4.6.5 Effect of Spraying Conditions and Delivery System on Binder Selection
Bitumen is normally a plastic solid at ambient temperatures. To enable application as a sprayed seal it is liquefied by heating, cutting back or emulsification. The viscosity of the binder varies with the binder type, delivery system and ambient conditions. Two of the parameters to be considered in choosing a binder delivery system are the likely pavement temperature at the time of spraying and the maximum gradient of the surface being sealed. Guidance on practical limits for these variables is provided in Table 4.4 and Table 4.5.
The minimum temperatures in Table 4.4 do not apply to SAMI seals, as is it recommended that little to no cutter is used even at low temperatures due to the potential to detrimentally effect to the properties of the overlaying asphalt. Cold weather trafficking of the SAMI seal should be avoided, due to the risk of stripping from poor adhesion between the heavily modified binder and aggregate. Once overlying asphalt is laid, heat from the mix will activate the SAMI allowing the binder and aggregate to bond.
|Binder type||Minimum pavement temperature (°C)|
|with cutter oil||without cutter oil|
|Polymer modified binder(1)||20||45(2)|
|Crumb rubber bitumen||20||45(2)|
- Does not apply to SAMI seals.
- This will vary by binder type and could be as high as 65 °C in some cases.
|Binder type||Maximum gradient (%)(1)|
|Hot cutback bitumen||10|
|Polymer modified binder||> 12(2)|
- The gradient will also depend to some extent on the existing surface texture and surface temperatures.
- Depends on the PMB type and concentration.
- Depends on the percentage of residual bitumen content, with higher residual bitumen content or polymer modified emulsions preferred for steeper grades.