Effects of Hydrated Lime on Moisture Susceptibility of Asphalt Concrete

Moisture damage is one of the most critical distresses affecting asphalt pavement. The use of mineral fillers such as hydrated lime can increase the moisture resistance of the asphalt mixtures. In this study, the effects of hydrated lime (HL) and nano hydrated lime (NHL) were investigated on the moisture susceptibility of modified asphalt mixtures. The asphalt mixtures were modified using HL and NHL with varying percentages of each additive. The indirect tensile strength (ITS) test was performed to investigate moisture susceptibility. This test was carried out with three types of asphalt mixtures including, normal asphalt mixture without additives, modified asphalt mixture with HL, and modified with NHL. The results showed that the addition of HL and NHL as mineral fillers enhanced the performance of the mixtures against moisture damage. The highest increase in ITS value under dry conditions was observed in the modified mixtures with 20% HL and 20% NHL, and under wet conditions it was observed at 30% HL and 20% NHL. The results also indicated that the addition of 20% HL and 30% NHL increased the value of tensile strength ratio (TSR) by 78.6% and 70.6%, respectively.


INTRODUCTION
Safe, comfortable, functional, and economical road network infrastructure is a priority for every developed country. Flexible asphalt pavement is the most commonly used due to quick construction, repair work is fairly easy, and No joints required during installation. Flexible asphalt mixtures are visco-elastic materials where the rate of load application and temperature have a great influence on their behaviour. In the flexible asphalt mixtures where good quality aggregates and asphalt cement materials were used, the main contributing factors to the pavement distresses may be the environment (e.g., temperature, water, and air) and traffic loading. These factors commonly cause distresses such permanent deformation (rutting), moisture damage, thermal cracking, and fatigue cracking. In general, conventional asphalt mixtures have a limited capacity under a wide range of traffic loads and temperatures. Therefore, asphalt mixtures are usually modified to face load and temperature challenges. Modified asphalt mixtures can bring a real advantage in road construction, by improving pavement performance as well as extending the pavement life.
Nano-materials enhance the properties of asphalt mixtures in terms of supporting these against common pavement distresses. Nano-materials having a very high specific surface area, which creates a strong network in bitumen and this often increases fatigue resistance, rutting resistance, and resilient modulus of asphalt mixtures (1)(2)(3)(4). The effects of Nano-materials on the performance of asphalt concrete have been investigated by many researchers such as (5-10).
Taherkhani and Tajdini (9) investigated the effect of Hydrated Lime (HL) and Nano-Silica (NS) on the mechanical properties of asphalt mixtures. The study showed that both additives improve the indirect tensile strength (ITS), resistance to freezing thaw, and resistance to fatigue cracks of the mixtures. The study also showed that more improvement was achieved with NS than with HL. However, other studies have found that additives such as HL and Nanohydrated lime (NHL) are very effective to protect the asphalt concrete against moisture damage, stripping, and rutting (3,4,(11)(12)(13)(14)(15). Gilani,Hosseinian (12) found that the NHL increased the resistance of asphalt concrete to fatigue and moisture damage. Moreover, the highest increase in fatigue life and indirect tensile strength (ITS) was observed in modified asphalt mixtures. Razavi and Kavussi (16) investigated the performance of the modified asphalt mixtures after adding HL and NHL. The results concluded that the use of 20% HL and 4% NHL had great resistance against damage caused by moisture and the values of tensile strength ratio (TSR) had increased to 60%.
The main objective of the current study is to investigate and evaluate the effects of HL and NHL in improving the resistance of asphalt mixtures against pavement distresses such as moisture damage. For this objective, the indirect tensile strength test was performed on asphalt mixtures. This test was carried out with three types of asphalt mixtures including, normal asphalt mixture without additives, modified asphalt mixture with HL, and modified with NHL.

Materials
In this research, the asphalt cement with a 60/70 penetration grade was adopted, which had been provided from the Jordan petroleum refinery corporation. In order to characterize the properties of the asphalt cement, a number of laboratory tests such as specific gravity test, penetration test, softening point test, flash point test, and ductility test were performed. The specifications of the asphalt cement are presented in Table 1. Natural crushed coarse and fine aggregates were used in this research. Table 2 shown the final gradation chosen for the aggregate mixture. Also, Figure 1 shows the gradation of the chosen aggregate and the specification band. The grading curve of the chosen aggregate mixture indicates that the gradient of this mixture conforms to the specified limits.
The HL, calcium hydroxide [Ca(OH) 2 ], additive was obtained from locally available resources, while NHL additive was produced from HL in the laboratory using a planetary ball mill. Based on the results of previous researches, the HL and NHL additives quantity were chosen as 5%, 10%, 20%, and 30% by the weight of bitumen. To mix the additives (i.e HL and NHL filler) and asphalt binder, the asphalt binder was heated to fluid condition   (150-160 °C) then the additive was slowly poured into a high-speed shear laboratory mixer with 2500-3000 rpm for 1 hour to ensure uniform distribution of additive in asphalt binder (3,17).

The indirect tensile strength test
The indirect tensile strength (ITS) test was conducted according to AASHO T 283 method to examine the performance of modified asphaltic mixtures against damage caused by moisture. In order to perform the test, eighteen samples were made in wet conditions and eighteen in dry conditions. The asphalt mixture samples included two without additives, eight with different proportions of HL, and eight with different proportions of NHL. Two samples were used in each test condition. In total, 36 samples were made in this research. In the ITS test, loading was performed at a constant rate of 50.8 mm/min (2 in/min) vertical deformation at 25 ᵒC until the samples break. The amount of loading was recorded at the moment of failure. The ITS value in kPa was estimated using the following equation (9) (1) where: F is the vertical load at the moment of failure in N, t is asphalt sample thickness in mm, and d is diameter of asphalt sample in mm.
Moisture susceptibility of the compacted samples is estimated by tensile strength ratio (TSR) using the following equation (9,18) where: ITS wet is the average value of the ITS in a wet condition (kPa), and ITS dry is the average value of the ITS in a dry condition (kPa).

RESULTS AND DISCUSSION
The average loading values obtained from every two samples under the same condition were recorded at the moment of failure as shown in Table 3. These were utilized to estimate the ITS values for control asphalt mixtures (without additives) and modified asphalt mixtures. Figure 2 shows the ITS values on the wet and dry conditions with different additive proportions. As can be seen, the addition of 20% HL and 20% NHL (by weight of bitumen) to the mixture resulted in increase ITS values of asphalt mixtures, in dry condition, by 22.3% and 30.4% respectively. Similarly, the addition of 30% HL and 20% NHL (by weight of bitumen) to the mixture resulted in increase ITS values of asphalt mixtures, in wet condition, by 33.3% and 59.4% respectively. These results may be attributed to the increasing  Figure  2, the use of HL and NHL as bitumen modifier in asphalt mixtures leads to increase TSR values. The TSR values for all modified asphalt mixtures are higher than TSR values for control mixtures (with no additives). The TSR values of 70.6% and 78.6% were obtained by adding 30% HL and 20% NHL, respectively. Whereas, TSR value of 64.3% was obtained for control mixture. These results indicate that the resistance of asphalt mixtures to the harmful effect of water increases with increasing HL and NHL. The results also indicated that the NHL was highly effective in improving the performance of asphalt mixtures against harmful moisture, as compared to control and HL modified mixtures.

CONCLUSIONS
The performance of asphalt mixtures modified with HL and NHL mineral fillers was investigated based on the ITS results. The following brief conclusions were drawn based on the experimental results, which valid over the test conditions and the range of additives used in this study.

1.
The results showed that asphalt modifiers such as HL and NHL helps the asphalt mixture to resist moisture damage under the wet condition. 2. The ITS values of the modified mixtures containing HL or NHL in the dry conditions were higher than for wet conditions. 3. The addition of 20% of HL and 20% of NHL to bitumen increased ITS values of asphalt mixtures, in dry conditions, by 22.3% and 30.4% respectively. 4. The addition of 30% HL and 20% NHL to bitumen increased ITS values of asphalt mixtures, in wet conditions, by 33.3% and 59.4% respectively. 5. The resistance of the NHL modified mixtures against moisture damage was higher compared to mixtures containing HL. The TSR values of 78.6% and 70.6% were obtained by adding 20% of NHL and 30% of HL, respectively. 6. Upon increasing NHL in mixtures at more than 20%, the trend of TSR values showed decreased. This can be attributed to introducing higher amount of NHL material, which has a larger specific surface area, resulting to insufficient amount of binder in mixture. As a result, stiffness of the asphalt mixture increases, air voids content increases, and the compaction of the asphalt mixtures become more difficult.