Mix design tests

Testing

The usual general testing systematic for asphalt construction methods is also applicable to the Stone Mastic Asphalt construction method and is consists of

• Mix design testing

• Internal control testing

• Check testing.

The special focus here lies in the mix design tests.

This is clearly stated in the "General Circular for Road Construction No. 29/1998" issued by the federal minister of transportation in Germany. This circular was introduced with the new edition of the ZTV Asphalt that included modifications and additions.
Here word for word:

"With regard to the high requirements on the performance characteristics of asphalt courses for roads of the construction classes SV and I as well as for traffic areas with special demands according to Section 1.3 of the ZTV Asphalt-StB 94, the mix design test takes on special significance. Here above and beyond the parameters of the mix design tests as per ZTV Asphalt-StB 94, additional assessments of the volumetric properties of the respective mixtures and tests in the form of extended mix design tests are necessary. These tests can be modified to meet the special requirements for the type and properties of the mineral aggregate, the type and grade of binder, and if necessary, the additives and the particle size distribution of the mineral aggregate mixture.

The test report of these mix design tests should not only give the information about the proposed composition of the asphalt, it must also give information about the deformation resistance, compactibility and behavior at very low temperatures. Extended mix design tests are, for such construction measures, a permanent component of the construction contract.

"The information on deformation resistance is in the foreground. Exact establishment of how this is to be tested or if it must be tested, does not exist in Germany at this time. Usually two test procedures are used:

• the mono-axial compressive oscillating test as per the "Technical testing regulations for asphalt in road construction TPA - StB, Part: Mono-axial compressive oscillating test - determination of the deformation behavior of roller asphalt during heat, 1999 Issue" and

• the rut formation test as per DIN EN 12697-22 "Test methods for hot mix asphalt - Wheel tracking".

For both tests, there are limits with regard to the indicators - particularly since there is not sufficient experience and assessment background. That is why the results of these tests are not suited for contractual requirements.

In his article "The Experience to date with Extended Mix Design Tests of Asphalt" presented at the FGSV-Asphalt Road Conference 1999 in Kassel, Graf wrote:

"If the results of dynamic tests on asphalt concrete, Stone Mastic Asphalt and porous asphalt which are done under identical testing conditions are compared, the conclusion must be reached that asphalt concrete is more stable than SMA, while the use of porous asphalt is completely inadvisable. This is because in the dynamic tests under identical conditions, porous asphalt fares the worst. On the other hand, however, we know that porous asphalt in practice performs well with regard to rut formations. The question is: Is there a legitimate transferability between laboratory results of dynamic testing to practical performance? Both test procedures, rut formation testing and dynamic pressure threshold testing, in their current forms, only give information on the viscosity characteristics of the mortar or the mastic. The most important factor for the results is, first of all, the hardness of the binder used and then, the amount of binder as well as the storage density of the specimen dependent on the compaction work and compaction temperature. We have not been able to quantify the individual variables."

Graf goes on to say:

"Neither of the two test procedures is currently able to point out the effects of differing proportions of the individual particle size classes in the chippings range or even the effects of the angularity of gravel chippings for the purpose of fine optimization. Even the influence of the binder content is not always clearly evident. It is generally true, however, that when favorable results are attained with regard to a high deformation resistance in the dynamic testing, than a higher deformation resistance can also be expected in practice. But we know that Stone Mastic Asphalt 0/8 S in combination with B 50/70 which shows no problems with regard to rut formations in practice, does not do especially well in the dynamic testing. This is even more true when using PmB 65."

"Furthermore the problem of the production of a homogeneous specimen should be addressed, irrespective of whether or not it is a Marshall specimen, testing slab or other specimen. Despite all the high-tech help, one cannot help currently feeling that the quality of the specimen or testing slab and the skill of the laboratory assistant when placing the mixture in the compaction dies, is connected. To exaggerate, we are testing the quality and the skills of the laboratory assistants to see if they can produce homogeneous and consistent specimens."

"Last but not least is the question of the test for stability with the dynamic tests in water immersion. Such a test quickly changes into a further test of the adhesion between the stone and binder and thus is limited as a test for the prediction of asphalt stability. This is mentioned in various publications.

From the above, it can be deduced that the tests for determining deformation behavior surely give valuable information, but, are not the "cure all" to solve all the problems of rut formation.

Graf correctly points out that a consideration of the volumetric parameters of asphalt is especially important and should always be done.

The question of the compactibility of Stone Mastic Asphalt, taken from the circular from the federal minister for transportation, is, according to experience only seldom relevant. For the test of compactibility, there is in Germany, an "instruction guide for the determination of the compactibility of rolled asphalt with the help of the Marshall procedure", worked out by the FGSV (German Road and Transportation Research Association) which describes the differing, varied expensive procedure.

In practice, in road construction, there are, according to experience, fewer problems with compactibility especially with Stone Mastic Asphalt, than with the correct and adequate compaction or compaction work. More on this topic can be found in the preceding Section 12.

Finally the question of behavior at very low temperatures is addressed. Speaking from experience, there are no special problems in Germany, when binders according to the ZTV are used. Nevertheless, the valid "Technical testing procedures: Behavior of asphalt at very low temperatures" from the FGSV are referred to. After all this property can have a totally different significance in other countries with other climatic conditions and other bitumen qualities. In conclusion, it must be stressed that the results of all these extended tests of Stone Mastic Asphalt have, it is true, significance for the evaluation of suitability and quality, but due to the above-mentioned difficulties, are not suitable for contractual specifications.