Zero Shear Viscosity

In Europe, attention had been paid on a parameter called Zero-Shear-Viscosity (ZSV) to evaluate rutting resistance of binders (Le Hir et al., 2003).

The concept of ZSV emerged based on the assumption that only linear viscoelastic behaviour occurs under wheel loading in rational pavement design. At this condition bitumen deforms slowly without any change in the structure maintaining an equilibrium state. The corresponding viscosity at this stage (ZSV) is independent on the shear rate (Liao, 2007).

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In the DSR creep-recovery testing, the creep compliance J can be divided into three parts (Figure 2.7):

J J J J (2.1)

Where is the strain, and is the constant stress, J is the elastic part, J is the delayed elastic part, and J is the viscous part.

Figure 2.7 ZSV from creep and creep recovery

In the loading steady state region the ZSV is equal to the inverse of the slope, and in the recovery part the ZSV can be estimated from the residual compliance J , mathematically given by (Desmazes et al., 2000):

ZSV lim (2.2)

ZSV lim (2.3)

There are other methods to estimate the ZSV using various test types (creep, creep-recovery, viscometry, and sinusoidal oscillation). The following section reviews important studies with different testing techniques to obtain ZSV.

Desmazes et al. (2000) suggested a protocol of long creep-recovery test to determine a consistent ZSV value. The test involved 4 successive cycles and the ZSV was measured in the creep and recovery parts. If the difference of

Compliance

Time

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ZSV between sequential cycles was more than 20% then the number of cycles was increased up to a maximum of 8 cycles. A range of 30 to 300 Pa shear stresses was applied to two conventional grade bitumens and 20-50 Pa for two styrene butadiene styrene (SBS) PMBs at 40⁰C. For conventional bitumen, the same viscosity value (ZSV) was obtained at 30 and 300 Pa with less than 20% difference between cycles. However, for binder with high modified content it was difficult to decide on the right operational condition as steady state was not reached or the test consumed impractical waiting times.

To avoid this, De Visscher et al. (2004) proposed an alternative method to determine ZSV in a shorter time creep test. For pure 70/100 pen bitumen and for 1 hour loading followed by 1 hour recovery at 500 Pa and 40⁰C, the difference between the ZSVs determined from creep and recovery was found to be only 0.1%. In addition, the difference between ZSV measured within the last 15 minutes and 1 hour was less than 0.2%. For highly modified SBS binder 4 hours waiting time was not enough and the repeatability was poor.

More importantly, the applied high shear stress may exceed the linearity region especially with PMBs.

Giuliani F. (2006) examined the effect of stress and loading time on estimating ZSV in a long creep test within the last 15 minutes. Pure 50/70 pen binder and crumb rubber modified (CRM) binder (16% and 20%

concentrations by mass of bitumen) were tested at 60⁰C with loading times of 1 hour for pure binder and up to 8 hours for the CRM. For pure bitumen, increasing the stress from 10 to 100 Pa, the difference in ZSVs was less than

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20% while a considerable variation was obtained with CRM. Additionally, the difference between the ZSVs calculated at different loading times didn’t stabilise to less than 20% for CRM as shown in Figure 2.8.

Figure 2.8 Comparison between ZSV of CRM at different loading times (Giuliani F., 2006)

Le Hir et al. (2003) compared ZSV values obtained in long creep and frequency sweep tests on different base, SBS, and ethylene vinyl acetate

‘EVA’ (with different contents) binders. The ZSV was measured in frequency sweep under the assumption of loss modulus over angular frequency equals to ZSV at a very low frequency ( G ZSV). ZSV is then extrapolated

as illustrated in Figure 2.9.

Figure 2.9 Extrapolation of ZSV from frequency sweep test results (Anderson et al., 2002)

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Comparison between the ZSVs using different test methods displayed a good correlation (less than 20% difference) except with the modified EVA binder as the properties of the binder changed due to the melting of some EVA crystalline parts during testing. However, at low temperature and for many modified binders the extrapolation is unreliable because occasionally the curve is curvilinear.

Anderson et al. (2002) utilised the Cross model to estimate ZSV from frequency sweep test. The Cross model is given by:

Where is the complex viscosity, is the complex ZSV, K and m are constants calculated by nonlinear regression. Different models were suggested to estimate ZSV, details of these models can be found elsewhere (Liao et al., 2011, Anwar Parvez et al., 2014).

Comparison between different methods (Extrapolation, Cross model, and long creep tests) to estimate ZSV showed a good agreement of less than 20%

difference except for some PMBs whose steady state was not reached in creep test.

However, when the same comparison was made by Biro et al. (2009) for highly modified binder with Sasobit and Asphamin the agreement was very poor and was attributed to the inability of the Cross model to predict ZSV as the curve didn’t reach a plateau even at very low frequency levels (0.01 Hz).

Also Vlachovicova et al. (2007) examined the ZSV values obtained from

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different loading/unloading of creep/recovery times and the variation between ZSV values of SBS PMB was attributed to the time-dependent microstructure attitude.

Thus, although using ZSV concept has been widely utilised, different testing procedures were suggested with a lack of consistency. The fundamental problem with ZSV is associated with highly modified binders which behave as a solid struggling to reach a steady state condition. In addition, each testing technique was customised based on accommodating a specific type of binder rather than a standardised procedure. Therefore, the applicability of these tests is still narrow and limited (D'Angelo et al., 2007a).