Aggregates are stockpiled in the vicinity of the job and precoating is usually carried out using a front end loader. Limited use is made of specialist mixing plant for precoating stone aggregates.
Like priming there has a move away from using tar based stone precoating fluids for HSE reasons. For more information on bituminous based precoating fluids refer to Sabita Manual 26.
Spreading of aggregates on Provincial and National roads is done using self propelled chip spreaders which are fed by tipper trucks driving in reverse. Limited amount of chip spreading is done using tipper trucks with ‘buckeye’ attachments driving in reverse.
Rolling of freshly spread single seal aggregate on a polymer modified binder is done using 27 ton pneumatic tyred roller with 6 – 8 passes.
Rolling with a steel drum roller is not recommended as this can lead to crushing of the aggregate leaving unsightly marks on the black precoated aggregate.
In the case of the construction of a double seal the first application of aggregate is rolled in a single pass with an 8 to 10 ton steel drum roller followed by 3 to 6 passes with a 27 ton pneumatic tyred roller.
It has been found that when rolling commences with a pneumatic tyred roller immediately after application of the aggregate, emulsion pick-up occurs on the rubber tyres, leading to a build-up of aggregate on the rubber tyres.
If rolling with a steel drum is not possible, the emulsion should be allowed to break slightly before rolling with a pneumatic tyred roller commences.
After the rolling is completed, all excess aggregate is removed from the surface by sweeping with a rotary broom.
Joints between sprays
Many seal failures result from poor longitudinal joints. This illustrates the importance of including the position of longitudinal joints in the overall design considerations.
Normally joints are placed in the same line as the road marking lines. Some deviation from this convention applies to high volume roads with surfaced shoulders. The joint between the slow (truck) lane and the shoulder is best placed to the outside of the lane edge line as truck traffic does wander over the edge line and could result in a bleeding joint.
The number of nozzles on the binder distributor will dictate maximum spray widths. A 6m wide spray bar will allow the application of binder on the shoulder and slow lane in a single pass, thus negating the need for a joint. However, this is only done when the same design application rate applies to both the shoulder and the slow lane.
There are many opinions relating to joints between sprays, but they all relate to the amount of binder overlap required to ensure full binder coverage over the joint. In order to prevent poor joints especially in the case of low binder applications and low traffic volumes over the joints, the following practices are carried out:
Opening to traffic
When constructing single or double seals, it is advantageous to have the completed seal open to traffic at least 2 hours before the road temperature drops below 25 ºC, in the case of decreasing temperatures.
Conversely in the case of high summer road temperatures it is not advisable to open the completed seal to traffic if the road temperature is higher than the binder softening point, minus 15 ºC (e.g. For S-E2 with a softening point of 60º C, do not open the road to traffic until the road temperature has dropped to below 45 ºC).
The initial traffic during and shortly after construction (during the first warm season) has a major influence on the performance of the seal. Apart from the Average Annual Daily Traffic and percentage of heavy vehicles, often available from road authorities, information should be obtained regarding seasonal variations and potential hauling contracts expected.
When possible, and unless designed for: