A Study of Expansion and Contraction in a Pavement Consisting of Prestressed Concrete Panels Interconnected in Place

This thesis investigation is a study of the temperature expansion and contraction characteristics of a composite pavement constructed as a continuous pavement. The composite pavement consists of precast, prestressed concrete panels interconnected with grout keys and special panel connectors. A thin layer of asphaltic concrete is placed on the panels to provide a smooth riding surface and protection for the panels. This pavement was developed at South Dakota State University by Emil R. Hargett as a means of providing a more economical pavement for the high traffic volumes and heavy wheel loads that are now carried by our modern highways. The continuous composite pavement, described in this thesis, consists of prestressed concrete panels 6 ft. x 24 ft. and laid in a longitudinal pattern as shown in Figure 1. These panels are interconnected with grout keys and tongue and fork connectors as shown in Figure 2. Favorable results were obtained by Kruse (1) from a laboratory investigation of the structural performance of this type of pavement under repetitive wheel loads. The success of this new type of pavement depends in part on the performance of the grouted joints and connectors under varying temperature conditions. Temperature changes within the pavement will cause corresponding volume changes of major concern in a continuous pavement. Therefore, the use of this type of payment poses a problem of the distribution of the expansion and contraction caused by changes in temperature. A study of the forces caused by the temperature expansion and contraction of this type of pavement is the primary objective of this investigation

Searching for binary central stars of planetary nebulae with Kepler

The Kepler Observatory offers unprecedented photometric precision (<1 mmag) and cadence for monitoring the central stars of planetary nebulae, allowing the detection of tiny periodic light curve variations, a possible signature of binarity. With this precision free from the observational gaps dictated by weather and lunar cycles, we are able to detect companions at much larger separations and with much smaller radii than ever before. We have been awarded observing time to obtain light-curves of the central stars of the six confirmed and possible planetary nebulae in the Kepler field, including the newly discovered object Kn 61, at cadences of both 30 min and 1 min. Of these six objects, we could confirm for three a periodic variability consistent with binarity. Two others are variables, but the initial data set presents only weak periodicities. For the central star of Kn 61, Kepler data will be available in the near future

The Planetary Nebula A39: An Observational Benchmark for Numerical Modeling of Photoionized Plasmas

Galactic nebulae are the main probes for the chemical evolution of the interstellar medium. Yet, recent observations have shown that chemical abundances determined from recombination and collisionally excited emission lines can differ by as much as an order of magnitude in some planetary nebulae (PNs). Many PNs have complex geometries and morphological evidence for interactions from stellar winds, and it is not clear to what extent winds, inhomogeneities, or shocked gas affect the observed spectrum. There currently is no full explanation for this discrepancy, which brings into question whether we understand the physical state of these low-density plasmas at all. This paper presents new spectroscopy from the KPNO Mayall 4 m telescope and imagery from the WIYN 3.5 m telescope of A39, a large PN with an exceptionally simple geometry. It appears to be a limb-brightened spherical shell, the simplest possible nebula. There is little evidence for external interactions, so this is the case in which photoionization simulations should be in near-perfect agreement with observation. We combine optical and UV spectroscopy to form a composite spectrum and compare this with photoionization models. No problems were encountered in reproducing the observed spectrum, although even this simple object has two distinct emission-line regions and exhibits several anomalies. A39 was too faint to detect the crucial heavy-element recombination lines in our data set, so it was not possible to compare collisional and recombination abundances. We predict the spectrum over a broad range of bandpasses to facilitate future observations that may probe deeper than our instrumentation allowed