FTC : Durability and Service Life of Concrete under the Influence of Freeze-Thaw Cycles combined with Chloride Penetration
Technical Committee FTC
Deputy Chair: Dr. Peng ZHANG
° At this moment there are many different ways to design and predict service life of reinforced concrete structures. Three deteriorating processes are mainly considered separately: (a) number of freeze-thaw cycles, (b) chloride penetration through the concrete cover to reach the reinforcing steel, and (c) time dependent carbonation. For all three processes national and international standards exist. But the predicted service life is rarely reached in practice. Chloride penetration is usually described by a diffusion process although migration of chloride is a very complex process including adsorption on the internal surface of hardened cement paste, convection with capillary absorbed salt solution with a pronounced filter effect, and chemical reactions. In addition in practice there is a great number of drying and rewetting cycles every year. With the results published in the final report of RILEM TC 246-TDC it could be shown that service life of reinforced concrete structures can be considerably shortened under the influence of combined actions, as for instance chloride penetration and an applied mechanical load. That means that prediction of service life based on one deteriorating process only cannot be considered to be conservative. It could be shown that chloride penetration is accelerated significantly in case a tensile stress is applied. One conclusion of RILEM TC 246-TDC is that the influence of combined actions has to be studied in order to make service life prediction more realistic. There is a great number of potential combinations and step by step the most influential combinations will have to be investigated. With this new RILEM TC the influence of a combination of freeze-thaw cycles, applied mechanical load, and chloride penetration shall be investigated. Freeze-thaw cycles modify the structure of the nano-pores. If freeze-thaw cycles are imposed under applied load, the material becomes anisotropic. There is a dominant direction for chloride penetration. Based on the expected results durability prediction will become more realistic and service life can be extended systematically. The expected results of the new RILEM TC will also have an enormous economical effect.
° Durability of concrete under combined freeze-thaw cycles, chloride penetration and mechanical load shall be studied experimentally in different laboratories. It is planned to carry out comparative test series, which shall be predefined in detail. Different types of concrete shall be included, such as standard concrete, high strength concrete, and fibre reinforced concrete. Results of different test series shall be analysed and compared with existing or newly developed materials models. Conclusions of the obtained results shall be compared with observations of real structures in different climatic environment.
This project may be considered to be a follow up project and a supplement of RILEM TC 246-TDC and it shall be carried out in close contact with the new RILEM TC chaired by Professor Nele De Belie: Carbonation of Concrete with Supplementary Cementitious Materials, and two newly proposed RILEM TCs, (1) Controlled Expansion of Concrete by Adding MgO-based Expansive Agents Taking the Combined Influence of Composition and Size of Concrete Elements into Consideration, and (2) Test Method for Concrete Durability under Combined Role of Sulphate and Chloride Ions.
° The rate of chloride penetration is accelerated in the direction parallel to the applied stress and it is slowed down in the direction normal to the applied stress. This effect is relevant for the prediction of service life of reinforced concrete structures, but it has been neglected totally so far. This specific influence on chloride penetration shall be considered in particular.
° RILEM TC 246-TDC was a first attempt to study the influence of combined actions such as applied load and chloride diffusion on durability and service life of reinforced concrete structures. It was clear, right from the beginning, that many more results will be needed to make service life design under the influence of combined actions more realistic and more reliable. This new RILEM TC can contribute essential new results and a number of similar TCs must follow to provide us with the necessary data for well-based reliable service life design of reinforced concrete structures.
Terms of reference
The elaboration of the proposed research program together with a detailed analysis of the results will most probably need a period of five years. The first part will consist of an in depth literature survey and the establishment of a specific bibliography. It is planned that comparative test series shall be run in at least six internationally renowned laboratories. These laboratories shall be selected from different countries all over the world. Well prepared round robin tests are envisaged. Some of the proposed tests can be run in the framework of Master and/or PhD theses. The relevance of this project is quite obvious. Service life of most existing structures is too short. In many countries excessive repair cost slows down further development of the infrastructure. If combined actions will be taken into consideration in the future, service life prediction will become more realistic and as a consequence service life can be extended substantially. The direct economic and industrial relevance is obvious. A major drawback of concrete structures, i.e. the limited service life, and the enormous cost for maintenance and repair, can be improved significantly by taking the complex deterioration processes into consideration during design.
Detailed working programme
- Establishment of a comprehensive bibliography of publications on the influence of the combination of freeze-thaw cycles and chloride penetration on service life of reinforced concrete structures.
- Detailed planning of comparative test series with participation of all partners.
- Carrying out the planned comparative test series (round-robin testing) in different laboratories and careful evaluation of all results obtained. First the different actions such as freeze-thaw cycles and chloride penetration shall be studied consecutively. At a later stage simultaneous actions will be considered in addition.
- Refinement of the test procedure if necessary based on results obtained.
- Planning and carrying out of a second series of comparative tests.
- In parallel it is planned to develop a numerical model to describe the observed deterioration of the material in order to better understand and interpret correctly the experimental results obtained.
- Preparation of a final report including recommendations on how to increase durability and service life of reinforced concrete structures in aggressive environment and in particular under the influence of chloride penetration and mechanical load.
- Publications describing and discussing major results obtained.
- Preparation and publication of recommendations for extended service life of reinforced concrete structures situated in aggressive environment.
- A close link with results from former RILEM TC 246-TDC will be a strong basis of the continuing tests series. This proposed TC may be considered to be a continuation of the former TC. A new RILEM TC on carbonation of concrete with supplementary cementitious materials (TC-CCC) chaired by Professor Nele De Belie has just been set up. The work of this TC will include quantification of the effect of mechanical loads on the carbonation resistance of SCM-containing concrete.
- A future RILEM TC on combined action, specialising on the influence of sulphate penetration under the influence of an applied load will be proposed by colleagues from Southeast University, Nanjing, (the group of Professor Miao Changwen). Close contacts with these new and already existing TCs will be maintained throughout the work of this new TC. Results will be of special interest to international associations such as fib and ISO but at the same time for national standardizing bodies.
- The most obvious benefit will be development of more realistic durability design and as a consequence substantial prolongation of repair free service life of reinforced concrete structures.
- The new TC will produce in particular:
- A comprehensive bibliography,
- Appropriate new test methods and recommendations for more realistic durability design
- A final state-of-the-art report with recommendations for practical applications
- The necessary basis for more realistic durability design
- The new TC will organise several symposia during its lifetime and an international conference to present and discuss major results towards the end of its activities.
Group of users
- Testing laboratories,
- Practitioners, engineers for service life design in particular.
- Standardizing and codifying agents.
Specific use of the results
The anticipated results will be of particular interest for
- Practitioners as they will be enabled to design more durable reinforced concrete structures in the future
- Municipal and governmental planning offices as the service life cost of buildings and structures will be substantially reduced.
- The community in general, as cost of service life and maintenance will be reduced considerably.