Effect of zinc on prestressing steel. Technical report prepared by Task Group February 2012

Transcription

1 This document is the intellectual property of the fib International Federation for Structural Concrete. All rights reserved.

2 Effect of zinc on prestressing steel Technical report prepared by Task Group 9.17 February 2012

3 Subject to priorities defined by the Technical Council and the Presidium, the results of fib s work in Commissions and Task Groups are published in a continuously numbered series of technical publications called 'Bulletins'. The following categories are used: category minimum approval procedure required prior to publication Technical Report State-of-Art Report Manual, Guide (to good practice) or Recommendation Model Code approved by a Task Group and the Chairpersons of the Commission approved by a Commission approved by the Technical Council of fib approved by the General Assembly of fib Any publication not having met the above requirements will be clearly identified as preliminary draft. This Bulletin N 64 was approved as an fib Technical Report by Commission 9 in January This report was drafted by Task Group 9.17, Prestressing steel and zinc, in Commission 9, Reinforcing and prestressing materials and systems: Hans Rudolf Ganz* (Convener, VSL Switzerland) Werner Brand (DSI, Germany), Toshiro Kido (Sumitomo (SEI) Steel Wire Corp., Japan), Larry Krauser (General Technologies Inc., USA), Erik Mellier (Freyssinet International, France), Ulf Nürnberger* (MPA Stuttgart Otto-Graf-Institut, Germany), Shoji Shirahama (Shinko Wire Company, Japan), Hans van Beurden (Nedri-Spanstaal, The Netherlands) Corresponding members: P. Brevet (France), Alain Chabert (A.C. Consultant, France), Christian Gläser (DSI, Germany), Dieter Jungwirth (Germany), Andor Windisch (Germany) * Main author Full address details of Task Group members are available to fib members through the online services section at Cover image: Kochertal Bridge, Germany fédération internationale du béton (fib), 2012 Although the International Federation for Structural Concrete fib fédération internationale du béton does its best to ensure that any information given is accurate, no liability or responsibility of any kind (including liability for negligence) is accepted in this respect by the organisation, its members, servants or agents. All rights reserved. No part of this publication may be reproduced, modified, translated, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission. First published in 2012 by the International Federation for Structural Concrete (fib) Postal address: Case Postale 88, CH-1015 Lausanne, Switzerland Street address: Federal Institute of Technology Lausanne EPFL, Section Génie Civil Tel Fax fib@epfl.ch ISSN ISBN Printed by DCC Document Competence Center Siegmar Kästl e.k., Germany

4 Foreword The use of zinc on or near prestressing steel has been a concern for designers, engineers, and organizations involved in concrete structures management for some time now. Around the world, scarce and sometimes apparently conflicting information is available. This technical report has not only the merit of regrouping the available information, but it presents, in a pragmatic way, conditions in which zinc near prestressing steel in an alkaline environment may be of concern, and highlights six relevant parameters for assessing zinc corrosion sensitivity on a case-by-case basis. It is hoped that this document will be helpful to those interested in the maintenance and durability of prestressed concrete structures. I would like to acknowledge the contributions made by all members of the Working Group. In particular, this document would not have been possible without the initiative of Dr. Hans R. Ganz and the dedicated work as well as expertise of Professor Ulf Nürnberger. In addition, the work by Laura Thommen-Vidale, fib secretariat, on the final editing of the report is greatly appreciated. Josée Bastien Chair of fib Commission 9, Reinforcing and prestressing materials and systems fib Bulletin 64: Effect of zinc on prestressing steel iii

5 Contents Foreword 1 Introduction 1 2 Summary of relevant parameters on the risk of hydrogen-induced stress corrosion cracking of prestressing steel Hydrogen evolution at the surface of prestressing steel Hydrogen evolution caused by steel corrosion Hydrogen evolution caused by zinc corrosion Zinc corrosion in the atmosphere Zinc corrosion in fresh cementitious grout or concrete Hydrogen evolution caused by grout admixtures Susceptibility of prestressing steel to hydrogen General Testing FIP test DIBt test Thickness of zinc coating on prestressing steel Level of tensile stress in prestressing steel Detailing aspects Distance of prestressing steel to embedded galvanized components Ratio of surface area of prestressing steel (cathode) versus zinc (anode) 11 3 Assessment of particular applications 11 4 Summary and conclusions Relevant parameters on the risk of hydrogen-induced stress corrosion cracking Conclusions for particular applications 16 5 References 17 iv fib Bulletin 64: Effect of zinc on prestressing steel

6 1 Introduction Galvanization of prestressing steels is well understood and well specified in national and international standards, see [1-5]. The hot-dip galvanizing process is commonly used for prestressing steels and has the following main stages: cleaning with acids (pickling), initial drawing to an intermediate diameter and cleaning of surface (in case of prestressing wires), hot-dip galvanizing at C, and final drawing (in case of prestressing wires). The above process leads to some reduction in tensile strength of prestressing steel. This reduction is typically compensated by starting the wire drawing process with rod of higher strength (or utilizing larger rod for drawing) so that at the conclusion of the process, the dimensions and minimum strength specified for equivalent non-galvanized prestressing wire are achieved. During cleaning with acids (pickling) hydrogen is available and may enter the prestressing steel. Once the zinc coating is applied, this hydrogen is trapped inside the prestressing steel. For a well-controlled process following proven procedures, pickling during galvanizing of prestressing steel does not introduce significant amounts of hydrogen. This aspect will be further discussed in Section 2 of this report. In the case of prestressing bars zinc overlay can be produced (after a proper cleaning, see above) by batch galvanizing. The use of zinc close to or on prestressing steels placed in an alkaline environment is controversial and different practices have been established in different countries [1]. This controversy applies in particular to the following two applications: 1. The use of bare (non-coated) prestressing steels in direct or indirect contact with galvanized components, such as galvanized ducts for post-tensioning tendons, galvanized inserts or galvanized reinforcing steel near prestressing steel in pretensioned members. 2. The use of galvanized prestressing steel in grouted post-tensioning tendons or in pretensioned concrete members. The following are examples of statements given in selected standards and recommendations: Post-Tensioning Institute (PTI) states [6]: Galvanized prestressing strand may be used in contact with cement grout provided the steel has been manufactured in accordance with the latest ASTM A416, BS5896, or EN10138 standards. Experience has shown that strand manufactured to these standards is not susceptible to hydrogen embrittlement. French standard NFA [2] for galvanized high-strength wires and strands for prestressed concrete states: Research showed that the risk of hydrogen embrittlement is very limited as corrosion tests showed that a period of hydrogen evolution of about 8 hours has no effect on galvanized prestressing steel. German standard DIN1045 [7] requires that there shall be a minimum spacing of 20 mm of concrete between prestressing tendons and galvanized inserts or galvanized reinforcing steels, and that in addition, no metallic conducting connection may exist between tendon and inserts or reinforcing steel. The use of galvanized prestressing steel in structural concrete (or inside grout) is prohibited. fib Bulletin 64: Effect of zinc on prestressing steel 1

7 European research report COST 534 [8] comments regarding the application of galvanized prestressing steels: This research showed that the risk of hydrogen embrittlement is very limited, as corrosion tests showed that a period of hydrogen evolution of about 8 hours has no effect on galvanized prestressing steel. The committee for pren Zinc and zinc alloy coated prestressing steel wires and strands [3] discusses whether galvanized cold-drawn wires and strands used in structural concrete may get into contact with concrete and grout without particular precautions. The above-mentioned requirement of the German standard DIN 1045 [7] and other concerns regarding the use of galvanized prestressing steels, galvanized ducts for tendons or galvanized inserts or components in contact with the prestressing steel are mainly based on the assumption that the hydrogen formed on the zinc surface during corrosion for a short period of time represents a risk of hydrogen-induced stress corrosion for the prestressing steel [9]. Basic research [10, 11] has demonstrated that hydrogen-induced stress corrosion may indeed occur with prestressing steels that are sensitive to hydrogen. In fact, direct contact of galvanized components with prestressing steel which is sensitive to hydrogen and which therefore is no longer approved in Germany, has led to damage and failure of prestressing tendons in some isolated cases in practice [1, 12]. The use of galvanizing for high-strength steels exposed to atmosphere is less controversial. However, under certain conditions hydrogen may also develop and potentially damage the high-strength steel. High-strength galvanized steel components used in other applications such as wire ropes used in bridge construction [13] or as fastenings [14] have suffered occasionally from hydrogen-induced failures. This process is discussed in Section This report provides a summary and evaluation of the relevant parameters on the corrosion behaviour of high-strength wires, strands and bars when in contact with zinc. Both, tendons made of prestressing steels used in structural concrete and tendons under atmospheric conditions will be considered. The main applications of high-strength steels in contact with zinc considered here are the following: A Zinc-coated high-strength steel for applications in atmospheric corrosion conditions (Fig. 1). B Zinc-coated prestressing steel embedded in fresh cementitious grout or concrete with or without defects in the zinc coating (Fig. 2). C Bare prestressing steel (no zinc coating) embedded in fresh cementitious grout or concrete in direct contact with, or indirect contact in the immediate vicinity of galvanized components embedded in the grout or concrete (e.g. galvanized ducts, galvanized reinforcing steel, galvanized fastenings/dowels, etc.) (Fig. 3). 2 1 Introduction

8 This document is the intellectual property of the fib International Federation for Structural Concrete. All rights reserved. Fig. 1: Fractures of galvanized wires of a locked-coil cable within the saddle on a pylon Fig. 2: Zinc-coated prestressing steel embedded in cementitious material; defects in the zinc coating after anchoring in wedges (Note: Wedge bites in strand subject to a stress of 45% UTS) Fig. 3: Bare prestressing steel embedded in grout in contact with a galvanized duct fib Bulletin 64: Effect of zinc on prestressing steel 3

9 2 Summary of relevant parameters on the risk of hydrogen-induced stress corrosion cracking of prestressing steels The actual risk for hydrogen-induced stress corrosion cracking of prestressing steels in contact with zinc depends on several parameters: corrosion conditions, in particular the duration and intensity of the hydrogen evolution; susceptibility of the prestressing steel for hydrogen-induced stress corrosion cracking; thickness of the zinc coating on the prestressing steel; level of tensile stress in the prestressing steel; detailing aspects of the actual construction such as the ratio of surface of zinc versus the surface of bare non-galvanized prestressing steel, and whether there is direct contact of zinc-coated elements with bare prestressing steel or indirect contact through cementitious grout or concrete; and type of cement (alkali and chromate content). These main parameters are discussed in the following sections. 2.1 Hydrogen evolution at the surface of prestressing steel Hydrogen evolution at the surface of the prestressing steel may be due to either corrosion of the prestressing steel or corrosion of zinc, which may be either a coating on the prestressing steel or zinc on other steel components that are in direct or indirect contact with the prestressing steel. The hydrogen may penetrate into the prestressing steel in the form of atoms, which may lead to cracking and subsequent failure. However, particular corrosion conditions are required for hydrogen to be generated as a consequence of steel corrosion or zinc corrosion Hydrogen evolution caused by steel corrosion [9, 15] Steel corrosion with hydrogen evolution is possible in acid aqueous solutions (ph <7). In this case, the anodic dissolution of steel Fe Fe e - is primarily compensated by the cathodic reaction of hydrogen discharge H + + e - H in which atomic hydrogen, H, is created and may be taken up by steel. This process can play a role during pickling in acid baths before galvanizing where, during a limited period of time of acid corrosion, hydrogen develops and is introduced into the prestressing steel. Process and duration (time for release of hydrogen) are different for different types of prestressing steels. Once galvanizing is applied, this hydrogen remains entrapped and may migrate to locations with stress concentrations. If concentrations exceed a certain level (depending on the type of prestressing steel) delayed rupture may occur after prestressing, depending of the sensitivity of the steel. The amount of hydrogen produced 4 2 Summary of relevant parameters on the risk of hydrogen-induced stress corrosion cracking

10 during pickling may be very high. Therefore, the pickling process of prestressing steels should be kept as short as possible (maximum 5 minutes). The risk is lower when pickling for a shorter period with higher acid concentration than when pickling for a longer duration with lower acid concentration. In any case, suitable inhibitors must be used during pickling. In applications of structural concrete, aqueous solutions are typically alkaline. Therefore, the conditions for hydrogen evolution are unfavourable. However, if there is chloride-induced pitting or crevice corrosion, acid solutions necessary for hydrogen evolution may develop in the corrosion pits or crevices, caused by the hydrolysis of corrosion products. In the region of corrosion pits and crevices the ph value may drop to values of about 4 to 5 because of the formation of hydrochloric acid, caused by the hydrolysis of the Fe 2+ ions in the solution: FeCl H 2 O Fe (OH) HCl It is therefore absolutely necessary to protect high-strength prestressing steels from the combination of local corrosion attack in contact with aqueous solutions containing chlorides. There is an increased risk due to hydrogen evolution if the prestressing steel is corroding in an alkaline medium while in contact with galvanized reinforcing steel or with zinc-coated embedded components. Under very negative potential (below the so-called equilibrium potential of the hydrogen evolution reaction which may occur, e.g. after cathodic polarization) hydrogen evolution may also occur due to water decomposition (see the discussion below on hydrogen evolution caused by zinc corrosion). Such extreme polarization is possible with non-suitable cathodic protection procedures and when corroding prestressing steel is in contact with zinc. The use of bare prestressing steel with galvanized duct or rebar should therefore be avoided in cases where there is a significant risk of corrosion of prestressing steel due to chlorides during service life of the structure Hydrogen evolution caused by zinc corrosion [15] Zinc coatings are only suitable for protecting steel against corrosion if the zinc can react at a given application with the environment to create corrosion-protecting surface layers or passive layers: Exposed to the atmosphere, zinc hydroxide is formed due to humidity and then reacts with carbon dioxide of the air to form basic zinc carbonate. This zinc carbonate is barely soluble in water and therefore, creates a protective surface layer. Embedded in fresh cementitious grout or concrete, zinc hydroxide and zinc oxide are formed, which then react with calcium hydroxide of the fresh concrete or grout to form the barely soluble calcium hydroxozincate (Ca [Zn (OH) 3 ] 2 2H 2 O). This causes the passivation of zinc exposed to fresh concrete or grout. This passive film protects the zinc and reduces further zinc corrosion. By the formation of a protective layer, the prestressing steel beneath the zinc coating is also protected against corrosion. Over time, the protective layer may be dissolved depending on the aggressivity of the environment. During this process fresh zinc from the coating continues to react, leading to widespread corrosion of zinc and resulting in a progressive dissolution of the zinc layer. Under certain conditions (see Section 3) this protective layer either: forms only with some delay this is particularly the case when zinc-coated components are exposed to highly alkaline fresh concrete or cementitious grout, or fib Bulletin 64: Effect of zinc on prestressing steel 5

11 is destroyed this is the case when zinc-coated components are used in the atmosphere combined with permanent humidity and no air circulation. Particularly critical is zinc corrosion in crevices (crevice corrosion). Under the above unfavourable conditions in which zinc is not protected by surface layers, active zinc corrosion with formation of hydrogen is possible. In this active corrosion process, zinc is dissolved (anodic reaction) as Zn Zn e - For both the above cases (corrosion in the atmosphere and exposure to aqueous highly alkaline media) with limited supply of oxygen (e.g. crevice corrosion) a highly negative corrosion potential will develop on the zinc surface which may lead to water decomposition. As a consequence, the cathodic reaction of the corrosion process is the formation of atomic hydrogen: 2H 2 O + 2e - 2H + 2OH - This atomic hydrogen may penetrate into the prestressing steel and, if further unfavourable conditions exist (see Sections 2.2 to 2.5), damage the prestressing steel. Whether or not such atmospheric corrosion conditions or exposure to highly alkaline fresh concrete and grout lead to a risk for the prestressing steel due to hydrogen depends therefore, primarily on the corrosion or passivation behaviour of the zinc and on the corrosive environment of the particular application (see Section 3) Zinc corrosion in the atmosphere Such corrosion conditions may occur for example in locked-coil cable construction [15]. Locked-coil cables are not used for prestressing, neither in case of pretensioned nor of posttensioned structures. Locked-coil cables are mentioned here because they use high-strength galvanized wires and because damage has been observed. The particular conditions for such damage are discussed here to highlight differences with respect to exposure to fresh mortar or concrete. In case of locked-coil cable constructions, frequent or quasi permanent wetting of the surface of the galvanized high-strength steel with a water film, loose contact of the galvanized high-strength steel with porous, permanently wet material, or formation of crevices in the zinc coating in contact with another metallic or non-metallic material prevent the formation of corrosion-protecting passivating surface layers. The rate of the corrosion process of zinc under these conditions is significantly higher than with exposure to environments with excess air. Under such unfavourable conditions zinc may locally be rapidly consumed. As a consequence of lack of oxygen in the electrolyte of the crevices, the corrosion potential of the non-noble zinc is in the range of water decomposition which forms hydrogen. The high-strength steel surface (cathode) and the zinc (anode) form a corrosion element at the location of local zinc dissolution. As a consequence, the bare surfaces of highstrength steel are polarized cathodically into the range for water decomposition. Since the formation of hydrogen on the zinc is significantly impaired, this decomposition now occurs directly on the high-strength steel. As a consequence, hydrogen-induced stress corrosion may occur in the locked-coil cable. Actual damage of this type has been observed in the area of deviation saddles, clamping plates and anchorages of locked-coil cables used for supporting shells/roofs and bridges [13]. However, the conclusions for this type of damage are not applicable to the conditions in fresh cementitious grout or concrete since in this latter case hydrogen exposure of the prestressing steel is only possible for a short period of time. 6 2 Summary of relevant parameters on the risk of hydrogen-induced stress corrosion cracking

12 Zinc corrosion in fresh cementitious grout or concrete The underlying process of zinc corrosion in fresh mortar or concrete has been described in the literature [16, 17]. Zinc coatings may corrode when in contact with fresh concrete or fresh cementitious grout/mortar for a limited period of time (hours to a few days). During this initial corrosion between 5 and 10 µm of zinc are consumed and hydrogen may develop. With the hydration of the fresh concrete or grout, passivating surface layers made of calcium hydroxozincate are formed. As a consequence, the initially very low (negative) corrosion potential of zinc increases and leaves the range where water decomposition is possible. This stops the zinc corrosion and hydrogen evolution within a short period of time. It is evident that a measurable deposition of hydrogen on the prestressing steel when in contact with zinc only occurs over a limited time period (maximum a few days) immediately after placement of fresh concrete or grout. This is the primary difference to the hydrogen evolution caused by steel corrosion (Section 2.1.1) which may represent a permanent process. It should be noted that the evolution of hydrogen in fresh cementitious grout or concrete at the interface between the galvanized prestressing steel and the concrete, simultaneous with the process of passivation by a film of calcium hydroxozincate, may slightly reduce the adhesive bond of prestressing steel to grout or concrete at an early age of hydration [17, 18]. The formation of calcium hydroxyzincate and the hydrogen that is released at the interface between the steel and the concrete may delay the setting and reduce the early strength of the concrete close to the prestressing steel surface. Hydrogen, however, loosens the surrounding structure of the concrete, resulting in reduced friction for early bond action and shearing strength between the galvanized steel and concrete. Through further reactions the zincate is included in the hydration products by physical and chemical reactions, which leads to a higher strength of the concrete close to the coating surface, and finally to a bond between zinc and concrete or grout comparable to that obtained with non-galvanized prestressing steel [19]. The duration of active zinc corrosion and the intensity of hydrogen evolution in fresh cementitious grout or concrete at the surface of prestressing steel depend on the passivation behaviour of the zinc coating in the alkaline environment. This behaviour is mainly determined by the properties of the cement. The main influencing factors are the content of alkali and chromate in the cement [17]: The duration and intensity of active zinc corrosion (until the potential is sufficiently positive) and temporary hydrogen evolution increase with the alkali content of the cement (Na 2 O-equivalent) and with increasing ph-value of the aqueous solution in the pores. However, even with highly alkaline cements, passivation occurs within a few days and hydrogen evolution is restricted to a relatively short period of time. Increased chromate content of the cement or addition of water-soluble chromates into the concrete or grout batching water have a favourable effect on initial zinc corrosion in fresh concrete or grout. Chromium (VI) chromate is highly soluble in fresh concrete or grout. It hinders and shortens initial lowering of the corrosion potential into the range of water decomposition through an early formation of a passivating chromate surface layer. As a consequence, initial zinc corrosion and hydrogen evolution is reduced and even stopped with a certain value of chromate content. In order to reduce the health hazard of workers processing cement/fresh concrete/fresh grout, the chromate content of European (and elsewhere) cements has been reduced to values below 2 ppm since Therefore, today s zinc coatings in contact with highly alkaline environment of fresh concrete or grout with hydrogen evolution may corrode more severely and over slightly longer periods of time than previously used cements with higher chromate contents. fib Bulletin 64: Effect of zinc on prestressing steel 7

13 Hydrogen evolution caused by grout admixtures Grouts for prestressing tendons use admixtures that typically contain a mixture of plasticizers and fillers. Grout admixtures ensure adequate viscosity to fill voids in the duct, reduce the water demand and the risk of bleed and segregation. Aluminium powder is added to some admixtures to provide a moderate expansion of the grout and thus supposedly ensuring complete filling of the tendon ducts. The aluminium powder reacts with the alkaline liquid grout producing hydrogen. The hydrogen acts as expanding or foaming agent and increases the volume of the liquid grout. However, the hydrogen produced during the reaction of the aluminium with the liquid alkaline grout does not get into contact with the prestressing steel since the time for the atomic hydrogen to be combined into harmless hydrogen molecules is significantly shorter (several factors of ten) than the time required to penetrate the prestressing steel. Tests [20] confirmed that even with non-galvanized prestressing steels, no increase of hydrogen content in the prestressing steel and no change of mechanical properties occurred. 2.2 Susceptibility of prestressing steel to hydrogen General Prestressing steels are either: cold drawn wire, cold drawn strand, or high-strength smooth or ribbed bars (micro-alloyed and tempered from the rolling heat). Hydrogen-induced stress corrosion only occurs with prestressing steels, which are susceptible to hydrogen. Prestressing steel is considered more susceptible to hydro-gen the lower the critical hydrogen content is which can lead to cracking under a given level of tensile stress. The risk of hydrogen-induced stress corrosion failure increases in susceptible prestressing steels with increasing hydrogen content in the prestressing steel and with increasing level of tensile stress [21]. Substantial variability in the susceptibility to hydrogen-induced stress corrosion cracking can be observed in different types of steels. Depending on composition and/or thermal treatment, the sensitivity of the prestressing steel structure to hydrogen increases in the following order: pearlitic grades, quenched and tempered, bainitic steels formed by continuous cooling, and finally martensitic structures. Because of its high sensitivity, martensitic steel should not be used where there is any risk of hydrogen-induced stress corrosion cracking. Based on existing knowledge and experience at the time of preparation of this report and [9], the typically used cold-drawn wires and strands are considered as comparatively nonsusceptible to hydrogen. This is primarily due to the particular structure of the prestressing steel produced by the cold-drawing process. However, it has been reported that the susceptibility increases significantly with the actual tensile strength of the prestressing steel [15, 21]. In accordance with L = C / (σ 3 R m 9 ) the time to failure, L, decreases with the 3 rd power of the tensile stress, σ, and the 9 th power of the actual tensile strength, R m. The coefficient C is a material parameter depending on the durability of the prestressing steel in the particular media to which it is exposed. Therefore the effect of the actual tensile strength is more significant than the effect of the actual tensile stress. 8 2 Summary of relevant parameters on the risk of hydrogen-induced stress corrosion cracking

14 Based on earlier investigations [9, 15] a critical actual tensile strength beyond which prestressing steel wires and strands may become susceptible to a rather long charging with hydrogen was proposed as about 2000 MPa. However, the authors of this report have received information from a Japanese manufacturer with recent test results for prestressing strand of 2230 MPa actual tensile strength, which showed similar time to failure as strand with tensile strength just below 2000 MPa. For hot-rolled bars the critical actual tensile strength is expected to be at about 1250 to 1400 MPa depending on actual chemical composition and heat treatment. Cold-drawn wires and strands with actual tensile strength of 2000 MPa or below and hot rolled bars with pearlitic structure and with a strength below 1400 MPa are in general sufficiently non-susceptible to low hydrogen exposure created by zinc corrosion over a limited period of time Testing FIP test From the evaluation of the different methods it was concluded that the test in the concentrated ammonium thiocyanate solution is a promising test to assess the susceptibility of prestressing steels to hydrogen-induced stress corrosion. This method is called the FIP stress corrosion test [22] and uses the following test conditions: corrosive medium: 200 g NH 4 SCN in 800 g H 2 O, Temperature 50 C, loading: constant load of 80% of the actual ultimate tensile strength, result: Lifetime to fracture of specimen (minimum and median values) is measured under these test conditions. The FIP test is specified for example in ISO standard (corrosion test with solution A), [23]. However, experience has shown that thresholds for the time to failure have to be defined for each individual particular type of prestressing steel, since a general limit value for prestressing steels does not exist. These thresholds depend on parameters such as diameter and type of prestressing steel. The FIP test therefore, is recommended to be used as quality control test for ongoing production of prestressing steels for which sufficient experience on the susceptibility exist. The FIP test, however, provides insufficient information to assess and safely judge the susceptibility of new types of prestressing steel DIBt test Various examples of failures of prestressed concrete structures caused by hydrogeninduced stress corrosion cracking that occurred at the beginning of the 1980s [24] led to the development of the so-called DIBt-stress corrosion test. In this test the prestressing steel specimens in the as-received state are stressed to 80% of the specified tensile strength at 50 C. According to the results from chemical analyses of on-site samples of water taken from prestressing ducts, the electrolyte solution for the test contains mol/l chloride, mol/l sulfate and mol/l thiocyanate (ph = 7.0). The SCN - ions promote the hydrogen uptake and thus make conditions more severe than in real conditions in practice. The DIBT test is specified for example in ISO standard (corrosion test with solution B) [23]. fib Bulletin 64: Effect of zinc on prestressing steel 9

15 Experience from testing of quenched and tempered wires, cold-drawn wires and strands, and bars, [9, 24], has shown that specimens made of susceptible materials fail at testing times of less than 2000 h. Hence, an absolute test performance threshold of 2000 h has been specified for the time to failure of prestressing steels in the DIBt test. It is also considered advantageous that the test solution closely simulates the corrosion conditions around the prestressing steels within the ducts in construction conditions. Therefore, the DIBt test is recommended as initial approval test for the susceptibility to hydrogen-induced stress corrosion cracking of any new type of prestressing steel. During approval testing, parallel tests with the FIP test should be performed in order to determine the median and minimum life-time values as reference for the later quality control testing during ongoing production. While the DIBT test provides a means to distinguish between prestressing steels which are susceptible or non-susceptible to hydrogen-induced stress corrosion cracking, it does not permit a direct ranking of different prestressing steels with respect to susceptibility [25]. 2.3 Thickness of zinc coating on prestressing steel The thickness of the zinc coating on continuously galvanized cold-drawn wires and strands is in the range of 30 to 50 µm. Prestressing bars are batch galvanized with a zinc thickness on the order of 100 µm. Atomic hydrogen from the corrosion process, generated on the zinc, is recombined to harmless molecular hydrogen provided that the zinc coating has no defects which extend through the coating to the surface of the prestressing steel. The permeability of intact zinc coatings for hydrogen is negligibly small and does not permit hydrogen-induced stress corrosion failure [26]. However, hydrogen ingress into the prestressing steel is possible if the zinc coating is too thin and/or locally damaged and/or removed through the corrosion process. 2.4 Level of tensile stress in prestressing steel As discussed in Section 2.2 the risk for hydrogen-induced stress corrosion increases approximately with the 3 rd power of the actual tensile stress. 2.5 Detailing aspects Hydrogen can only penetrate the prestressing steel if the hydrogen is deposited on a bare steel surface, i.e. without zinc coating. This can occur with: bare prestressing steel (non-galvanized) in direct or indirect contact with galvanized components, or galvanized prestressing steel in which the zinc coating is damaged or locally removed due to mechanical damage or local corrosion. Under the above two conditions, a corrosion element is formed in which the zinc acts as an anode and the prestressing steel acts as a cathode. For the case of non-galvanized prestressing steel embedded in fresh cementitious grout or concrete, in direct or indirect contact with embedded galvanized steel components, the distance of the prestressing steel from the galvanized components and the ratio of the surface area of zinc (anode) to prestressing steel (cathode) are the relevant detailing parameters. As mentioned earlier, the formation of hydrogen is limited to the period until the grout or concrete has set (hours to a few days) Summary of relevant parameters on the risk of hydrogen-induced stress corrosion cracking

16 2.5.1 Distance of prestressing steel to embedded galvanized components In accordance with investigations [16], variations of the distance in the vicinity of the prestressing steel have only a marginal effect on polarization in cases of indirect contact in fresh concrete or cementitious grout. Therefore, it is not possible to define a safe distance beyond which there is no risk for hydrogen evolution, due to the high conductivity of fresh concrete and grout Ratio of surface area of prestressing steel (cathode) versus zinc (anode) [16] The ratio of surface area of prestressing steel to zinc represents an important parameter for the polarization of the prestressing steel. Hydrogen evolution is possible (low potential of polarization of prestressing steel) for a short period of time if the surface area of zinc is larger than the surface area of prestressing steel. However, hydrogen evolution is significantly slowed or prevented (high potential of polarization of prestressing steel) if the surface area of the prestressing steel is significantly larger than the surface of zinc. At surface area ratios of prestressing steel/zinc of 10:1 (cement type CEM I) or more than 5:1 (cement type CEM IIIB) significantly positive potentials apply (outside the range of water dissolution) such that hydrogen evolution can no longer occur. In conclusion, large bare non-galvanized areas of prestressing steel in contact with small areas of corroding zinc are not susceptible to hydrogen evolution. 3 Assessment of particular applications This chapter reviews and assesses the risks involved for applications A to C presented in Section 1. A - Zinc-coated high-strength steel for applications in atmospheric corrosion conditions The underlying process of zinc corrosion in the atmosphere is described in Section The overall assessment for risk of hydrogen-induced stress corrosion for application Type A is: Design details that result in quasi permanently wet surfaces of zinc-coated high-strength cables with limited oxygen supply represent unfavourable corrosion conditions with active zinc corrosion and hydrogen evolution (crevice corrosion). These conditions may last for long periods of time. Because of this potentially long exposure there is a higher risk of damage due to hydrogen than with prestressing steel embedded in fresh cementitious grout or concrete, although it has lower stress levels. Cables of cable-supported structures exposed to atmospheric conditions (e.g. suspension bridge cables, and cable-stayed bridge cables) have significantly lower tensile stress levels than typical pretensioned or post-tensioned prestressing steel in concrete construction. In conclusion, the risk for hydrogen-induced stress corrosion may be significantly higher in cables subjected to atmospheric environment than for tendons in prestressed concrete. This risk is particularly high if deviators, connectors/clamps, and anchorage zones are not well detailed and therefore corrosion conditions with quasi permanently wet surfaces and limited oxygen supply may occur. fib Bulletin 64: Effect of zinc on prestressing steel 11

17 B - Zinc-coated prestressing steel embedded in fresh cementitious grout or concrete with or without defects in the zinc coating The underlying process of zinc corrosion in fresh mortar or concrete is described in Section Conditions specific to application B are: On the favourable side, an intact and sufficiently thick zinc coating on the prestressing steel does not permit migration of hydrogen to the prestressing steel. In such cases, there is no risk of damage due to hydrogen, although it has high stress levels. Unfavourable conditions apply when galvanized prestressing steels have defects in the coating (e.g. as a consequence of installation or in the region of anchorages with wedge bites). Hydrogen may penetrate the prestressing steel at locations of such defects. Corrosion conditions are mainly unfavourable when the zinc surface area is much larger than the exposed prestressing steel surface area, since the polarization of the prestressing steel reaches particularly low potentials. However, conditions are less critical if the level of stress in the prestressing steel is relatively low, see Section 2.2. C - Bare prestressing steel (no zinc coating) embedded in fresh cementitious grout or concrete in direct contact with, or indirect contact in the immediate vicinity of galvanized components embedded in the grout or concrete The underlying process of zinc corrosion in fresh mortar or concrete is described in Section Conditions specific to application C are: For direct or indirect contact of non-galvanized prestressing steel with galvanized embedded components, hydrogen evolution on prestressing steel is possible for a short period of time (hours to a few days), when the ratio of surface area of prestressing steel to zinc is smaller than 10:1 (for cement type CEM I) or smaller than 5:1 (for cement type CEM IIIB). With typical tendons of bare, non-galvanized prestressing steel in the vicinity of galvanized components, the surface area of the prestressing steel is typically several times larger than the surface of galvanized components. This applies in particular for multistrand or multi-wire tendons inside galvanized ducts where the steel/duct surface ratio varies between about 5 and 10 for small and large tendon sizes, respectively. For example: (1) Tendon of 7 strands of 15.7 mm inside a 55 mm diameter duct: surface area of individual wires A pw = mm 2 /m; surface area of duct A d = mm 2 /m; hence A pw / A d =4.7 (2) Tendon of 37 strands of 15.7 mm inside a 130 mm diameter duct: surface area of individual wires A pw = mm 2 /m; surface area of duct A d = mm 2 /m; hence A pw / A d =10.4 Therefore, in view of the short period of exposure to hydrogen combined with the beneficial effect of the typically large surface ratio of prestressing steel to zinc, the conditions of Application C are considered rather favourable and the risk of hydrogen deposition is significantly reduced if not eliminated Assessment of particular applicatons

18 B & C - Zinc-coated prestressing steel embedded in fresh cementitious grout or concrete with or without defects in the zinc coating Bare prestressing steel (no zinc coating) embedded in cementitious grout or concrete in direct contact with, or indirect contact in the immediate vicinity of galvanized components embedded in the grout or concrete Aspects which are identical for the above two groups of applications are considered together in this section. Galvanized coatings on prestressing steel and galvanized components such as galvanized ducts embedded in fresh concrete or filled with fresh cementitious grout generate hydrogen only for a short period of time (hours to few days). Once the zinc is passivated the formation of hydrogen due to water dissolution is no longer possible. Hence, in general the prestressing steel is exposed only initially to hydrogen. Prestressing steel is more exposed to hydrogen evolution with cements containing very high alkali contents and simultaneously low chromate contents. The deposition of hydrogen will particularly occur in the areas of defects in the zinc coating or alternatively on the bare non-galvanized prestressing steel in contact with zinc, and may last for about 1 to 2 days. The issue of short-term hydrogen evolution on the zinc coating has slightly worsened with the introduction of governmental (European) legislation permitting only use of low chromate content cements. The prestressing steel is exposed to hydrogen only for a short period of time (1 to 2 days) when zinc is in contact with fresh grout even if cements with reduced content of chromates are used [27]. Therefore, conditions are significantly more favourable than for corroding high-strength steel in air. When prestressing steel however, suffers pitting corrosion, e.g. under the influence of chlorides, conditions for hydrogen evolution and stress corrosion are permanently present. Experience from practice and experimental investigations [8, 16, 25] has confirmed that the hydrogen uptake into prestressing steel is significantly lower than expected in general. This applies both to applications of galvanized prestressing steel with defects in the coating, and to non-galvanized prestressing steel in direct or indirect contact with galvanized components embedded in fresh concrete or cementitious grout. Conditions for Application B (zinc coating with defects) are in general more unfavourable than those for Application C (bare prestressing steel in contact with galvanized components) due to the influence of surface area ratio of prestressing steel to zinc (the intensity of hydrogen evolution reduces with increasing ratio of surface area of steel to zinc). Tendons in prestressed concrete are stressed to significantly higher levels than cables in cable-supported structures exposed to atmospheric environment. The risk of stress corrosion cracking is raised by these higher stresses if other necessary conditions for this corrosion form occur simultaneously. The vast majority of today s prestressing steels are made of cold-drawn wires and strands with tensile strengths of 2000 MPa or below. These types of prestressing steels are considered to be relatively insensitive to hydrogen exposure because of their particular structure formed by the cold-drawing process. The final question is whether, considering all above-mentioned unfavourable conditions (contact of prestressing steel to zinc, low surface area ratio of prestressing steel to zinc, use of cements with low chromate content, high tensile strength), an exposure of proven prestressing steels to hydrogen in fresh grout or concrete for a short period of time may lead to irreversible damage in the prestressing steel. fib Bulletin 64: Effect of zinc on prestressing steel 13

19 In the opinion of the authors the answer to this question is no, in particular when also considering the investigations [16]: Durability tests were carried out with prestressing steels (cold drawn wires and strands and quenched and tempered wires) subject to a stress level of 95% of the 0.2% proof stress and exposed to solutions of calcium hydroxide inside wet concrete. These prestressing steels were subjected to a very negative potential of mv (Ag/AgCl) over a period of 48 hours, conditions as may occur on zinc when corroding in fresh grout or concrete under most unfavourable conditions. Current density measurement performed simultaneously proved that the prestressing steels were exposed to very high amounts of hydrogen. During a realistic period of exposure no failures and no change of the mechanical properties of today s approved prestressing steels were observed. However, failures occurred in the same tests with prestressing steels that are known to be sensitive to hydrogen and are no longer approved. 4 Summary and conclusions 4.1 Relevant parameters on the risk of hydrogen-induced stress corrosion cracking This report examines the relevant parameters on the corrosion behaviour of high-strength steels and prestressing steels (wires, strands and bars) when in contact with zinc. These are: A Zinc-coated high-strength steel for applications under atmospheric corrosion conditions. B Zinc-coated prestressing steel embedded in fresh cementitious grout or concrete with or without defects in the zinc coating. C Bare prestressing steel embedded in fresh cementitious grout or concrete in contact with galvanized components embedded in the grout or concrete (e.g. galvanized ducts, galvanized reinforcing steel, galvanized fastenings/dowels, etc). The actual risk for hydrogen-induced stress corrosion of prestressing steels in contact with zinc depends on the following six parameters: 1) Corrosion conditions, in particular the duration and intensity of the hydrogen evolution Hydrogen evolution is possible under the following conditions: The prestressing steel is corroding and in contact with zinc. Direct contact of bare prestressing steel with galvanized duct or rebar should, therefore, be avoided in cases where there is a risk of corrosion of prestressing steel due to chlorides during the service life of the structure. The protective layer of zinc coating is destroyed. This is the case when zinc-coated components are used in the atmosphere combined with permanent humidity and no air circulation. Zinc corrosion in crevices (crevice corrosion) is particularly critical. Under the above unfavourable conditions, in which zinc is not protected by surface layers, active zinc corrosion with formation of hydrogen is possible over extended periods of time Summary and conclusions

20 2) Type of cement (alkali and chromate content) The duration of active zinc corrosion and hydrogen evolution in fresh cementitious grout or concrete are mainly influenced by the alkali and chromate content of the cement: Temporary hydrogen evolution increases with the alkali content of the cement (Na 2 O- equivalent). Increased chromate content of the cement or the addition of water-soluble chromates into the concrete or grout batching water reduces the initial zinc corrosion and hydrogen evolution through the early formation of a passivating chromate surface layer. The application of chromate reduced cements causes the opposite, and extends somewhat the duration of exposure to hydrogen. 3) Susceptibility of the prestressing steel for hydrogen-induced stress corrosion cracking Hydrogen-induced stress corrosion only occurs with prestressing steels that are susceptible to hydrogen. The susceptibility increases significantly with the tensile strength of the prestressing steel. Cold-drawn wires and strands with actual tensile strength of 2000 MPa or less and hot rolled bars with pearlitic structure and with a strength less than 1400 MPa are in general sufficiently non-susceptible to low hydrogen exposure created by zinc corrosion over a limited period of time. 4) The thickness of the zinc coating on the prestressing steel Atomic hydrogen from the corrosion process, generated on the zinc, can enter the steel at defects which extend through the coating to the surface of the prestressing steel. The permeability of intact zinc coatings for hydrogen is negligibly small and does not permit hydrogen-induced stress corrosion failure. However, hydrogen ingress into the prestressing steel is possible if the zinc coating is too thin and/or removed through the corrosion process. 5) The level of tensile stress in the prestressing steel The risk for hydrogen-induced stress corrosion increases with the actual tensile stress. However, the effect of the actual tensile strength is more significant than the effect of the actual tensile stress. 6) Detailing aspects such as the ratio of surface of zinc versus the surface of bare prestressing steel and distance of the prestressing steel from the galvanized components The duration (hours to few days) and intensity of hydrogen evolution decrease with increasing ratio of prestressing steel/zinc surface areas. Large bare non-galvanized areas of prestressing steel in contact with small areas of corroding zinc are particularly favourable with respect to hydrogen evolution. At prestressing steel/zinc surface area ratios of 10:1 (cement type CEM I) or more than 5:1 (cement type CEM IIIB) hydrogen evolution cannot occur. Variations of the distance of the prestressing steel from electrically connected galvanized components in the vicinity of the prestressing steel has only a marginal effect on hydrogen evolution. fib Bulletin 64: Effect of zinc on prestressing steel 15