Gussasphalts are nowadays tested by a method, used at the time indentations of about 3 to 6 mm were common. The precision of this method is not sufficient enough for today's highly stressed objects with indentations of less than 2 mm.
The test of dynamic indentation is exceptionally suited for optimizing mix designs and for defining targets often used in practice. Currently, there is missing a significant and reliable method for the continuous quality control.
A new method should be developed by evaluation different parameters, similar to the previous test with cylindrical samples.
The aim of that project is to develop an uncomplicated method for testing the static indentation. As an improvement comparing to the previous standardized method, there should be achieved a wider range of results concerning different kinds of hard Gussasphalt. In addition, the repeatability of the method should be enhanced.
The following procedure is planned :
- Systematic analysis of the previous measurement results at about 100 investigations
- Definition of Test Specimens:
The following criteria are considered:
- Ratio maximum grain / specimen dimensions
- Handling (for example, dimensional stability of the formwork)
- Evaluation of the test parameters by systematic investigations
The potential of every single test parameter, such as temperature, load duration, imposed load etc., was estimated on the basis of well-known literature and the experiences of the testing body. As a result of that a choice of certain parameters was made in order to use them for the actual laboratory work. The study was carried out with eight different types of mastic asphalt, which differed in maximum grain size, kind of binder material and indentation depth.
It could be proved that it is possible to shorten the test duration without loss of meaningful results. For the evaluation of the "static indentation depth" two characteristic values are acquired: the indentation depth after 30 minutes, as well as the increase of the indentation depth after additionally 30 minutes. With that, the test duration is 60 minutes, regardless of the time which is needed for the heating process and the preload. The evaluation after 15 minutes and additionally 15 minutes (entire duration 30 minutes) shows, according to expectation, smaller values. After 15 minutes 85% of the standardised value (after 30 minutes) are reached, in fact with a high precision; the coefficient of variation is only 3%.
Temperature and applied load
Both, the test temperature and the applied load were varied. Beside the standardised temperature of 40° C there had been tests with 55° C, too. The standardised applied load of 525 N was raised to 650 N, and 775 N. The reason for having chosen that weight was, because for the standardised test the load (525 N) is applied by four rings at 125 N ( plus weight of the frame, 25 N ). So, in the laboratory it was possible to increase the load by adding more standard-rings. This variation of the test requirements was carried out with eight types of mastic asphalt. At a test temperature of 55° C and a load of 650 N the best results were achieved.
On the basis of the carried out parameter study the test parameters were redefined (55°C/650N). In total 24 types of mastic asphalt were tested in view of the static indentation under standard conditions (40° C/525 N) as well as under changed test requirements (55°C/650N) . It turned out, that the new test requirements showed an improved spread of the results and therefore a better differentiation. Regarding the modified test method, the span between the lowest and the highest value is 60 % greater than the span of the standardised test method.