187-SOC : Experimental determination of the stress-crack opening curve for concrete in tension
Technical Committee 187-SOC
Deputy Chair: Dr. Ravindra GETTU
The determination of fracture properties of concrete in tension has been the subject of study of previous RILEM TC committees (RILEM TC 50-FMC, RILEM TC 89-FMT, RILEM TC 51-ALC, RILEM TC 78-MCA). From their work, four RILEM recommendations issued: FMC 1-Determination of the fracture energy of mortar and concrete by means of three-point bend tests on notched beams, 1985; FMC 2- Determination of fracture parameters (KIcs and CTODc) of plain concrete using three-point bend tests, 1990; FMC 3-Size-effect method for determining fracture energy and process zone size of concrete, 1990; and AAC 13.1-Determination of the specific fracture energy and strain softening of AAC, 1992. However, as their titles suggest, these recommendations mostly address the determination of one or two fracture parameters, and ignore the fact that the fracture of concrete is progressive, which implies that the complete description of fracture requires the definition of a continuous response rather than one or two isolated parameters. This response, sometimes called the softening curve, is a basic ingredient in almost any detailed model of fracture for concrete, be it in a discrete crack approach (fictitious or cohesive crack) or in a continuum approach (smeared crack, crack-band, gradient, or nonlocal models). Over the last decade, various experimental methods, of different degrees of complexity, have been used by several groups around the world, none of which has been given a final widely accepted status. In the Workshop on Quantitatively Evaluation Methods for Toughness and Softening Properties of Concrete, held in Gifu on October 11-12, 1998, previous to the FraMCoS 3 Conference, it was generally agreed that it was time to make a careful analysis of the available methods so that a useful, general purpose method could be agreed and proposed to the laboratories working in this field. Professor Folker H. Wittmann suggested that the most efficient way achieve this goal was to promote a RILEM Technical Committee with the main purpose of issuing a pre-standard, i.e., a RILEM recommendation. Accordingly, the objective of the proposed Committee is specifically: to develop a RILEM Recommendation for the experimental determination of the stress-crack opening curve for concrete. This implies that the theoretical framework for the development is known from the onset, and that the essential task of the committee is to find the most efficient and reliable ways of performing the experimental measurements and carrying out the calculations for data reduction (back analysis or inverse analysis).
Terms of reference
The committee work will have a duration of 4 years. The committee will start working on the basis of existing experimental and numerical results, and will analyze testing methods proposed by individuals or research groups. The committee will develop an outline for the proposals and publish it within the first 6 months. A 6 months term will follow to allow individuals or groups to propose candidate methods. The commitee will analyze each of the proposals based on existing experimental and numerical data, for a maximum period of 1 year. In this term the committee should decide which methods are promising, write a detailed implementation of them, and decide and propose the general lines of a round robin if necessary. Based on these results, the committee will propose the details of a round robin to test the proposed methods. The preparation and development of the round robin will last 1 year, approximately. The analysis of results, including a draft of the report should then be written in the term of 6 months. The final versions would be finished in 6 more months and presented in a workshop coincident with one of the main conferences on fracture of concrete.
Detailed working programme
To achieve the proposed goal, the committee will first define a conceptual framework for the development of the pre-standard, consisting essentially in a detailed outline of the aspect to be addressed by the pre-standard both in its experimental (measurement) and numerical (software) aspects. From this, minimum requirements will be set to accept a potential procedure as a candidate for examination. Then a period will be opened to allow any researcher or group to make specific proposals for methods. After this, the committee will examine the proposed methods and decide which of them deserve deeper examination. Next, each of the proposed methods will be analyzed in depth. The first task for each will be to make a first evaluation of the viability of the proposed methods, particularly on their reliability, based on existing experimental and numerical results. If one method appears to be promising, the committee will write a detailed practical implementation of it (following the previously developed conceptual framework), and decide whether a round robin is necessary to assess the final performance of the method. After finishing the evaluation of the proposed methods, a round-robin will be carried out to evaluate the performance and relative merits of the various proposals. The round robin has to be able to evaluate both the experimental and numerical aspects of each of the proposed testing methods, and to compare between them. After the round robin , the committee will decide which of the methods, or merging of methods, can be the object of a pre-standard, and the final proposal will be written. The committee will publish the proposal and a report. The report will describe in detail the various methods analyzed, their theoretical and experimental support, the results and implications of the round robin, and the reasons that led to the final proposal(s). The committee will close with a workshop celebrated in conjunction with a major international conference on fracture of concrete. Since experience shows that frequent committee meetings are difficult to achieve with adequate attendance, most of the work has to be done by e-mail, which is a flexible and fast method of cooperative work. One full committee meeting will be scheduled every year, coincident with one international conference on related subjects.
The activities of this committee extend basically the work of the former RILEM TC 50-FMC, and may profit from the results of RILEM TCs 51-ALC and 78-MCA It is marginally related to TC QFS (Size Effect and Scaling of Quasibrittle Fracture). Its work has to build on previous conclusions of ACI Committee 446 and of the Workshops held in Cardiff in July 1995 (previous to FraMCoS 2) and in Gifu in October 1998 (previous to FraMCoS 3). A committee from the Japan Concrete Institute chaired by Prof. Kitsutaka is working on the same subject, although with a more limited scope. The RILEM committee will coordinate with the JCI committee through common committee members and by direct communication between the chairmen.
The achievements will be as follows: 1. At least one pre-standard for the determination of the softening stress-crack opening curve. If feasible, this pre-standard will include alternative methods of different degrees of complexity. If this was seen to lead to an excessively intricate text, various pre-standards should be written to face the various levels of complexity. 2. A report explaining in detail the various methods analyzed, their theoretical and experimental support, the results and implications of the round robin, and the reasons that led to the final proposal(s). 3. The proceedings of the workshop.
Group of users
University laboratories, research institutes, testing laboratories and advanced structural design units.
Specific use of the results
The results will serve the community providing a method for testing whose reliability has been tested and can be used at two different, but important levels: (1) for the determination of fracture properties for any given particular concrete, and (2) as a reference method for concrete characterization in research on fracture of concrete and concrete structures.