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NCHRP Project 14-30 3 CHAPTER 1 Background, Project Objectives, and Research Approach 1.1 Introduction Of the approximately 614,000 bridges in the FHWA National Bridge Inventory, about 170,000 are steel bridges that rely on protection from corrosion damage by thin film coatings or paints. Those coatings have a finite lifespan in relation to the steel they protect. Over time, they degrade eventually requiring repair or replacement. Maintenance of protective coatings on steel bridges is a major problem for transportation agencies, especially state highway agencies. Many lack the resources to continually re-paint their bridges in a timely manner. As a consequence, they will require costly demand maintenance for steel replacement or incur increased structural risk due to loss-of-section to critical structural components. The coating deterioration process is accelerated at highly stressed bridge locations that typically fail earlier than others subjected to more moderate exposures. The primary stresses for bridge coatings are extended time of wetness (TOW), soluble salt contamination, condensation/evaporation cycles and ultra-violet (UV) radiation from sunlight. Premature coating failures may also be attributed to variations in substrate condition, application, material variability in the resulting film, and inadequate/excessive film build. Frequently, bridge coatings will experience small areas of localized failure, while most of the coating remains intact and capable of years of additional service. In those situations, the failed coating areas can be repaired by recoating without affecting the balance of the intact existing coating (i.e., spot painting). The most cost-effective approach for state highway agencies to address bridge maintenance painting is to employ a âtoolkitâ containing the four maintenance painting options: 1) removal and replacement, 2 overcoating, 3) zone painting, and 4) spot painting. Spot painting is the lowest first-cost option (in terms of total cost) for restoring overall coating integrity and protection on many bridges. In determining the appropriate maintenance painting option, selection of spot painting depends, in part, upon the number, areas and disposition of coating failure sites to be repaired, along with the condition/type of the intact existing paint. A vital factor in selecting the use of spot painting is the identification of proper spot repair coatings. Repair coatings must possess application characteristics that match the level of worker skill and supervisor knowledge of the paint crews using them. They must also perform well, often over marginally prepared substrates and match or exceed the subsequent performance of the existing coating in areas where it is performing acceptably. A practical evaluation procedure is needed to identify spot coatings that can be used successfully over a variety of existing bridge coating types. 1.2 Project Objectives The objective of this research was to produce guidelines promoting the use of spot painting to extend the service life of existing coatings on steel bridge. Those guidelines included: Develop a methodology to identify suitable spot coatings - Accelerated laboratory testing procedures were to be used to:
NCHRP Project 14-30 4 ⢠Qualify spot coatings for spot painting repairs over specific existing coatings on bridges, and ⢠Assess their potential service lives for those applications. Provide guidance for state highway agency officials to deploy spot painting - Identify commonly employed structural painting methods that constitute: ⢠Best practices for spot painting on bridges incorporating the selection process, proper materials handling, cleaning, soluble salt remediation, mechanical surface preparation, verification of ambient conditions, coatings application, and compliance with worker safety and environmental regulations, and ⢠Best practices for cleaning/mechanical surface preparation to the SSPC-SP 3 standard. Address key issues in spot coatings selection - Review the existing coatings industry knowledge base to: ⢠Assess the advantages and limitations of generic coatings/systems for spot painting, ⢠Summarize application characteristics of spot coatings (including worker safety issues) that affect ease of use (user-friendliness and adaptability) for spot painting by state highway agency maintenance personnel, ⢠Assess non-traditional coatings/materials used to supplant or supplement conventional spot coatings for spot painting, ⢠Identify climatic conditions (e.g. environments, prevailing weather) affecting spot coating application and service performance, ⢠Recommend selection procedure of spot coatings for use over specific existing coatings with regard to compatibility issues and precautions in painting over weathered materials, ⢠Identify worker safety and environmental remediation issues, and ⢠Determine service performance of spot coatings over existing coating types and performance matching based upon condition of existing coatings. 1.3 KTC Research Approach The Kentucky Transportation Center used a six-step approach to address the goals of this project. The primary goal (first step) was to develop a test method that can be used to assess existing coating systems to be used for spot painting over common generic types of coating systems used on bridges (Chapter 2 of this report). That method was applied to evaluate a range of coatings, tapes, and greases used for spot painting/repair of localized failed coatings using the aforementioned common coating types serving as existing bridge coatings and determine their behavior when subjected to accelerated laboratory tests replicating several common service conditions on bridges (i.e. direct UV exposure with condensation/evaporation cycles and sheltered sites with inconsequential UV exposure but extended TOW). An important factor was the preconditioning of steel samples to replicate rusted steel with soluble salt contamination combined with surface preparation by SSPC-SP 3, âPower Tool Cleaning.â The second step was the review the large body of pertinent documents (reports, presentations, papers, guidance documents, standards, specifications and regulations) and
NCHRP Project 14-30 5 preparation of guidance to assist state highway agencies in deploying in-house forces (or contractors) to spot paint bridges effectively and efficiently. This work was addressed in the interim report to the NCHRP project review panel. The third was the analysis of a DOT/bridge owner survey distributed by KTC researchers. In addition, the findings of three other bridge painting surveys, one prepared and distributed by KTC in 1991, another in 2012, and a survey prepared and distributed by a consultant for MnDOT in 2013 were analyzed. Collectively, the surveys identified past and current agency practices relative to coatings decision making, scoping, and application. Barriers for wider deployment of spot painting by state highway agencies and other bridge owners were identified and addressed. The specific survey findings were provided in the interim report to the NCHRP project review panel and used indirectly to prepare the final NCHRP 14-30 guidance document. The fourth was the performance of field spot painting using the SSPC-SP 3 surface preparation method and the coatings evaluated in the laboratory accelerated performance testing. Concurrent with that work, initial field assessments of existing coatings on the subject bridges were performed by KTC researchers using procedures identified in the literature review. The fifth was the identification of best practices for state highway agency officials and personnel to: prepare budgets/warrants for deploying spot painting, identify necessary equipment and materials, address worker safety and environmental issues, and evaluate the acceptability of the completed work. Those are presented in a companion guidance report (Hopwood et al. 2018). The sixth and final step was to provide the information to support wider use of spot painting in the form of practical and implementable guidance documents targeted at specific agency audiences (as previously noted above). Some of that information was garnered from Phase 1 work (the interim report to the NCHRP project review panel) and the Phase 2 laboratory and field investigations provided in this report.
