235-CTC : Corrosion initiating chloride threshold concentrations in concrete

Technical Committee 235-CTC

General Information

Chair: Prof. Luping TANG
Deputy Chair: Mr. Jens Mejer FREDERIKSEN
Activity starting in: 2009
Activity ending in: 2016
Cluster D

Subject matter

The chloride concentration in concrete at which corrosion of reinforcement is initiated is an important parameter in assessing and re-assessing the service lifetime or the residual service lifetime of structural concrete. It has been well known for many years that this parameter is vital, but nevertheless there only exists scarce data in the literature on how to determine this parameter.

Although TC 213-MAI partly focussed on the problem of chloride threshold values, its main objective is 'the preparation of a State of the Art Report on existing models for both the design phase or for existing structures damaged by corrosion structures as well as the compilation of tests and sensors suitable to be used in practice for such modelling'. Obviously, it is not the specific objective for TC 213-MAI to work deeply on the parameter chloride threshold. After near four years’ activities, TC 213-MAI will soon finalise its work in the Spring of 2009. This could be a very good starting point for a new TC to focus on this complicated and vital parameter the Chloride Threshold concentration for Corrosion initiation (CTC).

Modelling or prediction of the service life is meaningless without knowledge on this parameter. So far not even the definition of the term 'chloride threshold' is clear and no generally accepted test method exists. Engineers are forced to make educated guesses on the magnitude of the parameter in order to make their service life estimations meaningful.

Nevertheless in practice the acceptable value is appointed with very little effort and often 0.05 % by mass of concrete or 0.4-0.6 % by mass of cement is appointed as the values to use as acceptance level. A practice where these values are doubled in wet areas is gaining ground, but still no scientific knowledge on the subject has been established. On the other hand, lower threshold values based on limited laboratory investigations are often used to the concrete with additional materials such as silica fume and fly ash, although these additional materials may significantly improve the pore structures and consequently improve the resistance to corrosion process. A further comprehensive study is obviously needed in order to promote and make sure of the use of additional materials in concrete with regard to its durability especially the resistance to reinforcement corrosion.

There is an urgent need to start focussing on the subject. This is due to the fact that service life predictions and estimations are being demanded more and more often. As will be reported by TC 213-MAI, well known service life models do exist - they are all trying to model the level of chloride contamination around the reinforcement after a certain exposure, but none of them have a neither scientific nor experimental basis for defining the end point of the calculation.

Discussions of an acceptable level for the chloride penetration parameter (according to some well defined standard test method) are frequent, but the same attention is not put on the threshold level for corrosion initiation. Several theoretical and practical analyses have shown that the uncertainty of the threshold level is just as important as the level of the chloride transport parameter (a diffusion coefficient of some kind), but still only little effort has been put into a 'universal solution'.

The knowledge of the parameter (Ccrit) is equally relevant for new structures (on the project state) as well as for existing structures.

Terms of reference

The TC CTC will comprise experts selected worldwide from membership organisations that have been active in searching for a better understanding, testing and modelling of chloride threshold for applications to service life design and redesign of concrete structures. It will include universities, research institutes or centres, material suppliers, contractors, engineering consultants, building owners, public agencies, etc. The members will also be recruited from previous RILEM TCs, and participants in international conferences, symposiums and workshops on the topic dealing with chloride transport, reinforcement corrosion and service life modelling.

The committee work will imply bibliographical research for state-of-the-art report (STAR) and organisation of round robin tests (RRT).

Estimated duration of the committee work is 4 years. The new committee can start its activities in the middle of 2009, immediately after the end of the activities of TC 213-MAI on chloride ingress modelling.

The committee work will imply bibliographical research for state-of-the-art report (STAR) and organisation of round robin tests (RRT).

Estimated duration of the committee work is 4 years. The new committee can start its activities in the end of 2007 or the beginning of 2008.

Detailed working programme

The committee work will be carried out with close links to on-going research projects. The main activities and provisional time schedule is as follows.

Activities Months from start Form
-  Mobilising TC work and establishing consensus on strategy and limitation of work
- Based on TC 213-MAI’s work, further collecting data and test methods from literature and previous projects,  identifying on-going projects dealing with chloride threshold and defining contents of STAR
-  Planning 1st Workshop
-  Selecting test methods and defining extent of RRTs and identifying potential laboratories.
-  First draft STAR on chloride threshold concentrations
-  Organising RRTs
-  Second draft STAR and 1st Workshop
-  Final version of STAR in ready to print format, analysis and discussion test results from RRTs and planning 2nd Workshop
- First draft report of RRTs and Recommendations of test methods
-  Second draft RRTs report and recommendations on test methods
-  Final versions of RRTs report and recommendations of test methods
-  2nd Workshop on chloride threshold-– and final summar

Technical environment

This TC links naturally to the previous work of TC 178 TMC (closed) and TC 213-MAI (to be closed) and it will be a natural continuation of these.

This TC will work closely in cooperation with another potential TC proposed by Dr Björn Johannesson with the title: Multi-component transport and chemical equilibrium in cement based material.

This TC will possibly cooperate, or at least exchange the information with TC CNM Reinforced concrete in the context of nuclear waste management.

Expected achievements

The TC CTC will seek to produce
-  Definitions
-  Terminology
-  Suggestions for test methods
-  Round Robin test
-  A state-of-the-art report
- Two workshops on the specific topic
-  Suggestions for implementation and gain of experience

Group of users

The targeted groups are owners of large buildings and infrastructures, testing laboratories, practitioners and engineering consultants.

Specific use of the results

The results of the TC CTC are used in models for service life predictions (scientific or not). If the Ccrit can be determined more precisely a lot of repair works may be postponed or planned with much more precision. For new structures the value of the service life predictions will improve leading to more optimal designs (concrete cover) and concrete compositions. Furthermore introduction of new and greener raw materials for manufacturing concrete can be introduced on a safer scientific/experimental basis.