This time, let us add that 20% removed from 2/5.6 mm chippings to 8/11.2 mm chippings (that makes 25% + 20% = 45%), leaving the content of fraction 5.6/8 the same as in Stage I (25%). The composition of the new mix is shown in Table 6.8, while its gradation curve is shown in Figure 6.6.
The achieved mix falls between the upper and lower gradation limits. Obviously it still needs some refining within the sand fraction, but at the moment the main topic is the coarse aggregate. Perfectionists would say that a bit of "messing about" with the coarse fraction could be useful, by lowering the gradation curve even more on the 8 mm sieve, for example. But the question is, is it worth it?
After all, lowering the gradation curve on the 8 mm sieve will increase the share of particles larger than 8 mm, which means a more coarse gradation. The mix with a predominant share of the fraction 8/11.2 will make a strong skeleton; however, it will be characterized by a high value of voids in mineral aggregate (VMA) that requires a high binder content to achieve a suitably low content of air voids. We also obtain better (deeper) macrotexture, which means better skid resistance with high speed measurements. The down side is that such a mixture will have considerably higher permeability. So is this worth doing?
Despite this, let us make the last correction of the mix.
Example of the SMA Mix with an Uneven Distribution of the Coarse aggregate Fraction among Three Fractions— predominantly chippings 8/11.2 (example II, stage 3)
component of aggregate mixture content, (m/m)
Filler 10%
Crushed sand 0/2 15%
Coarse aggregate 2/5.6 mm 5%
Coarse aggregate 5.6/8 mm 25%
Coarse aggregate 8/11.2 mm 45%
0
Sieve, # mm
FIGuRE 6.6 Example of SMA mix with an uneven distribution of the coarse aggregate fraction among three fractions—the effect of the decrease in quantity of aggregate 2/5.6 and the supplement of 20% aggregate 8/11.2 (Example II, Stage 3).
