265-TDK : Testing methods for determination of double-K criterion for crack propagation in concrete
Technical Committee 265-TDK
Deputy Chair: Yao WU
Activity ending in: 2018
The task of the new Technical Committee is to deal with the double-K criterion which describes crack formation in concrete (or other quasi-brittle materials). The double-K criterion has been developed in the nineties of last century. The double-K constitute two material parameters, the first is called Kcini which defines the onset of cracking, and the second is called Kcun which defines the onset of unstable cracking or failure. Both are derived from linear-elastic fracture mechanics principles. The two material parameters characterize the mechanical behaviour of concrete. It has been shown that the double-K does not depend on the size of the specimens. Many comparative tests have been performed with specimen sizes varying in a large range and no size effect could be found. So, the determination of double-K is straight forward. For the determination of double-K, the specimen geometry can be chosen according to the availability in the laboratory. Tests have been performed on three-point bending notched beams, on compact-tension specimens, on uniaxial tension specimens, and on wedge-splitting specimens. The testing of double-K can be done on every universal testing equipment, even without displacement control. One does not need loading-unloading cycles.
The double-K criterion has been used to assess the safety of large concrete structures like dams. The Chinese standard number DL/T 5332-2005 defines testing methods for the determination of double-K. It has been introduced in the People’s Republic of China in 2005.
The aim of the new TC is to make the knowledge of double-K internationally available by preparing a RILEM recommendation.
Terms of reference
It is estimated that the new TC needs about 4 years to finalize the work.
The members will come mainly from universities and research institutes. They will be selected by the chairman and the secretary of the TC. General information will be given on relevant symposia and RILEM members will be called for support.
To validate the testing method of double-K, a round robin champagne will be organized.
Detailed working programme
1 Literature survey
The literature will be screened with respect to fracture-mechanics testing with special emphasis on double-K criterion.
2 Round-robin testing
Double-K criterion testing has been performed in Stuttgart (Germany), Dalian, Hangzhou (China), Jamshedpur and Kharagpur (India), maybe at other places as well which are not yet detected. At the places mentioned, the results show good agreement as the testing method is concerned. However, a draft recommendation should be validated before it is finalized. Therefore, a round-robin campaign is proposed with at least two laboratories.
First option: The specimens are made at one place and shipped to the other testing site(s). Two types of concrete will be used.
Second option: Two types of concrete will be specified. The type of cement and addition, the water-cement ratio, the type of aggregate, the curing condition, the testing age will be prescribed. The testing labs will follow the instructions as punctually as possible and produce the concrete with local materials.
In both cases the testing results will be evaluated and compared with each other.
A recommendation will be prepared which defines the testing methods for double-K criterion. There will be several specimen shapes, such as three-point bending, wedge splitting, compact tension, and uniaxial tension. The methods how to analyze the test will be given in formulas. The recommendation will comprise the following paragraphs: background of double-K criterion, dimensions of specimens, fabrication of specimens, testing apparatus, measuring devices, test procedure, test results and calculations, test report, references.
A workshop is planned for the presentation of the TC’s work and of the draft recommendation. Other experts who deal with fracture mechanics of concrete will be invited. Maybe, the workshop could be a joint workshop of RILEM and ACI. The proceedings of the workshop will help to bring the recommendation into its final shape.
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The double-K is a new approach of fracture mechanics of concrete. The previous TC 50-FMC worked on the determination of the fracture energy of mortar and concrete, TC 89-FMT dealt with the determination of fracture parameters of the two-parameter fracture model and with the size-effect model for determining fracture energy and process-zone size of concrete. Active TCs are not involved in fracture-mechanics items. So, there is no overlap with other TCs.
The ACI Committee 446 “Fracture Mechanics” is dealing with other subjects.
As RILEM is concerned there was always great interest in mechanical properties and fracture of concrete. The new TC would continue the great tradition. Further on, RILEM recommendations often serve as pre-normative standards for international standardization bodies.
The TC will produce a recommendation which can be applied by everybody in the world. It is anticipated that the recommendation serves as a model for an international standard.
Towards the end of the work, a workshop will be organized for dissemination of the results. In the meantime, the TC will report during the Technical day of a RILEM week.
Group of users
The users of the recommendation are academics and testing laboratories in research and industry.
Specific use of the results
The recommendation makes it possible for everybody to measure fracture-parameters of concrete in an easy way. Nowadays, fracture parameters are gained in rather sophisticated manner with displacement-controlled testing devices. The new approach is more economic.