PERFORMANCE EVALUATION SYSTEM FOR BITUMINOUS BINDERS

Transcription

1 PERFORMANCE EVALUATION SYSTEM FOR BITUMINOUS BINDERS A. Vanelstraete, W. Teugels, Belgian Road Research Centre, Belgium, Nynas Bitumen, Belgium Abstract Understanding the behaviour and performance of bituminous binders in asphalt mixtures is a well-researched topic. The developments by the US SHRP in the beginning of 90s and the follow-up in Europe have triggered the interest towards performance related requirements of bituminous binders. The present paper presents the results of a common research project between Nynas and the Belgian Road Research Centre (BRRC) with the objective to draft a tentative performance evaluation system for binders. Use was made of a common database of BRRC and Nynas and of literature results. It could be concluded that: Reliable performance indicators can be defined for bearing capacity, permanent deformation and low temperature cracking. They are based on classical parameters, such as penetration, and on rheological measurements. The ranges of each of these indicators could be indicated for a large number of paving grade bitumens, as a function of penetration class. The impact of the indicator on the performance could be determined. A realistic number of performance levels as well as a tentative evaluation system could be proposed. Further research is needed in the field of fatigue cracking, thermal cracking, reflective cracking and resistance to ravelling and fretting. It is concluded that it is too early to define binder performance indicators, mainly because of lack of indicators showing good correlation with field and/or laboratory mix performance. 1. Introduction The SHRP research program in the US has led to the first performance related specifications for bituminous paving binders. In Europe, a first step has been completed by harmonising the existing, largely empirical, national specifications. The second step consists in replacing these specifications by performance related European standards. A new CEN technical committee TC336 was therefore created. Triggered by these actions, Nynas and BBRC started a common research project at the end of 1999 to gather all relevant information and to work out a proposal for a Performance evaluation system for bituminous paving binders. The definition of a performance evaluation system for bituminous binders implies the following consecutive steps: Definition of performance requirements, with selection of relevant requirements for binders. Definition of performance indicators for binders. These are the binder properties related to each of these performance requirements. This step includes the selection of a test method with appropriate test conditions and test results. The impact of the binder performance indicator on the performance requirement. The ranges of values for pure and modified binders. The definition of realistic performance classes. These should take into account the precision of the test method and the impact of the binder indicator on the performance. 2. Requirements for binder and mix The definition of performance indicators implies as a first step that the performance requirements for the binder are clearly defined. Apart from the production requirements (for pumping and storage of binders), they are of course closely related to the performance requirements of the mix. A distinction is made between functional requirements (to ensure the mix its performance on site) and handling requirements (for manufacturing and placing). Table 1 indicates in grey which requirements for the mix are not or are less influenced by the binder. Regarding table 1, it is noted that all functional requirements are subjected to short term ageing (production) and long term ageing (time, temperature, water, UV, oxygen). A trial to make a ranking of the importance of the requirements, made us conclude that this depends on many factors: the climatic conditions, traffic, the position in the road structure, the type of mix, some special circumstances (e.g. the presence of a cracked underlayer, high shear,...), on economic considerations, political decisions,... Although ranking is not possible in a general overview as table 1, it was considered interesting to make a distinction between general functional requirements (these are necessary in all cases) and more specific functional requirements (these are only important under particular circumstances). In [1] we gave an inventory of circumstances making a given general VANELSTRAETE, Performance evaluation, 1/6

2 functional requirement more important and the circumstances leading to the necessity of a given specific requirement. In the same paper [1] a qualitative ranking of the importance of each requirement was made according to mixture type. The requirements for binder and mix defined earlier, with the impact of circumstances, mixture type and changes in mix composition are implemented in a preliminary software developed in Microsoft Visual Basic. Although at this moment only indicative, this software shows how a binder performance system could be used and permits: to rank mixes for given requirements, to select mixes that are most suitable to fulfil given requirements, and to eliminate the mixes with high risks in performance, to give a rating for a binder according to mixture type for a given requirement. Table 1 : Requirements for binder and mix (grey : less or not influenced by binder) PERFORMANCE REQUIREMENTS FOR BINDER AND MIX Production requirements for binder Handling requirements (manufacturing and placing) Pumping Workability and compactibility Storage behaviour (homogeneity) Resistance to segregation Emission level General functional requirements for More specific functional requirements for i i Stiffness Resistance to wear due to studded tyres Resistance to fatigue cracking Resistance to reflective cracking Resistance to low temperature cracking Resistance to shear induced cracking Resistance to thermal cracking Hydraulic conductivity Resistance to permanent deformation Noise absorption Resistance to ravelling fretting Light reflection Cohesive strength (*) Skid resistance (*) global property, however considered as a kind of primary property of a mix, which should be fulfilled before examining the other properties. Also, there is a general feeling that it gives an indication of the binder-aggregate contact. 3. Candidate performance indicators for binders Several functional binder properties have been examined with respect to their correlation with asphalt performance. This was based on literature as well as on exploitation of common data. 3.1 Performance indicators for bearing capacity It was shown in [2] that the stiffness modulus of the mix, needed for structural calculations, can be sufficiently well predicted from the binder stiffness and the mix composition. For pure binders, the binder stiffness can be sufficiently well predicted from its penetration and Ring&Ball temperature (or Penetration Index) and hence are for pure binders candidate indicators to assess the bearing capacity. As a result of the stiffening effect of the polymer, the stiffness modulus of a PmB is higher than that of its base bitumen. However, if stiffness values for PmBs are compared to these of pure binders having comparable penetrations it was found in this project that the stiffnesses of PmBs were comparable to those of the pure binders, for the temperature range considered in structural calculations. For pure binders and PmBs, we found a clear relation (Saal s relation [3]) between the penetration and the binder stiffness for the conditions of the penetration test (25 C and 0.4s) for a large number of binders, available from literature and own data. This is represented in figure 1. VANELSTRAETE, Performance evaluation, 2/6

3 Complex Modulus (G*) (kpa) Figure 1: CRR-OCW G* (25 C Hz) - Penetration at 25 C 100 Pure Bitumen - set 1 Pure Bitumen - set 2 PmB-EVA - set 1 PmB-EVA - set 2 PmB-SBS - set 1 PmB-SBS - set 2 Other Bitumen - set 2 Rubberized - set 2 Saal relation Penetration at 25 C (mm/10) Relation between penetration (25 C, 5s) and G* at the same conditions for a large number of pure and modified binders. However, unlike pure binders, the stiffness of a PmB at any temperature-frequency - combination can no longer be predicted from its penetration value and Ring and Ball temperature. For a more precise characterisation of the PmB, measurements of G* are necessary. For PmBs, the following performance indicators seem therefore relevant for bearing capacity: penetration at 25 C. G*- values, measured at one or more defined temperatures at a given frequency that are relevant for the climatic and traffic conditions considered in structural calculations. The impact of the binder stiffness on the bearing capacity was studied with NYNAS NOAH- software. Two flexible road structures, commonly used in Belgium, one corresponding to very high traffic and one for relatively low traffic, were considered, as well as small variants in layer thickness. Two climatic zones were considered: oceanic and Mediterranean climate. Pure and modified binders belonging to the categories B35-50 and B50-70 have been selected for the study; they are often used for these bituminous mixtures for the climatic conditions considered. A 100 kn single wheel axle load with a 0.7 MPa pressure was taken as loading. 60 km/h was taken as mean traffic speed. The bearing capacity is evaluated through the vertical deformation, ε z, at the bottom of the sandy base. The following evaluation law is used for the calculation of the associated lifetime, N: ε z = 1,1 X 10-2 N -0,23 Figure 2 shows the impact of the binder type on the bearing capacity, for oceanic climate and high traffic road structures. The study led to following conclusions: The use of binders (pure binders or PmBs) with the same penetration, but with different temperature susceptibilities corresponding to realistic extreme values, leads to a maximum difference of a factor of two in lifetime or to a maximum difference of 2.5 cm in asphalt thickness (compare B50-54 to B50-48). Binders belonging to the same penetration class, but corresponding to extreme values within a given penetration class lead to a difference in lifetime of roughly a factor of 2 to 3. This corresponds to 2 to 3 cm in asphalt layer thickness. The impact of the binder is largest for thick bituminous pavements Number of load applications Total asphalt thickness (cm) Figure 2: Impact of the binder on the bearing capacity for the heavy trafic road structure and oceanic climate. VANELSTRAETE, Performance evaluation, 3/6

4 3.2 Performance indicators for resistance to permanent deformation It is essential to realise that the resistance to permanent deformation of a given mix depends on the complete mix composition. Besides the type and quantity of binder, the granular skeleton and the interaction between binder and aggregates are important. Hence, a prediction of rutting behaviour based on the binder properties alone is impossible. From the literature study, it can be concluded that G*/sinδ - values for binders correlate well with rutting test results (of course for the same mix composition), except for binders with a high delayed elasticity. Their resistance to permanent deformation is underestimated. It is more and more believed that the behaviour of binders towards rutting should be studied by preference by tests evaluating the long time-scale behaviour, e.g. in creep flow or creep-recovery experiments. In particular, measurements of the dissipated viscous deformation, as reflected in the zero shear viscosity, seem relevant. The results are however still limited in this field. G*/sinδ at a high temperature or, alternatively, the temperature at which G*/sin δ equals 1 kpa are candidate performance indicators. The advantage of the latter temperature parameter is that it shows less dispersion. The disadvantage of the temperature parameter is that the tests are more time consuming, because results are needed at different temperatures. A reproducibility standard deviation on G*/sin δ of % was found in the high temperature range [4]; this corresponds to a precision of about 1.5 C for the temperature parameter. This is comparable to the precision of the softening point R&B test. A plot of G*/sin δ - values at high temperatures (e.g. 60 C and 1,5 Hz) as a function of penetration [5] shows that for the same penetration, the G*/sin δ - value of a PmB can be up to a factor of five higher than for pure binders. Taking into account the precision of the correlation between G*/sin δ and rutting behaviour on the one hand and the variation of G*/sinδ - values on the other hand, leads to the conclusion that maximum three performance levels could be defined, for a given penetration class. 3.3 Performance indicators for resistance to fatigue cracking One of the major problems related to the fatigue phenomena is that the results of laboratory fatigue tests vary largely with the applied test methods and testing conditions. Due to the varying effects in the fatigue test methods, mix characteristics, traffic loads, rest periods/healing and environmental variables, the relevance of the prediction of the field fatigue is often limited. Methods of measuring fatigue on binders have been proposed. Further laboratory studies and field validations are however required, to show whether these tests accurately measure the fatigue-related performance properties. Several binder properties have been considered, but none of these parameters correlated well with mix fatigue. Therefore, it can be concluded that there is currently no fatigue related binder property available. 3.4 Performance indicators for resistance to low temperature cracking From the literature study it results that the bending beam rheometer test seems to be the most appropriate test to predict low temperature behaviour, for unmodified as well as for polymer modified binders. The highest limiting temperature resulting from S(60s) = 300 MPa or m(60s) = 0,3 can be used as performance indicator. It seems that it is too early to consider the direct tensile test results, as more data are needed. Fraass breaking temperature cannot be used to predict low temperature cracking. Even for unmodified binders only a very weak trend with BBR and mix fracture properties was observed. Figure 3 shows the BBR-limiting temperatures for a large variety of unaged binders as a function of penetration. The data result from a literature study and from own available data. Detailed information can be found in [6]. It can be concluded that: The limiting BBR-temperature changes with penetration class: the higher the penetration the lower (more negative) the limiting temperatures. Going from a penetration 10 to 100 leads to changes of 15 C to 20 C in limiting BBRtemperature. Large differences in the limiting BBR-temperature of about 10 C are observed within a given penetration class. If binders of the same penetration class are compared, unaged polymer modified binders generally show lower (more negative) limiting BBR-temperatures than pure binders. PmBs would therefore perform better at low temperatures than pure binders of the same penetration class: the differences in limiting BBR-temperatures are however limited and 5 C at maximum. This effect seems less pronounced after ageing [6], but insufficient data are available at this moment to conclude. There is no clear trend when polymer addition is favourable, it depends greatly on the polymer-binder pair. Taking into account the precision [4] and the variations within a given penetration class, a maximum of three performance classes could be defined per penetration class. VANELSTRAETE, Performance evaluation, 4/6

5 Highest limiting BBR - Temperature ( C) CRR-OCW No PMB, unaged not blown PMB, unaged Penetration Figure 3: Highest limiting BBR-temperature versus penetration 4. Conclusions In this project an inventory was made of the necessary requirements for the mix that are affected by the binder. Because of the various factors involved in assessing the influence of the binder on the final mix performance, a binder specification system can only be defined in combination with application guidelines connecting the binder to the mix and other circumstances. An inventory was therefore made of the circumstances that make certain requirements more important. A qualitative rating per requirement was also made, according to mixture type. A preliminary software has been developed. Concerning the binder indicators, we came to following conclusions: Preferably, the binder properties should be measured on material after short-term ageing. However, at this moment supportive data are insufficient. Reliable performance indicators can be defined for bearing capacity, permanent deformation and low temperature cracking: - G* for bearing capacity (intermediate temperature range). - G*/sinδ for rutting (high temperature); zero shear viscosity is to be considered as a very interesting alternative, but research is needed. - BBR limiting temperature (low temperature). Further research is needed in the field of fatigue cracking, thermal cracking, reflective cracking and resistance to ravelling and fretting. It is too early to define binder performance indicators for these requirements, mainly because of lack of indicators showing good correlation with field and/or laboratory mix performance. A basic outline for a potential binder specification framework, based on functional binder properties as given in table 2 can be proposed. As basic class indicator represented in the first column, several parameters can be chosen. However, taking into account that current standard asphalt structures are generally designed based on a certain penetration range, one should consider this. Within each (penetration) class, and for each binder indicator represented in the first row, several sub-levels could be defined. These sub-levels distinguish different levels of performance within a same binder (penetration) class. The number of these sub-levels is depending on the relevance of the property towards performance, taking into account the test precision, the correlation, the range within a given class, the impact of the parameter on performance. For example, according to the conclusions of this study: G*/sinδ as high temperature binder indicator with 2 to 3 levels within a penetration range of 30 mm/10 ; Limiting BBR-temperature as low temperature binder indicator with 2 to 3 levels within a penetration range of 30 mm/10. For the definition of the various levels, it is yet too early, as there is no uniform test protocol for the various more complex test methods. This process of harmonisation of the test methods is ongoing within CEN. Table 2: Scheme of a potential binder specification framework High Temperature Low Temperature Property Property. Other Functional Binder Property Cla ss A Z B VANELSTRAETE, Performance evaluation, 5/6

6 M Acknowledgments The authors thank B. Eckmann (Eurovia - France), but working for NYNAS at the start of this project) for the interesting discussions and ideas put forward as the initial leader of this research project for NYNAS. Many thanks also to H. Soenen (NYNAS) for working out the low temperature part and to T. Tanghe (NYNAS) for the calculations with the NOAH software. Thanks to P. Peaureaux (BRRC) for his help in setting up the database. References 1. Vanelstraete A. and Teugels W., Performance requirements for binder and mix, Paper prepared for Kielce - conference, Francken L., Vanelstraete A. and Verhasselt A., Long term ageing of pure and modified bitumen: influence of the rheological properties and relation with the mechanical performance of asphalt mixtures, ISAP-Conference, Seattle, Saal R. N. J., Labout J. W. A., Rheological properties of asphalts. Rheology (Ed. Eidrich) vol II CH9 pp , Academic Press Inc. New York, Vanelstraete A., Sybilski D. and others, Results of the RILEM round robin test on bituminous binder rheology, Eurasphalt & Eurobitume Conference, pp , Nynas-BRRC Research Cooperation, The design of performance evaluation system for bituminous paving binders, Soenen H. and Vanelstraete A., Performance indicators for low temperature cracking, this conference, VANELSTRAETE, Performance evaluation, 6/6