ASPECTOS ÉTICOS “ NO CORRESPONDE”

9.3.1 Permanent Deformation 9.3.1.1 Oscillation ZSV Test

Although the oscillation test has shown good correlation, the conditions have not been fixed, particularly for equiviscous temperatures. Therefore, the exact test conditions (temperature and frequency) need to be established that would give results that correlate best with asphalt rutting. The following guidelines would be used as the starting point:

• The tests should find the equiviscous temperature for a zero shear viscosity of 2000 Pa.s.

• Alternatively, the test temperature would be as close as possible to the temperature at which rutting occurs.

• The test frequencies would be between 0,01 Hz and 0,001 Hz, which have led to good correlations whilst still being practical.

The investigation would involve identifying a selection of sites with known level of traffic and histories of deformation achieved for which the precise aggregate skeleton and binder are known. Ideally, the aggregate skeletons would be identical whilst the deformations and the binders would differ significantly. A series of oscillation ZSV tests would be carried out under differing test conditions and the results analysed to maximise the correlation between the test results and the observed deformations after allowing for the traffic. If, as expected, there are a range of aggregate skeletons, some standard asphalt tests for deformation resistance (wheel tracking and/or cyclic compression) would be needed to identify the contribution of the skeleton and, hence, allow for it when maximising the correlation.

If insufficient field data can be found, the site data would have to be developed using accelerated load testing (ALT) of sites or trial lengths with known mixtures. However, ALT will not identify any effect of changes of the binder properties with time. There is a strong possibility that ALT will be necessary.

9.3.1.2 Creep ZSV Test and/or Repeated Creep Test

If the oscillation ZSV test is not taken up as the measure for permanent deformation for any reason, the best alternative is the creep ZSV test with the repeated creep test as an

interesting candidate. However, these tests require that the procedures are refined and a precision exercise undertaken on the finalised procedure for them to be viable tests.

The creep ZSV test would need to be repeated on a variety of binders under differing conditions, in particular duration and deformation. The comparative performance of those binders in asphalt mixtures, based on standard asphalt tests for deformation resistance (wheel tracking and/or cyclic compression), would be used to optimise the conditions for relevance, practicality and precision. An inter-laboratory procedure would be undertaken using at least six binders (three PMBs with a high polymer content) in order to assess the resulting repeatability and reproducibility.

The repeated creep test would need an inter-laboratory precision exercise with at least six binders (three PMBs with a high polymer content) included in order to assess the resulting repeatability and reproducibility.

9.3.2 Stiffness

There is sufficient data to validate the relationship between DSR binder stiffness and the mixture stiffness, particularly when using the same temperature and frequency conditions.

However, the durability implications and any relationship to pavement performance are effectively missing. Therefore, there is justification to undertake further research in order to understand the measures required to identify long-term changes in mixture stiffness from binder properties.

Ideally, a series of aggregate skeletons with a number of different binders would be laid on a road in a series of trial section over identical formations. The structural stiffness would be monitored regularly on core or other samples and any change or differences related back to DSR measurements made on the binders made unaged, after RTFOT and after RTFOT and PAV and/or RCAT. However, this procedure would take many years to produce the results.

Therefore, existing sites would need to be found with the results and with known binders that could be replicated for the DSR tests. It is uncertain how much data could be obtained from such sites.

9.3.3 Low Temperature Cracking 9.3.3.1 Direct Tensile Test

Despite promising results, the precision of the DTT still remains to be improved. Therefore, the test would need to be repeated on a variety of binders under differing conditions. The comparative performance of those binders in asphalt mixtures, based on standard asphalt tests for low temperature cracking (TST, RT, TSRST and/or UTST), would be used to

optimise the conditions for relevance, practicality and precision. An inter-laboratory precision exercise would be undertaken using at least six binders (including three PMBs with a high polymer content) in order to assess the resulting repeatability and reproducibility.

9.3.3.2 Critical Cracking Temperature

The concept of critical cracking temperature, which is a combination of BBR and DTT results to determine a low-temperature parameter called the critical cracking temperature, Tcr, has shown great promise but has still to be validated.

Sites with known asphalt mixtures would have to be found that have developed low-temperature cracking, ideally together with similar sites using different mixtures and/or binders where low-temperature cracking has not developed. The data that can be collected on the in service performance would be compared with the binder properties to ascertain the extent to which the tests can be used to predict low-temperature cracking.

If insufficient field data can be found, the site data would have to be developed using ALT of sites or trial lengths with known mixtures. However, ALT will not identify any effect of changes of the binder properties with time. There is a strong possibility that ALT will be necessary.

An inter-laboratory precision exercise will not be required because the precision should be calculable from the precision of the BBR and DTT results used to determine the value.

9.3.3.3 Fracture Toughness Test

Data from the fracture toughness test is limited but the fracture parameters do appear to be independent and, thus, may be complementary to other reasonable low temperature binder properties, such as the parameters from the DTT. Research is needed to establish how both types of parameters can be combined to predict the low temperature behaviour of asphalt mixtures.

At least three different aggregate skeletons with six different binders (including three PMBs with a high polymer content), giving a total of 18 mixtures, would be tested for the standard asphalt tests for low temperature cracking (TST, RT, TSRST and/or UTST). The binders would also be tested for fracture toughness and DTT. The relevant parameters from Fracture Toughness and DTT would be correlated to the asphalt properties to identify if a combined parameter could be used to give a better predictor and, if so, whether the extra test time/cost is worth the improvement gained.

If the research is positive, an inter-laboratory precision exercise would be undertaken using at least six binders (three PMBs with a high polymer content) in order to assess the resulting repeatability and reproducibility for the fracture toughness test.

9.3.3.4 Field Performance

With regard to both the critical cracking temperature (BBR and DTT) and the FTT parameters, there are insufficient data on their correlation with the field performance and further research would be necessary before any definitive conclusion can be drawn. In particular, data are needed on the performance characterisation of modified bitumen.

Sites with known asphalt mixtures would have to be found that have developed low-temperature cracking, ideally together with similar sites using different mixtures and/or binders where low-temperature cracking has not developed. The data that can be collected on the in service performance would be compared with the binder properties to ascertain the extent to which the tests can be used to predict low-temperature cracking.

If insufficient field data can be found, the site data would have to be developed using ALT of sites or trial lengths with known mixtures. However, ALT will not identify any effect of changes of the binder properties with time. There is a strong possibility that ALT will be necessary.

9.3.4 Fatigue Cracking

Further research is needed for a more explicit correlation between the bitumen fatigue and mixture fatigue at the number of cycles to achieve a 50 % reduction in G*. Validation with field performance is critical because there is a lack of information relating laboratory fatigue behaviour with performance in practice.

Asphalt mixtures with a wide range of polymer types and polymer contents, as well as paving grade bitumen, would be manufactured and tested for asphalt fatigue using the two point trapezoidal and four point bending regimes. The results for the number of cycles to achieve a 50 % reduction in G* would be correlated with the similar value for the binder tested for bitumen fatigue. Where possible, experience from sites with the mixtures tested should be compared with the test results, but there are unlikely for many such sites.

9.3.5 Adhesion

9.3.5.1 Control Aggregates

Adhesion is a property of both the binder and the aggregate to which the binder has to adhere. Therefore, any test needs to involve an aggregate, either the actual aggregate to be used or standard aggregates that can be used to develop a ranking for binders. For a European, or other international, standard, any standard aggregate could not be limited to a single quarry or location. Therefore, they need to be either a mineral (e.g. silica), a

manufactured product (e.g. aluminium oxide) or a widely available aggregate type (e.g.

limestone), any of which could be restricted by demanding specific properties.

Different options for one or more standard aggregates would be investigated by selecting several sources of the option and undertaking the candidate adhesion tests with the same binder to identify whether the range of sources added excessive uncertainty in the result. If the majority of the results produce consistent results but there are outliers, the reason for those outliers would need to be ascertained and limits applied to the aggregate that would exclude them.

9.3.5.2 Field Performance

The majority of adhesion tests lack data on their correlation with the field performance and further research would be necessary before any definitive conclusion can be drawn. In particular, data are needed on the performance characterisation of modified bitumen.

Sites with known asphalt mixtures have been found to develop adhesion problems would need to be found, ideally together with similar sites using different mixtures and/or binders where adhesion problems have not developed. The data that can be collected on the in service performance would be compared with the adhesion test results to ascertain the extent to which the tests can be used to predict adhesion and its retention.

If insufficient field data can be found, the site data would have to be developed using ALT of sites or trial lengths with known mixtures. However, ALT will not identify any effect of changes of the binder properties with time. There is a strong possibility that ALT will be necessary.

9.3.6 Proposal for Further Work

The items of further work described above can be categorised into three levels of desirability, together with an idea of the time required for the work as short-term (1-2 years), medium-term (2 to 5 years) or long-medium-term (over 5 years), as follows:

Essential:

Deformation resistance Oscillation ZSV test Section 9.3.1.1 Short-term Low temperature cracking Critical cracking temperature Section 9.3.3.2 Medium-term Adhesion Control aggregates Section 9.3.5.1 Medium-term Important:

Low temperature cracking Direct tensile test Section 9.3.3.1 Medium-term Low temperature cracking Fracture toughness test Section 9.3.3.3 Medium-term

Fatigue Bitumen fatigue test Section 0 Medium-term

Adhesion Relationship with site data Section 9.3.5.2 Long-term

Desirable:

Deformation resistance Creep ZSV/repeated creep tests Section 9.3.1.2 Medium-term Stiffness DSR relationship with site data Section 9.3.2 Long-term Low temperature cracking Relationship with site data Section 9.3.3.4 Long-term