Effects of specimen thickness and fiber length on tensile and cracking behavior of UHPFRC: Uniaxial tensile test and micromechanical modeling

This study aims to investigate the effects of specimen thickness and fiber length on the tensile and cracking behaviors of ultra-high-performance fiber-reinforced concrete (UHPFRC). To this end, a uniaxial tensile test was conducted with three specimen thicknesses (30, 50, and 100 mm) and two fiber lengths (13 and 20 mm), and the fiber orientation, dispersion and specimen void were quantitatively evaluated based on image recognition. The test results indicated that fiber orientation was improved with the decreased specimen thickness and increased fiber length. Meanwhile, the initial cracking and peak stress, capacity to limit cracking were enhanced with the decreased specimen thickness. A modified prediction model considering the wall effect, flattening and squeezing effect was developed to predict the probability density function p(θ) of fiber orientation angle. Additionally, the uniformity factor μ₂ was introduced to predict crack number, and the relationship between the μ₂ and parameter ψ = (V_f × l_f/d_f)/t was determined. Furthermore, a model was developed to convert the main crack width into uniaxial tensile strain. All models and relationships were validated using test data. A micromechanical model that considered the predicted p(θ) and conversion model was established to predict the uniaxial tensile response of UHPFRC, which was also successfully validated using test data.