Asphalt Institutes' Key Factors for Bond Coat Success - Bond Coat Best Practices
Bond Coat Manuals of Practice
Conclusions from FHWA Guidelines for Using Prime and Tack Coats (2005)
1. The purpose of tack coat is to ensure bond between the existing pavement surface and a new pavement surface.
2. A loss of bond between HMA layers can cause crescent-shaped slippage cracks or debonding to occur, leading to reduced pavement life.
3. Prior to tack application the surface should be clean, dry and free from loose material.
4. Applying tack is not a substitute for properly cleaning the existing HMA surface.
5. Tack coat should be applied in a thin coat and uniformly cover the entire surface, including all vertical surfaces of joints and structures. Too little tack coat can cause debonding and too much tack coat can cause slippage.
6. If possible, all traffic should be kept of tacked surfaces.
7. Tack should be applied to old existing HMA surfaces and PCC surfaces.
8. Tack has been successfully deleted between new lifts of HMA when the existing surface is still clean and tacky.
9. There is not complete agreement regarding the requirement that tack coat be allowed to break and set before placing the HMA layer.
10. Many factors were shown to affect laboratory interface shear strength, including rate of shear, magnitude of normal force, temperature and joint construction.
11. In a few studies, tacked surfaces were shown to have slightly lower interface shear strengths than untacked surfaces. However, in these studies the statistical significance of the difference in interface shear strength was not reported. In reports where the statistical significance of the differences in interface shear strength was evaluated, tacked interfaces were either stronger or not significantly different from untacked interfaces.
12. The higher the viscosities of the bituminous binder in the tack, the higher the reported interface shear strength.
13. At typically specified application rates, application rate had little effect on interface shearstrength. Higher than recommended application rates resulted in slightly lower interface shear strengths.
14. Diluted slow set emulsions are typically used for tack coat.