3.3.1 Sample Preparation
According to the above discussions, the sample preparation has enormous effect on the viscosity of the asphalt mastics. Therefore, to reduce errors in measuring the viscosity of the mastics as well as creating the possibility of comparison, the sample preparation should be standardized. Because of the importance of the volumetric characteristics of the fillers and bitumen, the amount of filler content was calculated by volume instead of weight.
To make sure for all samples that the filler size distribution is the same and segregation does not have any effect on the filler size, sufficient amount of filler for all samples were collected and stirred to ensure a representative filler distribution. Then, the filler was divided into different portions for the mastic samples. At that point, the filler was placed in an oven for 24 hours to get completely dry and also heated up to the mixing temperature. To mix the bitumen with the filler, the bitumen was also heated to the mixing temperature. For this 200 g of bitumen was placed in the oven for one hour.
All mastic samples were mixed at the same temperature (140˚C), to reduce the temperature effect of the mixing procedure. This temperature was chosen such that it would be appropriate for mixing the bitumen and fillers at all percentages. Mixing of the bitumen with the fillers was done with a mechanical high shear mixer. During this process attention was placed on creating a homogeneous mastic and avoiding filler agglomeration as much as possible. To prevent adding air bubbles into the mix, the filler was gradually spread in the bitumen during the mixing. The mastic was kept in an oven at 140˚C for 2 hours to give the samples time to release any remaining air bubbles. To ensure a homogeneous mixture, the mastic was mixed again at the relevant test temperature before pouring mastic into the cup of the viscometer.
3.3.2 Test conditions
As a test temperature, 100˚C was chosen. This temperature was selected as the optimum temperature viscosity test for bituminous mastic after several trials. The main reason to note this as the optimum is that, at this temperature, the filler sedimentation speed is moderate and the mastic has more time to be tested as a homogeneous material and still give a repeatable result. Furthermore, at this temperature, all tested mastics had sufficient fluidity, even at high filler concentrations. Finally, the tested mastics still show behaviour in the hydrodynamic regime at this temperature [14].
The used viscometer must be able to apply a constant shear stress in each measurement since, for a Newtonian material, at each shear stress only one shear rate, and consequently, one viscosity will be determined. So for a correct reading it is essential to apply a constant shear stress when the instrument is collecting data. The range of the applied shear rate was chosen to be appropriate to the mastic’s flow behaviour.
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3.3.3 Test set-up
To satisfy all the above conditions, a rotational co-axial viscometer was utilized. Due to varying flow behaviour of the tested mastics with the different filler concentrations, it was considered that accurate measurement should be possible when mastics display a different type of behaviour such as Bingham flow, shear thinning or thixotropy.
The geometry and the gap between inner and outer cylinder were chosen very carefully to avoid the influence of the boundaries on the measurements. The DIN standard (3219:1993(E)) gives the geometry of inner cylinder (rotor) for Newtonian materials (Figure 5). In this standard, the equations used to convert the raw experimental results to viscosity are based on the assumptions of dealing with a Newtonian material. From these assumptions, the limitations of using the cylindrical rotor are considerable. As long as mastics behave like a Newtonian material, the shear gradient can be considered linear in the gap of the coaxial cylinder. And the radiuses of inner and outer cylinder were chosen such as to create a 1.0 mm gap to reduce the plug zone.
When increasing the filler concentration, mastic starts to behave more non-Newtonian.
From the particular filler concentration at which this behaviour is noted, the use of the cylindrical rotor to measure the viscosity is no longer appropriate since a boundary bias starts to occur and due to difficulties for the inner cylinder to be placed inside the stiffer mastics. In addition to these, slippage on the wall of the inner cylinder must be prevented, the risk of which becomes quite high for the cylindrical geometry at the higher concentration.
For these reasons, a vane shaped rotor (Figure 5-d) was chosen in this study for the higher filler concentrations. The range of filler concentrations for using the two rotors can vary from one type of mastic to the other and it is highly depended on the filler size. From the tested mastics, however, most of the mastics showed that around 20% filler concentration by volume can be a suitable boundary for shifting from cylindrical rotor to a vane shaped rotor.
This threshold could also become a material parameter which later on could be used for comparison and/or design of mastics.
To avoid the slippage on the wall of outer cylinder as much as possible, an outer cylinder with small grooves on the inner wall of the cup was used (Figure 5-b). Considering the important effect of temperature on the asphalt mastic viscosity, having an accurate control on the temperature is essential for any viscosity measurement. For this reason, the whole system was placed in the Peltier Concentric Cylinder Jacket to provide the required temperature. The Peltier jacket with associate cooler can provide a range of temperature from 0˚C to 150˚C.
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Figure 5. a) The TA Rheometer (viscometer), b) grooved cup, c) cylindrical rotor and d) vane shaped rotor.
3.3.4 Viscosity measurements
The procedure for running the viscometry test was defined according to the mastic properties, after running some simple tests. With both the cylindrical and vane shaped rotors, tests were performed on shear rate control from 5 × 10−3 rad/s to 0.4 rad/s for most tests and were varied occasionally for some samples to optimize the measurement. The continuous-ramp procedure was designed into two steps, with first a continuous increase of the shear rate, followed by a continuous decrease.
As mentioned earlier, at elevated temperatures the particles have a tendency to sediment rather rapidly. For this reason, the testing time was held as short as possible to avoid sedimentation of the fillers in the bitumen. Effectively, each ramp took 120 seconds during which period 180 measurements for the shear rate sweep were made. For each mastic type, at least three tests were done to ensure that a repeatable and reliable result was found.
Eventually, the relative viscosity for all samples at any given shear rate were computed by normalizing the viscosity of sample with the viscosity of the neat bitumen.
3.4 Summary
In this chapter, first the need for a test to measure the viscosity of the asphalt mastics at high temperatures was discussed. Secondly, focus was places on identifying the dominant factors that may impact the test results and that need to be controlled in the protocol. In this, the stages of sample production and test procedure should be carefully controlled.
Additionally, the geometry of the viscometer must be such that it is consistent with the nature of the asphalt mastics.
According to the developed protocol for the samples preparation, filler samples must first be thoroughly homogenized to avoid inconsistent filler gradations in subsequent tests.
Samples should be mixed at a temperature at which i) mastic is totally fluid and ii) it is possible to produce all mastics, specially those with higher filler concentrations. Mixing time
(a) (b) (c) (d)
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should be long enough to ensure the homogeneity of the mastics (i.e the distribution of the filler inside the mastic) and kept constant for all samples. Mastics at the higher filler concentration are expected to behave as non-Newtonian materials. Thus choosing the correct geometry of the testing equipment that can capture this is rather important. In the developed protocol, a coaxial concentric cylinder is suggested for measuring the mastic at higher temperature. Based on extensive investigations with a wide variety of mastic types, it was concluded that the rotor used should be adjusted to the filler level. As will be further explained in the next chapter, the results showed that at low filler concentration a cylindrical rotor is appropriate and at higher filler concentration a vane-shaped one should be chosen.
The concentration level at which the rotor type should be changed, was identified as the critical concentration and was seen at approximately 20% for all tested mastics (also discussed in detail in the next chapter). In addition, to avoid the zero shear zone (or plug zone), the gap between the inner and outer cylinder should be as small as possible and is suggested at 1mm.
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Discussion of Experimental Results
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4 Discussion of Experimental Results
To investigate the effect of the fillers on the mastics viscosity, an extensive laboratory program was performed. Filler types were carefully selected to investigate the influence of their specific characteristics (e.g shape or mineralogy). After the calibration and validation procedure, an extensive set of samples were tested for their viscosity as a function of filler concentration and types. Additionally, X-Ray computed tomography was utilized to investigate agglomeration potential of the fillers inside the mastics, which can lead to effectively different filler size.