Effect of specimen size and loading conditions on spalling of concrete

Different qualities of concrete have been fire tested using different geometries of the specimens as well as different load levels and load configurations. The main objective with the study was to examine a test methodology consisting of a full‐scale test and different small scale‐tests for determining the probability of spalling and the amount of spalling of fire exposed concrete structures. A reference specimen was defined as a one‐sided fire exposed slab with the dimensions 1800 × 1200 mm² giving an exposed area of 1500 × 1200 mm². A number of concrete qualities with different probabilities for spalling, were tested using the reference specimen. These tests showed that the reference specimens worked well giving the expected test results. Small specimens were manufactured in different shapes with the same concrete as the one used in the reference tests. These small specimens were tested either at the same time as the reference specimens in the large furnace or afterwards on a small‐scale furnace where the fire exposed surface was 450 × 360 mm². The test results clearly show the increased probability and the increased amount of spalling by using external compressive loading. The results also show that by using pre‐stress through bars or wires the load can be lost due to heating of the bars/wires which results in a decreased amount of spalling. The boundary of the specimen also affects the amount of spalling. The spalling around the edges was in all tests less than the spalling on the central parts of the exposed area. It could also be noted that the spalling did not pass completely through any of the specimens. The reason for this is probably that the water/vapour could migrate out from the unexposed surface of the specimen. Copyright © 2006 John Wiley & Sons, Ltd.     

Sensor Support Systems for Asymmetric Threat Countermeasures

In the past, primary focus has been given to novel sensor elements for deployment against urban terrorists and in limited force engagements. The issue explored in this paper is the adequacy of electronic system support for these new sensing elements. For example, ad hoc distributed networks must lie dormant for long periods of time and ldquocome aliverdquo when threats are nearby. This presents a unique challenge in the storage, generation, and management of power. In this paper, we demonstrate designs of processor algorithms and telecommunication protocols that alleviate current power-system shortcomings for these stationary networks. These advances include: 1) low-power protocols for data fusion and fault tolerance and 2) system-level energy modeling and analysis. As a concrete example, we define a distributed sensor support system for line crossing recognition. We demonstrate that threat detection is a system-level problem. Single elements of the system chain individually have small impact on overall performance. Through the development of a preamplifier/amplifier chain for optimum signal-to-noise (S/N) ratio, we show the degree to which system-level architecture can improve reliable detection. Specifically, the use of sensor redundancy to improve performance is analyzed from a statistical basis.     

A threshold area ratio of organic to conventional agriculture causes recurrent pathogen outbreaks in organic agriculture

Conventional agriculture uses herbicides, pesticides, and chemical fertilizers that have the potential to pollute the surrounding land, air and water. Organic agriculture tries to avoid using these and promotes an environmentally friendly approach to agriculture. Instead of relying on herbicides, pesticides and chemical fertilizers, organic agriculture promotes a whole system approach to managing weeds, pests and nutrients, while regulating permitted amendments. In this paper, we consider the effect of increasing the total area of agricultural land under organic practices, against a background of conventional agriculture. We hypothesized that at a regional scale, organic agriculture plots benefit from existing in a background of conventional agriculture, that maintains low levels of pathogens through pesticide applications. We model pathogen dispersal with a diffusive logistic equation in which the growth/death rate is spatially heterogeneous. We find that if the ratio of the organic plots to conventional plots remains below a certain threshold l_c, the pest population is kept small. Above this threshold, the pest population in the organic plots grows rapidly. In this case, the area in organic agriculture will act as a source of pest to the surrounding region, and will always infect organic plots as they become more closely spaced. Repeated localized epidemics of pest outbreaks threaten global food security by reducing crop yields and increasing price volatility. We recommend that regional estimates of this threshold are necessary to manage the growth of organic agriculture region by region.     

Using the TPS method for determining the thermal properties of concrete and wood at elevated temperature

The transient plane source (TPS) method is shown to be very promising for determining thermal properties of materials at room temperature as well as temperatures up to 700°C. To investigate the applicability of the method it has been used in the study for determining thermal properties of wood (spruce) and concrete. Conductivity (λ) and diffusivity (α) were determined simultaneously. The thermal properties thus obtained have been compared with some values found in literature. The paper also presents results where calculations using properties obtained with the TPS method are compared with fire test measurements. The results are very encouraging. Copyright © 2005 John Wiley & Sons, Ltd.     

A temperature fluctuation equation model dedicated to the computation of turbulent thermal layers in high Reynolds internal flows

The paper deals with the modelling of temperature fluctuations in the fresh gases for the simulation of turbulent internal flows. For this purpose, a transport equation for the sensible enthalpy variance is treated. The proposed dynamic model for the scalar dissipation rate incorporates the effect of the turbulent Reynolds number. The wall closure is based on a non-isothermal formulation and accounts for the non-equilibrium state of the boundary layer. The developed models are implemented into the compressible code IFP-C3D. Results of the computations are successfully compared with experiments and DNS data for a slightly heated jet and moderate non-isothermal walls.     

Phasor estimation from phasorlets

The mathematical formulation for estimating phasors from phasorlets, obtained from signal segments of a fraction of a cycle, is established and the frequency responses of the phasorlet generator filters are evaluated. From the theory and the numerical simulations it is concluded that, when the signal segment involved in the phasorlet generation corresponds to a pure sinusoidal signal, this estimation technique offers the quickest measurement of its phasor, improving the speed of the well known one-cycle Fourier filter estimate. The new estimates were found very useful for detecting and locating abrupt changes in amplitude or phase between two sinusoidal states. However, before transients or impure sinusoids, the exactness and the dynamic behavior of this estimation method (which includes the one-cycle Fourier filter) will depend in great extent on the applied noise extraction technique, given the huge sensitivity to nonsinusoidal signals of the phasorlet generator filters.     

Including ampere measurements in generalized state estimators

Conventional state estimators (SEs) cannot take advantage of all available measurements, like currents flowing through circuit breakers and individual external injections. Current measurements are frequent at the lower voltage levels and their loss implies a reduction of the global redundancy and hence a deterioration of the SE filtering capability. In this paper, a methodology is presented allowing all current measurements to be included in the model. A generalized state estimator is used in which substations are compactly represented by means of an implicit model. To illustrate the performance of the proposed formulation, several simulations are presented. Results are validated by checking the accuracy of the estimated states and by showing the improved ability to detect and identify bad data and topology errors.     

Using multiple objective tabu search and grammars to model and solve multi-objective flexible job shop scheduling problems

A linguistic-based meta-heuristic modeling and solution approach for solving the flexible job shop scheduling problem (FJSSP) is presented in this study. FJSSP is an extension of the classical job-shop scheduling problem. The problem definition is to assign each operation to a machine out of a set of capable machines (the routing problem) and to order the operations on the machines (the sequencing problem), such that predefined performance measures are optimized. In this research, the scope of the problem is widened by taking into account the alternative process plans for each part (process plan selection problem). Probabilistic selection of alternative process plans and machines are also considered. The FJSSP is presented as a grammar and the productions in the grammar are defined as controls (Baykasolu, 2002). Using these controls and Giffler and Thompson's (1960) priority rule-based heuristic along with the multiple objective tabu search algorithm of Baykasolu et al. (1999) FJSSP is solved. This novel approach simplifies the modeling process of the FJSSP and enables usage of existing job shop scheduling algorithms for its fast solution. Instead of scheduling job shops with inflexible algorithms that cannot take into account the flexibility which is available in the job shop, the present algorithm is developed which can take into account the flexibility during scheduling. Such an approach will considerably increase the responsiveness of the job shops.     

A high-order curvature compensation technique for bandgap voltage reference using subthreshold MOSFETs

A bandgap voltage reference with high-order curvature compensation is presented in this study. It exploits subtraction and derivative equalisation of currents generated from two complementary NMOS and PMOS bandgap references (BGRs) using subthreshold MOSFETs. By equating the derivative with respect to temperature of the two currents, generated by the complementary bandgaps, and subtracting these currents, an accurate high-order curvature compensation is achieved. To overcome problems due to the limited input common-mode range of opamps used in BGRs, a transimpedance amplifier with new accurate current compensation that tracks the temperature variation is proposed. This bandgap is implemented using the 0.18 μm CMOS process with a supply voltage as low as 0.7 V. At 0.8 V power supply and an output reference voltage of 386 mV, the proposed circuit achieves a temperature coefficient of 19 ppm/°C from 0 to 130°C. The power consumption is 119 μW and the power supply reduction ratio is 24 dB at 1 kHz.     

A new ant colony optimization based algorithm for data allocation problem in distributed databases

The Performance and the efficiency of a distributed database system depend highly on the way data are allocated to the sites. The NP-completeness of the data allocation problem and the large size of its real occurrence, call for employing a fast and scalable heuristic algorithm. In this paper, we address the data allocation problem in terms of minimizing two different types of data transmission across the network, i.e., data transmissions due to site-fragment dependencies and those caused by inter-fragment dependencies. We propose a new heuristic algorithm which is based on the ant colony optimization meta-heuristic, with regards to the applied strategies for query optimization and integrity enforcement. The goal is to design an efficient data allocation scheme to minimize the total transaction response time under memory capacity constraints of the sites. Experimental tests indicate that our algorithm is capable of producing near- optimal solutions within a reasonable time. The results also reveal the flexibility and scalability of the proposed algorithm.