MCT : Multi-component transport and chemical equilibrium in cement based materials
Technical Committee MCT
Deputy Chair: Dr. Yoshifumi HOSOKAWA
Activity ending in: 2016
Recently principles and approaches, as used for many years in geotechnical transport and chemical equilibrium modeling, has been suggested and adopted for describing concrete durability. The general framework of such geotechnical models is relevant for concrete. However, special phenomena such as the chemical equilibrium condition for the C-S-H and the effect of electrical double layers on the transport and chemical equilibrium conditions needs special attention and further research efforts.
In order to promote the development of this kind of research it would be beneficial to try to gather the expertise in the field of durability modeling related to the above mentioned geotechnical approach. A TC of RILEM should be a good forum for researchers to discuss the development of these kinds of models and their verifying experiments needed.
Deterioration processes in cement based materials such as chloride penetration, sulfate attack, carbonation, leaching and alkali silica reactions are all described by quite complicated underlying mechanisms. These mechanisms are only partly understood and still need to be examined in order to obtain a greater knowledge of the materials in question. Especially, the coupling between the different separate processes is of essential interest since such couplings always are present in practice.
Due to the complexity of the problem at hand a powerful strategy is needed both in terms of the fundamental hypothesis building process and in terms of the experimentation process. Only in cases when these two processes work in harmony relevant scientific models and general understanding of the occurring processes can be obtained.
The proposed project neither deal with a new subject or a new approach, but is rather an attempt to solidify and develop the novel tentative advanced coupled transport and chemical equilibrium models established the last few years by different research groups around the world.
The project will be limited to consider the theoretical and experimental study of moisture and ionic constituent transport with account of chemical equilibrium of cement based materials and will therefore not include mechanical effects such as cracking and reinforcement corrosion effects. The results from the work to be performed will, however, be presented in a way so that it can be of direct use to research work and applications dealing with subjects falling outside the present scope.
No direct attention will be paid to the early age when the material undergoes significant hydration. The study will rather be directed towards the description of the behavior of well hydrated material even though some interesting insights of the hydration process can be obtained from the existing chemical part of the models to be analyzed.
The properties of main importance of this investigation are the physical state of water in the pores of the materials and the different ionic constituents dissolved therein. Further, the chemical equilibrium between ions bound into solid cement minerals and the ions being dissolved in the pore water is of paramount interest. It is also believed that the charged surfaces of the C-S-H play an important role when studying chemical equilibrium of cement based materials. Besides the equilibrium conditions the transport properties are of interest since they, loosely speaking, determines the rate of the deterioration process. In this respect a stringent description of the pore geometry and the effect of surface effects on the diffusion behavior become crucial.
Any relevant scientific investigation of the present problem needs advanced numerical methods making comparison between the hypotheses under consideration with experimental data points possible. No direct effort will be placed on developing such numerical methods otherwise than assuring that the adopted methods are stringently defined and documented.
The verification of the models needs experimental results from controlled laboratory tests involving the analysis of chemical equilibrium and analysis of penetration profiles of different elements by, for example, using advanced electron probe microscope analysis or similar techniques. The state of the water can, for example, be analyzed using existing dynamic vapor sorption equipments and results. However, essentially only existing experimental results will be analyzed in the proposed TC.
Terms of reference
The objectives of the project is to refine the status of the newly developed existing multi-component transport and chemical equilibrium models at hand, in terms of both developing new theoretical bases and defining new relevant experiments in line with the hypothesis of interest. By this approach it is believed that it is possible to come closer to realizing a useful tool for predicting durability of cement based materials by computer simulations.
For the theoretical part knowledge from different disciplines will be used. For example, statistical mechanics can be used to understand the underlying chemical processes involved in, for example, the development of electrical diffuse double layers and its effect on chemical binding and ionic diffusion in the water filled pore system of the material.
Evaluation of existing controlled laboratory test will be used exclusively in order to define the boundary conditions and other important conditions as exact as possible.
The aim is to collect as much relevant information and knowledge as well as to coordinate ongoing research activities. By this a state-of-the-art report should be produced including recommendations on how to implement durability models into computer codes. Further, an important gain of new knowledge, obtained in the frame of the proposed project is expected to be implemented. It is expected that conclusions from the project will be presented within about 4 to 5 working years.
The members to be recruited in the TC are recognized experts in the area of interest. In order to include geographically related durability issues and different general practices the TC will include representatives from mainly Europe, Japan and North America as well as relevant spheres of influences of respective geographical areas. The inclusion of industrial partners in the committee is sought in the sense that relevant recommendations and implementations are performed.
Detailed working programme
Besides the main purpose of writing a state-of-the art report in the TC project it is also of highly importance to create a creative environment among the members of the committee making important new findings within the research area of interest possible. Such new potential findings should be implemented and accessible for practicing engineers. In order to reach the goals the work will contain the following tentative stages:
1. Precision of the scope of the TC project in terms of detailed approaches, level of complexity etc.
2. Studies of existing methods and approaches.
3. Identifications of limitations regarding numerical approaches of existing methods.
4. Identification of lack of knowledge.
5. Preparation of a detailed and comprehensive state-of-the-art report.
6. Organization of a RILEM meeting in which the results of the state-of –the –art is scrutinized.
7. Suggestions on how to implementation new findings into durability computer codes.
8. Preparation of a final report presenting the new results and conclusions from the investigations performed. Recommendations on how to proceed with the development of physically based computer based models for durability will be produced.
The program can be subjected to minor changes depending on the results and knowledge gain in the process of preparing the state-of-the-art-report
The work of the committee is related to a number of other technical committees of RILEM. The following RILEM committees are identified to be relevant for the present project:
1. Use of concrete in radioactive waste disposal facilities (202-RWD).
2. Reinforced concrete in the context of nuclear waste management (-CNM).
3. Model assisted integral service life prediction of steel reinforced concrete structures with respect to corrosion and induced damage (213-MAI).
4. Durability of self-compacting concrete (205-DSC).
5. Performance of cement-based materials in aggressive environment (211-PAE)
The communication within different TC of RILEM will be assured by involving members who represents more than one TC. Naturally, collaborations with other organizations, companies and committees outside RILEM will also be promoted. Therefore, in order to promote the development the TC will support and take advantage of knowledge developed by organizations such as fib and ASTM.
The scientific achievements are among other things expected to consist of consistent analysis shredding light on how the micro-structural properties affect the chemical and transport related processes. Further, such fundamental analysis are expected to be presented in such a way making consistent implementations into macroscopic continuum models possible. Depending on the out come of the state-of-the art-report accurate suggestions on how to solve the coupled set of differential equation numerically may be of interest.
The more general expected achievements can be listed as follows:
1. Production of a state-of-the-art report.
2. Recommendations concerning details on how to implement and model transport related durability problems by computer simulations (3 to 10 pages to be published in Materials and Structures).
3. Organization of a RILEM workshop.
A special dedicated conference will be organized solely dealing with summarizing the TC project.
Group of users
Due to the broad aspect of the subject of the proposed TC it is expected to be of interest to both academia and industrial firms. A successful implementation of improvements into existing computer simulation tools for predicting the transport and chemical equilibrium related durability of cement based materials would potentially be of great interest to, especially, cement producing companies, engineering firms, contractors but also for educational purposes.
Specific use of the results
It is supposed that an increased knowledge gained in the project, and the use of advance physically based simulations tools developed would results in reduced costs through the use of improved materials and design and production methods. A successfully mediation of the knowledge gained by the work of the TC may potentially results in extended service life of cement based material constructions.