ARRB 2 cm
Road 90

Sprayed Seal Information Centre

overview | key documents | Austroads | ARRB | VicRoads | Standards | RTA NSW | MRWA | DTEI | AAPA
NZ TA | Roading NZ | NZ book | practice | history
overview | design | types & materials | construction | workshop |
workshop | photo gallery |
subglobal6 link | subglobal6 link | subglobal6 link | subglobal6 link | subglobal6 link | subglobal6 link | subglobal6 link
subglobal7 link | subglobal7 link | subglobal7 link | subglobal7 link | subglobal7 link | subglobal7 link | subglobal7 link
subglobal8 link | subglobal8 link | subglobal8 link | subglobal8 link | subglobal8 link | subglobal8 link | subglobal8 link

Aggregate spreading and rolling - South Africa

Spreading

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

rollingRolling 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.

 
With the construction of a seal using emulsion, a minimum number of passes with an 8 to 10 ton steel drum roller is recommended after spreading of the aggregate. Rolling should be discontinued when excessive crushing occurs under the rollers.

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:

  • A string line is used to mark the joint position;
  • The first spray is applied so that there is 100% binder coverage up to the string line;
  • The second spray is applied so that there is at least 100% application coverage over the full joint width;
  • Chip the first spray and clean up neatly up to the string line;
  • Do not leave a joint exposed for longer than 24 hours, and ifpossible close all longitudinal joints on the same day;
  • Keep traffic off incomplete joints. In the case of a coarse texture on the centre line, an additional diluted emulsion spray can be applied to a width of 500mm.

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.


In addition, cognisance should be taken of the traffic split per lane and direction, construction traffic movement, the construction process and how the road traffic will be accommodated.

When possible, and unless designed for:

  • Dual direction traffic should not be accommodated on the previous day’s seal;
  • Stop/Go control positions should not be selected at steep grades;
  • Single seals on intersections and access roads should be protected; and
  • New seals should not be opened to traffic if cold temperatures are expected.
About Us | Contact Us | ©2010 ARRB Group
modified 4/10/2010