275 research outputs found
Balancing Producer Fairness and Efficiency via Prior-Weighted Rating System Design
Online marketplaces use rating systems to promote the discovery of
high-quality products. However, these systems also lead to high variance in
producers' economic outcomes: a new producer who sells high-quality items, may
unluckily receive one low rating early on, negatively impacting their future
popularity. We investigate the design of rating systems that balance the goals
of identifying high-quality products (efficiency) and minimizing the variance
in economic outcomes of producers of similar quality (individual producer
fairness).
We show that there is a trade-off between these two goals: rating systems
that promote efficiency are necessarily less individually fair to producers. We
introduce prior-weighted rating systems as an approach to managing this
trade-off. Informally, the system we propose sets a system-wide prior for the
quality of an incoming product; subsequently, the system updates that prior to
a posterior for each producer's quality based on user-generated ratings over
time. We show theoretically that in markets where products accrue reviews at an
equal rate, the strength of the rating system's prior determines the operating
point on the identified trade-off: the stronger the prior, the more the
marketplace discounts early ratings data (increasing individual fairness), but
the slower the platform is in learning about true item quality (so efficiency
suffers). We further analyze this trade-off in a responsive market where
customers make decisions based on historical ratings. Through calibrated
simulations, we show that the choice of prior strength mediates the same
efficiency-consistency trade-off in this setting. Overall, we demonstrate that
by tuning the prior as a design choice in a prior-weighted rating system,
platforms can be intentional about the balance between efficiency and producer
fairness.Comment: 12 pages, 8 figures, submitted to TheWebConf 202
Mechanical properties of old concrete—UHPFC interface
The uniqueness of Ultra-High Performance Fiber Concrete (UHPFC) is its extremely low porosity gives its low permeability and high durability, making it potentially suitable for rehabilitation and retrofitting reinforced concrete structures or for use as a new construction material. This experimental study was performed to assess the bond strength between UHPFC as a repair material and Normal Concrete (NC) substrate as an old material; split tensile strength and slant shear tests were performed to quantify the bond strength in indirect tension and shear respectively, also the correlation between split tensile strength and slant shear were studied. The result showed that UHPFC has been cured by steam, gives high bond strength at the early age of the repair process, and interacts well with the surface of NC, as a result the failure occurred mostly in the NC substrate. A good correlation between the slant shear test results and the split tensile test results has been observed
Compressive Stress-Strain Behavior of Composite Ordinary and Reactive Powder Concrete
The deterioration of reinforced concrete structures is a major social problem. To minimize this problem and ensure effective structural management, the number and extent of repair interventions must be kept at the lowest probable level. Good bond is one of the main requirements for successful repair. The main aim of this study was to investigate the compressive stress-strain behaviour of the composite specimens consist of ordinary concrete (OC) substrate as old concrete and reactive powder concrete (RPC) as a retrofitting material, by using different types of OC substrate surface preparation methods. The results showed that the composite OC/RPC specimens were able to behave closely to individual OC, in the case of using OC substrate with surface prepared by sand blasted
The relationship between substrate roughness parameters and bond strength of ultra high-performance fiber concrete
The bonding that exists between the old concrete and the new concrete depends largely on the quality of substrate surface preparation. The accurate representation of substrate surface roughness can help determine very precisely the correct bonding behavior. In this work, an experimental investigation was carried out to quantify the normal concrete (NC) substrate roughness parameters and evaluate their relationship with the bonding performance of ultra high-performance fiber concrete (UHPFC), used as a repair material. The bond strength was quantified based on the results of the pull-off test, splitting cylinder tensile test, and the slant shear test. Three types of NC substrate surface preparation were used: as-cast (without surface preparation) as reference, wire-brushed, and sand-blasted (SB); the roughness of which was determined using an optical three-dimensional (3D) surface metrology device (Alicona
Utilization of ultra-high performance fibre concrete (UHPFC) for rehabilitation–a review
Under normal circumstances, reinforced concrete structures (RCS) show excellent performance in terms of durability and structural behaviour except for the zones that are subjected to severe mechanical or cyclic loading and aggressive environmental conditions. Therefore the methods of rehabilitation or strengthening of these zones should be reliable, effective and economical. Today, many scientists, academics and engineers understood the extremely low porosity and low permeability characteristics of ultra high performance fibre concrete (UHPFC) giving its enhanced durability over high performance concrete (HPC), thus making it potentially suitable for rehabilitation and retrofitting problematic RCS. The advantages of utilising the technology of UHPFC in repairing works includes (i) decrease the working time needed for the rehabilitation works; and (ii) increase the serviceability and durability to an extent where
Flexural Strength Behavior of Composite UHPFC-Existing Concrete
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Ultra high performance fiber concrete (UHPFC) is an advanced formula concrete that is proven to be more superior than conventional concrete because it embrace the qualities of steel and concrete. Therefore UHPFC properties which include high durability and strength are fully exploited in the research of rehabilitation and strengthening in concrete and even non-concrete structures. This article presents the findings of an experimental study carried out to examine the bonding strength behaviour between normal concrete (NC) substrate and UHPFC as a repair material, under flexural strength test by using third-point loading beam test method. Three types of NC substrate surface preparation were used: as-cast (without surface preparation) as a reference, wire-brushed, and sand-blasted. The flexural test results clearly indicated that all failures occurred through the NC substrate and no
Dynamics of the rotational degrees of freedom in a supercooled liquid of diatomic molecules
Using molecular dynamics computer simulations, we investigate the dynamics of
the rotational degrees of freedom in a supercooled system composed of rigid,
diatomic molecules. The interaction between the molecules is given by the sum
of interaction-site potentials of the Lennard-Jones type. In agreement with
mode-coupling theory (MCT), we find that the relaxation times of the
orientational time correlation functions C_1^(s), C_2^(s) and C_1 show at low
temperatures a power-law with the same critical temperature T_c, and which is
also identical to the critical temperature for the translational degrees of
freedom. In contrast to MCT we find, however, that for these correlators the
time-temperature superposition principle does not hold well and that also the
critical exponent gamma depends on the correlator. We also study the
temperature dependence of the rotational diffusion constant D_r and demonstrate
that at high temperatures D_r is proportional to the translational diffusion
constant D and that when the system starts to become supercooled the former
shows an Arrhenius behavior whereas the latter exhibits a power-law dependence.
We discuss the origin for the difference in the temperature dependence of D (or
the relaxation times of C_l^(s) and D_r. Finally we present results which show
that at low temperatures 180 degree flips of the molecule are an important
component of the relaxation dynamics for the orientational degrees of freedom.Comment: 17 pages of RevTex, 12 figure
Some remarks on the low-energy excitations in glasses: interpretation of Boson peak data
Cyclic Density Functional Theory : A route to the first principles simulation of bending in nanostructures
We formulate and implement Cyclic Density Functional Theory (Cyclic DFT) -- a
self-consistent first principles simulation method for nanostructures with
cyclic symmetries. Using arguments based on Group Representation Theory, we
rigorously demonstrate that the Kohn-Sham eigenvalue problem for such systems
can be reduced to a fundamental domain (or cyclic unit cell) augmented with
cyclic-Bloch boundary conditions. Analogously, the equations of electrostatics
appearing in Kohn-Sham theory can be reduced to the fundamental domain
augmented with cyclic boundary conditions. By making use of this symmetry cell
reduction, we show that the electronic ground-state energy and the
Hellmann-Feynman forces on the atoms can be calculated using quantities defined
over the fundamental domain. We develop a symmetry-adapted finite-difference
discretization scheme to obtain a fully functional numerical realization of the
proposed approach. We verify that our formulation and implementation of Cyclic
DFT is both accurate and efficient through selected examples.
The connection of cyclic symmetries with uniform bending deformations
provides an elegant route to the ab-initio study of bending in nanostructures
using Cyclic DFT. As a demonstration of this capability, we simulate the
uniform bending of a silicene nanoribbon and obtain its energy-curvature
relationship from first principles. A self-consistent ab-initio simulation of
this nature is unprecedented and well outside the scope of any other systematic
first principles method in existence. Our simulations reveal that the bending
stiffness of the silicene nanoribbon is intermediate between that of graphene
and molybdenum disulphide. We describe several future avenues and applications
of Cyclic DFT, including its extension to the study of non-uniform bending
deformations and its possible use in the study of the nanoscale flexoelectric
effect.Comment: Version 3 of the manuscript, Accepted for publication in Journal of
the Mechanics and Physics of Solids,
http://www.sciencedirect.com/science/article/pii/S002250961630368
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