Background to Spray Seal Design

Spray seal is a process that involves the application of a bituminous binder followed by the spreading of stone at a nominal size, primarily used to act as a waterproofing layer but is also used for a number of applications, including:

1. a) Wearing Course

Spray seals are a proven cost effective wearing course, often used in low traffic applications, typically in the form of single coat or double coat configurations.

1. b) Priming Course

Spray seals used as a ‘priming’ course are commonly used in granular pavements to provide a bonding interlayer for asphalt or spray seal wearing course to adhere to. When used as a priming course, the inclusion of cutter is typically used to reduce the viscosity of the bituminous material to penetrate the granular material.

1. c) Strain Alleviating Membrane (SAM)

Spray seals are often used on new and existing pavements that are subject to a high likelihood of reflective cracking behaviour through the surfacing. When used as a SAM, these seals are designed to have a higher binder application rate and commonly adopt polymer modified bitumen to provided additional interlayer elasticity and increased crack retardation.

The process of determining the suitable treatment and application rate is based around due consideration of individual project and environmental factors, including:

• Background Traffic Volumes – Is inversely correlated to the binder application rate and represents the ‘embedment’ potential of the seal. It is used in design procedures to reduce premature embedment under higher traffic
• Flakiness and Average Least Dimension (ALD) – The ALD is the characteristic vertical dimension of the stone once rolled and the flakiness is indicative of the stone geometry highlighting the binder retention requirements and interlock potential between two or more seals.
• Temperature – Temperature can be the single most detrimental effect on a spray seal, particularly in when used in a wearing course application. The selection of the binder gives consideration to the binder’s softening point and the anticipated temperature insitu. If this softening point is surpassed, the seal can lead to ‘flushing’ of the bitumen.

Using these variables, the Austroads spray seal design method calculates a bitumen application rate and an aggregate spread rate that can be standardised to changes in binder, including emulsions and polymer modified binders.

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For more information on Global Road Technology Polymer Modified Bitumen please contact GRT.