This lecture will discuss those aggregate properties that are associated specifically with HMA.
Superpave requirements are based on both the traffic level and the depth of the layer below the surface. The crushing requirements for low traffic volumes are low or none, regardless of depth. As traffic levels increase, so do the required percentages of crushed faces. There is a higher level of crushing required in the upper 100 mm of the pavement because this is the region which is subjected to the highest shear due to traffic loads. Higher shear forces require a higher level of resistance to shear.
Currently, the 5:1 ratio is used for Superpave requirements. However, research is being conducted to determine if the 3:1 ratio would be a better parameter. A higher percentage is being considered if the ratio is reduced.
As with the coarse aggregate angularity, the fine aggregate angularity requirements are selected for a given traffic level and depth below the surface.
The requirement changes with depth.
For HMA pavements these are the definitions for gradations.
To specify aggregate gradation, two additional features are added to the 0.45 chart: control points and a suggested restricted zone. Control points function as master ranges through which gradations must pass. They are placed on the nominal maximum size, an intermediate size and the dust size. The restricted zone (optional) resides along the maximum density gradation between the intermediate size (either 4.75 or 2.36 mm) and the 0.3 mm size. Gradations that pass through the restricted zone have often been called “humped gradations” because of the characteristic hump in the grading curve that passes through the restricted zone. In most cases, a humped gradation indicates a mixture that possesses too much fine sand in relation to total sand. This type of gradation can result in tender mix behavior, which is manifested by a mixture that is difficult to compact during construction and offers reduced resistance to permanent deformation during its performance life. Gradations that violate the restricted zone may depend too much on asphalt binder stiffness rather than internal aggregate structure to achieve mixture shear strength. This can make these mixtures sensitive to asphalt content and can easily become plastic. It should be noted that a number of states have achieved good performance with gradations that pass through the restricted zone. Therefore, this gradation limitation is now considered a recommendation if there is no or a poor performance history with an aggregate source.
The term used to describe the cumulative frequency distribution of aggregate particle sizes is the design aggregate structure . A design aggregate structure that lies between the control points and avoids the restricted zone meets the requirements of Superpave with respect to gradation. Superpave defines five mixture types as defined by their nominal maximum aggregate size:
These are the five gradation designations developed for Superpave. The gradation is referred to by its nominal maximum aggregate size.
The Superpave gradation requirements for the control points and the suggested restricted zone are shown on the next five slides.
Senior/Graduate HMA Course Superpave Aggregate Properties Currently Required for HMAAggregate Superpave Aggregates 1
Coarse Aggregate Angularity Criteria Traffic Depth from Surface Millions of ESALs < 100 mm (4”) > 100 mm < 0.3 55/-- --/-- <1 65/-- --/-- <3 75/-- 50/-- < 10 85/80 60/-- < 30 95/90 80/75 < 100 100/100 95/90 > 100 100/100 100/100First number denotes % with one or more fractured facesSecond number denotes % with two or more fractured faces Aggregate Superpave Aggregates 2
100 Aggregate Size Definitions 100100 99 90 • Nominal Maximum Aggregate 89 72 Size 72 65 – one size larger than the first sieve 65 48 to retain more than 10% 48 36 • Maximum Aggregate Size 36 22 – one size larger than nominal 22 15 maximum size 15 9 9 4 4 Aggregate Superpave Aggregates 6
Percent Passing100 max density line restricted zone control point nom max max size size 0 .075 .3 2.36 4.75 9.5 12.5 19.0 Sieve Size (mm) Raised to 0.45 PowerAggregate Superpave Aggregates 7