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Publication Number:  FHWA-HRT-14-083    Date:  September 2014
Publication Number: FHWA-HRT-14-083
Date: September 2014


Slip and Creep of Thermal Spray Coatings

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FHWA Contact: Justin Ocel, HRDI-40, (202) 493-3080, justin.ocel@dot.gov



All steel bridge systems and their components need some level of corrosion protection to assure a serviceable life. One of two approaches is typically used: either the bridge component is fabricated from a corrosion-resistant alloy, or the steel is coated for protection. The most common coating practice is use of a multilayered paint system over a zinc-rich primer. Other coating alternatives for corrosion protection are hot-dip zinc galvanization and thermal spray coatings (TSC). Both galvanization and TSCs offer better long-term corrosion protection than zinc-bearing paint systems in severe environments. For this reason, these alternative-coating systems need to be mainstreamed for the protection of steel bridges.

In addition to corrosion resistance, the coating must be compatible with use in high-strength bolted connections. The American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) Bridge Design Specifications require bolted connections be designed as “slip-critical” if the connection is subjected to “stress reversal, heavy impact loads, severe vibration or located where stress and strain due to joint slippage would be detrimental to the serviceability of the structure…”(1) Slip-critical connections rely on the clamping force from the bolts to develop frictional shear stresses as the means to transfer force from one element to the next. This construction is in contrast to bearing connections, in which the individual connection elements bear on the bolt and the force is transferred through shear stresses in the bolt itself. In the design of a slip-critical connection, the engineer must select a “frictional slip coefficient” between the layers of a connection to calculate the slip resistance. AASHTO refers to this frictional value as a “surface condition factor,” although in this TechBrief, it will be referred to as the “slip coefficient.” The engineer does not specify an exact slip coefficient; rather, the AASHTO LRFD Bridge Design Specifications provide three different categories (Class A, B, and C) from which the engineer can choose.

Class A surfaces have a minimum slip coefficient of 0.33, which can be achieved with unpainted, clean mill scale. Class B surfaces have a minimum slip coefficient of 0.50, which can be achieved with unpainted, blast-cleaned surfaces. In lieu of having bare steel on the slip surface, certified coatings applied over a blast-cleaned surface that demonstrates Class A or B performance may also be used. Class C surfaces also have a minimum slip coefficient of 0.33 but are only applicable for hot-dip galvanized coatings and are outside the scope of this TechBrief.

Coatings applied over blast-cleaned surfaces must be demonstrated through testing to achieve either Class A or B slip resistance and be certified as such. From the perspective of the bridge fabricator, there may be advantages to using slip-certified coatings in the faying surfaces of slip-critical connections. For instance, if the bridge will be painted, then it will have to be blast-cleaned prior to paint application, and primers should be applied shortly after blast-cleaning before the steel can flash rust. If the primer has been certified to provide a certain slip coefficient, then the entire piece can be primed without masking off the areas of the faying surfaces, a time-consuming step that adds cost to the overall fabrication of the bridge. The AASHTO LRFD Bridge Design Specifications say nothing about the use of TSCs on the faying surface. That is not to say they cannot be used, but because they are not directly referenced, there may be an aversion to specifying their use because of their unknown slip resistance.

As shown in figure 1, application of TSCs is analogous to painting, but the spray is droplets of molten metal. At the application gun, wire stock is melted with either a flame or an electric arc, and compressed air sheds the molten pool into a spray of droplets. The droplets are propelled toward the surface, where the molten droplets land on the surface and solidify. Hot-dip galvanizing produces a solid layer of zinc chemically bonded to the steel substrate, but a TSC of zinc leaves a porous layer of zinc that is only adhered to the steel substrate. For this reason, it is critical that TSCs are applied over a blast-cleaned surface with an angular anchor profile so the droplets can interlock with the roughened steel surface. Because the droplets also randomly form over each other, TSCs are inherently porous, and current practice recommends using topcoat sealant to fill the voids and prevent moisture infiltration.

This TechBrief introduces limited data on the slip coefficients developed by both sealed and unsealed TSCs.

FHWA-HRT-14-083 PDF Cover


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