Piezoelectric composites have been investigated for use in a variety of areas, including flow control, structural control, energy harvesting, and fuel ignition systems. While many of the investigations conducted in these areas have utilized traditional piezo actuation systems, such as unimorphs or stack actuators, a growing number of research groups are examining the increased performance derived from the mechanical advantage, and enhanced domain rotation, found in prestressed unimorph designs. Prestressed devices, like Thunder® and LIPCA, have been shown well suited for a number of applications; however, the price associated with these devices can often prevent them from being implemented. In an effort to produce a low cost unimorph device that possesses a performance-enhancing curved form, the present investigation presents a novel technique for manufacturing prestressed piezoelectric actuators that are capable of meeting the same high displacement and load bearing capabilities exhibited by conventional prestressed devices. The newly proposed mechanically prestressed composite device, or MPC, is similar in form and function to well-documented thermally prestressed devices like Thunder®. However, rather than deriving its characteristic curved form from a thermally induced stress, the present class of devices relies on the resorting force incited in the piezoelectric ceramic upon adhesion to a mechanically deformed substrate to provide both the performance-enhancing prestress and final form of the device. To aid in refinement of the newly proposed design, beam theory was used to model the stress developed within the device. The model allowed designers to investigate the limitations imposed on the performance-enhancing curved form of the device by the stresses developed in the ceramic as a result of the curvature. Findings derived from the model were experimentally verified before a finalized design was specified for the composite, and a number of devices were manufactured. An initial characterization of the device was carried out based on the composite's response to mechanical and electrical loading. By determining the slope of the electrically and mechanically induced displacement response of the device, the investigation was able to define the center displacement constant and effective spring constant of the unimorph. These parameters not only allow designers to predict the displacement that will occur in response to a given electric field or tensile load, but also to allow for comparison with various devices. In the present investigation, the performance characteristics of mechanically prestressed composites were assessed as a function of substrate thicknesses and adhesive properties. With composites constructed using substrates approximately 9.2cm in length, devices were found to have typical center displacement constants on the order of 1.59 to 7.78kV/mm2 while retaining an effective stiffness between 4.5 to 7.5N/mm. These values were found to be similar to the .71 to 3.85kV/mm2 center displacement constants demonstrated by similarly sized and shaped Thunder® devices, which posses an effective stiffness in the range of 10 to 16.3N/mm. A comprehensive presentation of the test methods and procedures used to determine these values, along with other performance characteristics, are provided.

No description available.

Although design codes have accepted lightweight concrete as a suitable structural material for nearly 50 years, there is still a good deal of uncertainty as to how to calculate the strength of this material when designing for shear in beams.  Design codes tend to penalize lightweight concrete due to its lower tensile strength and smoother interface along the shear cracks.  In this study, there were twelve tests on six full-scale, prestressed girders with composite decks designed to provide answers to some of those uncertainties.  The variables considered were concrete density, concrete compressive strength, effective shear depth, shear span-to-effective depth ratio, the amount of shear reinforcement, and the composite cross-sectional area.  Results show that the sand-lightweight concrete girders exceeded the expected shear strength according to the 2010 AASHTO LRFD Bridge Specifications.  Compared to normal weight concrete, sand-lightweight concrete performed reasonably well, and therefore, does not need a lightweight modifier when designing for shear.  However, a reliability analysis of the sand-lightweight girders in this study as well as twelve previous experiments indicate that there should be two different strength reduction factors for the shear design of sand-lightweight concrete depending on which shear design procedures are used in the 2010 AASHTO LRFD Bridge Design Specifications.  For the General Procedure as well as the guidelines outline in Appendix B5, the strength reduction factor should be increased from 0.70 to 1.00.  For the Simplified Procedure, that factor should be 0.75. / Ph. D.

Precast, prestressed concrete panels (PCPs) are used in 85% of bridges in Texas. The goal of this thesis is to reduce collinear cracking (cracks propagating parallel to strands) in PCPs. One way to reduce collinear cracking would be to reduce the initial prestress force. In design, TxDOT conservatively assumes total prestress losses of 45 ksi. Based on eight panel specimens, instrumented and fabricated at two different precast plants in Texas, actual prestress losses were measured as at most 25 ksi. This difference (about 20 ksi) is consistent with a reduction in initial prestress force from 16.1 kips per strand to 14.4 kips per strand. Another way to reduce collinear cracking would be to provide additional transverse reinforcement in the end regions of the panels. By comparing crack spacings and crack widths in current and modified panel specimens, it was found that additional reinforcement consisting of one or two #3 bars placed transverse to strands at panel ends would effectively control collinear cracking in PCPs. / text

The Utah Transportation Center (UTC), in partnership with the Mountain Plains Consortium (MPC), sponsored a study to investigate the differences in prestress losses and temperature gradients in a concrete deck bulb tee girder bridge. The Millville Bridge was built as an access point to the Ridgeline High School in Millville, Utah. The bridge was built in 2016 and presently supports two lanes of traffic. Changes in prestress were measured with a total of 16 vibrating wire strain gauges located at four cross-sections. Temperature gradients were measured with a total of 50 thermocouples located at five cross-section locations, four of which were shared locations with the vibrating wire strain gauges. These instruments were placed at the mid-span and end of an exterior and center girder to effectively measure the bridge response in one quarter of the bridge superstructure. These instruments were placed in the precast plant and tied to the reinforcing steel before the concrete was poured. The prestress loss recordings were initiated before the prestressing strands were released. The thermocouple data for Girder 1 began to be recorded before the initial casting of the girder concrete. The thermocouple data for Girder 5 was not recorded during casting and curing of the girder concrete, but was started before the curing blanket was removed in the casting yard. All data was recorded until February 29, 2016. Prestress losses at the girder mid-span and temperature gradients were compared with code recommended values according to the AASHTO bridge design specifications.

Key words: End zone, prestress transfer, wire tendon, transmission length, pull-in, plain wire, indented wire, concrete strength, size of wire, gradual release, sudden release, shock release, time dependent effects.An empirical investigation into the transfer of prestress force from wire tendons to concrete in the end zones of pretensioned prestressed concrete beams was accomplished in this project. The experimental tests featured 56 small scale prestressed concrete beams.Some of the factors influencing prestress transfer which were considered in the current tests are as follows:(a) type of release - gradual, sudden or shock(b) surface condition of the wire - plain or indented(c) size of the wire(d) concrete compressive strength at the time of transfer(e) time dependent effectsMost of the tests involved gradual release of steel tendons with the prestressing force transferred in approximately ten equal increments. Sudden release in a single step was achieved by allowing the supporting abutments to retract rapidly. Shock release was implemented in some beams by angle grinding the wires. The type of release which gave the best quality of prestress transfer was gradual release. This was followed by sudden and shock releases respectively.There were four types of wires used in the laboratory tests: namely the 5 mm dia. Plain, 5 mm dia. Chevron indented, 7 mm dia. Plain and 7 mm dia. Belgian indented wires. Transmission lengths were determined from strain distributions for these wires. Pull-ins of the wire tendons at the ends of the beams were also measured.There was significant scatter in the experimental data. Different ranges of transmission lengths and pull-ins were obtained for the various types of wires used.Three equations were derived for the 5 mm dia. Plain, 5 mm dia. Chevron and 7 mm dia. Plain wires, which linearly correlated pull-ins to the transmission lengths. ++ / These relationships provide a qualitative and quantitative method of indirectly monitoring for the transmission lengths through the measurements of pull-in.Statistical inference tests proved that indented wires were superior in performance compared to plain wires, but the differences were more apparent for the pull-ins than for the transmission lengths.Comparisons on the influence of tendon size substantiated that greater pull-ins occurred for larger wires but the differences were not significant for the transmission lengths.For concrete strength at the time of transfer of less than 32 MPa, the transmission lengths and pull-ins were significantly larger than those for higher strengths. It is recommended that concrete strength at transfer be at least 32 MPa for pretensioned prestressed concrete.Apart from the maturity and strength of concrete, the quality of a mix also influenced the transmission length and there was limited data to suggest that a better grade mix despite having lower strength at a more tender age could outperform a lower grade mix with greater strength released after a longer curing period.Formulae for plain and indented wires were found by dimensional analysis which correlated the transmission length to the diameter of wire tendon and the stress/strength ratio of the prestressed beams.Pull-ins increased significantly over 6 months but the changes in the transmission lengths were small. Normalised longitudinal strain distributions did not indicate that transmission lengths would remain unchanged over time.

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The new Ashton Bridge is a concrete tied-arch structure with a cable-supported deck, which spans 110 metres below the arching ribs. The tie-beam members, connecting the arch ribs, each have six longitudinal tendons that have primarily straight profiles. The author set out to validate the structural behaviour of the tie-beams, after the post-tensioning construction stage. This objective was achieved by validating selected finite element model parameters with field conducted tests. The input parameter, which is the prestress loading onto the structure, was validated with tendon elongation measurements and tendon lift-off tests. The output parameter, which is the strain and displacement response of the structure, was verified by measuring the elastic deck shortening and the strain gauge readings. Lower tendon extensions were encountered during tensioning. This required calibration of the friction coefficients and model updating. Lift-off tests and deck shortening measurements provided and order size estimation of the structural behaviour, but was not adequate for model validation. The strain gauge readings showed a close correlation with the expected strain state of the structure and offered insight into the behaviour of the structure during post-tensioning. The methods described in this dissertation may be used for validating the structural behaviour of concrete bridges subject to post-tensioning. Suggestions for improving tendon lift-off tests and deck shortening measurements are also presented.

No description available.

In this dissertation, we will study solvent-vapor induced spherulitic growth in multicomponent thin films modeled as prestressed elastic solids. The interface between the crystalline phase and the amorphous phase will be treated as an evolving thermodynamic system and no diffusion of any component will be considered. The dissertation is divided into three parts. In Part I we will determine necessary conditions of thermodynamic equilibrium between the two solid phases, the inter- face, and the vapor. In Part II we will derive the thermodynamic driving force for spherulitic growth in multicomponent elastic thin films. In Part III we will investigate the effect of prestress on the directional dependence of the growth. There a formula that delineates how the prestress affects the shape of the spherulite will be proposed.