COMPARISON OF TESTING MEHODS FOR DETERMINATION OF ASPHALT CONTENT FOR HIGH LOSS AGGREGATES

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1 Paper Id: 132 COMPARISON OF TESTING MEHODS FOR DETERMINATION OF ASPHALT CONTENT FOR HIGH LOSS AGGREGATES Vikash Kumar 1, Aravind Krishna Swamy 2 1 BTech Student,Indian Institute of Technology Delhi, New Delhi, India, vikash026@gmail.com 2 Assistant Professor, Indian Institute of Technology, New Delhi,India, akswamy@civil.iitd.ac.in Abstract Centrifuge extraction method is used widely for the determination of asphalt content of asphalt concrete in India. This process involves usage of solvents that are expensive, hazardous and create disposal problems. Long term exposure to these chemicals may lead to health issues. An alternative method used to determine asphalt content is usage of ignition oven. In this process, loose asphalt concrete mixture is heated to elevated temperature. The asphalt binder part is burnt leaving behind the aggregates. The difference between initial weight of loose mixture and final weight ofaggregates gives the binder content. Due to continuous circulation of hot air within the chamber, a part of fine aggregates is also lost during this process. Further, certain varieties of aggregates are prone to break down and lead to particle size degradation at high temperature. These aggregates are commonly referred to as high loss aggregates. Thus, any conclusion based on ignition oven test results for mixtures containing high loss aggregates might be misleading.in this research, limestone was used in the preparation of asphalt concrete mixtures and tested for binder content under controlled conditions. It was found that time consumed by ignition oven depends on binder properties and initial oventemperature. With least average error, the ignition oven proved to be the best method. Early estimation of the binder content canbe made by ignition oven method when compared to other techniques. Thereby, ignition oven proved to be the most environmental friendly and better option among all methods taken into consideration. Keywords Binder content determination; Centrifuge extractor; Ignition oven; 1. INTRODUCTION The determination of binder content in asphalt concrete has been used by pavement construction industry as a part of quality assurance and quality control procedures. One of commonly used technique to determine asphalt content in Indiais using centrifuge extractor along with solvents like Benzene, N-Propyl Bromide (npb), EthyleneTrichloride. The procedure involves submerging loose asphalt concrete mixture in any of the above mentioned solvents for certain amount of time. Submerging in solvent for extended period of time would give sufficient time for the solvent to dissolve asphalt binder coated on aggregates. This mixture is filled into centrifuge extraction bowl and is rotated with higher speed. This centrifugal action separates solvent with bitumen from the aggregate. This process is repeated 2-3 times until clear solvent is obtained. The

2 difference between initial weight of loose asphalt concrete mixtureand final weight of dry aggregates indicate actual binder content in the mixture. A large quantity of these solvents is consumed during this process. As these solvents are expensive and hazardous to the environment, they need to be treated before their disposal to the environment (Brown et al. 1995). The alternative method used to determine asphalt from loose asphalt concrete mixtures is to burn the binder using anignition oven at elevated temperature. The aggregates are heated to C ( F) by blowing hot air or using infrared heater. During this process, binder part is burnt leaving behind the aggregates. The difference between initial weight of loose mixture and final weight of aggregate indicates the amount of binder used in preparing the mixture. Due to continuous circulation of hot air, a part of fine aggregates is also lost during this process. Also, certain varieties of aggregates are prone break down and leading to particle size degradation at high temperature. These aggregates are commonly referred to as high loss aggregates. Thus, any conclusion based on ignition oven result using high loss aggregate might be erroneous. Unlike the conventional heating in the ignition oven where hot air heats the sample present in the chamber by convection, Infrared ignition oven uses infrared rays to heat the sample directly without heating the chamber. Infrared rays excite the molecules in the samples producing heat. The heated sample may transfer some heat to the chamber. The rest procedure is same as that of the ignition oven (Hurley and Prowell, 2005). The nuclear gauge uses fast moving neutrons to determine the asphalt content in the asphalt concrete. The neutrons collide with the elements like hydrogen, carbon and silicon and slow down after repeated collision. Hydrogen slows down the neutrons with least number of collisions to reach the thermal level. Thus neutron is primarily slowed down by hydrogen present in the asphalt. The number of collisions are measured and based on calibration equations actual asphalt content is measured. However nuclear gauges are very sensitive towards the nature of the aggregates, asphalt binder type, moisture content and mixture properties (Alattar and Al-Qadi 1996). This research evaluated the effectiveness of ignition oven and centrifuge extraction method for determining the percentage of binder in the asphalt concrete mixture for high loss aggregates.the effectiveness of method was evaluated through parameters like testing time, and accuracy. 2. MATERIALS AND METHODS To evaluate accuracy, issues with different binder content determination techniques, asphalt concrete mixtures were prepared in lab under controlled conditions. This section describes the test methods used in this research. 2.1 Materials and mixture preparation While preparing asphalt concrete mixtures, limestone which is considered as high loss aggregate was used. Limestone is a sedimentary rock which mainly consists of mineral calcite (crystal form of calcium carbonate). Three types of binders namely unmodified binder (Penetration

3 grade 60-70), Polymer Modified Binder (PMB 40) (Penetration grade 40) and Polymer Modified Binder (PMB 70)(Penetration grade 70) were used to evaluate effect of binder. Initially, the aggregates from stockpile was taken and sieved to obtain aggregates of similar size.these similar sized aggregates were stored in different bins. In this research, two aggregate gradations were used and the same is presented in Figure 1. These gradations were chosen from upper bound and lower bound specifications for Semi-Dense Bituminous Concrete (SDBC) as per MORTH guidelines. Total weight of loose asphalt concrete mixture was fixed as 1200g as per ASTM D specifications. For a particular binder type and aggregate, mixtures were prepared at five binder contents. With the gradation of mixture and binder content known, the weight of aggregate from individual bins was calculated. The aggregates were recombined to achieve exact gradation. 100 Cummulative % passing gradation 1 gradation Sieve size (in mm) Fig.1. Particle size distribution curve used for preparing asphalt concrete mixture The batched aggregates in previous step were heated in a metal pan. Once the aggregate reached the mixing temperature, pre-calculated amount of hot asphalt binder was added. The pan was reheated over the gas flame and mixed thoroughly until clear coat of binder was seen over aggregate. Then, hot mixture was poured intoa clean metal pan and left for cooling. This mixture was left exposed to room temperature for at least 24 hours before further testing. 2.2 Ignition oven testing The ignition oven was heated to the preset temperature of C. The lumps in loose mixture are broken lightly and its exact weight was noted. This weight reading was fed into the ignition

4 oven before the test was started. The loose mixture was placed into the chamber of the ignition oven and test was started. Due to circulation of hot air, and high temperature inside the oven, asphalt binder was burnt.the actual weight of mixture during burning process was recorded using inbuilt weighting mechanism.the difference in initial weight of mixture and weight of hot mixture at any time indicated mass loss due to burning of asphalt binder. The test was terminated when three consecutive readings of mass loss were same. The leftover aggregates from ignition chamber were taken out and cooled down to room temperature. Further, sieve analysis was performed using leftover aggregate to find aggregate gradation again. 2.3 Centrifuge extractor testing The weight of prepared loose mixture (~1200g) was noted and placed into the centrifuge extractorbowl. Sufficient amount of solvent (Benzene or npb) was poured into the centrifuge extractor bowl so that the loose mixturewas completely submerged in the solvent. The mixture along with solvent was left undisturbed for minutes, so that asphalt dissolves into the solvent. Aggregates and solvent was separated using centrifuge extractor.further,the process of addition of solvent and centrifuge extraction was repeated 2-3 times until the coating of asphalt was removed and the solvent being removed becomes clear. The aggregates were removed from the bowl and left for drying (for 24 hours) to remove the solvents. The difference between initial weight of asphalt concrete mixture and final weight of dry aggregates indicate actual binder content in the mixture. 3. OBSERVATIONS 3.1 Correction factor determination To quantify aggregate lost due to air circulation, and breakdown of aggregate, one blank mixture was tested using ignition oven. The aggregates were mixed according to specified gradation keeping the total weight constant i.e. 1200g. The test was run similar to the ignition oven testing method. The hot air inside the ignition oven removed a portion of finer particles. The final reading of ignition oven indicated actual aggregate content lost just by circulation of air and aggregate breakdown. This reading was used as correction factor for different gradations which was subsequently subtracted from all the obtained readings from the ignition oven accordingly. The correction factor is 0.2 and 0.1 for gradation-1 and gradation-2 respectively. There are no such requirements of correction factor in centrifuge extractor. Figure 2 depicts the percent of aggregate lost by hot air circulation with respect to time as obtained from the ignition oven.

5 Mass Loss (%) gradation-1 gradation time (in min) Fig. 2. Blank aggregate ignition oven results for determination of correction factor 3.2 Effect of binder content and testing time on mass loss Mass loss of mixture (as percentage) was obtained from ignition oven for different types of binder and at different binder content for different gradations i.e. gradation 1 and gradation 2. The variation of percentage mass loss with time for different binder contents for grade binder with gradation 1 is shownin Figure 2. Mass Loss (%) % BC time (in min) Fig. 3. Variation of mass loss with time at different binder content for PMB70 binder of gradation-1

6 As the loose asphalt concrete mixture is heated, the binder is burnt leaving behind the aggregate part. Thus, mass of the mixture would reduce with time. When all binder part is burnt, noncombustible part i.e. aggregate will remain. Thus the percentage mass loss calculated increases initially and reaches a plateau value after certain time.the percentage mass loss from this plateau region was used to predict binder content of the mixture. At any given time, the amount of binder burnt should be more in case of higher binder content mixture. Thus, curve for higher binder content mixture should be always above lower binder content mixture. The same is seen in Figure Actual binder content and R 2 determination through calibration Corrected readings of predicted binder content were obtained after deduction of temperature compensation factor and correction factor for blank aggregates from ignition oven predicted binder content. Temperature compensation factor takes into account the loss in aggregate mass pertaining to chemical changes in the aggregates at high temperatures. These corrected readings (predicted binder content from ignition oven) values were cross plotted against actual binder content with all three binders shown in Figure 4. A straight line was fitted for each data set to obtain calibration curve. This calibration curve can be used to calculate actual binder content in the mixture. Regression equations thus obtained for all cases are presented in Table 1. Actual Binder Content (%) grade pmb 40 pmb Predicted Binder Content from Ignition Oven (%) Fig. 4. Actual against predicted binder content from Ignition oven for gradation-1

7 Table 1. R 2 value of regression equations of various processes for gradation-1 Method Grade PMB 40 PMB 70 Ignition oven y = 0.882x (R² = 0.991) Using benzene y = 1.036x (R² = 0.975) Using npb y = 0.932x (R² = 0.965) y = 1.093x (R² = 0.997) y = 0.939x (R² = 0.991) y = 1.056x (R² = 0.997) y = 1.087x (R² = 0.999) y = 0.929x (R² = 1) y = 0.945x (R² = 0.986) Regression equations (through slope and intercept term coefficients) presented in Table 1indicatesthat over predicted/under predicted binder content when compared to actual binder content and predicted binder content from ignition oven. This indicates extraction technique adopted impacts binder content values determined.thus any calibration curve developed would be unique for a given mixture and its constituent material properties. 3.4 Gradation Analysis after determining binder content Gradation of aggregates was checked after testing the mixture for its binder content using ignition oven. The gradations before and after testing are plotted on same graph for a particular gradation, binder content and binder grade. One of the plots of 5 percent PMB 40 binder (gradation-2) is shown in Figure 4.As seen in Figure 4, the aggregate gradation changes after testing by any method. In case of solvent extraction method, very fine aggregate particles get mixed with solvent and are lost. However in case of ignition oven method, (i) coarse aggregates breakdown at high temperature and (ii) finer aggregate particles are lost due to air circulation. Visual examination of data from other mixtures indicated that binder grade, binder content and gradation have a bearing on aggregate degradation properties. The change in aggregate gradation was seen to be more dependent on binder content in the mixture rather than the type of the mixture. The major changes were observed in aggregates retained on 4.75 mm sieve. There was a significant loss in the aggregates of sizes between mm. This increasedpercentage of aggregates retained on sieves lower than 4.75 mm sieve. The amounts of fines showed significant increase in both the gradations. This can be observed in Figures 5 for gradation-2.

8 100 Cummulative % passing initial final (IO) final benzene final NPB Sieve size (in mm) Fig. 5. Change in aggregate gradation using 5 percent PMB 40 binder for gradation-2 4. CONCLUSIONS The shape of curves of percentage mass loss for higher bitumen content was much different from those of lower bitumen content. This indicates that burning mechanism of mixtures is dependent on aggregate type and binder content. During ignition oven testing, stiffer binder requires higher temperature to burn off the binder from asphalt concrete mixture. Thus, more time is required to reach this higher temperature. Time consumed to reach higher temperature depends on the initial temperature of ignition oven and the type of bitumen used. Among the binders used in this research, PMB 70 being softest binder, it required the least time. However PMB 40 being the stiffest binder, mixtures prepared using this binder required the highest time to complete the test. The plots of percentage mass loss verses time reached plateau value between minutes. As mass loss is directly related to percentage of asphalt burnt, an early approximation of the asphalt content can be made. This approach minimizes the testing time and can be used where much of accuracy is not needed. Also, mixture with higher contents of PMB70 (around 8 %) caught fire as soon as it was kept in the heated ignition oven. So this method cannot be used for testing mixtures having higher bitumen contents especially when they are polymer modified bitumen. One reason can be presence of high volatile materials in polymers.

9 It was found that aggregate degradation (reduction in particle size) took place during ignition oven testing. In ignition oven testing most of the loss occurred in the aggregates whose size was in between 9.5 and 4.75 mm and the loss which occurred in this range of aggregates accounted for the gain in weight the aggregates of lower sieve size mostly in between 0.3 mm and mm. Total weight of the aggregates at the end of test was less than the original weight used for preparing the mixture in every method. This loss can be easily observed in the very fine grained particles of less than mm size which are either burned or fly away during ignition oven testing or were removed with the solvent in solvent extraction method. This accounts for higher binder content being determined by both the methods. However the error in predicting binder content is much higher in case of solvent extraction technique using either of the solvents. References 1. Alattar, I. A., and I. L. Al-Qadi (1996).Prediction of Asphalt Cement Content of Hot-Mix Asphalt Specimens Using Nuclear Gauges.Transport Research Record 1545, TRB, National Research Council, Washington, D.C., pp American Society for Testing Materials (ASTM).Standard Test Methods for Quantitative Extraction of Bitumen from Bituminous Paving Mixtures, D (2005). 3. American Society for Testing Materials (ASTM). Standard Test Method for Asphalt content of Hot-Mix Asphalt by Ignition Method, D (2010). 4. Behrens, M. L., B. I. Dvorak, and W. E. Woldt (1999).Comparison of Asphalt extraction Procedures Implications of Hidden Environmental and Liability costs.transport Research Record 1661, TRB, National Research Council, Washington, D.C., pp Brown, E. R., N. E. Murphy, L. Yu, and S. Mager (1995).Historical development of Asphalt Determination by the Ignition Method.Journal of theassociation of Asphalt Paving Technologists, Vol. 64, pp Brown, E. R., and S. Mager (1996).Round-Robin Study of Asphalt-concrete by Ignition.Transport Research Record 1543, TRB, National Research Council, Washington, D.C., pp Hurley, G. C., and B. D. Prowell (2003).Evaluation of Infrared Ignition Furnace for Determination of Asphalt Content.Transport Research Record 1861, TRB, National Research Council, Washington, D.C., pp Hurley, G. C., and B. D. Prowell (2005).Refinement of the Hot-Mix Asphalt Ignition Method for High-Loss Aggregates.Transport Research Record 1907, TRB, National Research Council, Washington, D.C., pp Mallick, R. B., and E. R. Brown (1999).Development of a Method for Early Prediction of the Asphalt content of Hot-Mix Asphalt by Ignition test. Transport Research Record 1654, TRB, National Research Council, Washington, D.C., pp Prowell, B. D., and J. Youtcheff (2000). Effect of Lime on Ignition Furnace Calibration.Transport Research Record 1712, TRB, National Research Council, Washington, D.C., pp

10 11. Sebaaly, P. E., O. L. Mageid, M. Dunn, and W. Brinkmeyer (2001) rameters on the Correction factors of the Ignition Oven test, Journal of Testing and Evaluation. 12. Thakur, S. E., J. Han, W. K. Chong, and R. L. Parsons (2011). Comparison of Properties of RAP Aggregates Extracted by Ignition and Centrifuge Methods. Geo-frontiers, ASCE, pp

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