7,897 research outputs found
Using A Centrifuge For Quality Control Of Pre-Wetted Lightweight Aggregate In Internally Cured Concrete
Early age shrinkage of cementitious systems can result in an increased potential for cracking which can lead to a reduction in service life. Early age shrinkage cracking can be particularly problematic for high strength concretes, which are often specified due to their high strength and low permeability. However, these high strength concretes frequently exhibit a reduction in the internal relative humidity (RH) due to the hydration reaction (chemical shrinkage) and self-desiccation which results in a bulk shrinkage, termed autogenous shrinkage, which is substantial at early ages. Due to the low permeability of these concretes, standard external curing is not always efficient in addressing this reduction in internal RH since the penetration of water can be limited. Internal curing has been developed to reduce autogenous shrinkage. Internally cured mixtures use internal reservoirs filled with fluid (generally water) that release this fluid at appropriate times to counteract the effects of self-desiccation thereby maintaining a high internal RH. Internally cured concrete is frequently produced in North America using pre-wetted lightweight aggregate. One important aspect associated with preparing quality internally cured concrete is being able to determine the absorbed moisture and surface moisture associated with the lightweight aggregate which enables aggregate moisture corrections to be made for the concrete mixture. This thesis represents work performed to develop a test method using a centrifuge to determine the moisture state of pre-wetted fine lightweight aggregate. The results of the test method are then used in a series of worksheets that were developed to assist field technicians when performing the tests and applying the results to a mixture design. Additionally, research was performed on superabsorbent polymers to assess their ability to be used as an internal curing reservoir
Documentation of the INDOT Experience and Construction of the Bridge Decks Containing Internal Curing in 2013
The Indiana Department of Transportation (INDOT) constructed four bridge decks utilizing internally cured, high performance concrete (IC HPC) during the summer of 2013. These decks implement research findings from the research presented in the FHWA/IN/JTRP-2010/10 report where internal curing was proposed as one method to reduce the potential for shrinkage cracking, leading to improved durability. The objective of this research was to document the construction of the four IC HPC bridge decks that were constructed in Indiana during 2013 and quantify the properties and performance of these decks. This report contains documentation of the production and construction of IC HPC concrete for the four bridge decks in this study. In addition, samples of the IC HPC used in construction were compared with a reference high performance concrete (HPC) which did not utilize internal curing. These samples were transported to the laboratory where the mechanical properties, resistance to chloride migration, and potential for shrinkage and cracking was assessed. Using experimental results and mixture proportions, the diffusion based service life of the bridge decks was able to be estimated. Collectively, the results indicate that the IC HPC mixtures that were produced as a part of this study exhibit the potential to more than triple the service life of the typical bridge deck in Indiana while reducing the early age autogenous shrinkage by more than 80% compared to non-internally cured concretes
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Reactions of C+ + Cl-, Br-, and I--A comparison of theory and experiment.
Rate constants for the reactions of C+ + Cl-, Br-, and I- were measured at 300 K using the variable electron and neutral density electron attachment mass spectrometry technique in a flowing afterglow Langmuir probe apparatus. Upper bounds of <10-8 cm3 s-1 were found for the reaction of C+ with Br- and I-, and a rate constant of 4.2 ± 1.1 × 10-9 cm3 s-1 was measured for the reaction with Cl-. The C+ + Cl- mutual neutralization reaction was studied theoretically from first principles, and a rate constant of 3.9 × 10-10 cm3 s-1, an order of magnitude smaller than experiment, was obtained with spin-orbit interactions included using a semiempirical model. The discrepancy between the measured and calculated rate constants could be explained by the fact that in the experiment, the total loss of C+ ions was measured, while the theoretical treatment did not include the associative ionization channel. The charge transfer was found to take place at small internuclear distances, and the spin-orbit interaction was found to have a minor effect on the rate constant
High-energy emission from jet-clump interactions in microquasars
High-mass microquasars are binary systems consisting of a massive star and an
accreting compact object from which relativistic jets are launched. There is
considerable observational evidence that winds of massive stars are clumpy.
Individual clumps may interact with the jets in high-mass microquasars to
produce outbursts of high-energy emission. Gamma-ray flares have been detected
in some high-mass X-ray binaries, such as Cygnus X-1, and probably in LS 5039
and LS I+61 303. We predict the high-energy emission produced by the
interaction between a jet and a clump of the stellar wind in a high-mass
microquasar. Assuming a hydrodynamic scenario for the jet-clump interaction, we
calculate the spectral energy distributions produced by the dominant
non-thermal processes: relativistic bremsstrahlung, synchrotron and inverse
Compton radiation, for leptons, and for hadrons, proton-proton collisions.
Significant levels of emission in X-rays (synchrotron), high-energy gamma rays
(inverse Compton), and very high-energy gamma rays (from the decay of neutral
pions) are predicted, with luminosities in the different domains in the range ~
10^{32}-10^{35} erg/s. The spectral energy distributions vary strongly
depending on the specific conditions. Jet-clump interactions may be detectable
at high and very high energies, and provide an explanation for the fast TeV
variability found in some high-mass X-ray binary systems.
Our model can help to infer information about the properties of jets and
clumpy winds by means of high-sensitivity gamma-ray astronomy.Comment: Accepted for publication in A&A (10 pages, 8 figures
Polarization and photometric observations of the gamma-ray blazar PG 1553+113
We present the results of an observational photo-polarimetry campaign of the
blazar PG 1553+113 at optical wavelengths. The blazar was recently detected at
very high energies (> 100 GeV) by the H.E.S.S and MAGIC gamma-ray Cherenkov
telescopes.
Our high-temporal resolution data show significant variations in the linear
polarization percentage and position angle at inter-night time-scales, while at
shorter (intra-night) time-scales both parameters varied less significantly, if
at all. Changes in the polarization angle seem to be common in gamma-ray
emitting blazars. Simultaneous differential photometry (through the B and R
bands) shows no significant variability in the total optical flux. We provide B
and R magnitudes, along with a finding chart, for a set of field stars suitable
for differential photometry.Comment: 4 pages, 3 figures. To be published by Astronomy and Astrophysic
Non-Detection of Gravitationally Redshifted Absorption Lines in the X-ray Burst Spectra of GS 1826-24
During a 200 ks observation with the XMM-Newton Reflection Grating
Spectrometer, we detected 16 type-I X-ray bursts from GS 1826-24. We combined
the burst spectra in an attempt to measure the gravitational redshifts from the
surface of the neutron star. We divided the composite GS 1826-24 burst spectrum
into three groups based on the blackbody temperature during the bursts. The
spectra do not show any obvious discrete absorption lines. We compare our
observations with those of EXO 0748-676.Comment: 4 pages, 4 figures; accepted for publication in ApJ
Jamming and Fluctuations in Granular Drag
We investigate the dynamic evolution of jamming in granular media through
fluctuations in the granular drag force. The successive collapse and formation
of jammed states give a stick-slip nature to the fluctuations which is
independent of the contact surface between the grains and the dragged object --
thus implying that the stress-induced collapse is nucleated in the bulk of the
granular sample. We also find that while the fluctuations are periodic at small
depths, they become "stepped" at large depths, a transition which we interpret
as a consequence of the long-range nature of the force chains.Comment: 7 pages, 4 figures, RevTe
Determining the Moisture Content of Pre-Wetted Lightweight Aggregate: Assessing the Variability of the Paper Towel and Centrifuge Methods
Internally cured (IC) concrete is frequently produced in North America using pre-wetted lightweight aggregate (LWA). One important aspect associated with the production of quality IC concrete is the accurate determination of the moisture content, including absorbed moisture and surface moisture of the LWA. Knowledge of the moisture content enables aggregate moisture corrections to be made for the concrete mixture, thereby enabling an accurate water-to-cement ratio to be maintained. Two methods for determining the moisture content of LWA include the specified ASTM C1761-13b “paper towel method” and a method that uses a centrifuge (Miller, Barrett, Zander, & Weiss, 2014). There are limited data available on the variability associated with either of these approaches when the test is performed by multiple users. In this study, the absorption of four commercially available LWAs was tested by a single operator in a single laboratory using the centrifuge method. In addition, the absorption of three commercially available LWAs was tested by 25 users performing both experimental methods. This article provides an estimation of precision associated with both a single operator and multiple operators performing both the paper towel method and the centrifuge method to find the absorption of pre-wetted lightweight fine aggregate
Closed-Form Bayesian Inferences for the Logit Model via Polynomial Expansions
Articles in Marketing and choice literatures have demonstrated the need for
incorporating person-level heterogeneity into behavioral models (e.g., logit
models for multiple binary outcomes as studied here). However, the logit
likelihood extended with a population distribution of heterogeneity doesn't
yield closed-form inferences, and therefore numerical integration techniques
are relied upon (e.g., MCMC methods).
We present here an alternative, closed-form Bayesian inferences for the logit
model, which we obtain by approximating the logit likelihood via a polynomial
expansion, and then positing a distribution of heterogeneity from a flexible
family that is now conjugate and integrable. For problems where the response
coefficients are independent, choosing the Gamma distribution leads to rapidly
convergent closed-form expansions; if there are correlations among the
coefficients one can still obtain rapidly convergent closed-form expansions by
positing a distribution of heterogeneity from a Multivariate Gamma
distribution. The solution then comes from the moment generating function of
the Multivariate Gamma distribution or in general from the multivariate
heterogeneity distribution assumed.
Closed-form Bayesian inferences, derivatives (useful for elasticity
calculations), population distribution parameter estimates (useful for
summarization) and starting values (useful for complicated algorithms) are
hence directly available. Two simulation studies demonstrate the efficacy of
our approach.Comment: 30 pages, 2 figures, corrected some typos. Appears in Quantitative
Marketing and Economics vol 4 (2006), no. 2, 173--20
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