Tack is an adhesive term that relates to a sort of stickiness. A tack bond is a surface bond created by highly mobile or highly polar material. This is NOT what is meant by a tack coat in asphalt technology.
In the AEMA emulsion guidelines its defined as "A spray application of Asphalt Emulsion applied to an existing asphaltic Portland cement concrete pavement prior to a new asphalt overlay, or patching to eliminate slippage planes and provide a bond between new and existing pavement layers".
Sometimes erroneously called a prime coat a tack coat is solely to ensure a good bond on the next layer.
It need not be done with asphalt emulsion. In some countries cutback asphalt ( asphalt dissolved in solvents such as kerosene or diesel) are used and even penetration grade ( 200pen) asphalt is sprayed neat.
The residual binder is the key to the overall tack coat performance. However retained solvent or over application can have severe consequences.2. How does a Tack Coat work?
The tack coat is a very thin surface. The tack coat is sprayed using a tack coater spray system. It should be an even coat over the entire substrate to be overlaid.
The hot mixture placed on top should soften the tack coat layer allowing it to partially fill surface voids in the hotmix. So that when compaction occurs it becomes partially interlocked with the hot mix layer.
If used before a microsurfacing the tack coat will not be softened but the emulsion from the microsurfacing will wet the tack coat creating an asphalt/asphalt bond. ( Note in most cases tack coat is NOT required for microsurfacing except where the surface is a Portland cement concrete which is highly polished).
Films of asphalt have a stability based on their cohesion versus the thickness of the layer. To act as a good tack coat the layer must not shear sideways under traffic.
This means that thick films have relatively less cohesion. So over applied tack coats will cause a shear plane resulting is slippage or shear cracks in the hotmix.
Thus application rate is VERY important. If the surface is absorbent or open textured then more tack coat will be required. The aim is always to get a layer of membrane about 0.1-0.2mm thick.
For stiffer materials such as asphalt rubber or polymer modified emulsions the layers may be thicker. However as these materials are expensive there is little point.
An exception is where the overlay is open graded or has high voids levels. In such instances e.g. Novachip® or thin bonded layers, higher levels of tack coat are used. This is almost always polymer modified to ensure cohesion is sufficient to prevent shear slippage.3. Suitable Materials For Tack Coat
Emulsions are the preferred tack coat material. To achieve the low application rates required emulsions are ideal as they can be diluted.
The emulsion should break before the hotmix is applied. For this reason rapid set emulsions are used in some countries . However CRS emulsions with some emulsifiers s are not stable to dilution and slow set or medium set emulsions are used.
Where adhesion is problematic or where a thicker layer is desirable, latex modification may be required.
Suitable emulsions are:
- CRS-1 ( stable)
- CSS-1 30% emulsion
MC, SC, cutbacks have been used with success.
Where neat asphalt is used a difficulty in ensuring the application rate is usual and difficult to overcome.
The emulsion is diluted with water, the compatibility must be checked. It is important to do a compatibility test as follows.
- Fill a can (1 lt) half full with emulsion
- Add required amount of water
- Mix with a non metal spatula
- Examine for lumping by pouring through a 150um prewetted sieve
- Wash sieve thoroughly- any balls of material? How much?
- If more than 1% or so there may be an incompatibility problem- call manufacturer.
The water should always be added to the emulsion- NOT the other way around.
Do not store the diluted emulsion. Use it straight away (that day).
Warmed water may be used at 25-50C if desired. This will lower viscosity and make handling easier.
As in most processes that involve application to an existing surface to achieve adhesion requires a clean substrate. The road must be thoroughly swept so that all loose material and dust is removed.
Heavy oil drippings must be removed with detergent , burning or dig out.
Sometimes a light water spray on the surface is beneficial ( 0.6 lt/m2). This will improve flow of the material into the voids, however it should not be flooded.b. Equipment
The equipment for tack coating is not critical as long as it sprays uniformly. A chip seal type sprayer is ideal. Recommended spray nozzles are 4-5mm.
Emulsion may be heated to 50C
5. Tack Coats are not required sometimes:
Traffic control to slow vehicles should be maintained until the emulsion has cured to an extent that it will not pick up on the tyres of construction equipment. Pick up on tyres will cause bald spots on the surface and poor bonding.
If the surface has been primed, i.e. it is a new construction a tack coat is not necessary. If the surface has been stabilized with an asphalt based dust suppressant a tack coat is not required.
If a new membrane SAMI seal has been applied only a light tack coat is required.
Microsurfacing or slurry application on PCC pavements that are worn and absorbent will not require a tack coat. The slurry or microsurfacing mixture can be designed to ensure that the emulsion wets the surface of the pavement to create the bond required. In Slurry Surfacings of all types the mixture is highly mobile at spreading. Even under higher temperature conditions a spray of water is all that is needed.Tack coats are not tacky and do not really coat. They are a bridge layer between an existing surface and a new surface course. When the new surface course is a chip seal or slurry they will not be required. Where it is a hot mix or cold mix a tack coat is critical.
Where tack Coats may not be needed: primed surfaces, stabilized surfaces, chip sealed surfaces, membrane seals.