1 Performance Characteristics of High RAP Bio-Modified Asphalt Mixtures Walaa S. Mogawer, Ph.D., P.E. – Corresponding Author Civil and Environmental Engineering Department Highway Sustainability Research Center (HSRC) University of Massachusetts Dartmouth 151 Martine Street – Room 131, Fall River, MA 02723 Phone: (508) 910-9824 Fax: (508) 999-9120 Email:
[email protected] Ellie H. Fini, Ph.D., P.E. Department of Civil Engineering North Carolina A&T State University 447 McNair Hall, Greensboro, NC 27411 Phone: (336) 334-7737 ext. 665 Fax: (336) 334-7126 Email:
[email protected] Alexander J. Austerman, P.E. Highway Sustainability Research Center (HSRC) University of Massachusetts Dartmouth 151 Martine Street – Room 124, Fall River, MA 02723 Phone: (508) 910-9805 Fax: (508) 999-9120 Email:
[email protected] Abbas Booshehrian Highway Sustainability Research Center (HSRC) University of Massachusetts Dartmouth 151 Martine Street – Room 124, Fall River, MA 02723 Phone: (508) 910-9865 Fax: (508) 999-9120 Email:
[email protected] Boubacar Zada Department of Civil Engineering North Carolina A&T State University 447 McNair Hall, Greensboro, NC 27411 Phone: (336) 334-7737 Fax: (336) 334-7126 Email:
[email protected] Submission Date: November 15th, 2011 Word Count = 5,205 5 Tables x 250 Words = 1,250 4 Figures x 250 Words = 1,000 Total Equivalent Word Count = 7,455
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ABSTRACT The purpose of this study was to evaluate the effect of a bio-modified binder on the performance and workability of asphalt mixtures designed with and without a high Reclaimed Asphalt Pavement (RAP) content. The bio-modified binder was prepared by modifying a virgin asphalt binder with biobinder produced from swine manure. The four mixtures that were designed and evaluated were: a control mixture incorporating virgin materials, the control mixture incorporating 40% RAP, the control mixture incorporating the bio-modified binder and the control mixture incorporating the bio-modified binder and 40% RAP. The effect of the bio-modified binder on the stiffness and workability of the control mixture with and without RAP was evaluated by measuring the dynamic modulus and the torque resistance of the mixtures respectively. The performance of each mixture was evaluated for fatigue cracking (Overlay Tester) and moisture susceptibility/rutting potential (Hamburg Wheel Tracking Device). Finally, the degree of blending between the virgin and RAP binders was evaluated for each mixture. The data indicated that the addition of bio-modified binder helped reduce the stiffness of the control mixture with 40% RAP to a level closer to the stiffness of the same mixture without RAP. Correspondingly, the mixture workability for the control mixture with 40% RAP was improved. The data indicated that the bio-modified binder improved the cracking characteristics and had no negative effect on the moisture susceptibility/rutting characteristics of the control mixture with 40% RAP. Overall, data indicated that there was a good degree of blending between the virgin/bio-modified and RAP binders. BACKGROUND Recently, the asphalt paving industry has been confronted by many new challenges. At the forefront of these challenges is the increased price of petroleum based products, which has led to subsequent increased costs for asphalt binders and Hot Mix Asphalt (HMA) mixtures. The second significant challenge is to produce more environmentally friendly mixtures because the current methods of HMA production require a large quantity of energy and produces considerable amounts of gas and volatile emissions. In response to these challenges, the asphalt paving industry has been actively seeking technologies that can assist in producing cost effective and environmental friendly mixtures. A logical approach to achieve cost effective and environmental friendly mixtures is to use larger amounts of readily available recycled materials like Reclaimed Asphalt Pavements (RAP). RAP is HMA mixture that has been reclaimed from an existing roadway and is comprised of aggregates and aged binder. Accordingly, RAP can replace a portion of the aggregates and asphalt binder in a newly designed HMA mixture, therefore resulting in cost savings. RAP is generally used in surface asphalt mixtures at a percentage of no more than 20 percent, although using higher percentages will lead to more cost savings. Another innovative approach to produce cost effective and environmental friendly mixtures is to use bio-binder as an alternative to petroleum based asphalts. Bio-binder is derived from non-petroleum based renewable resources like wood and corn. Recently, research efforts have suggested using a biobinder along with the petroleum based asphalt to produce a bio-modified binder (1, 2, 3). Generally, five percent of the weight of asphalt binder is replaced by the bio-binder (1, 2). Currently, there are several ongoing research projects with the objective of using higher percentages of bio-binder. Ultimately, if supported through research, it would be ideal to use bio-modified binder instead of petroleum based asphalt binders. A previous research study has indicated that a bio-binder produced from swine manure has the ability to improve asphalt binder low temperature properties as well as compactibility of HMA (1, 2). In that study the bio-binder was produced using a thermo chemical liquefaction process transforming the volatile solids into oil under pressure in an anoxic, aqueous environment. The oil was further processed to produce bio-binder to be blended with typical asphalt binders (1, 2). The research was conducted to measure the effect of this bio-binder on the rheological properties of asphalt binder (2). A bio-modified binder was prepared by replacing 5% by weight of the asphalt binder with the bio-binder. It was found that the viscosity of the bio-modified binder was significantly lower than that of the base non-modified binder. This reduction in viscosity may increase the mixture workability and durability. Additionally,
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viscosity reduction will also allow for the mixtures to be mixed and compacted at lower temperatures which in turn will reduce binder aging and emissions (gas and volatile). The research results also showed significant decrease in stiffness and increase in relaxation capability at low temperatures, which implies improvement in the low temperature properties and reduction in low temperature cracking of the biomodified binder. These added rheological properties of the bio-modified binder make this type of binder well suited to use with high-RAP content HMA if a good degree of blending occurs between the RAP binder and the bio-modified binder. Developing high-RAP content HMA using the bio-modified binder has the potential to meet the industry challenge of producing cost effective and environmentally friendly mixtures. However, a concern associated with the use of higher RAP contents is that the resultant mixture might be too stiff and consequently might be prone to failures in the field (1, 2, 4). The aged binder in the RAP is what leads to the increased mixture stiffness. Accordingly, research and specifications have suggested/recommended the use of a softer virgin binder than would typically be specified for the same mixture when no or low RAP is incorporated. The performance of mixtures with high RAP contents will depend heavily on the degree of blending between the softer virgin binder and the RAP binder. If no blending between the virgin and the RAP binders occur, the mixture can be susceptible to rutting due the use of a softer virgin binder. If good blending occurs with the RAP binder, a bio-modified binder would be an ideal potential candidate to use as the softer binder for high-RAP content mixtures. The lower viscosity exhibited by the bio-modified binder can help increase the mixture workability and durability which can be decreased by the inclusion of high percentages of RAP. Also, the improvements to the asphalt binder rheological properties exhibited by the bio-modified binder may help restore the fatigue and low temperature cracking resistance of the mixture which is reduced due to the incorporation of RAP. The study presented herein focused on designing high-RAP content HMA using a bio-modified binder. The performance of the mixture in terms of stiffness, moisture susceptibility, rutting and fatigue cracking was evaluated. Furthermore, the effect of bio-modified binder on the workability and degree of particle coating of the mixture was also measured. Finally, the degree of blending between the RAP binder and the bio-modified binder was evaluated. OBJECTIVES The research study presented in this paper focused on evaluating the effect of a bio-modified binder on the performance and workability of high-RAP HMA. Also, the degree of blending between the RAP binder and the bio-modified binder was measured. Specifically, the objectives were: 1. Develop four HMA mixtures: a control mixture using only virgin materials, the control mixture incorporating 40% RAP, the control mixture using the bio-modified binder and virgin materials, and the control mixture incorporating 40% RAP and using the bio-modified binder. 2. Determine whether good blending or poor blending occur between the RAP binder and the control binder. 3. Determine whether good or poor blending occur between the RAP binder and the bio-modified binder. 4. Determine the effect of the bio-modified binder on the stiffness, fatigue cracking, moisture susceptibility, and rutting of the mixtures. 5. Measure degree of particle coating and the workability of all the mixtures to determine if the bio-modified binder improved the coating and workability of high-RAP mixtures. EXPERIMENTAL PLAN In order to achieve the objectives of the study, an experimental plan was developed as shown in Figure 1. The plan included designing a 9.5 mm Superpave mixture using a PG52-28 asphalt binder. This mixture was named the control mixture. The control mixture was redesigned by incorporating 40% RAP. Next, the PG52-28 binder was modified by incorporating five percent bio-binder to produce the bio-modified binder. The control and the control with 40 percent RAP mixtures were redesigned using the bio-modified
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binder. The effect of the bio-modified binder on the performance, coating, and workability of the control and the control with RAP mixtures was evaluated. Furthermore, the effect of the bio-modified binder on the degree of blending/mixing of the RAP binder with the added binder was evaluated using a methodology recently developed by Bonaquist (5). The methodology compares the measured mixture dynamic modulus |E*| to a mixture dynamic modulus |E*| predicted from the extracted/recovered mixture binder complex shear modulus G* utilizing the Hirsch model. The extracted binder is assumed to represent complete blending of the RAP and added binders. PG52-28 + 5% Bio-Binder (Bio Modified Binder)
PG52-28 Binder
9.5 mm Superpave Mixture
Virgin Aggregates
Control Mixture (No RAP)
40% RAP Mixture
Reclaimed Asphalt Pavement (RAP)
Control Mixture + Bio Modified Binder (No RAP)
40% RAP Mixture + Bio Modified Binder
Testing
Extract Binder from Each Mixture Develop Partial Binder G* Master Curve & Predict |E*| Using the Hirsch Model
Mixture Performance Testing
Stiffness Dynamic Modulus |E*|
Fatigue Cracking Overlay Tester (OT)
Moisture Susceptibility & Rutting Hamburg Wheel Tracking Device (HWTD)
Workability Asphalt Workability Device (AWD)
Compare Measured |E*| and Predicted |E*| Evaluate Effect of Bio-Modified Binder on Degree of Blending
FIGURE 1 Experimental plan. MATERIALS Base Binder A PG52-28 binder obtained from a local asphalt supplier was utilized for control mixture designs. It was the softest grade available that met the low temperature requirement of a PGXX-28 binder that is typically specified in the Northeast. It was used in an attempt to offset the potential mixture stiffening due to the
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use of high percentage of RAP in the mixtures. Based on the viscosity of the binder, the mixture mixing temperature was 144ºC (291ºF) and the compaction temperature was 132ºC (270ºF). Subsequent binder testing in the laboratory indicated that the control binder was a PG58-28 with a continuous grade of PG59.4-30.4. The binder will continue to be denoted as PG52-28 for the remainder of this paper. Bio Modified Binder Bio-binder derived from swine manure was added to the base PG52-28 asphalt binder at a dosage of 5% by weight of asphalt binder to create the bio-modified binder. Due to the reduced viscosity of the biomodified binder, the mixture mixing temperature was 124ºC (255ºF) and the compaction temperature was 113ºC (235ºF). Aggregates and RAP The aggregates utilized were from a crushed stone source in Wrentham, Massachusetts. Four aggregate stockpiles were obtained: 9.5 mm crushed stone, natural sand, stone sand, and stone dust. RAP was obtained from the same contractor. Each aggregate stockpile and the RAP were tested to determine their properties which are shown in Table 1. TABLE 1 Average Virgin Aggregate/RAP Stockpile Properties and Mixture Gradations Mixture Aggregate and RAP Properties Gradations 9.5 mm Sieve Natural Stone Stone 40% 9.5mm RAP* Control Superpave Size Sand Sand Dust RAP Specification 19.0 mm 100 100 100 100 100 100 100 12.5 mm 100 100 100 100 100 99.7 99.8 100 min. 9.5 mm 100 100 10 100 100 97.1 97.8 90-100 4.75 mm 74.1 99.7 99.8 99.7 74.1 66.8 64.5 90 max. 2.36 mm 57.8 98.3 83.7 83.7 57.8 47.8 45.3 32-67 1.18 mm 45.5 93.3 54.3 57.1 45.5 33.5 32.6 0.600 mm 34.4 73.3 33.8 38.6 34.4 23.0 22.9 0.300 mm 22.4 29.7 19.0 24.9 22.4 13.3 13.6 0.150 mm 13.5 4.8 9.4 15.9 13.5 7.1 7.1 0.075 mm 9.1 0.9 4.3 10.9 9.1 4.4 4.7 2-10 Bulk Specific Gravity, Gsb 2.638 2.638 2.624 2.644 2.629 (AASHTO T84/T85) 0.76 Absorption, % 0.76 0.45 0.53 0.60 *Note RAP Binder Content = 5.95% - = Not Applicable MIXTURE DESIGN A control mixture and control mixture incorporating 40% RAP were designed using the PG52-28 and redesigned using the bio-modified binder. Each mixture had the same aggregate gradation and was developed to meet the requirements for a 9.5 mm Superpave mixture in accordance with AASHTO M323 “Superpave Volumetric Mix Design” and AASHTO R35 “Superpave Volumetric Design for Hot Mix Asphalt” (6). The design gradations for each mixture are shown in Table 1. The design Equivalent Single Axle Loads (ESALs) for this project was selected as 0.3 to
