For decades, state and regional agencies have used waterproofing membrane systems on bridge decks to prevent corrosion. Designers typically cover these membranes with a layer of asphalt concrete pavement (ACP) to protect the waterproofing and provide a surface for vehicular traffic. However, despite this long track record, regional differences in material specifications, construction details, and climate have led to mixed results. In some cases, moisture-related damage to the pavement or poor adhesion between the pavement and waterproofing can lead to premature failure.
There is little consensus in the industry about how to address these adhesion problems on bridge decks, which pose special challenges that differ from typical roadway construction. This has led some agencies to stop using waterproofing membranes altogether (opting for less effective or more expensive alternatives), while others continue on successfully.
Researching New Test Methods
There is no nationally recognized standard to define adhesion requirements for ACP systems with waterproofing membranes. To help improve our understanding of how these assemblies perform, my colleague Tony Khoury and I, in partnership with Jonathan Haydu of Bridge Preservation LLC, conducted laboratory testing here at SGH to evaluate the interlayer shear strength of ACP and waterproofing membrane systems in different assemblies. We published this research as part of a longer series in the Civil + Structural Engineer magazine article, “Shear Resistance of Pavement and Waterproofing Systems.”
In our research, we adapted a standard test method (AASHTO TP114-15) typically used to test the shear strength of two ACP layers to assess the strength between ACP and waterproofing. We prepared thirty-eight cylindrical specimens for shear testing consisting of a cored concrete base with lab-compacted ACP on top. We introduced several combinations of membranes and tack coats in the samples and evaluated their related effects. Our testing demonstrated the differences in shear bond strength that can be achieved by using different tack coats. It also showed the difference in shear bond strength of pavement applied over a waterproofing membrane versus pavement over bare concrete.
The shear bond at the ACP-to-membrane interface is likely affected by many variables, including:
- ACP mix design and compaction
- Tack coat type and application rate
- Environmental conditions
- Waterproofing membrane chemistry, stiffness, and surfacing
- Construction workmanship
Results and Next Steps
Waterproofing membranes help to prevent corrosion, but can also reduce the shear strength at the interface between pavement and the underlying concrete structure. Many of these assemblies can perform adequately in the field, but project teams need to take special care during design and construction. Further testing, research, and standards development regarding shear strength in these assemblies can contribute to the durability of bridges.