ACI SP-308-2016 Chloride Thresholds and Limits for New Construction.pdf

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1、An ACI Technical Publication SYMPOSIUM VOLUMESP-308Chloride Thresholds and Limits for New ConstructionEditors:David Tepke, David Trejo, and O. Burkan IsgorChloride Thresholds and Limits for New ConstructionSP-308Editors:David Tepke, David Trejo, and O. Burkan Isgor Discussion is welcomed for all mat

2、erials published in this issue and will appear ten months from this journals date if the discussion is received within four months of the papers print publication. Discussion of material received after specified dates will be considered individually for publication or private response. ACI Standards

3、 published in ACI Journals for public comment have discussion due dates printed with the Standard.The Institute is not responsible for the statements or opinions expressed in its publications. Institute publications are not able to, nor intended to, supplant individual training, responsibility, or j

4、udgment of the user, or the supplier, of the information presented.The papers in this volume have been reviewed under Institute publication procedures by individuals expert in the subject areas of the papers.Copyright 2016AMERICAN CONCRETE INSTITUTE38800 Country Club Dr.Farmington Hills, Michigan 48

5、331All rights reserved, including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or written or oral, or recording for sound or visual reproduction or for use in any knowledge or retri

6、eval system or device, unless permission in writing is obtained from the copyright proprietors.Printed in the United States of AmericaEditorial production: Gail TatumISBN-13: 978-1-942727-94-1First printing, June 2016PrefaceThe detrimental influence of chlorides on the corrosion of reinforcing steel

7、 in concrete has been widely documented. The literature clearly shows that chloride concentration at the steel level must exceed a critical chloride threshold to initiate active corrosion of reinforcement embedded in concrete. It is now well accepted that this critical chloride threshold is not a un

8、ique value, but rather a range that depends on several factors. Regardless, placing concrete with chloride concentrations above the critical chloride threshold for a particular situation would result in active corrosion of the reinforcement and is therefore undesirable. Unnecessarily restrictive lim

9、its, however, can lead to preclusion of some otherwise acceptable materials or require use of supplemental materials or alternative mixture designs that may increase costs or impact sustainability. Thus, there is a need from a practical standpoint to establish conservative, yet reasonable, limits so

10、 that the effects of corrosion can be managed without undue restrictions. ACI documents place limits on the amount of chlorides that can be incorporated into new concrete these limits are referred to as the allowable admixed chloride limits.Documents published by ACI Committees 201 and 222 currently

11、 recommend limiting admixed chlorides based on a mass percentage of the portland cement in the concrete mixture. Other documents, such as ACI 318, limit the admixed chlorides based on weight percentage of cement. With the movement of the industry towards greener systems, the inclusion of supplementa

12、ry cementitious materials (SCMs) as part of the cement could be beneficial. SCMs, however, when used in large quantities, have been reported to decrease the pH of the pore solution, which may lower the critical chloride threshold values. If the critical chloride threshold values for concrete systems

13、 containing only portland cement are different than the critical chloride threshold values for systems containing portland cement and SCMs, the published allowable admixed chloride limits may not be applicable. A further complication in establishing values exists due to performance-based cements in

14、which the specific amounts of SCMs might not be known to the specifier.This special publication (SP), based on two technical sessions held during the Fall 2015 Concrete Convention and Exposition in Denver, CO, November 8-11, 2015, addresses challenges associated with allowable admixed chloride limit

15、s, critical chloride thresholds, testing for the critical chloride threshold, binding of chlorides in different systems, and how admixed chlorides influence service life. Authors and presenters from North America and Europe provided a variety of perspectives, experiences, and opinions. Based on the

16、presentations, the open discussion that followed the presentations, and the papers in this SP, evidence indicates that allowable chloride limits should be based on cementitious materials content including both portland cement and SCMs. However, because research on the amount of chlorides required to

17、 initiate corrosion in systems containing high SCM replacement levels suggests that there may be upper limits at which the inclusion is appropriate, it was suggested that it may be appropriate to place limits on the replacement percentages of SCMs used for calculations of cement content when determi

18、ning allowable admixed chloride limits. Although the Denver sessions and the papers in this SP provide a significant move forward on better defining allowable chloride limits and likely allow for refinement of current recommendations in ACI documents, more research is needed.On behalf of ACI Committ

19、ees 201 and 222, the editors sincerely thank all authors and presenters for their efforts and contributions to the presentations, open forum, and this SP volume. Special thanks are extended to the peer reviewers of the manuscripts for their constructive comments and recommendations. The editors are

20、also indebted to the ACI staff for their assistance in organizing the sessions, organizing the open forum, and in preparing this volume. The editors earnestly hope that this symposium and SP volume will serve as a valuable resource to those searching for data, guidance, and better clarity on allowab

21、le admixed chloride limits in concrete.EditorsDavid TepkeConsultantSKA Consulting Engineers Inc.David TrejoProfessor Chloride induced corrosion; Critical chloride content; Durability, Reinforcement corrosion; Service life modelling; Supplementary cementitious materials; Sustainabilit Ueli M. Angst a

22、nd Bernhard Elsener 1.2 Ueli M. Angst is a lecturer at the Institute for Building Materials at ETH Zurich, Switzerland, as well as a consultant for the Swiss Society for Corrosion Protection (SGK). He studied civil engineering at ETH Zurich and received his PhD from the Norwegian University of Scien

23、ce and Technology in Norway in 2011. His research interests include corrosion of metals, methods and sensors for condition assessment and laboratory measurements, and durability of reinforced and prestressed concrete. Bernhard Elsener is professor at ETH Zurich, Institute for Building Materials, Zur

24、ich, Switzerland. He studied materials science at the Department of Chemistry at ETH Zurich and received his PhD from ETH Zurich, Switzerland in 1982. His research interests include corrosion of metals in general, electrochemical techniques, sensors, condition assessment and durability of reinforced

25、 and post-tensioned structures, and corrosion of steel in concrete. INTRODUCTION In many countries around the world, portland cement has for decades been the most used type of cement to build reinforced concrete infrastructure. In recent years, however, portland cement has increasingly been substitu

26、ted with supplementary cementitious materials (SCMs). Table 1 illustrates this by means of the example of Switzerland. While for some of these materials there may be positive track records from certain countries for instance for ground granulated blast furnace slag that has been used in the Netherla

27、nds for roughly a century the diversity of cementitious materials and mineral binders to be used in the future is expected to increase, owing to, amongst others, reasons of local availability of raw materials (Angst et al. 2012). The main motivation for reducing the “clinker factor”, i.e. to increas

28、ingly substitute portland cement clinker partially with SCMs, is on the one hand to reduce the environmental impact, particularly by reducing the carbon footprint, and on the other hand to improve the durability. Achieving sustainability clearly not only requires decreasing the environmental footpri

29、nt of the materials at the time of their production, but to combine this with long and maintenance free service lives of the structures in their actual exposure environments. Due to the lack of long-term experience on the field performance of modern materials, we are forced to make predictions. This

30、 is done based on theoretical reasoning and on accelerated laboratory (or field) testing, believed to provide a basis to extrapolate the short-term behaviour to long-term and field conditions. Essentially, these efforts consist in adapting concepts established with experience from portland cement an

31、d applying them to the new materials. The conceptual understanding of degradation due to chloride-induced corrosion of reinforcing steel in concrete evolved in the second half of the last century and has since then essentially been unchanged. The widespread hypothesis is that the chloride concentrat

32、ion at the surface of the embedded steel is the by far most important parameter, and hence that initiation of corrosion can be predicted by reducing the entire problem to a question of chloride concentrations only. This is illustrated by the extensive research efforts performed over the last 60 year

33、s, seeking to determine the so-called chloride threshold value or critical chloride content (Angst et al. 2009), i.e. to specify a threshold concentration below which, conceptually, there is no corrosion and above which corrosion occurs. The belief that such a threshold exists is also the root of al

34、l research efforts to measure and model chloride ingress into concrete. This issue received considerable research attention over the last decades, and a high number of test methods have been devised internationally for assessing the ability of concrete to resist chloride penetration, as e.g. reviewe

35、d in Tang et al. (2012). Table 1 Types of cement sold in Switzerland in the years 1996, 2006, and 2015, illustrating the sharp transition from Portland cement to blended cements; data from (CemSuisse). Cement type Designation (acc. to EN 197-1) 1996 2006 2015 Portland cement CEM I 87% 37% 13% Blende

36、d Portland cement CEM II 11% 60% 83% Slag cement CEM III 1% 2% 1%Other 2% 2% 4%Chloride Threshold Values in Concrete A Look Back and Ahead 1.3 The consequence of the concept of the chloride threshold value is that materials such as cement types or concrete formulations are commonly regarded as param

37、eters, i.e. that chloride ingress properties and chloride threshold values can be determined by test methods for these different materials in order to predict the time to corrosion initiation. This paper addresses first the usefulness of this established concept by evaluating the experience availabl

38、e for portland cement systems. Subsequently, it is critically discussed whether the concept can be applied to modern materials, particularly SCMs, and which factors are currently seen to potentially handicap this application. Finally, suggestions for future research are made. EXPERIENCE FROM PORTLAN

39、D CEMENT SYSTEMS Literature data Laboratory and field dataIn a literature review performed by Angst et al. (2009), the variability of published chloride threshold values was found to be extremely high. Values determined in laboratory studies scattered from almost 0 to more than 8% chloride by mass o

40、f binder if all cement types are considered; if only studies with portland cement (excluding low C3A cements) are considered, the results span still from almost 0 to more than 3% chloride by mass of cement. No systematic trends could be found such as with respect to water/cement ratios, methods of a

41、ccelerating chloride ingress, storage conditions (e.g. humidity), and corrosion detection methods. It may also be worth mentioning that within this huge span of reported laboratory chloride threshold values, the results of most individual studies covered only a small fraction of the entire span. In

42、other words, some studies tended to give results in the lower range of the literature span, some were on an intermediate level, and some studies yielded results in the higher end of the literature span. Thus, the entire variability in the literature seems to arise from the combination of different e

43、xperimental parameters used in each study, i.e. from the measurement method. Considering data from field exposure sites or from measurements on real structures shows a better agreement between individual reports, but still a large variability. In the 1970s and 1980s, a number of studies reported tha

44、t chloride threshold values scattered in the range 0.1 to 2% chloride by weight of cement (Stratfull et al. 1975, Vassie 1984, Treadaway et al. 1989). It can be assumed that all of these studies dealt with portland cement. Figure 1 shows a compilation of literature data providing information on the

45、statistical distribution of the chloride threshold value. Note that data on the probability of corrosion initiation at certain chloride concentrations is scarce, and that not many more such references can be found in the literature. The data shown in Figure 1 as triangles stems from laboratory studi

46、es. In the work of Zimmermann (2000) ribbed carbon steel rebars (diameter 8 mm (0.315 inches), length 65 mm (2.56 inches) were embedded in mortar (w/c=0.6). The specimens were immersed in a chloride solution; initiation of corrosion was determined by monitoring the steel potential. In another labora

47、tory study (Breit 2001), similar specimens were used (rebar diameter 10 mm (0.39 inches), length 55 mm (2.17 inches), smooth steel bars; w/c=0.50.6), but they were not left at their open circuit potential during immersion in chloride solution, but polarized to relatively positive potentials (+0.5 V

48、vs. SHE). Both experimental setups can only mimic reality to a certain extent; differences in this regard have been discussed in (Angst et al. 2011b). Nevertheless, both setups permit to accurately detect corrosion onset and thus, the determined chloride content at the time of corrosion initiation,

49、is considered to represent well the chloride threshold value (under the given conditions). In contrast, the well-known data from field experience reported by Vassie (1984) (bridge structures in the UK) may include larger uncertainties since the time of corrosion initiation was not accurately known. Nevertheless, the results stem from real structures and thus, they are free from artefacts that typically arise from the way specimens are produced and exposed in accelerated laboratory testing (Angst et al. 2009). Figure 1 also shows the distribution of the chloride threshold value