Recent evidence in Washington State indicates that de-bonding of HMA surface layers may become a significant problem. “De-bonding” describes a condition where adjacent layers of HMA lose adhesion to one another and can become separated. Typically, design and construction practice is to build in a certain amount of bonding, however the appropriate amount, testing and techniques are still under debate. For WSDOT pavements, which are generally thick and long-lasting, this de-bonding is thought to be more prevalent between the surface layer (usually applied as a preservation overlay) and underlying layers. This de-bonding may contribute to early failure in HMA pavement surface layer, which can increase pavement preservation costs. This study gathers initial evidence on de-bonding in Washington State and attempt to define the problem scope and potential performance impacts. Specifically it attempts (1) determine if de-bonding occurs, (2) identify possible de-bonding mechanisms, (3) define the scope of de-bonding in WSDOT pavements, (4) determine de-bonding impacts on pavement performance, and (5) identify the role of tack coats in de-bonding.
Evidence examined in this study includes:
- Published research on HMA layer bonding and the role of tack coat over the last 30 years.
- Core logs from 3,402 cores across the state from the late 1990s to the early 2000s. Some of these core logs document de-bonding while others document in tact cores or do not have supporting documentation.
- Construction observations and Washington State Pavement Management System (WSPMS) data from 17 observed construction projects between 1999 and 2004.
- Three case studies from the WSDOT North Central region on projects that showed de-bonding either in pre-construction cores or during construction.
Based on examination of the evidence, the following conclusions can be drawn about HMA layer de-bonding in Washington State:
- De-bonding exists and does occur in Washington State.
- De-bonding is most likely caused by (1) poor tack coat between layers, or (2) water infiltration due to distress or inadequate compaction.
- It is difficult to estimate the extent of de-bonding in Washington. Based on core logs reviewed a reasonable estimate is that it occurs in some form on at least 10% of WSDOT jobs. Due to its localized nature it is unlikely that searches through large aggregate databases like WSPMS can identify it through surrogate indicators.
- Evidence is inconclusive on whether or not de-bonding reduces pavement life in Washington State. Theory and an observation at NCAT suggest that it does but statistics from the core logs and WSPMS hint at shorter pavement life but are not convincing.
The following recommendations are made to minimize the occurrence and detrimental impact of HMA layer de-bonding:
- Do not dilute tack coat.
- Continue to allow CSS-1, CSS-1h and STE-1 as tack coat emulsions.
- Continue to apply tack coat between all HMA layers, new construction or not.
- Adopt a test for tack coat application rate and uniformity and use it.
- Investigate new methods to reduce/eliminate tack tracking.
- Pay for tack coat as a separate bid item.
- Adopt a specification to remove thin de-bonded layers after milling.
Right now it is just a DRAFT version.