The dissociation of the binder from mineral aggregates is causing several kinds of damages. According the Swiss standard SN 640'925, typical damages are:
- Loss of material, deterioration of the surface
- Cracking, due to weakness of adhesion combined with mechanical stress
- Insufficient interlayer adhesion and blister formation
In case of extended dissociation the bearing capacity of the layer is lost
The standardized procedure to determine the coverage of aggregates in Switzerland (SN 671'960) should be validated. Mixtures having a good defined coverage are produced The specimens are on the one hand immersed in water at specified conditions and finally tested mechanically, on the other hand directly tested. This allows determining the water sensitivity, which can be correlated with the coverage.
The objective is to provide determination of the critical coverage below that mechanical strength will be insufficient. Furthermore it is planned to provide evaluating a proposal for a specification regarding coverage.
This part describes the tests carried out. Can be applied, since the examinations provided in very different degrees to be known and also carried out very specifically geared partly to this research their use, they are presented in different level of detail. In the extreme case, reference is made to the relevant standard. It is therefore selected a three-part description, the individual parts are inserted as needed:
- Motivation: Where sense explains why this test has been applied and what is expected of her.
- Description: This description pointing to the standard, possibly supplemented by information on modifications of the process. In little-known testing a full test description is given.
- Review: Some procedures were only preliminary at already there showed that the objectives cannot be achieved.
The research organizations were authorized to evaluate the practical relevance of the values determined according to the Swiss Standard 670‘460 and to propose limiting values and requirements.
Originally it was planned to determine the behaviour of a fine graded mixture series with increasing percentage of a mineral aggregate with bad affinity properties using specific mechanical test methods.
First tests using such methods rapidly showed that the differences between compacted mixtures with mineral aggregates with high affinity and mixtures with mineral aggregate with low affinity were negligible. With that finding at least one aim of the research was reached since the requirement of 80% affinity is enough and that the affinity does not seem to have a great importance.
Instead of validating this finding through numerous testing the research organizations decided to alter the testing program in order to come up with a more differentiated evaluation.
The following test methods showed very good results:
- Water storage for 72h at a temperature of 60°C followed by an Indirect Tensile Test. This test method is simple and according the results from the research shows the potential to replace the ITSR, which does not differentiate enough.
- For scientific evaluations the water sensitivity test using the CAST as described in [GUBLER 05] can be recommended. This method is cumbersome, but gives inside into the material behaviour.
The applied test methods cover different combinations of water interaction and mechanical loading on asphalt specimens. Some of these combinations show a tendency towards damage of compacted mixtures in case of mineral aggregates with low binder coverage, although this tendency is not severe enough to justify increasing the binder coverage of mineral aggregates.
The common Swiss requirement for the binder coverage of 80% should therefore be retained and not increased. This is true for binders, tested with a specified combination of crushed stone as well as for the testing of a mixture specific combination of binder and mineral aggregates.