Table of Contents

4.5.2 Maximising Packing Properties of Dense Graded Mixes

Dense graded asphalt mixes use a continuous grading to facilitate packing of aggregate particles to achieve:

  • flexibility
  • durability
  • structural stiffness
  • deformation resistance
  • low permeability.

The maximum density grading gives the theoretical densest particle packing and minimum voids in the mineral aggregate (VMA) condition of the mix (the total voids within the mass of compacted aggregate).

For a particular maximum aggregate size, the maximum density grading may be determined by the Fuller equation (Equation 1).

  \[\mathrm{P  =  100}\left(  \frac{\mathrm{d}}{\mathrm{D}}  \right)^{\mathrm{n}}\]1
 P=total percentage passing a given sieve 
 d=size of sieve opening 
 D=maximum size of aggregate 
 n=grading exponent 

The original Fuller equation used a grading exponent of 0.5. Further work has determined that an exponent of 0.45 provides the greatest density and lowest VMA. Increasing the exponent to say 0.6 provides an asphalt mix with slightly increased VMA and coarser texture. Decreasing the exponent also increases VMA but results in a finer textured mix. Higher exponents such as 0.8 or 0.9 tend towards an open graded mix.

The Fuller equation has proved to give impractical proportions of filler for some mixes. Another equation has been developed to adjust for the percentage filler content (Equation 2).

  \[\mathrm{P}\mathrm{ }\mathrm{=}\mathrm{ }\frac{ \left( \mathrm{100} - \mathrm{F} \right)\mathrm{ }\mathrm{*}\mathrm{ }\mathrm{(}\mathrm{d}^{\mathrm{n}} - \mathrm{0.075}^{\mathrm{n}}\mathrm{)}}{\mathrm{(}\mathrm{D}^{\mathrm{n}} - \mathrm{0.075}^{\mathrm{n}}\mathrm{)}}\mathrm{ }\mathrm{+}\mathrm{ }\mathrm{F}\]2
 F=the percentage of filler 
 P, d, D and n=are as above 

Maximising the density of the mix with a particle size distribution that closely follows the maximum density relationship may result in a VMA that is too low. A low VMA may not provide enough space for the required air voids and a reasonable volume of binder to provide cohesion, durability and fatigue resistance in the mix. Overfilling the VMA with binder in such circumstances can lead to mixes with low air voids and poor rutting resistance.

Deviations from the maximum density curve generally result in higher VMA. Significant deviations from the maximum density curve can, however, result in a reduction in mix stability. It can also result in a significant change in finished surface texture by becoming 'gap' graded. It is desirable, therefore, that changes to the aggregate grading combination to achieve higher VMA maintain a continuous distribution and interlock of aggregate particle sizes.