High temperature properties of modified bituminous binders produced in Poland

. Bitumen used for the construction of road pavements should exhibit adequate resistance to loads at high operating temperatures. An effective method of improving the properties of bitumen is their modification. To evaluate the high-temperature properties of bitumen, standard consistency tests and advanced rheological tests under dynamic load are conducted. The main purpose of the article is to assess the high temperature properties of modified binders produced by two major Polish producers. The assessment was based on the tests results of modified binders in the range of the standard softening point temperature test and tests in the dynamic shear rheometer (DSR) according to the requirements of Strategic Highway Research Program (SHRP) and Multiple Stress Creep Recovery Test (MSCR). It has been shown that due to the highest operating temperatures of road pavements in Poland, produced bituminous binders meet the requirements of resistance to permanent deformation with a great reserve.


Introduction
Bitumen used for the construction of road pavements should have an appropriate quality that ensures durability of the road construction [1]. The properties of bitumen as viscoelastic materials depend on the temperature and load time and vary from elastic at low temperatures and/or short load to viscous at high temperatures and/or long load times [2,3]. Bituminous binders characterized by inadequate high temperature properties may contribute to the destruction of the pavement with permanent deformations [4,5]. The most frequently used method of improving high-temperature properties of bitumen is their modification with polymers. These binders are commonly used in Poland for road construction that carry a high and very heavy traffic load [6,7].
To evaluate the viscoelastic properties of bitumen, the standard tests of consistency and cohesion as well as advanced rheological tests are used. One of the better methods of investigating properties at high operating temperatures is the dynamic load test, which allows to determine complex modules G* and the phase angle δ between stress and strain. The expression G*/sinδ allows to assess the stiffness of binders in terms of deformability at high temperatures in the summer and to avoid permanent deformation of the pavement at these temperatures or under long-term load, a higher shear modulus G* and a lower phase angle δ, i.e. binder should have a high proportion of the elastic part [8,9]. The method to evaluate these properties of binders is a test in the dynamic shear rheometer (DSR). According to the Superpave assessment criteria, bitumen with high resistant to the permanent deformation at the highest operating temperatures should meet the requirements G*/sinδ≥1.0 kPa for non-aged asphalt and G*/sinδ≥2.2 kPa for short term aged binders by RTFO procedure [3,9].
For modified bitumen, a better assessment of high temperature properties is a Multiple Stress Creep Recovery Test (MSCR) [10,11]. The MSCR test is characterized by high temperature properties of bitumen, the effectiveness of bitumen modification with polymers and allows the determination of the maximum functional type of PG [12,13,14]. The MSCR test allows to determine the influence of bituminous binders on the resistance of asphalt mixtures to permanent deformation in road pavement [15], depending on the category of traffic load, which is expressed as follows [16]: • S -standard traffic level, road pavement with a load less than 10 million ESAL (equivalent single axle loads), roads equivalent to traffic categories KR1, KR2, KR3 and partly KR4 may be considered equivalent in Poland, • H -heavy traffic level, load of the road pavement with traffic from 10 to 30 million ESAL, which corresponds in Poland to the traffic category KR4 -KR5, • V -very heavy traffic level -traffic load exceeding 30 million ESAL, which corresponds to the KR5 -KR6 traffic category in Poland, • E -extreme heavy traffic level -traffic load exceeding 30 million ESAL with a standing traffic loading -speed below 20km / h, which may correspond to the KR6 -KR7 traffic category in Poland.
In 2009-2011, on order for the Polish General Directorate for National Roads and Motorways, the Warsaw University of Technology carried out research on the technical characteristics of road and modified bitumens produced in Poland. On the basis of the results of the research a high variation in the properties of binders was found, both in individual types of binders as well as in producer groups. Part of the tested modified binders was characterized by a non-homogeneous phase of polymer dispersion in microstructure research and unfavourable elastic properties (the value of resilience below 50%) [3]. In the questionnaire survey carried out as part of the research project RID -Development of Road Innovation "Road asphalts and modified in Polish climatic conditions", road works contractors and investors also pointed to problems related to the quality of polymer modified binders produced [17].

Objective and scope of research
The main purpose of the work presented in this paper was to assess the properties of high temperature modified bitumens produced by two major Polish producers. The evaluation was based on the results of tests of modified binders in the range of the standard softening point temperature by R&B method and the tests under dynamic load in the DSR dynamic shear rheometer according to the requirements of SHRP and MSCR.
The softening point temperature of modified bitumen was determined in accordance with PN-EN 1427. The DSR test performed in accordance with the PN-EN 14770 standard defines a complex G*modulus and phase angle δ between stress and the strain. The viscoelastic properties of the binder were tested under a sinusoidal (cyclic) load at various temperatures from 10 °C to 94 °C. Two types of samples of the same binders were tested, i.e. original binder and aging binder according to RTFO. The weight of the test sample was 1 g, the angular velocity 10 rad s =1.59 Hz. The diameter of the plate φ and the thickness of the adhesive layer d is for the original binders and binders after aging RTFOT equal to: φ25 mm, d=1.00 mm. The MSCR test was performed in a dynamic shear (DSR) rheometer according to the AASHTO TP70 standard procedure (parallel plates with a diameter of 25 mm, 1 mm gap). At 64 °C, adopted as the highest seven-day average by type of functional PG in Poland, samples of asphalt binders after RTFO short-term aging were subjected to a repeated constant load of 0.1 kPa and 3.2 kPa in 1 second and then after each load cycle the material was relaxed during 9 seconds. A single cycle lasted 10 seconds and was repeated ten times.
The multiple stress creep recovery (MSCR) test determined the percentage average elastic recovery R and the non-recoverable creep compliance Jnr, which were calculated according to formulas 1, 2 and 3 R = ∑εri(τ,N)/10 (1) In those formulas the following symbols are used : τ -stress, N -number of cycles, εri =(ε1i -ε10i)*100 ⁄ ε1i -relative strain increment after the one cycle ε1i -strain increment after the first second cycle (at the end of the phase of creep) ε10i -strain increment after the 10 second cycle (at the end of the relaxation phase).
In order to evaluate the resistance to permanent deformation of bituminous binders according to AASHTO MP 19-10, the requirement for the percentage difference of the non-recoverable creep compliance part of the Jnr,diff calculated baseds on the Jnr test results at a stress of 3.2 kPa and 0.1 kPa (4) was also included.
According to the AASHTO MP 19-10 standard, the content of polymer in modified asphalt was evaluated (Fig. 1) [13]. Figure 1 shows the dependence of the percentage average elastic recovery R on nonrocoverable creep compliance Jnr, a curve with the equation R=29.371·Jnr -0.2633 was applied. Bituminous binders whose percentage average elastic recovery at 3.2 kPa will assume values from the set above the curve, contain the appropriate amount of polymer and characterize the modified binder. Whereas bitumens, whose percentage average elastic recovery assumes values from the set below the curve, characterize unmodified or polymer modified binders in insufficient quantity.

Results and analysis
The results of the softening point temperature by R&B method for binders before aging and after RTFO aging are shown in Table 1. It can be noticed that in groups of hardness of modified bitumens there is a high variation in the results of the tests of binders, both before and after aging, ranging from 5°C to 10°C. It should also be stated that modified bitumens meet the standard requirements in terms of softening point temperature.
The determination of the complex modulus G* and the phase shift angle δ between stress and strain as a function of temperature is a good assessment of the viscoelastic properties of the bituminous binders. The value of phase angle δ close to 0°C indicates the total stiffening of the binder, which leads to fracture toughness temperature. At high operating temperatures due to the possibility of liquefaction of the binder, the phase shift angle should not reach 90°C.
The results of the complex module δ and the phase angle tests are presented on Fig. 2. and Fig. 3.   Fig. 2. Dynamic modulus G* as a function of temperature  The table 3. shows the requirements for resistance to permanent deformation of bituminous binders determined based on the MSCR test. Generally, it can be stated that the percentage average elastic recovery R determines the elastic properties of the binder, whereas the non-recoverable creep compliance Jnr is characterized by the binder's susceptibility to the formation of permanent deformations. The higher value of the no recoverable creep compliance results in lower resistance to permanent deformation. Requiring a percentage difference of the no recoverable creep compliance part Jnr,diff expresses the binder's sensitivity to load change (it is recommended that it is not greater than 75%), and the lower Jnr,diff value in the MSCR test is beneficial.
Based on the results of the tests given in the table 2, in terms of elastic properties (R), the tested binders can be classified in the following groups from more to less resistant on permanent deformation:   (table 2.) it should be stated that all tested binders shown very low values of the Jnr modulus (less than 0,5 kPa -1 ). Among these binders, the highest susceptibility to accumulating permanent deformations exhibits PMB 45/80-55 binder. On the other hand, the highest resistance to permanent deformations among the tested modified binders is characterized by PMB 25/55-60, previously included in the group with high elasticity. Similarly, the PMB 45/80-65 binder (Jnr3.2 = 0.05 kPa-1), which showed the highest elasticity among the modified bitumen tested, should be highly evaluated in terms of resistance to deformation. Binder PMB 25/55-60 CR, PMB 45/80-55 CR and PMB 10/40-65 can be classified as modified binders with medium resistance to permanent deformation of modified binders. When comparing Jnr test results at 3.2 kPa-1 with AASHTO MP 19-10 requirements, all tested binder should be qualified for resistance to permanent deformation as useful for road surfaces for class E of extreme traffic loads. Fig. 6. presents a graphical classification of the tested binders on the basis of the percentage average elastic recovery R in function of no recoverable creep compliance Jnr. Fig. 6. shows that all modified bitumens, in addition to the PMB 10/40-65 binder, have a sufficient elastomer content. Percent elastic deformation for the PMB 10/40-65 binder is below the curve determined by the formula y = 29.371x -0.2633 and therefore the binder should be classified as modified bitumen with insufficient elastomer content.

Conclusions
Based on the conducted tests and analysis of the polymer modified bitumen, the following conclusions can be drawn: 1. In Poland, binders are produced of various hardness as modified with SBS polymer and modified both by polymer and rubber additives. Tested binders have been characterized by appropriate properties in the range of softening point temperature assessment, both before and after short-term aging.
2. Viscoelastic properties of modified binders, determined by the complex modulus and the phase angle, indicate high resistance of these binders to permanent deformation.
3. According to the MSCR evaluation, all tested modified binders should be qualified in terms of resistance to permanent deformation as useful for road pavements for E category of extremely high traffic. 4. In terms of the highest operating temperatures of road pavements in Poland, produced bituminous binders meet the requirements of resistance to permanent deformation with a great reserve.