Current Issue : January - March Volume : 2011 Issue Number : 1 Articles : 7 Articles
In this paper, work on air-coupled impact-echo is described. The method is applied to locate and to determine the depth of simulated delamination and void defects in a concrete slab. A small (6mm diameter) measurement microphone is used to sense impact-echo response without contacting the surface of the concrete. Ambient acoustic noise effects are significantly reduced by a specially-designed sound insulation device. Test results show that air-coupled sensors work as effectively as do contact impact-echo sensors when proper impactors are used. An air-coupled impact-echo scan was conducted over the entire slab area, and the defects are located in a generated 2-D contour image. Experiments show that the areal size of near-surface delamination defects can be accurately determined if the scan spacing is smaller than 1/2 of the expected defect size. The study presented in this paper shows air-coupled sensing offers an approach for rapid and effective evaluation of concrete deck structures through imaging....
Temperature and doping dependencies of electron mobility in InP, GaP and Ga0.52In0.48P structures have been calculated using an iteravive technique. The following scattering mechanisims, i.e, impurity, polar optical phonon, acoustic phonon and piezoelectric are inculded in the calculation. Ionized imurity scattering has been treated beyound the Born approximation using the phase-shift analysis. It is found that the electron mobility decreases monotonically as the temperature increases from 100K to 500K for each material which is depended to their band structures characteristics. The low temperature value of electron mobilty increases significantly with increasing doping concentration. The iterative results are in fair agreement with other recent calculations obtained using the relaxation-time approximation and experimental methods....
This paper describes a decision fusion strategy that can integrate multiple individual damage detection measures to form a new measure, and the new measure has higher probability of correct detection than any individual measure. The method to compute the probability of correct selection is presented to measure the system performance of the fusion system that includes the presented fusion strategy. And parametric sensitive studies on system performance are also conducted. The superiority of the fusion strategy herein is that it can be extended to deal with the multiresolution subdecision or blind adaptive detection, and corresponding methodologies are also provided. Finally, an experimental setup was fabricated, whereby the vibration properties of damaged and undamaged structures were measured. The experimental results with the undamaged structural model provide information for producing an improved theoretical and numerical model via model updating techniques. Three existing vibration-based damage detection methods with varied resolutions were utilized to identify the damage that occurred in the structure, based on the experimental results. Then the decision fusion strategy was implemented to join the subdecisions from these three methods. The fused results are shown to be superior to those from single method....
This paper surveys different material combinations and applications in the field of mineral-based strengthening of concrete structures. Focus is placed on mechanical behaviour on material and component levels in different cementitious composites; with the intention of systematically maping the applicable materials and material combinations for mineral-based strengthening. A comprehensive description of a particular strengthening system developed in Sweden and Denmark, denominated as Mineralbased Composites (MBCs), together with tests from composite material properties to structural elements is given. From tests and\r\nsurvey it can be concluded that the use of mineral-based strengthening system can be effectively used to increase the load bearing capacity of the strengthened structure. The paper concludes with suggestions on further development in the field of mineral-based strengthening....
The performance of water-reducing admixtures used in concrete is affected by the weather conditions to which the concrete mixture is exposed. The most used WRAs are lignosulfonate, naphthalene, and polycarboxylate. However, they react differently to weather conditions, especially to air temperature. Therefore, it can be useful to evaluate how temperature affects admixture performance. In this study, the performance of three admixtures (naphthalene, lignosulfonate, and polycarboxylate) was evaluated at 15, 25, and 35?C by means of the flow table test, mixture air content, and compressive strength. Moreover, mixture temperature was monitored and time-temperature curves were plotted in order to assess whether the admixtures affected cement hydration reactions at different temperatures. The final results indicate that an increase in temperature leads to an increase in saturation dosage; lignosulfonate had the most pronounced retarding effect, followed by polycarboxylate, and finally, naphthalene, and considering the weather conditions in the area where the study was carried, the final finding would be that the naphthalene-based admixture had the best performance....
This paper investigates the porosity and the mechanical strength of an Autoclaved Clayey Cellular Concrete (ACCC) with the binder produced with 75wt% kaolinite clay and 25wt% Portland cement. Aluminum powder was used as foaming agent, from 0.2wt% to 0.8wt%, producing specimens with different porosities. The results show that the specimens with higher content of aluminum presented pore coalescence, which can explain the lower porosity of these samples. The porosities obtained with the aluminum contents used in the study were high (approximately 80%), what accounts for the low mechanical strength of the investigated cellular concretes (maximum of 0.62MPa). Nevertheless, comparing the results obtained in this study to the ones for low temperature clayey aerated concrete with similar compositions, it can be observed that autoclaving is effective for increasing the material mechanical strength....
In the context of a growing need for safety and reliability in Civil Engineering, acoustic methods of nondestructive testing provide answers to a real industrial need. Linear indicators (wave speed and attenuation) exhibit a limited sensitivity, unlike nonlinear ones which usually have a far greater dynamic range. This paper illustrates the potential of these indicators, and evaluates its potential for in situ applications. Concrete, a structurally heterogeneous and volumetrically, mechanically damaged material, is an example of a class of materials that exhibit strong multiple scattering as well as significant elastic nonlinear response. In the context of stress monitoring in pre-stressed structures, we show that intense scattering can be applied to robustly determine velocity changes at progressively increasing applied stress using coda wave interferometry and thereby extract nonlinear coefficients. In a second part, we demonstrate the high sensitivity of nonlinear parameters to thermal damage as regard with linear ones. Then, the influence of water content and porosity on these indicators is quantified allowing to uncouple the effect of damage from environmental or structural parameters....
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