Phd Degree / Doktora

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Material Model Calibration of Fiber Reinforced Concrete Using Deep Neural Network
(01. Izmir Institute of Technology, 2023) Yaşayanlar, Yonca; Saatcı, Selçuk; Erdem, Tahir Kemal; Saatcı, Selçuk; Erdem, Tahir Kemal
The numerical modeling of fiber reinforced concrete (FRC) structures is quite challenging due to the material's heterogeneous and anisotropic nature. The majority of material models that are suitable for regular concrete are not able to account for the FRC material's increased tensile capacity and ductility. In this study, a calibration method is proposed that is simple and effective for modeling FRC structures using a selected concrete material model. The Karagozian and Case (K&C) material model in LS-DYNA is capable of representing the ductile nature of FRC, and its parameters related to tensile behavior were calibrated to reflect the tensile-softening behavior. The calibration process was executed using the uniaxial direct tension test results of two different FRC mixtures. In addition, single element numerical models were constructed using LS-DYNA under uniaxial tension. The tensile parameters of K&C were varied over a wide range using single elements to form a database. Then, a Deep Neural Network (DNN) was constructed to pass the database through and find the K&C parameters that best matched the experimental uniaxial test results. The proposed methodology was tested under static and high-strain rate loading conditions, and the results were compared to the experimental findings. The performance of the DNN-achieved parameters was found to be satisfactory. The results showed that the DNN-calibrated parameters were able to accurately predict the behavior of FRC structures under static and dynamic loading conditions.
Properties of Hybrid Fiber Reinforced Concrete for Impact Loading
(Izmir Institute of Technology, 2021) Alami, Mohammad Musa; Saatçi, Selçuk; Erdem, Tahir Kemal
Concrete is a brittle material and does not have significant energy absorption capacity before its fracture. Adding fibers to a concrete mix increases its ductility. Recently, there is significant development in the concrete technology to produce a concrete that can exhibit deflection hardening and show high energy absorption capacity. In this thesis, two kinds of cement based composites with high energy absorption capacity were studied: 1. Engineered Cementitious Composites (ECC). This material can exhibit deflection hardening under bending and it is produced only with synthetic fibers and fine aggregate, 2. Hybrid Fiber Reinforced Concrete (HyFRC). This material can exhibit deflection hardening under bending. It was produced with fine and coarse aggregates and hybrid fibers (both steel and synthetic fibers). The experimental program of this study consists of two main stages. The first stage is to design these composites and test their basic properties in fresh and hardened states, such as compressive strength, flexural behavior, freezing-thawing resistance, chloride ion permeability and sorptivity. In the second stage, dynamic tests (drop tests on small size specimens and pendulum impact tests on real size new generation road concrete barriers with a selected HyFRC mixture) were carried out to determine their energy absorption capacities. Based on the ECC results, fly ash/cement ratio of 1.2 and 20% perlite replacement of sand were selected for HyFRC mixtures. According to the mechanical behavior and durability test results of HyFRC, ST3,0.75_P0.25_D16 mixture (steel fiber type= ST3, steel fiber volume=0.75%, PVA volume=0.25%, Dmax=16mm) was found to have the best performance, and accordingly, this composite was selected for the real-size barrier pendulum test. The same mixture without fibers was also tested under pendulum test as control normal concrete since the present road barriers in the market do not employ fibers. As a result of this study, the HyFRC barrier was found to perform higher impact resistance.
Examination of Bonding Between Successive Lifts of Concretes Having Different Thixotropy Levels by Surface Scanning
(Izmir Institute of Technology, 2020) Ersöz, Hasan Yavuz; Erdem, Tahir Kemal
Unexpected delays in casting can result in strength losses and should be examined considering the thixotropy of the concrete. The effects of thixotropy and delays in casting (0, 45, and 90 min) on concrete bonding strength and on the surface properties were examined. Firstly, 15 self-consolidating concretes (SCC) having different compositions with three different water/cement ratios (0.36, 0.41 and 0.46) three different fine/all aggregate ratios (0.45, 0.50 and 0.55) and two slump flow diameters (60 cm and 70 cm) were produced. Slump flow, visual stability index, T50, V-funnel, static sieve segregation, rheology, and thixotropy tests were carried out on fresh concrete. Considering the results obtained so far, three mixtures were selected to have high, low, and moderate thixotropy for subsequent tests on hardened concrete specimens. Five different roughening patterns were applied on the concrete specimens' interlayer zones by using wooden frames. Slant shear, bi-surface shear, and pull-off tests were conducted on the cylindrical, cubic and drilled specimens to assess bond strength, respectively. Slant shear test gave the highest bonding test results. The lowest results were obtained for the pull-off test. Lateral surfaces of the specimens were scanned with two different lenses. 2D and 3D scanning procedures were adapted with those lenses. Pattern and thixotropy effect was investigated. Lowest void area was obtained for high thixotropy. Highest compressive strengths were obtained for mid thixotropy and high thixotropy for slant shear and pull-off tests respectively. Image J and Matlab were used to the analyze images of the surfaces. Ultrasonic pulse velocity test was conducted on all specimens, moderate thixotropy gave the highest results for Ultrasonic Pulse Velocity test.

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