Aggregate Gradiation

The molecule size dispersion, or degree, of a total is one of the most unbelievably compelling conglomerate aspects in certifying how it will perform as an asphalt material. In HMA, degree makes figure practically each critical property combining solidness, stability, toughness, penetrability, workability, exhaustion insusceptibility, frictional insusceptibility and invulnerability to dampness harm (Roberts et al., 1996[1]). In PCC, degree makes figure sturdiness, porosity, workability, concrete and water necessities, solidness, and shrinkage. In light of this, degree is an essential concern in HMA and PCC intermingle objective and consequently most bureaus point out permissible conglomerate degrees for both.

Max Aggregate Testing

Greatest conglomerate size can alter HMA, PCC and base/subbase courses in a few methods. In HMA, precariousness could effect from unnecessarily minor most extreme sizes; and abject workability or isolation would effect from too much impressive greatest sizes (Roberts et al., 1996[1]). In PCC, huge greatest sizes would not be able to fit among strengthening bar openings, but they will ordinarily build PCC solidness on account of the water-bond degree might be brought down. ASTM C 125 describes the most extreme total size in one of a few ways:

Maximum size. The smallest sieve through which 100 percent of the aggregate sample particles pass.Superpave defines the maximum aggregate size as “one sieve larger than the nominal maximum size” (Roberts et al., 1996^[1]).
Nominal maximum size. The largest sieve that retains some of the aggregate particles but generally not more than 10 percent by weight. Superpave defines nominal maximum aggregate size as “one sieve size larger than the first sieve to retain more than 10 percent of the material” (Roberts et al., 1996^[1]).

Thus, it is important to specify whether “maximum size” or “nominal maximum size” is being referenced.

Desired Gradation

Gradation has a profound effect on material performance. But what is the best gradation? This is a complicated question, the answer to which will vary depending upon the material (HMA or PCC), its desired characteristics, loading, environmental, material, structural and mix property inputs. Therefore, gradation requirements for specific HMA and PCC mixes are discussed in their respective pavement type sections. This section presents some basic guidelines applicable to common dense-graded mixes.

It might be reasonable to believe that the best gradation is one that produces the maximum density. This would involve a particle arrangement where smaller particles are packed between the larger particles, which reduces the void space between particles. This creates more particle-to-particle contact, which in HMA would increase stability and reduce water infiltration. In PCC, this reduced void space reduces the amount of cement paste required. However, some minimum amount of void space is necessary to:

Provide adequate volume for the binder (asphalt binder or portland cement) to occupy.
Promote rapid drainage and resistance to frost action for base and subbase courses.

Therefore, although it may not be the “best” aggregate gradation, a maximum density gradation does provide a common reference. A widely used equation to describe a maximum density gradation was developed by Fuller and Thompson in 1907. Their basic equation is: