Caracterización reológica avanzada de betunes tradicionales y modificados utilizados actualmente en Chile Advanced rheological characterization of traditional and modified bitumens currently employed in Chile

The purpose of this research was to achieve an advanced rheological characterization and classification for a set of modified and non-modified asphaltic bitumens currently employed in Chile. Six asphaltic bitumens were employed, which are locally produced by three asphalt manufacturer companies. Each company provided a traditional bitumen and a modified bitumen. The three traditional bitumens were classified as CA 24 and, SBS-modified bitumens were classified as 60-80. Rheological characterization tests were carried out by employing a dynamic shear rheometer and a bending beam rheometer on bitumens under original condition; primary ageing (RTFOT) and secondary ageing (PAV). Consequently, performance level, creep and recovery tests, master curves and Black ́s diagrams were determined for each bitumens. Besides, diverse and complete traditional tests were performed to the whole set of bitumens, in accordance to the aforementioned ageing levels. The rheological characterization enabled the determination of bitumen essential properties at high temperatures, intermediate temperatures and low temperatures. These properties together with an adequate climatologically zoning of Chile could be employed to elaborate an updated specification for bitumens in our country. It was also demonstrated that traditional tests are only useful if they are carried out under the following conditions: high temperatures, intermediate temperatures, low temperatures, original binder, primary aged binder (RTFOT) and secondary aged binder (PAV). Only by executing tests under the aforementioned conditions, it is actually possible to achieve information and results comparable to the ones provided by Superpave.


Introduction
Asphaltic bitumen specifications in Chile are of an empirical nature and they consider measurements for an incomplete set of material properties (Code MINVU 2008Highways Manual 2010). Some essential material properties -which are not currently characterized -correspond to behavior at low temperatures, material secondary ageing and essential mechanical properties such as dynamic modulus and phase angle (Delgadillo et al., 2005, Delgadillo et al., 2006.

Under the Fondef D09I1174 project framework: "Development of Asphaltic Resurfacing Materials Employed for Damaged Pavements Restoration", The Federico Santa Maria University implemented an advanced laboratory characterization of bitumen and asphaltic mixes. The equipment employed in this work consists of a Dynamic Shear Rheometer, which was used together with the Bending Beam Rheometer for the advanced characterization of bitumens.
Bitumen characterization is one of the first stages in the project. Bitumens showing the best performance at this stage were selected for their utilization on asphaltic mixes characterization in the second stage, which is currently under execution.

Classification according to performance level (pg xx-yy) superpave 2.1 Test Conditions
Classification tests including all bitumens according to their performance levels, were developed by using the Dynamic Shear Rheometer (DSR) and by following the procedure described in section 8.302.00 of the Chilean Highways Manual, as well as the ASTM D7175-08 standard. Each asphaltic bitumen was tested under original condition and with primary ageing (RTFOT) and secondary ageing (PAV). For  The test stops when it achieves a G*/sin (δ) minimum parameter value and a G*sin (δ) maximum parameter value. G* is the complex shear module and δ is the phase angle.

Master curves
Dynamic modulus is an essential property of asphaltic bitumen and asphaltic mixes. The latter is one of the most relevant input data for flexible pavements mechanistic design by means of the utilization of multi-layer elastic models (Wahr et al., 2008. Up-dated estimation formulas used to calculate dynamic modulus (Christensen et al., 2003, Bari et al., 2006 employ the complex modulus to estimate the mix modulus. Master curves were developed for different bitumens, which were obtained from complex modulus tests at different temperatures and frequencies.

Test Conditions
Master curves were designed considering an asphaltic bitumen under normal conditions, with primary ageing (RTFOT) and secondary ageing (PAV The behavior of complex shear modulus (G*) shows greater variations at low frequencies, but it tends to keep high frequencies. Therefore, each frequency scan was carried out at a logarithmic scale, in order to cover the greatest amount of low frequencies for measurements.
There are not requirements for stresses levels to be applied during a frequency scan test, excepting that they must be at an elastic range and they have to be executed at constant deformation. A temperature at 5°C, frequency at 10 [rad/ seg] and a variable deformation range from 0.1% up to100% were considered for these frequency scan tests. Consequently, traditional and modified asphaltic bitumens show a linear behavior up to 1% of deformation levels. That is the reason why a constant deformation of 0.5% was considered for frequency scan tests.

Elaboration of master curves
They were elaborated at a referential temperature of 20°C, shift-factors were manually determined for each temperature at a 5-55 [°C] range, every 5°C. Figure 1 shows the master curves obtained for each one of the six asphaltic bitumens under original conditions at a logarithmic scale. Primary ageing (RTFOT) and secondary ageing (PAV) graphs are not shown due to space restrictions, but they can be obtained from the original research job (González, 2012).

Comments on Master Curves
Traditional bitumens I and III have a similar behavior, particularly when they have a secondary ageing (PAV). Besides, bitumen III has the highest thermal susceptibility. Bitumen II has high G* values during a great portion of the analyzed frequency range, especially for frequencies lower than 100 [rad/seg].

Modified bitumens I and III have a similar behavior achieving G* values quite similar for all analyzed temperatures.
The modified bitumen II stands out for having the less thermal susceptibility of all tested bitumens, which is an expected characteristic for asphalts.
The ageing on bitumens provokes an increase of stiffness on bitumens, which is observed with the increase of complex shear modulus (G*). There is an outstanding difference when comparing G* values with secondary ageing (PAV) to bitumens under normal conditions. The effect of primary ageing is less evident and in the case of traditional bitumen and modified bitumen III, it is practically insignificant.

Black´s diagrams
Black´s diagrams are phase angle graphs versus complex modulus. They are useful tools for identifying bitumens and modifier agent types. Besides, they enable the identification of potential measurement errors.

Test Conditions
Test conditions are the same as those employed for the elaboration of Master curves because both parameters are obtained from the same frequency scan. Black diagrams were elaborated considering bitumen under original condition, with primary ageing (RTFOT) and secondary ageing (PAV).

Comments on Black´s Diagrams
Black´s Diagram satisfactorily fulfilled its purpose, because it was an effective tool for determining measurement errors (Marastenou et al., 2001, Airey et al., 2002 and it showed the effects of polymeric additives of modified bitumens. The greatest measurement errors source was generated as a consequence of the geometry change of test rheometer (parallel plates of 8 and 25 mm), which is evidenced by observing the Black´s Diagramming for Modified Bitumen mix 60-80 from manufacturer 2, under original conditions. It shows that developed measurements carried out with the 8mm-plate are vertically displaced in regards to measurements performed with the parallel 25mm-plate. An example is shown on Figure  2, where the change of test geometry generates a vertical distance indicated by Black´s Diagram.

Test Conditions
A Dynamic Shear Rheometer (DSR) was employed, which carried out five measurements on each bitumen. This procedure was developed in accordance with ASTM D7405-10a standard. Tests were applied on primary ageing bitumens (RTFOT). This test was executed at a temperature of 64°C, which is considered representative of high temperatures.
In accordance with ASTM D7405-10a standard, the test consists of twenty intervals, each one having a loading cycle and an off-time cycle. The first ten intervals are executed by applying a stress of 100 [Pa] in each loading cycle, while for the following ten intervals a stress of 3200 [Pa] is applied during loading cycles.

Comments on Creep and Recovery Test
There are no significant differences among traditional bitumens and, bitumen II is the one showing a slight better performance. However, there is a significant difference among modified bitumens, where bitumen mix II shows deformation levels lower than 50% compared to the deformation of bitumen I, which is the worst performance.
Modified bitumens show deformations significantly lower than traditional bitumens. This is mainly because of the existing deformation recovery taking place at off-time cycles, while traditional bitumens do not show such recovery.

Traditional tests 6.1 Test Conditions
Traditional tests were executed on the total amount of studied bitumens. In the case of traditional bitumens CA 24, they were tested under three different conditions: original condition, primary ageing (RTFOT) and secondary ageing (PAV). These three ageing stages of bitumen were considered because in this way it is possible to representatively characterize the behavior during the bitumen life span.
In the case of modified bitumens 60-80, modified with SBS, they are also tested under three different conditions: original condition, primary ageing (RTFOT) and secondary ageing (PAV). A storage stability test was also carried out for this bitumen type.
The results are shown on Table 3 and 4, respectively.   (Kandhal 1977) standard, it would indicate a potential lower fatigue cracking performance throughout time. By analyzing absolute viscosity at high temperatures, it can be observed that three bitumens are within a viscosity range of 3000 ± 600 poises, which is the specified viscosity for an AC 30 according to the ASTM D3381-05 standard. As far as yielding point is concerned, the three bitumens showed similar values, with variations of 1°C between the highest and the lowest of them.  (bitumen III,43dmm) and the hardest (bitumen I, 36 dmm), but always within a low range. Bending performance at 15°C is still within an acceptable range for bitumens I and III, but quite lower for bitumen II, which value is at 11. The latter value indicates a high cracking susceptibility in a near future for bitumen II, considering that a value lower than 10 is considered as negative (Kandhal 1977).

Betún Asfáltico Tradicional -CA 24/Traditional
In secondary ageing condition (PAV), penetration differences are also quite similar to RTFOT condition. Once again bending performance of bitumen II happens to be the lowest, delivering a value of 4, which in accordance to the existing literature (Kandhal 1977)

Conclusions
The SuperPave performance characterization enabled the determination of essential properties (complex modulus and phase angle) of bitumens at high temperatures, intermediate temperatures and low temperatures. The three traditional bitumens were classified as PG64-22. Two modified bitumens were classified as PG70-22 and one as PG70-28. However, such difference is not quite significant if the exact temperatures of fulfillment parameters in the specification are considered, instead of the proposed discrete cathegories (-27 y -29°C, respectively). These PG classifications levels, together with a proper climatologically zoning of Chile could be used to elaborate an up-dated bitumens specification for our country.
Master curves elaborated for each bitumens enables the determination of binders stiffness and elasticity at any temperature and loading frequency combination. The complex model of each bitumen can be used, for example, for the estimation of the dynamical modulus of the asphaltic layer in pavement designs by means of elastic multi-layers programs. The master curves also enabled the visualization of thermal susceptibility for modified bitumens.
Black´s diagrams became a very useful tool for starting up the dynamic shear rheometer, because they enabled the development of data analysis and the detection of measurement errors, which were consequently corrected. Besides, by means of these diagrams it was possible to detect the use of SBS modifier agent in modified binders.
Creep and recovery tests enabled us to witness relevant existing elasticity differences between traditional and modified bitumens, when they were subject to repetitive loads, such as road pavement traffic loads. This elastic recovery turns into less permanent deformations on modified bitumens at high temperatures.