Evaluation of Permanent Deformation and Fatigue using Lime Modified Binder

Hot mix asphalt (HMA) pavements are exposed to repeatedly variations in traffic loads and environmental conditions. When imposed stresses are coupled with environmental actions that are caused owing to frequent traffic loads, a reduction in pavement life occur due to moisture damage. Resultantly, it formed permanent deformation and cracking failures. In order to avert this harm, anti-stripping additive are used to increase adhesion of the aggregate-asphalt interface. In this research, lime -the most common solid anti stripping additiveis used .The study targets to evaluate the permanent deformation and fatigue cracking of lime modified asphalt mixtures through Hamburg Wheel Tracking (HWT) laboratory test and Indirect Tensile Fatigue Test (ITFT). The overall findings concluded that lime modified mixtures shows better results than that of unmodified asphalt mixtures in performance test.


III. RESEARCH METHODOLOGY
The research was conducted in three phases. Initially, different aggregate sizes including 0-5mm, 5-10mm, 10-20mm were collected through Margalla Quarry. Bitumen source was Attock Refinery Limited (ARL) and penetration grade 60/70 was selected to be used. The reason for selecting the penetration grade 60/70 is that it is the commonly used bitumen grade across Pakistan. After selection of material, next task was to characterize the material according to reference specifications.
For both gradations, the samples were prepared according to superpave mix design manual (SP-2) in order to determine optimum asphalt content. In this way, binder with 3-4.5% with an interval of 0.5 is used and samples were prepared in gyratory compacter. Each sample weights approximately 4500 gram. The volumetric parameters theoretical maximum specific gravity Gmm, effective specific gravity Gse, Bulk specific gravity Gmb and %Gmm of prepared specimens were measured, verified in light of Superpave mix design criteria and finally optimum asphalt contents were determined.
In the second phase, for determining the optimum asphalt content, bituminous paving mixes were prepared according to the method explained in Asphalt Institute's Superpave mix design manual (SP-2). HMA samples were prepared using 0%, 1%, 1.5% and 2% of lime. These percentages of lime were added in account of total dry weight of aggregate. The optimum asphalt content for each was determined by repeating the Superpave mix design procedure two times. The volumetric properties of mix including, air voids (Va), voids in mineral aggregates (VMA) and voids filled with asphalt (VFA) were determined using their respective formulae after determination of theoretical maximum specific gravity (Gmm) and bulk specific gravity (Gmb). Theoretical maximum specific gravity (Gmm) and bulk specific gravity (Gmb) were determined in accordance with AASHTO T209 and AASHTO T166 respectively.
The size of the samples prepared in the gyratory compactor used is 6 inch (150 mm) in diameter and 7 inch (177.8 mm) in height. After the samples were compacted using the gyratory compactor the samples were left for 24hr to come to the room temperature. Once the samples were at room temperature core cutting machine accompanied by the saw cutting machine was used to core out 4 inch (100 mm) diameter specimens from the 6 inch (150 mm) samples. Further the saw cutting machine was used to cut the specimens into the required thickness, at least 1.57 inch (40 mm) for Indirect Tensile Fatigue Test as instructed in EN 12697 -24 and 150 mm for wheel tracker test respectively.
In the third phase, the samples were tested in order to find the rut depth and number of cycles to failure of the specified specimen using Hamburg Wheel Tracker (HWT) and Universal Testing Machine (UTM). HWT was performed according to AASHTO T 324-04 standard and the number of passes on the specimens was fixed to 20,000. Wet mode of wheel tracker device was selected at 40 degree centigrade. Finally the test was run and wheel started moving to and fro on the mounted specimen. One complete to and fro movement of the wheel was taken as 2 passes. The indirect tensile fatigue test was conducted according to EN-12697-24 standard on the cylindrical shaped samples to characterize modified and unmodified HMA mixes under repeated load applied with constant load mode. The cylindrical shaped test samples are subjected to repeated compressive load in the vertical direction. . The testing was performed for 25 °C and 40°C with a load of 2500N. The samples were tested in Universal Testing Machine UTM 25.

IV. ANALYSIS AND RESULTS
Test results of Indirect Tensile Fatigue test and wheel tracker tests are presented in this section. Statistical Analysis done on Indirect Tensile Fatigue test in order to obtain the significant factors is also presented in this section.
A. Wheel Tracker Test Rutting can be assessed by comparing the rut depths obtained for controlled mixtures of both gradations with the lime-modified mixtures. Result of Hamburg Wheel Tracker test of 19mm gradation is shown in figure 1. It is cogent from the figure that for 19mm gradation, the rut depth value of modified sample is 3.57mm for sample 1 and 3.63mm for sample 2. On taking average of both the values, the average rut depth became 3.6mm. On the other hand, the values of unmodified samples were 4.27mm and 5.02mm and their average became 4.645mm. Conclusively, it showed that average improvement in 19mm gradation with 1.5% lime was notes as 22.04%. The graphical illustrations of 19mm gradation test results are shown in figure 1.

Statistical Analysis of Indirect Tensile Fatigue Test
The statistical analysis of ITFT data with and without lime modification was performed by considering three factors i.e. gradation, test temperature and lime percentage each with two levels. Therefore, 2 3 full factorial design of experiment was performed using MINITAB-15 software. Table 1 shows the factors that have been considered in the factorial design with their high and low levels and abbreviations.

5.
Significant Effects In terms of Normal probability plot and Pareto plot generated using Minitab 15 software, the factors and interaction of factors, which are most significant and affect fatigue cracking of asphalt mixtures, are also shown. Figure 7 shows the Pareto plot having a reference line with red color which shows that beyond this reference line a significant variable came up and have greater effect on the fatigue cracking. It is obvious that, temperature showed significant result and have greater influence on fatigue cracking of lab prepared mixtures at 5% significance level. The other plot is the normal probability plot which also shows the significant main effect as shown in figure 8 respectively. In the normal probability plot the factors or interactions away from the reference line are significant at 5% significance level and the factors which are near the reference line or on the reference line, are insignificant.

Main Effect Plots
The effects of gradation, temperature and lime %age of lab Prepared specimens are shown in figure 9 respectively. The graph between temperature and fatigue cracking reveals that with increase in temperature the number of cycle to failure decreases.
The graph between fatigue cracking and gradation indicates direct relationship i.e. the number to cycle failure increases if nominal maximum aggregate size increases. So from this analysis it is quite obvious that the temperature has greater effect on fatigue cracking as in the below figure, it is clear that the slope of temperature vs number of cycles is greater. Moreover, nominal maximum aggregate size has also greater impact on fatigue cracking as it also showed greater slope. At last, modifier also showed impact on fatigue cracking behavior as its slope in the figure is also liner and inclined that showing effects on fatigue cracking. whereas 22% improvement in fatigue cracking is also observed for 25mm gradation.  At 40 o C, improvement in fatigue cracking is observed in 19mm gradation in lime modified mixtures whereas 22% improvement in fatigue cracking is also observed for 25mm gradation.  Statistical analysis shows that temperature is the most significant factor which affects the indirect tensile fatigue test values followed by gradation and lime content.