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Structural Research Series 382PDF


Title:Fracture Mechanisms of Concrete Under Static, Sustained, and Repeated Compressive Loads
Author(s):Diaz, Samuel I.; Hilsdorf, Hubert K.
Subject(s):Fracture mechanics.
Concrete --Fracture.
Abstract:The investigation was concerned with the study of the mechanisms of crack propagation in plain concrete when subjected to static, high sustained or high repeated compressive loads. The objectives were: (1) to study the nature of progressive crack growth of concrete under such loading conditions, and (2) to formulate a conceptual model which describes crack extension at a given stress, as well as the manner in which failure of concrete takes place. The first objective was accomplished by developing a technique which allowed continued observation of progressive crack growth on the surface of plain concrete prisms under load. Strains were recorded throughout the life of each specimen. The effect of the load history, the maximum stress level, and the surface condition of the specimen on the nature of crack growth were investigated. On the basis of the study of surface crack growth and the examination of failed specimens it was possible to formulate a conceptual model which explains the failure mechanism and the various stages in the process of crack growth. The validity of the conceptual model was tested by performing model studies on simple cracked systems. The study of surface crack growth showed that cracks in concrete prior to loading have random orientation and are concentrated mainly around voids and aggregated-mortar interfaces. Cracking which results from load application is oriented closer to the load direction. Cracking is originally restricted to the interfaces between paste and aggregate, but if the applied load is increased beyond a given stress level, if the load is sustained long enough, or if the specimen is subjected to a certain number of cycles of the applied load, the interface cracks extend into the mortar. As the time under load increases, cracks increase in number and continue to extend. Their orientation becomes progressively closer to the direction of the applied load. Frequently, small cracks coalesce to form larger cracks. Often, the cracks prefer to grow along nearby interfaces rather than through mortar. Most specimens showed large inclined cracks at failure which are continuous and which traverse the entire or major part of the specimen. Therefore, it was proposed that failure of concrete occurs if the conditions for the formation of an inclined failure surface exit. Model studies on simplified crack systems support this proposition. It was found in these studies that sufficiently close cracks, with various relative positions to one another, in most cases, coalesce at a certain critical stress as a result of crack interaction and eventually pass through the entire specimen so that an inclined failure surface will be formed. Similarly, failure of concrete results from the progressive joining of small cracks until a crack large enough is formed which will lead to spontaneous crack extension, and eventually, to the formation of a failure surface. The formation of an inclined failure surface in a short-time test on plain concrete may be delayed or prevented by a continuous reduction of the applied load, thereby allowing the straining of the specimen into the descending portion of the stress-strain diagram. This reduction of the load along with the arresting action of the aggregates restrains unstable crack growth of the critical crack and, consequently, failure surface formation.
Issue Date:1971-08
Publisher:University of Illinois Engineering Experiment Station. College of Engineering. University of Illinois at Urbana-Champaign.
Series/Report:Civil Engineering Studies SRS-382
Genre:Technical Report
Sponsor:The National Science Foundation Research Grant No. GK-1808
Date Available in IDEALS:2009-12-08
Identifier in Online Catalog:2074114

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