Evaluations of Nano-Sized Hydrated Lime on the Moisture Susceptibility of Hot Mix Asphalt Mixtures

The objective of this study was to evaluate the effect of nano-sized hydrated lime on the moisture susceptibility of the hot mix asphalt (HMA) mixtures in terms of three methodologies to introduce into the mixtures. The experimental design for this study included the utilizations of one binder source (PG 64-22), three aggregate sources and three different methods introducing the lime. A total of 12 types of HMA mixtures and 72 specimens were fabricated and tested in this study. The performed properties include indirect tensile strength (ITS), tensile strength ratio (TSR), flow, and toughness. The results indicated that the nano-sized lime exhibits better moisture resistance. Introducing process of the nano-sized lime will produce difference in moisture susceptibility.

The objective of this study was to evaluate the effect of nano-sized hydrated lime on the moisture susceptibility of HMA mixtures in terms of three methodologies to introduce into the mixtures, i.e., nano lime modified with asphalt binder, nano lime mixed with water, and nano lime blended with aggregate directly. Experiments were carried out to use the following testing procedures such as indirect tensile strength (ITS), tensile strength ratio (TSR), flow, and toughness.

EXPERIMENTAL MATERIALS AND TEST PROCEDURE Materials
The experimental design detailed in this study included the use of one binder grade (PG 64-22), and three aggregate sources (designated as A, B, and C). The engineering properties of coarse and fine aggregate sources are shown in Table 1. Aggregate A (schist) is a metamorphic rock while aggregate source C (granite) is composed predominantly of quartz and potassium feldspar. Aggregate B has larger percentage values of Al 2 O 3 and SiO 2 than aggregate A. Coarse aggregate A has the highest LA abrasion loss percentage and absorption while aggregate C has the lowest. The LA abrasion machine produced super fine nano lime powder was dispersed in acetone by sonicating for 20 minutes. The resulting suspension was dropped onto a clean Si substrate and dried under air. The prepared samples were coated with a very thin layer of Au to increase the conductivity to get clear SEM images of the sample morphology. Hydrated lime is commonly used for anti-stripping of the mixture by being added to the aggregate (1% by weight of dry aggregate).
In this study, three different methodologies were used to produce nano lime mixtures. One regular method was to blend nano lime with the dried aggregate and then mixed with 5% water (by weight of dry aggregate) to make nano lime coat the aggregate completely. It was referred as DNL, second method was to add 1% nano lime to 5% water to generate the slurry, which was mixed with the aggregate. It was denoted as SNL. These aggregates produced from these two approaches were oven dried before mixing. Third method is to add nano lime into asphalt binder and blend for 30 minutes at a speed of 700 rpm and a temperature of 163°C. The modified nano lime (MNL) binder was used to mix with the aggregate in the laboratory.
Thermal-Assisted Field Emission SEM provided by Georgia Institute of Technology was used to take images of nano lime. It is a state-of-the-art equipment that can yield 1 nm resolution at 20 kv and 3 nm at 1kv with operating voltage ranges from 200v to 30kv. The images of these nano lime are shown in Figure 1.

Mix Design, Sample fabrication and testing
The mix design included the aggregates used for a 12.5 mm mixture that satisfied the specifications set forth by the South Carolina Department of Transportation (SCDOT). The design aggregate gradations for each aggregate source were the same when using different methods. The gradations of three aggregates between low and up ranges defined by SCDOT are presented in Table 2. Obviously, the passing percentages of these three gradations are generally similar, and thus the effects of aggregate on the mixtures can be neglected in this study. Superpave mix design defines that the laboratory mixing and compaction temperatures can be determined by using a plot of viscosity versus temperature. The mixing temperatures of 152°C were employed to blend asphalt binder and aggregate. The compaction temperature of 145 o C was used in this study regardless of produced nano lime method and aggregate type.
In Superpave mix design, the optimum binder content (OBC) was defined as the amount of binder required to achieve 3.5-4.0% air voids in accordance with SCDOT volumetric specifications. The detailed mix designs are shown in Table 3. It can be noted that the OBC value of the mixture from aggregate A is higher than these values of mixtures from aggregates B and C. The mixture from aggregate B has the lowest OBC value of 4.8% greater than minimum value of 4.5% set forth by SCDOT. The voids in mineral aggregate (VMA) and voids filled with asphalt binder (VFA)

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Advanced Building Materials and Sustainable Architecture values of all mixtures are higher than 14.5% and between 70-80%, respectively. In addition, the ratios of dust to asphalt contents of mixtures are in the range of 0.6-1.20. Therefore, all mixtures followed SCDOT Superpave mix design specifications in this study. It should be noted that overall mixtures from various aggregate sources used the same regular hydrated lime content of 1%. After the mix designs were completed, for each aggregate, these three blended nano limes were used to make six Superpave gyratory compacted specimens (150mm in diameter and 95mm in height) were prepared with 7 ± 1% air voids, and then the samples were tested at 25 °C (77 °F) to determine the ITS, flow and toughness values. Three of the samples were tested in dry condition and the other three in wet condition. The wet samples were conditioned in accordance with SC T 70, Laboratory Determination of Moisture Susceptibility (SCDOT Test Procedures, 2007). The evaluated parameters included ITS, TSR, and toughness.

EXPERIMENTAL RESULTS AND DISCUSSIONS
To study the effects of three blended nano lime methods on the ITS, flow and toughness values of mixes, analysis of variance (ANOVA) was performed to test the null hypothesis that the sample means (ITS, flow, and toughness of each treatment) are not significantly different from each other at a 5% level of significance.

Gyration number analysis
For gyratory asphalt samples, an increase of gyration number typically reduces its air void content. SC T 70 indicates that the ITS sample should be compacted to 7 ± 1.0% air voids before testing. The gyration number required to reach the target air voids of each mixture is shown in Figure 2. It can be observed that mixtures from aggregate B have the greatest gyration number. In other words, more compaction efforts are required to achieve target air voids for mixtures from aggregate B. However, mixtures from aggregate C generally have the lowest gyration number in this study. In terms of the methodology of using nano lime in the mixture, it can be found that, compared to regular hydrated lime mixtures (CL), nano lime mixtures from aggregate A have slightly greater gyration numbers, however, nano lime mixtures from aggregate C show an opposite trend since these nano lime mixtures have lower gyration numbers. No obvious gyration number trends are found in terms of three methodologies in general. One possible reason is that the asphalt binder content and aggregate shape play a key role in affecting the compaction of mixtures

ITS analysis
The ITS test is often used to evaluate the moisture susceptibility of an asphalt mixture. A higher ITS values typically indicate that the mixture will perform well with a good resistance to moisture damage. At the same time, mixtures that are able to tolerate higher strain prior to failure are more likely to resist cracking than those unable to tolerate high strains.
The dry ITS results shown in Figure 3(a) indicate that the ITS values of specimens from aggregate B are higher while mixtures from aggregates A and C show lower ITS values regardless of the method of using nano lime. With respect to the lime type, it can be noted that mixtures with MNL generally have slightly lower or similar dry ITS values with CL mixtures but generally lower than DNL and SNL mixtures. In most cases, mixtures with DNL have relatively the highest dry ITS values. Similarly, in Figure 3(b), it can be observed that mixtures from aggregate B have the greatest wet ITS values. In addition, MNL mixtures have close or less wet ITS with CL mixtures while the wet ITS value of DNL and SNL mixtures are generally higher regardless of aggregate type. Furthermore, Figure 3(b) indicates that mixtures from aggregates A and C generally have similar wet ITS values. All mixtures have ITS values greater than 448 kPa in this study, the minimum required as per the SCDOT specifications. Statistical analysis shown in Table 4 illustrate that there is no significant difference in dry and wet ITS value amongst any mixtures made from four lime types.

TSR analysis
Tensile strength ratio is usually used to identify the ratio of wet ITS to dry ITS values and avoid the moisture induced damage in dry and wet condition. TSR results are presented in Figure 4. It can be noted that all mixtures have TSR values higher than 80% (the minimum value set forth by AASHTO) regardless of lime and aggregate types. TSR values of mixtures from aggregate B are the highest as using nano lime. In most cases, TRS values from MNL, DNL, and SNL satisfy the requirements by specification.

Deformation analysis
The deformation (flow) resistance of dry ITS specimens, a measure of the material's resistance to permanent deformation in service and related to its stiffness [8], was used for moisture susceptibility analysis of the mixtures [8]. The flow is the total deformation value from the beginning of loading until the loads begins to decrease. As shown in Figure 5(a), the deformation results indicate that, in general, the mixtures from aggregate B show lower dry flow values than mixtures from aggregates A and C. Beside the aggregate properties, another contributing reason is the fact that mixtures made from aggregates A and C had higher optimum asphalt binder contents. In addition, in most cases, mixtures with nano lime have greater flow values than regular hydrated lime in this study. Amongst three nano lime mixtures, MNL mixtures generally have slight higher dry flow values than DNL and SNL mixtures. Statistical analysis shown in Table 4 indicates that  Table 4

Toughness analysis
Toughness is defined as the area under the tensile stress-deformation curve up to a deformation of twice that incurred at maximum tensile stress [3,8]. The toughness results of dry ITS specimens are shown in Figure 6(a) and statistical analysis is presented in Table 4. It can be noted that, in most cases, the dry toughness values of specimens from aggregates A and C are similar regardless of nano lime or regular lime, however, mixtures from aggregate C with nano lime have lower toughness values than mixtures with regular lime form same aggregate. In addition, the toughness values of mixtures with MNL, DNL, and SNL generally are close. Moreover, the wet toughness values of mixtures exhibit similar trends with dry toughness values. Statistical analysis in Table 4 illustrates that there are no significant difference in dry and wet toughness values between any two mixtures such as mixtures containing nano lime or regular lime. Advanced Building Materials and Sustainable Architecture

FINDINGS AND CONCLUSIONS
The following conclusions were drawn based upon the experimental results obtained from the mixtures with or without nano lime in terms of three methodologies of producing nano lime mixtures: