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Transportation Research Record: Journal of the Transportation Research Board

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Research article

First published online January 8, 2019

A Method to Quantify Reclaimed Asphalt Pavement Binder Availability (Effective RAP Binder) in Recycled Asphalt Mixes

Fawaz Kaseer [email protected], Edith Arámbula-Mercado, and Amy Epps MartinView all authors and affiliations

https://doi.org/10.1177/0361198118821366

Abstract

State highway agencies recognize the environmental and economic benefits of utilizing reclaimed asphalt pavement (RAP) in asphalt mixes. Currently, most agencies assume all of the RAP binder content is available for mix design purposes. However, the percentage of available or effective RAP binder in the asphalt mix is usually less than 100% and not quantified, which could yield dry asphalt mix with a high air void content, potentially leading to premature distress. The term available or effective RAP binder refers to the binder that is released from the RAP, becomes fluid, and blends with virgin binder under typical mixing temperatures. This study proposes a method to estimate the RAP binder availability factor (BAF) which can be used to adjust the virgin binder content in RAP mixes to ensure that the mix design optimum binder content is achieved. In this method, asphalt mixes were prepared so that, after mixing and conditioning, the RAP material can be separated from the virgin aggregate, which allows for a thorough evaluation of the extent of RAP binder availability in the asphalt mix. This method was verified in a preliminary experiment and then used to estimate the BAF of RAP from different sources, and a correlation between RAP BAF and the high temperature performance grade (PG) of each RAP source was established. Finally, factors affecting the RAP BAF were also evaluated such as mixing temperature, conditioning period, the use of recycling agents (or rejuvenators), and the method of adding the recycling agent to the mix.

Reclaimed asphalt pavement (RAP) is used extensively in asphalt mixes due to its environmental and economic benefits. These benefits are achieved by replacing a portion of the expensive virgin binder and aggregate in recycled asphalt mixes. The quantity of RAP binder in the asphalt mix is typically represented as asphalt binder replacement (ABR) or recycled binder ratio (RBR). Both terms are used to define the percentage of RAP binder by weight with respect to the total binder by weight in the asphalt mix. However, the quantity of effective RAP binder in the asphalt mix is usually unknown, which raises concerns due to its ultimate effect on performance. The term effective RAP binder refers to the binder that is released from the RAP, becomes fluid, and blends with the virgin binder under typical mixing temperatures. Other terms used include RAP binder contribution, RAP binder activation, degree of RAP activation, RAP working binder, and RAP binder availability. The latter will be used in this study.

When discussing RAP binder availability, it is important to distinguish between RAP binder availability and RAP binder blending (or degree of blending). Some authors use both terms interchangeably, however, the first indicates the amount of RAP binder that becomes fluid and is released in the mix, while the second indicates how well or to what extent the RAP binder and the virgin binder blend in the mix. If there is no active or available RAP binder, the blending would be nil. However, even if the RAP binder is fully available, complete and homogeneous blending between the RAP binder and the virgin binder may not occur. Nevertheless, the more fluid and active the RAP binder is, the more blending is expected to occur since the active or available RAP binder is expected to dissipate uniformly in the asphalt mix through mechanical mixing at elevated temperature.

RAP binder availability is typically addressed through one of three assumptions: (a) 0% availability, where the RAP acts as a “black rock”; (b) 100% availability, where all the RAP binder becomes fluid and is available to blend with the virgin binder; or (c) partial availability, where a portion of the RAP binder becomes fluid and is available to blend with the virgin binder. Although rarely measured, it is generally accepted that the third assumption is more realistic. Many studies have consistently shown that, when RAP is mixed with virgin binder and aggregates at elevated mixing temperatures, the RAP binder is partially available (1–5); that is, somewhere between 0% and 100% availability occurs in the asphalt mix. However, in a recent survey in NCHRP Synthesis 495 (6), 77% of the state highway agencies that responded reported that they consider 100% RAP binder availability, and thus they reduce the virgin binder content in the asphalt mix by the RAP binder content. About 6% of the respondents in this same survey considered 0% RAP binder availability, and approximately 17% considered partial RAP binder availability, assuming around 75% of the RAP binder is available (6).

Designing asphalt mixes with the assumption of 100% availability could result in asphalt mixes with less total binder content than the selected optimum from the mix design. In this case, coatability issues may arise resulting in a dry asphalt mix with a high air void content; potentially leading to cracking, raveling, or premature moisture damage. On the contrary, designing asphalt mixes with the assumption of 0% availability could result in soft mixes with potential rutting problems, due to possibly excessive total binder content.

RAP binder availability and blending with virgin binder was first addressed in NCHRP Project 9-12 (1) which aimed to determine whether the RAP acted like a “black rock” or whether some of the RAP binder blended with the virgin binder. The authors prepared three types of specimens simulating the degree of blending as follows: (a) blending RAP, virgin aggregate, and virgin binder, as in actual practice; (b) removing all RAP binder and blending the virgin binder with the recovered RAP aggregate and virgin aggregate, simulating 0% blending; and (c) removing all RAP binder, physically blending the extracted and recovered RAP binder with the virgin binder, and then combining the blended binder with the virgin aggregate, simulating 100% blending. Superpave shear tests and indirect tensile creep and strength tests indicated that the RAP did not act like a black rock and partial blending occurred to a significant extent. The limitation of this approach is the RAP binder extraction and recovery process, since it is well known that this process can affect the binder properties, and consequently, the indirect tensile creep and strength test results.

Bonaquist (2) developed an approach for evaluating RAP binder availability and blending using five steps: (a) measure the dynamic modulus (|E*|) of the asphalt mix (with RAP); (b) extract and recover the binder from the mix; (c) measure the recovered binder shear modulus (|G*|) using the dynamic shear rheometer (DSR); (d) estimate |E*| based on measured |G*| using the Hirsh model; and (e) compare the estimated |E*| to measured |E*|. The authors assumed that overlapping or similar values indicated 100% RAP binder availability and blending; otherwise, partial RAP binder availability and blending occurred. This approach has been advocated for evaluating the binder blending issue but still has some limitations. In addition to the issue of the binder extraction and recovery process, this approach cannot determine, or estimate, how much RAP binder is available and blended (as a percentage). Furthermore, |E*| is an important property of the asphalt mix that measures the response under loading, but even if there is no blending, the measured |E*| values may be close to those of mixes with partial blending, as reported in other studies (3, 7).

D’Angelo et al. (2011) investigated the extent of RAP binder availability using the aggregate size exclusion method (4). In this method, the RAP has a designated size in the mix and the virgin aggregates have a different designated size. The authors employed asphalt mixes containing only two distinct fractions, virgin aggregates and RAP, which could be separated easily by sieving. After mixing with the virgin binder, the RAP was separated from the virgin aggregate, which allowed for investigation of whether the binder content was the same for both materials. If the RAP had a higher binder content than the virgin aggregate, then the RAP binder was not fully available to blend with the virgin binder. In this case, most of the RAP acts like a black rock and the virgin binder coats the RAP as it does any other aggregate particle.

Previous studies have suggested that the use of recycling agents (or rejuvenators) can help activate the hardened RAP binder and mitigate its stiffness (8, 9), increasing its availability and ability to blend with the virgin binder (10). RAP binder availability has been studied and debated for a long time but to date there is no standard test or method to determine accurately, or at least estimate, how much RAP binder is active and available in the asphalt mix. The ability to quantify the percentage of available RAP is critical in determining the actual virgin binder content that needs to be added to the asphalt mix to satisfy the optimum binder content determined by mix design.

Objectives

The objectives of this study are:

•

To propose a method to determine, or estimate, the percentage of active and available RAP binder in an asphalt mix.

•

To investigate the effect of certain factors such as mixing temperature, conditioning period, RAP material source, recycling agent addition, and the method of addition on the RAP binder availability.

Methodology

The following methodology is proposed to estimate the RAP binder availability based on an evaluation of asphalt mixes with specific sizes of virgin aggregate and RAP:

•

Prepare the virgin asphalt mix using: (a) virgin binder and (b) virgin aggregate with three distinct fractions: a coarse size (passing the 1/2” sieve and retained on the 3/8” sieve), an intermediate size (passing the 3/8” sieve and retained on the No. 4 sieve), and fine sizes (a combination of material passing the No. 4 sieve and retained on the No. 8, and passing the No. 8 sieve and retained on the No. 30 sieve).

•

Condition the loose asphalt mix in the oven for 2 hours at 135°C to simulate short-term aging.

•

Sieve the loose asphalt mix to separate the coated particles into the different sizes. The sieving process should be performed while the loose mix and the sieves are reasonably hot.

•

Determine the binder content of each fraction using the ignition oven per AASHTO T 308, and label the binder content of the intermediate size aggregate (retained on the No. 4 sieve) as Reference P_b.

•

Prepare the RAP asphalt mix using: (a) virgin binder, (b) virgin aggregate with two distinct fractions: a coarse size (passing the 1/2” sieve and retained on the 3/8” sieve), and fine sizes (a combination of material passing the No. 4 sieve and retained on the No. 8, and passing the No. 8 sieve and retained on the No. 30 sieve), and (c) RAP of intermediate size (passing the 3/8” sieve and retained on the No. 4 sieve).

•

Repeat steps 2 through 4, and label the binder content of the particles retained on the No. 4 sieve (RAP material) as RAP′ P_b.

The minimum recommended asphalt mix mass is 4,000 gram to obtain two replicates for the ignition oven. Figure 1a provides an illustration of the proposed method. The binder content of the individual sizes of coated RAP (RAP′ P_b) and coated virgin aggregate (Reference P_b) provides significant insight into the amount of RAP binder that is active and available.

Figure 1. (a) Summary of the proposed method and (b) possible scenarios for RAP binder availability.

To understand the methodology, consider an example of a virgin asphalt mix consisting of virgin binder and virgin aggregate with distinct fractions with the percentage retained of each fraction (by weight of total aggregate) of 28% (3/8”), 30% (No. 4), 28% (No. 8), and 14% (No. 30). The total binder content of this mix is 4.5%. The measured binder contents for each sieve size, after the ignition oven, are 2.7%, 4.0% (Reference P_b), and 6.1% for sieves No. 3/8, No. 4, and (No. 8 + No. 30), respectively. The coarse aggregate is expected to absorb less binder than the intermediate and fine aggregate sizes due to smaller surface area (11). The Reference P_b value is only valid for this particular mix, with its specific virgin aggregate type and gradation, and the total binder content.

When using RAP (with a 4.5% binder content) to prepare a RAP asphalt mix with 0.3 RBR (i.e., 30% RAP binder and 70% virgin binder) and a total binder content the same as in the virgin asphalt mix (4.5%), the total binder content consists of 3.15% virgin binder (70%) plus 1.35% RAP binder (30%). Therefore, the virgin binder contents in each sieve size of aggregate should be close to 70% of the values measured in the virgin mix with 100% virgin binder content; that is, 1.9% (3/8”), 2.8% (No. 4), and 4.3% (No. 8 + No. 30). These values were confirmed by preparing the same virgin mix but with 3.15% binder content and determining the binder content for each sieve using the ignition oven. The addition of the RAP binder should complete the binder content for each sieve size of aggregate to 2.7% (3/8”), 4.0% (No. 4), and 6.1% (No. 8 + No. 30).

In this RAP asphalt mix, the RAP′ P_b (binder content of RAP retained on the No. 4 sieve) is measured, and the following three outcomes are plausible depending on how much RAP binder is active or available:

1. Scenario 1: RAP′ P_b = Reference P_b (= 4.0% in this example)

The coated RAP particles have the same binder content as the coated virgin aggregate particles on the No. 4 sieve. This would imply that the RAP binder is fully released, and completely active and available in the mix, and the total binder composite (virgin and RAP binders) was evenly distributed within the mix. This scenario would represent 100% RAP binder availability as illustrated in Figure 1b.

2. Scenario 2: RAP′ P_b = [1−RBR] ×Reference P_b+ RAP binder content (= 7.3% in this example)

The coated RAP particles have significantly more binder content than the coated virgin aggregate particles on the No. 4 sieve in the virgin mix, and this difference is equal to the RAP binder content. This would imply that the RAP binder is acting as a “black rock” and is not available in the mix. In other words, only the virgin binder was evenly distributed within the mix (between the virgin aggregate and the RAP). This scenario would represent 0% RAP binder availability as illustrated in Figure 1b. In this example, since the contribution from the virgin binder equals 2.8% (at 70% of the total binder, as calculated and verified previously when only the virgin binder is available), the RAP′ P_b will approach 7.3% (2.8% + 4.5%). Again, this value is only valid for these particular mixes, with their specific virgin aggregate type and gradation, RAP binder content, and the total binder content in the mix.

3. Scenario 3: Reference P_b < RAP′ P_b < (Reference P_b+ RAP binder content)

The coated RAP particles in the RAP mix have more binder content than the coated virgin aggregate particles on the No. 4 sieve in the virgin mix, but this difference is less than the RAP binder content. This represents partial binder availability as illustrated in Figure 1b.

Therefore, the concept behind this method is that if there is no difference in binder contents between the coated RAP particles and the coated virgin aggregate particles (both retained on the No. 4 sieve), there is 100% RAP binder availability since the RAP binder is fully released, and completely active and available in the mix. However, if the coated RAP particles have a higher binder content than the coated virgin aggregate particles, then the binder in the RAP is not fully released and not fully active and available in the mix. Depending on the difference between the binder content of these particles, the RAP binder availability can be calculated.

To calculate the % RAP binder availability, a linear relationship between the two extremes can be used: scenario 1 when RAP′ P_b equals 4.0% in this example, which represents 100% availability, and scenario 2 when RAP′ P_b equals 7.3% in this example, which represents 0% availability, as shown in Equation 1 and Figure 2:

R A P B A F (%) = m \times R A P' P_{b} + b

(1)

Figure 2. Example relationship between BAF and *RAP′ P_b*.

where RAP BAF (%) is the RAP binder availability factor; m is the slope (–30.3 in this example); RAP′ P_b is the binder content of RAP particles retained on the No. 4 sieve; and b is the intercept (221.2 in this example). From this relationship, a binder availability factor (BAF) for a given Reference P_b and RAP′ P_b can be calculated. The RAP BAF is the percentage of available (effective) RAP binder in the asphalt mix, and can be used to adjust the virgin binder content in asphalt mixes with RAP, to ensure that the total optimum (active) binder content as prescribed in the mix design is achieved.

The slope and intercept values are dependent on both the virgin and the RAP asphalt mixes (total binder content and aggregate type and gradation), while RAP′ P_b is dependent on the RAP binder availability. Therefore, as long as the virgin and RAP asphalt mixes have the same total binder content and aggregate type and gradation, Equation 1 can be used to calculate the BAF. Noticeably, the value of the slope and intercept will proportionally change with the RAP binder content (i.e., using a different RAP source), but that will have no effect on BAF calculation. In the 0% availability case (scenario 2 with RAP′ P_b equal to 7.3% in this example), RAP′ P_b will always equal Reference P_b+RAP binder content.

Limitations of the Method

The RAP BAF was estimated based on the binder content of individual fractions of the asphalt mix, which provide a reasonable approximation. There are two main limitations to the method that may increase or decrease the actual RAP binder availability:

Absorbed RAP binder: Even if the RAP binder is very soft and completely fluid, active, and available in the asphalt mix, there will always be some RAP binder that is absorbed by the RAP aggregate. Thus it will be almost impossible to obtain 100% RAP binder availability with this method and the resulting values will likely be somewhat lower than actual RAP binder availability values.

Aggregate gradation: The RAP and aggregate fractions retained on the No. 4 sieve are used in this method to represent the entire RAP source and aggregate gradation in the asphalt mix. However, RAP materials typically include a variety of sizes, mostly intermediate and fine, and less coarse. Smaller RAP sizes are expected to yield higher RAP binder availability due to their larger available surface area, higher binder content, and thicker binder film. Therefore, using the No. 4 sieve in this method may result in lower than actual RAP binder availability values. Moreover, the RAP binder performance grade (PG) varies among different RAP sizes (of the same RAP source), and that will also affect the RAP binder availability values, as discussed subsequently.

Method Verification

This method was initially verified by preparing asphalt mixes and aging them at various levels to create artificial RAP (i.e., laboratory aged) materials. The artificial RAP was produced by mixing a PG 64-22 virgin binder with virgin aggregate fractions retained on the No. 4 sieve at a binder content of 4.5% to simulate RAP particles retained on the No. 4 sieve. This artificial RAP was then aged in the laboratory according to the following protocols:

•

No aging: labeled as RAP 1 and representing a soft RAP.

•

5 days at 110°C (230°F): labeled as RAP 2 and representing a stiff RAP.

•

10 days at 110°C (230°F): labeled as RAP 3 and representing a very stiff RAP.

•

10 days at 110°C (230°F) plus 3 days at 150°C (302°F): labeled as RAP 4 and representing an extremely stiff RAP.

The BAF of each artificial RAP was calculated, using the method described above, by preparing virgin and RAP (artificial) asphalt mixes with virgin aggregate from Texas (limestone) with the percentages retained by weight of the total aggregate equal to 28% (3/8”), 30% (No. 4), 28% (No. 8), and 14% (No. 30). The RBR in the RAP asphalt mixes was 0.3, and the total binder content in both asphalt mixes was 4.5%. In the virgin asphalt mix, the Reference P_b was 4.0% as determined using the ignition oven (steps 1–4). In the artificial RAP mixes, steps 5–6 were followed for each different artificial RAP, and the RAP′ P_b values were also determined using the ignition oven.

Figure 3a shows the RAP′ P_b values for the artificial RAPs. As expected, the soft RAP (RAP 1) had a slightly higher binder content (RAP′ P_b) than Reference P_b (4.27% versus 4.0%), while the extremely stiff RAP (RAP 4) had a much higher binder content (RAP′ P_b) than Reference P_b (6.01% versus 4.0%). This resulted in higher BAF values for RAP 1 compared with RAP 4, as shown in Figure 3b. As expected, the BAF value has a negative correlation with RAP stiffness (or extent of aging): the softer the RAP binder, the higher the BAF.

Figure 3. (a) *RAP′ P_b* values and (b) BAF values for asphalt mixes with artificial RAPs.

Factors Affecting RAP BAF

After verifying the proposed method to estimate the RAP BAF using artificial RAP prepared in the laboratory, the method was used to estimate the RAP BAF of actual RAP materials from different sources in the U.S.: Texas (TX), Florida (FL), Indiana (IN), New Hampshire (NH), Nevada (NV), Delaware (DE), and Wisconsin (WI). These materials were utilized to evaluate the impact of the following variables on the RAP BAF:

•

Mixing temperature and short-term conditioning period

•

RAP source and RAP binder PG

•

Recycling agent addition and the method of addition

The BAF of each RAP was calculated, using the proposed method, by preparing virgin and RAP asphalt mixes. The virgin asphalt mixes were prepared with a WI PG 58-28 virgin binder and virgin aggregate from Wisconsin (crushed rocks from Muskego, WI) with the percentages retained by weight of the total aggregate equal to 28% (3/8”), 30% (No. 4), 28% (No. 8), and 14% (No. 30). The RAP asphalt mixes were prepared with the same virgin binder and aggregate (excluding the No. 4 sieve that was replaced by the RAP of the same size), and the RBR for the RAP mixes was about 0.3. The total binder content in both asphalt mixes was 4.5%. Since the RAP binder content was not the same for the different RAP sources, some RAP asphalt mixes had a little bit less, or more, RAP binder than others, in order to maintain the same total binder content in all RAP asphalt mixes. It was important to maintain the same total binder content for all RAP asphalt mixes to match the virgin asphalt mix in order to keep the calculations of RAP′ P_b and Reference P_b valid within the same total binder content.

After following the proposed method in steps 1–6, ignition oven results showed that the Reference P_b in the virgin asphalt mix was 4.4%. In the RAP asphalt mixes, the RAP′ P_b values varied among different RAP sources. Two replicates were prepared and considered for each asphalt mix.

Mixing Temperature and Short-Term Conditioning Period

Production (or mixing) temperature of asphalt mixes depends on the viscosity of asphalt binders and how well the asphalt binder coats the aggregates. For virgin asphalt mixes without RAP, general rules are well established. For example, the production temperature for a virgin mix with a PG 58-28 binder (as used in this study) is approximately 141–147°C (287–297°F) at which the virgin binder can easily flow and coat the virgin aggregates (12). However, it is not certain that this mixing temperature range is adequate for RAP asphalt mixes to ensure that the RAP binder is released, becomes fluid, and blends with the virgin binder.

Asphalt mixes were prepared at two mixing temperatures: 140 and 150°C. Figure 4a shows the results of RAP BAF versus mixing temperature. The error bars on each column represent ± one standard deviation from the average BAF value of the two replicates. It is clear that mixing temperature plays a dominant role in increasing the RAP BAF; the higher the mixing temperature, the higher the BAF. This is expected since higher mixing temperatures help soften the RAP binder, becoming more fluid and facilitating blending with the virgin binder.

Figure 4. (a) The effect of mixing temperature on RAP BAF and (b) the effect of short-term conditioning period on RAP BAF.

Figure 4b shows the estimated RAP BAF of two different short-term conditioning periods (2 hours versus 4 hours): in both cases, mixing and condition temperatures were 150 and 135°C, respectively. It seems that extending the short-term conditioning to 4 hours slightly increased the RAP BAF of FL, IN, and DE RAP sources, but statistically, there was no difference between 2 hours versus 4 hours short-term conditioning time.

RAP Source and RAP Binder PG

To evaluate the effect of RAP PG on the BAF, the recycled binders from the different RAP sources were extracted in accordance with ASTM D 2172 (test method A: centrifuge extraction), and then recovered in accordance with ASTM D 5404 using the rotary evaporator. Rheological characterization was performed on the recovered RAP binders using the DSR to obtain the high-temperature PG (PGH) of each RAP binder, per AASHTO M 320, as an indication of RAP binder stiffness and the extent of aging.

Figure 5a and b shows the results of RAP BAF versus RAP binder PGH at 140 and 150°C mixing temperatures. A clear trend is observed in both cases: the lower the RAP binder PGH, the higher the BAF. Therefore, when mixing at 140°C, for instance, it is estimated that only 50% of the TX RAP binder will be active and available in the mix, as compared with 80% for the WI RAP. However, if the mixing temperature is increased to 150°C, the availability of the RAP binder from TX and WI will increase to about 70% and 95%, respectively.

Figure 5. RAP BAF versus RAP PGH at (a) 140°C and (b) 150°C mixing temperature.

Recycling Agent Addition and the Method of Addition

Previous studies have shown the effectiveness of recycling agents in softening the RAP binder and improving the performance of recycled asphalt mixes by mitigating the stiffness and brittleness of the RAP binder (10, 13–21). To evaluate the effect of recycling agent addition on the RAP BAF, a modified vegetable oil was added to the RAP asphalt mixes at a dose of 5%. To evaluate the method of recycling agent addition, the recycling agent was added to the virgin binder prior to mixing with virgin aggregate and RAP in one set of RAP asphalt mixes, while in another set the recycling agent was added directly to the RAP (at room temperature for about 5 minutes) prior to mixing with virgin aggregate and virgin binder.

Figure 6a shows that including the recycling agent in the asphalt mix clearly increased the RAP BAF for most RAP sources at 140°C mixing temperature. However, the method of adding the recycling agent to the RAP directly, as opposed to mixing it with the virgin binder, did not show any significant effect on the RAP BAF. This could be due to the fact that the recycling agent was added to the RAP just five minutes before mixing, and thus there was not sufficient time for the recycling agent to diffuse into the RAP binder. In a report published by the National Asphalt Pavement Association (NAPA) in 2015 (22) to discuss practices in Japan in high RAP asphalt pavements, it was reported that recycling agents were mixed directly with the heated RAP in a small pugmill, and the hot rejuvenated RAP was then transferred to a surge bin to give additional conditioning time (2–3 hours). The merit of this approach is that it allows the recycling agent to quickly diffuse into the softened aged RAP binder. This practice would increase the RAP BAF.

Figure 6. The effect of addition of recycling agent and the method of addition on RAP BAF at (a) 140°C and (b) 150°C mixing temperature.

Figure 6b shows, however, that adding the recycling agent slightly increased the RAP BAF, at 150°C mixing temperature, but did not show any statistical difference except for the TX and FL RAP sources. This would indicate that the recycling agent addition had more impact on the RAP BAF at low mixing temperatures than higher mixing temperatures. However, although increasing mixing temperature and adding a recycling agent had equivalent effects on the RAP BAF, the recycling agent had more benefit in softening the RAP binder and improving its rheology without additional aging at the higher mixing temperature. The method of adding the recycling agent to the RAP directly, as opposed to mixing it with the virgin binder at 150°C mixing temperature, also did not show any significant effect on RAP BAF.

Conclusions and Recommendations

This study proposed a method to estimate the BAF of RAP. Since not all of the binder is released from the RAP, becomes fluid, and blends with the virgin binder under typical mixing temperatures―as is commonly assumed—the BAF can be used to adjust the virgin binder content in recycled mixes to ensure that the mix design optimum binder content is achieved. In the proposed method, asphalt mixes were prepared so that, after mixing and conditioning, the RAP material could be separated from the virgin aggregate, allowing for a thorough evaluation of the extent of RAP binder availability in the recycled asphalt mix.

The following conclusions are drawn based on the proposed method:

•

The RAP BAF ranged from 50% to 95% depending on RAP source and mixing temperature: the lower the RAP binder PGH, the higher the BAF, and the higher the mixing temperature, the higher the BAF.

•

Extending the short-term conditioning from 2 to 4 hours did not significantly increase the RAP BAF.

•

Adding the recycling agent clearly increased the RAP BAF for most RAP sources at a lower mixing temperature (140°C), but did not significantly increase the RAP BAF at a higher mixing temperature (150°C).

•

The method of adding the recycling agent to the RAP directly, as opposed to mixing it with the virgin binder, did not show any significant effect on RAP BAF, but time and temperature of marination were not explored.

The RAP BAF was estimated using the proposed method based on the binder content of individual fractions of the mix, which provides a reasonable estimate of the percentage of active/available RAP binder. Using the measured BAF from this method is more appropriate than the rough estimate of 75% used by many state Departments of Transport, as reported by NCHRP Synthesis 495 (6).

Besides binder availability, the degree of blending of the RAP and virgin binders needs to be investigated since it will affect asphalt mix performance, and stiffness or cracking resistance testing should always be performed on recycled asphalt mixes to evaluate the effect of RAP BAF and the degree of blending. Other variables that may affect the RAP BAF and the degree of blending that need to be investigated include mixing time and different RAP/aggregate gradation.

Acknowledgments

This study was performed under the currently ongoing National Cooperative Highway Research Program (NCHRP) Project 09-58: The Effects of Recycling Agents on Asphalt Mixtures with High RAS and RAP Binder Ratios. The authors acknowledge Thomas Henz and Geoffrey Giannone of the Texas A&M Transportation Institute for their efforts and contributions in the laboratory.

Footnote

The Standing Committee on Non-Binder Components of Asphalt Mixtures (AFK30) peer-reviewed this paper (19-00711).

This paper does not constitute a standard or specification, nor is it intended for design, construction, bidding, contracting, or permit purposes. Trade names are used solely for information and not for product endorsement. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of NCHRP.

References

1. McDaniel R., Anderson R. M. NCHRP Report 452: Recommended Use of Reclaimed Asphalt Pavement in the Superpave Mix Design Method: Technician’s Manual. HRB, National Research Council, Washington, D.C., 2001.

Abstract

Objectives

Methodology

Limitations of the Method

Method Verification

Factors Affecting RAP BAF

Mixing Temperature and Short-Term Conditioning Period

RAP Source and RAP Binder PG

Recycling Agent Addition and the Method of Addition

Conclusions and Recommendations

Acknowledgments

Footnote

References

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