This is a National Science Foundation (NSF) funded project. It has a three year window which began on September 1, 1995, and ends August 30, 1998. Thanks to a generous extension by NSF, the project can continue until May 31, 1999.

The primary investigator is Professor Anthony R. Ingraffea. Jim Hanson is the graduate student in charge of the research. Many staff members and other students have helped.

Hypothesis:

A valid fracture toughness, K_Ic, for concrete can be estimated from Level I testing (applies Linear Elastic Fracture Mechanics, LEFM) and/or Level II testing (sub-size specimen adjustment) of standard cylinders in a round double beam (short rod) configuration.

Concrete design and evaluation has reached the level in practice that engineers are preparing to conduct analysis and design considering fracture mechanics. In order to accurately predict a structure's behavior, the fracture toughness, K_Ic, must be known.

A draft recommendation for measuring the fracture toughness of concrete already exists. The recommendation is by The International Union of Testing and Research Laboratories for Materials and Structures (RILEM). It entails testing beams up to 1.2 m long. Such a test is not practical for use by commercial labs in the United States. In addition, such a test can not be applied to samples taken from an existing structure.

A joint ACI/SEM Coordinating Committee on Testing Standards for Fracture Properties of Concrete is currently investigating specimen geometries and test methods for a fracture toughness testing standard to be used in the United States. A subcommittee of this group is designated to investigate testing methods using cylindrical specimens. Professor Ingraffea is chairman of this subcommittee.

The goal of this research, therefore, is to determine the feasibility of using the standard 6" by 12" compression cylinder, ASTM C39-94, in a round double beam configuration for measuring concrete fracture toughness.

Method:

In order to ascertain whether or not the round double beam specimen can produce a valid fracture toughness for concrete, we are performing laboratory tests on three sizes of round double beam specimens and three sizes of single edge notch beam specimens according to the RILEM recommendations.

To test our understanding of the mechanics of the specimen, we are performing three dimensional numerical simulations. These simulations consider the cohesive stresses transfered across the process zone as damage progresses through the specimen. The parameters that we use for the cohesive stresses are called the "tension softening diagram". The goal of the simulations is to reproduce the load versus displacement behavior observed in the laboratory specimens.

To further assist in understanding what is a "valid" fracture toughness, we have organized an international round robin test program to compare fracture toughness measurements from various specimen geometries, loading configurations and data reduction techniques.

Related Papers
Hanson, J. H., 2000, "Proposed Standard Test Method for Round Double Beam Fracture Toughness of Concrete," Research Report, 00-1, Department of Structural Engineering, Cornell University, Ithaca, NY, Jan.

Hanson, J. H. and Ingraffea, A. R., 1998, "Behavior of Concrete Round Double Beam Fracture Toughness Test Specimens," Proceedings, Third International Conference on Fracture Mechanics of Concrete and Concrete Structures, International Association of Fracture Mechanics for Concrete and Construction Standards (IAFraMCoS), Gifu, Japan, Vol. 1, pp 441-452.

Hanson, J. H. and Ingraffea, A. R., 1997, "Standards for Fracture Toughness Testing of Rock and Manufactured Ceramics: What Can We Learn for Concrete?," Cement, Concrete, and Aggregates, CCAGDP, Vol. 19, No. 2, Dec., pp. 103-111.

Hanson, J. H., 1997, "The Short Rod: A Potential Standard Test Method for Determining the Fracture Toughness of Concrete," Proceedings, 19th Annual Technical and Career Conference, Society of Hispanic Professional Engineers, pp. 63-70.

Related Presentations:

Ingraffea, A. R., "Three-Dimensional Cohesive Crack Simulations of Concrete Round Double Beam Test Specimens," co-authored with J. H. Hanson, Computational Concrete Mechanics Session, Fifth US National Congress on Computational Mechanics, Boulder, CO, August 6, 1999.

Hanson, J. H., "Round Double Beam Fracture Toughness Testing of Concrete," co-authored with A. R. Ingraffea, Fracture Parameters Session, Third International Conference on Fracture Mechanics of Concrete and Concrete Structures, Gifu, Japan, October 15, 1998.

Hanson, J. H., "Round Double Beam Fracture Toughness Testing of Concrete: Specimen Behavior Issues," co-authored with A. R. Ingraffea, Research in Progress Session, American Concrete Institute Convention, Atlanta, GA, November 10, 1997.

Hanson, J. H., "Fracture Toughness Testing of Concrete: Are the Short Rod Geometry and Test Methods Valid?," co-authored with A. R. Ingraffea, Research in Progress Session, American Concrete Institute Convention, Seattle, WA, April 7, 1997.

Hanson, J. H., "The Short Rod: A Potential Standard Test Method for Determining the Fracture Toughness of Concrete," Graduate Paper Session, 19th National Technical and Career Conference, Society of Hispanic Professional Engineers, Philadelphia, PA, January 30, 1997.

Hanson, J. H., "Standards for Fracture Toughness Testing of Rock and Manufactured Ceramics: What Can We Learn for Concrete?," co-authored with A. R. Ingraffea, Symposium on Concrete Fracture Mechanics Standards, American Society for Testing and Materials Convention, New Orleans, LA, December 9, 1996.

Last updated on January 4, 2000.