ASPHALTIC CONCRETE EVALUATION FOR MECHANISTIC PAVEMENT DESIGN

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 8, August 2018, pp , Article ID: IJCIET_09_08_049 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed ASPHALTIC CONCRETE EVALUATION FOR MECHANISTIC PAVEMENT DESIGN A.K. Arshad, E. Shaffie, F. Ismail Institute for Infrastructure Engineering and Sustainable Management (IIESM), Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia W. Hashim, Z. Abd Rahman Faculty of Civil Engineering, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia ABSTRACT Resilient modulus of asphaltic concrete is an important parameter used in the mechanistic design of flexible pavements. Laboratory testing of two common types of asphaltic concrete in Malaysia (ACW 20 and ACB 28) was carried out on Marshall specimens (63.5 mm high x 102 mm diameter) using the indirect tensile test according to ASTM 4123 to determine the resilient modulus values at temperatures of 25, 30, 35, 40 and 45 C. These temperatures were chosen to reflect pavement temperatures that exist in Malaysia. In addition, the effects of other factors such as bitumen content, variations in the grading of the mix and different bitumen types (pen 60/70 and 80/100) were investigated. A regression analysis was then conducted to determine the relationship between the resilient modulus and temperature, bitumen content, grading and bitumen type. Log regression models were found to provide the best fit and these are recommended for use in the determination of resilient modulus values of asphaltic concrete for mechanistic design of flexible pavements. Key words: Asphaltic Concrete, Pavement Materials, Resilient Modulus, Flexible Pavement Design, Mechanistic Pavement Design. Cite this Article: A.K. Arshad, E. Shaffie, F. Ismail, W. Hashim, Z. Abd Rahman, Asphaltic Concrete Evaluation for Mechanistic Pavement Design. International Journal of Civil Engineering and Technology, 9(8), 2018, pp INTRODUCTION In Malaysia, a typical flexible pavement consists of asphaltic concrete wearing and binder course, crushed aggregate base layer and sand subbase layer. The stiffness of the asphaltic concrete wearing and binder course is a major parameter in the design of the flexible pavement structure [1]. The stiffness of asphaltic concrete is measured in terms of the elastic modulus. It can be determined through laboratory testing or predicted using nomographs or formulae. According to Huang [2], the elastic modulus that should be used in flexible editor@iaeme.com

2 A.K. Arshad, E. Shaffie, F. Ismail, W. Hashim, Z. Abd Rahman pavement design is the resilient modulus (M R ), which is defined as the ratio of the deviator stress (σ d ) and the recoverable strain (ε r ): M R σ d = ε r The resilient modulus testing described in this paper was conducted using the indirect tension test according to ASTM D-4123 [3]. In this method, a minimum of three specimens was tested, each having a minimum height of 50.8 mm (2 inches) and a minimum diameter of mm (4 inches). Repeated haversine loading was applied at temperatures of 5, 25 and 40 C (41, 77 and 104 F) and at the recommended load period of 0.1 seconds followed by a rest period of 0.9 seconds at each temperature [4]. Each specimen was tested twice (the other at 90 to the first test position) The main advantage of the indirect tensile test is that it uses specimens of Marshall size which can be readily fabricated in the laboratory and easily cored from pavements for in situ specimens. Recent advances in laboratory equipment have made it affordable for laboratories to acquire lower-cost machines such as the Materials Testing Apparatus (MATTA) and the Nottingham Asphalt Tester [5]. However, the indirect tensile test can produce inconsistent results. According to Brown and Foo [6], these variations are due to experimental error, orientation variation and sample variation. Most laboratory standards set the temperature for resilient modulus testing of asphalt at 20 or 25 C, which represent the moderate pavement service temperature conditions which normally occur in the UK (BS DD ) [7] or the USA (ASTM D4123). Past research has suggested that pavement temperatures in Malaysia rarely fall below 30 C in daylight hours, and usually lie within the range of C [8]. In order to realistically depict Malaysian conditions, therefore, laboratory testing should be conducted at temperatures of 35 or 40 C [9]. It is also preferable that the material be tested over the range of pavement service temperature which actually occurs in Malaysia (25-45 C) so that the behaviour can be investigated over the entire range. This paper presents the findings of laboratory resilient modulus testing conducted at five temperatures (25, 30, 35, 40 and 45 C) on Malaysian asphaltic concrete mixes, specifically the ACW 20 wearing course and the ACB 28 binder course mixes. Three different asphalt gradings and two types of bitumen (60/70 and 80/100 pen) were tested. Multiple regression analysis was then conducted to develop a predictive model relating the resilient modulus with the parameters studied: temperature, bitumen content, grading and bitumen type. 2. MATERIALS AND METHODS 2.1. Materials Granite aggregates sourced from a quarry in Cheras, Malaysia was used in this study. The aggregate grading used was in accordance with the gradation specified in the Public Works Department of Malaysia s Standard Specifications for Road Works,as shown in Table 1 [10]. Three different grading lines located within the grading envelope were used (Figure 1): mid (midway between the upper and lower grading lines), mid 25% (between mid and lower grading lines) and mid +25% (between mid and upper grading lines). Bitumen of grades and penetration were used for both asphaltic concrete mixes. Prior to testing, the samples were conditioned in an environmental chamber for 24 hours at 25 C editor@iaeme.com

3 Asphaltic Concrete Evaluation for Mechanistic Pavement Design Table 1 Grading Envelope for ACW 20 and ACB 28 mixes [8] Sieve Size (mm) BS Sieve ACW 20 Wearing Course (% passing by ACB 28 Binder Course (% passing by weight) weight) Figure 1 Grading lines and envelope for ACW 20 mix 2.2. Equipment Figure 2a UTM-5P testing machine Figure 2b Indirect tensile test jig editor@iaeme.com

4 A.K. Arshad, E. Shaffie, F. Ismail, W. Hashim, Z. Abd Rahman The UTM-5P servo, pneumatically-controlled testing machine was used for the resilient modulus testing. The UTM-5P machine was located inside an environmental chamber which could be set to the required testing temperatures with a range of 15 to 60 C. For the repeated load indirect tensile test for resilient modulus, a testing jig equipped with integrated LVDT holders was used to hold the asphalt specimens (Figures 2a and 2b) Testing Procedure Marshall-sized specimens (approximately 102 mm in diameter and 63.5 mm high) of the the ACW 20 and ACB 28 mixes were prepared in the laboratory. Prior to testing, the samples were conditioned in an environmental chamber for 24 hours at 25 C. Resilient modulus testing was then carried out in accordance with ASTM The haversine-shaped load pulse was set at 0.1 seconds and the rest period was 0.9 seconds. A total of five conditioning pulses and 20 load pulses were applied in each test. The maximum applied load and Poisson s ratio were set as shown in Table 2. Table 2 Applied Load and Poisson s ratio Temperature 25 C 30 C & 35 C 40 C & 45 C Load (N) Poisson s ratio The samples were then conditioned and testing was repeated at temperatures of 30, 35, 40 and 45 C, whereby the maximum test load and Poisson s ratio were set as above. The resilient modulus (in MPa) was determined using the following formula: Resilient Modulus (E RT ) = P( ν RT +0.27) / tδh T where P = repeated load (N) t = thickness of specimen (mm) ν RT = total resilient Poisson s ratio ΔH T = total recoverable horizontal deformation (mm) 3. RESULT AND DISCUSSION This section presents the results, analysis and discussion of the laboratory resilient modulus testing carried out on the asphaltic concrete specimens 3.1. Determination of Optimum Binder Content Table 3 shows the optimum binder content (OBC) for each type of asphalt specimens tested for 60/70 and 80/100 pen bitumen and mid, mid 25% and mid +25% gradings. Table 3 Optimum Binder Content for ACW 20 and ACB 28 Asphalt Type ACW 20 ACB 28 Bitumen Pen 60/70 80/100 60/70 80/100 Grading Type mid 25% mid mid +25% It can be seen that, for mixes with grading mid -25% has the lowest OBC for both mix types (ACW 20 and ACB 28). This is possibly due to the lower fines content (passing the editor@iaeme.com

5 Asphaltic Concrete Evaluation for Mechanistic Pavement Design μm sieve) than the other mixes. Also, mixes with the 60/70 pen bitumen had a slightly lower bitumen content than the mixes with the 80/100 pen bitumen. This is most likely due to the lower binder content at the maximum stability and the required flow for mixes with the 60/70 pen bitumen. For the purposes of analysis and discussion the OBC values for the ACW 20 and ACB 28 mixes were assumed to be approximately 5.0% and 4.5% respectively Effect of Bitumen Content Figure 3 shows the relationship between the resilient modulus and temperature at the various bitumen contents for the ACW 20 mix (mid grading) and the 80/100 pen bitumen. The following observations were made: the resilient modulus values decreased as the binder content increased from 4.5 to 6.5%; for a particular bitumen content, as the temperature increased from 25 to 45 C, the resilient modulus values decreased exponentially. This was also observed for all the other ACW 20 mixes with different gradings (mid 25% and mid +25%) and also for mixes using the 60/70 pen bitumen. Similar relationship were also obtained for the ACB 28 mix BITUMEN CONTENT 4.5% y = 68185e x R 2 = Resilient Modulus (MPa) % 5.5% 6.0% 6.5% y = 67337e x R 2 = y = 60767e x R 2 = y = 44367e x R 2 = y = 44481e x R 2 = Temperature (oc) Figure 3 Effect of increasing temperature and bitumen content on resilient modulus of ACW 20 (80/100 pen) mix 3.3. Effect of Variation in Grading The effect of varying aggregate gradation at optimum bitumen content is shown in Figure 4. Using ACW 20 with 60/70 pen bitumen as example and bitumen content of 5.0%, it could be seen that mix curve is located above the mid mix curve while the mid 25% mix curve is located below the mid mix curve. Similar trend was also observed for ACW 20 using 60/70 pen bitumen and ACB 28 using both 80/100 and 60/70 pen bitumen editor@iaeme.com

6 A.K. Arshad, E. Shaffie, F. Ismail, W. Hashim, Z. Abd Rahman y = 58514e x Resilient Modulus (MPa) Mid Mid-25% R 2 = y = 51619e x R 2 = y = 48193e x R 2 = Temperature ( o C) Figure 4 Comparison between different gradings of ACW 20 (60/70 pen) mix at OBC 5.0% 3.4. Effect of Bitumen Penetration Figure 5 compares the resilient modulus for the different bitumen 60/70 and 80/100 penetrations. It can be seen that the resilient modulus for the harder bitumen grade (60/70 pen) was higher than the softer bitumen grade (80/100 pen). This suggests that the use of a harder bitumen type will produce stiffer mixes (i.e. higher resilient modulus values) Resilient Modulus (MPa) pen80/100 y = 39929e x R 2 = pen60/70 y = 58514e x R 2 = Temperature (oc) Figure 5 Comparison of resilient modulus for 60/70 and 80/100 pen bitumen at OBC 5.0% for ACW 20 mix (mid +25% grading) 3.5. Effect of Different Maximum Aggregate Size Figure 6 compares the resilient modulus of the mixes with different maximum aggregate sizes at their respective optimum binder content: ACB 28, which had a maximum aggregate size of 28 mm and an optimum binder content of 4.5%, and ACW 20, which had a maximum aggregate size of 20 mm and an optimum binder content of 5.0%. It can be seen that the ACB 28 curve is above the ACW 20 curve, therefore suggesting that the resilient modulus values editor@iaeme.com

7 Asphaltic Concrete Evaluation for Mechanistic Pavement Design for the larger maximum aggregate sizes were higher at their respective optimum binder content. Similar trends were also observed when comparing ACW 20 and ACB 28 using 60/70 pen. bitumen. This is consistent with the research carried out by Alam (1996) who observed that mixes with larger aggregate sizes have higher resilient modulus values than mixes with smaller sizes aggregates. Another possible reason for this is the lower optimum binder content of ACB28 than ACW Resilient Modulus (MPa) ACWC20mm OBC 5.0% y = 39929e x R 2 = ACBC28mm OBC 4.5% y = 62909e x R 2 = Temperature ( o C) Figure 6 Comparison of resilient modulus of ACW 20 and ACB 28 mixes (80/100 pen bitumen and mid +25% grading) 4. REGRESSION MODELS FOR ASPHALTIC CONCRETE Multiple regression models were developed to predict the resilient modulus values for the ACW 20 and ACB 28 mixes with variables including temperature, bitumen content, fines content and bitumen penetration. A full regression model that included all the independent variables was developed. The variables considered included bitumen content, temperature, the amount of fines passing the 75 um sieve (to represent different gradings) and bitumen type. The multiple regression analysis was carried out using MINITAB software and the best subsets regression approach. To obtain the best model for a given number of variables, a bestsubsets regression was performed. The best subsets regression approach evaluates the best subsets of models for a given number of independent variables. The final model chosen was the model with the highest adjusted r 2 value. Analysis indicated that a log regression model gave a better fit than the pure linear model, and the r 2 values for both models were above 90%. The following log models were therefore selected: ACB 28 Binder course Regression Model Ln (M R ) = (BinCon) 0.104(Temp) (%Fines) (BinPen) (or M R (MPa) = e (BinCon) (Temp) (% Fines) (BinPen) ) (r 2 = 0.93; r 2 (adj) = 0.93) ACW 20 Wearing Course Regression Model Ln (M R ) = (BinCon) 0.108(Temp) (% Fines) (BinPen) (or M R (MPa) = e (BinCon) 0.108(Temp) (%Fines) (BinPen) ) (r 2 = 0.94; r 2 (adj) = 0.94) editor@iaeme.com

8 A.K. Arshad, E. Shaffie, F. Ismail, W. Hashim, Z. Abd Rahman where M R = Resilient Modulus (MPa) BinCon = binder content (4-6% for ACB28; % for ACW 20) Temp = temperature of the asphalt (between 25 and 45 C) % Fines = amount of fines passing the 75 um sieve (3-7% for ACB28; 4-8% for ACW20) BinPen = binder penetration (65 for 60/70 pen; 90 for 80/100pen). The resilient modulus to be used can then be determined using the resilient modulustemperature relationship model. The following example used the resilient modulus log regression model developed for the ACW 20 mix. Assuming a binder content of 5.0%, a % fines of 6% (mid-gradation), a 60/70 pen binder and a pavement temperature of 37 C, the resilient modulus was calculated as follows: (BinCon) (Temp) (%Fines) (BinPen) M R (MPa) = e (5.0) 0.108(37) (6) (65) M R = e M R = 1,334 MPa For the ACB 28 mix, assuming a binder content of 4.7%, a % fines of 5.5% (midgradation), a 60/70 pen binder and a pavement temperature of 35.3 C (calculated for 60mm thick ACB 28, laid below a layer of 40 mm thick ACW 20), the resilient modulus was calculated as follows: (BinCon) (Temp) (%Fines) (BinPen) M R (MPa) = e (4.7) 0.104(35.3) (5.5) (65) M R = e M R = 1,757 MPa To obtain the resilient modulus value for the combined thickness of asphaltic concrete layers (ACW 20 and ACB 28), the following formula is commonly used [11]: where M Req = Resilient Modulus value of the combined asphalt layers h 1 = thickness of the first asphalt layer (ACW 20 in this case), mm h 2 = thickness of the second asphalt layer (ACB 28 in this case), mm M R1 = Resilient Modulus value of the first asphalt layer, MPa M R2 = Resilient Modulus value of the second asphalt layer, MPa Assuming a combined asphaltic concrete thickness of 100mm (ACW 20 of 40 mm and ACB 28 of 60 mm), the resilient modulus value of the combined asphaltic concrete layers was calculated as follows: M Req = 1,578 MPa editor@iaeme.com

9 Asphaltic Concrete Evaluation for Mechanistic Pavement Design 5. CONCLUSIONS Malaysia has high air and asphalt temperatures all year which results in lower resilient modulus values than temperate countries such as the US and UK. This in turn affects the structural strength of the pavement. This paper has presented the results of an investigation of the effects of high temperatures on the resilient modulus of typical Malaysian asphaltic concrete mixes (ACW 20 and ACB 28), particularly with respect to variations in bitumen content, grading and bitumen type. The research indicated that lower bitumen content, the use of bitumen with lower penetration value and the use of aggregate gradings with higher fines content increased the resilient modulus value of the asphalt. ACKNOWLEDGEMENTS Special thanks to the Research Management Institute (RMI) of Universiti Teknologi MARA for providing the financial support under the Dana Kecemerlangan fund. The authors would like to thank the Faculty of Civil Engineering, Universiti Teknologi MARA, Shah Alam, Malaysia for providing the experimental facilities and to all technicians at Highway and Traffic Engineering Laboratory. REFERENCES [1] PWD Malaysia, Manual for the Structural Design of Flexible Pavement (ATJ 5/85 rev.2013). Kuala Lumpur: Public Works Department of Malaysia, [2] Huang, Y.H., Pavement Analysis and Design, 2 nd ed., Englewood Cliffs: Prentice-Hall, [3] American Society for Testing and Materials, Annual Book of ASTM Standards, Vol : Road and Paving Materials. Indirect tension test for resilient modulus of bituminous mixtures. Section D4123, [4] Mallick, R.B. & El-Korchi, T., Pavement Engineering: Principles and Practice, 2 nd ed., Cleveland.: CRC Press, [5] Brown, S.F., Design of heavy duty pavements. Hot Mix Asphalt Technology, 1(4), 1996, pp [6] Brown, E.R. and Foo, K.Y., Evaluation of variability in resilient modulus test results (ASTM D 4123) NCAT Report No National Center for Asphalt Technology, Auburn University, [7] British Standards Institution, Method for the determination of the indirect tensile stiffness modulus of bituminous mixtures. DD213, BSI, London, [8] Bulman, J.N. and Smith, H.R., Pavement performance and deflection studies on Malaysian Roads. TRRL Laboratory Report 795, [9] PWD Malaysia, Interim guide to the evaluation and rehabilitation of flexible road pavements, [10] PWD Malaysia, Standard Specification for Road Works (JKR/SPJ/1988), Public Works Department of Malaysia, [11] Asphalt Institute, Thickness Design Manual (MS-1) (9th Ed.). Asphalt Institute, Lexington KY., editor@iaeme.com