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原文传递 EVALUATION OF ROCKY POINT VIADUCT CONCRETE BEAM

题名：	EVALUATION OF ROCKY POINT VIADUCT CONCRETE BEAM
作者：	Stephen D. Cramer, Bernard S. Covino, Jr., Gordon R. Holcomb, Sophie J. Bullard, James H. Russell, Cheryl M. Dahlin, Cathy A. Summers Albany Research Center, U. S. Department of Energy, Albany OR 97321 And H. Martin Laylor and Steven M. Soltesz Oregon Department of Transportation, Salem OR 97301
关键词：	bridge, cathodic protection, CP, corrosion, chloride, diffusion, environment, reinforced concrete, reinforcing bar, rebar
摘要：	This study was intended to determine why it was necessary to replace the Rocky Point Viaduct (1954 - 1994), on the Oregon coast south of Pott Orford, after a period of service that was much shorter than that of many other reinforced concrete bridges on the Oregon coast; to identify construction practices that may have contributed to its early failure; and to identify evaluation methods that are most effective for characterizing bridge condition. A 14.3 m (47 foot) section of concrete Beam A1 and bridge deck between the north abutment and pier 1 was removed from the seaward side of the Viaduct for evaluation of; (I) concrete properties; (2) reinforcing bar (rebar) corrosion; and (3) chloride migration in the concrete. The harsh environment at the site was an important factor in the premature failure of the Viaduct. This led to substantial salt penetration of the concrete to the depth of the shear stirrups and square rebar. The severity of chloride penetration of the concrete was such that neither the original concrete cover nor subsequent efforts to repair the structure and prevent further corrosion could mitigate its effects on rebar corrosion. Shear stirrups had inadequate concrete cover for the severity of the environment. Early corrosion damage could be predicted from chloride depth profiles, concrete cover depths, and the rebar corrosion threshold. Application of the patch concrete merely delayed, but not for long, the occurrence of highly corrosive conditions around shear stirrups and the square rebar. Other efforts at preventing further corrosion damage (corrosion resistant coatings on the rebar, coal tar and linseed oil barrier coatings) were ineffective. Chloride profiling, using concrete powder samples taken at small depth increments, was one of the most powerful techniques Oregon DOT has for assessing the condition of bridges on the Oregon coast. The other was half-cell potential mapping. Chloride profiles, when fit by Pick’s second law of diffusion, yield an effective diffusion coefficient, D, for chloride in concrete and a surface chloride concentration, Co, characteristic of the bridge-environment combination. Sampling to a depth of 1S cm (6 inches) in 1.3 cm (0.5 inch) increments would adequately define the chloride profile for bridges on the Oregon coasL The D and Co obtained can be used to assess the severity of environmental conditions, to look back in time to understand performance problems related to corrosion damage, or to look forward in time to anticipate maintenance and repair needs. Once good values of D and Co are obtained, chloride measurement using concrete powder sampling may not be necessary, except o;. a long-term basis (every 10 to 15 years) to check predictions. Impressed Current Cathodic Protection (ICCP) over a period equivalent to 15 years service at Oregon coastal bridge conditions resulted in significant extraction of Cl from around the outer rebar. Chloride migration under the influence of *e potential gradient dominated migration due to the concentration gradient. ICCP shifts the rebar to a state of diminished corrosion, i.e., a protected state, gradually reduces the aggressiveness of the concrete environment surrounding the rebar by reducing chloride ion concentrations, and increases alkalinity at the rebar surface so that the rebar may eventually return to a naturally passive state. Bridge construction practices should consider, in addition to quality concrete and adequate cover, the aggressiveness of the site environment, including meteorological conditions and washing and sheltering that affect chloride deposition. Assessment of site environmental conditions would be a useful to bridge design to assure a structure would achieve the desired service life. Such an assessment recognizes that a wide range of microclimates exist on the Oregon coast. Site environments could be characterized by one of the following; measurement of atmospheric corrosion rates for mild steel, measurement of salt deposition rates, or determination of C» from chloride profiling an existing or nearby structure. Chloride profiles for coastal bridges and a knowledge of corrosion initiation threshold chloride levels provide solid evidence supporting the selection by Oregon DOT of stainless steel rebar and microsilica (high-performance) concrete for use in new bridge construction on the Oregon coast.
报告类型：	科技报告