The Use of Prime Coat and Tack Coat in Road Construction

The Use of Prime Coat  and Tack Coat in Road Construction

There are two kinds of road pavements:

1. Flexible Pavement

2. Rigid Pavement

Rigid pavements have a high flexural rigidity and are made up of three major layers: subgrade, base course, and concrete slab.

Flexible pavements have low flexural rigidity and are made up of four main layers: sub-grade, sub-base, base course, and asphalt. Furthermore, Prime Coat and Tack Coat are applied within the pavement structure of flexible pavements. Here is a quick overview of these two types of coats, as well as their requirements and applications in pavement building.

Prime Coat

Prime Coat:

Prime Coat refers to the layer between Asphalt Course and Crushed Aggregate Base Course (CABC). Its objective is to bind the loose CABC aggregates so that they can be prepared for the future construction activity of laying the asphalt layer. Prime Coat also prevents water from rising to the Asphalt layer by preventing capillary action in CABC. It should be remembered that asphalt and water do not mix, and pavement should always be built to drain away any water that comes into contact with asphalt.

The prime coat serves to limit the passage of water that rises from the embankment owing to capillary action. The primer coat also serves as a binder for the asphalt and CABC. Excess Prime Coat, on the other hand, can result in inadequate bonding between CABC and Asphalt Course and potentially change the properties of Asphalt, such as a reduction in air spaces. The effective link between Asphalt and CABC is critical since it aids in minimizing longitudinal shear stress gradients caused by accelerating and breaking cars, especially if the Asphalt layer is thin, i.e., < 4 in.

Prime coat materials include cut back asphalt and emulsified asphalt (Diluted)

Rates of application:

Asphalt Cutback:                0.65 to 1.75 Litres / Sqm

Asphalt Emulsified:             2.3 to 6.8     Litres / Sqm

The lower application limit is for CABC that is very tight and has a high fines content, while the maximum limit is for very porous CABC.

Prime Coat should be kept undisturbed for 24 hours to allow it to permeate down into the CABC and not form a pond.

Tack Coat

TACK COAT:

Tack Coat refers to the layer between Asphalt Base Course (ABC) and Asphalt Wearing Course (AWC). Tack Coat is also put between the concrete deck slab of bridges and the AWC installed over it, or between the concrete slab of Rigid Pavement and the AWC.

Tack Coat's goal is to create a link between AWC and ABC (or concrete slab). If Tack Coat is not used, AWC will slip under traffic load and the pavement will fail owing to AWC spalling. When AWC is laid immediately over ABC, the bitumen within AWC is insufficient to produce a long-term effective link between the two layers. As a result, after rainfall and water penetration beneath the AWC layer, the link between AWC and ABC will cease to exist.

At this point, AWC will begin to slip, and slippage cracks will become visible, eventually leading to spalling. It should be noted that too much Tack Coat will cause AWC slippage; thus, its application rate should be controlled according to site conditions.

Tack Coat materials include cut back asphalt and emulsified asphalt (Diluted)

Rates of application:

Asphalt Cutback:                0.20 to 0.40 Litres / Sqm

Asphalt Emulsified:             0.25 to 0.70 Litres / Sqm

After the Tack Coat has dried and become "tacky," AWC should be applied over the ABC.

General Specifications, National Highway Authority (Pakistan) (1998)