221-SHC : Self-healing phenomena in cement-based materials
Technical Committee 221-SHC
Deputy Chair: Dr. Mario DE ROOIJ
Activity ending in: 2013
Centuries-old buildings have been said to have survived these centuries because of the inherent self-healing capacity of the binders used for cementing building blocks together. This self-healing capacity has never been termed like that. It was just a well-known fact that was seen as the forgiveness of nature rather than self-healing of inherently smart building materials.
Today the field of self-healing materials is considered a new area of materials research. It was in 2001 that Professor Scott White from the University of Illinois published his results on self-healing in polymer-based systems by microencapsulated healing agents. His and related research in other fields of materials science was the result of an initiative by NASA launched amongst selected top institutes in the USA in 1996. Since then the field is developing rapidly. Since healing presupposes the presence of a defect and a defect generally emerges at a very small scale, probably at the nanoscale, it is not surprising that self-healing is one of the promising application fields of nanotechnology.
White's work has been said to be preceded by studies at the same university on self-healing of cracked cementitious materials. Although microcracks, and even macrocracks, in cement-based systems should not necessarily be judged damage, they can be judged as defects indeed. One of the first works on self-healing of cementitious materials has been published even more than 20 years ago. It was shown that so called Kaiser effect (absence of acoustic emission, which is usually observed at repeated loading of structural element, until the load exceeds the previously achieved level) disappears for concrete kept under water for a long time before a new loading. Self-healing of macrocracks has been studied quite intensively in the past decade in view of recovery of tightness of liquid retaining structures. Recently self-healing of microcracks has been suggested the reason why the diffusion coefficient of concrete in marine structures reduces with time. The aforementioned examples illustrate that self-healing of cementitious materials is not a completely new issue. Thus far, however, the self-healing capacity of cement-based materials has been considered as something "extra". We could call it passive self-healing, since it was not a designed feature of the material, but an inherent property of it. It would be worthwhile, however, if we could create self-healing properties of cement-based materials by design. Developing concepts for self-healing by design, or active self-healing, is considered the main topic and challenge of the new RILEM committee. With respect to the idea of self-healing an in view of consistency in the discussion on this issue the following quote might help to focus the scope of the committee:
"Self-healing materials are man-made materials, which have the built-in capability to repair structural damage autonomously or with the minimal help of an external stimulus. The initial stage of failure in materials is often caused by the occurrence of small microcracks throughout the material. In a self-healing material the occurrence of these microcracks is 'recognized' in some way. Subsequently, mobile species, e.g. atoms, have to be triggered to move to these places and perform their self-healing capacity. These processes are ideally triggered by the occurrence of damage itself, in which case we call it an autonomous self-healing event. In practice one could well imagine that also self-healing that is triggered by an external stimulus greatly enhances the reliability and durability of materials. These notions lead to the distinctions between an ideal self-healing material and a minimal self-healing material. The research in the field of self-healing materials will develop between these to extreme cases; starting somewhere close to the minimal case with a strong drive and effort to come close to the ideal case".
The proposed Committee has two main objects:
(1) Preparation and publication of State-of-the-Art Report, and
(2) Preparation of an international RILEM symposium
Terms of reference
The scope of the TC is new. At present, there is no interaction between activities by national, regional and international associations (fib, ACI, IABSE, etc). At the same time, depending on the progress in the TC work, the contacts with other RILEM TCs, such as TC-ACD "Acoustic Emission and Related NDE Techniques for Crack Detection and Damage Evaluation in Concrete"; TC-CCD "Concrete Cracking and its Relation to Durability: Integrating Material Properties with Structural Performance", TC-DSC "Durability of Self-Compacting Concrete", TC-197 NCM "Nanotechnology in Construction Materials"; TC-203 RHM "Repair Mortars for Historic Masonry" and TC-PAE "Performance of Cement-Based Materials in Aggressive Aqueous Environments", will be established, and a cooperation with the above listed RILEM TCs is planned.
The work of the Committee will stimulate new activities in RILEM and will serve as a trigger for using the self-healing concepts for studying other (non-cement) construction materials.
Detailed working programme
The working stages of the committee's work are following:
Compilation of State-of-the-Art Report
In this STAR and historic overview of self-healing phenomena in building materials in general and in cement-based systems in particular should be given. Different types and concepts of self-healing will be listed and explained. This STAR should end up with suggestions for the formation of special task groups for studying particular self-healing concepts and the potential of them in more detail. The duration of this stage: 1 - 2 years.
Formation of special task groups
Based on the suggestions from the STAR, task groups could be installed. They will have a long-term mission (roughly 2-3 years). The task group activities are planned for 2007-2008.
Preparation of an international RILEM symposium
In the first year of the task group activities the scope of an international symposium has to be worked out. The tentative time of organizing the symposium is end 2008 or early 2009.
Final report of the RILEM committee
The committee should end their work with issuing a final report. It could be that the symposium proceedings can be used for this. The final report is expected in 2009.
The proposed TC members are academic and researchers. Some of them are end users of the models to be developed for simulation or description of self-healing mechanisms. The technical committee would involve interdisciplinary fields that are linked with the "core business" of RILEM and would utilize developments of several existing RILEM committees.
As main outcomes from the TC is foreseen:
- A State-of-the-Art Report on self-healing phenomena in building materials in general and in cement-based systems in particular;
- International RILEM symposium "Self-healing of Cement-Based Materials";
- Final Report of the TC.
Group of users
This TC will address researchers in the field and technologists working in cement and concrete industry. Among the addressed clients are also developers of the conceptual and numerical models and end users. Finally, the main achievements in the TC work are expected to be implemented in teaching process in order to demonstrate the students the realistic picture of the combined destruction and structure formation.
Specific use of the results
Knowledge transfer from the RILEM TC into academy and research is going to result in using the self-healing concepts in design, and maintenance of concrete structures. The places/cases/technologies, which can be useful for implementation of self-healing concepts in civil engineering practice, will be defined. Application of self-healing concepts in design and construction/repair practice can bring an economical benefit for the society: knowledge and utilization of the potential of materials to self-healing can economy material, power and labour resources needed for repair of self-healing defects. Finally, the safety and reliability of engineering structures made of self-healing cementitious materials are improved, which has an additional cost equivalent.