5.3.1 Average Least Dimension (ALD)
The concept of an aggregate particle tending to lie with its least dimension vertical is central to the volumetric design of a sprayed seal.
The least dimension is defined as the smallest dimension of a particle when placed on a horizontal surface (Figure 5.1). The shape is most stable when lying with its least dimension (A) vertical. If placed with the width (B) or the depth (C) of the shape vertical, it would require less energy to knock the aggregate particle over so that the least dimension (A) was again vertical, particularly if the particle is other than cubic. Thus, in a seal, the final orientation of most particles is such that the least dimension is near vertical, providing that there is sufficient room for the particles to re-align.
The least dimension may be only marginally smaller than the other two dimensions, as in the case of almost cubical aggregates, or can be much less in the case of flaky aggregates.
ALD can be determined by:
- direct measurement (AS 1141.20.1 for 10 mm or larger nominal size or AS 1141.20.2 for 5 mm and 7 mm nominal sizes)
- AS 1141.20.3 involving calculating (or using a nomograph) the ALD from the grading, median size, and flakiness index
- NZS 4407 using Test Method 3.13: The Size and Shape of Aggregate Particles.
ALD is the critical parameter in the Austroads sprayed seal design procedures. It is used to calculate both the aggregate spread rate and the design binder application rate. The design procedures assume that, for 10 mm and larger, only a single layer of aggregate particles adheres to the binder film, but for 7 mm and smaller aggregates, the aggregate layer can often be two (or more) aggregate particles in thickness.
The importance of using a representative ALD cannot be overemphasised. Poor sampling techniques and/or inaccurate testing procedures to determine ALD will result in incorrect aggregate design spread rates and inaccurate design binder application rates.
It is poor practice to use assigned or nominal values of ALD for a particular nominal size of aggregate. ALD of individual aggregate samples can vary by up to 17% within normally specified ranges of grading and flakiness index, resulting in an equivalent variation in the design rates of basic binder application and aggregate spread rate. It is thus important to separate aggregate at stockpiles with significantly different ALDs (> 1.0 mm) and monitor the ALD of aggregate stockpiles regularly within a sufficiently comprehensive sampling scheme to progressively evaluate whether seal design changes are warranted.