The creep and shrinkage of concrete and the relaxation of cables cause long-term redistribution of internal forces and time-dependent deformations in concrete structures, steel-concrete composite structures and concrete cable-stayed bridges. They should be properly modelled for accurate prediction of their long-term behaviour and its effect on instantaneous structural responses at service. An equivalent stress relaxation model is proposed for prestressing tendons based on the intrinsic stress relaxation, from which the equivalent creep coefficients can be obtained through a recursive algorithm. Based on the equivalent stress relaxation model, an accurate finite element analysis of time-dependent behaviour by time integration has been devised considering concrete creep, concrete shrinkage and cable relaxation. Concrete members are modelled by beam-column elements while tendons are modelled by truss elements with nodes connected to the beam axis by perpendicular rigid arms. It is found that the proposed relaxation model with time integration can provide a reliable method as well as benchmark solutions for time-dependent analysis. The numerical results obtained indicate that the interactions among these factors should be properly considered in analysing the long-term performance of concrete bridges. Although time integration provides a reliable method for time-dependent analysis, both the computing time and memory requirement increase drastically with the number of time steps as the time-dependent strains of concrete and tendons within a time interval depend on the loading history up to that time. It is therefore necessary to develop a more efficient method to conduct time-dependent analysis. The relaxation-adjusted elasticity modulus is introduced on the basis of equivalent creep coefficients of tendons. Then, an efficient tendon sub-element is put forward to cope with cables with arbitrary profiles. Finally, a more general single-step method is devised using the classical age-adjusted elasticity modulus to account for external loading and creep effect, the shrinkage-adjusted elasticity modulus to consider shrinkage effect and its interaction with concrete creep, and the relaxation-adjusted elasticity modulus to consider the effect of cable relaxation based on the finite element method. The numerical results obtained indicate not only the accuracy of the single-step method but also the significance of interaction among various time-varying factors. Based on the time integration or single-step method, a systematic method is developed to monitor the long-term variations of dynamic properties of cable-stayed bridges taking into account various time-varying factors and geometric nonlinearities. Numerical studies show that, although geometric nonlinearities tend to reduce the natural frequencies, the time-dependent behaviour of concrete more than offsets it and tends to increase the natural frequencies in the long run. A generic method is further presented to investigate the long-term dynamic response of vehicle-bridge interaction systems taking account of time-dependent behaviour. The vehicles are represented by a combination of mass-spring-damper systems while the bridge is modelled by finite elements. The surface roughness of bridge deck is simulated by spectral representation method and introduced to the coupled system properly. Based on the method, the individual and combined effects of various time-varying factors are studied in detail using various numerical examples. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

External sulfate attack is a complex process that can result in concrete structures deteriorating prematurely. This dissertation describes a comprehensive evaluation of factors influencing external sulfate attack and includes both laboratory and field investigations. Significant emphasis was placed on evaluating the sulfate resistance of mortar and concrete containing high-calcium fly ash (Class C fly ash as per ASTM C 618). This investigation showed that these fly ashes generally reduced the sulfate resistance of mortar and concrete, but that sulfate resistance was possible through the incorporation of other supplementary cementing materials (silica fume and ultra fine fly ash) into these mixtures. Another key area of emphasis in this project was the correlation between accelerated laboratory tests and outdoor exposure site performance. The behavior of concrete exposed to various sulfate salts (sodium, magnesium, and calcium) was evaluated, both in static immersion tests and in outdoor sulfate trenches. It was found that the distress outdoors was exacerbated by physical sulfate attack, especially when concrete was exposed to sodium sulfate. It was found that lowering the water-tocementitious ratio (w/cm) and incorporating appropriate dosages of suitable supplementary cementing materials improved resistance to both chemical and physical forms of sulfate attack. Lastly, a comprehensive study of bridges throughout Texas discovered concrete structures suffering from possible external sulfate attack. The use of analytical techniques such as x-ray diffraction (XRD) and scanning electron microscopy (SEM) were used throughout the study to determine the presence of hydration products that may form with mortars and concrete specimens. Findings from this research will be implemented in specifications in Texas for the use of Class C fly ash use in concrete in areas that may have sulfate soils and groundwater.

No description available.

Synthetic fiber reinforced concrete sample sets were exposed to two different environments. One set, of six samples, was exposed to filtered seawater in the lab with wet and dry cycles, while the other set of samples was exposed, on a barge, to the marine environment, in the intracoastal waterways, at SeaTech. The samples were exposed for 8 months, and then removed for experimental and mechanical testing. Upon removal, the barge samples were photographed to observe surface organisms that were attached to each sample. The barge samples, after cleaning, were then exposed to UV light to observe surface bacteria. The barge samples were also taken to Harbor Branch facility for DNA testing, and then sent in for sequencing. This sequencing was used to identify the organisms that were present inside the concrete samples. An Indirect Tensile Strength Test, IDT, was performed on both sets of samples to observe the first crack, max load, and fracture toughness of each sample. The Barge samples had a lower first crack, max load, and fracture toughness, which means that it took less force to break these samples, than the Seawater samples. As the fiber content increased, the Seawater samples grew stronger, while the Barge samples grew weaker. Also, as the fiber content increased, the biodiversity found on the surface of the Barge samples increased as well. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection

M.Tech. (Civil Engineering) / Every year, millions of Rands are being spent in rehabilitation, repairs and maintenance of reinforced concrete structures projects around the country due to corrosion. There are a number of studies and investigations that have been done recently to address the effect of corrosion on reinforced concrete structures. Concrete provides an ideal environment for steel, supplying both physical and chemical protection from corrosive attack. The effect of corrosion on reinforced concrete structures contributes to the reduction of steel cross sectional area, weakening the bond strength between steel and concrete and thereby reducing the ductility, deflection capacities and load carrying capacity of the structure. In this research, five series of three samples each of reinforced concrete beams were fabricated, some with reduced cross-sectional area and with exposed bars to simulate loss of bonding through spalling. The structural performance of these beams was tested in terms of maximum load carrying capacity, deflections and ductility ratio. The main conclusions are as follows:  As little as 1 % loss in mass of tension steel resulted in a load carrying capacity decrease of 6.9 %  Beam deflection increased by a factor of 1.5 times when the steel mass loss level was 14%  5 % steel loss in mass resulted in a 16.5 % decrease in relative ductility.

Yes / To develop high deterioration resistance concrete for marine infrastructures, two types of nano TiO2 (NT) including anatase phase NT and silica surface-treated rutile phase NT were incorporated into concrete. The fabricated NT modified concrete was then put into the marine environment for 21 months in this study. The effects and mechanisms of two types of NT on the deterioration of concrete in the marine environment were investigated from three aspects, including seawater physical and biological and chemical actions on concrete with NT. Under the seawater physical action, the exposed degree of coarse sand particles on the surface of control concrete is greater than that of concrete with NT. Owing to the microorganism biodegradation property of NT, the elimination and inhibition rates of concrete with NT on microorganisms can reach up to 76.98% and 96.81%, respectively. In addition, the surface biofilm thickness of concrete can be reduced by 49.13% due to the inclusion of NT. In the aspect of seawater chemical action, NT can increase the pH value inside concrete by 0.81, increase the degree of polymerization of C-S-H gel, and improve the interfacial transition zone between cement paste and aggregate in concrete. Compared to concrete with anatase phase NT, silica surface-treated rutile phase NT is more effective in improving the deterioration resistance of concrete in the marine environment. It can be concluded that incorporating NT can inhibit the deterioration of concrete in the marine environment.

Chloride diffusivity in high performance concrete is influenced by the exposure environment, aside from the concrete mixture properties like, water to cementitious ratio (w/cm) and presence of add-on pozzolans. In this study, a set of concrete specimens (eleven-different concrete mixtures) were cast and exposed to three different environmental conditions (Tidal, Splash and Barge) in which the solution was seawater or brackish water. These exposures simulated environmental field conditions. After the specimens had been wet cured for 32 days (on average), the specimens were exposed to three different field simulation conditions for up to 54 months. The specimens under the field simulated conditions were cored at 6, 10, 18, 30 and 54 months at four elevations and then the chloride profiles were obtained from the cores. The apparent diffusivity values for each profile were calculated based on Fick’s 2nd law. Then, the aging factor “m” was calculated by regression analysis of the diffusivity values vs. time (days) plotted in the log10-log10 scale. This was done for samples exposed to the three different exposure conditions and then the results were compared side-by-side. First, the “m” values were calculated using the exposure duration. Then, to study the effect of including the curing time on “m” value, the curing time was added to the exposure time and a new calculation and “m” value was obtained and compared with the previous results. Moreover, upon inspecting the chloride diffusivity values vs. time plots, it was observed that in some cases, a number of data points showed significantly higher or lower values in comparison with the rest of the data points. It was decided to recalculate the “m” values for these cases, and to only use selected data points instead of all data points (i.e., remove outlier data points). In terms of chloride diffusivity value, it was found that in most cases the specimens with higher water to cementitious (w/cm) ratio showed higher diffusivity, as expected. Further, the presence of pozzolans had a noticeable impact on the chloride diffusivity by decreasing the diffusion rate due to microstructure changes that occurred with time. In terms of “m” values, the result for the field simulated conditions showed a range of “m” values dependent on the specimen’s mixture composition and the elevation at which the specimens were cored. It was observed that the chloride diffusivity declined with time and after a certain amount of time (in this research, almost after 30 months) the diffusivity reduction became small and a transition in the slope of the diffusivity trend appeared in a number of cases. After the transition, the diffusivity trend reached either a plateau zone or continued with a significantly lower slope, depending on the time, composition and exposure. It was found that the specimens under tidal and splash field simulation conditions that had only fly ash in their mixtures showed higher “m” values when compared with samples that contained fly ash and silica fume or fifty percent slag. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2019. / FAU Electronic Theses and Dissertations Collection

The purpose of this study was to investigate the performance to chloride penetration of specimens made with three base compositions (three different supplementary cementitious materials) and water to cementitious ratios of 0.35, 0.41, or 0.47. The specimens were subjected to bulk diffusion test or full immersion. The mixes were exposed to 0.1 M, 0.6 M, or 2.8 M sodium chloride solution for different periods of time. Also, partially immersed specimens were exposed to indoor and outdoor exposures (tidal, splash, barge). Chloride concentration profiles were obtained and the apparent diffusion coefficient was calculated. The skin effect was found only on some chloride profiles exposed to 0.1 M sodium chloride solution. The chloride binding capacity was calculated; specimens with 20% Fly Ash and 8% Silica Fume had the highest binding capacity (70.99%). The apparent diffusivity coefficient was found to be dependent on the curing regime as well as the water to cement ratio. The correlation between effective resistivity and apparent diffusion coefficient was determined. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2014. / FAU Electronic Theses and Dissertations Collection

The FDOT has been using supplementary cementitious materials while constructing steel reinforced concrete marine bridge structures for over 3 decades. Previous findings indicated that such additions in concrete mix makes the concrete more durable. To better understand corrosion propagation of rebar in high performance concrete: mature concrete samples that were made (2008/2009) with Portland cement, a binary mix, a ternary mix and recently prepared (April 2016 with 50% OPC + 50% slag and 80% OPC + 20% Fly ash) concrete samples were considered. None of these concretes had any admixed chloride to start with. An accelerated chloride transport process was used to drive chloride ions into the concrete so that chlorides reach and exceed thechloride threshold at the rebar surface and initiate corrosion. Electrochemical measurements were taken at regular intervals (during and after the electro-migration process) to observe the corrosion propagation in each sample. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2017. / FAU Electronic Theses and Dissertations Collection

This study investigates corrosion initiation and propagation in instrumented specimens obtained from segments of dry-cast reinforced concrete pipes. Potential, LPR and EIS measurements were carried out. During the propagation stage in different exposures, reinforcement eventually reached negative potentials values, which suggest mass transfer limitations. So far these specimens show no visual signs of corrosion such as cracks or corrosion products with one exception; where corrosion products have reached the surface. Moreover, the apparent corrosion rate values obtained suggest high corrosion rate. No crack appearance so far, could be explained by the high porosity of the specimens; the corrosion products are filling these pores. It is speculated that although, there might be mass transfer limitations present, the current demanded by the anode is being balanced by a larger cathode area due to macrocell effects, since the high moisture conditions likely reduced the concrete resistivity and increased the throwing power. / by Hariharan Balasubramanian. / Thesis (M.S.C.S.)--Florida Atlantic University, 2013. / Includes bibliography. / Mode of access: World Wide Web. / System requirements: Adobe Reader.