262-SCI : Characteristics of the steel/concrete interface and their effect on initiation of chloride-induced reinforcement corrosion
Technical Committee 262-SCI
Deputy Chair: Prof. Mette GEIKER
Activity ending in: 2021
This new proposal is approved by TAC but will be ratified officially by Bureau in March 2015. Nevertheless it can start its activity now.
Material: Reinforced concrete
Structures: Reinforced concrete structures exposed to chloride environments
Phenomena / specific aspect: Initiation of chloride-induced reinforcement corrosion; critical chloride content; durability; service live predictions (for a more detailed description, see below in section 2)
Level of investigation: Literature survey, theoretical study, bringing together experience (for a more detailed description, see sections 3 and 4)
Approach: This TC is based on a new approach to the topic of initiation of chloride-inducedreinforcement corrosion (for a more detailed description, see sections 2 and 3)
Background and motivation:
It is well recognized that the presence of chlorides at the steel surface of reinforcement in concrete can promote initiation of pitting corrosion. Due to the localized nature of this type of corrosion, pitting corrosion is often associated with high corrosion rates and thus high local loss of steel cross section.
For service life predictions and assessments, it has become common practice to use the concept of the critical chloride content (Ccrit). The critical chloride content (chloride threshold) is defined as the chloride content at which corrosion initiates. The concept is based on measuring (when assessing existing structures) or predicting (e.g. in the design stage of new structures) the chloride content at the steel depth and comparing this value with Ccrit. Hereby it is assumed that the phenomenon of chloride-induced corrosion initiation can be reduced solely to a question of chloride concentrations.
Over the past ca. 50 years, considerable efforts have been made with regard to the determination of the value and/or distribution of Ccrit. A thorough review of the existing literature  on Ccrit has shown the following:
- The values for Ccrit reported in the literature scatter over a large range, starting from almost 0 to more than 3% chloride by weight of binder.
- There is no agreement on a test method / experimental setup to determine Ccrit.
- Numerous parameters affect Ccrit; the three most important influencing parameters have been identified to be
a) the electrochemical potential of the reinforcement steel
b) the pH of the concrete pore solution
c) the conditions at the steel/concrete interface.
The first two of the three main influencing parameters on Ccrit, viz. a) the potential and b) the pH, are parameters that can reasonably well be measured or quantified. The third influencing parameter, viz. c) the “conditions at the steel/concrete interface”, is more difficult to assess / quantify.
When discussing conditions at the steel/concrete interface and their role on corrosion initiation, the term “defects” is frequently used. These “defects” or irregularities are recognized to offer preferable corrosion initiation sites. However, there exist many different types of defects and their influence varies. Examples are air voids / macro pores , the bleed-water zone at the underside of reinforcement steel [3-5], but also cracks or the presence of aggregates in direct contact with the steel. Another example is the surface condition of the steel, viz. the presence or absence of mill-scale or rust layers that is known to affect corrosion initiation, see e.g. Refs. [6, 7] or the experience currently made within the work of RILEM TC 235-CTC. A certain“defect” may also act differently depending on the exposure conditions. For instance, air voids promote corrosion when they contain water. In case they are air-filled, they will presumably not act as corrosion initiation sites. The relevance of moisture conditions for the influence of voids on corrosion initiation is highlighted by on-going research of the current applicants [unpublished work].
There is indication that for predicting initiation of corrosion, reducing the problem to comparing the chloride content at the steel to the parameter Ccrit does not adequately describe reality. Our hypothesis is that the local conditions at the steel/concrete interface strongly affect the location and tolerable chloride concentration for initiation of pitting corrosion. It has in fact not yet been demonstrated that the chloride concentration at the steel is the most dominant parameter controlling corrosion initiation –more dominant than e.g. irregularities at the steel/concrete interface.
Aims and objectives
Thus, this proposed TC aims at
A) Summarizing the state of the art concerning different conditions at the steel/concrete interface and their possible effect on chloride-induced corrosion initiation
B) Identifying the related areas that require more research in order to improve the understanding of the conditions leading to chloride-induced corrosion
C) Summarizing existing methods suitable to characterize the conditions at the steel/concrete interface – both during or after execution (as quality control / assurance) and assessment of existing structures.
D) If possible, make recommendations concerning methods that may be applied in the laboratory or on-site in order to characterize the steel/surface conditions
 U. Angst, B. Elsener, C.K. Larsen, Ø. Vennesland, Critical chloride content in reinforced concrete - A review, Cem Concr Res, 39 (2009) 1122-1138.
 B. Reddy, Influence of the steel-concrete interface on the chloride threshold level, PhD thesis, Imperial College, London, 2001.
 U. Angst, Chloride induced reinforcement corrosion in concrete. Concept of critical chloride content - methods and mechanisms, doctoral thesis 2011:113, ISBN 978-82-471-2762-9, Norwegian University of Science and Technology, NTNU, 2011.
 A.T. Horne, I.G. Richardson, R.M.D. Brydson, Quantitative analysis of the microstructure of interfaces in steel reinforced concrete, Cem. Concr. Res., 37 (2007) 1613-1623.
 T.A. Soylev, R. Francois, Quality of steel-concrete interface and corrosion of reinforcing steel, Cem. Concr. Res., 33 (2003) 1407-1415.
 L.T. Mammoliti, L.C. Brown, C.M. Hansson, B.B. Hope, The influence of surface finish of reinforcing steel and pH of the test solution on the chloride threshold concentration for corrosion initiation in synthetic pore solutions., Cem Concr Res, 26 (1996) 545-550.
 L. Li, A.A. Sagüés, Chloride corrosion threshold of reinforcing steel in alkaline solutions - Open-circuit immersion tests., Corrosion, 57 (2001) 19-28.
Terms of reference
Time necessary and provisional time schedule:
The estimated duration of the TC work is 4 years. The start of the TC activities is possible from spring 2015 onwards. More details on the time schedule and working programme are given in “Methodology” below and in section 4.
The proposed TC will bring together experts from the field of chloride-induced corrosion and the field of characterization of defects occurring at interfacial transition zones. Members will be recruited primarily from academia, but also members from the industry (consultant engineers, testing institutes, etc.) are anticipated.(Compare section 9)
The committee will prepare a state of the art report (STAR). The STAR will be based on both documented results from the literature and theoretical, conceptual reasoning within the committee. In addition to bibliographical research, the committee work will consist in exchange of experience and results, including unpublished results. In case the state of the art indicates that the currently known methods are not applicable for the specific purpose or that they are not (yet) capable of delivering the information in the present context, the committee will also consider performing own experimental investigations (in the laboratory or on real structures), given that participating laboratories or institutes have the capacity for this.
In addition to common committee meetings (presumably on a 6 months interval basis), it is planned to organize two workshops with invited presentations on selected topics in order to stimulate and focus the discussion.
A more detailed description of the planned STAR is given in section 6.
Detailed working programme
First two years:
• The TC work will be started with a 1st workshop. The workshop will be devoted to documented experience with different conditions/defects/irregularities at the steel/concrete interface and their effect on chloride-induced corrosion initiation. The workshop will consist of presentations on selected aspects and have a strong focus of discussion.
• In subsequent 2–3 committee meetings the TC will work on the STAR concerning aims A and B outlined in section 1.
• Milestone after 2 years: one workshop held and part I of the STAR completed.
Years 3 and 4:
• A 2nd workshop will be organized; this time on methods to characterize the conditions/defects/irregularities at the steel/concrete interface that were identified to be relevant within the first part of the committee work (aims A and B outlined in section 1). The workshop will again consist of presentations on selected aspects and have a strong focus of discussion.
• The remaining 2–3 committee meetings will be used to work on the second part of the STAR (concerning aims C and D outlined in section 1). In case the committee identifies the need for additional results, e.g. specific aspects lacking in the available literature and experience, complimentary laboratory or on-site testing may be undertaken.
• Milestone after 4 years: two workshops held and STAR completed.
The objective of this proposed TC is within the scope of the RILEM mission. The TC would most appropriately be assigned to cluster B (Transport and Deterioration Mechanisms) as it deals with chloride-induced corrosion, which is one of the major deterioration mechanisms of reinforced concrete. Nevertheless, the topic of this TC is also relevant for clusters A (Material Processing and Characterization), C (Structural Performance and Design), and D (Service Life and Environmental Impact Assessment). Exchange of information is thus planned with current and future TC activities within these clusters.
The work of the TC will be linked to the previous RILEM TCs, particularly TC 108-ICC “Interface between cement paste and concrete constituents” (closed), TC 213-MAI “Model assisted integral service life prediction of steel reinforced concrete structures with respect to corrosion induced damage” (closed), TC 207-INR ”Interpretation of NDT results and assessment of RC structures”, and 235-CTC “Corrosion initiating chloride threshold concentrations in concrete” (running, to be closed withinca. 1 year). However, it is not a direct continuation of the work of either ones of them, but rather a complimentary approach. Particularly with respect to TC 235-CTC, it was made sure with the chair, Tang Luping, that there is no conflict of interest with the presently proposed TC.
• State-of-the-art report:
This report will comprise a list of types of different conditions, irregularities or defects that can be present at the steel/concrete interface and an assessment of their effect on corrosion initiation. The report will also comprise a compilation of available methods for assessing the conditions and documenting the defects at the steel/concrete interface. The methods will include microscopic techniques as well as non-destructive test methods such as electrochemical measurements that have the potential of being applied on-site (e.g. for quality control / assurance during/after execution or for condition assessment of existing structures).
Based on the STAR, recommendations will be made for areas needing more research work in order to improve the understanding of the conditions for chloride-induced corrosion initiation. This will act as guidance for future research activities. If possible, recommendations will be made concerning methods that may be applied in the laboratory or onsite in order to characterize the steel/surface conditions.
• Two international RILEM workshops
• 1–2 summary papers published in scientific journals (preferably Materials and Structures)
Group of users
The targeted group of users is primarily researchers. However, depending on the results, a better understanding of the conditions for corrosion initiation and eventually proposed measurement methods may also have a direct impact on the practice of durability design and condition assessment, viz. also be useful for owners of infrastructure, testing laboratories, orconsulting engineers.
Specific use of the results
It is expected that the new approach to the topic of initiation of chloride-induced corrosion will enhance research in this relevant and frequently addressed topic. At present, defects at the steel/concrete interface are in models for predicting corrosion initiation taken into account via a statistical variability of the critical chloride content. In this approach characteristics of the steel/concrete interface are lumped together with all kinds of uncertainties such as experimental inaccuracies etc. Being able to not only identify but also improve the quantification of the most important influencing parameter(s) on corrosion initiation will thus significantly narrow the uncertainties currently associated with predictive models.
Thus, the outcome will on the long-term be relevant for both assessing the condition of existing, chloride-exposed structures and predicting the service life. Finally, identifying critical characteristics inherent to the steel/concrete interface (voids, bleed-waterzone, presence of rust, etc.) may also stimulate research with respect to material optimization / tailoring (for instance mix design to minimize the presence of corrosion-promoting characteristics).