What is Stripping?

Stripping with reference roads made of asphalt/ bituminous binders, refers to the "loss of bond between the binder and the aggregate." This bond between the stone /aggregate and the binder is the most important factor influencing the road / pavement life and durability.

Main Reasons / Causes of Stripping

The primary causes of stripping in roads are :

  • Water – In the form of rain, snow, ground water, sea water, moisture etc.
  • Nature of Aggregate. Certain types of aggregate do not bond or adhere well with bituminous binders due to their composition. This aggregate are generally "water – loving" and are called "hydrophilic aggregate". These aggregate are highly prone to stripping in the presence of water.
  • High air voids and low density of Hot Mix.
  • Improper or no drainage system of the road / highway / pavement.
  • Poor workmanship and weak initial binder – aggregate bond.
  • Construction weather and area climate
  • Traffic load

Water - It's Damaging Effect.

Water is the most damaging factors responsible for premature failure of bituminous pavements. The various mechanisms by which water affects the pavements are :

  • Water form a thin, invisible film around the aggregate in the form of moisture during road construction. This obstructs the complete, strong coating of aggregate by bitumen. Therefore, when the initial bond itself is weak, the pavement is unable to take normal stress and disintegrates
  • In post construction period, when water intrudes the surface of the pavement and penetrates between aggregate and bitumen film, it starts the process of stripping the binder from the aggregate.
  • Pore pressure, which is the increased pressure caused by the circulation of trapped water through the void structure of aggregate also initiates stripping. The water may be present due to ground water ingress or surface intrusion.
  • Hydraulic scouring which occurs on surface courses because of a compression tension cycle caused by the interaction of tire pressure with surface water

Types of Aggregate and Binder Bond or Adhesion – Active Adhesion & Passive Adhesion

The coating and adhesion which takes place between the binder and aggregate at the time of "first contact", in the presence of water, is called "Active Adhesion" in HMA.

When the bitumen has the property to coat even wet aggregate, it is called good active adhesion. This is essential at the time of construction, as this would result in a complete bond between bitumen and aggregate. Even thin film of moisture would cause improper weak bonding.

The maintenance and retention strength of the adhesion of aggregate/ binder, is called Passive Adhesion. This is the ability of the bond to withstand influences which cause it to break.

Anti Stripping Agents /Additives - Definition, Functioning and Explanation

Anti stripping Agents are chemicals, whose primary function is to increase the adhesion between the binder and aggregate. The main goal of anti-stripping additives, is to increase the strength and durability of the adhesion between aggregate and bituminous binders.

The adhesion promoter works in 3 ways :

  • They reduce the surface tension of binder.
  • They change the charge of the aggregate.
  • They act as strong bridge between aggregate and binder.

The amine-based anti stripping products are made of 2 parts :

  • The amine-head has an affinity for the aggregate and strongly sticks with it.
  • The 2nd part, is a hydrocarbon tail, which bonds with the binder ( Same family).

This results in a bridging – bonding , adhesion promoting effect.

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