Experimental Compressive Behavior and Numerical Modeling of Unconfined and Confined Ultra-High Performance Concrete
Ultra-high performance concrete (UHPC) is commonly defined as a cementitious material, reinforced by fiber, which has compressive and sustained post-cracking tensile strength of more than 21.7 ksi (150 MPa) and 0.72 ksi (5 MPa), respectively. The outstanding mechanical behavior of UHPC is owed to its particular mixture. UHPC is a combination of Portland cement, silica fume, fine sand, ground quartz, high-range water reducer (superplasticizer), less than 0.25 water-to-cement ratio, and fibers (mostly steel fibers).Fibers play a crucial role in enhancement of the strength and ductility of UHPC in both tension and compression. Addition of fibers make the material to fail in a fairly ductile manner in compression (compared with material with no fiber) as well as providing ductility and energy absorption in tension. In the recent decades, UHPC has been used in some small-scale applications while it has the potential to be used in larger scales to build sustainable and seismic resistant structures. Currently, there is no comprehensive design guidelines or standards that can be used yet in the analysis and design of UHPC structures under various loading conditions. Thus, the overall goal of this doctoral study is to contribute towards future design guides by providing a better understanding of the compressive behavior of UHPC; at the material level through experimental testing of unconfined and confined cylinders, and at the structural level through numerical modeling.This study consists of three main parts. First, a series of uniaxial compression tests was conducted on more than 130 unconfined and confined UHPC cylinders with varied ratios of steel fibers and transverse reinforcements to evaluate linear and nonlinear behavior of UHPC in the pre- and post-peak regions. The individual and contributory confining effects of steel fibers and transverse reinforcement was quantitatively investigated and reported in terms of the full stress-strain curves, modulus of elasticity, stress and strain at peak strength, and post-peak strains. In the second part, compression test results were utilized to evaluate the validity of several existing confinement models for normal strength, high strength, and fiber reinforced concrete, in predicting the uniaxial compressive behavior of UHPC. This evaluation rendered the reviewed existing confinement models inappropriate for UHPC where they commonly overestimate the modulus of elasticity, peak compressive strength, and post-peak behavior of confined UHPC and urged the need for a newconfinement model for UHPC. Therefore, an exclusive new confinement model was developed for UHPC which accounts for the combined effects of steel fibers and spirals on compression behavior of UHPC. The proposed model was evaluated using additionalexperimental data and is shown to adequately represent the uniaxial compressive behavior and full stress-strain curves of both unconfined and confined UHPC with transverse reinforcement.In the last part of the study, the uniaxial stress-strain relationships of UHPC either from the literature or present study were employed to numerically simulate the UHPC material behavior for 3D finite element modeling purposes. The total strain crack model, as readilyimplemented in the general-purpose software FEA DIANA, was utilized with user-defined input to model the UHPC constitutive material behavior. A two-column bridge bent was selected as a prototype case study and was simulated using different UHPC material modelsin DIANA. The overall behavior, failure mechanism, and effect of design details and material model sensitivity on force and displacement capacities of the UHPC bridge bent were assessed. According to the overall observed response from the finite element analysisat the structural level, UHPC columns are shown to feature a ductile behavior and get be still used for seismic applications in light of current design code requirements. The study also provides foundational work that could be extended to inform future designs andoptimize structural components through careful reinforcement design in terms of selecting best ratios of steel fibers, and longitudinal and transverse reinforcements.
Compression, Confinement, Finite Element Modeling, Seismic, Ultra High Performance Concrete
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