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NCHRP Project 14-30 1 SUMMARY Spot Painting to Extend Highway Bridge Coating Life Bridge coatings are the primary means of corrosion protection for steel bridges in the United States. Maintaining those coatings is a major challenge for State highway agencies that are faced with many bridges requiring repainting and constrained budgets that limit the amount of painting work they can perform. If the coatings are not preserved in a timely manner, they can fail and expose the steel bridge members to corrosion damage. That can lead to repairs or risks posed by corrosion losses to key structural members. Most bridge coatings tend to fail prematurely in localized areas and spot painting can be used to repair those. That will restore the lost corrosion protection and extend the service lives of existing bridge coatings, often at a fraction of the cost of a complete bridge repainting. However, many state highway agencies do not perform spot painting primarily due to performance concerns and lack of familiarity with its proper utilization and execution. This research was initiated to provide guidance to state highway agencies on âbest practicesâ for employing spot painting in a cost-effective, safe, and environmentally compliant manner. That guidance, less the evaluation method employed in this report, is incorporated in a companion document (Hopwood et al. 2018). One component of this project was to develop laboratory procedures for accelerated testing of coatings to qualify them for use as spot repairs to specific existing coatings on bridges and assess their potential service lives. A key facet was the focus on the SSPC-SP 3, âPower Tool Cleaningâ method for preparing damaged steel surfaces/existing coatings to receive applications of spot coatings. That method provides an acceptable level of surface cleanliness to promote reasonable spot coating performance under atmospheric exposure in mild or moderate climates. It can be utilized by a range of workers from in-house bridge maintenance crews to experienced painting contractor personnel. The KTC at the University of Kentucky developed a testing protocol that replicated typical spot failures on steel test panels using three types of coatings to duplicate existing bridge coatings. Corroded areas on the panels received surface preparation using the SSPC-SP 3 standard. The test panels were subsequently coated with six conventional 2-coat liquid-applied coating systems amenable to the SSPC-SP 3 standard and several non-traditional coatings -- a grease and a tape. Those were intended to assess both the test procedures employed and the performance of the coatings over typical substrates receiving spot coatings in the field. The coated test panels were subjected to conventional accelerated weathering and corrosion laboratory tests to stress the coatings and determine their performance under conditions that simulated several common types of coating exposure on bridges. The test panels were periodically evaluated for damage by rusting and blistering. At the end of the testing program, the coated flat panels had been exposed to 5,000 hours of testing which proved sufficient to fail lower-performing coatings and validate the test protocols as being suitable for screening coatings for use on bridge spot painting projects using SP 3 surface preparation and common (existing) bridge coatings or, at least, provide comparative performance data. A field test protocol was developed during preparation of the guidance document that enabled field researchers to conduct experimental spot painting tests on several bridges using those procedures in conjunction with the use of surface preparation employing the SSPC-SP 3 standard and coatings evaluated in the laboratory tests. Three Kentucky Transportation Cabinet
NCHRP Project 14-30 2 (KYTC) bridges were selected for testing which had coatings containing lead, requiring KTC to address the appropriate worker safety and environmental protection actions to achieve regulatory compliance. Prior to performing the field tests, coatings assessments were conducted to measure key parameters on the bridges that impacted the performance of maintenance painting work and the selection of spot painting as a viable option using the field test protocol. Based on those assessments, field researchers mobilized and applied spot coating repairs to the three bridges. The work encompassed two areas on one bridge with different problems â disbonding of the existing coatings (alkyds) and corrosion under a leaking deck joint. The spot coating work in the first area was intended to repair the failed existing coating and halt further disbonding. Spot work in the second area was intended to repair the failed existing coating and stop further corrosion. Work on the other two (twin) bridges was also intended to repair failed existing coatings and stop further corrosion. Work on those bridges addressed corrosion issues occurring with a different type of existing coating system than on the first bridge â an inorganic zinc/vinyl. The field spot painting work was performed in accordance with the KTC spot painting protocol and was successfully completed without incident. The field spot coating tests were evaluated 14 months after application and most were performing satisfactorily. The coatings had halted progressive failures of existing coatings in the disbonding problem areas. In the other areas with corrosion, there was evidence of rust-back to varying degrees with the conventional coatings, but in most cases, it was limited and the coatings were effective in halting widespread corrosion damage. More time would be required to determine the long-term performance of the field spot coatings and better correlate their performance with the laboratory test results. The research performed provides guidance for state highway agencies to address the use of spot painting as a viable maintenance painting option for bridge coatings. It further provides a laboratory protocol for assessing candidate spot coating systems. It also demonstrates a spot painting method that could be applied by relatively inexperienced state highway agency bridge crews to affect spot repairs of damaged coatings on steel bridges using minimal procedures such as SSPC-SP 3, âPower Tool Cleaningâ and brush application of liquid-applied coatings. Troweling and wall paper hanging procedures were used by KTC researchers to apply the grease and tape respectively. Those methods are also amenable for adoption by state highway bridge crews.
