Component Materials Selection Guidelines and Evaluation Tools for the Development of Durable and Balanced Mixtures with High Recycled Asphalt Materials Contents

Abstract

Motivations for using recycled asphalt materials (RAM) in new asphalt mixtures include economic savings in material costs, conservation of natural resources, and preservation of landfill space. However, the use of RAM requires careful engineering of a unique combination of materials (specific component materials at selected proportions) to achieve adequate durability and avoid premature failure. Currently, state departments of transportation (DOTs) limit or prohibit the use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in new asphalt mixtures, commonly expressed in terms of the recycled binder ratio (RBR). This limitation arises from the lack of guidance for designing durable mixtures with RAM, particularly those with high RAM contents. This study aims to propose a comprehensive draft American Association of State Highway and Transportation Official (AASHTO) standard practice with practical tools and guidelines for evaluating and designing durable asphalt mixtures with 0.3–0.5 RBR. Through extensive laboratory testing involving different component materials used in asphalt mixtures with varying RBRs, this study successfully designed mixtures with different RAM contents that are resistant to the most prevalent distresses (cracking, rutting) and durability concerns (raveling, moisture susceptibility). The results helped identify important parameters of component materials that influence performance, such as the quantity and quality of virgin and RAM binders and aggregate type. Overall, this paper describes proposed revisions to an existing draft AASHTO standard practice, providing examples that support each proposed modification. These modifications include guidelines for selecting and proportioning component materials and a framework that includes a stepwise approach with tests, protocols, and thresholds tied to climatic zones.

Keywords

asphalt materials selection and mix design recycled asphalt materials balanced/performance engineered mixture design asphalt mixture performance tests cracking fracture and durability moisture damage testing,standard practice

The amount of recycled asphalt materials (RAM) incorporated into new asphalt mixtures has increased significantly over the last 15 years. According to the most recent survey by the National Asphalt Pavement Association (NAPA), the amount of reclaimed asphalt pavement (RAP) incorporated into new asphalt mixtures based on percentage by weight of mixture gradually increased from 15.6% in 2009 to 22.2% in 2022, representing an increase of 42% ( 1 ). Similarly, the amount of recycled asphalt shingles (RAS) utilized in new asphalt mixtures increased by 7% between 2021 and 2022, achieving an average RAS percentage by weight of 0.15% in 2022. The use of these RAM produced materials savings of approximately $4.7 billion in 2022 and preserved more than 68 million cubic yards of landfill space. As asphalt recycling continues to increase, there are more opportunities to obtain greater economic and environmental benefits. However, these benefits can only be fully achieved by engineering asphalt mixtures with RAM using a unique combination of materials (specific component materials at selected proportions). As a result of the uncertain durability of asphalt mixtures with RAM, state departments of transportation (DOTs) limit RAP or RAS contents or the associated recycled binder ratio (RBR) that captures RAM content and corresponding RAM binder content in the mixture ( 2 ). This limitation can be overcome by new or revised guidelines and practical tools for designing durable asphalt mixtures with RAM, specifically for those with higher RAM contents or RBRs.

The current mix design specifications focus on determining sufficient quantity of binder in the asphalt mixture based on volumetric requirements from the Superpave volumetric mix design method. The Superpave procedure is defined by the American Association of State Highways and Transportation Official (AASHTO) standard practice R 35 and specification M 323 ( 3 , 4 ). However, increasing RAM contents introduces uncertainty concerning the effective binder from RAM contributing to the total quantity in the asphalt mixture and its interaction with other component materials. Balanced mix design (BMD) per AASHTO PP 105 is an enhanced method that incorporates two or more mixture performance tests, such as rutting and cracking tests, into the mix design process ( 5 , 6 ). BMD has been rapidly adopted in the United States to overcome the limitations of the volumetric mix design method ( 7 ). Particularly for asphalt mixtures with RAM, BMD provides an indication of the quality of the virgin and RAM binders and their interactions with other component materials or additives to improve performance if used ( 8 ). Additives and other mitigation strategies, namely high RAM or moisture strategies, are typically implemented by producers to mitigate the intrinsic brittleness of RAM and support its use in asphalt mixtures ( 2 ). Common strategies include incorporating a softer binder concerning the performance grade (PG) for either high-or low-temperature performance grades (PGH, PGL) or both, polymer-modified asphalt (PMA) binder, additives (e.g., warm-mix asphalt [WMA] products, anti-stripping agents [ASA] such as liquid anti-stripping [LAS] agents or lime, and recycling agents [RA]), reducing RBR, and increasing the effective binder content by designing asphalt mixtures with different gradations or alternate mix design requirements ( 9 – 14 ). These requirements include reducing number of gyrations, increasing minimum voids in the mineral aggregate, lowering design air voids, or applying discount factors to the binder from RAM contributing to the overall binder in the asphalt mixture, also known as decreased recycled binder availability (RBA) ( 15 ).

As part of the effort to develop guidelines for the use of RAP in asphalt mixtures, previous studies have proposed changes to the AASHTO R 35 and M 323 Superpave standards. McDaniel and Anderson ( 16 ) found that asphalt mixtures with RAM contents from 10% to 20% could be used without changing the virgin binder grade. If mixtures included more than 20% RAP, recovery and testing of the recovered binder from RAP was recommended along with blending charts to determine the PG of the binder blend or the maximum amount of RAP that should be used in the asphalt mixture. Copeland ( 17 ) recommended determining the moisture content of RAP, RAP binder content, RAP aggregate gradation, and RAP aggregate properties, including bulk specific gravity, for mix design purposes. Other recommendations include careful selection of the virgin binder in asphalt mixtures with RAP contents greater than 25% and performance assessment through evaluation of rutting, cracking, moisture, and fatigue resistance, in addition to volumetric requirements. West et al. ( 18 ) evaluated asphalt mixtures with RAP contents in the range of 25%–50% and proposed minor but relevant changes. Their study proposed the term RAP binder ratio, which refers to the amount of binder from RAP that contributes to the overall binder content in the asphalt mixture. Therefore, mixtures with high RAP content were redefined as those with RAP binder ratios greater than 0.25. Additional recommendations include testing for the evaluation of moisture resistance regardless of the RAP content, rutting resistance if the mixture incorporates a softer virgin binder, and cracking resistance at low temperatures for high RBR mixtures to be used in climates prone to thermal cracking.

Although changes from previous studies were adopted in the current Superpave standards oriented to meet volumetric requirements, there is no uniform AASHTO standard practice for assessing component materials and their proportions within a BMD framework, particularly in asphalt mixtures with high RBRs between 0.3 and 0.5. Therefore, more recent studies have proposed new guidelines and tools to standardize the evaluation of asphalt mixtures with RAM following the new BMD method. National Cooperative Highway Research Program (NCHRP) project 09-58 produced the first draft AASHTO standard practice for the characterization of asphalt mixtures with high RAM contents and RAs ( 19 ). A summary of the main aspects incorporated in the first draft AASHTO standard practice was reported by Epps Martin et al. ( 20 ). The main aspects include guidelines for component materials selection and mixture performance evaluation. This first draft AASHTO standard practice was later modified based on the findings from laboratory and field performance evaluations obtained in NCHRP project 20-44(24) and reported by Leavitt et al. ( 21 , 22 ). The main revisions included an improved method for determining an appropriate RA dose, different aging protocols based on climate, recommendations when using a softer binder or a RA as a high RAM strategy, and consideration for decreasing RBA, given its relevance in mixture performance.

NCHRP project 09-65 followed on these previous efforts with the following objective: 1. Revise the first draft AASHTO standard practice developed in NCHRP project 09-58 and revised in NCHRP project 20-44(24) to address durability and 2. Provide a more comprehensive AASHTO standard practice for the design and evaluation of durable asphalt mixtures regardless of RAM content or incorporation of additives. Based on the laboratory test results of different materials combinations evaluated in this study, the proposed AASHTO standard practice includes expanded and revised aspects, listed as follows:

Component material selection and proportioning guidelines for virgin binders, RAM binders, aggregates, and additives.

Binder blend evaluation tools including blend preparation procedures, binder aging protocols, and thresholds for adequate performance.

Mixture performance evaluation tools including specimen fabrication procedures, mixture aging and moisture conditioning protocols, and thresholds for adequate performance tied to climatic zones.

This paper summarizes the comprehensive laboratory testing results and main findings from NCHRP project 09-65 that facilitate the engineering of durable asphalt mixtures with RAM and allow for the development of evaluation tools included in the proposed AASHTO standard practice. These tools and guidelines provide a framework for assessing the resistance of asphalt mixtures with RAM to the most prevalent pavement distresses (cracking, rutting) and durability concerns (raveling, moisture susceptibility) and aim to guide transportation agencies and producers in the implementation of asphalt mixtures with high RAM contents. A more detailed analysis of the cracking performance, moisture, and raveling resistance results can be found in previous papers by Bairgi et al. ( 23 ) and Montañez et al. ( 24 ).

Materials and Methods

The component materials included in this study encompass different virgin aggregates, binders, and RAMs that represent 10 existing mix designs with typical and high RBRs, defined for this study as less than 0.3 and between 0.3 and 0.5, respectively. These component materials and mix designs are typically produced in the north/freeze and south/no freeze climatic zones in the United States, divided at approximately 40°N latitude, as suggested in NCHRP project 09-52A ( 25 ). Moisture-resistant and moisture-susceptible aggregates were gathered from each climatic zone, as well as the associated RAMs, to produce the mix designs with varying RBRs shown in Table 1. Asphalt mixtures requiring an ASA, either lime or LAS, to improve moisture resistance were classified as mixtures with moisture-susceptible aggregates. Thus, the existing mix designs represent four aggregate sources, four RAP sources, and two RAS sources as follows:

North moisture-resistant (NR) aggregate with northeast (NE) RAP and NE RAS,

North moisture-susceptible (NS) aggregate with midwest (MidW) RAP and MidW RAS,

South moisture-resistant (SR) aggregate with southeast1 (SE1) RAP, and

South moisture-susceptible (SS) aggregate with southeast2 (SE2) RAP.

In addition, as shown in Table 1, each group of mixtures (NR, NS, SR, and SS) utilizes combinations of the same stockpiles in different proportions to produce different aggregate blends and varying RBRs. Finally, information related to water absorption capacity was gathered for each aggregate stockpile from each source. This information was obtained from DOT online databases because it was utilized to examine the interaction between the component materials properties and mixture performance.

Table 1.

Existing Mix Designs and Component Materials with Varying Recycled Binder Ratios (RBRs)

Aggregate	RAM source	RAM^a (RBR)	Add.^b	Pb^c (%)	Virgin aggregate blend (%)^d
Aggregate	RAM source	RAM^a (RBR)	Add.^b	Pb^c (%)	Agg. #1	Agg. #2	Agg. #3	Agg. #4	Agg. #5	Agg. #6
North/freeze moisture-resistant mixtures (NR)
Aggregate absorption capacity (Agg_Abs)^e					0.55	1.00	0.60	na	na	na
Moisture-resistant	NE RAP	25% (0.21)	WMA	5.95	23	37	15	na	na	na
	NE RAP	41% (0.37)	na	5.61	21	28	10	na	na	na
	NE RAP + RAS	26 + 4% (0.22 + 0.22 = 0.44)	na	5.85	34	26	10	na	na	na
North/freeze moisture-susceptible mixtures (NS)
Aggregate absorption capacity (Agg_Abs)^e					0.88	0.58	0.88	0.87	0.88	0.54
Moisture-susceptible	MidW RAP	25% (0.20)	na	5.20	16	14	13	17	15	na
	MidW RAP	35% (0.29)	na	5.20	13	na	20	na	18	14
	MidW RAP + RAS	28 + 2% (0.23 + 0.10 = 0.33)	LAS	5.20	13	na	17	na	40	na
South/no freeze moisture-resistant mixtures (SR)
Aggregate absorption capacity (Agg_Abs)^e					0.80	0.50	0.80	na	na	na
Moisture-resistant^f	SE1 RAP	20% (0.16)	na	5.79	47	22	10	na	na	na
	SE1 RAP	35% (0.29)	na	5.75	40	24	na	na	na	na
South/no freeze moisture-susceptible mixtures (SS)
Aggregate absorption capacity (Agg_Abs)^e					0.55	0.55	0.55	0.55	na	na
Moisture-susceptible^g	SE2 RAP	20% (0.22)	Lime	5.15	27	21.1	20	11	na	na
	SE2 RAP	35% (0.39)	Lime	5.15	28	na	29.1	7	na	na

Note: Agg. = aggregate; MidW = midwest; NE = northeast; RAM = recycled asphalt materials; RAP = reclaimed asphalt pavement; RAS = recycled asphalt shingles; SE1 = southeast1; SE2 = southeast2; WMA = warm-mix asphalt; LAS = liquid anti-stripping; na = not applicable.

RAM percentage by total weight of aggregate.

Additive shown in the existing mix design.

Total binder content in the asphalt mixture.

Each virgin aggregate stockpile for blending is different for each aggregate source.

Refers to water absorption capacity by AASHTO T 84 and T 85.

Includes 1.0% of baghouse fines by weight of aggregate.

Includes 0.9% of lime by weight of aggregate.

Table 2 presents a list of binders used in this study, including the virgin and RAM binders. These binders were characterized with Superpave PG rheological parameters determined using the dynamic shear rheometer (DSR) and bending beam rheometer (BBR) according to AASHTO M 320. Climate-based PG 58-28 and PG 64-22 virgin binders with relatively low ductility or poor relaxation properties were used as control binders for each climatic zone. In this study, low-ductility binders refer to those with ΔT_c values less than +0.5°C. The ΔT_c value is measured after rolling thin film oven (RTFO) per AASHTO T 240 and 20 h of pressure aging vessel (PAV) conditioning per AASHTO R 28 and represents the difference between the continuous critical PGL based on stiffness (S) and relaxation (m). Alternate virgin binders were utilized as a high RAM strategy to improve performance, if needed. Alternate binders include binders with the same PG but from a different source with improved ductility and relaxation properties (i.e., ΔT_c values greater than +0.5°C), softer binders in regard to PG (climate-based control binder shifted by 6°C in both PGH and PGL) with high ΔT_c (greater than +0.5°C), and PMA binders with PG values associated with the climatic zone. The RAM binders were extracted and recovered utilizing the automatic asphalt extraction system according to the American Society for Testing and Materials (ASTM) D8159 and characterized following Superpave PG protocols.

Table 2.

Characteristics of Virgin (Control and Alternate) and Recycled Asphalt Material (RAM) Binders

Binder classification	Continuous PGH (°C)	Continuous PGL (°C)	PG	ΔT_c (°C)
North/freeze
PG58-28Low (control, low ΔT_c)	59.6	−28.6	58–28	+0.4
PG58-28High (high ΔT_c)	60.3	−28.8	58–28	+2.0
PG52-34 (softer, high ΔT_c)	53.6	−35.5	52–34	+2.5
PG70-28 (PMA)	72.3	−31.4	70–28	−0.6
NE RAP	92.6	−9.5	88–4	−7.8
NE RAS	152.3	+33.7^a	148 + 32^a	−58.6
MidW RAP	84.8	−17.6	82–16	−7.4
MidW RAS	151.0	+44.0^a	148 + 44^a	−94.1
South/no freeze
PG64-22Low (control, low ΔT_c)	65.7	−25.3	64–22	−1.4
PG64-22High (high ΔT_c)	65.0	−26.4	64–22	+1.3
PG58-28 (softer, high ΔT_c)	60.3	−28.8	58–28	+2.0
PG76-22 (PMA)	77.4	−24.6	76–22	−1.0
SE1 RAP	126.7	0.3	124 + 2	−12.8
SE2 RAP	99.3	−11.4	94–10	−9.9

Note: MidW = midwest; NE = northeast; PGH = high-temperature performance grade; PGL = low-temperature performance grade; PMA = polymer-modified asphalt; PG = performance grade; RAP = reclaimed asphalt pavement; RAS = recycled asphalt shingles.

Estimated based on 4 mm DSR results for reference only.

To evaluate the performance of virgin and RAM binders for use in asphalt mixtures with typical and high RBRs, different binder testing and analysis tools involving rheological parameters at different aging conditions were used. The tools are listed in Table 3, including the corresponding durability issue associated with each parameter, and images of the equipment for laboratory testing are shown in Figure 1. These tools were adopted from the first draft AASHTO standard practice developed in NCHRP project 09-58 ( 20 ) and from those proposed in NCHRP project 09-59 ( 26 ). Similarly, a comprehensive set of mixture testing and analysis tools presented in Table 4 and shown in Figure 2 was utilized in this study to evaluate the interaction of different component materials in different combinations and the resistance of asphalt mixtures with RAM to the most prevalent distresses (cracking, rutting) and durability concerns (raveling, moisture susceptibility). A labeling convention system was adopted to report the performance results. The system includes the aggregate type and associated RAM sources, along with the RBR reflecting the corresponding RAM content and RAM binder content (Table 1), virgin binder PG and ΔT_c (Table 2), and any additional high RAM or moisture strategy. For example, a high 0.29 RBR mixture with NS aggregate and associated RAM source (MidW RAP), incorporating a PG 58-28 virgin binder with low ΔT_c and decreased RBA as a high RAM strategy, was labeled as NS_0.29_58-28Low_RBA.

Table 3.

Analysis Tools for Binder Evaluation

Parameter	Definition	Potential durability issue	Standard method and/or reference
High-temperature, original and short-term aging
PGH	High-temperature performance grade	Rutting	AASHTO T 315, M 320
Intermediate-temperature, track with aging
G-R_{15°C, 0.005rad/s}	Glover-Rowe parameter is a rheological index calculated at 15°C and 0.005 rad/s from master curve analysis, where G-R = $\| G^{*} \| co s^{2} δ / \sin δ$	Cracking	AASHTO T 315Glover et al. ( 27 ), Rowe ( 28 )
G-R_10rad/s	Alternative G-R determined at 10 rad/s and specific intermediate temperature based on the binder PGL	Cracking	AASHTO T 315Christensen and Tran ( 26 )
R-value	Shape factor utilizing the Christensen-Anderson model at 15°C after setting complex modulus (\|G*\|) values to be greater than 10⁵ Pa with the G_g (glassy modulus) allowed to vary	Cracking	AASHTO T 315Anderson et al. ( 29 )
Low-temperature, short- plus long- term aging
ΔT_c	Difference in critical low temperatures when binder stiffness equals 300 MPa and m-value equals 0.30	Cracking	AASHTO R 118Anderson et al. ( 30 )

Note: AASHTO = American Association of State Highways and Transportation Official.

Figure 1.

Equipment for binder testing (a) dynamic shear rheometer (DSR) for high-temperature performance grade (PGH), G-R_{15°C, 0.005rad/s}, G-R_10rad/s, and R-value, (b) bending beam rheometer (BBR) for low-temperature performance grade (PGL) and ΔT_c..

Table 4.

Analysis Tools for Mixture Performance Evaluation

Parameter	Definition	Potential durability issue	Standard method and/or reference
High-temperature, short-term aging (underwater)
N_12.5	Passes to reach 12.5 mm rut depth	Rutting	AASHTO T 324
SIP	Intersection of the lines that best fit the creep and stripping phases in a HWTT curve	Moisture	AASHTO T 324
SN	Maximum passes before adhesive failure occurs calculated as the inflection point in a HWTT curve	Moisture	AASHTO T 324Yin et al. ( 31 )
Intermediate-temperature, short-term aging (with or without freeze–thaw cycle)
TSR	Ratio of the average tensile strength for the specimens under wet condition to the average in dry condition	Moisture	AASHTO T 283
Intermediate-temperature, short- and/or long-term aging (with or without freeze–thaw cycle)
%AL	Percentage loss as a result of abrasion after short-term aging determined based on the initial mass of the specimen	Raveling	AASHTO T 401
Cantabro ratio	Ratio of abrasion loss for conditioned specimens to unconditioned specimens	Raveling	AASHTO T 401
Intermediate-temperature, short- plus long-term aging
CT_index	Cracking resistance index calculated from the fracture energy, slope, and displacement at 75% of the post-peak load in a load–displacement curve	Cracking	ASTM D8225
Low-temperature, short term aging
G_f	Fracture resistance expressed as the energy required to create a unit surface area of a crack	Cracking	ASTM D7313

Note: AASHTO = American Association of State Highways and Transportation Official; ASTM = American Society for Testing and Materials; HWTT = Hamburg wheel tracking test; SIP = stripping inflection point; SN = stripping number; TSR = tensile strength ratio; %AL = percent loss as a result of abrasion.

Figure 2.

Equipment for mixture testing (a) Hamburg wheel tracking test (HWTT) for N_12.5, stripping inflection point (SIP), stripping number (SN), (b) modified Lottman for tensile strength ratio (TSR), (c) Cantabro for the percent loss as a result of abrasion (%AL) and Cantabro ratio, (d) indirect tensile asphalt cracking test (IDEAL-CT) for cracking tolerance index (CT_Index), and (e) disk-shaped compact tension test (DCT) for fracture energy (G_f).

Before mixture specimen fabrication, the virgin aggregates were oven-dried at 110 ± 5°C to a constant mass, and RAM was air-dried in thin layers, 2 to 3 in. thick. Materials preparation consisted of preheating the virgin binder at the mixing temperature for 2 h before mixing until it was adequately fluid. The virgin aggregates were preheated 25°C above the mixing temperature for at least 3 h. RAP was preheated at 135°C for 1.5 to 3 h before mixing, whereas RAS was not preheated. If the asphalt mixture included a RA or LAS agent, these additives were carefully blended in the virgin binder for one minute using a mixing drill approximately 10 min before mixing. RA doses were determined according to supplier recommendations and the selection method developed and revised in NCHRP projects 09–58 ( 19 ) and 20–44(24) ( 21 ). A LAS dose of 0.5%, based on the total weight of binder, was recommended by the supplier. If decreased RBA was considered as a high RAM strategy, the corrected optimum asphalt content (COAC) was calculated to improve cracking resistance, which is currently part of the Georgia and South Carolina DOT specifications ( 32 , 33 ). The COAC was determined using 85% and 75% decreased RBA for RAP and RAS, respectively, based on survey results from seven state agencies as part of TxDOT project 0-7062 ( 34 ).

To produce an asphalt mixture with typical RAP, high RAP, or high RAP/RAS RBRs; virgin aggregates, RAP and/or RAS, and any dry additives were initially combined and dry-mixed. The asphalt mixture was then prepared by mixing the virgin binder or a blend of virgin binder and additives (RA and/or LAS) with the dry mix using a mechanical mixer for one minute or until the aggregate was thoroughly coated. A short-term oven aging (STOA) protocol of 4 h at 135°C was applied to the loose mixture before compaction per the previous AASHTO R 30. Then, if applicable, a long-term oven aging (LTOA) protocol tied to the climatic zone was applied. The LTOA protocols consisted of loose mixture aging for 6 h at 135°C or 3 days at 95°C for north/freeze climates, as suggested by Bahia et al. ( 35 ), and 8 h at 135°C or 5 days at 95°C for south/no freeze climates, as indicated by Chen et al. ( 36 ). Aging protocols at 95°C were used for asphalt mixtures with PMA binders to avoid the potential thermal degradation of polymers at 135°C.

The mixture specimens were fabricated using the Superpave gyratory compactor after completion of the STOA or STOA plus LTOA protocols. The specimens were prepared to a target air void content of 7.0 ± 0.5% for laboratory testing and evaluation of cracking and moisture resistance, with additional evaluation of rutting resistance to confirm balanced performance. The durability parameters and tests included the cracking tolerance index (CT_Index) by the indirect tensile asphalt cracking test (IDEAL-CT) and fracture energy (G_f) by the disk-shaped compact tension test (DCT) to evaluate cracking resistance, the number of passes to 12.5 mm rut depth (N_12.5), stripping inflection point (SIP), and stripping number (SN) from the Hamburg wheel tracking test (HWTT) to assess rutting simultaneously with moisture resistance, and the tensile strength ratio (TSR) by indirect tensile strength tests (IDT) before and after moisture conditioning to assess moisture resistance. Similarly, the Cantabro test was conducted for additional specimens fabricated at 4.0 ± 0.5% air voids to evaluate raveling resistance by the percent loss as a result of abrasion (%AL) and the ratio of abrasion loss for conditioned to unconditioned specimens (Cantabro ratio). To determine the TSR or Cantabro ratio, moisture conditioning protocols tied to the climatic zone were conducted on compacted specimens, which included vacuum-water saturation and a freeze–thaw cycle for the north/freeze mixtures, and then hot water soaking for all mixtures according to AASHTO T 283.

Laboratory testing was conducted after completion of the aging and moisture protocols associated with the climatic zone. The asphalt mixtures were evaluated by comparing the durability parameters results to the suggested thresholds selected based on current thresholds in standard methods, previous research, and ongoing projects. In addition, the mixtures were evaluated with respect to other factors, such as the component materials or mixture properties, to identify potential relationships. A summary of the testing conducted, conditioning protocols, and thresholds utilized for mixture evaluation is presented in Table 5.

Table 5.

Laboratory Testing, Conditioning Protocols (Aging, Moisture), and Thresholds for Asphalt Mixture Evaluation

Test	Parameter	Climatic zone	Test temp. (°C)	Aging protocol	Moisture protocol	Suggested threshold
Cracking
IDEAL-CT (D8225)	CT_Index	North/freeze	25	STOA + 6 h at 135°C (or 3 days at 95°C for PMA binders)	None	≥30
IDEAL-CT (D8225)	CT_Index	South/no freeze	25	STOA + 8 h at 135°C (or 5 days at 95°C for PMA binders)	None	≥20
DCT (D7313)	G_f	North/freeze	PGL+10	STOA + 6 h at 135°C (or 3 days at 95°C for PMA binders)	None	≥400 J/m²
DCT (D7313)	G_f	South/no freeze	PGL+10	STOA + 8 h at 135°C (or 5 days at 95°C for PMA binders)	None	≥400 J/m²
Rutting
HWTT (T 324)	N_12.5	North/freeze	45	STOA	Underwater	≥10,000 passes
HWTT (T 324)	N_12.5	South/no freeze	50	STOA	Underwater	≥10,000 passes
Moisture susceptibility
Modified Lottman (T 283)	TSR	North/freeze	25	STOA	T 283 with freeze–thaw	≥80%
Modified Lottman (T 283)	TSR	South/no freeze	25	STOA	T 283 without freeze–thaw	≥80%
HWTT (T 324)	SIP	North/freeze	45	STOA	Underwater	≥9,000 passes
	SIP	South/no freeze	50	STOA	Underwater	≥9,000 passes
	SN	North/freeze	45	STOA	Underwater	≥2,000 passes
	SN	South/no freeze	50	STOA	Underwater	≥2,000 passes
Raveling
Cantabro (T 401)	%AL	North/freeze and south/no freeze	25	STOA	None	≤10%
	Cantabro Ratio	North/freeze	25	STOA + 6 h at 135°C (or 3 days at 95°C for PMA binders) + T 283 with freeze–thaw		≤1.75
	Cantabro Ratio	outh/no freeze	25	STOA + 8 h at 135°C (or 5 days at 95°C for PMA binders) + T 283 without freeze–thaw		≤1.75

Note: CT_Index = cracking tolerance index; DCT = disk-shaped compact tension test; IDEAL-CT = indirect tensile asphalt cracking test; G_f = fracture energy; HWTT = Hamburg wheel tracking test; PMA = polymer-modified asphalt; SIP = stripping inflection point; SN = stripping number; STOA = short-term oven aging; TSR = tensile strength ratio; %AL = percent loss as a result of abrasion; temp. = temperature.

Findings—Component Materials Selection and Proportioning Guidelines

The results from this study led to the revision of the proposed component materials selection guidelines included in the first draft AASHTO standard practice by Epps Martin et al. ( 19 , 20 ). These guidelines aim to provide tools to support the selection of component materials for use in asphalt mixtures with RAM (Table 6). The guidelines include suggested thresholds to offer general guidance and do not intend to exclude the use of component materials that fall outside these limits. Asphalt mixtures with RAM can incorporate component materials that do not meet the suggested thresholds; however, achieving all performance requirements for adequate durability may be more challenging.

Table 6.

Component Materials Selection Guidelines

Parameter	Component materials suggested thresholds					Potential durability issue	Standard method and/or reference
Parameter	Virgin binder	RAP	RAS	Recycling agent	Aggregate	Potential durability issue	Standard method and/or reference
Original
Agg_Abs^a	na	na	na	na	<1.0	Moisture	AASHTO T 84, T 85
High-temperature, short-term aging^b
PGH	≤64°C	≤100°C	≤150°C	na	na	Rutting, cracking, moisture	AASHTO M 320
Intermediate-temperature, track with aging^{b, c}
G-R_10rad/s^d	≤5,000 kPa after RTFO + PAV20,≤8,000 kPa after RTFO + PAV40 ^c	na	na	na	na	Cracking	AASHTO T 315Christensen and Tran ( 26 )
R-value	1.5 ≤ R-value ≤ 2.5 after RTFO + PAV20,2.0 ≤ R-value ≤ 3.2 after RTFO + PAV40 ^c	na	na	na	na	Cracking	AASHTO T 315Christensen and Tran ( 26 )
Low-temperature, short- and long-term aging^c
ΔT_c	≥−3.5°C after RTFO + PAV20	≥−7.5°C after RTFO + PAV20	na	na	na	Cracking	AASHTO R 118
Proportioning
RBR	na	≤0.5 RBR (RAP_BR+RAS_BR)	≤0.2 RAS_BR	na	na	Cracking
Dose	na	na	na	≤maximum without sacrificing rutting and moisture resistance^e	na	Rutting, moisture

Note: AASHTO = American Association of State Highways and Transportation Official; RAP = reclaimed asphalt pavement; RAS = recycled asphalt shingles; RBR = recycled binder ratio; na = not applicable.

Aggregate absorption refers to water absorption by AASHTO T 84 and T 85.

Original binder and rolling thin-film oven (RTFO) aged by AASHTO T 240.

Pressure aging vessel (PAV) conditioning (20 h or 40 h) at 100°C by AASHTO R 28.

Test temperature based on binder PGL and limits according to Christensen and Tran (26).

Percent by total binder in the blend/asphalt mixture.

The guidelines are related to the component materials properties and proportions and the associated potential durability issue. Additionally, the guidelines are provided as a system with thresholds applicable to all cases in which data are available. In particular, the new and revised guidelines, shown with a gray background in Table 6, are related to binder and aggregate properties and the proportioning of recycled materials and RAs. Similarly, the G-R_10rad/s and R-value thresholds shown with a gray background are suggested as intermediate-temperature parameters for binder evaluation after RTFO and PAV aging based on the recommendations from NCHRP project 09-59 ( 26 ), which evaluated the relationship between properties of asphalt binders and fatigue performance of asphalt mixtures. Additional details and analysis that support these thresholds can be found in the papers by Bairgi et al. ( 23 ) and Montañez et al. ( 24 ).

Figure 3 presents the interaction plots with the TSR results for the typical and high RBR control mixtures (i.e., without strategies) previously reported by Montañez et al. ( 24 ), which allowed for the development of parameters to preliminarily assess the moisture resistance of asphalt mixtures with RAM. All the interaction plots presented in this study include ‘T’, ‘H’, and ‘RAS’ labels to represent asphalt mixtures with typical RAP, high RAP, and high RAP/RAS RBRs, respectively, for each combination of materials (NR, NS, SR, SS) represented by color and symbol. Additional letters specify the high RAM strategy if used, including softer binder (SF), decreased RBA, and RA.

Figure 3.

(a) Tensile strength ratio (TSR) versus maximum blend% × aggregate absorption capacity (Agg_Abs) and (b) TSR versus total binder content (Pb)/blend high-temperature performance grade (PGH) (×100) ratios for typical and high recycled binder ratio (RBR) control mixtures ( 24 ). Color online only.

The suggested aggregate absorption capacity (Agg_Abs) threshold in Table 6 includes all aggregate stockpiles utilized in this study, with values ranging from 0.5 to 1.0, as shown in Table 1. Additional guidance was noted in the proposed AASHTO standard practice to consider the combined influence of the aggregate absorption capacity (Aggs_Abs) and its proportion in blending (Blend%) on moisture resistance using the Blend% × Agg_Abs screening tool previously proposed by Montañez et al. ( 24 ) and presented in Figure 3a. The results demonstrated a clear influence of the selected aggregate sources and proportions based on failing TSR values (TSR < 80%) for asphalt mixtures including an aggregate stockpile that represented more than 30% of the total aggregate blend with an absorption capacity from 0.8 to 1.0 (falling within the lower right quadrant or red area), also expressed as those with a maximum product of aggregate proportion and absorption capacity greater than or equal to 30 (Blend% × Agg_Abs ≥ 30).

Considering that some asphalt mixtures passed the aggregate criteria (Blend% × Agg_Abs < 30) but continued to fail the TSR (falling within the lower left quadrant in yellow), Montañez et al. ( 24 ) proposed an additional screening tool. This tool considers the influence of binder quantity and quality on moisture resistance and corresponds to the ratio of the total binder content in the asphalt mixture (Pb) to the estimated binder blend PGH (Blend PGH), multiplied by 100. As shown in Figure 3b, the asphalt mixtures with Pb/Blend PGH (×100) ratios lower than 7.5, representing those with binder issues as a result of either low binder contents or stiff binder blends caused by RAM binders with high PGH values, failed TSR and fell within the lower left quadrant in red. Some mixtures without binder issues (Pb/Blend PGH (×100) ≥7.5) still failed TSR and fell within the lower right quadrant in yellow, as observed for the NR and SR mixtures with typical RBR. This is likely because the TSR of these mixtures was influenced by the aggregate proportion and absorption capacity, as shown in Figure 3a. As expected, asphalt mixtures with aggregate and binder issues simultaneously failed TSR. Thus, the results demonstrated the utility of the proposed aggregate absorption threshold (Agg_Abs < 1.0) and support recommendations for controlling high absorptive aggregate proportions (i.e., more than 30%) and carefully designing asphalt mixtures with either low binder contents or very stiff RAM sources with Pb/Blend PGH (×100) <7.5 to control moisture resistance issues.

The PGH suggested thresholds after RTFO for virgin binders (less than or equal to 64°C) and RAM binders (less than or equal to 100°C for RAP and 150°C for RAS) were adopted from the first draft AASHTO standard practice by Epps Martin et al. ( 19 , 20 ) and revised based on the results in this study. The threshold for virgin binders excludes PMA binders because PMAs did not significantly improve the CT_Index, as reported by Bairgi et al. ( 23 ), and their characterization and that of corresponding asphalt mixtures may require additional evaluation to properly capture performance ( 26 ). The PGH threshold for RAP excludes the very stiff SE1 RAP source, with a PGH value of 126.7, based on the effect of its stiffness on the cracking performance of the asphalt mixtures fabricated with this source. Figure 4 shows the CT_Index results from the IDEAL-CT test for the typical and high RBR control mixtures evaluated, previously reported by Bairgi et al. ( 23 ), with the corresponding thresholds and error bars indicating one positive and one negative standard deviation from the average CT_Index. The results were statistically compared at 95% confidence using Tukey’s Honestly Significant Differences (HSD), and the letters outside each bar represent Tukey’s HSD results, where asphalt mixtures with the same letter are considered statistically similar. Tukey’s HSD pairwise comparisons for both typical and high RBR mixtures, presented in Figure 4, a and b , respectively, showed that the SR mixtures (SR_0.16_64-22Low, SR_0.29_64-22Low) exhibited statistically different CT_Index values compared with the NR and NS mixtures. Thus, the results demonstrated that the low cracking resistance of the SR mixtures may be because of the very stiff RAP source incorporated (SE 1 RAP), although the mixtures have different virgin binders and aggregates that may also influence cracking resistance.

Figure 4.

Cracking tolerance index (CT_Index) results for (a) typical recycled binder ratio (RBR) and (b) high RBR control mixtures ( 23 ).

Figure 5 also highlights the impact of incorporating a very stiff RAP source on cracking resistance, along with the total binder content in the asphalt mixture (Pb). Interaction plots with the CT_Index results from the IDEAL-CT test for the NR, NS, and SR mixtures evaluated for cracking resistance are shown, as previously obtained and analyzed by Bairgi et al. ( 23 ) and Montañez et al. ( 24 ). The CT_Index values are related to the binder quantity and quality, expressed as the Pb/Blend PGH (×100) ratio, for mixtures without and with high RAM strategies, if needed. The Pb values for mixtures with decreased RBA applied as the high RAM strategy are increased to account for the inactive RAM binder. In general, the results highlight the effect of either low binder contents or stiff binder blends caused by RAM binders with high PGH values, resulting in low Pb/Blend PGH (×100) ratios and failure to meet the CT_Index threshold (lower left quadrant in red). Based on these results, guidance was noted in the proposed AASHTO standard practice, indicating that cracking resistance increases when the Pb/Blend PGH ratio increases (i.e., greater than 8.0). Figure 5 also illustrates the impact of incorporating very stiff RAP on performance, with the SR mixtures exhibiting worse cracking (lower CT_Index values) either without or with high RAM strategies compared with the NR and NS mixtures. Similarly, the SR mixture with typical RBR required a combination of two high RAM strategies (softer binder + decreased RBA) to pass the CT_Index threshold. Likewise, the SR mixture with high RBR required the highest RA dose among all mixtures (7% by weight of total binder) to meet the suggested threshold.

Figure 5.

Interaction plot for cracking tolerance index (CT_Index) results Pb/blend high-temperature performance grade (PGH) (×100) ratios (a) north moisture-resistant (NR), north moisture-susceptible (NS) mixtures without strategies, (b) NR, NS mixtures with strategies to improve cracking, (c) south moisture-resistant (SR) mixtures without strategies, and (d) SR mixtures with strategies to improve cracking ( 23 , 24 ).

The recommended RBR limits in Table 6 for RAP and RAS of 0.5 and 0.2, respectively, were adopted from the first draft AASHTO standard practice by Epps Martin et al. ( 19 , 20 ) and revised in this study based on the proportions of RAM utilized and existing DOT practices ( 37 , 38 ). Particularly, the RAS_BR was increased to 0.2 based on current DOT limits of 0.1 to 0.2 ( 38 ) and the adequate cracking performance of the NR mixture with 0.22 RAP RBR and 0.22 RAS RBR and decreased RBA as high RAM strategy presented in Figure 5b, with adequate rutting and moisture resistance as indicated subsequently.

Finally, the RA doses utilized in this study to improve cracking performance of the NR and SR mixtures with high RBR corresponded to 4.8% and 7.0% by weight of total binder, respectively. The doses were obtained from the dose selection method developed and revised in NCHRP projects 09-58 ( 19 ) and 20-44(24) ( 21 ), and included in the proposed AASHTO standard practice from this study. Because none of the RA doses created rutting or moisture resistance issues, no specific threshold for the RA dose was suggested in Table 6; however, it was specified to utilize the maximum cost-effective amount that leads to sufficient cracking resistance without adversely affecting rutting or moisture resistance.

Findings—Mixture Performance Evaluation

Considering that asphalt mixtures perform under the confluence of the effects of traffic and climatic conditions, the proposed AASHTO standard practice includes a durability assessment framework for the evaluation of mixtures with RAM. This framework uses mixture performance tests, aging and moisture protocols, and thresholds categorized by climatic zone (freeze and no freeze) based on the long-term pavement performance (LTPP) climatic zones presented in Figure 6 (wet–freeze, dry–freeze, wet–no freeze, and dry–no freeze).

Figure 6.

Long-term pavement performance (LTPP) climatic zones.

To incorporate existing test methods and current practices for the design of durable asphalt mixtures with RAM specific to the environmental conditions of individual projects, Table 7 presents the proposed durability assessment framework with analysis tools based on the climatic zone (freeze and no freeze) that balances mixture cracking resistance at intermediate and low temperatures and rutting resistance at high temperatures and considers moisture susceptibility. These tools were adopted from the first draft AASHTO standard practice by Epps Martin et al. ( 19 , 20 ) and include thresholds for different laboratory parameters associated with the most prevalent distresses and durability concerns (cracking, rutting, and moisture susceptibility), test temperatures, and aging and moisture protocols. The suggested thresholds are provided as a system, with requirements recommended for at least one high-temperature and one intermediate- or low-temperature test where data are available. Guidance is also provided for the evaluation of moisture resistance. In particular, thresholds for number of passes to reach 12.5 mm rut depth (N_12.5), mixture Glover-Rowe parameter (G-R_m), flexibility index (FI) from Illinois Flexibility Test (I-FIT), mixture flexural creep stiffness (S_m) and relaxation rate (m-value_m), and environmental cracking resistance index (CRI_Env) were included in the first draft AASHTO standard practice based on laboratory results tied to field performance from NCHRP project 09-58 that showed these tools are relevant and can help achieve adequate performance of high RBR mixtures ( 19 ).

Table 7.

Analysis Tools for Evaluation of Asphalt Mixtures with Recycled Asphalt Material (RAM)

Note: CRI_ENV = environmental cracking resistance index; CT_Index = cracking tolerance index; FI = flexibility index; G_f = fracture energy; G-R_m = mixture Glover-Rowe parameter; LTOA = long-term oven aging; m-value_m = mixture relaxation rate; PG = performance grade; SIP = stripping inflection point; S_m = mixture flexural creep stiffness; SN = stripping number; STOA = short-term oven aging; TSR = tensile strength ratio; na = not applicable.

Gray background indicates proposed mixture evaluation tools based on the results of this study.

LTOA protocol for compacted specimens as described in AASHTO R 30 (5 days at 85°C).

The results from this study were utilized to propose additional or revised mixture evaluation tools, shown with a gray background in Table 7, to assess the performance and durability of asphalt mixtures with RAM. The suggested thresholds were selected according to current specifications in standard methods, ongoing research, and existing specifications from the north and south DOTs defined based on evaluation of asphalt mixtures with acceptable durability. The HWTT per AASHTO T 324 was proposed for rutting evaluation at high temperature, including different thresholds based on the climate-based binder grade. Rutting specifications from Texas and Illinois DOTs were selected to establish the thresholds for N_12.5, as they are examples of warm (no freeze) and cold (freeze) climates, respectively, separated at 40°N latitude as suggested in NCHRP project 09-52A ( 25 ). The thresholds from Texas ( 40 ) were utilized for the no freeze climate with climate-based PG 64-XX binder and shifted to a colder binder grade for the freeze climate following the Illinois specifications ( 41 ). Similarly, different testing temperatures were included to capture the effect of climate based on existing state specifications for north and south DOTs, specifically 45°C for the freeze and 50°C for the no freeze climate.

To evaluate moisture resistance, the thresholds for the SIP and SN from HWTT developed by Yin et al. ( 42 ) were included. In addition, the TSR parameter per AASHTO T 283 was selected for assessing moisture resistance with or without a freeze–thaw cycle to represent climate differences. A minimum TSR of 80% was adopted, as recommended in the standard specification for Superpave volumetric mix design (AASHTO M 323). Both HWTT and TSR outcomes were determined after STOA of 4 h at 135°C per the existing AASHTO R 30 during the experiment design phase of this study.

The results of this study support the tools proposed for moisture resistance evaluation using HWTT or TSR, with TSR particularly added because it was found to be more sensitive to mixture components and proportions than moisture parameters from HWTT (SIP or SN). As an example, Figure 7a shows the HWTT results for the SR mixtures using an enlarged y-axis with maximum rut depth of 7.5 mm for improved visualization. Figure 7b shows a bar chart with the IDT results and TSR outcomes as labels, and Figure 7c shows the TSR results only. The HWTT results illustrated no stripping behavior for most of the SR mixtures with SIP results greater than 20,000 passes, with or without high RAM strategies, except for the asphalt mixture with a combined softer binder and decreased RBA strategy (SR_0.16_58-28High_RBA), which showed stripping but did not exceed the SIP or SN thresholds. Conversely, the TSR results captured low moisture resistance (TSR < 80%) for both typical and high RBR SR control mixtures (SR_0.16_64-22Low, SR_0.29_64-22Low), which improved after the implementation of either high RAM or moisture strategies. The interaction plot in Figure 7d shows the relationship between TSR by IDT and SIP obtained by HWTT and highlights that TSR is more sensitive to component materials, as evidenced by a wider range of TSR results ranging from 70% to 95% for mixtures with different RAP contents, binder type quality and quantity, and inclusion of additives, while exhibiting similar SIP results. Consequently, the proposed tools are based on the HWTT and TSR results from this study, but it is also suggested to continue using existing practices to assess moisture resistance if they are already incorporated into mix design practices or implement other criteria developed by state or local agencies to substitute those provided in this study. Additionally, considering that the mixtures evaluated in this study included low absorptive aggregates (Agg_Abs = 0.5-1.0), it is also suggested to implement both TSR and wet IDT parameters to evaluate moisture resistance in mixtures incorporating aggregates with higher absorption. Similarly, dry and wet IDT thresholds can also be implemented to control strength values that are reduced with the implementation of high RAM strategies, as evidenced in the results obtained for both typical and high RBR SR mixtures (Figure 7b), to better evaluate moisture resistance using TSR.

Figure 7.

Moisture resistance evaluation results for south moisture-resistant (SR) mixtures (a) Hamburg wheel tracking test (HWTT) curve, (b) dry and wet indirect tensile strength tests (IDT) and tensile strength ratio (TSR) as labels, (c) TSR, (d) TSR versus stripping inflection point (SIP) interaction plot ( 24 ).

The suggested thresholds to assess cracking resistance at intermediate and low temperatures, CT_Index from IDEAL-CT per ASTM D8225 and G_f from DCT per ASTM D7313, were adopted based on consistent correlations between laboratory and field results from previous studies conducted by the north and south DOTs. The CT_Index threshold of 20 after loose mixture LTOA of 8 h at 135°C for the no freeze climate was adopted from top-down cracking measured at the surface after 10 million equivalent single axle loads (ESALs), where this threshold discriminated asphalt mixtures with good cracking performance from those with moderate performance ( 43 ). The proposed CT_Index of 30 after loose mixture LTOA of 6 h at 135°C for the freeze climate was adopted based on a benchmarking study by the Wisconsin DOT, with approximately 15% of the existing mixtures expected to fail ( 44 , 45 ). Finally, the suggested G_f threshold of 400 J/m² after STOA follows the Minnesota DOT recommendations for projects with low to moderate degrees of thermal cracking, which was obtained from field thermal cracking data correlated with G_f data at a test temperature equal to climate-based PGL plus 10°C ( 46 ).

The proposed mixture evaluation tools were utilized for designing balanced and durable asphalt mixtures with RAM following the stepwise approach shown in Figure 8. The stepwise approach denotes the application of mixture performance tests in a specific order and after appropriate conditioning. It also includes potential strategies for mitigating cracking attributed to RAM and moisture susceptibility. The order for mixture performance evaluation corresponds to surface cracking at intermediate temperature, rutting, moisture susceptibility, and low-temperature cracking. Cracking resistance at intermediate temperature with no strategies is first evaluated by comparing the associated parameter (FI from I-FIT or CT_Index from IDEAL-CT) against the suggested threshold in Table 7, as this distress is most critical for asphalt mixtures with RAM. If cracking resistance needs to be improved, optional high RAM strategies can be employed to ensure satisfactory results. Bairgi et al. ( 23 ) found that no single strategy works for all asphalt mixtures, and strategies need to be used systematically until a passing result is obtained. The strategies listed in the stepwise approach and evaluated in this study include decreasing RBA from RAP/RAS, using an alternate binder, incorporating a RA, or a combination of these strategies. Using an adjusted aggregate gradation and reducing the RBR were not evaluated in this study but were also included based on positive results toward improving cracking resistance in other studies ( 44 , 47 ). Next, the application of the stepwise approach balances the asphalt mixture by ensuring rutting resistance, followed by an assessment of moisture susceptibility with application of moisture strategies if required, and a final assessment of low-temperature cracking for mixtures in the freeze climatic zone. Lime and LAS agents were included as moisture strategies and evaluated in this study. Other additives, such as WMA products, and selection of an aggregate source less susceptible to stripping were also included, based on previous research ( 12 , 48 , 49 ). The final product is a set of robust mixtures, defined as those with specific strategies that meet the suggested thresholds of each durability issue evaluated by the proposed stepwise approach.

Figure 8.

Stepwise approach for designing durable asphalt mixtures with recycled asphalt materials (RAM).

The results obtained from the application of the stepwise approach to the asphalt mixtures evaluated in this study are presented in Figure 9. Results are shown in single radar performance diagrams for the NR, NS, and SR mixtures to comprehensively evaluate the resistance of the high RBR mixtures with respect to multiple durability issues. These performance diagrams allow for visualization of the effects of increased RBR, additives, and mitigation strategies, with each spoke representing a performance parameter that improves durability when increasing radially outward. The diagrams aim to compare the high RBR mixtures incorporating mitigation strategies, if needed, with the typical RBR control mixture. The parameters included CT_Index from IDEAL-CT, G_f from DCT, rut depth (RD) at 10,000 passes from HWTT, TSR by IDT, and Cantabro Ratio from the Cantabro test. The RD at 10,000 passes was included because N_12.5 exceeded 20,000 passes in all the mixtures evaluated. The Cantabro Ratio was included in the diagrams for the NR and SR mixtures, as these results were available. The shaded area corresponds to the results for typical RBR mixtures, whereas dashed lines indicate the results for high RBR mixtures. Finally, the thick black line represents the suggested thresholds, and results located radially beyond this line indicate that the mixture passed the corresponding performance parameter. In this study, the stepwise approach was applied completely to the NR and SR mixtures, whereas it was partially applied to the NS mixtures.

Figure 9.

Performance diagram for robust mixtures and corresponding typical recycled binder ratio (RBR) control mixture (a) north moisture-susceptible (NR), (b) north moisture-susceptible (NS), and (c) south moisture-resistant (SR).

Figure 9a shows the results obtained for the NR mixtures after applying the stepwise approach. The results showed that LAS agent was required to improve the moisture resistance of the NR control mixture with typical RBR (NR_0.21_58-28Low_Control_LAS), but no high RAM strategies were required based on the cracking performance evaluated in the first step. The high 0.37 RAP RBR and 0.44 RAP/RAS RBR NR mixtures required RA and decreased RBA, respectively, as high RAM strategies to improve cracking. These robust mixtures, NR_0.37_58-28Low_RA and NR_0.44_58-28Low_RBA, exhibited a decreased CT_Index but improved or equivalent G_f, RD at 10,000 passes, TSR, and Cantabro Ratio compared with those of the typical 0.21 RBR NR control mixture. Thus, the application of the stepwise approach produced robust high RBR mixtures with better performance compared with the typical RBR mixture with LAS with almost all of the parameters shifted radially outward. This also means that for this materials combination improving cracking resistance of the high RBR mixtures at intermediate temperature resulted in a shift in the other performance parameters compared with the typical RBR control mixture but produced robust mixtures that met the suggested performance thresholds.

Figure 9b shows the NS control mixture with typical 0.20 RBR met the suggested CT_Index threshold without requiring a high RAM strategy in the first step of the approach (NS_0.20-58-28Low_Control). Conversely, the high 0.29 RBR mixture required the decreased RBA strategy to improve cracking resistance at intermediate temperature (NS_0.29_58-28Low_RBA). The cracking resistance evaluation at low temperature showed that this mixture did not meet the G_f suggested threshold (G_f ≥ 400 J/m² after STOA); however, G_f was measured after STOA and LTOA protocols in this study to observe the effect of more severe aging conditioning on low-temperature cracking performance. Further research is required to identify G_f thresholds that consider more critical conditions and account for aging, particularly for mixtures with high RBRs. Among all the parameters evaluated, the typical 0.20 RBR control mixture exhibited improved CT_Index, G_f, and TSR but reduced rutting resistance compared with the robust high 0.29 RBR mixture with decreased RBA. The high 0.33 RAP/RAS RBR mixture was assessed only for rutting and moisture resistance in this study (i.e., second and third steps of the approach) and required LAS to pass the suggested TSR threshold (NS_0.33_58-28Low_LAS). The low moisture resistance of this mixture demonstrates the negative impact of increased RBR with RAS and the use of moisture-susceptible aggregates. Once LAS was incorporated, this high 0.33 RAP/RAS RBR mixture with LAS showed similar RD at 10,000 passes and reduced TSR compared with the typical 0.20 RBR control mixture. In conclusion, for this materials combination, application of the stepwise approach produced high RBR NS mixtures with reduced performance compared with the typical 0.20 RBR control mixture because almost all of the parameters moved radially inward, but they still met the suggested thresholds and are expected to provide adequate performance.

Figure 9c illustrates that the SR control mixture with typical 0.16 RBR did not pass the suggested CT_Index threshold in the first step of the approach, and a combined high RAM strategy of softer binder and decreased RBA was required (SR_0.16_58-28High_RBA). Additional results for the typical 0.16 RBR control mixture without strategies (SR_0.16_64-22Low_Control), including RD and TSR, were determined for comparison. The robust 0.16 RBR mixture with combined high RAM strategies exhibited a slightly reduced rutting resistance but improved CT_Index and TSR compared with the typical 0.16 RBR control mixture without strategies. The high 0.29 RBR mixture required RA as a high RAM strategy to meet the CT_Index threshold in the first step of the approach (SR_0.29_64-22Low_RA). Comparing the two robust typical 0.16 and high 0.29 RBR mixtures, the robust high 0.29 RBR mixture with RA exhibited reduced CT_Index and Cantabro Ratio but improved TSR and rutting resistance compared with those for the robust typical 0.16 RBR with combined softer and decreased RBA strategies. Again, for this materials combination, the changes required to improve cracking resistance of both typical and high RBR mixtures at intermediate temperature represented a shift in some of the other parameters compared with the typical 0.16 RBR control mixture but also produced robust mixtures that met all of the suggested performance thresholds.

Table 8 summarizes the results obtained for the robust and control mixtures, with passing results indicated in bold and underlined text. The results for three NR and two SR robust mixtures (highlighted with gray background) indicated that these five asphalt mixtures passed the suggested performance thresholds. The results for three NS and two SS mixtures were also included because most of them also passed the suggested thresholds for all durability issues for which they were evaluated.

Table 8.

Overall Results for Typical and High Recycled Binder Ratio (RBR) Robust Mixtures (Gray Background) and Corresponding Typical RBR Control Mixtures

Note: CT_Index = cracking tolerance index; MidW = midwest; NE = northeast; RAM = recycled asphalt materials; RBA = recycled binder availability; RD = rut depth; SE = southeast; TSR = tensile strength ratio.

Bold and underlined text indicate test results passed the suggested performance threshold.

Results without ASA.

(—) = not included in the experimental design or not applicable based on the stepwise approach.

The application of the proposed framework and stepwise approach provided evidence to recommend the use of the most effective and practical high RAM strategies in the following order: decreasing RBA, using a softer binder grade, and incorporating a RA. Bairgi et al. ( 23 ) found that decreasing RBA or using a softer binder resulted in substantial changes for improving CT_Index. Decreasing RBA may be the easiest strategy to implement because it utilizes the same component materials, and any value less than 100% will improve cracking resistance. RAs were also found to be effective; however, this strategy might require additional effort to select the correct dose and conduct performance testing to verify compatibility issues that may arise. Asphalt mixtures with a RA in this study exhibited higher rut depths and decreased wet and dry IDT values, as observed in the moisture results for the high 0.29 RBR SR mixture with RA (SR_0.29_64-22Low_RA) shown in Figure 7. The proposed AASHTO standard practice includes step-by-step instructions concerning the application of decreased RBA or RAs as high RAM strategies mitigation strategies to improve cracking resistance.

Additional results from this study provided guidance toward the utilization of a RAP source exceeding the RAP PGH threshold suggested in Table 6. In this case, it is recommended to utilize a combination of strategies, either a softer binder together with decreased RBA or an alternate gradation together with decreased RBA, based on the results for the typical 0.16 RBR SR mixture. Similarly, LAS agents and lime were included as moisture strategies because they were effective in improving the moisture resistance of asphalt mixtures with RAM in this study. Finally, because raveling issues were not observed in the robust mixtures with mitigation strategies defined in steps one and two of the stepwise approach, checking the raveling resistance was not included in the proposed assessment framework and stepwise approach to simplify the asphalt mixture evaluation.

Findings—Cost Analysis

A relative cost analysis was performed to identify the costs associated with asphalt mixture production when engineering robust mixtures incorporating high RAM or moisture mitigation strategies. The costs associated with the mixing plant and equipment on-site, transportation to the project location, and quality control and assurance were not included. The cost per ton of asphalt mixture was calculated using approximate cost values for the binder, aggregate, RAM (RAP/RAS), and additives such as RA, LAS agent, and lime. Table 9 presents the representative price/cost for the component materials utilized in this analysis. The scenario considered reflects a high economic incentive situation, where the prices of binder and aggregate are relatively high, while the costs of RAP, RAS, and additives are relatively low. Figure 10 presents the approximate cost savings determined for each typical or high RBR robust mixture obtained from the application of the tools proposed in this study, compared with the typical RBR control mixture with no strategies.

Table 9.

Representative Price/Cost for Each Component Material Associated with Asphalt Mixture Production

Item	Unit	Price/cost (US$)
Item	Unit	Representative range	Representative value
Virgin binder	Per ton	400 to 800	450
Virgin aggregate	Per ton	12.00 to 15.00	13.00
RAP, RAS	Per ton	5.00 to 8.00	6.00
RA, LAS	Per ton	500 to 700	550
Lime	Per ton	150 to 250	200

Note: LAS = liquid anti-stripping; RA = recycling agent; RAP = reclaimed asphalt pavement; RAS = recycled asphalt shingles.

Figure 10.

Approximate savings (high-cost scenario) per ton of mixture compared with typical recycled binder ratio (RBR) control mixture with no strategies.

For the NR mixtures, the savings achieved by increasing the typical RAP RBR from 0.21 (NR_0.21_58-28Low) to a high 0.37 RAP RBR with RA required to improve cracking resistance (NR_0.37_58-28Low_RA) were more significant (almost 20%) than those obtained when increasing from 0.21 RAP RBR to a high 0.44 RAP/RAS RBR with decreased RBA strategy (NR_0.44_58-28Low_RBA), which were approximately 15%. The NS mixtures included lower RBRs than the NR mixtures, and reduced savings (around 5%) were obtained when increasing the typical RAP RBR from 0.20 (NS_0.20_58-28Low) to a high 0.29 RAP RBR with decreased RBA (NS_0.29_58-28Low_RBA). Larger savings (around 12%) were achieved when increasing from the typical 0.20 RAP RBR to a high 0.33 RAP/RAS RBR with LAS required to improve moisture resistance (NS_0.33_58-28Low_LAS) because this mixture was evaluated for moisture resistance only.

For the SR mixtures, increasing the typical RAP RBR from 0.16 (SR_0.16_64-22Low) to a high 0.29 with RA required for acceptable cracking performance (SR_0.29_64-22Low_RA) resulted in savings of 11%. Conversely, improving the cracking performance of the typical 0.16 RAP RBR mixture with a combination of softer binder and decreased RBA (SR_0.16_58-28High_RBA) showed no savings because it used the same amount of RAP and more binder, making it less attractive. Finally, the SS mixtures with moisture resistance evaluation only showed savings of approximately 16% when increasing the typical RAP RBR from 0.22 (SS_0.22_64-22Low) to a high 0.39 RAP RBR with lime as an ASA (SS_0.39_64-22Low_Lime), and no savings were obtained for the same typical 0.22 RAP RBR mixture with the added cost of lime to improve moisture resistance.

In general, cost savings were evident when virgin material costs were relatively high and RAP, RAS, and additives costs were relatively low, supporting the economic advantages of recycling in scenarios with elevated virgin material costs. Based on the results from this study, cost savings were higher for the NR mixtures with corresponding strategies because of their higher RBR values, as higher contents of RAM were incorporated, compared with the NS, SR, and SS mixtures. In addition, significant savings were achieved in the asphalt mixtures requiring an RA to improve cracking performance without affecting rutting or moisture resistance, but the high doses utilized in this study might not be practical. In conclusion, significant cost savings of 5%–20% per ton of mixture can be obtained when designing asphalt mixtures with RAM by applying the tools and guidelines proposed in this study.

Conclusions

This paper summarizes the findings derived from NCHRP project 09-65 that resulted in the development of a more comprehensive draft AASHTO standard practice for the design and evaluation of durable asphalt mixtures, regardless of RAM content or the presence of additives. The results of this study demonstrated that the development of robust mixtures implies careful selection of component material types and sources, balancing proportions of each component material, and adjusting the asphalt mixtures through the application of high RAM or moisture mitigation strategies to achieve balanced cracking and rutting performance without creating moisture or raveling issues. The results confirmed that the best high RAM or moisture strategy depends on the specific materials combination (specific component materials at selected proportions). Additionally, the high RAM strategies implemented in the materials combinations evaluated in this study did not create rutting, moisture, or raveling issues. Also, specific aggregate and binder parameters and proportions were shown to affect durability. Particularly, the quantity and quality of the virgin and RAM binders are both important with respect to influencing cracking and moisture resistance.

Comprehensive analysis across all asphalt mixtures and comparisons to suggested performance thresholds allowed for the development of revisions and modifications included in the proposed AASHTO standard practice. In general, the main updates and changes include the following:

Component material guidelines and evaluation tools for aggregates and binders, including aggregate water absorption and binder rheological parameters.

A framework with candidate mixture performance tests, conditioning protocols, and thresholds categorized by climatic zone (freeze or no freeze) to assess durability and resistance to common pavement distresses.

A stepwise approach to conduct performance evaluation in a specific order and develop robust durable asphalt mixtures with sufficient cracking resistance at intermediate temperature and balanced performance with respect to rutting, along with adequate moisture and raveling resistance, and low-temperature cracking resistance for mixtures in freeze climates.

Other aspects included in the proposed AASHTO standard practice, but not presented in detail in this paper, include guidance on binder blend and asphalt mixture preparation, guidelines for binder content adjustments when selecting decreased RBA as a high RAM strategy to improve performance, and a detailed RA dose selection method.

This study demonstrated that asphalt mixtures with RAM can be effectively evaluated and designed using the BMD approach, thereby increasing RAM utilization while producing cost-effective and durable asphalt mixtures. The suggested thresholds were suitable for screening component materials and assessing the binder and asphalt mixture performance. Further evaluation of mixtures with different materials is necessary to refine the suggested thresholds for screening component materials, particularly for the evaluation of moisture resistance in asphalt mixtures with RAM incorporating highly absorptive aggregates. Similarly, it is necessary to expand the proposed framework categorized by freeze or no freeze climates to include conditioning protocols and evaluation tools categorized by wet and dry climatic zones. Thus, single and multiple wet–dry and freeze–thaw cycles should be explored to include a wider range of U.S. climates and adequately represent field conditions. Lastly, it is important to note the discrepancies in the method utilized for STOA conditioning of loose mixture according to the standard practice AASHTO R 30. Considering that this standard was modified during the development of this study and that the revised STOA time is shorter than the time utilized, the suggested performance thresholds for evaluating asphalt mixtures after STOA might need to be verified.

Additional next steps include further investigation to produce other practical tools for screening component materials incorporated in asphalt mixtures with RAM and examine their interactions. In addition, the construction of field pilot projects and monitoring of active field projects are required to demonstrate the utility of the proposed guidelines and refine the binder and asphalt mixture performance thresholds suggested in this study. Furthermore, appropriate thresholds for the Flexibility Index (FI) parameter from the Illinois Flexibility Test (I-FIT) tied to freeze or no freeze climates need to be developed. Finally, additional research is required to assess performance under the combined effects of aging and moisture, improve characterization of PMAs and corresponding mixtures, and further evaluation of binders or binder blends at intermediate temperature tied to corresponding asphalt mixture performance.

Footnotes

Acknowledgements

The authors would like to thank the National Cooperative Highway Research Program for funding this study as part of NCHRP project 09-65.

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: A. Epps Martin, E. Arámbula-Mercado, N. Tran; data collection: J. Montañez, M. Verma, B. K. Bairgi, R. Moraes; analysis and interpretation of results: A. Epps Martin, E. Arámbula-Mercado, J. Montañez, N. Tran, F. Yin, M. Verma, B. K. Bairgi, R. Moraes, C. Rodezno; draft manuscript preparation: J. Montañez, A. Epps Martin, E. Arámbula-Mercado, F. Yin, R. Moraes, B. K. Bairgi. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The authors declared the following potential conflicts of interest with respect to the research, authorship, and/or publication of this article: Edith Arámbula-Mercado is a member of Transportation Research Record’s Editorial Board. All other authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research is part of the NCHRP project 09-65, which is part of the National Cooperative Highway Research Program (NCHRP). NCHRP is administered by the Transportation Research Board (TRB) and funded by participating member states of the American Association of State Highway and Transportation Officials (AASHTO). NCHRP also receives critical technical support from the Federal Highway Administration (FHWA), United States Department of Transportation.

ORCID iDs

Juliana Montañez

Amy Epps Martin

Edith Arámbula-Mercado

Madhav Verma

Biswajit Kumar Bairgi

Fan Yin

Raquel Moraes

Carolina Rodezno

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