Browsing by Subject "post-tensioning"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Analysis of Short and Long Term Deformations in a Continuous Precast Prestressed Concrete Girder(2014-12-09) Sarremejane, TristanA precast prestressed concrete girder using in-span splices to extend the span length is constructed to investigate performance under service and ultimate load conditions. Continuity is provided through the splices by a combination of mild steel reinforcement plus post-tensioned prestress. The thesis focuses on the study of short and long term deformations in the test specimen between the time the pretensioned prestressed segments were first cast, through splicing, deck construction and curing, and then initial testing. To support these observations, three creep frames are set up and shrinkage readings are taken. Previous research is reviewed to determine what models should be used for the analysis of the experimental results. A time-dependent Matlab program based on AAASHTO recommendations is developed to predict the prestress losses due to the short and long-term deformations. Experimental observations from the test specimen are compared to those predictions. The predictions by most models available for assessing long-term deformations due to creep and shrinkage are overestimated when compared to the experimental observations. Unreliable predictions of prestress losses due to long-term deformations may have significant repercussions on a long-span structure; an over-estimation may lead to a design being too conservative, while an under-estimation may lead to cracking and thereby excessive deflections under service loading. It appears that the over-estimation is, in part, due to the girder units being constructed with self-consolidating concrete (SCC). It is concluded that improved estimates of deformations for such structures composed of SCC girders can be achieved if a correction factor of 0.6 is applied to the AASHTO recommendations.Item Assessment of grouts for constructability and durability of post-tensioned bridges(2010-01-16) Kataria, SureshPost-tensioned (PT) bridge technology was first introduced in France in the 1930?s as described in the post-tensioned concrete bridges: Anglo-French liaison report by Highway Agency and is widely used in Europe and the US. PT bridge technology is advantageous over other bridge-type structures due to its larger span-to-depth ratio and reduced construction costs and time. This technology however faces several challenges due to potential corrosion of the prestressing steel. PT bridges constructed in the US during the 1970?s used cementitious grouts to fill the empty spaces in the PT ducts in order to protect the strands from corrosion. This grout in the ducts was intended to protect the strands from being attacked by aggressive agents and to prevent corrosion. A mixture of ASTM Type I cement and water was used as the grouting material for construction of PT bridges. In Texas, four major PT structures have been in place for more than 10 years. Recent investigations of the PT bridges in Texas did not identify any strand failures. However, the visual inspections identified voids in many of the ducts, especially at the ends of the bridge spans. These voids are believed to have been formed as a result of grout bleeding, poor grouting materials, and poor grouting techniques. One of the main performance requirements sought from PT grouts is their ability to fill existing voids in the existing ducts. Currently, many prepackaged grouts are available for PT application that are reported to not bleed and provide better flowability as compared to the older ASTM Type I cement grout. However, the current standard specifications for approving grout materials have limited requirements for evaluating the ?fillability? of these pre-packaged PT grouts. This research is being performed to provide modifications to the existing PT specifications such that PT repair grouts can be objectively assessed for fillability and long-term performance.Item Electrochemical characterization and time-variant structural reliability assessment of post-tensioned, segmental concrete bridges(2010-07-14) Pillai Gopalakrishnan, RadhakrisIn post-tensioned (PT) bridges, prestressing steel tendons are the major load carrying components. These tendons consist of strands, ducts, and cementitious grout that fill the interstitial space between the strands and ducts. However, inspections on PT bridges have reported the presence of voids, moisture, and chlorides inside grouted ducts as the major cause of accelerated corrosion of strands. Corrosion of the strands has resulted in PT bridge failures in Europe and tendon failures in the United States. As most of the PT bridges have high importance measures and the consequences of failure are significant, it is important to maintain high levels of safety and serviceability for these bridges. To meet this goal, bridge management authorities are in dire need of tools to quantify the long-term performance of these bridges. Time-variant structural reliability models can be useful tools to quantify the long-term performance of PT bridges. This doctoral dissertation presents the following results obtained from a comprehensive experimental and analytical program on the performance of PT bridges. 1) Electrochemical characteristics of PT systems 2) Probabilistic models for tension capacity of PT strands and wires exposed to various void and environmental conditions 3) Time-variant structural reliability models (based on bending moment and stress limit states) for PT bridges 4) Time-variant strength and service reliabilities of a typical PT bridge experiencing HS20 and HL93 loading conditions and different exposure conditions for a period of 75 years The experimental program included exposure of strand specimens to wet-dry and continuous-atmospheric conditions. These strand specimens were fabricated to mimic void and/or grout-air-strand (GAS) conditions inside the tendons. It was found that the GAS interface plays a major role in strand corrosion. The GAS interfaces that are typically located in the anchorage zones of harped PT girders or vertical PT columns can cause aggressive strand corrosion. At these locations, if voids are present and the environment is relatively dry, then limited corrosion of the strands occurs. However, if the presence of high relative humidity or uncontaminated and chloride-contaminated water exists at these interfaces, then corrosion activity can be high. The strands were exposed for a period of 12, 16, and 21 months, after which the remaining tension capacity was determined. The analytical program included the development of probabilistic strand capacity models (based on the experimental data) and the structural reliability models. The timevariant tension capacity predicted using the developed probabilistic models were reasonably consistent with the tendon failures observed in PT bridges in Florida and Virginia. The strength reliability model was developed based on the moment capacity and demand at midspan. Service reliability model was developed based on the allowable and applied stresses at midspan. Using these models, the time-variant strength and service reliabilities of a typical PT bridge were determined based on a set of pre-defined constant and random parameters representing void, material, exposure, prestress, structural loading, and other conditions. The strength and service reliabilities of PT bridges exposed to aggressive environmental conditions can drop below the recommended values at relatively young ages. In addition, under similar conditions the service reliability drops at a faster rate than the strength reliability.