Table of Contents

6.1.2 Adjustments to the Basic Voids Factor

The design voids factor, VF (L/m2/mm), is determined by adjusting the basic voids factor (Vf) to account for abnormal aggregate shape (Va) and the effect of traffic (Vt). These factors may be positive or negative and are cumulative.

Adjustment for aggregate shape (Va)

An adjustment, Va, is made to the basic voids factor (Vf) to account for variation in aggregate shape in accordance with Table 6.1. Flaky aggregates tend to produce seals with less voids due to their tendency to pack more tightly than cubical aggregates, consequently flaky particles require less binder.

Table 6.1: Adjustment to basic voids factor for aggregate shape (Va)

Aggregate typeAggregate shapeFlakiness index (%)Shape adjustment (Va)
Crushed or partly crushedVery flaky> 35Considered too flaky and not recommended for sealing
Flaky26 to 35–0.01
Angular16 to 25Nil
Cubic10 to 15+0.01
Very cubic(1)< 10+0.02
Not crushedRoundedn.a+0.01
  1. Not recommended for bottom layer of D/D seal as insufficient angularity does not promote interlock with top layer.

Adjustment for traffic effects (Vt)

The basic voids factors, Vf, described in Section 6.1.1, have been developed for an average mix of light and heavy vehicles in a free traffic flow situation. Where this assumption is not correct, an adjustment, Vt, needs to be made to compensate for variations in the traffic composition. This can include non‑trafficked areas, lanes with few heavy vehicles or for large proportions of heavy vehicles, channelisation or concentration of traffic, and slow‑moving heavy vehicles in climbing lanes or stop/start conditions (refer to Table 6.2).

Traffic normally wanders within traffic lanes resulting in wheelpath travel up to 1.2 m wide. Where traffic is constrained (channelled) from wandering such as on single-lane bridges, tight radius curves or narrow lane widths, an adjustment to the Vf must be made to reduce the risk of the seal bleeding. For example, a narrow single‑lane bridge may increase the effective traffic loading in the wheelpath by as much as threefold when the cumulative effects of combining lane volumes and constraining traffic to a confined path are taken into account.

Table 6.2: Adjustment (Vt) to basic voids factor for traffic effects

TrafficAdjustment to basic voids factor (L/m2/mm)
Flat or downhillSlow moving – climbing lanes
On overtaking lanes of multi‑lane rural roads where traffic is mainly cars with ≤ 10% of HV+0.010.00n.a.n.a.
Non-trafficked areas such as shoulders, medians, parking areas+0.02n.a.n.a.n.a.
0 to 15 EHV(%)Nil–0.01–0.01–0.02
16 to 25 EHV(%)–0.01–0.02–0.02–0.03
26 to 45 EHV(%)–0.02–0.03–0.03–0.04
46 to 65 EHV(%)–0.03–0.04–0.04–0.05
> 65 EHV(%)Use the process described in Section 5.2.7 for access roads to quarries, mining locations, etc.

n.a.: Not applicable.

EHV(%): Equivalent heavy vehicles, includes both standard heavy vehicles and large heavy vehicles (see Equation 2, Section 5.2.5).

* Channelisation – a system of controlling traffic by the introduction of an island or islands, or markings on a carriageway to direct traffic into predetermined paths, usually at an intersection or junction. This also applies to approaches to bridges and narrow culverts.

Possible short-term increases in traffic volumes such as during grain harvests and local field days may occur early in the life of the seal. Designers should consider this and may need to make some adjustment to the traffic volumes and design voids factor adopted in the design procedure.

Where a short-term traffic increase is only for a few days, it is preferable to defer the sealing work, for example to avoid a local annual field day or race meeting.

Seasonal variation, particularly an increase in large heavy vehicles such as large truck and trailer combinations, B‑doubles or larger combinations during hot summer periods, can affect the performance of a seal for up to two summers after construction. This applies mostly to annual crops harvested during the hotter weather conditions, stock transport and similar situations.

Consideration should be given to changing the type of treatment and/or binder if the increase is relatively large compared to the average annual daily traffic.

If unsure, design the rates of application for the normal and worst traffic cases before deciding on deferral of work, selection of an alternative treatment or selection of a final design.