. used Strategic Highway Research Program (SHRP) tests and showed that CNT modified binders have higher rutting resistance at high temperatures and higher cracking resistance at low temperatures. Other studies by Santagata et al. concluded that well-dispersed CNT within the asphalt matrix could result in the fatigue life improvement of asphalt binders. Also, they claimed that sonication might be a major factor in CNT-modified asphalt binder’s enhanced performance based on the oscillatory and Multiple Stress Creep Recovery (MSCR) test results on unaged and short-term aged binders. Positive impact of CNT on healing behaviors of bituminous materials was justified by another research of Santagata et al.
In this study, two kinds of aggregates (granite and limestone) and a neat asphalt binder of 60/70 by penetration from Tabriz mineral oil refinery were used to fabricate asphalt mixture samples. Chemical components and physical properties of the aggregates are given in Tables 1 and 2, respectively. Also, the selected gradation is presented in Table 3. It falls within the upper and lower limits of Iran Highway Asphalt Paving Code.
From Table 1, it can be seen that granite has a large percentage of silicon dioxide (SiO2) while calcium oxide (CaO) makes up a large portion of limestone ant the amount of SiO2 is very small. This conforms with the general recognition that limestone is alkaline aggregate and granite is acidic aggregate. It is well-established that the adhesion between asphalt and aggregate is very much dependent on aggregate properties.
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