Piezoelectric-based Crack Detection Techniques of Concrete Structures:Experimental Study

Feasibility of a wave propagation-based active crack detection technique for nondestructive evaluations （NDE） of concrete structures with surface bonded and embedded piezoelectric-ceramic （PZT） patches was studied. At first, the wave propagation mechanisms in concrete were analyzed. Then, an active sensing system with integrated actuators/sensors was constructed. One PZT patch was used as an actuator to generate high frequency waves, and the other PZT patches were used as sensors to detect the propagating wave. Scattered wave signals from the damage can be obtained by subtracting the baseline signal of the intact structure from the recorded signal of the damaged structure. In the experimental study, progressive cracked damage inflicted artificially on the plain concrete beam is assessed by using both lateral and thickness modes of the PZT patches. The results indicate that with the increasing number and severity of cracks, the magnitude of the sensor output decreases for the surface bonded PZT patches, and increases for the embedded PZT patches.

Numerical investigation of temperature gradient-induced thermal stress for steel–concrete composite bridge deck in suspension bridges

A3D finite element model(FEM)with realistic field measurements of temperature distributions is proposed to investigate the thermal stress variation in the steel–concrete composite bridge deck system.First,a brief literaturereview indicates that traditional thermal stress calculation in suspension bridges is based on the2D plane structure with simplified temperature profiles on bridges.Thus,a3D FEM is proposed for accurate stress analysis.The focus is on the incorporation of full field arbitrary temperature profile for the stress analysis.Following this,the effect of realistic temperature distribution on the structure is investigated in detail and an example using field measurements of Aizhai Bridge is integrated with the proposed3D FEM model.Parametric studies are used to illustrate the effect of different parameters on the thermal stress distribution in the bridge structure.Next,the discussion and comparison of the proposed methodology and simplified calculation method in the standard is given.The calculation difference and their potential impact on the structure are shown in detail.Finally,some conclusions and recommendations for future bridge analysis and design are given based on the proposed study.

Solution precision of concrete temperature fields with finite difference method

With the finite difference method to calculate the temperature distribution in mass concrete structures, the solution precision will increase with a smaller step size, at the cost of computational time. In view of the basic characteristics of the finite difference method, a simple yet powerful improvement is introduced. By multiplying the adiabatic temperature function with a correction factor, the precision of the solution can be assured without an increase in the computation time. In addition, the correction rules for three types of commonly used concrete hydration formulas are investigated.

Damage Detection in Reinforced Concrete Berthing Jetty Using a Plasticity Model Approach

A conventional method of damage modeling by a reduction in stiffness is insufficient to model the complex non-linear damage characteristics of concrete material accurately.In this research,the concrete damage plasticity constitutive model is used to develop the numerical model of a deck beam on a berthing jetty in the Abaqus finite element package.The model constitutes a solid section of 3D hexahedral brick elements for concrete material embedded with 2D quadrilateral surface elements as reinforcements.The model was validated against experimental results of a beam of comparable dimensions in a cited literature.The validated beam model is then used in a three-point load test configuration to demonstrate its applicability for preliminary numerical evaluation of damage detection strategy in marine concrete structural health monitoring.The natural frequency was identified to detect the presence of damage and mode shape curvature was found sensitive to the location of damage.

Fiber Concrete under Temperature Drop Load with Stochastic FEM

Plain concrete plate and fiber concrete plate subjected to temperature drop load were analyzed on stochastic finite element method (FEM).It is found that fibers can enhance concrete ability to resist tem- perature drop load for improving concrete's fracture energy and deferring the crack process.It is found for concrete not to improve apparently its tensile strength and fracture energy is recommended to be its appraisal parameter.

An Analytical Method to Detect the Coupling Damage Relationship of Concrete Subjected to Bending Fatigue and Temperature Actions

Based on the assumptions validated by the experiments,an analytical method to detect the coupling actions of bending fatigue and temperature on concrete was proposed.To this purpose,a coefficient denoted by f D （T）with the strain distributions caused by these two actions was defined.In terms of the known parameters and the fitted functions of strain,the explicit expression for f D （T）which develops in the way same as the law of temperature change in the body of specimens was obtained.Our experimental results indicate that the weigh fraction of temperature stress decreases in the coupling damage field with the fading temperature gradient,and consequently disclose the mutual influence between these two types of actions in the loading history.

Detecting Damage in Reinforced Concrete Beams Using Vibrational Characteristics

This paper evaluates two methods of diagnosing damage, Natural frequency and Stiffness-Frequency change-Based damage detection method in reinforced concrete beams under load using vibration characteristics such as natural frequency and mode shape. The research uses finite element method with crack damage instead of deleting or reducing the bearing capacity of the element like in previous studies. First, a theory of the damage diagnosis method based on the change of natural frequency and mode shape is presented. Next, the simulation results of reinforced concrete beams using ANSYS will be compared with the experiment. Particularly, the investigated damage cases are cracks in reinforced concrete beams under loads. Finally, we will evaluate the accuracy of the damage diagnosis methods and suggest the location of the vibration data and specify the failure threshold of the methods.

Crack detection for wading-concrete structures using water irrigation and electric heating

Cracking in wading-concrete structures has a worse impact on structural safety compared with conventional concrete structures.The accurate and timely monitoring of crack development plays a significant role in the safety of wading-concrete engineering.The heat-transfer rate near a crack is related to the flow velocity of the fluid in the crack.Based on this,a novel crack-identification method for underwater concrete structures is presented.This method uses water irrigation to generate seepage at the interface of a crack;then,the heat-dissipation rate in the crack area will increase because of the convective heat-transfer effect near the crack.Crack information can be identified by monitoring the cooling law and leakage flow near cracks.The proposed mobile crack-monitoring system consists of a heating system,temperature-measurement system,and irrigation system.A series of tests was conducted on a reinforcedconcrete beam using this system.The crack-discrimination indexψwas defined,according to the subsection characteristics of the heat-source cooling curve.The effects of the crack width,leakage flow,and relative positions of the heat source and crack onψwere studied.The results showed that the distribution characteristics ofψalong the monitoring line could accurately locate the crack,but not quantify the crack width.However,the leakage flow is sensitive to the crack width and can be used to identify it.

Temperature variation of concrete box girder bridge

On the basis of theoretical analysis and measurement on site,the temperature gradients and its variation on concrete box girder bridges resulting from ambient temperature,solar radiation,and structural temperature fields were discussed.With the help of the heat transfer theory and finite element method(FEM),the heat flux of the surface of concrete box girder was carried out,and all the heat fluxes were described as the convection.The results were compared with that from field measurements,and a good correlation was found.It was revealed that the methods,the parameters,the boundary condition used in this paper were reasonable.Last,some conclusions were obtained,which could provide the basis for box girder design under solar radiation.

基于知识图谱的鼓风机轴承温度智能预测

轴承温度是衡量鼓风机是否正常运行的重要指标之一.然而,轴承通常安装在狭小密闭的空间中,导致其温度难以实时准确检测.为了解决这个问题,设计了基于知识图谱的鼓风机轴承温度智能预测方法.利用统计方法分析鼓风机运行系统,获取与轴承温度相关的影响因素.结合运行机理和领域知识构建知识图谱,提取影响轴承温度的直接和间接特征变量.采用双模块模糊神经网络对知识图谱进行推理,实现对鼓风机轴温的实时准确预测.结果表明,基于知识图谱的鼓风机轴承温度智能预测方法可以准确地建模鼓风机系统,具有良好的温度预测能力.该项研究可以为轴承温度的实时监测和变化趋势预测提供支持.

一种温度补偿调焦红外系统丢包检测方法

在红外技术广泛应用于当前军事、安防、医疗等领域背景下,如何提高红外系统信号的传输质量和传输可靠性,成为了一个亟待解决的问题。本文通过研究温补技术在红外系统中的丢包应用问题,提出了一种失同步触发判断丢包检测方法,并对其性能进行了系统分析、测量和验证。本研究为提高红外系统信号传输质量和信号完整性、可靠性提供了一种新的解决思路和技术方法。

高温后混凝土单轴拉伸和压缩破坏规律及力学性能研究

本文对常温及高温作用后的混凝土进行单轴拉伸和压缩试验,同时使用数字图像相关方法(DIC)观测混凝土试件表面的变形演化过程,探究高温后混凝土拉伸和压缩过程中裂缝的变形演化及力学性能衰减规律。结果表明,随着温度升高,混凝土表面裂缝的萌生和扩展时间逐渐提前,破坏时表面应变集中区的宽度及相应的应变数值逐渐增大,裂缝扩展程度更加显著,当温度达到800℃时,甚至呈多裂缝扩展形态。裂缝的显著扩展增加了混凝土的耗能,使高温损伤后混凝土的延性增加。比较高温后混凝土拉伸和压缩应变云图可知,裂缝开裂引起的应变集中区宽度相近,但压缩试件的应变数值略高于拉伸试件。裂缝的萌生和扩展影响混凝土的力学性能,随着温度升高,混凝土抗拉强度、抗压强度及弹性模量不断下降,峰值应力处的应变逐渐增加;拉伸应力应变曲线由双阶段逐渐转变为三阶段,破坏特征由脆性转变为延性;而压缩应力应变曲线始终为三阶段,与单轴拉伸相比,压缩破坏的延性特征更加明显。

混凝土中氯离子含量研究的关键问题综述

氯离子是影响混凝土结构耐久性的重要因素,合理有效地控制混凝土氯离子含量能够保证建筑使用寿命。因此,充分了解氯离子含量在不同影响因素下的变化规律及作用机理尤为重要。综合评述了混凝土中氯离子含量在不同胶材体系、不同养护温度和不同龄期下的变化规律及其作用机理。其中,不同胶材体系对混凝土氯离子的固化能力不同,粉煤灰、矿渣粉、硅灰等辅助胶凝材料的氯离子结合能力与其活性成分和掺量存在直接关系;氯离子结合能力在1 d龄期前快速增长,28 d后趋于稳定;提高养护温度能够在一定程度上提高氯离子结合能力,但温度过高又会降低其结合能力。此外,测试方法及表征方式的多样化对氯离子含量的研究造成了一定困扰,因此,制定统一的标准对于后续进一步研究具有极其重要的意义。

基于ANN和XGB算法的锈蚀钢筋混凝土高温粘结强度预测方法

为准确评估锈蚀钢筋混凝土(CRC)结构在突发火灾下的结构承载力,锈蚀钢筋混凝土高温粘结强度的统一预测方法研究亟待开展。然而,粘结退化机理复杂,粘结因素众多,实验方法不能考虑所有粘结因素的相关复杂关系的影响。在现有大量试验数据的基础上,采用机器学习方法可以有效地通过数据建立输入和输出特征之间的回归关系。该文利用ANN和XGB两种机器学习算法建立了一个统一的锈蚀钢筋混凝土高温粘结强度预测模型。基于612组高温锈蚀钢筋混凝土的试验研究数据,进行模型训练和测试。结果表明:ML模型的预测结果与实验结果十分吻合。此外,针对机器学习算法本身存在的黑盒子问题,使用SHAP方法来解决锈蚀钢筋混凝土高温粘结强度预测过程中的模型可解释性问题。同时,还将ML模型的计算结果与三种理论计算公式的结果进行了比较,结果表明:ML模型具有明显的优势。新构建的混合机器学习模型很有可能成为准确评估CRC结构经受高温后的损伤程度问题的新选择。

常温养护超高性能混凝土弯拉性能及轴拉本构模型

根据水化反应方程和紧密堆积理论确定了常温养护下超高性能混凝土(UHPC)的基础配合比,基于此考虑水胶比和钢纤维掺量设计了8组UHPC抗弯试件。通过四点弯曲试验,分析了试件的受弯破坏形态、荷载-挠度曲线、弯拉特征参数和弯曲韧性等;基于试验结果采用倒推方法得到了UHPC轴拉应力-应变曲线,采用回归分析提出了考虑纤维特征的轴拉本构模型,并经过材料和构件两个层次的验证。结果表明:掺加钢纤维可抑制主裂缝的发展,从而明显改善UHPC的抗弯韧性,钢纤维掺量为2.0%的UHPC弯曲韧性指数达到116.9 J;随着水胶比增大,试件抗折强度和峰值挠度均呈下降趋势;增大钢纤维掺量明显提升了试件弯曲性能,掺入2.0%钢纤维的UHPC与未掺纤维相比,其初裂挠度和抗折强度分别提升157.14%和148.63%;当纤维含量为1.5%~2.0%时,试件具有良好的弯拉性能;水胶比对曲线平台段趋势影响不大,纤维掺量大于1.0%时曲线具有较明显的应变硬化特征,可保证UHPC良好的抗拉性能,其应变硬化特征随着纤维掺量增大而变得更明显;所提模型对UHPC受拉应力-应变关系具有较好的预测性。

条干均匀度检测方式的研究进展

为进一步提升条干均匀度检测的效率和自动化水平,降低环境温湿度、条干结构状态、光强等因素对条干均匀度检测方式的影响,分析了条干均匀度电容式检测、光电式检测及其他检测方式的原理和特点,讨论了各种检测方式在不同环境条件、检测速度、精度、有效检测区域等方面的优劣,总结了各种条干均匀度检测方式的研究重点与改进措施。认为:检测方式应朝着高精度、高速、在线检测的方向发展,以适应现代纺织对须条条干大定量、快速检测和现场检测的要求。

沙漠砂混凝土高温后强度预测及超声检测研究

为研究沙漠砂混凝土(DSC)高温后抗压性能,以替代率为0%、20%、40%、60%、80%和100%的沙漠砂等质量替代中砂配制DSC,通过试验研究受火温度和沙漠砂替代率(DSRR)对DSC回弹值、超声波速、抗压强度的影响;建立基于灰色系统理论的高温后DSC强度模型,给出DSC超声回弹综合法测强曲线和受火温度计算公式。结果表明,随着受火温度升高,DSC烧失率逐渐增加,表观颜色由深变浅,DSC抗压强度、回弹值、超声波速呈先增大后减小的趋势。随着DSRR增加,DSC回弹值及抗压强度呈先增大后减小的趋势,当DSRR为40%时,DSC抗压强度及回弹值达到最大。NSGM(1,3)模型预测性能优异,平均相对误差为8.6%,可应用于中长期DSC高温后抗压强度预测,给出的DSC超声回弹综合法测强曲线预测精度满足规范要求。

养护方式对混凝土结构温度场及裂缝影响分析

为了探究养护方式对混凝土结构温度场演化规律的影响,依托松江污水池改造工程,设置恒定喷淋、定时喷淋、自然养护、潮湿遮挡养护4种养护方式,研究4种方式下混凝土温度场演化规律,并深入分析温度应力规律及对裂缝产生的影响,指导工程实践。试验结果表明:大气温度及养护条件对结构表面温度有显著影响,自然养护条件下结构表面温度与大气温度数值方面比较接近,其余3种养护条件可显著减缓结构表面温度的变化,并且使最高温度下降了1.5~4.8℃;但对混凝土结构深部温度无明显影响,内部温差在1.5℃以内。自然养护区域温度应力最大,其余3种养护方式(恒定喷淋、定时喷淋、潮湿遮挡养护)最大温度应力分别降低了13.5%、12.9%、13.3%,进而降低了裂缝的产生机率。

自密实混凝土高温冷却后的抗压性能试验研究

为研究高温后自密实混凝土(简称SCC)抗压性能,制作了C35强度标准立方体混凝土试块48个,通过在不同温度下进行模拟火灾试验,分析了自然冷却和水冷却方式对高温后自密实混凝土试块的表观状态、抗压强度、峰值应力和破坏形态的影响。研究结果表明:两种冷却方式下试块的表观裂纹数目随着目标温度的升高而逐渐增多,试块在经历600℃和700℃高温后水冷却过程中,其边角出现明显剥落现象。两种冷却方式下试块的抗压强度呈现降低趋势,反常现象是300℃温度下采用自然冷却试块的抗压强度相较于100℃和200℃温度下的自然冷却试块有明显增强,增幅为8.1%和9.9%。自密实混凝土应力⁃应变曲线的峰值应力随着目标温度的升高而明显减小,所对应的峰值应变显著变大。最后,给出了自密实混凝土轴压应力⁃应变本构方程的参数取值,其拟合曲线能够与试验实测数据较好地吻合。

LNG底板大体积混凝土温度裂缝控制方法研究

基于深圳某下沉式LNG储罐基坑及相关地下结构工程,通过实测温度数据和数值模拟结果分析了不同浇筑方式和不同养护条件等因素对大体积混凝土温度、应力的影响及其变化规律,并分析了对工程最高温度、最大温升等规范要求参数的影响,验证了该基坑底板工程的大体积混凝土温度控制方法的有效性。