Investigations carried out in the Netherlands (Voskuilen, 2000) have proved that the gradation within the coarse aggregate fraction exerts an impact on the distribution of voids in a mix. Briefly, the conclusions drawn in the Netherlands are as follows: [25]
6.3.2.2.3 Determining the Size of Active Particles
After exercises in changing ratios within the coarse aggregate fraction, it is time to explore the question of the influence of the size of active particles on the mix. As we remember from Chapter 2, active grains are those making an aggregate structure that carries loads. The problem of actively setting up the SMA skeleton by particles of a certain fraction—say, 2/4 (or 2/5.6) mm—was discussed there. According to the German approach to SMA, that size of particle could be used for that purpose, though to a limited extent (as the German ratios suggest that in SMA 0/11 only one seventh of all coarse aggregates should be of size 2-5mm). According to the U. S. approach, this size should not be used, although that depends on the maximum size of the SMA aggregates, or NMAS. The lower limit sieve, from which active particles are counted, is called the breakpoint (BP) sieve in the United States.
 |
 |
The adopted classification in the United States—in NAPA SMA Guidelines QIS — 122—imposes lower size limits for coarse (active) particles based on NMAS as follows:
At any rate, coarse particles 2.36/4.75 mm (below 4.75) have been regarded as active ones in SMA 0/9.5 mm. In SMA 0/4.75 mm, the fraction 1.18/2.36 mm is also considered an active one (as are all larger ones). In coarser mixes, aggregates above
4.75 mm are regarded as active.
The selection of the BP sieve influences not only on the shape of the gradation curve but also the properties of SMA mixtures. Generally, the larger the BP sieve, the stronger the predominance of coarse (active) particles in a mix. One can safely say that the coarse aggregate fraction becomes more single sized as the discontinuity of gradation becomes stronger. When estimating gradation curves for various BP sieves, the conclusion can be drawn that the larger the BP sieve, the further the position of the breaking point of the gradation curve is moved to the right. And thus one can also say that the larger the BP sieve, the more open the mix and the more binder is required.
Results of some work in the United States (Cooley and Brown, 2006) justify saying that raising the size of the BP sieve results in the following consequences to the properties of an SMA mixture because it increases: [26]
• Permeability of the mixture—with the same content of voids in a compacted SMA mixture, the permeability is higher with a larger BP (for more on permeability, see Chapter 12).
