Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1992. / Vita. Abstract. Includes bibliographical references (leaves 52-54). Also available via the Internet.

In addition to immediate elastic deformations, concrete undergoes time-dependent deformations that must be considered in design. Creep is defined as the time-dependent deformation resulting from a sustained stress. Shrinkage deformation is the time-dependent strain that occurs in the absence of an applied load. The total strain of a concrete specimen is the sum of elastic, creep, and shrinkage strains. Several test beams for the Pinner's Point Bridge have been produced by Bayshore Concrete Products Corp., in Cape Charles, VA. These beams feature high strength concrete mix designs with specified 28-day compressive strengths of 55.2 MPa (8,000 psi) and 69.0 MPa (10,000 psi). These test beams were equipped with thermocouples to track interior concrete temperatures, and vibrating wire gages placed at the center of prestressing to record changes in strain. Laboratory creep and shrinkage testing was conducted on specimens prepared with identical materials and similar mixture proportions to those used at Bayshore. The temperature profile from the test beams during steam curing was used to produce match-cured specimens for laboratory testing. Two match cure batches were produced, along with two standard cure batches. Creep specimens from each batch were placed in the creep room and loaded to 30 percent of their after-cure compressive strength. The creep room had a temperature of 23.0 ± 1.7 °C (73.4 ± 3 ºF) and relative humidity of 50 ± 4 %. Companion shrinkage specimens were also placed in the creep room. Measurements were taken on the creep and shrinkage specimens using a Whittemore gage. Four cylinders were also equipped with embedded vibrating wire gages (VWGs) so that the interior and exterior strains could be compared. The Whittemore and VWG elastic and creep strains were similar, while the VWGs recorded significantly less shrinkage. The measured creep and shrinkage strains were compared to seven different models to determine which model was the most accurate. The models considered were ACI 209, ACI 209 modified by Huo, CEB Model Code 90, AASHTO-LRFD, Gardner GL2000, Tadros, and Bazant B3. The ACI 209 modified by Huo was most accurate in predicting time-dependent strains. / Master of Science

published_or_final_version / Civil and Structural Engineering / Master / Master of Philosophy

published_or_final_version / abstract / Civil Engineering / Doctoral / Doctor of Philosophy

El tema d'estudi d'aquesta tesi és l'eliminació biològica de nitrogen d'aigües amb alta càrrega d'amoníac, més concretament, el procés de nitrificació (l'oxidació de l'amoni a nitrat). Aquesta reacció en dos passos, catalitzada per dos tipus de microorganismes (AOB i NOB), pot patir problemes d'inhibicions per amoníac i àcid nitrós. Això és especialment important quan es tracta aigua amb alta concentració d'amoni i per tant, es necessita un control del procés adequat. Tot i així, aquestes inhibicions es poden utilitzar per a aconseguir nitrificació parcial (l'oxidació de l'amoni a nitrit), la qual combinada amb el procés de desnitrificació, aporta beneficis importants pel què fa a la utilització dels recursos.En aquesta tesi es mostra el desenvolupament i calibratge d'un model matemàtic per a descriure la cinètica i l'estequiometria de la nitrificació considerant les inhibicions mencionades anteriorment i tenint en compte els dos tipus de bacteris nitrificants i també els bacteris heteròtrofs. Es varen dissenyar experiments específics per a l'optimització dels paràmetres del model i amb l'ajuda d'eines d'identificabilitat de paràmetres, aquests experiments es van analitzar i millorar. Les constants d'afinitat pel substrat i els coeficients d'inhibició per substrat es van determinar dues vegades utilitzant biomasses diferents i es van obtenir resultats diferents. Això indicà que aquests paràmetres són variables i depenen de l'aclimatació de la biomassa. Es va utilitzar la tècnica d'hibridació fluorescent in situ (FISH) per a la detecció i quantificació de les fraccions bacterianes. Per a la detecció de la fluorescència es van utilitzar un microscopi d'epifluorescència, un microscopi confocal i un citòmetre de flux. Els resultats obtinguts es van comparar entre ells i es van discutir els seus avantatges i inconvenients tenint en compte la precisió dels resultats, la velocitat de l'anàlisi i la disponibilitat de l'equip. La millor metodologia va resultar ser l'observació del FISH amb el microscopi confocal encara que la citometria de flux no es va poder investigar prou a fons. El model matemàtic desenvolupat i calibrat en aquesta tesi es va utilitzar per a l'optimització de la posada en marxa d'un sistema de nitrificació completa. Es van optimitzar dues estratègies de control que posteriorment es van implementar experimentalment partint d'un inòcul procedent dels llots d'una estació de tractament d'aigües residuals urbanes. El controlador dissenyat es basava en la mesura de la velocitat del consum d'oxigen (OUR) en l'últim reactor del sistema i actuava sobre la càrrega d'entrada. Els resultats obtinguts es van comparar amb els resultats d'una posada en marxa amb control manual i es va demostrar que el control automàtic permet disminuir el temps de posada en marxa i augmentar l'estabilitat del procés. Posteriorment, els resultats experimentals es van simular amb el model matemàtic obtenint un bon ajust. Finalment, el nou model es va utilitzar per a fer prediccions del comportament del sistema a curt i llarg termini. L'enriquiment de la biomassa en microorganismes nitrificants es va comprovar mitjançant el FISH i la microscòpia confocal. La biomassa que es va obtenir després de la última posada en marxa del sistema es va utilitzar per a aconseguir la nitrificació parcial treballant a 25 ºC, 1.1 mg O2 L-1, pH de 8.3 i amb un set point d'OUR apropiat en el control automàtic. La nitrificació parcial es va mantenir de forma estable durant uns 120 dies amb una càrrega mitjana de 0.5 g N g-1 SSV d-1. L'anàlisi microbiològic amb FISH va demostrar que la població de NOB havia estat eliminada del sistema. Posteriorment, el sistema de control es va millorar amb l'adició de dues regles de control expert que van permetre l'operació estable del sistema davant d'importants pertorbacions externes. / Biological nitrogen removal of high-strength ammonium wastewater was studied in this thesis, particularly, the nitrification process (the oxidation of ammonium to nitrate). This two-step reaction, catalyzed by two kinds of bacteria (AOB and NOB), can suffer serious inhibition problems due to ammonia and nitrous acid when dealing with highly concentrated ammonium wastewater and therefore it requires adequate process control. However, these inhibitions can be used to achieve partial nitrification (the oxidation of ammonium to nitrite), which coupled to a denitrifying process leads to significant benefits in terms of use of resources.A mathematical model describing the kinetics and the stoichiometry of the nitrification process was developed and calibrated. It considered the aforementioned inhibitions and took into account both kinds of nitrifying bacteria and also heterotrophic bacteria. Specific experiments were designed for parameter estimation and parameter identifiability tools were used to analyze and improve them. Optimal experimental designs were used to calibrate most of the model parameters and the obtained values were compared with values found in the literature. Affinity constants for substrate and substrate inhibition coefficients were estimated twice using different sludges and, as a result, different values were found indicating that they change depending on the biomass acclimation. This model was coupled to the hydraulic model of the experimental system (pilot plant) and was implemented in Matlab ®. Fluorescence in situ hybridization (FISH) was used for bacterial fractions detection and quantification. Several equipments were used for fluorescence detection: an epifluorescence microscope, a confocal microscope and a flow cytometer. Biomass fractions were determined with each of the equipment and also with simulations. Obtained results were compared and the advantages and disadvantages of the tested methodologies were discussed considering the accuracy of the results, the speed of the analysis and the availability of the equipment. FISH combined with confocal microscopy turned out to be the best technique for nitrifying biomass quantification although flow cytometry could not be extensively investigated.The start-up of a complete nitrification system was optimized by means of mathematical simulation using the previously developed and calibrated method. Two automatic control strategies were optimized and implemented in the experimental system by using sludge from a municipal wastewater treatment plant as inoculum. The controller was based on the measurement of the oxygen uptake rate (OUR) in the last reactor of the system and actuated over the nitrogen loading rate. Results were compared with a start-up performed with manual control and it was demonstrated that automatic control decreased the length of the start-up and increased its stability. Then, experimental results were simulated with the nitrification model. Model predictions agreed well with experimental data. The final model was useful for both long- and short-term prediction. The sludge enrichment in nitrifying bacteria was checked with FISH and confocal microcopy.The nitrifying sludge obtained after the last start-up contained both AOB an NOB and was used to achieve partial nitrification. Some environmental conditions and the automatic control strategy were changed in order to inhibit NOB and wash them out of the system. Partial nitrification with an effluent devoid of nitrate was achieved at 25 ºC, 1.1 mg O2 L-1 and pH of 8.3 using the appropriate OUR set point for the automatic controller. Partial nitrification was run for 120 days with an averaged nitrogen loading rate of 0.5 g N g-1 VSS d-1. FISH analysis demonstrated that NOB were completely washed out. The control strategy was improved by the addition of two expert rules and stable operation was maintained even when external disturbances were provoked. Finally, a model-based study was performed to test the partial nitrification start-up strategy under different conditions and system configurations.

This thesis presents the results of experimental and numerical investigations carried out to assess the effects of tube bending and hydroforming on the crash performance of s-rail structures manufactured from three different advanced high strength steels, namely DDQ, HSLA350, and DP600. The main impetus for this project is to reduce vehicle weight through material substitution and, in order to do so, the effects of material strength on crashworthiness, as well as the interaction between forming processes and crash response must be well understood. To this end, in the current research, s-rails were fabricated through tube bending and hydroforming experiments conducted on DDQ, HSLA350, and DP600 steels with a nominal wall thickness of 1. 8mm, as well as HSLA350 steel with a nominal wall thickness of 1. 5mm. Impact experiments were subsequently performed on non-hydroformed and hydroformed s-rails to examine the effects of the forming processes and material substitution on the crushing loads and levels of absorbed energy. All forming and crash experiments were simulated using numerical finite element methods which provide additional insight into various aspects of the crash response of these structures. In particular, crash simulations were used to show the effects of work-hardening, material thickness changes, and residual stresses incurred during the forming operations. <br /><br /> The numerical tube bending simulations accurately predict the results of the tube bending and hydroforming processes for all materials, particularly for the DP600; the predictions for the DDQ material are the least accurate. Both simulations and experiments show that material thinning occurs on the tensile side of the bend, and material thickening on the compressive side of the bend; the level of thickness change is unaffected by material strength or initial material thickness. The low-pressure hydroforming process does not greatly affect the thickness and strain distributions of s-rails. <br /><br /> The crash simulations provide predictions that are in excellent accord with the measured results, with a maximum error of ±10% in the peak loads and energies; simulations of DP600 s-rails are the most accurate, while simulations of DDQ s-rails are the least accurate. Through simulations and experiments, it is shown that material thickness has the greatest effect on the crash performance of s-rail structures, while material strength plays a secondary role. A 20% increase in the wall thickness of HSLA350 s-rails amounts to a 47% increase in energy absorption. Substituting HSLA350 and DP600 steels in place of DDQ steel leads to increases in energy absorption of 31% and 64%, respectively, for corresponding increases in strength of 30% and 76%. Neglecting material strain-rate effects in the numerical models lowers the predicted peak loads and energies by roughly 15%. By performing a numerical parametric study, it is determined that a weight reduction of 22% is possible by substituting thinner-gauge DP600 s-rails in place of DDQ s-rails while maintaining the energy absorption of the structures.

This thesis presents the results of experimental and numerical investigations carried out to assess the effects of tube bending and hydroforming on the crash performance of s-rail structures manufactured from three different advanced high strength steels, namely DDQ, HSLA350, and DP600. The main impetus for this project is to reduce vehicle weight through material substitution and, in order to do so, the effects of material strength on crashworthiness, as well as the interaction between forming processes and crash response must be well understood. To this end, in the current research, s-rails were fabricated through tube bending and hydroforming experiments conducted on DDQ, HSLA350, and DP600 steels with a nominal wall thickness of 1. 8mm, as well as HSLA350 steel with a nominal wall thickness of 1. 5mm. Impact experiments were subsequently performed on non-hydroformed and hydroformed s-rails to examine the effects of the forming processes and material substitution on the crushing loads and levels of absorbed energy. All forming and crash experiments were simulated using numerical finite element methods which provide additional insight into various aspects of the crash response of these structures. In particular, crash simulations were used to show the effects of work-hardening, material thickness changes, and residual stresses incurred during the forming operations. <br /><br /> The numerical tube bending simulations accurately predict the results of the tube bending and hydroforming processes for all materials, particularly for the DP600; the predictions for the DDQ material are the least accurate. Both simulations and experiments show that material thinning occurs on the tensile side of the bend, and material thickening on the compressive side of the bend; the level of thickness change is unaffected by material strength or initial material thickness. The low-pressure hydroforming process does not greatly affect the thickness and strain distributions of s-rails. <br /><br /> The crash simulations provide predictions that are in excellent accord with the measured results, with a maximum error of ±10% in the peak loads and energies; simulations of DP600 s-rails are the most accurate, while simulations of DDQ s-rails are the least accurate. Through simulations and experiments, it is shown that material thickness has the greatest effect on the crash performance of s-rail structures, while material strength plays a secondary role. A 20% increase in the wall thickness of HSLA350 s-rails amounts to a 47% increase in energy absorption. Substituting HSLA350 and DP600 steels in place of DDQ steel leads to increases in energy absorption of 31% and 64%, respectively, for corresponding increases in strength of 30% and 76%. Neglecting material strain-rate effects in the numerical models lowers the predicted peak loads and energies by roughly 15%. By performing a numerical parametric study, it is determined that a weight reduction of 22% is possible by substituting thinner-gauge DP600 s-rails in place of DDQ s-rails while maintaining the energy absorption of the structures.

This investigation for the use of large volume of fly ash in concrete in combination with superplasticizer, was for the purpose of optimizing its mechanical properties while reducing its cost. Several concrete mixtures using coarse/fine aggregate ratio of 1.22 and aggregate/binder ratio of 5.1 were investigated. Fly ash was used as a partial replacement of type 10 Portland cement at levels ranging between 20-60% by weight of the total cementitious materials in the mixture. Use of superplasticizer allowed a reduction of the water/binder ratio to 0.28-0.33, while the K-slump of fresh concrete was kept at a practical level of 25%. The effect of fly ash on the development of the compressive strength of the hardened concrete was determined. The selection of a concrete mixture with an optimum fly-ash/cement ratio was based on compressive strength results and cost. Concrete with compressive strength levels of 50 MPa, applicable to mid-rise buildings, mine structural components and bridge construction, was obtainedby taking advantage of the water reducing properties of superplasticizers, and by replacing 50% of the cement with Class C fly ash. The 28-day compressive strength of the resultant concrete was approximately 80% of the strength of the identical control mixture containing no fly ash replacement of the cement; at 56 and 91 days, the strength of the resultant mixture improved and eventually became identical to that of the control mixture. The above results were achieved with a 10% reduction in cost, which is a significant savings for the construction industry. The selected mixture was tested for its engineering properties of strength, elasticity, shrinkage and creep, and the results were compared to the same properties of a control mixture. Creep and shrinkage are important concrete properties in prestressed and reinforced concrete structures. Time-dependent deformation of concrete due to creep and shrinkage, results in partial loss of the prestress force and produces significant changes in deflection. In reinforced concrete structures a slow growth of deflection with time may lead to eventual unsatisfactory performance of the structure. Creep and shrinkage of concrete are affected by time, stress intensity, temperature and humidity. In the present study it is indicated that fly ash concrete produced lower creep and drying shrinkage strains than the control concrete under sustained loads at room temperature while its creep increased with a rise in sustained temperatures. Durability tests were performed using freezing-and-thawing and sulphate resistance experiments. The results were compared to the same properties of a control mixture containing superplasticizer and 100% type 10 Portland cement. The frost resistance of fly ash concrete was found to be comparable tothe control mixture. The presence of a large volume of fly ash improved the sulphate resistance of the hardened concrete. Microstructural studies were concurrently conducted in order to determine and explain the effects of superplasticizer and fly ash in producing high performance concrete.

Test results are presented from an experimental program carried at the University of Texas at Austin aimed at evaluating the seismic performance of concrete columns reinforced with high-strength steel. Comparisons are made between the performance of columns reinforced with conventional Grade 60 steel, and the higher Grade 80. The high-strength steel used in this study is the result of a recent push in the U.S. to produce higher grade reinforcing bars with relatively high ductility. All steel used satisfied the specifications of ASTM A706. Column specimens were tested under constant axial load and reverse cyclic lateral loading until collapse. Columns performed in a similar manner, indicating that current limits on the yield strength of reinforcing bars in seismic applications could be raised to include Grade 80 A706 bars. Conclusions are drawn with respect to the effects of higher strength reinforcement on, member cracking, drift capacity, plasticity spread, plastic hinge performance, and strain demands on reinforcing bars. / text

This research study focuses on evaluating the design of HSC prestressed bridge girders. Specifically there were three major objectives. First, to determine the current state of practice for the design of HSC prestressed bridge girders. Second, to evaluate the controlling limit states for the design of HSC prestressed bridge girders and identify areas where some economy in design may be gained. Third, to conduct a preliminary assessment of the impact of raising critical design criteria with an objective of increasing the economy and potential span length of HSC prestressed girders. The first objective was accomplished through a literature search and survey. The literature search included review of design criteria for both the AASHTO Standard and LRFD Specifications. Review of relevant case studies of the performance of HSC prestressed bridge girders, as well all as of important design parameters for HSC were carried out. In addition, a survey was conducted to gather information and document critical aspect of current design practices for HSC prestressed bridges The second objective was accomplished by conducting a parametric study for single span HSC prestressed bridge girders to mainly investigate the controlling limit states for both the AASHTO Standard (2002) and LRFD (2002) Specifications. AASHTO Type IV and Texas U54 girder sections were considered. The effects of changes in concrete strength, strand diameter, girder spacing and span length were evaluated. Based on the results from the parametric study, the limiting design criteria for HSC prestressed U54 and Type IV girders using both the AASHTO Standard and LRFD Specifications for Highway Bridges were evaluated. Critical areas where some economy in design may be gained were identified. The third research objective was accomplished by evaluating the impact of raising the allowable tensile stress for service conditions. This stress limit was selected for further study based on the current limit for uncracked sections provided by the ACI 318 code (2002) and the limit used for a specific case study bridge (Ralls 1995). Recommendations for improving some critical areas of current bridge designs, as well as for increasing bridge span lengths, are given.