In classic SMA composition and in regulations introduced all over the world, the total content of grains smaller than 2 mm has generally been in the range of 15-30% (m/m). When we add the typical filler content (8-13%), we receive up to 22% from the sand fraction (0.063/2.0 mm). But when designing the content of fine aggregate in SMA, one should remember the increase in the content of fine particles during compaction due to crushing and wearing of the coarse particles.
Is the sand fraction desired in a mix? Looking at the shape of an example SMA gradation, we can imagine a mix designer adding all the permitted quantity of filler (i. e., approximately 13% [m/m]) instead of 0.063/2.0 material. This example is illustrated in Figure 6.13. As can be seen, the gradation curve stays within the limits up to the 0.85 mm sieve. Then it takes the “low route,” meaning that there are too few fine particles, which are needed for mastic creation. Undoubtedly, composing an SMA without the material 0.063/2.0 is not possible.
The sand fraction is indispensable because building a mastic with only filler grains makes achievement of the expected features of a newly designed SMA impossible. So is it possible to determine the best course of a gradation curve in the area below
100
0
Example of a SMA aggregate mix gradation without fine aggregate (0/2 mm).
2 mm? There is not an unequivocal answer to this question because a lot depends on the type and properties of filler and features of the 0.063/2.0 material, too. The intended use of the designed mix is also of great significance. However, the following is worth bearing in mind:
• Guiding the curve upward enhances the risk of closing the mix and raises the threat of excessive mastic and the appearance of fat spots.
• Guiding the curve downward enhances the risk of an excessive opening of the mix.
• Designing using the maximum quantity of filler and the minimum amount of the sand fraction is disadvantageous and risky.
• Care ought to be taken so that an increase of the sand fraction can be observed on subsequent sieves to supply enough material for making mastic.
• The quantity of filler should fluctuate around the middle of the allowable range (i. e., about 9-10% [m/m]) to make possible the collection of material on sieves smaller than 1.0 mm and to prevent the gradation curve from rising upward.
• Non manufactured (natural) sand may be applied only for SMA layers on roads with low traffic volumes.
