Evaluating concrete with high-performance steel fibers using double-punch testing

In order to quantify steel fiber-reinforced concrete (SFRC) mixtures for field applications, a material test capable of predicting the performance of SFRC for various fiber content and types is required. However, current test methods used to evaluate the structural properties of FRC (such as residual strength and toughness) are widely regarded as inadequate and complicated; a simple, accurate, and consistent test method is needed. It was determined that the Double-Punch Test (DPT), originally introduced for plain concrete, could be extended to fiberreinforced concrete to satisfy this industry need. Like most conventional methods, in the DPT, a specimen is stressed in indirect tension. A concrete cylinder is compressed vertically by steel punches located concentrically on the top and bottom cut surfaces. Initial tests showed distinct advantages in simplicity, reliability, and repeatability over current test methods for FRC. Benefits were largely due to the loading procedure and load distribution which resulted in multiple failure surfaces that captured the random distribution of fibers more effectively than current standardized test methods for FRC. Several variables including fiber manufacturer, fiber content, and testing equipment were evaluated to refine the testing procedure for straightforwardness and accuracy. In this paper, the results of over 350 double-punch tests are summarized.