In the process of asphalt pavement production in practice it is observed, that the state of polymer modified binders has altered in comparison to the original state as a result of thermal stress in the heating process. The main causes can be attributed to overheating and long storage or transportation time. In this process mainly the PMB is damaged since as a rule polymers are more temperature sensitive than bitumen. This leads to altered characteristics at low and/or high temperatures which affect directly the behaviour of the road pavement.
The investigation of the binder in the completed pavement is not an easy process and generally the binder has to be extracted. But there are legitimate doubts whether the PMB can be extracted in full. In addition to this fact the microstructure of the polymer framework in the PMB might be changed, resulting in altered binder properties.
Only PmB containing a polymer which is soluble in a solvent at room temperature can be analysed by gel permeation chromatography and can directly show the polymer degradation. In view of the presented situation a test method allowing the evaluation of the completed road pavement without binder extraction would be a major advantage.
In this project solutions are sought to determine and evaluate overheating of PmB after the pavement has been built avoiding the extraction process of the binder.
This research project investigated the question, whether an overheating of the binder can be proven directly on cores of the damaged asphalt pavement without the extraction process.
To address these points, hot mix asphalt was produced in the laboratory, artificially aged and tested by mechanical tests. To study the different ageing behaviour, dense and open graded asphalt concrete as well as mastic asphalt were produced in the laboratory.
Although mastic asphalt is only 10% of the hot mix production in Switzerland, this asphalt type was included in the research because of its much higher production temperatures, which make this material especially sensitive to thermal damage of the temperature sensitive polymer bitumen.
It has been found that it is not possible to show an overheating during production directly on asphalt cores without the extraction of the binder. In the case of asphalt mastic, it was observed that the stirring process in the lab produced larger amounts of fines, leading to an excessive stiffening of the bituminous mortar. This hardening is independent of the aging temperature and only influenced by the stirring time, which has been concluded from the determination of the filler.
In the case of asphalt concrete, a higher thermal stress stiffens the binder, which leads to higher marshall stability and cyclic compression test values. On the other hand, the higher binder stiffness decreases the compactibility. As a consequence, the air void content becomes higher, resulting in reduced marshall stability. It is difficult to reach any valid conclusion because of these opposing impact factors.
For these reasons, it became clear that it was not possible to reach the defined goals and the research team decided to abandon the project. Therefore, the planned tests on porous asphalt concrete were not carried out.