Basics of Prime Coat and Tack Coat in Road Construction


To start this article, I would like to go from the basics. There are two types of road pavements:

  1. Rigid Pavement
  2. Flexible Pavement

Rigid Pavements have high flexural stiffness and consist of three primary layers i.e. Sub-grade, Base Course and Concrete Slab.

Flexible Pavements have low flexural stiffness and consist of four primary layers i.e. Sub-grade, Sub-base, Base Course and Asphalt. Further, flexible pavements also have Prime Coat and Tack Coat applied within the structure of pavement. Following is the answer to a question asked by an engineer regarding the two types of coats. I decided to post the article for benefit of those young engineers who are not familiar with road design and construction. Here follows brief explanation of these two types of coats and their specifications and purposes in pavement construction.

The process of spraying bitumen emulsion as a prime coat

Prime Coat

The layer between Asphalt Course and Crushed Aggregate Base Course (CABC) is called Prime Coat. Its purpose is to bind the loose aggregates of CABC so that it can be prepared for subsequent construction activity of laying Asphalt layer. Prime Coat also serves the purpose of blocking capillary action in CABC so that water may not rise up to the Asphalt layer. It should be kept in mind that asphalt and water do not go well together, and pavement should always be designed to drain away any water which tends to come in contact with asphalt. Prime coat serves this purpose of blocking the path of water which rises from the embankment due to capillary action. Prime coat also acts as a binder for Asphalt and CABC. However excess Prime Coat, particularly if it has formed a pond, can result in ineffective bonding between CABC and Asphalt Course and even alter the properties of Asphalt i.e. reduction in air voids. Effective bond between Asphalt and CABC is very important as it helps in reducing longitudinal shear stress gradient due to accelerating and breaking vehicles, particularly if Asphalt layer is thin i.e. < 4 in.

Types of material used as Prime Coat: Cut Back Asphalt or Emulsified Asphalt (Diluted)

Application Rates:

  1. Cut Back Asphalt: 0.65 to 1.75 Litres / Sqm
  2. Emulsified Asphalt: 2.3 to 6.8 Litres / Sqm

Lower application limit pertains to CABC which is very tight and has high fines content while upper limit pertains to highly porous CABC.

Prime Coat should be left undisturbed for 24 hours so that it seeps down within the CABC and does not form a pond on CABC.

Recently engineers are eliminating the use of Prime Coat due to certain studies which indicate that Prime Coat does not play any significant part in stability of pavement structure. However, considering the fact that Prime Coat accounts for a tiny percentage of road cost, its use should not be discontinued. Particularly due to the fact that it has useful properties, although it may not contribute in the structural strength of the pavement.

Lightly application of bitumen emulsion on the surface

Tack Coat

The layer between Asphalt Base Course (ABC) and Asphalt Wearing Course (AWC) is called Tack Coat. Tack Coat is also applied between the concrete deck slab of bridges and AWC laid over it, or concrete slab of Rigid Pavement on which AWC is to be laid.

The purpose of Tack Coat is to form a bond between AWC and ABC (or concrete slab). If Tack Coat is not applied, AWC will tend to slip under traffic load and pavement will fail due to spalling of AWC. If AWC is directly placed over ABC, the bitumen within AWC is not enough to form a long term effective bond between the two layers. Hence, after rainfall and penetration of water beneath AWC layer, bond between AWC and ABC will cease to exist. At this stage AWC will start slipping and slippage cracks will become visible which will ultimately lead to spalling. It should be kept in mind that too much Tack Coat will also result in slippage of AWC, hence, its application rate should be adjusted as per site conditions.

Types of material used as Tack Coat: Cut Back Asphalt or Emulsified Asphalt (Diluted)

Application Rates:

  1. Cut Back Asphalt: 0.20 to 0.40 Litres / Sqm
  2. Emulsified Asphalt: 0.25 to 0.70 Litres / Sqm

AWC should be laid over the ABC after Tack Coat has dried and become “tacky”.

This Post Has 2 Comments

  1. Manuel Labor

    Very energetic post, I liked that bit. Will there be a part 2?

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