Assessment of Partial Debonding Practice for AASHTO Type Girders

Pretensioned girders have been commonly used in bridge construction for years. However, some problems remain that hinder the further application of longer and more heavily prestressed girders. The prestressing force can produce large stresses at both the top and bottom surfaces of the girders, especially near the ends where self-weight moments are minimal. Additionally, the transfer of large prestressing forces can cause local cracking. Partial debonding of straight strands, harping strands and/or adding top strands are three common approaches to mitigating such problems. However, harping is limited to those strands aligned with the member web, and adding top strands affects the overall stress state of the section. Comparatively, partial debonding is a simple and preferred approach. The total prestress force is introduced to the member gradually, reducing stress concentrations and associated end-region cracking. Even so, partial debonding decreases the longitudinal tension capacity particularly when a large number of strands is debonded. Excessive debonding, therefore, can also have detrimental effects of the flexure and shear capacity of the girder. This thesis aims to quantify the effects of partial debonding on initial girder stresses and ultimate girder capacity in an effort to identify acceptable prestressing strand debonding details. Two series of AASHTO Type III-VI girders having varying spans, amounts of prestressing and different debonding ratios are systematically analysed for their adherence and consistency with present AASHTO LRFD Specification requirements. The analyses use a purpose-written MATLAB program. Analytically obtained girder capacities are validated with initial design capacities from the PCI Bridge Design Manual. An individual case is presented in order to illustrate the analysis procedure. From this study, acceptable partial debonding ranges, satisfying AASHTO-prescribed stress limits, are obtained. Conclusions indicate that the upper limit for an acceptable debonding ratio may be increased from the AASHTO-prescribed 25% to perhaps 50%. However the results also indicate that this upper limit is a function of span length and may be greater for longer spans. In many cases no acceptable amount of debonding was found for shorter spans. Further parametric study is required to establish such a relationship and to extend the study to other girder shapes

Subcritical Crack Growth Induced by Stress Corrosion in Quasibrittle Materials

For concrete structures, a primary driver of deterioration shortening their lifespans is the damage growth resulting from coupled chemo-mechanical attack. Under sustained service load coupled with corrosion, stress corrosion cracking will happen and lead to subcritical crack growth (SCG) in concrete members. Thus, knowledge of the SCG in cement-based materials subject to concurrent physical and chemical attacks is of great importance for understanding and mitigating the chemo-mechanical deterioration in concrete structural members. In this thesis, the SCG in hardened cement pastes is investigated experimentally by a novel test approach aided with micro-characterization. Specimens of negative geometry are designed, which enable the use of load control to trigger stable crack propagation in hardened cement pastes. Specimens from the same batch of mixture (with water/cement ratio w/b = 0.35 and 0.40) are exposed to the same chemical condition and loaded at the same age for both the static fatigue and stress corrosion groups. The average trend and the associated variation of the dependence of crack velocity v on the stress intensity factor K at the crack tip are obtained by using a high-resolution microscopy system to trace the crack tip. Three distinctive regions are captured in the K-v curves of stress corrosion specimens, which is different from those in static fatigue. With the help of advanced techniques including SEM, AFM and Raman spectroscopy, the microstructure destruction and chemical composition change induced by the imposed chemo-mechanical attack are characterized at different stages. In addition to the physical insights for deeper understanding of the coupled effect of chemo-mechanical attack, these experimental results provide important macro- and microscopic benchmarks for numerical modeling. Moreover, anchored at the obtained experimental benchmarks, material modeling and numerical schemes are developed to approximate the coupled chemo-mechanical deterioration in cement-based materials. To utilize the unique physical or chemical laws involved in each individual deterioration process, a two-dimensional (2D) discrete model consisting of two lattice systems is constructed in this study to approximate the meso-structure of cementitious materials. These two lattice systems, one approximating the dissolution of cement matrix under calcium leaching and the other simulating the response of material skeleton to external loads, are interlinked by a common physical variable – the porosity of hardened cement pastes, which evolves with the interaction of skeleton cracking and cement dissolution. To reduce the computational cost, an artificial time scale, which allows coarse temporal discretization, is used in the numerical framework resting on a hybrid of implicit and explicit formulation. The model is implemented in ABAQUS and validated by the experimental results. The numerical results show that the proposed discrete model can realistically describe the SCG in hardened cement pastes subject to coupled mechanical damage and calcium leaching

An investigation on the best-fit models for sugarcane biomass estimation by Linear Mixed-Effect Modelling on Unmanned Aerial Vehicle-Based Multispectral Images: a case study of Australia

Due to the worldwide population growth and the increasing needs for sugar-based products, accurate estimation of sugarcane biomass is critical to the precise monitoring of sugarcane growth. This research aims to find the imperative predictors correspond to the random and fixed effects to improve the accuracy of wet and dry sugarcane biomass estimations by integrating ground data and multi-temporal images from Unmanned Aerial Vehicles (UAVs). The multispectral images and biomass measurements were obtained at different sugarcane growth stages from 12 plots with three nitrogen fertilizer treatments. Individual spectral bands and different combinations of the plots, growth stages, and nitrogen fertilizer treatments were investigated to address the issue of selecting the correct fixed and random effects for the modelling. A model selection strategy was applied to obtain the optimum fixed effects and their proportional contribution. The results showed that utilizing Green, Blue, and Near Infrared spectral bands on models rather than all bands improved model performance for wet and dry biomass estimates. Additionally, the combination of plots and growth stages outperformed all the candidates of random effects. The proposed model outperformed the Multiple Linear Regression (MLR), Generalized Linear Model (GLM), and Generalized Additive Model (GAM) for wet and dry sugarcane biomass, with coefficients of determination (R2) of 0.93 and 0.97, and Root Mean Square Error (RMSE) of 12.78 and 2.57 t/ha, respectively. This study indicates that the proposed model can accurately estimate sugarcane biomasses without relying on nitrogen fertilizers or the saturation/senescence problem of Vegetation Indices (VIs) in mature growth stages

Legume crop rotation suppressed nitrifying microbial community in a sugarcane cropping soil

Nitrifying microorganisms play an important role in nitrogen (N) cycling in agricultural soils as nitrification leads to accumulation of nitrate (NO3 −) that is readily lost through leaching and denitrification, particularly in high rainfall regions. Legume crop rotation in sugarcane farming systems can suppress soil pathogens and improve soil health, but its effects on soil nitrifying microorganisms are not well understood. Using shotgun metagenomic sequencing, we investigated the impact of two legume break crops, peanut (Arachis hypogaea) and soybean (Glycine max), on the nitrifying communities in a sugarcane cropping soil. Cropping with either legume substantially increased abundances of soil bacteria and archaea and altered the microbial community composition, but did not significantly alter species richness and evenness relative to a bare fallow treatment. The ammonia oxidisers were mostly archaeal rather than bacterial, and were 24–44% less abundant in the legume cropping soils compared to the bare fallow. Furthermore, abundances of the archaeal amoA gene encoding ammonia monooxygenase in the soybean and peanut cropping soils were only 30–35% of that in the bare fallow. These results warrant further investigation into the mechanisms driving responses of ammonia oxidising communities and their nitrification capacity in soil during legume cropping

Effects of toe-out and toe-in gaits on lower-extremity kinematics, dynamics, and electromyography

Toe-in and toe-out gait modifications have received increasing attention as an effective, conservative treatment for individuals without severe osteoarthritis because of its potential for improving knee adduction moment (KAM) and knee flexion moment (KFM). Although toe-in and toe-out gaits have positive effects on tibiofemoral (TF) joint pain in the short term, negative impacts on other joints of the lower extremity may arise. The main purpose of this study was to quantitatively compare the effects of foot progression angle (FPA) gait modification with normal walking speeds in healthy individuals on lower-extremity joint, ground reaction force (GRF), muscle electromyography, joint moment, and TF contact force. Experimental measurements using the Vicon system and multi-body dynamics musculoskeletal modelling using OpenSim were conducted in this study. Gait analysis of 12 subjects (n = 12) was conducted with natural gait, toe-in gait, and toe-out gait. One-way repeated measures of ANOVA (p < 0.05) with Tukey’s test was used for statistical analysis. Results showed that the toe-in and toe-out gait modifications decreased the max angle of knee flexion by 8.8 and 12.18 degrees respectively (p < 0.05) and the max angle of hip adduction by 1.28 and 0.99 degrees respectively (p < 0.05) compared to the natural gait. Changes of TF contact forces caused by FPA gait modifications were not statistically significant; however, the effect on KAM and KFM were significant (p < 0.05). KAM or combination of KAM and KFM can be used as surrogate measures for TF medial contact force. Toe-in and toe-out gait modifications could relieve knee joint pain probably due to redistribution of TF contact forces on medial and lateral condylar through changing lateral contact centers and shifting bilateral contact locations