The detrimental influence of chlorides on the corrosion of reinforcing steel 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 unique 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 limits, 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 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 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 supplementary 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 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 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 limits, 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 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 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 determining 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 Committees 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 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 allowable admixed chloride limits in concrete.

The detrimental influence of chlorides on the corrosion of reinforcing steel 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 unique 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 limits, 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 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 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 supplementary 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 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 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 limits, 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 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 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 determining 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 Committees 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 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 allowable admixed chloride limits in concrete.

The aim of this study was to determine the chloride threshold concentration of carbon steel rebar embedded in high performance concrete under exposure conditions relevant to the substructure of coastal bridge in Florida. The experiments were based upon a series of reinforced and non-reinforced concrete specimens that contained 1) 20, 35 and 50 percent cement replacement by fly ash, 2) 6, 15 and 27 percent cement replacement by silica fume, and 3) control specimens (no pozzolanic admixture). All specimens had a target w/cm ratio of 0.37. The specimens have been exposed to one week wet - one week dry ponding cycle with natural seawater since January, 1995. Rebar potential values were monitored with time in order to determine when corrosion initiated. The rebar of several specimens activated after 15.6 to 17 years of exposure, and selected specimens were terminated for forensic examination and extent of corrosion. Cores were obtained to determine the extent of chloride ingress, the apparent diffusion coefficient (Dapp) and concrete resistivity. The chloride concentration above the rebar trace was also measured on most of the terminated specimens. The value for Dapp was correlated against the corresponding measured resistivity. Specimens with 50 percent FA had the lowest Dapp but also the lowest chloride threshold.

Over the last 60 years, extensive research efforts aimed at determining the so-called chloride threshold value in reinforced concrete. The belief that such a threshold exists is the root of all efforts to measure and model chloride ingress into concrete. This paper addresses the usefulness of this established concept by evaluating the experience available for portland cement systems. Additionally, it is critically discussed whether the concept can be applied to modern materials, particularly SCMs. Finally, suggestions for future research are made. It is concluded that the pronounced stochastic nature of the chloride threshold currently permits only corrosion prognoses with large uncertainties. It is shown that even the most sophisticated chloride transport model in concrete will not significantly improve this. Instead of refining mass transport models, future research should thus aim at finally understanding the relevant parameters governing corrosion initiation in concrete. There is strong indication that a number of such parameters are overlooked in the current concept of the chloride threshold value. We believe that as long as initiation of chloride-induced corrosion is not fully understood, it does not make sense to continue applying the (unsuccessful) concept of the chloride threshold value to modern materials.

A thermodynamic modeling investigation was conducted to quantify free and bound chloride concentrations, and pore solution pH, in mixtures produced with different types of blended cements and admixed chlorides. Specifically, the validity of using total cementitious materials content, instead of cement content, as the basis for allowable admixed chloride limits in new construction was evaluated. The effects of replacing OPC with two types of fly ash (class C and F) and one type of slag at different replacement levels were investigated. Assuming chlorides are only chemically bound, the water soluble Cl-/OH- remained rather stable up to 20% class F fly ash, 30% class C fly ash, and 40% slag replacements. Beyond these replacement levels Cl-/OH- were shown to increase, in some cases rather sharply, for the mixtures that were studied in this investigation. The results of this investigation support the ACI committee recommendations for allowable chloride limits to be reported in terms of total cementitious materials content up to 20% class F fly ash, 30% class C fly ash, and 40% slag replacements. Since SCM replacements affect the binding capacity, water soluble chlorides, and the pH of pore solution simultaneously, it is recommended that allowable chloride limits are made in terms of Cl-/OH- rather than chloride contents alone.