Effect of ageing on bitumen chemistry and rheology

Construction and Building Materials 16 Ž2002. 15᎐22

Effect of ageing on bitumen chemistry and rheology Xiaohu LuU , Ulf Isacsson Di¨ ision of Highway Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received 15 October 2000; received in revised form 16 July 2001; accepted 25 October 2001

Abstract Effect of ageing on bitumen chemistry and rheology was studied. Seven bitumens were aged according to the thin film oven test ŽTFOT. and the rolling thin film oven test ŽRTFOT.. The binders were characterised using infrared spectroscopy, chromatography and dynamic mechanical analysis. Statistical correlation between different chemical parameters, as well as between chemical and rheological parameters, was examined. The relationship between TFOT and RTFOT was also investigated. It was observed that ageing influenced bitumen chemistry and rheology significantly. However, chemical and rheological changes were generally not consistent, and consequently, ageing susceptibility of bitumens may be ranked differently when different evaluation methods are used. Regardless of the type of the parameters measured, a strong correlation was observed between TFOT and RTFOT, and the two ageing procedures show similar severity. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Bitumens; Ageing; Chemical analysis; Rheological characterisation

1. Introduction Bitumen ageing is one of the principal factors causing the deterioration of asphalt pavements. Important ageing related modes of failure are traffic and thermally induced cracking, and ravelling. In bitumen ageing, two types of mechanisms are involved. The main ageing mechanism is an irreversible one, characterised by chemical changes of the binder, which in turn has an impact on the rheological properties. The processes contributing to this type of ageing include oxidation w1᎐3x, loss of volatile components w4,5x and exudation Žmigration of oily components from the bitumen into the aggregate. w6x. The second mechanism is a reversible process called physical hardening w7᎐9x. Physical hardening may be attributed to molecular structuring, i.e. the reorganisation of bitumen molecules Žor bitumen microstructures . to approach an optimum

U

Corresponding author. Tel.: q46-8-7908703; fax: q46-8-108124. E-mail address: [email protected] ŽX. Lu..

thermodynamic state under a specific set of conditions w1,10x. Bitumen ageing occurs during the mixing and construction process as well as during long-term service in the road. The circumstances at different ageing stages vary considerably. The factors affecting bitumen ageing include characteristics of the bitumen and its content in the mix, nature of aggregates and particle size distribution, void content of the mix, production related factors, temperature and time. All these factors operate at the same time, making the process of bitumen ageing very complex. As summarised in a state-of-theart report w11x, a number of laboratory methods have been used in the quantitative determination of bitumen ageing at various stages of the production process as well as in service. The simulation of field ageing entails increasing the temperature, decreasing bitumen film thickness, increasing oxygen pressure, or using combinations of these factors. Kinetics of bitumen ageing may vary with test conditions w3,12x. In this paper, two standardised methods, the thin film oven test ŽTFOT, ASTM D 1754. and the rolling

0950-0618r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 0 1 . 0 0 0 3 3 - 2

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thin film oven test ŽRTFOT, ASTM D 2872., were used to age bitumens. The effect of ageing on bitumen chemistry and rheology was investigated using infrared spectroscopy, chromatography and dynamic mechanical analysis. Correlation between chemical and rheological changes was examined.

2. Experimental 2.1. Bitumens Seven bitumens, denoted B1 to B7, were used in this study. The penetration grades of B1, B2 and B7 were 60, 85 and 370, respectively, and the other bitumens were penetration 180. The crude source of bitumens B1, B2, B3 and B7 was Venezuela, and the sources of B4, B5 and B6 were Mexico, Saudi Arabia and Russia, respectively. 2.2. Ageing procedures Ageing of bitumens was performed using TFOT ŽASTM D 1754. and RTFOT ŽASTM D 2872.. Standardised conditions, i.e. 163 ⬚C and 75 min for RTFOT,

and 163 ⬚C and 5 h for TFOT, were used. The aged bitumens were evaluated by measuring their rheological properties and chemical characteristics. 2.3. Fourier transform infrared (FTIR) spectroscopy A FTIR spectrometer, Infinity 60AR ŽMattson, resolution 0.125 cmy1 ., was used to determine the functional characteristics of bitumens before and after ageing. Five percent by weight solutions of bitumens were prepared in carbon disulfide. Blank Žsolvent. and sample scans were performed using circular sealed cells ŽZnSe windows and 1 mm thickness .. All spectra were obtained by 32 scans with 5% iris and 4 cmy1 resolution in wavenumbers ranging from 1900 to 500 cmy1 . 2.4. Thin-layer chromatography with flame ionization detection (TLC᎐FID) In the TLC᎐FID, 2% Žwrv. solutions of bitumens were prepared in dichloromethane, and 1 ␮ l sample solution spotted on chromarods using a spotter. The separation of bitumen into four generic fractions Žsaturates, aromatics, resins and asphaltenes . was performed by a three-stage development using n-heptane,

Table 1 Effect of ageing on bitumen composition Bitumens

IR absorbance

Generic fractions Ž%.

Carbonyl compounds

Sulfoxides

Saturates

Aromatics

Resins

Asphaltenes

B1-unaged B1-TFOT B1-RTFOT

3.68 4.52 4.34

0.71 0.84 0.79

10.3 10.7 8.8

54.9 51.2 53.4

23.5 27.6 26.0

11.3 10.5 11.4

B2-unaged B2-TFOT B2-RTFOT

3.55 4.91 4.88

0.73 0.90 0.91

10.7 10.4 9.8

56.7 48.2 49.7

21.2 28.5 27.2

11.4 12.9 13.3

B3-unaged B3-TFOT B3-RTFOT

3.86 5.53 5.45

0.68 0.90 0.92

14.2 13.0 12.0

62.5 47.6 49.8

13.2 27.8 25.6

10.1 11.6 12.6

B4-unaged B4-TFOT B4-RTFOT

0.49 1.40 1.49

0.65 1.21 1.00

9.3 10.0 9.0

62.7 52.5 60.0

19.1 29.2 22.7

8.9 8.3 9.2

B5-unaged B5-TFOT B5-RTFOT

0.49 1.24 1.31

0.62 1.05 1.13

9.0 8.9 10.2

64.2 53.0 53.2

19.3 30.2 29.6

7.5 7.9 7.0

B6-unaged B6-TFOT B6-RTFOT

4.97 5.94 5.72

0.67 1.14 0.91

17.1 17.3 17.1

54.6 44.9 43.8

20.9 29.8 30.1

7.4 8.0 9.0

B7-unaged B7-TFOT B7-RTFOT

4.91 5.45 5.45

0.76 0.83 0.81

15.2 13.8 13.3

54.1 50.6 52.2

22.6 25.7 24.2

8.1 9.9 10.3

X. Lu, U. Isacsson r Construction and Building Materials 16 (2002) 15᎐22

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toluene and dichloromethanermethanol Ž95r5 by volume., respectively. The fractions were determined by means of Iatroscan MK-5 analyzer ŽIatron Laboratories Inc., Tokyo, Japan.. 2.5. High performance gel permeation chromatography (HP-GPC) The HP-GPC system used was Waters 515 HPLC pump equipped with Waters 410 differential refractometer. Three ultra-styragel columns were arranged in ˚ The the order of pore size of 100, 500 and 500 A. system temperature was 35 ⬚C. In the analysis, 5% by weight solutions of bitumens were prepared in tetrahydrofuran ŽTHF. and 50 ␮ l sample solution was injected into the column. The flow rate of the THF mobile phase was 1 mlrmin. To calibrate the instrument, a series of polystyrene standards were used. 2.6. Dynamic mechanical analysis (DMA) DMA was carried out using a rheometer ŽRDA II, Rheometrics.. Temperature sweeps with 2 ⬚C increments Žfrom y30 to 135 ⬚C. were performed at 1 radrs and frequency sweeps were applied over the range of 0.1᎐100 radrs at 25 and 60 ⬚C. Parallel plates with ␾ 8 mmr1.5 mm gap and ␾ 25 mmr1 mm gap were used in the temperature ranges of y30 to 50 ⬚C and 40 to 135 ⬚C, respectively. The strains used varied with temperature to ensure tests were within the linear viscoelastic region.

3. Results and discussion 3.1. Functional groups As mentioned earlier, oxidation is the most important ageing mechanism. It can be verified and quantitatively measured by functional group analysis using FTIR. A typical infrared spectrogram is shown in Fig. 1. The absorbance bands at 1705 cmy1 are due to the C⫽O stretch in carbonyl compounds Že.g. ketones, carboxylic acids and anhydrides., while those at 1030 cmy1 are due to the S⫽O stretch in sulfoxides. Consequently, the peak areas at the two wavenumbers may be considered as concentration measures of carbonyl compounds and sulfoxides, respectively. As indicated in Table 1, carbonyl compounds and sulfoxides are formed in the process of TFOT and RTFOT and the degree of the oxidative changes is dependent on the bitumen. For most of the bitumens tested, there is a small difference between TFOT and RTFOT with regards to formation of the functional groups. The mechanism of bitumen oxidation is very complex. It could be that oxidation of methylene and

Fig. 1. Effect of ageing on bitumen FTIR sepctrogram.

degradation of unsaturated chains andror naphthenic rings of benzene systems lead to ketones and carboxylic acids, respectively, and oxidation of thio-ethers to sulfoxides. In addition, aromatization and chain scission may occur during oxidative ageing, which do not result in oxygen incorporation in the bitumen w1x. The functionalities formed should introduce an increase in the overall polarity of the bitumen, which in turn will influence bitumen rheology. 3.2. Generic fractions The effect of ageing on the chemical composition of bitumens was also studied using TLC᎐FID. Using this method, four generic fractions, namely saturates, aromatics, resins and asphaltenes, were determined. As indicated in Fig. 2 and Table 1, ageing decreases aromatics and at the same time increases the content of resins and asphaltenes. However, the content of saturates changes slightly due to their inert nature to oxygen. Since the fractionation of bitumens is mainly based on molecular polarity, the compositional changes should imply transformation of different fractions, i.e. aromatics ª resins ª asphaltenes. Similar to FTIR observation, a small difference is observed between TFOT and RTFOT as measured by changes in bitumen generic fractions ŽTable 1.. 3.3. GPC parameters In HP-GPC, the sample components are eluted in order of decreasing molecular weight. An example of HP-GPC profiles is given in Fig. 3, and accordingly, two GPC fractions, Fraction-I and Fraction-II, are evaluated. For unaged bitumens, the elution times of the two fractions are 15.5᎐19.5 and 19.5᎐29.0 min, respectively; the transition point of elution time Ž19.5 min. shifts approximately 0.5 min after bitumen ageing. The GPC results are summarised in Table 2. In determining

X. Lu, U. Isacsson r Construction and Building Materials 16 (2002) 15᎐22

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Fig. 2. Effect of ageing on bitumen TLC-FID chromatogram.

molecular weights, a series of polystyrene standards were used. The contents of fractions were determined by area normalization of chromatograms. As can be seen, during ageing, the content of Fraction-I Žlarge molecules. is increased at the expense Žoxidation. of Fraction-II. However, for most of the bitumens, the weight average Ž Mw . and number average Ž Mn . molecular weight of the two fractions change slightly after ageing. On the other hand, increases in molecular weight and polydispersity Ž MwrMn . are observed for the whole bitumen system when subjected

Fig. 3. Effect of ageing on bitumen GPC chromatogram.

to ageing; this is due to the content changes of Fractions-I and -II. Using GPC analysis, it can be demonstrated that association of smaller molecules with higher polarity may contribute to the high molecular weight fraction of the bitumen w13,14x. This means that the increased content of Fraction-I may also be indicative of the formation of highly polar functional groups during ageing. Table 2 also indicates that TFOT and RTFOT result

Table 2 Effect of ageing on bitumen GPC parameters Bitumens

Mn

Mw

Mw rMn

Mn -I

Mn -II

Mw -I

Mw -II

I-%

II-%

B1-unaged B1-TFOT B1-RTFOT

699 753 776

3970 4760 5070

5.69 6.32 6.53

10 600 9100 9500

449 425 424

12 900 12 600 13 300

1280 1080 1100

17.1 28.8 28.6

82.9 71.2 71.4

B2-unaged B2-TFOT B2-RTFOT

840 890 792

4170 5280 4980

4.96 5.94 6.29

11 900 10 400 9130

685 640 569

14 800 14 400 13 200

1580 1360 1190

19.3 29.9 31.8

80.7 70.1 68.2

B3-unaged B3-TFOT B3-RTFOT

670 742 753

3320 4430 4580

4.95 5.97 6.08

10 000 8940 8810

547 526 526

12 300 12 200 12 200

1340 1170 1140

18.0 29.4 30.9

82.0 70.6 69.1

B4-unaged B4-TFOT B4-RTFOT

817 868 864

2860 3410 3460

3.50 3.93 4.01

11 380 11 000 10 600

729 737 716

12 900 12 800 12 700

1630 1590 1520

10.9 16.2 17.3

89.1 83.8 82.7

B5-unaged B5-TFOT B5-RTFOT

750 800 798

3040 3580 3640

4.05 4.47 4.56

11 200 10 800 10 900

660 667 662

13 000 12 900 13 000

1580 1550 1550

12.7 17.8 18.2

87.3 82.2 81.8

B6-unageda B6-TFOT B6-RTFOT

726 759 745

6040 6840 6660

8.31 9.01 8.95

8780 8610 8320

508 505 498

12 100 15 100 11 700

1130 884 1080

32.1 35.9 35.9

67.9 64.1 64.1

B7-unaged B7-TFOT B7-RTFOT

537 580 583

3270 4370 4580

6.09 7.54 7.86

10 700 9430 9210

546 546 540

13 600 12 800 12 900

1420 1270 1220

20.9 30.3 33.0

79.1 69.7 67.0

a

Fraction-I of unaged and aged bitumen B6 consists of two narrow peaks.

X. Lu, U. Isacsson r Construction and Building Materials 16 (2002) 15᎐22

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modulus Ž GU ., storage modulus Ž G⬘. and the loss modulus Ž G⬙ . increase and the phase angle Ž ␦ . decreases. These indicate that ageing makes the mechanical properties of the bitumen more solid-like. The magnitude of the changes is dependent on bitumen type and evaluation conditions. For example, at 25 ⬚C and 10 radrs, the complex modulus may increase by 1 ŽB6. to 105% ŽB7. and phase angle may decrease by 3 ŽB6. to 10% ŽB1. after ageing. On the other hand, ageing has minimal effect on dynamic moduli Ž GU , G⬘ and G⬙ . at low temperatures Ž- 0 ⬚C.. A significant decrease in phase angle is generally observed at temperatures of 0᎐70 ⬚C ŽFig. 5.. Since phase angle is a measure of the ratio between loss modulus and storage modulus Žtan ␦ s G⬙rG ⬘., the decreased phase angle implies that ageing leads to a greater increase in storage modulus than in loss modulus. However, at high temperatures ŽG 40 ⬚C., the increase in complex modulus is mainly attributed to the increased viscous component Ž G⬙ .. Fig. 5 also indicates that at a temperature range of 0᎐50 ⬚C the dynamic moduli become less susceptible to temperature after ageing. In the plot of phase angle versus temperature, three DMA characteristic temperatures, T15 , T45 and T75 , can be defined. T45 is the temperature at which phase angle Table 3 Complex modulus and phase angle at 10 radrs and two different temperatures Complex modulus ŽPa.

Phase angle Ž⬚.

25 ⬚C

60 ⬚C

25 ⬚C

60 ⬚C

B1-unaged B1-TFOT B1-RTFOT

8.39E5 1.53E6 1.37E6

2.77E3 4.97E3 5.25E3

69.7 62.4 62.7

85.4 82.3 81.7

in similar changes in GPC parameters ŽFig. 4., and ageing sensitivity is dependent on the bitumens. The results are consistent with those obtained using FTIR and TLC᎐FID.

B2-unaged B2-TFOT B2-RTFOT

5.65E5 8.47E5 8.97E5

1.92E3 2.99E3 3.82E3

72.7 66.3 65.1

86.2 84.0 82.9

3.4. Rheological measurements

B3-unaged B3-TFOT B3-RTFOT

1.29E5 2.50E5 2.49E5

6.86E2 1.10E3 1.43E3

77.3 71.4 70.6

88.0 85.8 85.7

B4-unaged B4-TFOT B4-RTFOT

1.62E5 2.52E5 2.62E5

6.43E2 8.13E2 1.02E3

75.6 70.5 70.5

88.5 87.3 87.1

B5-unaged B5-TFOT B5-RTFOT

2.22E5 3.79E5 3.70E5

7.75E2 1.77E3 1.76E3

75.0 69.7 69.6

88.2 87.3 86.7

B6-unaged B6-TFOT B6-RTFOT

1.66E5 1.87E5 1.67E5

4.75E2 7.51E2 1.12E3

61.1 57.6 59.3

85.1 82.7 82.3

B7-unaged B7-TFOT B7-RTFOT

4.37E4 8.98E4 9.02E4

2.25E2 4.60E2 4.35E2

78.5 75.2 75.3

89.1 87.8 87.9

Bitumens

Fig. 4. Correlation between TFOT and RTFOT as evaluated by GPC.

Bitumen is a viscoelastic material, which displays either elastic or viscous behaviour, depending on temperature and time of loading. At sufficiently low temperatures andror short loading times, bitumen behaves essentially as an elastic solid. As temperature increases andror loading time increases, the viscous property of bitumen becomes more obvious. At sufficiently high temperatures andror long loading times, bitumen is essentially a Newtonian liquid, and can be described by a shear rate independent viscosity value. The temperature and time dependence of bitumen rheology may be changed by the process of ageing, as illustrated in Figs. 5 and 6, as well as in Table 3. The results show that, with ageing, the complex

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X. Lu, U. Isacsson r Construction and Building Materials 16 (2002) 15᎐22 Table 4 Effect of ageing on DMA characteristic temperatures

Fig. 5. Dynamic modulus and phase angle as functions of temperature at 1 radrs for bitumen B1 before and after TFOT.

equals 45 ⬚ Ži.e. loss modulus and storage modulus are equal. while T15 and T75 represent the temperatures at which phase angles equal 15 and 75 ⬚, respectively. The elastic contribution Ž G⬘. to bitumen complex modulus Ž GU . would be dominant at temperatures lower than T15 , since in this case G⬙ is less than 27% of G⬘ and GU s 'G⬘ 2 q G⬙ 2 f G⬘. On the other hand, bitumen is essentially fluid as the temperature exceeds T75 , be-

Bitumens

T15 Ž⬚C.

B1-unaged B1-TFOT B1-RTFOT

y17 y15 y16

1 4 4

28 38 39

B2-unaged B2-TFOT B2-RTFOT

y18 y17 y18

y2 0 0

23 34 34

B3-unaged B3-TFOT B3-RTFOT

y23 y22 y22

y9 y6 y6

14 24 28

B4-unaged B4-TFOT B4-RTFOT

y24 y24 y23

y4 y2 y1

18 26 26

B5-unaged B5-TFOT B5-RTFOT

y22 y22 y22

y2 1 1

18 27 28

B6-unaged B6-TFOT B6-RTFOT

᎐a ᎐ ᎐

4 6 6

34 39 40

B7-unaged B7-TFOT B7-RTFOT

y26 y25 y25

y13 y11 y11

7 18 19

a

T45 Ž⬚C.

T75 Ž⬚C.

Lower than y30 ⬚C.

cause of the negligible contribution of G⬘ to GU . The interval between T75 and T15 can be considered as a temperature range of binder viscoelastic behaviour. Table 4 shows the effect of ageing on T15 , T45 and T75 . The temperatures were obtained by using temperature sweeps at a frequency of 1 radrs. As expected, T45 and T75 are largely dependent on the bitumen and increase with ageing. Differences in T45 and T75 between the unaged bitumens are as large as 17 and 41, respectively, and ageing may increase T45 and T75 by 3 and 10 ⬚C, respectively. T15 also depends on sourcertype of the bitumens and is slightly influenced by ageing ŽTable 4.. Statistical investigation Žlevel of significance 0.01. indicated that T15 , T45 and T75 relate to the weight average molecular weight Ž Mw .. The correlation coefficients are 0.62, 0.55 and 0.76, respectively. DMA measurements also show a good relationship between TFOT and RTFOT. Examples of correlation are given in Fig. 7. For the bitumens studied, TFOT and RTFOT show similar severity. 3.5. Ageing index

Fig. 6. Complex modulus and phase angle as functions of frequency at 25 ⬚C for bitumen B1 before and after TFOT.

Ageing susceptibility of bitumens may be evaluated

X. Lu, U. Isacsson r Construction and Building Materials 16 (2002) 15᎐22

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binder to that of the original binder. Ageing indices obtained using chemical and rheological measurements are shown in Table 5. Relationships between different ageing indices have been examined. It was found that good statistical correlation Ž R s 0.93. exists between GU and Mw ratios, however, no correlation or very weak correlation exists between other ageing indices. This observation suggests that the ageing susceptibility of bitumens would be ranked differently when a different ageing indices are used. It also indicates that the chemical and rheological changes of bitumens during the process of ageing are not necessarily consistent.

4. Conclusions

Ageing influences bitumen chemistry and rheology significantly. Chemical changes include the formation of carbonyl compounds and sulfoxides, transformation of generic fractions, and increases in amount of large molecules Žor molecular association., molecular weight and polydispersity. As a result of the chemical changes, the mechanical properties of aged bitumens become more solid-like, as indicated by increased complex modulus and decreased phase angle. However, the chemical and rheological changes are generally not consistent. Consequently, ageing susceptibility of bitumens may be ranked differently when different evaluation methods are used. Using either chemical analyses or rheological measurements, a strong correlation is observed between TFOT and RTFOT, and the two methods show similar severity.

Fig. 7. DMA comparison of TFOT and RTFOT aged bitumens.

by means of an ageing index, which is defined as the ratio of a chemical or physical parameter of the aged

Table 5 Ageing indices obtained using chemical and rheological measurements Bitumens

B1

B2

B3

B4

B5

B6

B7

TFOT ageing index Carbonyls ratio Sulfoxides ratio Resins ratio Mw ratio GPC Fraction-I ratio GU ratio Ž25 ⬚C, 10 radrs. GU ratio Ž60 ⬚C, 10 radrs.

1.23 1.18 1.17 1.20 1.68 1.82 1.79

1.38 1.23 1.35 1.27 1.55 1.50 1.56

1.43 1.32 2.11 1.33 1.63 1.94 1.60

2.86 1.86 1.53 1.19 1.49 1.56 1.26

2.53 1.69 1.56 1.18 1.40 1.71 2.28

1.20 1.70 1.43 1.13 1.07 1.13 1.58

1.10 1.09 1.14 1.34 1.45 2.05 2.04

RTFOT ageing indexa Carbonyls ratio Sulfoxides ratio Resins ratio Mw ratio GPC Fraction-I ratio GU ratio Ž25 ⬚C, 10 radrs. GU ratio Ž60 ⬚C, 10 radrs.

1.18 1.11 1.11 1.28 1.67 1.63 1.90

1.37 1.25 1.28 1.20 1.65 1.59 1.99

1.41 1.35 1.93 1.38 1.72 1.93 2.08

3.04 1.54 1.19 1.21 1.59 1.62 1.59

2.67 1.82 1.53 1.20 1.43 1.67 2.27

1.15 1.36 1.44 1.10 1.08 1.01 1.58

1.10 1.07 1.07 1.40 1.58 2.06 1.93

a

a

Ageing index is defined as the ratio of a chemical or rheological parameter after and before ageing.

22

X. Lu, U. Isacsson r Construction and Building Materials 16 (2002) 15᎐22

Acknowledgements The authors would like to thank Jonas Ekblad, Britt Wideman and Clarissa Villalobos for their laboratory assistance during the testing programme. References w1x Branthaver JF, Petersen JC, Robertson RE, Duvall JJ, Kim SS, Harnsberger PM, Mill T, Ensley EK, Barbour FA, Schabron JF. Binder Characterization and Evaluation, vol. 2: Chemistry, SHRP-A-368. Washington, DC: National Research Council, 1993. w2x Tuffour YA, Ishai I. The diffusion model and asphalt agehardening. Proc. Assoc. Asphalt Paving Technol. 1990;59:73᎐92. w3x Petersen JC, Branthaver JF, Robertson RE, Harnsberger PM, Duvall JJ, Ensley EK. Effects of physicochemical factors on asphalt oxidation kinetics. Transport. Res. Record 1993; 1391:1᎐10. w4x Traxler RN. Relation between asphalt composition and hardening by volatilization and oxidation. Proc. Assoc. Asphalt Paving Technol. 1961;30:359᎐377. w5x van Gooswilligen G, De Bats FTh, Harrison T. Quality of paving grade bitumen ᎏ a practical approach in terms of functional tests, Proceedings of 4th Eurobitume Symposium, vol. I, Madrid, October 1989, pp. 290᎐297.

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