Tack coat asphalt

Types of tack coat asphalt

A tack coat is a thin layer of asphalt binder applied to an existing pavement surface to bind the new overlay of the asphalt to the existing surface. There are a few types of tack coats, such as emulsified asphalt tack coat, SS-1h, diluted CSS-1h, and hot applied tack coat.

• Emulsified asphalt tack coat: The emulsified asphalt tack coat is a mixture of water, asphalt, plus an emulsifying agent. It provides a uniform spray application while heating the material. The emulsified asphalt is divided into two categories: cationic and anionic. The most commonly used cationic emulsions are SS-1h and CSS-1h, while anionic emulsions include AC-1h. The cationic SS-1h is a straight-run, low residual, slow setting used as a tack coat between layers of pavement or for surface sealing. The cationic diluted CSS-1h is a diluted version of CSS-1h, used when soil and surfacing are absorbent. In addition to being used as a tackifier, it can act as a fog seal. AC-1h is an anionic emulsified asphalt used for fog seal coats.
• Diluted emulsified asphalt: Diluted emulsified asphalt is a tack coat. Water is mixed with emulsified asphalt to create a diluted product. This process makes the material easier to apply and absorb. The diluted emulsified asphalt is often used for surfaces to absorb the tack coat and is ideal for spraying applications. Some common diluted emulsified asphalts include CSS-1h and CSS-1.
• Hot applied tack coat: Hot applied tack coats are typically applied between layers of pavement to improve bonding. They are often applied in the field, ensuring that there is uniform bonding between layers. Hot applied tack coats are mainly used for bonding new asphalt layers to existing surfaces and are typically used for larger projects where strong adhesion is necessary.

Specifications and Maintenance of Tack Coat Asphalt

Understanding the performance and repair needs of tack coat asphalt is vital for getting the most out of it.

• Consumption

  The volume of asphalt glue used depends on the surface it is applied to. On a rough surface, more glue (0.5–1.5 liters per square meter) is needed than on a smooth one (0.3–0.5 liters per square meter). Surfaces like old asphalt, concrete, and prior tack-coated layers require different amounts. A bonding strength of 60-80% is acceptable when using the minimum amounts.

• Environmental Conditions

  Asphalt glue should only be applied in dry weather where the temperature is above 5 °C (41 °F). Surfaces must be clean of debris, wetness, dirt, and pollutants to allow proper adhesion. Prior heating of surfaces by sunlight is helpful. Tack glue must avoid contact with rain during the first 24 hours post-application to prevent washout. Higher temperatures (up to 60 °C or 140 °F) are acceptable for the glue to be effective. Low humidity (below 50%) is ideal for quick solvent evaporation and bonding strength. Heat sources like propane or acetylene torches assist surface drying if conditions are not ideal. Refrain from using flammable sources on tar or dirt surfaces. The following table summarizes the optimum conditions for tack coat usage:

  Table: Optimum Conditions for Tack Coat Usage

  • Parameter
  • Tack Coat
  • Value
  • Surface Temperature

  60 °C

  • Ambient Temperature

  41 °C

  • Humidity

  50%

• Application Equipment

  A uniform application of asphalt glue is required. This can be accomplished with a brush, roller, or spray machine. When using a brush or roller, a 40 cm (16 in) wide brush is acceptable. For spray applications, a spray trailer with an airless paint sprayer capable of 3000 psi is required. The spray nozzle size must be 0.5-0.7, and a pressure of 1000-1500 psi is acceptable. Nozzles with a higher number in a range produce smaller droplets but better coverage. The spray creates more uniform and consistent application, which improves the coating's quality and performance.

• Surface Preparation

  Proper surface preparation is important for good glue coating and bonding. Oil and other materials must be washed off with a degreaser or steam, followed by high-pressure water cleaning. The surfaces to bond must be clean, dry, sound, and free of dirt and debris. Any potholes and cracks must be filled before applying the tack coat. An air compressor should blow out any moisture and dirt in holes and cracks before filling them. The surface to be coated must also be free from any moisture or standing water. Ambient conditions must be followed for the surface to be dry. Crack filling with fiber-deck sealer creates a smooth surface that maximizes the surface area for the glue. Carrying out these steps leads to a better quality asphalt glue tack coating.

Scenarios of using tack coat asphalt

There are several scenarios in which a tack coat may be applicable. Here are some of them:

• Overlay on milled surface

  A tack coat is often required when putting a new layer of asphalt on a surface that has been milled. The asphalt milling pays a rough surface that needs to be smoother. The rough surface acts as an ideal candidate for an overlay.

• Overlay on alligator cracks

  In some cases, an asphalt surface may develop alligator cracks. Rather than completely removing the surface and starting all over again, contractors may find an overlay to be a more cost-effective solution. By applying a tack coat and then an overlay, the new asphalt can bond well and provide a longer-lasting surface.

• Overlay on a non-milled surface

  When a road surface isn't too damaged, an overlay can improve it. However, for the overlay to be effective, a tack coat must be applied first. The tack coat will ensure that the new asphalt overlay bonds properly and provides a good surface for drivers.

• New road construction

  In some cases, a new road is being built. Once the base has been established, a thin tack coat may be applied before the final layer of asphalt is laid. The tack coat will help bind the last layer of asphalt to the layers below, ensuring a strong and stable road.

• Repaired area

  Over time, asphalt roads may develop potholes or cracks that need repairing. Asphalt patches can sometimes be applied without completely replacing the road. Before the patch is applied, a tack coat should be laid down to ensure the patch bonds well with the existing asphalt.

• Joint treatment

  When two different asphalt surfaces meet, a joint is formed. Joints are often weak points in asphalt surfaces and can lead to cracking and water infiltration if not properly treated. A tack coat can be used to bond different asphalt surfaces together, reducing the chance of joints failing and water infiltrating.

How to choose tack coat asphalt

• Surface condition:

  The state of the base or surface to be treated is a crucial component when selecting the tack coat. Variables like cleanliness, dryness, texture, and absorptive qualities of the surface should be analyzed. For example, surface contamination like dust or oil may necessitate the choice of a tack coat that works effectively on contaminated surfaces or pre-treatment strategies.

• Emulsion vs. asphalt:

  Whether to use an asphalt emulsion or a diluted liquid asphalt must be decided. Asphalt emulsions offer benefits like quick setting times and low-temperature requirements, making them suitable for various conditions.

• Application method:

  The preferred application technique should be compatible with the chosen tack coat. Whether hand spraying for more controlled application or machine spraying for large-scale uniform application, selecting a method that fits project requirements is crucial.

• Temperature and weather:

  The prevailing temperature and weather conditions during the application phase are essential factors to consider. Some tack coats perform better under specific temperature ranges, while others have advantages in humid or dry conditions.

• Project constraints:

  Consideration of factors like budget constraints, project timeline, and accessibility of materials is crucial. Selecting a tack coat that aligns with project limitations ensures efficiency and cost-effectiveness.

• Performance requirements:

  Ultimately, selecting the ideal asphalt tack coat depends on performance requirements such as bond strength, durability, and resistance to moisture-induced stripping. By considering these factors, a suitable tack coat can be selected that meets the specific needs of the paving project.

FAQ

Q1: What is the difference between tack coat and prime coat asphalt?

A1: The prime coat is characterized by deep penetration into the base layer, while the tack coat is a thin, sticky layer applied to provide a bonding layer between two asphalt layers. The prime coat is usually employed when damp or dry untreated granular substrates are present, while the tack coat is used to ensure bonding between two asphalt layers.

Q2: Does tack coat bind to dust?

A2: Dust can severely impair the bond between the tack coat and the surface, resulting in stripping and delamination. Therefore, one should always aim for a clean and sound surface to which the tack can bind. Nevertheless, some researches suggest that surface dust may sometimes enhance the strength of the tack by creating a pliable interface.

Q3: Does the asphalt tack coat weaken over time?

A3: It is possible for the bond established by the asphalt tack coat to weaken over time, leading to potential delamination between the layers. However, the asphalt material is known for its durability, and with proper maintenance, any weakening of the tack coat can be easily mitigated.

Q4: Is asphalt emulsions the same as asphalt cement?

A4: No. Tack coat is often referred to as asphalt emulsion. While both are composed of the same fundamental material, they differ in their physical and chemical properties.