. DEFINITION OF AN SMA SKELETON ACCORDING

to the original german method

The original German approach (by Dr. Zichner) to designing SMA aggregate blends is based on having adequate ratios of various aggregate fractions. In that context, the stone-to-stone contact is neither specifically analyzed nor controlled.

Since the beginning of the SMA concept, the ratios established by Dr. Zichner have been only slightly changed. Thus we may emphasize clearly that Dr. Zichner’s mixture is a really genuine SMA. The contemporary weight ratios of coarse aggre­gate fractions preferred in Germany (Druschner and Schafer, 2000) are shown in Table 2.1.

In the German design of the SMA coarse aggregate skeleton, all fractions of aggregate bigger than 2 mm are used. For example, for an SMA 0/11 we take not only aggregate sizes 8/11 and 5/8 but 2/5 mm as well. Manipulation of the ratios of these different sizes is required (see Table 2.1) to provide the desired skeleton by minimizing the share of 2/5 mm aggregate down to one part in seven (15%) of coarse aggregates’ mass fraction. Consequently the German SMA gradation curves have no sharp breaks related to the absence of successive aggregate fractions and do not exclude any fractions larger than 2 mm (i. e., all fractions of coarse aggregates are used). In a way, most of the original German gradations are дийш’-continuous grad­ings (all fractions are present in the mix), with a minimal share of specific coarse aggregate fractions. It should be emphasized that in most German guidelines the amount of coarse aggregates (bigger than 2 mm) are not very high. For example, for SMA 0/11 the lower limit of this fraction has been changed from 75% (in 1984) to 73% (in 2001). There were many reasons for this change, including better com — pactability, lower permeability, and improved rutting resistance.

The success of SMAs designed according to the ratios of aggregate sizes presented in Table 2.1 has been proven through long-term pavement evaluations, although these ratios have been refined now and then (within a limited scope). So the strength of the

German method is a designed SMA skeleton composition based on long observation and experimentation. (See Chapter 7 for a detailed description of the method used in Germany.)