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Publication Number:  FHWA-HRT-14-093    Date:  December 2014
Publication Number: FHWA-HRT-14-093
Date: December 2014


Interlaboratory Variability of Slip Coefficient Testing for Bridge Coatings


Steel bridge components are frequently secured together with high-strength bolts to facilitate erection of the entire bridge. In applications where the connection is subjected to tension or reversal loads, the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design Bridge Design Specifications (LRFD BDS) requires that they be designed as a "slip-critical" connection.(1) Slip-critical connections are a class of bolted connections where the transfer of force from one element to the next is through friction rather than bearing on the bolts. To develop the required frictional resistance, two elements are required: a minimum clamping force from the bolts that is ensured through proper installation techniques and a surface with a guaranteed minimum level of slip resistance, which can also be referred to as a "friction coefficient" or "surface condition factor" per AASHTO.

The AASHTO LRFD BDS specifies three possible surface condition factors, which are referred to by class (A through C). Class A has a surface condition factor of 0.33, while class B has a surface factor of 0.50. Class C only pertains to galvanized surfaces and is not relevant to this report. Typically, most designers would specify a class B slip resistance because it requires the least number of bolts to meet the design requirements. An unpainted and blast-cleaned surface can achieve class B slip resistance, but if a coating is applied on the faying surface, that coating must demonstrate that it can meet class B slip resistance. Coating manufacturers demonstrate this property through testing. For bridges, coating systems are regularly evaluated through the AASHTO National Transportation Product Evaluation Program (NTPEP). One of the evaluation criteria of NTPEP is qualifying the slip resistance of primers used in bridge coating systems, as it is the zinc-rich primer that will be the coating on the faying surface.

The test to evaluate slip resistance is specified in the Research Council on Structural Connections’ (RCSC) Specification for Structural Joints Using High-Strength Bolts.(2) Appendix A of the publication defines the testing method to determine the surface condition factor for coatings used on steel. In the rolling evaluation cycles run by NTPEP, while inorganic zinc-rich primers tend to easily pass class B slip, organic zinc-rich primers have shown mixed and inconsistent performance in slip testing. In recent years, there has been a propensity of organic zinc-rich primers to fail class B slip resistance where previously they demonstrated class B slip resistance. The new data have been anecdotally described to "just barely fail" class B slip resistance, though much of the data were proprietary being paid for by paint manufacturers and not available for public consumption. Depending on the entity, the differences in slip resistance have been blamed on the testing agencies, the coating manufacturer, and/or the test method. This report presents an objective view of the RCSC test method.


The objective of this study was to take a comprehensive but neutral look at slip resistance of organic zinc-rich primers to understand the variables that control results and cause variation or error in the slip testing results. To understand the variability associated with the test method, this work focuses on the RCSC test method through an interlaboratory variability testing regime of four different labs, five different organic zinc-rich primers, and two coating thicknesses. The goal of the testing was to make recommendations for a more robust testing procedure.


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