Materials and Structures (2011) 44:911–921 DOI 10.1617/s11527-010-9675-8

ORIGINAL ARTICLE

Effects of recycling agents on aged asphalt binders and reclaimed asphalt concrete Ping-Sien Lin • Tung-Lin Wu • Chi-Wen Chang Bang-Yan Chou

•

Received: 2 September 2009 / Accepted: 25 October 2010 / Published online: 2 November 2010  RILEM 2010

Abstract This paper aims to study the effect of adding different ratios (from 10 to 40%) of three recycling agents (RAs), including RA-25, RA-75 and RA-250, to the reclaimed asphalt binder (RAB) with a viscosity of 42800 poises and also to the reclaimed asphalt concrete (RAC) according to the Marshall mix design method. The study includes a variety of tests designed to determine the difference between the three RAs in terms of penetration, viscosity, softening point, ductility, toughness of the asphalt binder, as well as indirect tensile strength, and stability value of Marshall specimens. The results show that adding the RA increased the cohesiveness of RAB and thus improved the applicability of RAB. Of the three RAs in this study, RA-25 offered the best performance when added to asphalt binder. This study proposes a recycling model to predict the changes in RAB viscosity when adding RAs. The results of this model show a close fit with experimental data from laboratory tests. This model allows highway engineers to estimate the amount of RA added to aged binder. Marshall tests show that the RA-75 specimen had higher indirect tensile strength and stability value than the RA-25 and RA-250 specimens. Based on overall performance and cost

P.-S. Lin  T.-L. Wu (&)  C.-W. Chang  B.-Y. Chou Department of Civil Engineering, National Chung Hsing University, Taichung, Taiwan, ROC e-mail: [email protected]

comparisons among the three RAs, this study regards RA-75 as the RA of choice. Keywords Recycling agent  Viscosity  RAB  RAP  Recycling model

1 Introduction The use of reclaimed asphalt concrete (RAC) has become a prevalent trend in the field of pavement engineering. Not only does RAC abide by governmental policies on conserving natural resources, but also addresses the shortage of construction material, which is now a serious problem in Taiwan [9]. Regulating the quality of RAC is an essential step to maximizing the optimal use of reclaimed asphalt pavement (RAP). Due to the government’s environmental protection policy, RAP is sometimes available free of charge [4]. The U.S.A. regenerates more than 50 million tons of pavement material every year [5]. Development of technologies for recycling RAP, which started some decades ago in Japan, is still in progress [14]. When RAP contains hard and aged asphalt, it is difficult to recover the aged asphalt. Moreover, some RAC used in recycled asphalt concrete is already multi-recycled material. Recycling agents (RAs) are suitable for either highly oxidized material or for mixtures containing a large percentage of RAP [16].

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Therefore, using an RA is usually the most effective way to recycle asphalt [7, 14]. Using the appropriate type and quantity of RA is extremely important to improving the efficiency and convenience of recycling asphalt concrete plant. Moreover, the RA percentage may be crucial for the properties of the blended aged asphalt [16]. However, contractors often use softer asphalt instead of RAs in recycled asphalt concrete production because the price of RAs is higher in Taiwan. The chemical components of the RA or asphalt include asphaltenes (A), polar aromatics (PA), naphthalene aromatics (NA), and saturates (S) [3]. RAs are introduced in the recycling of aged normal binders to compensate for the decrease in the small molecular fractions (saturates and aromatics) [15]. While blended RA mixes have usual levels of aromatics, they have higher saturate levels and lower viscosities than typical mixes. The asphaltenes function as solution thickeners, while the saturate and naphthene aromatic fractions improve fluidity by plasticizing the solid polar aromatic and asphaltene fractions. The asphaltenes are the main viscositybuilding components [3, 6]. Therefore, RA must redisperse the asphaltenes of aged asphalt. In other words, recycled asphalt binder has a reduced amount of asphaltenes. Researchers have developed several types of RAs to meet different industry requirements (ASTM D4552-92). Although ASTM D4552 has already suggested various types of RAs, their suitability for different regional types of RAP requires further study. Recycling RAP is essentially the process of recovering the aged asphalt around aggregates using a fresh asphalt binder either alone or together with an RA [14]. Since the bituminous material in RAP has a high viscosity, an RA may be necessary as a pavement recycling material. If the viscosity of reclaimed asphalt binder (RAB) is too high, the pavement company must add an RA to meet viscosity specifications [9]. Previous research indicates that it is possible to produce a good asphalt binder from considerably hardened pavement material using the proper RA [11, 19]. However, researchers do not fully understand the effects of different types of RAs on RAB and RAC in Taiwan. A better RA can react with retrieved binder rapidly, achieving a high quality of recycled binder. However, little information is available regarding

Materials and Structures (2011) 44:911–921

what RA functions improve the recovery of the physical properties of aging asphalt, and thus increase the quality of the RAC. Therefore, the type and quantity of RAs requires further study for good quality control. The choice of RA grade depends on the amount and hardness of the asphalt in the aged pavement. Moreover, different RABs may reduce the viscosity differently as the producer mixes in different types of RAs, and viscosity data often reveals other characteristics of the asphalt. This study includes laboratory tests to understand the influences of RA25, RA-75, and RA-250 on the physical and mechanical properties of RAB and RAC. This study also presents a recycling model that serves as a reference for highway engineers using RA.

2 Test materials 2.1 Binder and aggregate The experiments in this study selected RAP materials milled using pavement from Nangang’s industrial zone in Taiwan. A solution of aged binder was extracted from RAP material by centrifuge extractor in accordance with ASTM D2172. The RAB, belonging to AC-20 and extracted from the RAP, had a viscosity of 42800 poises. Table 1 shows that the properties of fresh AC-20 asphalt cement meet the specified requirements (ASTM D3381). The aggregates of the Marshall specimens included new aggregates, reclaimed coarse aggregates, and fine aggregates. The Public Construction Commission (PCC) of the Executive Yuan in Taiwan Table 1 IVc aggregate gradation Asphalt cement properties

Specification

Testing

Penetration (0.1 mm)

Min 60

67

Viscosity (60C, poise)

2000 ± 400

2210

Viscosity (135C, cSt)

Min 300

472

Flash point (C)

Min 232

344

Specific gravity (25C)

–

1.03

Ductility (25C, 5 cm/min, cm)

Min 100

100?

Softening point (C)

40–60

55

Solubility (%)

Min 99.0

99.7

Loss on heating (%)

Max 0.5

0.02

Materials and Structures (2011) 44:911–921

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3 Test methods

Table 2 Test results of asphalt cement properties Sieve size (mm)

Passing (%)

Lower–upper limits (%)

25

100

100

19

96

90–100

9.5 4.75

67 54

56–80 33–65

2.36

39

23–49

0.3

15

5–19

0.075

5.2

2–8

stipulates that the addition of RAP to RAC cannot exceed 40% [13]. Therefore, this study used 35% RAP content and 5% asphalt content. The asphalt contents of the coarse RAP and fine RAP were 3.6 and 4.0%, respectively. This aggregate gradation complied with the hot mix asphalt (HMA) IVc gradation specified by the Ministry of Transportation and Communications (MTC, Taiwan), as Table 2 shows [12]. 2.2 RA The study analyzes three RAs, RA-25, RA-75, and RA-250, made from different proportions of fragrance oil, mineral oil, synthetic resin, and polyamide. The three types of RAs were purchased from a commercial supply company in Taiwan. The RA supplies the fragrant oil lost in the chemical reaction of asphalt with oxygen in the air. Adding RA reduces the consistency and improves the flow behavior of the asphalt at low temperatures, restoring the penetration, and consistency of aged asphalt. The physical properties of RAs used in the study comply with the specified requirements (ASTM D4552-92), as Table 3 shows.

3.1 RAB mixed with RA For years, the asphalt industry has used penetration, viscosity, softening point, ductility, and toughness to depict the physical properties of asphalt binders [8]. Softening point and penetration tests show the hardness and the softening properties of the asphalt [17]. This study assesses all these properties to determine the effects of adding RA to asphalt binders. After extracting the recycled asphalt, RA was added to RAB, followed by tests for penetration, viscosity, softening point, ductility, and toughness. The following discussion presents the methods of this study. A standard needle vertically penetrating a sample under a certain loading time and temperature was used for the penetration test of bituminous material. The results of this study represent the penetration distance in tenths of a millimeter. The most common conditions are 100 g penetrated for 5 s at a temperature of 25C (ASTM D5). A Brookfield Company rotational viscometer was used to measure the viscosity of asphalt at 60C (ASTM D4402-02) and determine its flow behavior under a certain temperature. This test also reveals the mixing and compacting temperature. Testing the softening point involves the determination of the point (from 30 to 157C) at which asphalt softens using a ring-and-ball apparatus immersed in distilled water (ASTM D36). The ductility of bituminous material was tested by pulling apart the two ends of a briquet specimen at a specified speed of 5 cm/min and at a specified temperature of 25C, and then measuring the distance between the two ends when the material broke apart (ASTM D113). This investigation performed the tests mentioned above on RAB specimens with the addition of

Table 3 Physical properties of RA Properties of RA

RA-25 Specification

RA-75 Testing

Specification

RA-250 Testing

Specification

Testing

Viscosity (60C, cSt)

901–4500

3620

4501–12500

6340

12501–37500

31900

Flash point (C)

Min 219

223

Min 219

225

Min 219

230

Specific gravity

Value of report

0.986

Value of report

0.991

Value of report

0.993

Quality loss after TFOT test (%)

Max 3

1.66

Max 3

1.7

Max 3

1.76

Viscosity ratio (%)

Max 3

1.687

Max 3

1.835

Max 3

2.07

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RA, RA-25, RA-75, and RA-250 in proportions of 10, 20, 30–40% by total weight of binder, respectively. 36 g of asphalt was poured into a container with an interior diameter of 55 mm and a depth of 35 mm to measure its toughness by pulling it at a specified speed of 50 cm/min and at a temperature of 25C until the material broke (ASTM D5801). When mixing RAB, specimens with a viscosity of 42800 poises were used for RA-25, RA-75, and RA-250 with a desired final viscosity of 2000 poises. The results of this test reveal the tensile strength and toughness of the aged asphalt with the addition of different RAs.

respectively, for the first stability value tests on the Marshall specimens (ASTM D6927). For the next stability value tests on the Marshall specimens added RA-25, RA-75, and RA-250 at 10, 18, and 36%, respectively. The viscosity of RAB mixed with RA remained at 2000 poises. The test then added different types and amounts of RA to compare the differences in stability.

3.2 Marshall specimen mixed with RA

The test results illustrated in Fig. 1 show that the penetration of RAB without the addition of RA is 20 (0.1 mm). Adding different proportions of RA increased the penetration, indicating that RA boosts the penetration of the same aging asphalt. This also shows a change in penetration based on different amounts of RA. These test results are similar to those obtained by Hui-Ling Xu [19]. As expected, results indicate that RA efficiently softens aged asphalt binder [16]. This study conducted a one-way ANOVA analysis to assess the effect of amount and types of RAs on

The indirect tensile strength test applies compressive loads along a diametrical plane through two opposite loading strips. This type of loading produces a relatively uniform tensile stress which acts perpendicularly to the applied load plane, and the specimen usually fails by splitting along the loaded plane [18]. This test was applied at a speed of 5.1 cm/min and at a temperature of 25C. This test automatically recorded the duration of the test load and deformation values until splitting occurred (ASTM D6931). Viscosity, aggregate interlock, asphalt binder, and gradation curves influence indirect tensile strength. To avoid the influence of the gradation curve on indirect tensile strength, this study used a single gradation curve. RA-25, RA-75, and RA-250 were used to determine the influences of different RAs on RAC. Test results indicate that adding RA-25, RA-75, and RA-250 in different proportions of 10, 18, and 36%, respectively, achieved the viscosity goal of 2000 poises for an asphalt binder mixed with RA. The proportions above were added by the total weight of the binder, which included fresh and aged asphalt. This is unlike the process in Fig. 2, where the proportion of adding RA included only aged asphalt. In other words, Fig. 2 was not used to decide the proportion of adding RA for the Marshall specimen. Therefore, this study performed the indirect tensile strength test on the Marshall specimens with the addition of these three RAs (RA-25, RA-75, and RA-250) at rates of 10, 18, and 36%, respectively. To determine the influence of RA on stability value, RA-25, RA-75, and RA-250 were added in different proportions ranging from 10, 20, 30, to 40%,

4 Results and analysis 4.1 Penetration analysis

ANOVA Source Proportion Type Error Total SS

SS

DF

MS

F

3409.600

4

852.400

44.863

P-value 0.000

430.000

2

215.000

11.316

0.005

152.000

8

19.000

3991.600

14

Fig. 1 Plots of penetration of RAB with RA along with the ANOVA for differentiating effects of proportion and type of RA

Materials and Structures (2011) 44:911–921

penetration performance. This analysis assessed the significant difference between RAs for the five added proportions and three types. Results reveal that a significant difference exists between the added proportions and the different types of RAs, as Fig. 1 shows. Analysis also shows that after separately adding the three types of RAs, RA-25 significantly increased penetration, indicating that the relatively low viscosity of RA-25 contributes to increased penetration. According to the specification of ASTM D3381-83 (Table 2), the minimum penetration of AC-20 in general pavement of highway is 60 (0.1 mm). Figure 1 shows that RA-25, RA-75, RA-250, when the added in quantities of approximately 24, 37 and over 40%, respectively, increased the penetration of RAB to 60 (0.1 mm).

915

ANOVA Source Proportion Type Error Total SS

4.2 Viscosity analysis Viscosity is the ability of a liquid to resist flow. Therefore, RAB with high viscosity has difficulty flowing, while RAB with low viscosity tends toward the state of a Newtonian liquid [1, 10]. Figure 2 shows that the initial viscosity was 42800 poises, and then changed with the amount of RA added to the specimen. The viscosity of recycled asphalt decreased as the proportion of RA increased, indicating that adding RA reduced the viscosity of RAB effectively. This result is similar to the results of HuiLing Xu [19] and Shen et al. [15]. Moreover, the viscosity of the binder containing the RA decreased linearly as the RA percentages fewer than 10% increased, which is also similar to the results obtained by Shen et al. [15]. This study includes a one-way ANOVA analysis to assess the effect of amount and types of RAs on viscosity. This analysis assessed the significant difference between RAs for the five added proportions and three types. Results reveal that a significant difference exists between the added proportions and the different types of RAs, as Fig. 2 shows. Test results also show that recycled asphalt with RA-25 added declined the most. Because the viscosity of RA-25 is relatively low, it tends to soften asphalt faster than any other RAs. Chen and Huang [2] developed an aging model for asphalt. Their model adequately expresses the change of viscosity in paving binders. Test results from the

SS

DF

MS

F

3276224000

4

819056000

43.979

P-value 0.000

240148000

2

120074000

6.447

0.021

148992000

8

18624000

3665364000

14

Fig. 2 Plots of viscosity of RAB with RA along with the ANOVA for differentiating effects of proportion and type of RA

current study show a similar trend of change as compared with the research by Chen and Huang [2]. Therefore, this study proposes the following recycling model for predicting the change in RAB viscosity as follows: X 0 ðw Þ ¼

dX ðwÞ ¼ aX 2 ðwÞ  bX ðwÞ 0 5 w 5 0:4 dw ð1Þ

where X(w) is the viscosity of RAB with an RA content of w, X0 (w) is the rate of change in X with content w of RA, and a, b are constants (with positive values). Note that, in this equation, the value of viscosity decreases as the proportion of RA increases. Equation 1 implies that the maximum rate of change occurs at the start of the process. The rate of change then decreases to the value as w is 0.4. To solve this equation, replace the constant a in Eq. 1 with kr, and replace b with r. Then, rewrite the equation in the following form: X 0 ðwÞ ¼ krX 2 ðwÞ  rX ðwÞ

ð2Þ

The solution to this differential equation involves separating the variables (X and w) and integrating both sides of the equation. Then, apply Eq. 2 to RAB recycling as follows:

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X ðw Þ ¼

r kr þ cerw

ð3Þ

where c is the integration constant. When w equals zero, RAB is aged and unrecycled. At this state, constant k and c are as follows: r c¼  kr ð4Þ X0 where X0 is the initial value of X, i.e., content of w = 0. After substituting c, rewrite the solution in Eq. 3 as follows: X ðw Þ ¼ kþ

h

1 X0

1

i  k erw

ð5Þ

Define CRI ¼

X0:4 X0

where X0 is the viscosity of aged asphalt without the addition of RA, and X0.4 is the viscosity of aged asphalt with the addition of RA in 40% by the weight of the aged asphalt. The certain recycling index (CRI) is the ratio of X(w) to the viscosity of RAB with RA contents of w = 0.4 and 0.0, respectively. In other words, CRI represents the final decrease in X with increasing content of RA. When the content of RA becomes 40%, the X value is equal to CRI times X0. The parameter r indicates a measure of the decrease rate in X over the range of CRI. Therefore, r varies with different types of RA, and assists in comparisons of the viscosity reduction rate. when w ¼ 0:4;

k¼

1  CRI e0:4r X0 ð1  e0:4r ÞCRI

ð6Þ

This equation can be rewritten as follows: X ðw Þ ¼

X0 ð1  e0:4r ÞCRI 1  CRI e0:4r þ ðCRI  1Þerw

ð7Þ

Equation 7 was rewritten and normalized as follows: RI ¼

X0  X ðwÞ ðCRI  1Þðerw  1Þ ¼ X0 1  CRI e0:4r þ ðCRI  1Þerw ð8Þ

The recycling index (RI) is the normalized value of the variation between X0 and X(w) by X0. RI provides a practical way to compare the recycling

Table 4 Recycling parameters R2

Types of RA

CRI

r

RA-25

0.00701

12.2

0.98

RA-75

0.05841

7.1

0.99

RA-250

0.10047

8.6

0.99

effort for different RABs with RA-25, RA-75, or RA-250. A trial-and-error procedure determined the recycling parameter for RAB. A spreadsheet, used for necessary calculations, aided this process. Equation 7 simulated the recycling behavior of RAB in the laboratory test. Table 4 indicates that the coefficients of determination (R2) are all above 0.98, showing that the predicted values agree well with testing values. The recycling parameter r for RAB with the addition of RA-25, RA-75, or RA-250 had values of 12.2, 7.1, and 8.6, respectively. These values represent significant differences in the performance of these three RAs. 4.3 Softening point analysis The softening point test determines the temperature of the asphalt binder when it reaches flow state. The softening point is also an index of when road pavement deforms due to high temperatures. The temperature sensitivity of asphalt is relatively low when the softening point is high. Figure 3 shows that the initial softening point of the retrieved asphalt was 68C, and decreased proportionally as the RA increased. These results are similar to those obtained by Hui-Ling Xu [19]. This study includes a one-way ANOVA analysis to assess the effect of amount and types of RAs on softening point performance. This analysis assessed the significant differences between RAs for the five added proportions and three types. The results reveal that a significant difference exists between added the proportions and the different types of RAs, as Fig. 3 shows. Increasing RA-25, RA-75, and RA-250 by 10% changes the softening point by about 4.7, 4.8, and 4.0C, respectively. Results also reveal that RAB with RA-25 or RA-75 added has a relatively greater change in the softening point than RA-250, indicating that RA-25 and RA-75 can soften RAB more

Materials and Structures (2011) 44:911–921

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ANOVA Source

SS

DF

MS

F

660.233

4

165.058

170.017

0.000

ANOVA

Type

19.233

2

9.617

9.906

0.007

Source

Error

7.767

8

0.971

Proportion

Total SS

687.233

P-value

14

Fig. 3 Plots of softening point of RAB with RA along with the ANOVA for differentiating effects of proportion and type of RA

effectively than RA-250. These results also imply that RA-25 and RA-75, which have lower viscosity than RA-250, have higher temperature sensitivity and a greater softening effect on RAB [19]. 4.4 Ductility analysis A ductility test, which expresses the toughness and cohesion of bituminous materials, measures the cohesiveness of asphalt. Figure 4 reveals that adding 10% of the RA significantly improved the ductility for all three types. Moreover, when more than 20% of the RA was added, the ductility was higher than 100 cm for all three RAs. This shows that RA effectively improved the ductility of RAB and increased the cohesiveness of RAB. These results are similar to those obtained by Hui-Ling Xu [19]. This study includes one-way ANOVA analysis to assess the effect of various amounts and types of RAs on ductility. The analysis assessed the significant differences between RAs for the five added proportions and three types. The results reveal that a difference exists between added proportions of RAs, as Fig. 4 shows. Figure 4 also shows that no difference exists between different types of RAs, and adding more than 20% of the RA might have no effect on ductility. Results also show that adding RA-25 and RA-75 improves the ductility of RAB more than adding RA-250.

Proportion Type Error Total SS

SS

DF

MS

F

22766.400

4

5691.600

360.228

P-value 0.000

31.600

2

15.800

1.000

0.410

126.400

8

15.800

22924.400

14

Fig. 4 Plots of ductility of RAB with RA along with the ANOVA for differentiating effects of proportion and type of RA

4.5 Toughness analysis Deformation increased as tensile force increased when the bituminous materials were pulled, and tensile force decreased as deformation increased until the sample could no longer bear the increasing tensile forces. The area below the curve of tensile force and deformation represents the strain energy of bituminous materials, which also represents toughness (ASTM D5801). Figure 5 shows that the tensile strength of initial RAB exceeded 77 kg. Adding the three RAs

Fig. 5 Tensile strength of RAB with RA

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Fig. 6 Tensile deformation of RAB with RA Fig. 7 Toughness of RAB with RA

decreased the tensile strength of RAB, with the greatest reduction for RA-25, followed by RA-75, and then RA-250. The initial retrieved asphalt had a viscosity of 42800 poises, appeared hard in shape, and was relatively fragile. In this condition, it easily fractures due to fragility, and its tensile deformation is extremely short. Figure 6 shows that adding the RA improved the degree of ductility and extension, indicating that RA contributes to alleviating fragility in retrieved asphalt. Hui-Ling Xu [19] showed that the degree of ductility and extension of asphalt after adding the RA is better than for AC-20. Adding RA increases cohesiveness, reduces the probability of fatigue failure, and reduces the viscosity of RAB more effectively than combining RAB with fresh asphalt. Using RA is better than using AC-20. The toughness of asphalt material is crucial because tensile stress constantly occurs at the top of the pavement, adjacent to the edges of automobile tires. If the tensile strength or toughness of the asphalt binder is not sufficient, the pavement can easily deform and experience fatigue deterioration under repetitive loading. Figure 7 shows the difference in RAB toughness after adding RA-25, RA-75, and RA-250. RAB toughness was greatest with the addition of RA-250. This suggests that pavement has better resistance to deformation and fatigue under repetitive loading using RA-250.

indicates its resistance to tensile stress. The indirect tensile strength St (kPa) can be calculated by Eq. 9: St ¼

2Pmax ptd

where Pmax is the maximum applied load (kN), t is thickness of the specimen (m), d is diameter of the specimen (m). Figure 8 illustrates the variation of indirect tensile strength of Marshall specimens with an addition of different types of RAs. Under the same conditions (viscosity of 2000 poises), the indirect tensile strength of the RA-75 mixture was higher than the indirect tensile strength achieved by the other RAs. RA-75 reduced the viscosity of the recycled asphalt concrete and restored the characteristics of fresh asphalt better than the other RAs. Thus, adding RA-75 reduces the tendency of cracking in asphalt pavement.

4.6 Indirect tensile strength analysis The indirect tensile strength test determines the tensile, or ‘‘cracking,’’ properties of asphalt concrete [18]. The indirect tensile strength of asphalt concrete

ð9Þ

Fig. 8 Indirect tensile strength of RAC with RA

Materials and Structures (2011) 44:911–921

Fig. 9 Stability value of RAC with RA Table 5 Stability value with the same viscosity Types of RA

Percentage of RA (%)

Stability value (kg)

RA-25

10

1293

RA-75

18

1588

RA-250

36

1418

4.7 Stability value analysis Figure 9 shows the results of the stability value test. Each of the RAs exhibited a different reduction in the stability value because the differing percentages of RA added produced different viscosities. The stability value of RA-25 was less than the other two under the same conditions. The viscosity of RA-25 was lower than the others were. Since RA-250 had higher viscosity, its asphalt binder was fragile and harder than the others are, and its stability value was higher due to the high stiffness of the asphalt binder. Table 5 shows the stability value of the Marshall specimens. The stability value of RA-75 was better than the stability values of RA-25 and RA-250. 4.8 Discussion The statistical results of this study show significant effects of adding three types of RAs to aged asphalt, with different added percentage below 20%. In other words, the physico-chemical reaction between RA and asphalt plays an important role in pavement performance. The RA and fresh binder should be a rejuvenator, allowing the final mix of aged and fresh

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binder to have an acceptable consistency and a sound chemical constitution. Therefore, RA should be highly aromatic. Nevertheless, the reaction mechanism is very complex and further research is needed. Chui-Te Chiu and Yi-Hsiang Chiu [3] showed that as the binder ages, the asphaltene content also increases, naphthene aromatics decreases, and saturates remain the same. A higher content of asphaltenes and polar aromatics causes higher viscosity. The polar aromatic fraction imparts ductility to the asphalt. Moreover, RA shows lower polar aromatics content and asphaltenes. The addition of RA in aged asphalt could have beneficial effects on its physical properties. In particular, RA-75 has lower asphaltenes and polar aromatics than RA-250. In other words, adding RA75 to RAB could achieve better effects than adding RA-250, which agrees with the results of previous research. Equation 5 shows that k might be the function of the content w of RA. For example, k equals an arbitrary number,

X0 X0:2 e0:2r X0 X0:2 ð1e0:2r Þ

and

1CRI e0:4r X0 ð1e0:4r ÞCRI

with

w zero being 0.2 and 0.4, respectively. X0.2 is the value of X when w = 0.2. This study uses Eq. 7 when w 2 0.4. Table 4 shows that the recycling parameter r for RA-250 lays between the recycling parameters for RA-25 and RA-75, which is inconsistent with CRI trend of lowest to highest for RA-25, RA-75, and RA250, respectively. The fact that the viscosity of RAB with RA-250 added still undergoes certain changes yields the above results. For instance, in Fig. 3, the slope of RA-250 at w = 0.4 is not yet close to 0, meaning the viscosity at w = 0.4 is continuously changing. In other words, the size of CRI and rvalues exhibit a consistent trend for the model received by k when the magnitude of w exceeds 0.4. Based on the trend above, RA-250 is not economical in comparison with RA-25 and RA-75 because steadying the viscosity change rate requires the addition of a greater amount of RA-250. This study adopts the score method to compare the performance of RAB with the addition of three RAs: RA-25, RA-75, and RA-250. The assessment items include the penetration, viscosity, softening point, ductility, and toughness of the asphalt binder, as well as the stability value and indirect tensile strength of the Marshall specimens. Besides, the economy of using RA under the same viscosity goal of the Marshall test has been accounted for. The scores of

920

Materials and Structures (2011) 44:911–921

Table 6 Scores for properties of asphalt binder Items of assessment

Table 8 The total assessment for performance of RAC with the addition of RA

Types of RA RA-25

RA-75

RA-250

RA-75

Table 6

12

11

1

Table 7

5.8

8.1

5

17.8

19.1

12

2

1

3

2

1

Viscosity

3

2

1

2

3

Ductility

2

3

1

Total score

Toughness

2

1

3

Rank

Total score

12

11

7

1

2

3

Rank

various items, except for economy, were graded as 3, 2, and 1 from the best to the worst. However, the economic values were graded as 3.8, 2.1, and 1 according to the inverse ratio of the RA content used in the Marshall specimens multiplied by the price. The prices of these three RAs, RA-25, RA-75 and RA-250, were approximately NT$50, NT$51, and NT$53 per kilogram, respectively. Tables 6 and 8 show that the overall appraisal for each item was based on the accumulated scores from the given information in these tables for the best binders to use. Table 6 shows that RA-25 achieved the optimum recovery of the physical properties of RAB. The economic value listed in Table 7 accounts for the amount of RA used to achieve the same viscosity goal of asphalt binder. RA-75 had the optimum recovery based on indirect tensile strength, stability value, and economy. Table 8 also shows that RA-75 had the best recovery considering all the test results and economic value. Therefore, this study regards RA-75 as the top choice for practical applications. Although this study adopted the ratio of 35% RAP, future studies should consider other percentages to determine the relationship between RAP and the Table 7 Scores for indirect tensile strength, stability value, and economy Items of assessment

Types of RA RA-25

Penetration Softening point

Items of assessment

Types of RA RA-25

RA-75

RA-250

Indirect tensile strength

1

3

2

Stability value

1

3

2

Economy

3.8

2.1

1

Total score

5.8

8.1

5

Rank

2

1

3

RA-250 7

3

quantity of RA. Other types of RAs, such as RA-5, are recommended for regeneration studies of highly viscous RAP in Taiwan [7]. Since this study took place in the central region of Taiwan, further studies should focus on other areas and different weather conditions to obtain a better understanding of salient factors and their relationships. Future research should also address the issue of multiple recycling. Currently available equipment and materials do not guarantee the quality of the RAC, which requires a great deal of effort to develop a quality product having the viscosity closest to that of traditional asphalt concrete. RAC with the addition of RA has better engineering properties than RAC without RA. Therefore, this study strongly recommends using RAP and RA to reduce pollution. The RAs adopted in this study are available locally for use in the production of asphalt concrete. Thus, these RAs eliminate the cost of importing material from foreign countries.

5 Conclusions This study reports the results of laboratory tests evaluating the effects of the physical properties of RAB with a viscosity of 42800 poises, and the engineering properties of RAC after adding RA-25, RA-75, and RA-250 to the RAB and the RAC. Based on the results above, this paper draws the following conclusions: •

•

Adding different amounts and types of RAs creates significant differences in the physical properties of RAB. RA-25 had the best recovery ability of the physical properties of RAB in these three RAs. Adding RA-75 increases the indirect tensile strength and the stability value of Marshall tests

Materials and Structures (2011) 44:911–921

•

as compared with RA-25 and RA-250. This indicates that RA-75 offers the most significant improvement in the resistance of asphalt pavement, which prolongs the life span of the road, reduces maintenance frequency, and is more costeffective in the end. This study develops a recycling model to express the change in viscosity for RAB when adding RA25, RA-75, or RA-250. The predicted values of the recycling model correspond well to the measured values obtained from laboratory tests. However, the recycling parameter r for RAB with the addition of RA-250 must be modified according to the test results when the addition amount is up to 40%.

Acknowledgments The authors wish to express their special thanks to Dr. C.R. Luo for his consultations regarding this research.

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