Investigating micro-crack and local crack formation and propagation in ultra-high performance concrete (UHPC)
Ultra-High Performance Concrete is a unique structural material growing in use in the United States and throughout the world due to its many benefits, such as high compressive and tensile strength, resistance to chloride ingress, and increased bond with reinforcement. However, its tensile behavior, particularly regarding the micro-cracks that form under tensile stress, is not well understood. Properly understanding the tensile behavior of UHPC is essential for safe and efficient design of UHPC structures subjected to flexural or shear action. The micro-cracks that form in UHPC under tensile load are regarded by many researchers as essential to UHPC ductility allowing UHPC to reach significantly higher strains before reaching its maximum tensile stress. Additionally, UHPC is assumed to remain durable, even past its maximum stress value, as micro-cracks widths are typically small enough to prevent damaging chloride penetration. However, when defining tensile behavior, traditional tensile test methods do not directly measure UHPC micro-cracking behavior under tensile loads, instead grouping the micro-cracking behavior with the global tensile behavior over a chosen gage length. Therefore, results regarding micro-cracking behavior in UHPC are traditionally derived from tests based on assumptions rather than measured directly.
The primary goal of this research is to understand the role of UHPC mix type, fiber type, and fiber content on micro-cracking behavior and address gaps in knowledge on the relationship between micro-crack and local-crack formation and propagation by directly measuring micro-crack strains. In this study, direct tensile tests are conducted on over 30 specimens composed of UHPC from three suppliers available in the United States market and containing combinations of five fiber types and various fiber contents. In general, the specimens tested show one of two tensile behaviors; one where specimens exhibit brittle behavior and one where specimens exhibit ductile behavior. Results show that fiber type affected which case of tensile behavior is observed. Based on tensile test results, limits for stress and strain are provided to prevent local crack formation and local crack growth for specimens with 1% and 2% fiber. Additionally, variation between specimens is discussed and fracture energy and characteristic length values are presented.