Showing papers on "Embedment published in 2022"

Metal foam-phase change material composites for thermal energy storage: A review of performance parameters

Abstract: Many efforts have been made to improve the weak thermal performance of phase change materials (PCMs). Among the common methods of such, embedment of open-cell metal foams (MFs) into PCMs offer unique opportunities to overcome the issue. The composites made of PCMs and MFs (PCM-MFs) have been extensively studied while behaviour of such composites is yet to be fully understood. There are a variety of parameters affecting thermophysical performance of PCM-MFs, some of which are well-known to date, and some are a subject of controversy. This study provides a detailed review of the parameters affecting thermophysical performance of PCM-MFs. In doing so and directed by literature, the most effective parameters are identified, and all their possible effects are investigated. The common observations along with the contradicting reports are pointed out and discussed in detail. Finally, the current gaps in the field are identified and opportunities for further research work to address them are discussed.

Time-Dependent Fragility Functions for Circular Tunnels in Soft Soils

Abstract: Fragility functions are used in the vulnerability analysis of structures considering different sources of uncertainties. In this research, a framework to develop time-dependent fragility functions for circular tunnels embedded in soft soils is proposed considering the impact of corrosion on the lining reinforcement. Typical shallow and deep circular tunnel sections in soft soils of Shanghai City are used as case studies. The seismic response of the tunnel lining was obtained based on a series of nonlinear dynamic analyses of the soil-tunnel system. The aging effect due to corrosion of the reinforcement bar is considered by decreasing the strength properties of the tunnel lining. Time-dependent fragility curves as a function of free-field peak ground velocity (PGV), as well as fragility surfaces in terms of PGV and service time t, are proposed for minor, moderate, and extensive damage states. The main sources of uncertainty are linked with the input motion and frequency content, the soil properties and response, the tunnel embedment depths, and the estimation of the damage levels. Results show an overall increase in the seismic fragility for both the shallow and deep tunnels over time, emphasizing the significant impact of aging effects on the performance of tunnels. The findings of this study provide an improved understanding of the performance of tunnels exposed to diverse hazards and hence facilitate the life-cycle seismic risk assessment and resilient designs of transport infrastructure.

Scour Propagation Rates around Offshore Pipelines Exposed to Currents by Applying Data-Driven Models

Abstract: Offshore pipelines are occasionally exposed to scouring processes; detrimental impacts on their safety are inevitable. The process of scouring propagation around offshore pipelines is naturally complex and is mainly due to currents and/or waves. There is a considerable demand for the safe design of offshore pipelines exposed to scouring phenomena. Therefore, scouring propagation patterns must be focused on. In the present research, machine learning (ML) models are applied to achieve equations for the prediction of the scouring propagation rate around pipelines due to currents. The approaching flow Froude number, the ratio of embedment depth to pipeline diameter, the Shields parameter, and the current angle of attack to the pipeline were considered the main dimensionless factors from the reliable literature. ML models were developed based on various setting parameters and optimization strategies coming from evolutionary and classification contents. Moreover, the explicit equations yielded from ML models were used to demonstrate how the proposed approaches are in harmony with experimental observations. The performance of ML models was assessed utilizing statistical benchmarks. The results revealed that the equations given by ML models provided reliable and physically consistent predictions of scouring propagation rates regarding their comparison with scouring tests.

Bond-slip behavior of steel reinforced UHPC under flexure: Experiment and prediction

Abstract: Ultra-high performance concrete (UHPC) is an advanced class of concrete materials that show superior mechanical and durability performance. While several studies have investigated the bond-slip behavior of steel reinforced UHPC using pullout tests, very limited information is available on the bond-slip behavior of reinforced UHPC from beam-type tests, which produce flexural stress states commonly seen in structural members. This study experimentally examines the local bond-slip behavior of reinforced UHPC using beam-end tests. The test variables include UHPC material designs (one low-cement, green UHPC mix and one typical UHPC mix), fiber volumes (0%, 0.5%, and 1.0%), cover thickness (16 mm–51 mm), steel bar sizes (16 mm–32 mm), and test ages (3 days and 50 days). A total of thirty-two tests are conducted. While the non-fiber UHPC specimens fail in a brittle manner due to splitting failure, all fiber-reinforced UHPC specimens exhibit confined splitting failure and ductile bond-slip responses. The peak bond strength is found to exhibit a linear relationship with the cover-thickness-to-bar-diameter ratio and the square root of UHPC compressive strength. Furthermore, a local bond-slip model is developed, which includes a new bond-strength prediction method that predicts the experimental results with a mean absolute error of 7.6%. Finally, this local bond-slip model is combined with a partial interaction model to explore the global bond stress variations under different embedment lengths and flexural states. Equations are proposed to predict the development length that can lead to reinforcement yielding with minimized bonded length or minimized global slip.

Nanoengineered smart cement composite for electrical impedance-based monitoring of corrosion progression in structures

Abstract: The aim of this study is to develop smart cement-based composite, as embedded sensor, for the detection and monitoring of corrosion induced damage in structures. A systematic method for development of the sensor through incorporation of functionalized multi-walled carbon nanotubes is presented. In order to develop an efficient sensor, the optimum dosage of CNTs is ascertained through the percolation threshold limit. Prior to employ the developed sensors for damage monitoring, effect of embedment depth, positioning of sensors, and exposure to the saline environment on the electrical characteristics are investigated. The efficacy of the developed sensors for corrosion monitoring is investigated using electrical impedance/conductance technique. The variation in conductance signature is quantified by statistical metrics and change in the bandwidth of the frequencies. The results illustrate the very promising performance of the developed novel cement composite based sensor for electrical impedance-based measurement. It can be effectively used as embedded sensors for quantitative evaluation and temporal tracking of corrosion in structures.

Pullout behaviour of inclined shallow plate anchors in sand

Abstract: This paper presents an experimental study on the pullout behaviour of inclined shallow plate anchors subjected to axial pull in sand. The 1g model tests were performed to examine the effects of anchor inclination and sand–anchor interface conditions on the load–displacement response and the associated failure and deformation mechanisms of plate anchors at various embedment ratios and sand densities. The anchor pullout capacity was found to increase continuously with the load inclination angle to the vertical (α), and the increase was more significant for α from 45° to 90°. The effect of sand–anchor interface conditions was negligible for horizontal plate anchors (α = 0°), but it became increasingly significant at larger inclination angles. The effects of these two factors both decreased with an increasing embedment ratio. Their influences on the failure and deformation mechanisms were measured and analysed using a digital image correlation (DIC) technique. Based on the test data and results available in the literature, a simple empirical method for the prediction of pullout resistance of inclined plate anchors in sand is calibrated and recommended.

Mechanical performance of inclined Dougong bracket sets under vertical load: Experimental tests and finite element calculation

Abstract: This paper presents an investigation on the mechanical performance of inclined Dougong bracket sets under the vertical load. Initial inclination angles of Dougongs are determined based on material test results and embedment mechanism theories. Four full-scale Dougongs with different initial inclination angles are tested under the vertical load. The failure patterns, load-displacement curves, load distribution and displacement analyses are disclosed. Moreover, finite element calculation is conducted, model validation, stress and strain analyses are illustrated. Parametric analyses considering initial inclination angles of Dougongs, compressive strength and elastic moduli of wood, and friction coefficients between embedded surfaces are also carried out. The results indicate that inclined Dougongs under the vertical load behave similarly to a timber block under compression in the perpendicular-to-grain direction. Increasing the initial inclination angles can enhance vertical stiffness to some extent, but the bearing capacities of the inclined Dougongs are not promoted. For inclined Dougongs, part of the load transferred in the width direction is gradually carried by other components in the overhanging direction. The inclined Dougong rotates under the vertical load, and as the load increases, the overall rotation becomes more obvious. Parametric studies show that as initial inclination angles increases, plastic stiffness decreases to some extent. With the increment of compressive strength, elastic moduli, and friction coefficients, ultimate displacement decreases but plastic stiffness obviously increases.

Experimental and Theoretical Research on the Bearing Mechanism of Group Anchors in Sand

Abstract: To investigate the bearing mechanism and efficiency of group anchors in sand, upward pulling tests of model group anchors were carried out for different conditions of the sand density, anchor burial depth ratio, and anchor spacing. The results show that the load-displacement relationships for group anchors is similar to that for single anchors, both being nonlinear for the same relative density and embedment ratio. The load-carrying capacity of group anchors is not a simple superposition of the capacities of two single anchors, but has a clear superposition effect, depending on the relative density, embedment ratio, and anchor spacing. The load carrying capacity increases with the anchor spacing up to a limiting critical value. The bearing mechanism of group anchors was qualitatively analyzed and quantitatively characterized using the strain field and shear stress field obtained through the digital image correlation. Adopting the test data and theoretical derivation, a critical anchor spacing equation is proposed and the relationship between critical anchor spacing, embedment ratio, and relative density is quantitatively characterized. Theory is proposed for predicting the group efficiency of group anchors with different configurations. Comparisons between the results of the developed model and experimental results reported in the literature show good agreement.

Mechanical performance of inclined Dougong bracket sets under vertical load: Experimental tests and finite element calculation

Abstract: This paper presents an investigation on the mechanical performance of inclined Dougong bracket sets under the vertical load. Initial inclination angles of Dougongs are determined based on material test results and embedment mechanism theories. Four full-scale Dougongs with different initial inclination angles are tested under the vertical load. The failure patterns, load-displacement curves, load distribution and displacement analyses are disclosed. Moreover, finite element calculation is conducted, model validation, stress and strain analyses are illustrated. Parametric analyses considering initial inclination angles of Dougongs, compressive strength and elastic moduli of wood, and friction coefficients between embedded surfaces are also carried out. The results indicate that inclined Dougongs under the vertical load behave similarly to a timber block under compression in the perpendicular-to-grain direction. Increasing the initial inclination angles can enhance vertical stiffness to some extent, but the bearing capacities of the inclined Dougongs are not promoted. For inclined Dougongs, part of the load transferred in the width direction is gradually carried by other components in the overhanging direction. The inclined Dougong rotates under the vertical load, and as the load increases, the overall rotation becomes more obvious. Parametric studies show that as initial inclination angles increases, plastic stiffness decreases to some extent. With the increment of compressive strength, elastic moduli, and friction coefficients, ultimate displacement decreases but plastic stiffness obviously increases.

Optimization of hydraulic fracturing with rod-shaped proppants for improved recovery in tight gas reservoirs

Abstract: Abstract Due to the increasing demand and importance of natural gas in the global energy mix, its expeditious recovery is crucial, especially from large-scale unconventional geo-resources. Hydraulic stimulation is an established means of productivity increase especially from tight gas reservoirs. The fracture conductivity generally depends on proppant properties, particularly the shape. Therefore, in this research, the effect of using rod-shaped proppants was investigated. Using rod-shaped proppants instead of conventional spherically shaped proppants, can make a significant difference. Due to the cylindrical shape, higher porosity and permeability are generated, resulting in better conductivity fractures. Thus, to analyze the effect of different proppant shapes on post-fracture performance, a production model was implemented in the FLAC3D plus -TMVOC framework. Later, an in-depth sensitivity analysis was performed to investigate the effects of the proppant shape, size, strength, and effective stress on the fracture aperture reduction and conductivity due to proppant deformation and embedment. The application to a generic model revealed that recovery can be increased by about 7% using aspect ratio 1 rod-shaped proppant with the same diameter as the spherical proppant. Then, increasing the rod-shaped proppant size from an aspect ratio of 1–10 can significantly increase the gas recovery by 13% but results in higher proppant deformation. Finally, the application of rod-shaped proppants to fracturing proposals in well x in a tight gas reservoir of Germany showed that the recovery could be significantly improved if spherical proppants are replaced with rod-shaped proppants.

Experimental and Numerical Investigation of the Performance of Piles and Suction Caissons Subjected to Inclined Cyclic Loading in Cohesive Soils

Abstract: Cyclic loading on caisson and pile anchors can be a significant design issue, particularly with respect to cumulative vertical displacements that can lead to reduction in embedment depth and load capacity of the anchor. Most existing design methodologies are based on monotonic load capacity, often accounting for potential soil strength degradation due to cyclic loading. Because the cyclic strength criterion is often based on cumulative strains, these approaches arguably implicitly consider cumulative deformations. However, cumulative plastic deformations under storm conditions should be investigated. Piles and caissons serving as anchors for floating offshore structures experience inclined cyclic loading. This paper presents (1) monotonic and cyclic loading tests in the vertical and lateral directions, and (2) a finite-element (FE) investigation of different aspect ratios (L/D=4, 5, and 6) of caissons subjected to different load angles (ψ=0° to 45° from the horizontal). Additionally, it presents a precise method to calibrate the constitutive model (nonlinear kinematic hardening model), where the calibrated model was match to monotonic and cyclic lateral and vertical loading tests. Nonuniform load amplitudes were used in the FE investigation. The primary finding from the investigation was that the influence of the cumulative displacements increases when increasing the load amplitudes, especially for the load inclinations of 0°–20° for the lateral displacements and 20°–45° for the vertical displacements.

Scale effects in the post-cracking behaviour of fibre-reinforced concrete beams

Abstract: Abstract The scale effects on the global structural response of fibre-reinforced concrete (FRC) beams subjected to bending are discussed in the framework of Fracture Mechanics by means of the Updated Bridged Crack Model (UBCM). This model predicts different post-cracking regimes depending on two dimensionless numbers: the reinforcement brittleness number , N P , which is related to the fibre volume fraction, V f ; and the pull-out brittleness number , N w , which is related to the fibre embedment length, w c . Both these dimensionless numbers depend on the beam depth, h , which, keeping the other variables to be constant, drives a ductile-to-brittle transition in the post-cracking regime of the composite. The critical value of the reinforcement brittleness number, N PC , allows for prediction of the minimum (critical) specimen size, h min , which, analogously to the minimum fibre volume fraction, V f,min , is required to achieve a stable post-cracking response. Numerical simulations are compared to experimental results reported in the scientific literature, in which FRC specimens, characterized by the same fibre volume fraction but different sizes, are tested in bending.