《NACE-01102-2002.pdf》由会员分享，可在线阅读，更多相关《NACE-01102-2002.pdf（23页珍藏版）》请在三一文库上搜索。
1、Item No. 24217 NACE International Publication 01102 This Technical Committee Report has been prepared by NACE International Task Group 046*on Cathodic Protection of Prestressed Concrete Elements. State-of-the-Art Report: Criteria for Cathodic Protection of Prestressed Concrete Structures February 20
2、02, NACE International This NACE International technical committee report represents a consensus of those individual members who have reviewed this document, its scope, and provisions. Its acceptance does not in any respect preclude anyone from manufacturing, marketing, purchasing, or using products
3、, processes, or procedures not included in this report. Nothing contained in this NACE International report is to be construed as granting any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or product covered by Letters Patent, or as indemn
4、ifying or protecting anyone against liability for infringement of Letters Patent. This report should in no way be interpreted as a restriction on the use of better procedures or materials not discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictab
5、le circumstances may negate the usefulness of this report in specific instances. NACE International assumes no responsibility 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
6、, and regulatory documents and for determining their applicability in relation to this report prior to its use. This NACE International report may not necessarily address all potential health and safety problems or environmental hazards associated with the use of materials, equipment, and/or operati
7、ons detailed or referred to within this report. Users of this NACE International report are also responsible for establishing appropriate health, safety, and environmental protection practices, in consultation with appropriate regulatory authorities if necessary, to achieve compliance with any exist
8、ing applicable regulatory requirements prior to the use of this report. CAUTIONARY NOTICE:The user is cautioned to obtain the latest edition of this report.NACE International reports are subject to periodic review, and may be revised or withdrawn at any time without prior notice.NACE reports are aut
9、omatically withdrawn if more than 10 years old.Purchasers of NACE International reports may receive current information on all NACE International publications by contacting the NACE International Membership Services Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1281228-62
10、00). Foreword This NACE International state-of-the-art report is intended for use by engineers when evaluating criteria whereby pre- stressedconcretestructuresandmemberscanbe protected from corrosion by means of cathodic protection (CP). Throughout this report reference is made to pertinent, availab
11、le standards. Of particular relevance are NACE Standards RP0187,1RP0290,2and RP0390.3Under certain circumstances, the CP system can either become a structural element or significantly affect the serviceability or structural performance of the prestressed concrete element. Therefore, a review of such
12、 impact from the CP system is typically made by a registered structural engineer. This technical committee report was prepared by Task Group (TG) 046 on Cathodic Protection of Prestressed Concrete Elements.This TG is composed of corrosion researchers, corrosion engineers, corrosion consultants, arch
13、itects, structure owners, and representatives of both industry and government.TG 046 is administered by Specific Technology Group (STG) 01 on Concrete and Rebar. It is also sponsored by STG 05 on Cathodic/Anodic Protection.This technical committee report is issued by NACE International under the aus
14、pices of STG 01. _ *Chairman William H. Hartt, Florida Atlantic University, Dania Beach, FL. Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 05/16/2007 00:57:00 MDTNo reproduction or networking permitted without
15、 license from IHS -,-,- NACE International 2 Introduction Types and Principles of Prestressed Concrete Prestressed concrete has evolved during the past four-plus decades to the point that it is now widely employed for transportation structures, buildings, pipelines, and other applications because of
16、 its technical viability and economic competitiveness.While concrete per se normally exhibits acceptable compressive strength, it is relatively weak in ten- sion. Therefore, embedded steel is added to accommodate tensile stresses.For structural applications, concrete is either reinforced or prestres
17、sed (or a combination of the two). For the former, bars are positioned in the formwork; and the concrete is poured and sets such that, neglecting dead weight and service loadings, no stresses are imparted by either component (steel or concrete) to the other. The principle of prestressed concrete is
18、based on tensioning of the steel in such a manner that it ultimately places the concrete in a state of residual compression. Consequently, service tensile loadings on the concrete, up to a certain level, act to reduce this compression; and tensile stresses are either less than would otherwise be the
19、 case or are avoided altogether. A basic introductory discussion of the two types of prestressed concrete, pretensioned and post- tensioned, is provided in this Introduction.For more detailed information, a standard text in the field can be consulted.4 Types and properties of prestressing steel.Impo
20、sition of adequate residual compression to a concrete member via prestressing steel uses steel of high strength, because its cross-section is generally small compared with that of the concrete and the net force in each component (steel and concrete) balances. The specification for prestressing steel
21、 strand is provided by ASTM(1)A 416.5Currently, most pre- stressing is in the form of spiral, seven-wire strand that is designated as either Grade 250 or Grade 270, in which the number refers to minimum ultimate strength in kilopounds per square inch (ksi) units (1 ksi = 1,000 psi 6.895 MPa). Histor
22、ically, bar as well as strand has been employed. Prestressing for concrete pipe is in the form of wire and is addressed by another standard (ASTM A 6486).In either case (strand or wire), strengthening is achieved by a carbon concentration near the eutectoid composition (0.77 w%) combined with cold d
23、rawing. Heat-treated (quenched and tempered) steel is not used because of its greater suscep- tibility to brittle fracture and environmental cracking. In the past, however, quenched and tempered material has been used in some countries.Otherwise, the above standards primarily address dimensions and
24、strength, with the means by which the requisite strength is achieved being left to the producer.Steel composition is typically considered to be important, because this influences the strengthening that is derived from cold drawing. Either plain carbon or micro- alloyed steel, with small amounts of e
25、ither chromium, vana- dium, or chromium plus vanadium, is commonly employed. Pretensioned concrete. Components in this class are norm- ally produced in a prefabrication yard and then transported to the construction site. Consequently, there are practical limits on member size.Beams, columns, and pil
26、ings are examples of components that are routinely pretensioned. Fabrication involves placement and pretensioning of the tendons(2)in a form bed, pouring the concrete, allowing the concrete to set, and, finally, removing the applied tensioning force on the tendon. The tendency for the tendon within
27、the hardened concrete to contract, once this force is removed, places the concrete in a state of residual compression. Posttensioned concrete.Components in this class are normally produced in place at the job site (an exception to this, segmental construction, is described below). Slabs for building
28、s and decks for parking garages are examples in which posttensioning is commonly employed.For fabric- ation, tendons are contained in ducts that are, in turn, positioned in the pouring forms.Consequently, only the duct and not the tendon is in direct contact with and is bonded to the concrete. Once
29、the concrete has achieved a prescribed strength, the tendons are tensioned and the loaded ends secured by collets in anchors. As is the case for pretensioning, compressive stresses are imparted to the concrete. In some cases a grout slurry is then pumped into the duct pore space (bonded posttensioni
30、ng), while no such measure is used in others (unbonded posttensioning). Tendons in unbonded posttensioned concrete are typically surface treated with grease that contains a corrosion inhibitor. Reinforcing steel has invariably been present in addition to the prestressing strands within prestressed c
31、oncrete mem- bers. This is done either to ensure integrity in areas of loc- ally high stress or to provide strength in the transverse direction, or both. Segmental construction is a special case of prestressed concrete in which individual pretensioned members are secured together into a larger assem
32、bly by posttensioning. Corrosion of Prestressing Steel in Concrete Corrosion mechanism.Typically, the cement paste in concrete and mortar is alkaline (pH 12.5 to 13.8), which facilitates formation and maintenance of a protective, pas- sive film; and a low corrosion rate generally results. How- ever,
33、 this protective film can be compromised by either carbonation or chlorides. Carbonation involves reaction of atmospheric carbon dioxide with hydroxides to form carbon- ates; and, as a consequence, pH is reduced to below the value for which steel is passive.Chlorides, on the other hand, arise from e
34、xposure of the concrete to deicing salts, a marine environment, high-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 stran
35、ds, or bars that, when tensioned, impart a compressive stress to the structure or structural member. Copyright NACE International Provided by IHS under license with NACELicensee=IHS Employees/1111111001, User=Wing, Bernie Not for Resale, 05/16/2007 00:57:00 MDTNo reproduction or networking permitted
36、 without license from IHS -,-,- NACE International 3 admixtures (alternatively, to a combination of these). While chlorides do not significantly alter pH of the pore water, they do react with and locally compromise the passive film. In either case (carbonation or chloride intrusion) and assuming tha
37、t moisture is present,7the resultant corrosion rate can be unacceptably high although limited by oxygen availability at cathodic sites according to Reaction (1): +OH2e2OHO 2 1 22 (1) Such corrosion leads to accumulation of solid reaction prod- ucts in the cement pore space near the embedded steel/ c
38、oncrete interface; and these give rise to tensile hoop stresses and, eventually, to concrete cracking and spalling. Several factors indicate that corrosion can be more signifi- cant for prestressing compared with reinforcing steel in concrete. First, corrosion on a single wire of a strand repre- sen
39、ts a greater percentage of cross-section loss than for a reinforcing bar of the same size as the strand.Second, local cross-section loss on a single wire can eventually cause it to fail by overload.This, in turn, leads to load transference to the remaining six wires such that an even smaller corrosi
40、on penetration on one or more of these wires can cause failure of the tendon. Once this occurs, the pre- stress is reduced over a certain volume of the concrete member; and additional stress is transmitted to adjacent tendons, which renders them increasingly susceptible to overload fracture.The same
41、 rationale applies to pre- stressed concrete pipe (PCP), but here load transference upon corrosion and fracture of a single wire is to adjacent wraps of the wire around the pipe rather than to remaining wires in a strand. In the specific case of prestressed concrete cylinder pipe (PCCP), the occurre
42、nce of corrosion has been related to the presence of voids and porosity around the wires, with greatest susceptibility occurring when chlorides are also present.8It has been projected that the localized environ- ment within such voids can differ from that of the pore water itself with pH values as l
43、ow as 6 having been reported.9 Failures from corrosion have been attributed to a combin- ation of physical and chemical conditions compromising the mortar alkalinity at the wire surface.This results in either corrosion and mortar disbondment at a rate determined by dissolved oxygen availability or,
44、in the absence of oxygen, hydrogen generation and embrittlement in the case of wire that has been rendered sensitive by dynamic strain aging.9 While only a relatively small fraction of the thousands of kilometers (miles) of in-place PCCP have ruptured, such failures, when they occur, are often sudde
45、n and cata- strophic and can result in loss of service, property damage, and injury and death.8 Types of exposure.Prestressed components and struc- tures encounter a variety of exposures, but in a general context these can be categorized as (1) atmospheric, (2) buried, (3) submerged, and (4) various
46、 combinations of (1) through (3). The corrosion rate of embedded steel in a part- icular situation is determined by (1) the extent of carbon- ation or chloride contamination (or both), (2) oxygen and moisture availability, (3) concrete quality, and (4) macrocell, galvanic, or stray electrical curren
47、ts.Fully submerged exposures are generally benign, even in seawater, because the flux of dissolved oxygen through the concrete cover to the steel is nil. Relatedly, low relative humidity atmospheric exposures are typically not severe because of the absence of moisture. On the other hand, embedded st
48、eel corrosion and corrosion-induced concrete deterioration are typically most severe in situations involving alternate wetting and drying, as arise in tidal marine exposures, buried appli- cations in which the structure or component is exposed to a changing water table, and in soils with variable ca
49、pacities for moisture retention. Influence of variables.Factors that influence corrosion of embedded 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 deleterious species (chlorides, for example), as well as water and oxygen, and the frequency and intensity of al