The Short Rod: a Potential Standard Test Method for Determining the Fracture Toughness of Concrete

The concrete community in the United States has reached the point where it needs a standard test for measuring the fracture toughness of concrete. A concrete standard might be obtainable by adapting existing standards for cemented carbides and rock. These standards use the short rod specimen geometry and two associated test methods. The combination of specimen geometry and test methods may have many advantages over other proposed standards for concrete. The material makeup of concrete is similar to cemented carbides and rock; therefore, the short rod geometry and associated test methods are worthwhile to explore for a standard test for concrete. INTRODUCTION As high performance concrete becomes stronger and more common in structural applications, the applicability of linear elastic fracture mechanics increases. Stress analysis is only half of the solution though. A material limit state must be compared with the analysis results to determine if the fracture strength has been exceeded. In linear elastic fracture mechanics (LEFM), this limit state is called the critical stress intensity factor, KIc, or fracture toughness. At this time KIc can not be calculated from a numerical model; it must be measured in a laboratory test. Proposed standard test methods for concrete have existed since 1985 [1-3]. However, most of these methods require the use of a large, rectangular test specimen. If concrete in an existing structure is to be evaluated, a cylindrical core will be removed. Therefore, a cylindrical test specimen would be more appropriate. One such test specimen is the short rod (Fig. 1). The short rod specimen and associated test methods have been a published standard for the fracture toughness testing of cemented carbides and rock since the late 1980’s. The American Society for Testing and Materials (ASTM) published “Standard Test Method for Short Rod Fracture Toughness of Cemented Carbides”, B771, in 1987 [4], and the International Society for Rock Mechanics (ISRM) published “Suggested Methods for Determining the Fracture Toughness of Rock” in 1988 [5]. This specimen has a chevron notch cut-out which leaves a triangular ligament to hold the two halves together. Its geometry, in conjunction with two levels of testing, may provide unique advantages over other proposed fracture toughness testing standards.