Strain Capacity of Strain‐Hardening UHP‐FRC Part I: Steel Fibers

Presented By: Dr. Antoine E Naaman, University of Michigan
The main focus of this study is to provide a better understanding of the tensile strain capacity of strain-hardening HP- and UHP-FRC composites with the objective to possibly improve it; more precisely, how to increase the value of strain, at the maximum post-cracking stress or tensile strength. In this Part I, composites with steel fibers are examined. Overall, at time of this writing, the maximum post-cracking tensile strength recorded in numerous investigations of UHP-FRC remains mostly below 15 MPa and the corresponding strain below 4/1000. Both values are significantly reduced when the specimen size increases, as is needed for real structural applications. The authors analyze test data on and from close to one hundred series of direct tensile tests carried out in more than thirty investigations. Several factors observed to influence the strain capacity are identified (including the matrix composition and compressive strength, specimen size tested, the length of the fiber relative to the specimen size, fiber orientation, fiber content or volume fraction, fiber parameters (length, diameter, surface deformations, surface coating…), and indirectly the specific surface of fiber reinforcement. However, independently of the numerous parameters encountered, two observations emerged beyond all others, namely: 1) the higher the post-cracking tensile strength is (whichever way it is achieved), the higher the corresponding tensile strain; and 2) fibers with slip-hardening bond characteristics lead to an increase in strain capacity. A rational explanation for these observations is provided. After evaluating desirable strain requirement for structural applications, potential solutions for improving are suggested.