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Slip and creep of thermal spray coatings (TechBrief).

Publication Number: FHWA-HRT-14-083

When used in high-strength bolted connections, coating systems to protect steel bridge systems and components from corrosion must meet required levels of slip and creep performance. The American Association of State Highway and Transportation Officials' specifications for load and resistance factor design of bridges require bolted connections to be designed as slip critical under certain circumstances, including heavy impact loads or severe vibration. Slip-critical connections rely on the clamping force from bolts to develop frictional shear stresses to transfer force from one bridge element to the next.

In severe environments, thermal spray coatings offer better long-term corrosion protection for steel bridge systems and their components than zinc-bearing paint systems. Yet these coatings have not been mainstreamed into practice, in part because of the unknown frictional resistance they provide for slip resistance within high-strength bolted connections. This technical brief discusses research aimed at understanding key variables that influence the slip and creep performance of thermal spray coatings in high-strength bolted connections. It introduces limited data on slip coefficients developed by sealed and unsealed coatings.

Researchers found that because of rough textures, unsealed zinc and zinc/aluminum alloy thermal spray coatings had no problems passing slip performance requirements. However, once the surface was sealed, neither coating system could meet slip performance criteria. Until further research can demonstrate slip-critical performance of sealed thermal spray coatings, the researchers recommend that slip-critical surfaces be either masked off during manufacturing to prevent sealing or assembled before coating sealers are applied.

This document is available to download at www.fhwa /bridge/l4083/index.cfm. Printed copies are available from the PDC.

Compiled by Lisa Jackson of FHWA's Office of Corporate Research, Technology, and Innovation Management

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Title Annotation:Communication Product Updates
Author:Jackson, Lisa
Publication:Public Roads
Geographic Code:1USA
Date:Mar 1, 2015
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