EFFECT OF SHEAR RATE ON BITUMEN VISCOSITY …aapaq.org/q/2011st/CAPSA2011/FA6_04_185_1114.pdf · Although temperatures at the pumping of bitumen and at the mixing and compaction of

10th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN A FRICA

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EFFECT OF SHEAR RATE ON BITUMEN VISCOSITY MEASUREMENTS – RELEVANCE TO HIGH TEMPERATURE

PROCESSING OF BITUMINOUS PRODUCTS

G.A.J. Mturi 1, S.E. Zoorob 2 and J. O’Connell 1

1CSIR Built Environment, Transport Infrastructure Engineering, Pretoria 0001, South Africa, E-mail: [email protected], E-mail: [email protected]

2Coventry University, Department of Built Environment, Coventry CV1 5FB, United Kingdom, E-mail: [email protected]

Abstract ASTM D4402 is the current standard test method for viscosity determination of bitumen at elevated temperatures (38-260°C), using a rotational viscometer. It specifies measurements to be done at a resisting torque range of 10-98% of maximum torque at a non-specified speed. This makes the test method neither a controlled shear rate (CSR) test nor a controlled shear stress (CSS) test. This paper discusses the limitations of rotational viscometry, especially when used for the determination of viscosity measurements for shear-thinning modified binders, as well as the advantages of dynamic shear rheometry (DSR) as an alternative method of viscosity measurement. Although temperatures at the pumping of bitumen and at the mixing and compaction of asphalt mixes are based on set binder viscosities, shear rates remain uncontrolled as the methods were established for unmodified binders with Newtonian flow at high temperatures. The implication of using uncontrolled shear rate viscosities for shear-thinning modified binders is that compaction and mixing temperatures can be overestimated. The current high temperatures used when processing shear-thinning modified binders have a significant effect on such issues as economics (energy costs), environment (carbon footprint) and binder/mix stiffness (ageing). 1. INTRODUCTION

In CSR rotational tests, the shear rate or rotational speed is preset or controlled. These tests are useful for liquids without a yield point and those to be used at a defined shear rate or flow conditions such as processing a liquid through a pipe. In CSS rotational tests, the shear stress or torque is the set parameter. These tests are useful in the determination of the yield point of a material (Mezger, 2006). In both CSR and CSS testing modes, viscosity is defined as:

γ/η &τ= (Eq.1) (with η, the viscosity or shear viscosity [Pa.s], τ, the shear stress [Pa] and γ& , the shear rate [s-1])


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Bituminous binders are assumed to behave like Newtonian fluids at high temperatures (>>100°C) where shear rate is proporti onal to applied shear stress and the viscosity is independent of shear rate (see Figures 1 and 2). These same binders can assume shear-thinning behaviour at lower temperatures (<100°C) where the viscosity decreases with an incr ease in shear rate (see Figures 3 and 4). In this case, the ratio of shear stress to shear rate varies with shear rate and is referred to as apparent viscosity.

Figure 1: Newtonian rheology: viscosity

vs. shear rate.

Figure 2: Newtonian rheology: shear

stress vs. shear rate.

Figure 3: Shear-thinning rheology:

viscosity vs. shear rate.

Figure 4: Shear-thinning rheology: shear

stress vs. shear rate. Bitumen viscosity at lower temperatures is a function of both temperature and shear rate. The extent will differ with binder stiffness and in the case of modified binders it will also depend on the modifier type and extent of modification. Figure 5 (Mezger, 2006) shows the viscosity function of an uncross-linked polymer with entangled macromolecules. Mturi et al. (2010) have shown the same model can be applied for bitumen. The figure shows 3 viscosity ranges on a double logarithmic plot.

• Range 1 represents the initial Newtonian range where the zero-shear viscosity (ηo) is the plateau or limiting value:

( )γηlimη 0γo &&→=

(Eq.2)

• Range 2 represents the shear-thinning range where viscosity is dependent on the shear rate:

( )γfη &= (Eq.3)

• Range 3 represents the second Newtonian range where the infinite-shear viscosity (η∞) is the plateau or limiting value:

( )γηlimη γ&

& ∞→∞ =

(Eq.4)


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Figure 5 : Viscosity function of an uncross-linked polymer with entangled

macromolecules (Mezger, 2006). Using dynamic and Brookfield viscosities, this paper aims to illustrate that when bitumen viscosity measurements are obtained at a lower shear rate range or shear rate (in the case of Range 2) than is representative of the shear rate during processing (compaction, mixing and pumping), they predict that higher temperatures are required for processing than necessary. Such higher temperatures would have a detrimental effect on binder durability and process sustainability. 2. DYNAMIC VISCOSITY WITH SHEAR RATE In this article, dynamic (or absolute) viscosity refers to complex viscosity (η*) determined from oscillatory tests. The complex viscosity is a vector sum of the viscous (η') and elastic (η'') portions of the bitumen.

( ) ( )2''2'* ηηη += (Eq.5)

Complex viscosities were determined using an Anton Paar Physica Smartpave Plus Dynamic Shear Rheometer (DSR) that uses a Peltier system with a parallel plate measuring configuration. This is achieved by initially conducting strain sweeps to obtain the linear viscoelastic range; then running shear rate sweeps while measuring complex viscosities. This was done while making sure that transient viscosity peaks did not occur in the curves due to time-dependent transition effects. Zoorob et al. (2010) have shown from low shear complex viscosity investigations that at an equiviscous temperature, elastomer (e.g. Styrene Butadiene Styrene - SBS) modified bitumen have no viscosity plateau (i.e. zero shear viscosity η₀) as opposed to unmodified bitumens which have a definite plateau at very low shear rates. Figure 6 confirms these observations in a plot of η* vs. γ& for three unmodified binders (i.e. 20/30, 40/50 and 60/70pen grade bitumen) and an SBS-modified binder at 70°C. The SBS-modified binder exhibited significant shear-thinning behaviour at all shear rates. Viscosities of these modified binders cannot therefore be measured with confidence at these low shear rates for such temperatures. The current testing of modified binders in South Africa is according to Technical Guideline 1 (2007): The use of Modified Binders in Road Construction. All Brookfield viscosity measurements in this manual are conveniently specified at high temperatures (≥ 165°C) where the binder is more Newtonian.


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Figure 6: Complex viscosity (η*) curves at various shear rates for a 20/30, 40/50,

60/70pen grade bitumen and an SBS-modified binder at 70°C. At shear rates between 0.0004 – 34s-1, the complex viscosity curves of unmodified bitumens are influenced by the stiffness of the binder (i.e. as a consequence of using harder binders and/or with ageing). Figures 7-10 show plots of complex viscosity results at 55°C and 70°C against shear rates for va rious binders (i.e. a 20/30pen, a 40/50pen, a 60/70pen grade bitumen and an SBS-modified bitumen) before and after Rolling Thin Film Oven Test (RTFOT)-ageing.

Figure 7: Complex viscosity curves at various shear rates for RTFOT-aged and

unaged 60/70 and 40/50pen grade bitumen at 55°C.


unaged 60/70 and 40/50pen grade bitumen at 70°C.


unaged 60/70, 40/50, 20/30pen grade bitumen and an SBS-modified binder at

55°C.

Figure 10: Complex viscosity curves at

various shear rates for RTFOT-aged and unaged 60/70, 40/50, 20/30pen grade

bitumen and an SBS-modified binder at 70°C.


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As expected, the more Newtonian behaviour was observed with the softer unaged binders, e.g. 60/70pen grade bitumen, in particular at the higher temperature of 70°C and at lower shear rates. On the other hand, pseudoplastic rheological behaviour was most prevalent for the RTFOT-aged 20/30pen grade bitumen, especially at the lower temperature of 55°C. As in Figure 6, the SBS-modified binder displays no viscosity plateau towards the lower shear rates. This behaviour is much more pronounced at 70°C (Figure 10) than at 55°C (Figure 9) i.e. with an increased different iation between the elastic properties of the polymer and bitumen phases. 3. BROOKFIELD VISCOSITY WITH SHEAR RATE SANS 307 specifies that viscosity be measured at 60°C and at 135°C according to standard test method ASTM D4402, using a rotational viscometer. This implies that at these temperatures and at the defined torque range, the viscosity of bitumen will not be affected by any changes in shear rate during measurement. The CSIR uses a DV-I+ model (with a Thermosel system) capable of measuring in the speed range of 0.3-100rpm for sample viscosities between 50mPa.s to 3.3x106 mPa.s. Viscosities of unaged 40/50pen grade bitumen were tested at various speeds whilst maintaining the torque range according to ASTM D4402 requirements. Any variation in viscosity due to non-Newtonian behaviour is expected at the lowest testing temperatures (i.e. at 60°C and/or at temperatures c lose to 60°C); as a result the samples were tested at 60°C, 80°C, 90°C and 110°C. Figures 11 and 12 present the plots of viscosity vs. shear rate and viscosity vs. shear stress for the unaged 40/50pen grade bitumen, respectively. The results show that viscosity measurements were independent of both shear rate and shear stress for this binder.

Figure 11: Plot of viscosity vs. shear rate for a 40/50pen grade bitumen at 60, 80,

90 and 110°C (S29 = Spindle 29 and S27 = Spindle 27).

Figure 12: Plot of viscosity vs. shear

stress for a 40/50pen grade bitumen at 60, 80, 90 and 110°C (S29 = Spindle 29

and S27 = Spindle 27).


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Figures 13 and 14 contain plots of viscosity vs. shear rate and viscosity vs. shear stress for unaged 20/30pen grade bitumen, respectively. The results also indicate Newtonian rheology with rotational viscosities being independent of both shear rate and shear stress for this binder.

Figure 13: Plot of viscosity vs. shear rate for a 20/30pen grade bitumen at 60, 80,

90 and 110°C (S29 = Spindle 29).


stress for a 20/30pen grade bitumen at 60, 80, 90 and 110°C (S29 = Spindle 29).

Unmodified bitumen is expected to have Newtonian behaviour at high temperatures since viscosity measurements occur at the third Newtonian range where infinite-shear viscosity (η∞) is the plateau or limiting value. Approaching lower temperatures, these binders adopt a more shear-thinning behaviour and viscosity measurements occur towards Range 2 ( ( )γfη &= ). But due to the high binder viscosity at low temperatures, viscosity measurements require very low speeds and a narrow range of shear rates in order to achieve the required torque range. Hence the expected viscosity variation with shear rate may not be obvious due to the limited range of shear rates covered. Additionally, Figures 7-10 show that the 40/50pen and 60/70pen grade bitumen are not very shear-thinning as compared to the other binders at the testing temperatures. Hence Brookfield viscosities are expected to show Newtonian behaviour for unmodified binders softer than the 40/50pen grade bitumen and so viscosity measurements, as per SANS 307 specification requirements, will not show variation with changes in shear rate and/or shear stress for those binders specified in SANS 307. The plots showing the relationship between viscosity with shear rate and shear stress for an SBS-modified binder are shown in Figures 15 and 16, respectively. This binder displays shear-thinning behaviour where viscosity decreases with an increase in shear rate. Brookfield viscosities were measured at the shear-thinning range where ( )γfη &= and are referred to as apparent viscosities. This viscosity dependence on shear rate reduces with an increase in temperature, as the sample adopts a more Newtonian rheology.


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Figure 15: Plot of viscosity vs. shear rate for the SBS-modified binder at 80, 90 and

110°C (S29 = Spindle 29).


stress for the SBS-modified binder at 80, 90 and 110°C (S29 = Spindle 29).

4. DYNAMIC VISCOSITY VERSUS BROOKFIELD VISCOSITY The viscosity-temperature plots herein combined rotational viscosities determined from the Brookfield viscometer and viscosities calculated from oscillatory tests, using the dynamic shear rheometer (DSR). The following relationship (NCHRP 1-37A, 2004) was used to obtain viscosity values in centipoise from oscillatory measurements:

4.8628

sinδ

1

10

G*η

=

(Eq.6)

where: η = bitumen viscosity (cP), G* = bitumen complex shear modulus at 10 rad/s (Pa), δ = phase angle (degrees). Figure 17 shows good correlation between viscosities obtained from rotational tests and those from oscillatory tests for both virgin and RTFOT-aged 40/50pen grade bitumen. Standard 40/50pen grade bitumen therefore follows the Cox/Merz relation where the shear viscosity, )γ(η & is identical to the complex viscosity, )(η* ω .

)γη()(η*&=ω (Eq.7)

This relation was found empirically for equal values of angular frequency (ω) in rad/s and shear rate ( γ& ) in s-1. The viscosity-temperature relationship of standard 20/30pen grade bitumen is shown in Figure 18 for an unaged and RTFOT-aged sample. This binder is stiffer than the 40/50pen grade bitumen and more shear-thinning at an equivalent temperature. It requires much lower shear rates than practically possible to measure bitumen viscosity at

oη and unachievable high shear rates to reach ∞η at the overlapping temperatures. Brookfield viscosities were therefore measured where ( )γfη &= and at the slowest speeds possible to achieve the required torque range at these temperatures. Nonetheless, viscosity measurements occurred at similar angular frequency and shear rates and the binder still followed the Cox/Merz relation (despite a slight deviation) as shown in Figure 18.


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Figure 17: Viscosity-temperature relationship of a virgin and RTFOT-aged 40/50pen

grade bitumen derived from complex viscosity and Brookfield viscosity data.

Figure 18: Viscosity-temperature relationship of a virgin and RTFOT-aged 20/30pen

grade bitumen derived from complex viscosity and Brookfield viscosity data. Figure 19 depicts the viscosity-temperature relationship of a virgin and RTFOT-aged SBS-modified binder. In this figure, the rotational viscosities at the overlapping temperatures portray a much stiffer binder. Rotational viscosities only start aligning with the oscillatory-derived viscosity curve (extrapolated to 165°C) at the temperature of 135°C for the virgin binder. An improved correlation between rotational and oscillatory-derived viscosities was observed for the RTFOT-aged SBS-modified binder, where alignment possibly starts from 110°C. These results re-affirm those presented in Figure 10 showing a flatter slope for the complex viscosity vs. shear rate curve of the SBS-modified binder with ageing.


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Figure 19: Viscosity-temperature relationship of a virgin and RTFOT-aged SBS-modified bitumen derived from complex viscosity and Brookfield viscosity data.

The viscosity relationships in Figures 17-19 can be explained through plots of frequency sweeps at 70°C for the three binders (una ged and after RTFOT-ageing) in terms of G’ and G” displayed in Figures 20-22. The 40/50pen and 20/30pen grade bitumen showing G”>G’ (see Figures 20 and 21, respectively) had slopes approaching a 2:1 ratio for the G’-curve and 1:1 for G”-curve towards the low frequency region. This is the expected rheology of uncross-linked material. For these binders the Cox/Merz relation would apply and the shear viscosity will be similar to the complex viscosity or oscillatory-derived viscosity as seen in Figures 17 and 18.

Figure 20: Frequency Sweeps of a virgin and RTFOT-aged 40/50pen grade bitumen

at 70°C in terms of G’ and G”.


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Figure 21: Frequency Sweeps of a virgin and RTFOT-aged 20/30pen grade bitumen

at 70°C in terms of G’ and G”. The unaged SBS-modified binder showed the G’-curve approaching a constant limiting value (showing G’=G”) at low frequencies indicating a cross-linked polymer (see Figure 22). For this binder, the Cox/Merz relation is not useful and would not apply, hence the deviation in viscosities observed in Figure 19. The SBS-modified binder had no plateau at the lower viscosities hence viscosity measurements occur where ( )γfη &= . The exponential increase in viscosity towards zero shear viscosity for these binders would mean that any two viscosities measured at different shear rates will have values a lot further apart than with unmodified bitumen. The RTFOT-aged curves in Figure 22 shows G”>G’ and the two curves are now almost parallel. The effect of thermal ageing (RTFOT-ageing) on the SBS-modified binder is likely polymer chain scissions and a reduction of the cross-linkages as the binder adopts the behaviour of the base bitumen. This resulted in an improved correlation between rotational and oscillatory-derived viscosities for the RTFOT-aged SBS-modified binder. Hence, at low shear rates there are significant differences in viscosities for modified binders. These differences cannot be detected with the current Brookfield testing procedures or through the supporting SANS 307 specification requirements. Rotational viscosity measurements will only correspond to oscillatory-derived viscosity measurements from those temperatures where )γη()(η*

&=ω and the Cox/Merz relation applies.


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Figure 22: Frequency Sweeps of a virgin and RTFOT-aged SBS-modified bitumen at

70°C in terms of G’ and G”. 5. EFFECT OF APPARENT VISCOSITIES ON HIGH TEMPERATU RE

PROCESSES Internationally, both the Superpave (Superior Performance Asphalt Pavements) and Marshall mix design stipulate (as per ASTM D1559) bitumen viscosity requirements of 0.17 and 0.28Pa.s at mixing and compaction of asphalt mixes, respectively. The national Technical Methods for Highways, TMH1 C2 also contains the same requirements. The mixing and compaction temperatures are obtained from viscosity-temperature relationships as per ASTM D2493. Although there is an obvious need to control the temperature of these processes by specifying the viscosity (Hensley and Palmer, 1998), shear rates remain uncontrolled as the methods were established for unmodified binders with Newtonian flow at these high temperatures. Modified binders have pseudoplastic rheology even at high temperatures, where their viscosity decreases with an increase in shear rate. Hence the standard ASTM D2493 procedure for determining mixing and compaction temperatures should be changed accordingly because this approach can result in much higher temperatures (Bahia et al., 1998) being specified for mixing and compaction operations which could possibly damage the binder. A modified approach requires the determination of shear rates of asphalt mixes during these high temperature processes, thereafter measuring bitumen viscosity at the corresponding shear rates. Yildirim et al. (2000, 2003) determined the shear rate inside a Superpave gyratory compactor (SGC) to be in the range 400-490s-1. Due to such high shear rates experienced during compaction, polymer modified binders (experiencing shear thinning) can be practically compacted at lower temperatures than those suggested by ASTM D2493 (i.e. at a target viscosity value of 0.28 Pa.s). This would also apply to the SBS-modified binder investigated; Figure 23 shows the binder having shear thinning behaviour even at 135°C and at 165°C. Yild rim et al. (2000) determined compaction temperatures to be in the range of about 10-40°C lower for such binders at the shear rate of compaction. Using similar arguments, Delgadillo and Bahia


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(2008) proposed a minimum temperature limit for compaction while maintaining adequate densification at a binder viscosity of 50Pa.s.

(a) (b) Figure 23: Plot of viscosity vs. shear rate for the SBS-modified binder (a) at 135°C

and (b) at 165°C, S29 = Spindle 29. In essence, these previous investigations have shown that a viscosity target up to 50Pa.s may be adequate for good compaction on site and, 400-490s-1 to be a realistic range of shear rates during the compaction process. Using Brookfield viscosity charts in Figures 15 and 23, the viscosity target range can be translated to the corresponding compaction temperature range. It is not possible to use the Brookfield viscometer to measure viscosity at very high shear rates, viscosity values were therefore extrapolated to the shear rates of compaction. Figure 15 shows that the viscosity of the shear-thinning SBS-modified binder will meet the target upper viscosity limit of 50Pa.s at 90°C by e xtrapolating to shear rates approaching 400-490s-1. By extrapolating the viscosity-temperature lines of Figure 23 at 165°C (using log-log scales) it is possible to show that at this temperature, the viscosity target of 0.28Pa.s is readily met. Thus it may be possible to propose for the SBS modified bitumen new lower and upper limits for compaction temperatures of 90 to 165°C, in contrast to ~170°C using the current ASTM D2493 approach which does not consider shear thinning. The compaction range estimated has only been included in this paper for debate purposes. Literature survey has shown varying recommendations with regards to the binder viscosity at compaction (Read et al., 2003) and that the actual shear rate inside a SGC could be a lot different to that obtained in the field (NCHRP Report 648, 2010). NCHRP Report 648 (2010) has additionally developed improved test methods for determining laboratory mixing and compaction temperatures for all binders taking shear-thinning into account. 6. CONCLUSION Ideally, the shear rates used during viscosity measurements should simulate those associated with the handling and application processes of bitumen.


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This article highlights that bitumen in general is a non-Newtonian fluid at certain temperatures. Hence, measured bitumen viscosity will be affected by changing torque (shear stress) and/or rotational speed (shear rate) at those temperatures. The extent of pseudo-plasticity varies between bitumen types and for different temperatures. Binder viscosity measurements as per the current SANS 307 specification requirements using ASTM D4402 are not affected by such behaviour. They are specified for unmodified bitumen at shear rates and temperatures ensuring a Newtonian viscosity range. However, viscosity measurements using ASTM D4402 for the calculation of mixing and compaction temperatures based on the ASTM D2493 approach are misleading. This method assumes bitumen viscosity is not affected by variation in shear rates. It has been shown in this investigation that lower compaction temperatures can be realistically employed using viscosities derived at the very high shear rates attained during the compaction process. Therefore, the standard ASTM D2493 procedure of determining mixing and compaction temperatures can result in much higher temperatures being specified for modified binders than is actually necessary for adequate compaction which could lead to damaging the binder. The same trends are also anticipated when using viscosity measurements to predict mixing and pumping temperatures for modified binders (exhibiting shear thinning) without taking into account the effects of shear rate on viscosity. The use of much lower temperatures will significantly reduce the energy and carbon footprint of these high temperature processes, thus improving the sustainability of the overall handling and use of modified binders. REFERENCES Mezger T.G., 2006. The Rheology Handbook: For Users of Rotational and Oscillatory Rheometers. Vincentz Network (Coatings Compenda), Second Revised Edition, Hannover, Germany. Mturi G., O'Connell J. and Zoorob S.E., 2010. Investigating the rheological characteristics of South African road bitumens. 29th Southern African Transport Conference, Transportation Research Board of the National Academies (TRB), Pretoria, South Africa. Zoorob S.E., Mturi G.A.J. and O’Connell J., 2010. Challenges in rheological characterisation of road bitumens. Southern African Society of Rheology (SASOR) Conference, Cape Town, South Africa. Technical Guideline: TG 1, 2007. The Use of Modified Bituminous Binders in Road Construction. Asphalt Academy, Second Edition, Pretoria, South Africa. National Cooperative Highway Research Program: NCHRP 1-37A, 2004. Guide for Mechanistic-Empirical Design of New and Rehabilitat ed Pavement Structures, Part 2: Design Inputs, Chapter 2: Material Characte rization. Transport Research Board, National Research Council, Illinois, United States.


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Technical Methods for Highways: TMH1, 1986. Standard Methods of Testing Road Construction Materials. National Institute for Transport and Road Research of the Council for Scientific and Industrial Research, Second Edition, Pretoria, South Africa. ASTM D2493, 2009. Standard Viscosity-Temperature Chart for Asphalts. ASTM International, West Conshohocken, PA, United States. Hensley J. and Palmer A., 1998. Establishing Hot Mix Asphalt Mixing and compaction Temperatures at the Project Level. Asphalt, Asphalt Institute, Volume 12, Lexington, KY. Bahia H.U., Hislop W.P., Zhai H. and Rangel A., 1998. Classification of asphalt binders into simple and complex binders. Journal of the Association of Asphalt Paving Technologists, Volume 67, 1-41. Yildrim Y., Solaimanian M. and Kennedy T., 2000. Mixing and Compaction Temperatures for Hot Mix Asphalt Concrete, Report N o. 1250-5. The University of Texas at Austin and Texas Department of Transportation, Texas, United States. Yildrim Y. and Kennedy T., 2003. Calculation of Shear Rate on Asphalt Binder in the Superpave Gyratory Compactor. Turkish Journal of Engineering & Environmental Sciences. Volume 27. 375-381. Delgadillo R. and Bahia H.U., 2008. Effect of temperature and pressure on hot mixed asphalt compaction: Field and laboratory stud y. Journal of Materials in Civil Engineering. Volume 20. Issue 6. 440-448. Read J., Whiteoak D. and Hunter R., 2003. The Shell Bitumen Handbook. Shell Bitumen, Fifth Edition, UK. National Cooperative Highway Research Program: NCHRP Report 648, 2010. Mixing and Compaction Temperatures of Asphalt Binde rs in Hot-Mix Asphalt. Transportation Research Board of the National Academies, Washington, D.C., United States. KEY WORDS Shear Rate, Shear-Thinning, Dynamic Viscosity, Brookfield Viscosity, Compaction Temperatures.

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EFFECT OF SHEAR RATE ON BITUMEN VISCOSITY …aapaq.org/q/2011st/CAPSA2011/FA6_04_185_1114.pdf · Although temperatures at the pumping of bitumen and at the mixing and compaction of