- Email: [email protected]
Relationships between bitumen chemistry and low temperature behaviour of asphalt
Constructionand BuildingMaterial, Vol. 11, No. 2, Pp. 83-91,1997
8 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0950-0618/97 $17.00 + 0.00
PII:30950-0618(97300008-1
Relationships between bitumen chemistry and low temperature behaviour of asphalt U. Isacsson* and Huayang Zeng’ Division of Highway Engineering, Kungl Tekniska Hogskolan, Royal Institute of Technology, S- 100 44 Stockholm, Sweden Received 18 September 1996; revised 10 March 1997; accepted 17 March 1997 A laboratory investigation of the relationships between bitumen chemistry and low temperature behaviour of asphalt mixtures is described. Five bitumens from four sources (Venezuela, Saudi Arabia, Mexico, Russia) and three different mixture types (dense graded, stone mastic and porous asphalt) were studied. Chemical characteristics of the binders were measured using Thin Layer Chromatography (TLC) and Gel Permeation Chromatography (GPO. Low temperature properties of asphalt characterised by the fracture temperature were measured using Thermal Stress Restrained Specimen Test (TSRST). Statistically significant relations between chemical characteristics of bitumens and fracture temperature of asphalt specimens are presented, indicating that the bitumen chemistry may have a significant influence on the low temperature behaviour of asphalt pavements. 0 1997 Elsevier Science Ltd. Keywords:
asphalt;
low temperature
cracking;
bitumen
Thermally induced cracking of asphalt pavement may be a problem in cold regions (low temperature cracking) as well as in areas which experience large extremes in daily temperatures (thermal fatigue cracking). Low temperature cracking of asphalt pavements is attributed to thermal stresses developed during cooling. If the stress is equal to or greater than the tensile strength of the pavement, a microcrack may develop on the pavement surface, and, after additional low temperature cycles, this crack will propagate downward through the pavement. Water entering the crack may freeze during winter and result in the formation of ice lenses, which in turn can produce frost heave. Pumping of fines through the crack may produce voids under the pavement, which means that the bearing capacity may be reduced. Consequently, the cracking process described above may cause poor ride quality, reduced service life and increased costs for maintenance of roads, unless measures are taken to decrease the risk of low temperature cracking. Over the years, the effect of a great number of different types of factors, such as material, environmental and pavement structure factors, on the low
*Correspondance to U. Isacsson ‘Presentaddress: Svedala, Dynapac S-371 23 Karlskrona, Sweden
Int. High Comp Centre,
chemistry
temperature behaviour of asphalt pavements has been identified’. Among material factors, the most important is probably the consistency of the binder. Several investigations have shown that binder consistency parameters such as stiffness2T3, viscosity4, penetration4 and softening point5 have a strong influence on low temperature cracking of asphalt mixtures. In this paper, investigations with the purpose of establishing relationships, if any, between bitumen chemistry and low temperature properties of asphalt, are described.
Materials and methods Binders and mixtures
Five plain bitumens from four different sources ((Venezuela (Laguna), Saudi Arabia, Mexico and Russia>> were used in this study. One of the Laguna binders was a B 85 bitumen, while the others were B 180. Characteristics of the bitumens (before mixing with aggregates) are given in Table 1. The aggregate consisted of a crushed granite material from Farsta, which was used in three types of mixtures, dense graded (ABT), stone mastic (ABS), and porous asphalt (ABD), respectively. Slabs were pre-
Box 504,
83
84 Table 1
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng Characteristics of bitumens used in this study Bitumen B 85 (Venezuela)
Parameter Pen., 25”C, 100 g, d mm Softening point, “C Dynamic viscosity at 6o”C, Ns/m’ Kinematic viscosity at 135”C, mm2/s
B180 (Venezuela)
86 46 185 356
181 38 66 208
pared in the laboratory using a laboratory mixer (Kalottikone OY, Finland) and a plate compactor (MAP, Spechbach-le-bas, France) with hvo rubber tyres (diameter 400 mm, width 80 mm). The mixing temperature was in the range 150-170°C and compaction temperature was in the range 140-155°C depending on binder and mixture type. The slabs were compacted to void contents in accordance with Swedish requirements. From each slab, five cylindrical specimens were horizontally extracted. The diameter of the specimen tested was 58 f 1 mm and the length 250 mm + 2 mm. The maximal particle size was 12 mm in all mixtures and the particle size distributions are shown in Figure 1. The nominal binder content was 6.2% by weight (ART), 5.8% by weight (MS) and 5.2% by weight (ABD), respectively.
B 180 (Saudi Arabia)
B 180 (Mexico)
B 180 (Russia)
160 40 59 222
164 40 73 260
187 42 32 159
Tensile stress restrained specimen test (TSRST) The TSRST system is schematically shown in Figure 2. The basic principle of the test system is to keep the length of the asphalt sample constant during cooling. A cylindrical specimen is mounted in the load frame. The temperature inside the environmental chamber is decreased during the test with the aid of vaporised liquid nitrogen or a refrigerating machine. As the specimen contracts, two linear variable differential transducers (LVDT) sense the movement and a signal is sent to the computer, which in turn causes the screw jack to stretch the specimen back to its original length. As the temperature continues to decrease, the thermal stress inside the sample increases until the specimen breaks. The tests were performed on unaged specimens and specimens aged 5, 25 and 100 days, respectively, at 85°C using a cooling rate of lO”C/h.
Test equipment and procedures Thin layer chromatography (TLC) In this study, the Thermal Stress Restrained Specimen Test (TSRST) was used to determine the low temperature cracking resistance of the asphalt mixtures. After recovering the binder from all the TSRST specimens using rotary evaporator technique, the chemical composition of the binders was characterised using Thin Layer Chromatography (TLC) and Gel Permeation Chromatography (GPC).
In TLC, the sample to be analysed is dissolved in a solvent and spotted at one end of a quartz rod coated with a thin layer of sintered silica. After developing the rods with suitable solvent(s) and removing the solvent(s) by heating, the rods are scanned with the aid of a flame ionisation detector (FID). Using this method, four different components of the binder (saturates, aromatics,
r -I - r -I
0.01
0.1
1
Aggregate size (mm) +ABTlZ Figure 1 Particle size distribution of the aggregate.
+ABSIZ
+ABDlZ
-t-i7
I-IT
i-1
Bitumen chemistry and low temperature behavior of asphalt: lJ. lsacsson and H. Zeng
85
AC Specimen
I
L
Figure 2
=7
Ikd Platen
Schematic of the TSRST system.
resins and asphaltenes) are determined (in per cent). In this study, 1.5% by weight of the sample solutions was prepared in dichloromethane and 1 ml solution spotted on the rod. Separation into components was performed toluene and dichloromethane/ using n-heptane, methanol (95:5, v/v), respectively. The test equipment used in this investigation is commercially available under the trade name IATROSCAN MK-5.
of the gel exclude molecules larger than a certain critical size, while smaller molecules can permeate the gel structure by diffusion. The sample components are eluted in order of decreasing size or molecular weight and, in this study, detected using UV absorption.
Test results Gel
permeation chromatography (GPC) Them1 stress restrained specimen test (TSRSTS)
In GPC, molecules are separated with respect to size by passing the sample through a stationary phase consisting of porous cross-linked polymeric gel. The pores
TSRST measures several parameters, such as fracture temperature, fracture stress, transition temperature,
StressRelaxation Frachue Temperature
-40
-35
-30
-25
-20
-15
Temperature (“C) Figure 3 Typical test results of TSRST.
-10
-5
0
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng
86
and slope of the stress-temperature curve below transition temperature (Figure 3). The most important parameter is fracture temperature. In this study, special attention is given to relationships between the fracture temperature of asphalt mixtures and the chemical composition of bitumens. Discussions of other TSRST parameters as well as detailed information on test results are found elsewhere’. In Figure 4, TSRST results obtained on stone mastic asphalt are illustrated. As can be seen, the fracture
Laguna 85
Laguna 180 -I-
temperature is affected not only by the degree of ageing, but also by the binder source/grade. For unaged samples, the maximum difference in fracture temperature due to different binders is about 5°C (Russia and Venezuela B 85). For the same source 0_aguna), the fracture temperature of B 180 is about 2°C lower than B 85. The resistance of asphalt to low temperature cracking increases with softness of the binder. Ageing leads to an increase in fracture temperature. After 100 day’s ageing at 85”C, the increase in fracture
Arabian
Mexico T
T
Russion T
404 BBlNoageing 05 days E325 days Ml00 days Figure 4
Effect of ageing
and asphalt
source
on fracture
Venezuela
temperature
Venezuela (Laguna 180)
(Laguna 85)
of ABS asphalt
specimens.
Saudi Arabia
Mexico
Russia
qNo ageing 05 days D25 days n lOO days Figure
5
Effect of ageing
and binder
source/grade
on the fracture
temperature
of porous
asphalt
specimens
(ABD).
Bitumen chemistty and low temperature behavior of asphalt: lJ. lsacsson and H. Zeng temperature ranges from 8 to WC, the mixture containing the Russian binder showing a slightly greater increase than the others. However, for unaged samples, the TSRST results on stone mastic asphalt mixtures indicate the best low temperature behaviour of mixtures containing Russian bitumen. Ageing of porous asphalt (ABD) is faster than ageing of the other two mixture types studied due to higher air void content (Figure 5). The increase in fracture temperature after 100 days’ ageing was only about 9°C for specimens containing Mexico bitumen (the corresponding difference for stone mastic asphalt was YC), but as high as about 23°C for mixtures containing Russian bitumen. For unaged samples, the maximum difference in fracture temperature due to different binders (Russia and Venezuela B 85) is somewhat larger (about 9°C) compared to dense graded mixtures. The effect of binder grade is found to be rather greater in porous asphalt compared to stone mastic asphalt; the difference in fracture temperature of unaged porous asphalt mixtures containing Laguna B 85 is about 5°C higher than the corresponding mixture containing Laguna B 180. This observation supports the common opinion that softer binders are favourable with regard to low temperature behaviour of asphalt pavements. In this case, it should also be noted that the ‘Russian’ mixtures show the best low temperature properties before ageing (fracture temperature about -35”C), as was the case for stone mastic asphalt. The TSRST results obtained on dense graded asphalt mixtures were similar to those obtained on stone mastic asphalt specimens.
Thin layer chromatography-flame ionisation detector (TLC-FID) tests A typical TLC-FID chromatogram
is given in Figure 6
and results obtained on Laguna B 180 recovered from porous asphalt specimens illustrated in Figure 7. As shown, the chemical composition of the binder varies with degree of ageing. Saturates are almost insensitive to ageing, while aromatics decrease, and resins and asphaltenes increase during ageing. For more detailed information regarding the TIC analyses, see 2&g’. Gel permeation chmmatogmphy (GPC) tests GPC profiles of bitumens recovered from porous asphalt containing binder from Venezuela (Laguna B 180) are compared in Figure 8. The diagram shows the GPC profile before and after ageing for 5,25 and 100 days, respectively. As can be seen, the ageing process causes an increase in the content of high molecular weight compounds and, at the same time, a decrease at the main peak. In Figure 9, GPC profiles of the 8ve bitumens recovered after 100 days’ ageing are given. The figure indicates that the greatest change in binder chemistry during ageing occurs in the Russian binder and the smallest change in the Saudi Arabian binder. Regression analysis Relationships between the chemical characteristics of bitumen obtained using TLC-FID and the fracture temperature of asphalt were investigated using linear regression analysis. The results are shown in Table 2. The correlation coefficients presented are calculated using test results obtained on 59 samples (three mixture types, four ageing periods and five bitumens make 60 combinations; one specimen failed before TSRST). The corresponding correlation coefficients of porous asphalt specimens (19 samples) are given in parentheses. All values given in Table 2 are statistically significant at a risk level of 5%. As shown, the sum of
Retention time Figure 6
Example of thin layer chromatography
test result.
a7
88
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng
Saturates 07
II 10
0
20
I
I
30
40
I
I 50
60
70
60
90
100
Degree of ageing (days) Figure 7
Effect of ageing on chemical composition of binder recovered from porous asphalt (Laguna B 180) using TLC.
Decreasingmolecularweight
14
15
16
17
18
19
20
Elutiin -Noageing’
Figure 8
21
22
23
24
25
26
27
time (minutes)
5 days -25
days ------lOOdays
GPC profiles of Laguna B 180 recovered from porous asphalt specimens before and after ageing up to 100 days at 85°C.
asphaltenes and resins, which can be considered as a measure of the content of high molecular components, shows the best correlation (R = 0.86 and R = 0.90, respectively). Among the mixture types studied, the greatest change in chemical composition of the binder occurs in porous asphalt. When analysing results obtained with this mixture type, a somewhat better correlation is established (Table 2). In Figure IO, one of these relationships is shown graphically. GPC profiles (Figures 8 and 9) can be used analytically by partitioning the area below the chromatograms into different portions based on elution time and then computing the area under the curve corresponding to the portions. In this investigation, a division into eight sub-areas, denoted X1-X8, was made. The details of the procedure are described elsewhere’.
An analysis of the GPC parameters (X1-X8) was conducted to determine whether relationships exist between these binder parameters (or combinations of parameters) and the corresponding fracture temperatures of asphalt mixtures determined using TSRST. In most cases, no or very poor correlation was observed. However, using the parameter X5 (corresponding to a molecular weight of about 400-7001, an excellent correlation was established, as shown in Figure II. The high molecular weight portion, as characterised by the GPC parameter (Xl + X2), was assumed to be a measure of the asphaltene content of the binder. This assumption seems reasonable when comparing with TLC-FID data; regression analysis showed a statistically significant linear relationship (risk level 5%) between (Xl + X2) data and TLC asphaltene contents.
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng
89
Decreasing molecular weight
14
I
I
15
16
.
I
17
18
19
20
21
I
22
23
24
25
26
27
Elutiontime (minutes) -Venezuela Figure 9
GPC profiles of five
B 85 -
-
- Venezuela B 180 -Saudi
-Mexico
- - - - - -Russia
binders recovered from porous asphalt after 100 days’ ageing.
Table 2 Linear correlation coefficients between chemical composition (TLC-FID) of recovered bitumen and fracture temperature of asphalt specimens of different mixture types (dense graded, stone mastic and porous asphalt) Component/combination of components
Linear correlation coefficient
Aromatics Resins Asphaltenes Asphaltenes + resins Asphaltenes x resins/aromatics Values in parentheses
Arabia -
0.71(0.80) 0.82 (0.89) 0.79 (0.83) 0.86 (0.90) 0.82 (0.89)
represent porous asphalt samples.
25s
c! I
24--
3
23..
y=-sQa+16.92 R=O.93
B 2 x
22..
$
21.-
g B
20--
x cJ
19.. 194 40
I -35
-30
-25
-20
-15
-10
-5
Fmduretewmiure('C)
Figure 11 GPC parameter ture.
70..
X5 as a function of fracture tempera-
55..
temperature, an interesting result was obtained. As illustrated in Figure 12, very good correlation is established for binders from Venezuela, Saudi Arabia and Mexico. However, the Russian binder deviates from the others in a significant way. This discrepancy is discussed below.
60.. 5s.. !N_. 45.. 40.. 35..
3ooJ -40
Discussion -35
-30
-25
-20
-15
and conclusions
-10
Fracture tsnnperature (‘C) Figure 10 Bitumen chemical composition (asphaltenes + resins) as a function of asphalt fracture temperature.
When evaluating a possible relationship between the ‘asphaltene’ content from GPC measurements, estimated in the manner just described, and the fracture
As described in a currently published state of the art6, different types of factors, such as material, environmental and pavement structure factors, may influence the behaviour of asphalt at low temperature. Among material factors, the properties of the binder are of the utmost importance. Research showing relationships between rheological properties of the binder, determined using empirical methods such as penetration
Bitumen chemistry and low temperature behavior of asphalt: il. lsacsson and H. Zeng
90
1
0 40
-35
-30
-25
-20
-15
-10
-5
Fracture tgnpecature (‘C)
Figure 12 GPC parameter temperature.
(Xl + X2) as a function of fracture
and softening point, and low temperature characteristics of the asphalt has been publishedx8. However, as far as is known by the authors, no investigations of relationships between bitumen chemistry and fracture properties of asphalt at low temperature have hitherto been carried out. To characterise the low temperature properties of asphalt concrete mixtures, Tensile Stress Restrained Specimen Test (TSRSTl was used. This method was first described by Monismith et ~1.~3 decades ago and has since been improved by other researchers, such as Arand and Jung and Vinson3. According to Vinson et aLlo, this test method actually simulates field conditions. A broader acceptance of a laboratory test method for characterisation of performance-related properties of asphalt pavements, such as resistance to low temperature cracking, requires results that relate with field observations. A field validation of the thermal stress restrained specimen test (TSRST) has been performed using five test roads and a full scale and fully controlled low temperature cracking test programme”. The main conclusion drawn from this investigation is that TSRST can be used to predict low temperature cracking of asphalt aggregate mixtures. From the chemical point of view, bitumen is a very complex material. It consists of a very great number of organic compounds structured in a complex way. In principle, it is impossible to obtain detailed knowledge of the chemical composition of a bitumen. A large number of methods have been proposed for characterisation of bitumen chemistry, of which several are based on the partition into different components (fractions). One such method is thin layer chromatography with flame ionisation detector (TLC-FID), which has been used in this study. TLC-FID was chosen due to the fact that this method is rapid; a complete determination of the four fractions (saturates, aromatics, resins and asphaltenes) of a sample can be performed in about 1 h. The repeatability of the test procedure also seems to be satisfactory; tentative investigations indicate variation coefficients of about 5% in determination of the different components. In contrast to TLC-PID, Gel
Permeation Chromatography (GPC) does not partition the bitumen sample into components, but provides a molecular size distribution of the bitumen. During service, an asphalt pavement becomes aged, that is, the brittleness of the material gradually increases due to physico-chemical changes in the binder. Four principal mechanisms are related to bitumen ageing: exudation, evaporation, oxidation and physical ageing, respectively, the most important being oxidation. In bitumen oxidative ageing, formation of functionalities containing oxygen as well as transformation between different binder components can be observed. Usually, the asphaltene content increases while the content of aromatics decreases (Figure 7). Saturates are inert to oxygen. Only slight changes in this fraction may occur due to volatilisation. The transformation between bitumen components in oxidative ageing may be written as follows’2*‘3 Aromatics * Resins * Asphaltenes This is in accordance with results obtained in this investigation. In this study, 85°C was chosen as the laboratory ageing temperature in accordance with results obtained in the Strategic Highway Research Program”. The asphalt specimens were stored at that temperature for up to 100 days. Such a long period of storage was used in order to make it possible to differentiate between the binders with regard to ageing properties. However, it is questionable whether the binder after 100 days of ageing at 85°C really is a bitumen in the full sense of the word. Rheological studies of recovered bitumens have also indicated a divergent pattern of results at 100 days’ ageing compared to ageing up to 25 days’. This investigation has demonstrated statistically significant relationships between chemical characteristics of bitumen obtained using thin layer chromatography (TLC) and gel permeation chromatography (GPC), respectively, and low temperature properties of asphalt (fracture temperature), as illustrated in Figures 10-12. Increased content of high molecular components (resins, asphalthenes) results in increased hardness of the binder, which in turn means higher fracture temperature (Figure 10). The same conclusion can be drawn when comparing GPC results with fracture temperatures, as shown in Figure 12. The discrepancy between the Russian bitumen and the other bitumens studied, as illustrated in Figure 12, can probably be explained by the fact that the content of the high molecular compounds (Xl + X2) of the original Russian binder is higher than the corresponding parameter of the other binders14. In Figure 11, the relationship between the chemistry (GPC parameter X5) of the recovered binders and the fracture temperature of asphalt concrete specimens determined by TSRST, is illustrated. No interpretation of this behaviour can be given. The range of molecular weight of parameter X5 (400-700) indicates that aromatics and saturates may be found in X5. However,
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng detailed investigations of the chemistry of the X5 portion are necessary to clarity the mechanisms behind this relationship. In summary, the investigations described in this paper indicate that bitumen chemistry may have a significant influence on the low temperature behaviour of asphalt pavements. Since very few, if any, studies on this topic have been published earlier, as far as the authors know, further investigations are needed before a more definitive conclusion can be drawn. As has been mentioned above, TSRST appears to be capable of predicting field performance of asphalt at low temperature. However, even in this area, more experience is required. Finally, correlation studies of laboratory and field tests are necessary before the possible practical importance of the results presented in this paper can be determined.
3
4
5
6
10
Acknowledgements The financial support received from the Swedish National Road Administration is gratefully acknowledged. GPC analyses and manufacturing of slabs by Nyr& AR, as well as skilful technical assistance in the laboratory by Jonas Ekblad and Xiaohu Lu, are greatly appreciated.
11
12
13
14
References 1 Zeng, H., On the low temperature cracking of asphalt pavements, PhD thesis, Division of Highway Engineering, Royal Institute of Technology, TRITA-IP FR 95-7, Stockholm, 1995. 2 McLeod, N. W., A 4-year survey of low temperature transverse
15
91
pavement cracking on three Ontario Test Roads. Proc. Assn Asphalt Paving Tech&, 1972,41,424-468. Jung, D. H. andvinson, T. S., Low temperature cracking resistance of asphalt concrete mixtures. Proc Assn. Asphalt Paving Technol, 1993,62, 54-92. Kandhal, P. S., Button, J. W., Davis, R. L, Ensley, E. K., Khosla, N. P. and Puzinauskas, V. P., Low temperature properties of paving asphalt cements. Transportation Research Board, National Research Council, 1988. Kanetva, H., Effects of asphalt properties on low temperature cracking of asphalt mixtures. 7th International Conference on Asphalt Pavements, Performance, Nottingham, 1992,2,95-107. Zeng, H. and Isacsson, U., Low temperature cracking of bituminous courses - state of the art. Division of Highway Engineering, Royal Institute of Technology, Report TRITA-IP-FR 95-10, Stockholm, 1995. Arand, W., Bewertungshintergrund zur Beurteilung von Walzasphalten bei Glte. Die Asphultstrasse, 1987, 3, 5-16. Arand, W., EinfluI3 der Zusammensetzung auf das Verhalten von Walzasphalten bei KiiIte. Die Asphaltstrasse, 1987,4, 7-20. Monismith, C. L., Secor, G. A. and Secor, K. E., Temperature induced stresses and deformations in asphalt concrete. Proc. Assn. Asphalt Paving Technol., 1965,34, 248-284. Vinson, T. S., Janoo, V. C. and Haas, R. C. G., Summary report on low temperature and thermal fatigue cracking. Strategic Highway Research Program SHRP-A/IR - 90-001, Washington, DC, 1989. Zubeck, H. K., Zeng, H., Vinson, T. S. and Janoo, V. C., Field validation of the thermal restrained specimen test - Six case histories. Transportation Research Record. National Research Council 1996, 1545, 67. Corbett, L. W. and Schweyer, H. E., Composition and rheology considerations in age hardening of bitumen. Pmt. Assn. Asphalt Paving Technol., 1981, SO, 571-582. Tallafigo, M. F., Evaluation of chemical composition of bitumen during oxidation in laboratory with the thin film oven test method. Proceedings of 5th Eurobitume Congress, Vol. IA, pp. 214-219, Stockholm, June 1993. Lu, X. and Isacsson, U., Compatibility and storage stability of styrene-butadiene-styrene copolymer modified bitumens. Division of Highway Engineering, Royal Institute of Technology, S-100 44 Stockholm. To be published in Materials and Structures. Bell, C. A., Fellin, M. J. and Wieder, A., Field validation of laboratory ageing procedures for asphalt aggregate mixtures. Proc. Assn. Asphalt Paving Technol., 1994, 63, 45-80.
8 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0950-0618/97 $17.00 + 0.00
PII:30950-0618(97300008-1
Relationships between bitumen chemistry and low temperature behaviour of asphalt U. Isacsson* and Huayang Zeng’ Division of Highway Engineering, Kungl Tekniska Hogskolan, Royal Institute of Technology, S- 100 44 Stockholm, Sweden Received 18 September 1996; revised 10 March 1997; accepted 17 March 1997 A laboratory investigation of the relationships between bitumen chemistry and low temperature behaviour of asphalt mixtures is described. Five bitumens from four sources (Venezuela, Saudi Arabia, Mexico, Russia) and three different mixture types (dense graded, stone mastic and porous asphalt) were studied. Chemical characteristics of the binders were measured using Thin Layer Chromatography (TLC) and Gel Permeation Chromatography (GPO. Low temperature properties of asphalt characterised by the fracture temperature were measured using Thermal Stress Restrained Specimen Test (TSRST). Statistically significant relations between chemical characteristics of bitumens and fracture temperature of asphalt specimens are presented, indicating that the bitumen chemistry may have a significant influence on the low temperature behaviour of asphalt pavements. 0 1997 Elsevier Science Ltd. Keywords:
asphalt;
low temperature
cracking;
bitumen
Thermally induced cracking of asphalt pavement may be a problem in cold regions (low temperature cracking) as well as in areas which experience large extremes in daily temperatures (thermal fatigue cracking). Low temperature cracking of asphalt pavements is attributed to thermal stresses developed during cooling. If the stress is equal to or greater than the tensile strength of the pavement, a microcrack may develop on the pavement surface, and, after additional low temperature cycles, this crack will propagate downward through the pavement. Water entering the crack may freeze during winter and result in the formation of ice lenses, which in turn can produce frost heave. Pumping of fines through the crack may produce voids under the pavement, which means that the bearing capacity may be reduced. Consequently, the cracking process described above may cause poor ride quality, reduced service life and increased costs for maintenance of roads, unless measures are taken to decrease the risk of low temperature cracking. Over the years, the effect of a great number of different types of factors, such as material, environmental and pavement structure factors, on the low
*Correspondance to U. Isacsson ‘Presentaddress: Svedala, Dynapac S-371 23 Karlskrona, Sweden
Int. High Comp Centre,
chemistry
temperature behaviour of asphalt pavements has been identified’. Among material factors, the most important is probably the consistency of the binder. Several investigations have shown that binder consistency parameters such as stiffness2T3, viscosity4, penetration4 and softening point5 have a strong influence on low temperature cracking of asphalt mixtures. In this paper, investigations with the purpose of establishing relationships, if any, between bitumen chemistry and low temperature properties of asphalt, are described.
Materials and methods Binders and mixtures
Five plain bitumens from four different sources ((Venezuela (Laguna), Saudi Arabia, Mexico and Russia>> were used in this study. One of the Laguna binders was a B 85 bitumen, while the others were B 180. Characteristics of the bitumens (before mixing with aggregates) are given in Table 1. The aggregate consisted of a crushed granite material from Farsta, which was used in three types of mixtures, dense graded (ABT), stone mastic (ABS), and porous asphalt (ABD), respectively. Slabs were pre-
Box 504,
83
84 Table 1
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng Characteristics of bitumens used in this study Bitumen B 85 (Venezuela)
Parameter Pen., 25”C, 100 g, d mm Softening point, “C Dynamic viscosity at 6o”C, Ns/m’ Kinematic viscosity at 135”C, mm2/s
B180 (Venezuela)
86 46 185 356
181 38 66 208
pared in the laboratory using a laboratory mixer (Kalottikone OY, Finland) and a plate compactor (MAP, Spechbach-le-bas, France) with hvo rubber tyres (diameter 400 mm, width 80 mm). The mixing temperature was in the range 150-170°C and compaction temperature was in the range 140-155°C depending on binder and mixture type. The slabs were compacted to void contents in accordance with Swedish requirements. From each slab, five cylindrical specimens were horizontally extracted. The diameter of the specimen tested was 58 f 1 mm and the length 250 mm + 2 mm. The maximal particle size was 12 mm in all mixtures and the particle size distributions are shown in Figure 1. The nominal binder content was 6.2% by weight (ART), 5.8% by weight (MS) and 5.2% by weight (ABD), respectively.
B 180 (Saudi Arabia)
B 180 (Mexico)
B 180 (Russia)
160 40 59 222
164 40 73 260
187 42 32 159
Tensile stress restrained specimen test (TSRST) The TSRST system is schematically shown in Figure 2. The basic principle of the test system is to keep the length of the asphalt sample constant during cooling. A cylindrical specimen is mounted in the load frame. The temperature inside the environmental chamber is decreased during the test with the aid of vaporised liquid nitrogen or a refrigerating machine. As the specimen contracts, two linear variable differential transducers (LVDT) sense the movement and a signal is sent to the computer, which in turn causes the screw jack to stretch the specimen back to its original length. As the temperature continues to decrease, the thermal stress inside the sample increases until the specimen breaks. The tests were performed on unaged specimens and specimens aged 5, 25 and 100 days, respectively, at 85°C using a cooling rate of lO”C/h.
Test equipment and procedures Thin layer chromatography (TLC) In this study, the Thermal Stress Restrained Specimen Test (TSRST) was used to determine the low temperature cracking resistance of the asphalt mixtures. After recovering the binder from all the TSRST specimens using rotary evaporator technique, the chemical composition of the binders was characterised using Thin Layer Chromatography (TLC) and Gel Permeation Chromatography (GPC).
In TLC, the sample to be analysed is dissolved in a solvent and spotted at one end of a quartz rod coated with a thin layer of sintered silica. After developing the rods with suitable solvent(s) and removing the solvent(s) by heating, the rods are scanned with the aid of a flame ionisation detector (FID). Using this method, four different components of the binder (saturates, aromatics,
r -I - r -I
0.01
0.1
1
Aggregate size (mm) +ABTlZ Figure 1 Particle size distribution of the aggregate.
+ABSIZ
+ABDlZ
-t-i7
I-IT
i-1
Bitumen chemistry and low temperature behavior of asphalt: lJ. lsacsson and H. Zeng
85
AC Specimen
I
L
Figure 2
=7
Ikd Platen
Schematic of the TSRST system.
resins and asphaltenes) are determined (in per cent). In this study, 1.5% by weight of the sample solutions was prepared in dichloromethane and 1 ml solution spotted on the rod. Separation into components was performed toluene and dichloromethane/ using n-heptane, methanol (95:5, v/v), respectively. The test equipment used in this investigation is commercially available under the trade name IATROSCAN MK-5.
of the gel exclude molecules larger than a certain critical size, while smaller molecules can permeate the gel structure by diffusion. The sample components are eluted in order of decreasing size or molecular weight and, in this study, detected using UV absorption.
Test results Gel
permeation chromatography (GPC) Them1 stress restrained specimen test (TSRSTS)
In GPC, molecules are separated with respect to size by passing the sample through a stationary phase consisting of porous cross-linked polymeric gel. The pores
TSRST measures several parameters, such as fracture temperature, fracture stress, transition temperature,
StressRelaxation Frachue Temperature
-40
-35
-30
-25
-20
-15
Temperature (“C) Figure 3 Typical test results of TSRST.
-10
-5
0
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng
86
and slope of the stress-temperature curve below transition temperature (Figure 3). The most important parameter is fracture temperature. In this study, special attention is given to relationships between the fracture temperature of asphalt mixtures and the chemical composition of bitumens. Discussions of other TSRST parameters as well as detailed information on test results are found elsewhere’. In Figure 4, TSRST results obtained on stone mastic asphalt are illustrated. As can be seen, the fracture
Laguna 85
Laguna 180 -I-
temperature is affected not only by the degree of ageing, but also by the binder source/grade. For unaged samples, the maximum difference in fracture temperature due to different binders is about 5°C (Russia and Venezuela B 85). For the same source 0_aguna), the fracture temperature of B 180 is about 2°C lower than B 85. The resistance of asphalt to low temperature cracking increases with softness of the binder. Ageing leads to an increase in fracture temperature. After 100 day’s ageing at 85”C, the increase in fracture
Arabian
Mexico T
T
Russion T
404 BBlNoageing 05 days E325 days Ml00 days Figure 4
Effect of ageing
and asphalt
source
on fracture
Venezuela
temperature
Venezuela (Laguna 180)
(Laguna 85)
of ABS asphalt
specimens.
Saudi Arabia
Mexico
Russia
qNo ageing 05 days D25 days n lOO days Figure
5
Effect of ageing
and binder
source/grade
on the fracture
temperature
of porous
asphalt
specimens
(ABD).
Bitumen chemistty and low temperature behavior of asphalt: lJ. lsacsson and H. Zeng temperature ranges from 8 to WC, the mixture containing the Russian binder showing a slightly greater increase than the others. However, for unaged samples, the TSRST results on stone mastic asphalt mixtures indicate the best low temperature behaviour of mixtures containing Russian bitumen. Ageing of porous asphalt (ABD) is faster than ageing of the other two mixture types studied due to higher air void content (Figure 5). The increase in fracture temperature after 100 days’ ageing was only about 9°C for specimens containing Mexico bitumen (the corresponding difference for stone mastic asphalt was YC), but as high as about 23°C for mixtures containing Russian bitumen. For unaged samples, the maximum difference in fracture temperature due to different binders (Russia and Venezuela B 85) is somewhat larger (about 9°C) compared to dense graded mixtures. The effect of binder grade is found to be rather greater in porous asphalt compared to stone mastic asphalt; the difference in fracture temperature of unaged porous asphalt mixtures containing Laguna B 85 is about 5°C higher than the corresponding mixture containing Laguna B 180. This observation supports the common opinion that softer binders are favourable with regard to low temperature behaviour of asphalt pavements. In this case, it should also be noted that the ‘Russian’ mixtures show the best low temperature properties before ageing (fracture temperature about -35”C), as was the case for stone mastic asphalt. The TSRST results obtained on dense graded asphalt mixtures were similar to those obtained on stone mastic asphalt specimens.
Thin layer chromatography-flame ionisation detector (TLC-FID) tests A typical TLC-FID chromatogram
is given in Figure 6
and results obtained on Laguna B 180 recovered from porous asphalt specimens illustrated in Figure 7. As shown, the chemical composition of the binder varies with degree of ageing. Saturates are almost insensitive to ageing, while aromatics decrease, and resins and asphaltenes increase during ageing. For more detailed information regarding the TIC analyses, see 2&g’. Gel permeation chmmatogmphy (GPC) tests GPC profiles of bitumens recovered from porous asphalt containing binder from Venezuela (Laguna B 180) are compared in Figure 8. The diagram shows the GPC profile before and after ageing for 5,25 and 100 days, respectively. As can be seen, the ageing process causes an increase in the content of high molecular weight compounds and, at the same time, a decrease at the main peak. In Figure 9, GPC profiles of the 8ve bitumens recovered after 100 days’ ageing are given. The figure indicates that the greatest change in binder chemistry during ageing occurs in the Russian binder and the smallest change in the Saudi Arabian binder. Regression analysis Relationships between the chemical characteristics of bitumen obtained using TLC-FID and the fracture temperature of asphalt were investigated using linear regression analysis. The results are shown in Table 2. The correlation coefficients presented are calculated using test results obtained on 59 samples (three mixture types, four ageing periods and five bitumens make 60 combinations; one specimen failed before TSRST). The corresponding correlation coefficients of porous asphalt specimens (19 samples) are given in parentheses. All values given in Table 2 are statistically significant at a risk level of 5%. As shown, the sum of
Retention time Figure 6
Example of thin layer chromatography
test result.
a7
88
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng
Saturates 07
II 10
0
20
I
I
30
40
I
I 50
60
70
60
90
100
Degree of ageing (days) Figure 7
Effect of ageing on chemical composition of binder recovered from porous asphalt (Laguna B 180) using TLC.
Decreasingmolecularweight
14
15
16
17
18
19
20
Elutiin -Noageing’
Figure 8
21
22
23
24
25
26
27
time (minutes)
5 days -25
days ------lOOdays
GPC profiles of Laguna B 180 recovered from porous asphalt specimens before and after ageing up to 100 days at 85°C.
asphaltenes and resins, which can be considered as a measure of the content of high molecular components, shows the best correlation (R = 0.86 and R = 0.90, respectively). Among the mixture types studied, the greatest change in chemical composition of the binder occurs in porous asphalt. When analysing results obtained with this mixture type, a somewhat better correlation is established (Table 2). In Figure IO, one of these relationships is shown graphically. GPC profiles (Figures 8 and 9) can be used analytically by partitioning the area below the chromatograms into different portions based on elution time and then computing the area under the curve corresponding to the portions. In this investigation, a division into eight sub-areas, denoted X1-X8, was made. The details of the procedure are described elsewhere’.
An analysis of the GPC parameters (X1-X8) was conducted to determine whether relationships exist between these binder parameters (or combinations of parameters) and the corresponding fracture temperatures of asphalt mixtures determined using TSRST. In most cases, no or very poor correlation was observed. However, using the parameter X5 (corresponding to a molecular weight of about 400-7001, an excellent correlation was established, as shown in Figure II. The high molecular weight portion, as characterised by the GPC parameter (Xl + X2), was assumed to be a measure of the asphaltene content of the binder. This assumption seems reasonable when comparing with TLC-FID data; regression analysis showed a statistically significant linear relationship (risk level 5%) between (Xl + X2) data and TLC asphaltene contents.
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng
89
Decreasing molecular weight
14
I
I
15
16
.
I
17
18
19
20
21
I
22
23
24
25
26
27
Elutiontime (minutes) -Venezuela Figure 9
GPC profiles of five
B 85 -
-
- Venezuela B 180 -Saudi
-Mexico
- - - - - -Russia
binders recovered from porous asphalt after 100 days’ ageing.
Table 2 Linear correlation coefficients between chemical composition (TLC-FID) of recovered bitumen and fracture temperature of asphalt specimens of different mixture types (dense graded, stone mastic and porous asphalt) Component/combination of components
Linear correlation coefficient
Aromatics Resins Asphaltenes Asphaltenes + resins Asphaltenes x resins/aromatics Values in parentheses
Arabia -
0.71(0.80) 0.82 (0.89) 0.79 (0.83) 0.86 (0.90) 0.82 (0.89)
represent porous asphalt samples.
25s
c! I
24--
3
23..
y=-sQa+16.92 R=O.93
B 2 x
22..
$
21.-
g B
20--
x cJ
19.. 194 40
I -35
-30
-25
-20
-15
-10
-5
Fmduretewmiure('C)
Figure 11 GPC parameter ture.
70..
X5 as a function of fracture tempera-
55..
temperature, an interesting result was obtained. As illustrated in Figure 12, very good correlation is established for binders from Venezuela, Saudi Arabia and Mexico. However, the Russian binder deviates from the others in a significant way. This discrepancy is discussed below.
60.. 5s.. !N_. 45.. 40.. 35..
3ooJ -40
Discussion -35
-30
-25
-20
-15
and conclusions
-10
Fracture tsnnperature (‘C) Figure 10 Bitumen chemical composition (asphaltenes + resins) as a function of asphalt fracture temperature.
When evaluating a possible relationship between the ‘asphaltene’ content from GPC measurements, estimated in the manner just described, and the fracture
As described in a currently published state of the art6, different types of factors, such as material, environmental and pavement structure factors, may influence the behaviour of asphalt at low temperature. Among material factors, the properties of the binder are of the utmost importance. Research showing relationships between rheological properties of the binder, determined using empirical methods such as penetration
Bitumen chemistry and low temperature behavior of asphalt: il. lsacsson and H. Zeng
90
1
0 40
-35
-30
-25
-20
-15
-10
-5
Fracture tgnpecature (‘C)
Figure 12 GPC parameter temperature.
(Xl + X2) as a function of fracture
and softening point, and low temperature characteristics of the asphalt has been publishedx8. However, as far as is known by the authors, no investigations of relationships between bitumen chemistry and fracture properties of asphalt at low temperature have hitherto been carried out. To characterise the low temperature properties of asphalt concrete mixtures, Tensile Stress Restrained Specimen Test (TSRSTl was used. This method was first described by Monismith et ~1.~3 decades ago and has since been improved by other researchers, such as Arand and Jung and Vinson3. According to Vinson et aLlo, this test method actually simulates field conditions. A broader acceptance of a laboratory test method for characterisation of performance-related properties of asphalt pavements, such as resistance to low temperature cracking, requires results that relate with field observations. A field validation of the thermal stress restrained specimen test (TSRST) has been performed using five test roads and a full scale and fully controlled low temperature cracking test programme”. The main conclusion drawn from this investigation is that TSRST can be used to predict low temperature cracking of asphalt aggregate mixtures. From the chemical point of view, bitumen is a very complex material. It consists of a very great number of organic compounds structured in a complex way. In principle, it is impossible to obtain detailed knowledge of the chemical composition of a bitumen. A large number of methods have been proposed for characterisation of bitumen chemistry, of which several are based on the partition into different components (fractions). One such method is thin layer chromatography with flame ionisation detector (TLC-FID), which has been used in this study. TLC-FID was chosen due to the fact that this method is rapid; a complete determination of the four fractions (saturates, aromatics, resins and asphaltenes) of a sample can be performed in about 1 h. The repeatability of the test procedure also seems to be satisfactory; tentative investigations indicate variation coefficients of about 5% in determination of the different components. In contrast to TLC-PID, Gel
Permeation Chromatography (GPC) does not partition the bitumen sample into components, but provides a molecular size distribution of the bitumen. During service, an asphalt pavement becomes aged, that is, the brittleness of the material gradually increases due to physico-chemical changes in the binder. Four principal mechanisms are related to bitumen ageing: exudation, evaporation, oxidation and physical ageing, respectively, the most important being oxidation. In bitumen oxidative ageing, formation of functionalities containing oxygen as well as transformation between different binder components can be observed. Usually, the asphaltene content increases while the content of aromatics decreases (Figure 7). Saturates are inert to oxygen. Only slight changes in this fraction may occur due to volatilisation. The transformation between bitumen components in oxidative ageing may be written as follows’2*‘3 Aromatics * Resins * Asphaltenes This is in accordance with results obtained in this investigation. In this study, 85°C was chosen as the laboratory ageing temperature in accordance with results obtained in the Strategic Highway Research Program”. The asphalt specimens were stored at that temperature for up to 100 days. Such a long period of storage was used in order to make it possible to differentiate between the binders with regard to ageing properties. However, it is questionable whether the binder after 100 days of ageing at 85°C really is a bitumen in the full sense of the word. Rheological studies of recovered bitumens have also indicated a divergent pattern of results at 100 days’ ageing compared to ageing up to 25 days’. This investigation has demonstrated statistically significant relationships between chemical characteristics of bitumen obtained using thin layer chromatography (TLC) and gel permeation chromatography (GPC), respectively, and low temperature properties of asphalt (fracture temperature), as illustrated in Figures 10-12. Increased content of high molecular components (resins, asphalthenes) results in increased hardness of the binder, which in turn means higher fracture temperature (Figure 10). The same conclusion can be drawn when comparing GPC results with fracture temperatures, as shown in Figure 12. The discrepancy between the Russian bitumen and the other bitumens studied, as illustrated in Figure 12, can probably be explained by the fact that the content of the high molecular compounds (Xl + X2) of the original Russian binder is higher than the corresponding parameter of the other binders14. In Figure 11, the relationship between the chemistry (GPC parameter X5) of the recovered binders and the fracture temperature of asphalt concrete specimens determined by TSRST, is illustrated. No interpretation of this behaviour can be given. The range of molecular weight of parameter X5 (400-700) indicates that aromatics and saturates may be found in X5. However,
Bitumen chemistry and low temperature behavior of asphalt: U. lsacsson and H. Zeng detailed investigations of the chemistry of the X5 portion are necessary to clarity the mechanisms behind this relationship. In summary, the investigations described in this paper indicate that bitumen chemistry may have a significant influence on the low temperature behaviour of asphalt pavements. Since very few, if any, studies on this topic have been published earlier, as far as the authors know, further investigations are needed before a more definitive conclusion can be drawn. As has been mentioned above, TSRST appears to be capable of predicting field performance of asphalt at low temperature. However, even in this area, more experience is required. Finally, correlation studies of laboratory and field tests are necessary before the possible practical importance of the results presented in this paper can be determined.
3
4
5
6
10
Acknowledgements The financial support received from the Swedish National Road Administration is gratefully acknowledged. GPC analyses and manufacturing of slabs by Nyr& AR, as well as skilful technical assistance in the laboratory by Jonas Ekblad and Xiaohu Lu, are greatly appreciated.
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References 1 Zeng, H., On the low temperature cracking of asphalt pavements, PhD thesis, Division of Highway Engineering, Royal Institute of Technology, TRITA-IP FR 95-7, Stockholm, 1995. 2 McLeod, N. W., A 4-year survey of low temperature transverse
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pavement cracking on three Ontario Test Roads. Proc. Assn Asphalt Paving Tech&, 1972,41,424-468. Jung, D. H. andvinson, T. S., Low temperature cracking resistance of asphalt concrete mixtures. Proc Assn. Asphalt Paving Technol, 1993,62, 54-92. Kandhal, P. S., Button, J. W., Davis, R. L, Ensley, E. K., Khosla, N. P. and Puzinauskas, V. P., Low temperature properties of paving asphalt cements. Transportation Research Board, National Research Council, 1988. Kanetva, H., Effects of asphalt properties on low temperature cracking of asphalt mixtures. 7th International Conference on Asphalt Pavements, Performance, Nottingham, 1992,2,95-107. Zeng, H. and Isacsson, U., Low temperature cracking of bituminous courses - state of the art. Division of Highway Engineering, Royal Institute of Technology, Report TRITA-IP-FR 95-10, Stockholm, 1995. Arand, W., Bewertungshintergrund zur Beurteilung von Walzasphalten bei Glte. Die Asphultstrasse, 1987, 3, 5-16. Arand, W., EinfluI3 der Zusammensetzung auf das Verhalten von Walzasphalten bei KiiIte. Die Asphaltstrasse, 1987,4, 7-20. Monismith, C. L., Secor, G. A. and Secor, K. E., Temperature induced stresses and deformations in asphalt concrete. Proc. Assn. Asphalt Paving Technol., 1965,34, 248-284. Vinson, T. S., Janoo, V. C. and Haas, R. C. G., Summary report on low temperature and thermal fatigue cracking. Strategic Highway Research Program SHRP-A/IR - 90-001, Washington, DC, 1989. Zubeck, H. K., Zeng, H., Vinson, T. S. and Janoo, V. C., Field validation of the thermal restrained specimen test - Six case histories. Transportation Research Record. National Research Council 1996, 1545, 67. Corbett, L. W. and Schweyer, H. E., Composition and rheology considerations in age hardening of bitumen. Pmt. Assn. Asphalt Paving Technol., 1981, SO, 571-582. Tallafigo, M. F., Evaluation of chemical composition of bitumen during oxidation in laboratory with the thin film oven test method. Proceedings of 5th Eurobitume Congress, Vol. IA, pp. 214-219, Stockholm, June 1993. Lu, X. and Isacsson, U., Compatibility and storage stability of styrene-butadiene-styrene copolymer modified bitumens. Division of Highway Engineering, Royal Institute of Technology, S-100 44 Stockholm. To be published in Materials and Structures. Bell, C. A., Fellin, M. J. and Wieder, A., Field validation of laboratory ageing procedures for asphalt aggregate mixtures. Proc. Assn. Asphalt Paving Technol., 1994, 63, 45-80.