All types of substrates exhibit strength formation and growth issues (Emerson). Strength gradually develops during curing, and the internal structure becomes increasingly dense. Experiments show that for semi-rigid substrates, applying tack coat before the cement (material: powdered hydraulic inorganic binder) has resulted in deeper penetration; however, the penetration effect decreases with age and strength increase.

Simultaneously, timely application of tack coat effectively retains moisture in the substrate, preventing evaporation and ensuring strength formation. The substrate surface is also less prone to loosening. Tack coat also protects semi-rigid substrates from cracking due to intense sunlight. Furthermore, since the substrate is no longer covered with water for curing, the higher internal temperature is beneficial for strength growth.

In general areas, where air humidity is lower than in Ejina (where the average relative humidity is only 5%), the tack coat should be applied when the substrate surface is slightly dry but before it has developed strength. Asphalt distributors use external additives (modifiers) such as rubber, resin, polymers, finely ground rubber powder, or other fillers to improve the properties of asphalt or asphalt mixtures. The materials used for spraying can be liquid asphalt, emulsified asphalt, or coal tar pitch. The spraying rate is generally the median or lower value specified in the standard. After spraying, the distributor requires minimal management.

Selection of Spraying Materials: Emulsified asphalt should be used because it is water-based and contains some moisture. The oil film formed after spraying better seals the moisture in the base layer, preventing moisture loss. The spraying rate for the tack coat is 1.2~1.5 L/m2, with the specific rate determined based on trial spraying. Crushed stone aggregate with a particle size of 2.36~4.75 mm should be sprayed on the surface, with a dosage that prevents sticking to the rollers.

Points to Note During Spraying Due to the low local air humidity, some water needs to be added after rolling to ensure the base surface remains moist. When watering, the water truck's nozzles should not spray directly onto the base; instead, the nozzles should be pointed upwards so that the water forms a mist and falls naturally. Immediately after watering, the emulsified asphalt should be applied using the equipment.