1、Item No. 24217NACE International Publication 01102This Technical Committee Report has been preparedby NACE International Task Group 046*onCathodic Protection of Prestressed Concrete Elements.State-of-the-Art Report: Criteria for CathodicProtection of Prestressed Concrete StructuresFebruary 2002, NAC
2、E InternationalThis NACE International technical committee report represents a consensus of those individual memberswho have reviewed this document, its scope, and provisions. Its acceptance does not in any respect precludeanyone from manufacturing, marketing, purchasing, or using products, processe
3、s, or procedures not includedin this report. Nothing contained in this NACE International report is to be construed as granting any right, byimplication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or productcovered by Letters Patent, or as indemnifying or pro
4、tecting anyone against liability for infringement of LettersPatent. This report should in no way be interpreted as a restriction on the use of better procedures or materialsnot discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictablecircumstances
5、 may negate the usefulness of this report in specific instances. NACE International assumes noresponsibility for the interpretation or use of this report by other parties.Users of this NACE International report are responsible for reviewing appropriate health, safety,environmental, and regulatory do
6、cuments and for determining their applicability in relation to this report prior toits use. This NACE International report may not necessarily address all potential health and safety problems orenvironmental hazards associated with the use of materials, equipment, and/or operations detailed or refer
7、redto within this report. Users of this NACE International report are also responsible for establishing appropriatehealth, safety, and environmental protection practices, in consultation with appropriate regulatory authorities ifnecessary, to achieve compliance with any existing applicable regulator
8、y requirements prior to the use of thisreport.CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACEInternational reports are subject to periodic review, and may be revised or withdrawn at any time without priornotice. NACE reports are automatically withdrawn if m
9、ore than 10 years old. Purchasers of NACEInternational reports may receive current information on all NACE International publications by contacting theNACE International Membership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906(telephone +1281228-6200).ForewordThis NACE Inter
10、national state-of-the-art report is intendedfor use by engineers when evaluating criteria whereby pre-stressed concrete structures and members can beprotected from corrosion by means of cathodic protection(CP). Throughout this report reference is made to pertinent,available standards. Of particular
11、relevance are NACEStandards RP0187,1RP0290,2and RP0390.3Undercertain circumstances, the CP system can either become astructural element or significantly affect the serviceability orstructural performance of the prestressed concrete element.Therefore, a review of such impact from the CP system istypi
12、cally made by a registered structural engineer.This technical committee report was prepared by TaskGroup (TG) 046 on Cathodic Protection of PrestressedConcrete Elements. This TG is composed of corrosionresearchers, corrosion engineers, corrosion consultants,architects, structure owners, and represen
13、tatives of bothindustry and government. TG 046 is administered bySpecific Technology Group (STG) 01 on Concrete andRebar. It is also sponsored by STG 05 on Cathodic/AnodicProtection. This technical committee report is issued byNACE International under the auspices of STG 01._*Chairman William H. Har
14、tt, Florida Atlantic University, Dania Beach, FL.NACE International2IntroductionTypes and Principles of Prestressed ConcretePrestressed concrete has evolved during the past four-plusdecades to the point that it is now widely employed fortransportation structures, buildings, pipelines, and otherappli
15、cations because of its technical viability and economiccompetitiveness. While concrete per se normally exhibitsacceptable compressive strength, it is relatively weak in ten-sion. Therefore, embedded steel is added to accommodatetensile stresses. For structural applications, concrete iseither reinfor
16、ced or prestressed (or a combination of thetwo). For the former, bars are positioned in the formwork;and the concrete is poured and sets such that, neglectingdead weight and service loadings, no stresses are impartedby either component (steel or concrete) to the other. Theprinciple of prestressed co
17、ncrete is based on tensioning ofthe steel in such a manner that it ultimately places theconcrete in a state of residual compression. Consequently,service tensile loadings on the concrete, up to a certainlevel, act to reduce this compression; and tensile stressesare either less than would otherwise b
18、e the case or areavoided altogether. A basic introductory discussion of thetwo types of prestressed concrete, pretensioned and post-tensioned, is provided in this Introduction.Formoredetailed information, a standard text in the field can beconsulted.4Types and properties of prestressing steel. Impos
19、ition ofadequate residual compression to a concrete member viaprestressing steel uses steel of high strength, because itscross-section is generally small compared with that of theconcrete and the net force in each component (steel andconcrete) balances. The specification for prestressing steelstrand
20、 is provided by ASTM(1)A 416.5Currently, most pre-stressing is in the form of spiral, seven-wire strand that isdesignated as either Grade 250 or Grade 270, in which thenumber refers to minimum ultimate strength in kilopoundsper square inch (ksi) units (1 ksi = 1,000 psi 6.895 MPa).Historically, bar
21、as well as strand has been employed.Prestressing for concrete pipe is in the form of wire and isaddressed by another standard (ASTM A 6486). In eithercase (strand or wire), strengthening is achieved by a carbonconcentration near the eutectoid composition (0.77 w%)combined with cold drawing. Heat-tre
22、ated (quenched andtempered) steel is not used because of its greater suscep-tibility to brittle fracture and environmental cracking. In thepast, however, quenched and tempered material has beenused in some countries. Otherwise, the above standardsprimarily address dimensions and strength, with the m
23、eansby which the requisite strength is achieved being left to theproducer. Steel composition is typically considered to beimportant, because this influences the strengthening that isderived from cold drawing. Either plain carbon or micro-alloyed steel, with small amounts of either chromium, vana-diu
24、m, or chromium plus vanadium, is commonly employed.Pretensioned concrete. Components in this class are norm-ally produced in a prefabrication yard and then transportedto the construction site. Consequently, there are practicallimits on member size. Beams, columns, and pilings areexamples of componen
25、ts that are routinely pretensioned.Fabrication involves placement and pretensioning of thetendons(2)in a form bed, pouring the concrete, allowing theconcrete to set, and, finally, removing the applied tensioningforce on the tendon. The tendency for the tendon within thehardened concrete to contract,
26、 once this force is removed,places the concrete in a state of residual compression.Posttensioned concrete. Components in this class arenormally produced in place at the job site (an exception tothis, segmental construction, is described below). Slabs forbuildings and decks for parking garages are ex
27、amples inwhich posttensioning is commonly employed. For fabric-ation, tendons are contained in ducts that are, in turn,positioned in the pouring forms. Consequently, only theduct and not the tendon is in direct contact with and isbonded to the concrete. Once the concrete has achieved aprescribed str
28、ength, the tendons are tensioned and theloaded ends secured by collets in anchors. As is the casefor pretensioning, compressive stresses are imparted to theconcrete. In some cases a grout slurry is then pumped intothe duct pore space (bonded posttensioning), while no suchmeasure is used in others (u
29、nbonded posttensioning).Tendons in unbonded posttensioned concrete are typicallysurface treated with grease that contains a corrosioninhibitor.Reinforcing steel has invariably been present in addition tothe prestressing strands within prestressed concrete mem-bers. This is done either to ensure inte
30、grity in areas of loc-ally high stress or to provide strength in the transversedirection, or both.Segmental construction is a special case of prestressedconcrete in which individual pretensioned members aresecured together into a larger assembly by posttensioning.Corrosion of Prestressing Steel in C
31、oncreteCorrosion mechanism. Typically, the cement paste inconcrete and mortar is alkaline (pH 12.5 to 13.8), whichfacilitates formation and maintenance of a protective, pas-sive film; and a low corrosion rate generally results. How-ever, this protective film can be compromised by eithercarbonation o
32、r chlorides. Carbonation involves reaction ofatmospheric carbon dioxide with hydroxides to form carbon-ates; and, as a consequence, pH is reduced to below thevalue for which steel is passive. Chlorides, on the otherhand, arise from exposure of the concrete to deicing salts, amarine environment, high
33、chloride soils and ground waters,or presence of this species in the raw materials including_(1)ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.(2)Tendons are high-strength steel strands, groups of strands, or bars that, when tensioned, impart a compressive stress to the struc
34、ture orstructural member.NACE International3admixtures (alternatively, to a combination of these). Whilechlorides do not significantly alter pH of the pore water, theydo react with and locally compromise the passive film. Ineither case (carbonation or chloride intrusion) and assumingthat moisture is
35、 present,7the resultant corrosion rate can beunacceptably high although limited by oxygen availability atcathodic sites according to Reaction (1):+ OH2e2OHO2122(1)Such corrosion leads to accumulation of solid reaction prod-ucts in the cement pore space near the embedded steel/concrete interface; and
36、 these give rise to tensile hoopstresses and, eventually, to concrete cracking and spalling.Several factors indicate that corrosion can be more signifi-cant for prestressing compared with reinforcing steel inconcrete. First, corrosion on a single wire of a strand repre-sents a greater percentage of
37、cross-section loss than for areinforcing bar of the same size as the strand. Second,local cross-section loss on a single wire can eventuallycause it to fail by overload. This, in turn, leads to loadtransference to the remaining six wires such that an evensmaller corrosion penetration on one or more
38、of these wirescan cause failure of the tendon. Once this occurs, the pre-stress is reduced over a certain volume of the concretemember; and additional stress is transmitted to adjacenttendons, which renders them increasingly susceptible tooverload fracture. The same rationale applies to pre-stressed
39、 concrete pipe (PCP), but here load transferenceupon corrosion and fracture of a single wire is to adjacentwraps of the wire around the pipe rather than to remainingwires in a strand.In the specific case of prestressed concrete cylinder pipe(PCCP), the occurrence of corrosion has been related tothe
40、presence of voids and porosity around the wires, withgreatest susceptibility occurring when chlorides are alsopresent.8It has been projected that the localized environ-ment within such voids can differ from that of the pore wateritself with pH values as low as 6 having been reported.9Failures from c
41、orrosion have been attributed to a combin-ation of physical and chemical conditions compromising themortar alkalinity at the wire surface. This results in eithercorrosion and mortar disbondment at a rate determined bydissolved oxygen availability or, in the absence of oxygen,hydrogen generation and
42、embrittlement in the case of wirethat has been rendered sensitive by dynamic strain aging.9While only a relatively small fraction of the thousands ofkilometers (miles) of in-place PCCP have ruptured, suchfailures, when they occur, are often sudden and cata-strophic and can result in loss of service,
43、 property damage,and injury and death.8Types of exposure. Prestressed components and struc-tures encounter a variety of exposures, but in a generalcontext these can be categorized as (1) atmospheric, (2)buried, (3) submerged, and (4) various combinations of (1)through (3). The corrosion rate of embe
44、dded steel in a part-icular situation is determined by (1) the extent of carbon-ation or chloride contamination (or both), (2) oxygen andmoisture availability, (3) concrete quality, and (4) macrocell,galvanic, or stray electrical currents. Fully submergedexposures are generally benign, even in seawa
45、ter, becausethe flux of dissolved oxygen through the concrete cover tothe steel is nil. Relatedly, low relative humidity atmosphericexposures are typically not severe because of the absenceof moisture. On the other hand, embedded steel corrosionand corrosion-induced concrete deterioration are typica
46、llymost severe in situations involving alternate wetting anddrying, as arise in tidal marine exposures, buried appli-cations in which the structure or component is exposed to achanging water table, and in soils with variable capacitiesfor moisture retention.Influence of variables. Factors that influ
47、ence corrosion ofembedded steel in concrete include (1) type of exposure,(2) inherent cement alkalinity, (3) concrete permeability, and(4) concrete resistivity. Parameters in the first of these cate-gories include temperature, concentration of deleteriousspecies (chlorides, for example), as well as
48、water andoxygen, and the frequency and intensity of alternate wettingand drying. Concrete permeability (item 3) is important be-cause it influences the transport of influential species(chloride, water, and oxygen) to the steel-concrete interface,whereas item 4 (resistivity) is critical to functionin
49、g of theelectrochemical cell(s), particularly when anodes andcathodes exist on a macroscopic scale, as is often the case.Past research investigations have concluded that as littleas 0.025 to 0.033% Cl-(concrete weight basis) can causelocalized loss of passivity for conventional reinforcing steelin concrete, thereby facilitating corrosion at a rate controlledby water and oxygen availability at cathodic sites.10, 11Higher values have been reported in the case of pre-stressing steel.12Related experiments involving simula
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