Wilkins Aquino

The influence of metakaolin and silica fume on the chemistry of alkali–silica reaction products

Abstract This investigation studies the influence of two mineral admixtures, silica fume (SF) and high-reactivity metakaolin (HRM), on the chemistry of alkali–silica reaction (ASR) products. Four different mortar bar mixes containing different combinations of high-alkali cement, alkali–inert dolomitic limestone, reactive Beltane opal, HRM, and SF were prepared and stored in a 1 N NaOH solution at 80°C (ASTM C 1260) for 21 days. Expansion of bar specimens was measured, and chemical analysis was performed at different ages using X-ray spectra and maps. Test results confirmed that HRM and SF significantly reduce expansion due to ASR. In addition, X-ray microanalysis showed that calcium content increases with time in ASR products. Furthermore, it was found that as ASR proceeded the calcium content of reaction products increased proportionally as the silica content decreased.

Inverse estimation of viscoelastic material properties for solids immersed in fluids using vibroacoustic techniques

This work presents an approach to inversely determine material properties for solids immersed in fluids through the use of steady-state dynamic response. The methodology uses measured acoustic pressure amplitudes in the fluid surrounding a structure being vibrated with a harmonic force to determine the parameters for elastic and viscoelastic material models. Steady-state dynamic finite element analysis is used to compute the frequency response function of homogeneous and heterogeneous solids. The frequency response is then used to inversely estimate material parameters. In order to solve the inverse problem, an optimization method is presented which combines the global search capabilities of the random search method with the reduced computational time of a surrogate model approach. Through numerical and laboratory experiments, this work shows that acoustic emissions hold sufficient information for quantifying both elastic and viscoelastic material behaviors. Furthermore, the examples show that the surroga...

An Inverse Problem Approach for Elasticity Imaging through Vibroacoustics

A methodology for estimating the spatial distribution of elastic moduli using the steady-state dynamic response of solids immersed in fluids is presented. The technique relies on the ensuing acoustic field from a remotely excited solid to inversely estimate the spatial distribution of Youngs modulus of biological structures (e.g., breast tissue). This work proposes the use of Gaussian radial basis functions (GRBF) to represent the spatial variation of elastic moduli. GRBF are shown to possess the advantage of representing smooth functions with quasi-compact support and can efficiently represent elastic moduli distributions such as those that occur in soft biological tissue in the presence of unhealthy tissue (e.g., tumors and calcifications). The direct problem consists of a coupled acoustic-structure interaction boundary-value problem solved in the frequency domain using the finite element method. The inverse problem is cast as an optimization problem in which the error functional is defined as a measure of discrepancy between an experimentally measured response and a finite element representation of the system. Nongradient based optimization algorithms are used to solve the resulting optimization problem. The feasibility of the proposed approach is demonstrated through a series of simulations and an experiment. For comparison purposes, the surface velocity response was also used for the inverse characterization as the measured response in place of the acoustic pressure.

A Review of Vibro-acoustography and its Applications in Medicine.

In recent years, several new techniques based on the radiation force of ultrasound have been developed. Vibro-acoustography is a speckle-free ultrasound based imaging modality that can visualize normal and abnormal soft tissue through mapping the acoustic response of the object to a harmonic radiation force induced by ultrasound. In vibro-acoustography, the ultrasound energy is converted from high ultrasound frequencies to a low acoustic frequency (acoustic emission) that is often two orders of magnitude smaller than the ultrasound frequency. The acoustic emission is normally detected by a hydrophone. In medical imaging, vibroacoustography has been tested on breast, prostate, arteries, liver, and thyroid. These studies have shown that vibro-acoustic data can be used for quantitative evaluation of elastic properties. This paper presents an overview of vibro-acoustography and its applications in the areas of biomedicine.

Seismic Retrofitting of Corroded Reinforced Concrete Columns Using Carbon Composites

This paper presents an experimental study on the behavior of large-scale corroded reinforced concrete columns and the feasibility of using carbon composites to restore their seismic effectiveness. Large-diameter reinforced concrete columns were corroded using external currents, repaired with different layouts of carbon composite material, and then tested to failure under lateral cyclic loading. Bond degradation due to corrosion dictated the losses in ductility and load capacities for the corroded columns. Findings showed that advanced composite materials are a viable alternative for the repair and seismic upgrading of corroded columns. Columns retrofitted with carbon composite wraps had load and ductility capacities matching or exceeding those expected for an undamaged seismically designed column. The test results also indicate that the use of external currents is a feasible way to induce corrosion in large-scale laboratory tests.

MOISTURE DISTRIBUTION IN PARTIALLY ENCLOSED CONCRETE

Most common deterioration problems with reinforced concrete, such as corrosion, alkali-silica reaction, freezing/thawing, sulfate attack, and others, are associated with the presence of elevated moisture contents in the concrete microstructure. An analytical and experimental study on moisture distribution in partially enclosed drying concrete is presented in this paper. The nonlinear diffusion theory was used along with relative humidity measurements to study the problem. Circular concrete columns jacketed with different patterns of impermeable bands were analyzed under simulated, fixed environmental conditions. The clear spacing of the bands was found to have negligible influence on the rate of moisture dissipation from areas covered by the impermeable bands. Conversely, the width of the bands was found to have a significant effect on the rate of drying from the covered areas. In addition, the effect of the area covered by the impermeable bands decreased with increasing depth from the surface of the concrete. To validate analytical results, concrete cylinders were cast, cured, enclosed partially or fully with impermeable bands, and then exposed to a fixed relative humidity environment. Moisture distribution within the cylinders over time was measured using a commercial hygrometer and compared to the numerical predictions. The experimental data confirms the findings from the numerical studies.

Identification of material properties of orthotropic elastic cylinders immersed in fluid using vibroacoustic techniques

A numerical study is presented to show the potential for using vibroacoustic-based experiments to identify elastic material properties of orthotropic cylindrical vessels immersed in fluids. Sensitivity analyses and a simulated inverse problem are shown to quantify the potential for material characterization through the use of acoustic emissions. For comparison purposes, the analyses are also shown with the normal component of the velocity at the surface of the cylinder as the measured response in place of the acoustic pressure. The simulated experiment consisted of an orthotropic cylinder immersed in water with an impact force applied to the surface of the cylinder. The material parameters of the cylinder considered in the analyses were the circumferential and longitudinal elastic moduli, and the in-plane shear modulus. The velocity response is shown to provide sufficient information for characterizing all three moduli from a single experiment. Alternatively, the acoustic pressure response is shown to provide sufficient information for characterizing only the two elastic moduli from a single experiment. The analyses show that the acoustic pressure response does not have sufficient sensitivity to the in-plane shear modulus for characterization purposes.

A source sensitivity approach for source localization in steady-state linear systems

Localizing sources in physical systems represents a class of inverse problems with broad scientific and engineering applications. This paper is concerned with the development of a non-iterative source sensitivity approach for the localization of sources in linear systems under steady-state. We show that our proposed approach can be applied to a broad class of physical problems, ranging from source localization in elastodynamics and acoustics to source detection in heat/mass transport problems. The source sensitivity field introduced in this paper represents the change of a cost functional caused by the appearance of an infinitesimal (or point) source in a given domain (or its boundary). In order to extract macroscopic inferences, we apply a threshold to the source sensitivity field in a way that parallels the application of the topological derivative concept in shape identification. We establish precise formulas for the source sensitivity field using a direct approach and a Lagrangian formulation. We show that computing the source sensitivity field entails just obtaining the solution of a single adjoint problem. Hence, the computational expense of obtaining the source sensitivity is of the same order as that of solving one forward problem. We illustrate the performance of the method through numerical examples drawn from the areas of elastodynamics, acoustics, and heat/mass transport. Our results show that our proposed approach could be used on its own as a source detection tool or to obtain initial guesses for more quantitative iterative gradient-based minimization strategies.

Measurement of biaxial mechanical properties of soft tubes and arteries using piezoelectric elements and sonometry.

Arterial elasticity has gained importance in recent decades because it has been shown to be an independent predictor of cardiovascular diseases. Several in vivo and ex vivo techniques have been developed to characterize the elastic properties of vessels. In vivo techniques tend to ignore the anisotropy of the mechanical properties in the vessel wall, and therefore fail to characterize elasticity in different directions. Ex vivo techniques have been focused on studying the mechanical properties in different axes. In this paper, we present a technique that uses piezoelectric elements to measure the elasticity of soft tubes and excised arteries in two directions while maintaining the natural structure of these vessels. This technique uses sonometry data from piezoelectric elements to measure the strain in the longitudinal and circumferential directions while the tubes/arteries are being pressurized. We conducted experiments on urethane tubes to evaluate the technique and compared the experimental results with mechanical testing done on the materials used for making the tubes. We then performed sonometry experiments on excised pig carotid arteries assuming that they are transversely isotropic materials. To evaluate the sensitivity of this technique to changes in the material properties, we changed the temperature of the saline bath in which the arteries were immersed. The calculated Youngs modulus from sonometry experiments for the urethane tubes and the mechanical testing values showed good agreement, deviating no more than 13.1%. The elasticity values from the excised arteries and the behavior with the temperature changed agreed with previous work done in similar arteries. Therefore, we propose this technique for nondestructive testing of the biaxial properties of soft material tubes and excised arteries in their natural physiological shape.

Thermal Safety of Vibro-Acoustography Using a Confocal Transducer

Vibro-acoustography (VA) is an imaging method that forms a two-dimensional (2-D) image by moving two cofocused ultrasound beams with slightly different frequencies over the object in a C-scan format and recording acoustic emission from the focal region at the difference frequency. This article studies tissue heating due to a VA scan using a concentric confocal transducer. The three-dimensional (3-D) ultrasound intensity field calculated by Field II is used with the bio-heat equation to estimate tissue heating due to ultrasound absorption. Results calculated with thermal conduction and with blood perfusion, with conduction and without perfusion and without conduction and without perfusion are compared. Maximum heating due to ultrasound absorption occurs in the transducers near-field and maximum temperature rise in soft tissue during a single VA scan is below 0.05 degrees C for all three attenuation coefficients evaluated: 0.3, 0.5 and 0.7 dB/cm/MHz. Transducer self-heating during a single VA scan measured by a thermocouple is less than 0.27 degrees C.