Blast-cleaning often mandates the construction of a containment around individual bridge connections and the process also increases the potential for over-blast damage of adjacent painted bridge surfaces that must be repaired.Larger connection surfaces are typically shop blast-cleaned and shop primed only with an inorganic or organic zinc-rich primer and then bolted together in the field using hot-dipped-galvanized (HDG) or mechanically galvanized bolt assemblies consisting of a nut, bolt and washer.Smaller shop-primed connections are sometimes bolted in the shop prior to shipping.
Field finish painting of bolted connections is typically accomplished using an epoxy intermediate coat followed by a urethane finish coat. However, some bridge owners specify the application of an organic zinc-rich primer on mechanically galvanized bolt assemblies prior to the epoxy intermediate and urethane finish. Because galvanized bolt assemblies (as opposed to plain black bolts) are typically used in conjunction with steel connections primed with zinc-rich primer in the shop, that is our focus here. Because the bolt assemblies have a more complex configuration and are spaced so closely, cleaning and painting these surfaces presents challenges that require more time, effort and expense to properly complete, versus other bridge surfaces. Proper painting of bridge connections is critical because these are the surfaces where premature coating problems often begin. Standard practices used by various bridge owners and recommended by paint manufacturers are addressed in this article. The socket attached to the torque wrench typically damages (i.e. As an example, Figure 1 shows damage and rusting along the two bottom rows of galvanized nuts that exceeds what is typically observed on bridge projects. In this case, the galvanizing was damaged by using too large of a socket when tightening was performed. Care must be taken to use the proper tools and techniques during installation. WJ-4, Waterjet Cleaning of Metals Light Cleaning would apply in this case. Field experience has shown that more intensive cleaning to remove all spot-rust results in little, if any, improvement in paint system performance and the extra time, expense and effort required to accomplish removal is not warranted. Inquiries to bridge owners and coating manufacturers confirmed that their standard practices and recommendations for field cleaning of bolted connections are consistent with the methods discussed previously. ![]() On one bridge project, concerns arose because field knife adhesion testing on HDG bolts caused forced separation of the system from the galvanizing, whereas the same paint on the surrounding inorganic zinc-primed steel could not be removed to the substrate. Further, laboratory adhesion testing on numerous painted test panels with galvanized bolts installed also showed that adhesion was weaker on the galvanized bolt assemblies even though the performance was excellent. Before field adhesion concerns are raised, the expectation should be known. Attempting to compare paint adhesion values on galvanized (particularly on smooth HDG) versus inorganic zinc-primed steel is likely not valid. ![]() Some of these solutions require post-rinsing and neutralization (usually with clean potable water) but no-rinse formulations are also available. Blast-cleaning can damage or remove the superior corrosion protection properties provided by the galvanizing on the bolt assemblies and the shop-applied zinc-rich primer on the connection plate.
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