Determining bond strength of micro-surfacing mixes

"June 2018."; "FHWA/OH-2018-9"--Technical report documentation page.; Includes bibliographical references (pages 13-14 and 74-76).; Phase 1 [Interim] Report;; Sponsoring agency: Ohio Department of Transportation; State Job NUmber 135491This report summarizes Phase 1 research work that was completed to document the current state-of-the-practice for using tack coat with micro-surfacing projects by state and provincial transportation agencies, summarize studies on the evaluation of bond strength between micro-surfacing mixes and existing pavement surface, and identify available tests that are used for measuring the interface bond strength to determine the most appropriate one to use in micro-surfacing applications. A national survey of transportation agencies was conducted to collect information from those agencies on using tack coat for micro-surfacing applications. The survey results indicated that there is no consensus among responding transportation agencies about the importance of using tack coat for micro-surfacing applications. Most agencies that use tack coat with micro-surfacing believe that it is critical for providing adequate bonding with the underlying surface, while the majority of the agencies that do not use tack coat believe that it is not needed, as adequate bonding can be provided by the emulsion in the micro-surfacing mix. However, some agencies that stopped using tack coat with micro-surfacing (such as Indiana DOT and Michigan DOT) have noted some debonding issues and decided or are considering adding a requirement for tack coat usage. The survey results also indicated that none of the responding agencies are performing field tests to evaluate the bonding strength of micro-surfacing mixes. The results of the literature review conducted in Phase 1 of this project indicated that achieving adequate bonding between the micro-surfacing mix and the existing pavement is important to ensure full load transfer between the micro-surfacing layer(s) and the existing pavement. In addition, some studies suggested that the agencies that do not require using tack coat with micro-surfacing assume that the consistency of the micro-surfacing mix permits it to be evenly spread over the pavement surface, forming an adequate bond to the surface. The literature review was used to identify several candidate tests for the evaluation of the bond strength of micro-surfacing mixes. Based on the results of Phase 1 it was recommended that the testing matrix for Phase 2 include the following variables: Existing surface conditions, micro-surfacing mix type, micro-surfacing mix residual binder content, tack coat application rate, and tack coat material type. It was also recommended that the variables be evaluated through a field-testing program. Furthermore, it was recommended to consider two types of field bond strength tests for Phase 2: pull-off and torque bonding tests