6.7.2 Influence of Traffic
Traffic will influence the packing of the aggregate and affect the void space to be filled with binder. Some adjustment in the spread rate is required for different design traffic volumes in order to achieve a satisfactory, tightly packed mosaic of aggregate after rolling and trafficking.
Aggregate rearranges during construction rolling into a more stable position with the least dimension tending towards vertical. This can only happen if there is sufficient space (provided by the aggregate spread rate) for the aggregate to move/rotate.
At the design spread rate, with an interlocked mosaic, particles provide mutual support and can thus provide greater resistance to the shearing and plucking action of traffic. If the spread rate is too heavy, the contact to contact mosaic may form in a more random orientation. The void volume in this random orientation is considerably higher (up to 25% more) than is the case where a large percentage of the particles lie with the least dimension vertical. If the aggregate spread rate is too light, the particles will not be able to form a fully interlocked mosaic resulting in a lower binder rise and likely reduced seal life.
Aggregates in a sprayed seal continue to reorientate under traffic. The rate of reorientation and amount of change in void volume is dependent on the traffic volume and, in particular, the number of heavy vehicles. This reorientation occurs mainly during the first one or two years of service. High traffic volumes result in the least dimension of nearly all particles being near vertical and interlocking with each other. The extent of reorientation is less at low traffic volumes resulting in greater random orientation of aggregate particles and greater void volume.
For the binder application rate to fill the voids within the aggregate mosaic to a depth of about two‑thirds up the aggregate, it is essential that the aggregate is spread at the design rate.
The aggregate spread rate for very low-traffic roads (≤ 200 v/l/d) can be about 5% heavier than that for more heavily trafficked roads to achieve both a satisfactory aggregate mosaic and to avoid pick‑up of binder by the aggregate spreader/trucks due to the relatively heavier rates of application of binder.
To achieve a satisfactory aggregate mosaic, the actual spread rates may have to be varied in practice by as much as ±10 m2/m3 from the design spread rate given below. It is not required to add an allowance for whip‑off to the design aggregate spread rate.
When ordering aggregate for works, stockpile wastage should be considered by including an additional allowance. A typical allowance for stockpile wastage is an additional 5 to 10% of aggregate above the quantity (tonnes or m3) required to cover the proposed works at the design spread rate.