United States Department of Transportation
i
Corrosion-Resistant Steel Fastener Assemblies for ASTM A709 Grade 50CR Steel Bridges
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2021-03-01
[PDF-4.98 MB]
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Edition:Final Report
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Abstract:The purpose of this study was to evaluate three types of corrosion-resistant steel fastener assemblies for use on ASTM A709 Grade 50CR (hereinafter “50CR”) steel bridges. The three types were ASTM A193 Grade B8 Class 2 (hereinafter “A193 B8-2”); Type 2205 duplex stainless steel (hereinafter “2205”); and bolts made from a martensitic chromium alloy (MCA). The evaluation of the corrosion-resistant steel fasteners included mechanical tests on individual elements of the fastener assembly, such as proof loading and wedge tests on bolts; proof tests on nuts; and hardness tests on bolts, nuts, and washers. Five types of lubricants were also evaluated in terms of chemistry and their effectiveness in reducing galling. Fastener assemblies were also subject to torqued tension and relaxation tests to evaluate their assembly performance. Long-term corrosion test samples of the bolts with 50CR steel were also placed at an exposure site to monitor over time. A cost analysis was also conducted to evaluate the cost of corrosion-resistant steel fastener assemblies relative to standard fastener assemblies. Test results showed that corrosion-resistant fastener assemblies exist for use with 50CR steel. Specified minimum bolt pretension values and installation parameters, such as the nut rotation for turn-of-nut installation, were determined for these corrosion-resistant fastener assemblies. The study showed that A193 B8-2 and 2205 bolts can be pretensioned to 30 kip whereas MCA bolts can be pretensioned to 49 kip. All corrosion-resistant nuts used in this study met their required specifications for proof and cone proof load tests. Testing confirmed that washer hardness is critical to a bolt’s installation performance. The 2205 and MCA washers performed well, but the 303 washers used in this study were insufficiently hardened, leading to poor performance of some A193 B8-2 bolts. A comparison to the findings of previous studies on A193 B8-2 bolts confirmed that harder 303 washers can lead to successful performance of A193 B8-2 bolts. Testing showed that one lubricant was much more effective in reducing galling than the other lubricants. The study also showed that corrosion-resistant bolts lack dimensional standards and their commercial and domestic availability needs to continue to be evaluated. A cost evaluation showed that A193 B8-2 and 2205 fastener assemblies can be expected to cost approximately 8 to 10 times more than galvanized A325 fastener assemblies. The study recommends that the Virginia Transportation Research Council continue evaluating corrosion-resistant steel fasteners, especially in terms of minimum hardness values for washers, dimensional standards, and domestic and commercial availability, and develop a research needs statement to propose accelerated corrosion testing on standard and corrosion-resistant steel fastener assemblies to determine their relative corrosion resistance. The Virginia Department of Transportation should develop a special provision for using corrosion-resistant steel fastener assemblies on its projects, including their use with 50CR steel and in corrosive environments. This special provision should consider allowable bolt/nut/washer combinations, dimensional requirements, acceptable hardness limits, allowable lubricants, acceptance testing, installation procedures, and specified minimum bolt pretension.
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