Key Steps in Asphalt Construction
The tack coat is an oil layer that must be sprayed within 6 hours after the base course is compacted. It uses emulsified asphalt PC-2, and the dosage is determined through trial spraying to ensure a penetration depth of at least 5mm. Subsequently, an emulsified asphalt PC-1 undersealing coat should be laid, using 1.0 liter per square meter, with aggregates of 0.5–1cm to ensure a thickness of at least 0.6cm. Before paving the asphalt concrete, a tack coat should be sprayed on specific areas above the undersealing coat, between the upper and lower layers, and on the sides of curbs, drains, manholes, and other structures. Emulsified asphalt PC-3 is used, with a dosage of 0.5 liters per square meter.
◆ Difference between Prime Coat and Sealing Coat
For highways and Class I roads in rainy and humid areas, where the asphalt surface layer has a high porosity and is prone to severe water seepage, or where vehicles need to pass through even though the asphalt surface layer cannot be laid immediately after the base course is laid, it is recommended to lay the under-sealing coat immediately after spraying the prime coat. It is crucial to clearly distinguish between the under-sealing coat and the prime coat: the under-sealing coat aims to seal the surface, while the prime coat requires penetration to a certain depth. Although some projects may attempt to use an impermeable prime coat topped with aggregate and sand as the under-sealing coat, this does not replace the function of the prime coat.
◆ Prime Coat Construction Details
In addition, slurry seal coats are commonly used for preventative maintenance on Class II and lower-grade roads and are also suitable as the under-sealing coat for newly constructed roads. The under-sealing coat, as a structural layer placed on the surface of a semi-rigid base course, protects the base course from damage by construction vehicles, promotes the curing of the semi-rigid material, prevents rainwater from seeping into the structural layers below the base course, and enhances the bond between the surface layer and the base course. Although there are various methods for applying a base coat, practice has proven that a single-layer asphalt surface treatment is an economical and effective approach.

◆ Tack Coat and Applicable Conditions
Finally, let's understand the specific functions and applicable conditions of asphalt prime coats, tack coats, and seal coats. The main function of a prime coat is to promote a good bond between the asphalt surface layer and the non-asphalt base course. This is achieved by applying emulsified asphalt, coal tar, or liquid asphalt to the base course to form a thin layer that penetrates the base course surface. When specific conditions are met, such as when the base course has reached the specified strength, smoothness, and dryness, a prime coat should be applied in a timely manner to ensure construction quality.
① A prime coat must be applied to graded gravel or graded crushed stone base courses in asphalt pavements.
② A prime coat is also required for soil layers stabilized by inorganic binders such as cement, lime, and fly ash, as well as for granular semi-rigid base courses.
◆ Seal Coat's Function
Next, we will discuss the function and applicable conditions of a tack coat. The main function of a tack coat is to enhance the adhesion between the asphalt layers of the pavement and to promote a strong bond between the asphalt layers and the cement concrete pavement. In specific situations, such as when it is necessary to improve the adhesion between pavement layers or prevent asphalt surface layer slippage, a tack coat of asphalt should be applied in a timely manner.
① Before laying the upper layer of a two- or three-layer hot-mix asphalt pavement, if the lower asphalt layer is contaminated, appropriate treatment is required.
② When adding a new asphalt layer on an old asphalt pavement layer, it is necessary to ensure the adhesion between layers.
③ When laying an asphalt surface layer on a cement concrete pavement, special attention should be paid to the application of the tack coat of asphalt to ensure a strong bond.
④ The sides of curbs, storm drains, manholes, etc., that come into contact with the newly laid asphalt mixture also need to be properly treated to prevent asphalt surface layer slippage.
◆ Application of Slurry Seal
Next, we will further discuss the function and applicable conditions of seal coats. The main purpose of a seal coat is to seal the voids on the pavement surface, preventing moisture from penetrating the surface layer or base layer, thereby ensuring pavement stability. The seal coat applied to the surface layer is called the upper seal coat, while the seal coat applied below the surface layer is called the lower seal coat. In specific situations, such as when sealing surface voids or preventing moisture infiltration, a top seal should be laid on the asphalt surface layer.
① When the asphalt surface layer has large voids, leading to water permeability problems.
② For old asphalt pavements with cracks or that have already been repaired, enhanced protection is required.
③ To improve the skid resistance of old asphalt pavements, a wearing course needs to be added.
④ For newly constructed asphalt pavements, a corresponding seal treatment is also necessary to lay a wearing course or protective course.
Next, we will further understand the function and applicable conditions of slurry seal. Slurry seal is a fluid asphalt mixture made by mixing appropriately graded stone chips, sand, or fillers with emulsified asphalt, admixtures, and water. It is evenly spread on the road surface to form a protective asphalt seal layer. When one of the above situations is encountered, it is appropriate to lay a bottom seal under the asphalt surface layer.
① In rainy areas, if the asphalt surface layer has wide voids, water permeability problems will become prominent.