A unified bond theory, probabilistic meso-scale modeling, and experimental validation of deformed steel rebar in normal strength concrete

Bond between deformed rebar and concrete is affected by rebar deformation pattern, concrete properties, concrete confinement, and rebar-concrete interfacial properties. Two distinct groups of bond models were traditionally developed based on the dominant effects of concrete splitting and near-interface shear-off failures. Their accuracy highly depended upon the test data sets selected in analysis and calibration. In this study, a unified bond model is proposed and developed based on an analogy to the indentation problem around the rib front of deformed rebar. This mechanics-based model can take into account the combined effect of concrete splitting and interface shear-off failures, resulting in average bond strengths for all practical scenarios. To understand the fracture process associated with bond failure, a probabilistic meso-scale model of concrete is proposed and its sensitivity to interface and confinement strengths are investigated. Both the mechanical and finite element models are validated with the available test data sets and are superior to existing models in prediction of average bond strength (\u3c 6% error) and crack spacing (\u3c 6% error). The validated bond model is applied to derive various interrelations among concrete crushing, concrete splitting, interfacial behavior, and the rib spacing-to-height ratio of deformed rebar. It can accurately predict the transition of failure modes from concrete splitting to rebar pullout and predict the effect of rebar surface characteristics as the rib spacing-to-height ratio increases. Based on the unified theory, a global bond model is proposed and developed by introducing bond-slip laws, and validated with testing of concrete beams with spliced reinforcement, achieving a load capacity prediction error of less than 26%. The optimal rebar parameters and concrete cover in structural designs can be derived from this study. --Abstract, page iii

Using far-field measurements for determining mixed-mode interactions at interfaces

Traction–separation relations (TSR) can be used to represent the interactions at a bimaterial interface during fracture and adhesion. The goal of this work is to develop a direct method to extract mixed-mode TSRs using only the far-field measurements. The first topic of the dissertation deals with extracting mixed-mode TSRs based on a combination of global and local measurements including load-displacement, crack extension, crack tip opening displacement, and fracture resistance curves. Mixed-mode interfacial fracture experiments were conducted using the end loaded split (ELS) configuration for a silicon-epoxy interface, where the epoxy thickness was used to control the phase angle of the fracture mode-mix. Infra-red crack opening interferometry (IR-COI) was used to measure the normal crack opening displacements. For the resistance curves, an approximate value of the J-integral was calculated based on a beam-on-elastic-foundation model that referenced the measured load-displacement data. A damage-based cohesive zone model with mixed-mode TSRs was then adopted in finite element analyses, with the interfacial properties determined directly from the experiments. With the mode-I fracture toughness from a previous study, the model was used to predict mixed-mode fracture of a silicon/epoxy interface for phase angles ranging from -42˚ to 0˚. Additional measurements would be necessary to further extend the reach of the model to mode-II dominant conditions. The second topic of the dissertation addresses characterization of interfacial interactions between copper through-silicon vias (TSV) and silicon substrates. A suitable choice of via length allowed a direct method to be implemented for determining the mode-II traction-separation relation between silicon and copper TSVs. This interface was loaded in a nano-indentation experiment on specimens with pre cracks that were fabricated using focused-ion-beam (FIB) milling. The elastic and plastic properties of the copper vias were characterized from micro-pillar compression experiments and associated finite element analyses. Analytical and numerical models were developed for extracting the parameters of traction-separation relation. The third topic of the dissertation explores a more general approach to directly extract the mixed-mode traction-separation relations using only far-field measurements from laminated beam specimens. Balanced laminated beam configurations were used to conduct the mixed-mode fracture experiment on a silicon-epoxy interface. The far-field measurements included the displacement at the middle of the bottom adherend at a point behind of the crack front, the force-displacement response, and the angle of rotation at the end of the top adherend. With these far-field measurements, the J integrals in mode-I and mode-II could be calculated separately when the ratio between the thickness and the bending stiffness is the same for both the top and bottom adherends. The local separations at the crack tip are also calculated using these far-field measurements. The traction-separation relations are then obtained directly through numerical differentiation of the obtained J integral with respect to the local separations. This method was validated by comparing to the local measurements of normal separation using IR-COI technique. The extracted normal and shear TSR showed decoupled behavior in damage initiation and evolution which indicate that it is impossible to model using potential-based TSRs. A promising attempt was made to subtract out the elastic deformations of the epoxy that was used in this work. This was achieved by conducting a cohesive zone analysis using an extracted pair of normal and shear traction-separation relations for a particular mode-mix without the epoxy between the adherends.Engineering Mechanic

Splice Performance Evaluation of Enamel-Coated Rebar for Structural Safety

This report summarizes the findings and results from an experimental study of vitreous enamel coating effects on the bond strength between deformed rebar and normal strength concrete. A total of 24 beam splice specimens were tested under four-point loading with four parameters investigated: bar size, lap splice length, coating, and confinement conditions. As the splice length increases, the ratio of bond strength between coated rebar and black rebar first increases from 1.0 to a maximum value of 1.44, and then decreases to 1.0. The maximum bond strength ratio corresponds to the near initial yielding of coated rebar. On the average, enamel coating can increase the bond strength of steel rebar in concrete by approximately 15%. A coating factor of 0.85 is thus recommended to take into account the enamel coating effect in lap splice designs, according to ACI and AASHTO bond strength equations

In Situ Mechanical Characterization of the Mixed- Mode Fracture Strength of the Cu/Si Interface for TSV Structures

In situ nanoindentation experiments have been widely adopted to characterize material behaviors of microelectronic devices. This work introduces the latest developments of nanoindentation experiments in the characterization of nonlinear material properties of 3D integrated microelectronic devices using the through-silicon via (TSV) technique. The elastic, plastic, and interfacial fracture behavior of the copper via and matrix via interface were characterized using small-scale specimens prepared with a focused ion beam (FIB) and nanoindentation experiments. A brittle interfacial fracture was found at the Cu/Si interface under mixed-mode loading with a phase angle ranging from 16.7° to 83.7°. The mixed-mode fracture strengths were extracted using the linear elastic fracture mechanics (LEFM) analysis and a fracture criterion was obtained by fitting the extracted data with the power-law function. The vectorial interfacial strength and toughness were found to be independent with the mode-mix

THE RELATIONSHIP BETWEEN REFORMED TEACHING AND STUDENTS’ CREATIVITY IN A CHINESE MIDDLE SCHOOL

Current education reform in both the United States and China promotes a reformed inquiry-based approach based on the constructivist learning theory. This study contributes to the research literature by exploring the relationship between reformed science teaching and students’ creativity. Chinese education is often criticized for a lack of creativity by some news media (Stack, 2011). This study was designed to explore the creativity of students and the extent to which inquiry instruction is used in the science classroom. The study used a convenience sample of two classes from a middle school located in Wuhu city, Anhui province, China. A total of 120 students and 3 science teachers participated. A mixed-methods research approach was adopted for integrated explanation. Student surveys, the Torrance Test of Creative Thinking (TTCT), Verbal, Reformed Teaching Observation Protocol (RTOP), and semi-structured interview were utilized as research tools for collecting quantitative and qualitative data. The findings indicate that there was a positive relationship between reformed teaching and students’ creativity (F (2, 117) = 19.760, p<.001). Classroom observation generally indicated that the participating teachers were skillful at promoting conceptual understanding and provoking high-level thinking. However, evidence of student-centered instruction was less apparent. The semi-structured interviews with participating teachers showed a positive attitude toward inquiry-based teaching but also revealed several challenges. The findings from the Verbal TTCT and classroom observation provided evidence of Chinese students’ creativity. Directions for future research are provided

Internal Structure and Breakage Behavior of Biogenic Carbonate Sand Grains

This Study Investigates the Mechanical Behavior of Biogenic Carbonate Sands from Puerto Rico at Grain-Scale Level. Micro-Computed Tomography Has Also Been Used to Get Insights on the Internal Structure of These Particles Before and after Loading. the Crushing Strength of These Particles Are Smaller Comparing to the Values Reported for Silica Sands. It Has Also Been Shown that These Particles Have Complex Internal Structure Including a Network of Pores Connected with Channels. This Study Also Demonstrates the Effect of Intragrain Structure of Biogenic Carbonate Sands and Shows How Internal Grain Structure Plays a Role on Particle Fracture