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ABSTRACT A summary on the disbandment of epoxy coating on reinforcing steel extracted from marine substructures in Florida is presented. The bridges examined in this study had been in service for periods ranging from 3 to 13 years. Disbandment was consistently observed in rebar in bridges older than 4 years. The disbandment developed even in the absence of significant chloride ion contamination of the concrete surrounding the rebar. Continued loss of adhesion was observed in most of the specimens even after prolonged storage (1 month to years) in a desiccator. Contamination of the epoxysteel interface was modest to very small, and the extent of contamination did not show any distinct correlation with loss of bond. \ INTRODUCTION This paper summarizes the FDOT-USF experience with coating disbandment of epoxy-coated rebar (ECR) extracted from bridge substructures. The ECR investigated was from actual production stock, manufactured between 1979 and 1989. While some new variations of rebar coatings are being presently made and beginning to be put in service, a very large inventory of structures (on the order of 100,000) have been built or rehabilitated in the U.S. and Canada with ECR manufactured using methods in force during the period mentioned above. There are no indications that the material examined in this investigation was not generally representative of the rebar used elsewhere during the same period. Attention to the problem of ECR corrosion developed first in 1986 with the detection of a corrosion span at the then 6-year old Long Key bridge in the Florida Keys. Numerous additional spans developed in subsequent years in that and other bridges built with ECR in the same geographical area [1-4]. By 1995, over 300 ECR corrosion related-spans are affecting 5 major bridges along U.S. 1 in the Florida Keys. Examination of the ECR in the concrete span regions showed that the epoxy coating itself was not visibly different from its condition prior to placement in concrete, but that significant corrosion of the steel had occurred beneath the coating. The coating could be easily separated by peeling it away from the corroding metal. The metal corrosion did not affect all the steel in the span area. Some portions of the steel were bright or only slightly darkened underneath the coating, but the coating could still be easily peeled off the metal. Examination of ECR extracted from surrounding regions of the substructure where no corrosion had developed showed nevertheless that significant loss of coating adhesion existed there as well. An extensive survey of Florida bridges built using ECR [2-4] showed that loss of coating adhesion to visibly uncorroded metal was widespread, independent of the amount of chloride contamination of the surrounding concrete. Two modes of coating disbandment were therefore observed, the first (A) associated with visible corrosion of the steel and the second (B) taking place in the absence of conspicuous corrosion. The evidence strongly suggested that mode B is a precursor to the development of corrosion and mode A disbandment, and this sequential development has been proposed as part of the overall mechanism of corrosion of ECR in concrete [1,2]. This paper concerns mode B disbandment. Mode B may be conceptually divided into submodes BW and BD (see Nomenclature). Mode BW or wet adhesion loss corresponds to the disbandment observed when the coating-metal surface system still retains a significant amount of the moisture prevalent in the concrete environment. Mode BD or dry adhesion loss [5] designates the loss of adhesion still observed after the rebar has been extracted from the concrete and allowed to dry in a desiccator for a period of several d
ABSTRACT The pH of the concrete pore solution is expected to be somewhat lower in concretes using pozzolanic additions than in concrete using unblended cements. Variations in pH pore solution may hold the key to explaining conflicting reports on the performance of galvanized rebar. To examine that factor, plain and galvanized rebars have been tested for over two years in concrete specimens made with cement type II, with various contents of fly ash and silica fume. Electrochemical impedance measurements and sensitive polarization techniques have been used to measure the rate of metal dissolution in the absence of chloride contamination at two different levels of concrete moisture. The plain steel specimens have shown little tendency for passivation in two of the cement compositions with the highest levels of pozzolanic addition. The galvanized steel passivated in all cases and showed apparent corrosion current densities of less than 0.3 µA near the end of the test. Implications of length of the corrosion initiation period and on current materials selection criteria are presented. INTRODUCTION Long term durability goals (75 years and more) have recently become common in construction of reinforced concrete highway structures. This poses new challenges in controlling reinforcement corrosion, which remains one of the most important limiting factors of the service life of concrete. Long-term corrosion control approaches are relying increasingly on extending the length of the initiation period of corrosion (the length of time in which the reinforcement surface is still in the passive condition) since the propagation stage (from the start of active corrosion until damage is externally observed) tends to last relatively few years. Galvanized reinforcing steel bars are being newly considered for long-term durability service in chloride-induced corrosion service, specially since adverse experience with epoxy coated rebar in marine substructures has created interest in examining alternative corrosion control methods. There are indications that the chloride concentration threshold for corrosion initiation of galvanized steel is greater than that for plain steel, with consequent extension in the length of the initiation period. However, exceptional stability of the galvanized layer inside concrete is required if the coating is expected to be effective after an initiation period that may well need to exceed 50 years. The chloride concentrations are small during the initiation period, but the galvanized layer is continuously exposed to an alkaline environment. Zn and Zn alloys exhibit anophoteric behavior, corroding actively in both acidic and likewise in highly alkaline media. In the absence of carbonation, the pH of concrete pore solutions is usually sufficiently high to prevent acidic corrosion, but excessive corrosion due to a too high pH is a distinct danger. In an extensive series of experiments Andrade and coworkers have shown that a continuous passivating layer of calcium hydroxyzirlcate forms on the surface of galvanized steel wheel the pH is 13.3 +0.1 or below. Modern concrete formulations for high durability in seawater service include commonly AASHTO Type II cement (for sulfate resistance), a low water to cement ratio, a high cement Factor with pozzolamic replacement. The latter consist of fly ash for reduced permeability and to reduce temperature rise in mass concrete applications) and, increasingly, microsilica for early strength and reduced permeability. The pozzolanic additions consume Ca(OH)2 and the reaction products may entrap alkali ions that would otherwise be present in the pore water. As a result, the pH of the pore water in concretes with silica fume and, fly ash additions can be significan
- Well Drilling > Casing and Cementing > Cement formulation (chemistry, properties) (1.00)
- Well Completion > Well Integrity > Subsurface corrosion (tubing, casing, completion equipment, conductor) (1.00)
- Facilities Design, Construction and Operation > Pipelines, Flowlines and Risers > Materials and corrosion (1.00)