Acoustic inspection of bond strength of steel-reinforced mortar after exposure to elevated temperatures

In order to evaluate the bond strength between the reinforcement and concrete after fire damage, a combination of acoustic through-transmission and pull-out tests were used. Previous studies have shown a 25% decrease in the ultrasonic pulse velocity at 90% of the maximum load at room temperature. The specimens were kept in the oven at an elevated temperature for 1, 2, or 3 h. They were then removed and cooled to room temperature. Inspection was conducted using a high-power ultrasonic pulse velocity system while a pull-out load was applied. The correlation between preheated temperature, acoustic wave velocity, and the applied load was analyzed. Initial results show that bond strength and pulse velocity decreased substantially as the temperature or the heating time increased.     

Ultrasonic evaluation of the physical and mechanical properties of granites

Masonry is the oldest building material that survived until today, being used all over the world and being present in the most impressive historical structures as an evidence of spirit of enterprise of ancient cultures. Conservation, rehabilitation and strengthening of the built heritage and protection of human lives are clear demands of modern societies. In this process, the use of nondestructive methods has become much common in the diagnosis of structural integrity of masonry elements.With respect to the evaluation of the stone condition, the ultrasonic pulse velocity is a simple and economical tool. Thus, the central issue of the present paper concerns the evaluation of the suitability of the ultrasonic pulse velocity method for describing the mechanical and physical properties of granites (range size between 0.1-4.0 mm and 0.3-16.5 mm) and for the assessment of its weathering state. The mechanical properties encompass the compressive and tensile strength and modulus of elasticity, and the physical properties include the density and porosity. For this purpose, measurements of the longitudinal ultrasonic pulse velocity with distinct natural frequency of the transducers were carried out on specimens with different size and shape. A discussion of the factors that induce variations on the ultrasonic velocity is also provided.Additionally, statistical correlations between ultrasonic pulse velocity and mechanical and physical properties of granites are presented and discussed. The major output of the work is the confirmation that ultrasonic pulse velocity can be effectively used as a simple and economical nondestructive method for a preliminary prediction of mechanical and physical properties, as well as a tool for the assessment of the weathering changes of granites that occur during the serviceable life. This is of much interest due to the usual difficulties in removing specimens for mechanical characterization.     

Humidity and aggregate content correction factors for air-coupled ultrasonic evaluation of concrete

This paper describes the use of non-contact ultrasound for the evaluation of concrete. Micromachined capacitance transducers are used to transmit ultrasonic longitudinal chirp signals through concrete samples using air as the coupling medium, and a pulse compression technique is then employed for measurement of time of flight through the sample. The effect on the ultrasonic wave speed of storing concrete samples, made with the same water/cement ratio, at different humidity levels is investigated. It is shown that there is a correlation between humidity and speed of sound, allowing a correction factor for humidity to be derived. A strong positive linear correlation between aggregate content and speed of sound was then observed; there was no obvious correlation between compressive strength and speed of sound. The results from the non-contact system are compared with that from a contact system, and conclusions drawn concerning coupling of energy into the samples.     

高温处理后活性粉末混凝土动力学行为及本构模型研究

 利用分离式霍普金森压杆系统,采用铅片作为整形器,分别对常温下及400、600、800 ℃高温处理后的活性粉末混凝土（Reactive Powder Concrete,RPC）试样进行单轴冲击压缩实验,研究高温后RPC材料的动态力学性能,建立高温处理后材料的率型本构模型。结果表明：经不同高温处理后的RPC材料的动态抗压强度和韧性指标均有较明显的应变率敏感性,而峰值应变、初始弹性模量受应变率影响不大;不同应变率下,400 ℃以上高温处理后RPC材料的单轴动态压缩力学性能有所降低。扫描电镜分析表明,高温处理后RPC材料微观结构的劣化是宏观力学性能降低的根本原因。对ZWT粘弹性本构模型进行了修正,修正后的模型适用于混凝土材料经高温处理后的率型本构关系的分析。     

Characterization and hardening of concrete with ultrasonic testing

In this study, we describe a technique which can be used to characterize some relevant properties of 26 cylindrical samples (15 x 30 cm2) of concrete. The characterization has been performed, according to Spanish regulations in force, by some destructive and ultrasound-based techniques using frequencies of 40 kHz. Samples were manufactured using different water/cement ratios (w/c), ranging from 0.48 to 0.80, in order to simulate different values of compressive strength at each sample. We have correlated the propagation velocity v of ultrasonic waves through the samples to compressive strength R values. As some other authors remark, there exists an exponential relationship between the two above parameters. We have found that a highly linear relationship is present between R and w/c concentration at the samples. Nevertheless, when the same linear model is adopted to describe the relationship between v and w/c, the value of r decreases significantly. Thus, we have performed a multiple regression analysis which takes into account the impact of different concrete constituents (water, cement, sand, etc.) on ultrasound propagation speed. One of the most relevant practical issues addressed in our study is the estimation of the hardening curve of concrete, which can be used to quantify the viability of applying the proposed method in a real scenario. Subsequently, we also show a detailed analysis of the temporal evolution of v and R through 61 days, beginning at the date where the samples were manufactured. After analyzing both parameters separately, a double reciprocal relationship is deduced. Using the above parameters, we develop an NDE-based model which can be used to estimate hardening time of concrete samples.     

Water content and its effect on ultrasound propagation in concrete--the possibility of NDE

It is known that water content or moisture affects the strength of concrete. The purpose of this study is to examine the possibility of the NDE of concrete from a knowledge of the relationship between water content and ultrasonic propagation in concrete. The results of measurements made on the ultrasound velocity and the frequency component on ultrasonic propagation as a function of the water content in concrete are reported. Test pieces of concrete made from common materials were made for the fundamental studies. The test piece dimensions were 10 cm in diameter and 20 cm in length. Test pieces were immersed in water for about 50 days to saturate them. To measure the effect of different water contents, test pieces were put in a drying chamber to change the amount of water between measurements. This procedure was repeated until the concrete was completely dried and the weight no longer changed. Water contents were defined as weight percentage to full dried state. Thus water content could be changed from 8% to 0%. Using the pulse transmission method, ultrasonic propagation in the frequency range 20 to 100 kHz was measured as a function of water content. The sound velocity varied gradually from 3000 m/s to 4500 m/s according to the water content. The frequency of maximum transmission also depended on the water content in this frequency range. It is considered that the ultrasonic NDE of concrete strength is feasible.     

A probabilistic mechanical model for prediction of aggregates’ size distribution effect on concrete compressive strength

To predict aggregates’ size distribution effect on the concrete compressive strength, a probabilistic mechanical model is proposed. Within this model, a Voronoi tessellation of a set of non-overlapping and rigid spherical aggregates is used to describe the concrete microstructure. Moreover, aggregates’ diameters are defined as statistical variables and their size distribution function is identified to the experimental sieve curve. Then, an inter-aggregate failure criterion is proposed to describe the compressive-shear crushing of the hardened cement paste when concrete is subjected to uniaxial compression. Using a homogenization approach based on statistical homogenization and on geometrical simplifications, an analytical formula predicting the concrete compressive strength is obtained. This formula highlights the effects of cement paste strength and aggregates’ size distribution and volume fraction on the concrete compressive strength. According to the proposed model, increasing the concrete strength for the same cement paste and the same aggregates’ volume fraction is obtained by decreasing both aggregates’ maximum size and the percentage of coarse aggregates. Finally, the validity of the model has been discussed through a comparison with experimental results (15 concrete compressive strengths ranging between 46 and 106 MPa) taken from literature and showing a good agreement with the model predictions.     

Effects of ultrasonic agitation on adhesion strength of micro electroforming Ni layer on Cu substrate

Micro electroforming is an important technology, which is widely used for fabricating micro metal devices in MEMS. The micro metal devices have the problem of poor adhesion strength, which has dramatically influenced the dimensional accuracy of the devices and seriously limited the development of the micro electroforming technology. In order to improve the adhesion strength, ultrasonic agitation method is applied during the micro electroforming process in this paper. To explore the effect of the ultrasonic agitation, micro electroforming experiments were carried out under ultrasonic and ultrasonic-free conditions. The effects of the ultrasonic agitation on the micro electroforming process were investigated by polarization and alternating current (a.c.) impedance methods. The real surface area of the electroforming layer was measured by cyclic voltammetry method. The compressive stress and the crystallite size of the electroforming layer were measured by X-ray Diffraction (XRD) method. The adhesion strength of the electroforming layer was measured by scratch test. The experimental results show that the imposition of the ultrasonic agitation decreases the polarization overpotential and increases the charge transfer process at the electrode–electrolyte interface during the electroforming process. The ultrasonic agitation increases the crystallite size and the real surface area, and reduces the compressive stress. Then the adhesion strength is improved about 47% by the ultrasonic agitation in average. In addition, mechanisms of the ultrasonic agitation improving the adhesion strength are originally explored in this paper. The mechanisms are that the ultrasonic agitation increases the crystallite size, which reduces the compressive stress. The lower the compressive stress is, the larger the adhesion strength is. Furthermore, the ultrasonic agitation increases the real surface area, enhances the mechanical interlocking strength and consequently increases the adhesion strength. This work contributes to fabricating the electroforming layer with large adhesion strength.     

A new nondestructive technique for measuring pressure in vessels by surface waves

Thin-walled pressure vessels are widely used in modern industry. It is important to measure pressure of the vessels. It has been proved that the velocity of ultrasonic propagation in the material could be affected by the stresses applied to it. The pressure measurement should consider the effect of temperature as the velocity of ultrasonic is affected by the temperature and thermo-stress is produced in the vessels wall. Furthermore, the propagation distance of ultrasonic is influenced by thermal deformation due to temperature changes. In consideration of these influencing factors, a modified model of pressure measurement has been developed, and according to this model, a reference method for the temperature compensation is presented. The relationship between the time delay and pressure was established through this method. The correlative time estimation method based on Hilbert-Huang Transform is presented to estimate the time delay. Therefore, a new method for measuring the pressure of thin-walled vessels nondestructively is presented. Two vessels made of different kinds of materials were used as specimens in our experiments. The results obtain from the tests are used to validate the modified model and demonstrate that the reference method is effective.     

NDT response of spectral analysis of surface wave method to multi-layer thin high-strength concrete structures

This study presents the results of the non-destructive testing using spectral analysis of surface waves (SASW) based on high-strength concrete materials. This SASW method was used to evaluate the compressive strength of single-layer high-strength concrete slabs through a correlation with the surface wave velocities. This paper also presents the relationship between the theoretical and experimental compact dispersion curves when the SASW test is applied to multi-layer thin high-strength concrete slab systems with a finite thickness. The test results show that the surface wave velocity profile obtained from the theoretical dispersion curve has lower values than the profile obtained from the experimental compact dispersion curve under the condition of a finite thickness due to different boundary conditions and reflections from the boundaries. Based on the measured response, an experimental study was conducted to examine if the dispersive characteristics of Rayleigh wave exist in the multi-layer high-strength concrete slab systems. This study can be utilized in examining structural elements of high-strength concrete structures and can also be applied in the integrity analysis of high-strength concrete structures with a finite thickness.     

Anomalous elastic behaviour of Bi-Pb-Ag composites

Bi-Pb and Bi-Pb-Ag superconducting composites have been prepared by the solid state reaction method. After the usual characterization, ultrasonic longitudinal velocity and attenuation studies have been undertaken over a temperature range 80–300 K by the pulse transmission technique. In contrast to normal solids, the ultrasonic velocities of both the samples in the temperature range 200-100 K are found to decrease with decreasing temperature (softening), followed by a velocity maximum. The samples are also found to exhibit longitudinal attenuation peaks at TEMPERATURES = 260, 160 and 120 K. An attempt has also been made to verify whether the Wachtman's equations can theoretically explain the low-temperature behaviour of the Young's modulus of these materials. A qualitative explanation for both the phenomena of softening of velocity as well as the occurrence of attenuation peaks is given.     

激光超声技术测量高温下蓝宝石单晶的弹性模量

利用纳秒激光脉冲所激发的超声纵波, 非接触地测量了蓝宝石单晶沿c轴方向的弹性模量 C₃₃随温度的变化关系. 结果表明, 在室温到1000 ℃ 的范围内, 蓝宝石的弹性模量C₃₃随温度T的升高而减小, 两者之间近似呈如下二次关系: C₃₃ = - 1.541× 10^-5T² - 0.021T + 498.3. 由于该方法利用激光烧蚀效应激发出了强度很大的纵波, 因此对弹性模量的测量具有较高的精度, 估算C₃₃的测量误差不超过0.1%.     

嵌入式超声传感器的混凝土损伤非线性检测研究*

对建筑结构在其生命周期中的损伤状态进行实时监测十分重要,借助埋在混凝土构件内部的压电传感器对结构进行无损检测是一种新型的检测手段。本文将压电传感器嵌入混凝土试件内部,通过超声二次谐波法对信号进行分析处理得到表征材料损伤的非线性参数,以此来监测混凝土的早期强度并对混凝土试件受压和钢筋混凝土梁受弯的不同损伤程度进行检测。结果证明,非线性参数可以反映混凝土养护过程的强度增长情况,也可以表征不同损伤形式下试件的损伤程度。     

Thermographic measurement of the effect of humidity in mortar porosity

The objective of this analysis is to examine the influence of the moisture in the porosity measurement by means of thermal non-destructive test and ultrasound techniques. It is possible to determine the concrete durability by the calculation of its porosity. Porosity is determined in an indirect way, measuring mortar diffusivity by means of active thermography. Using ultrasound techniques, the porosity is related with the ultrasonic propagation of velocity. The diffusivity has been calculated using the W.J. Parker equation. In the ultrasound technique, using the pulse transmission method, ultrasonic propagation velocity was measured as a function of the water content. The conclusions express the correlation between both methods.     

Assessment of strength properties of cemented paste backfill by ultrasonic pulse velocity test

Ultrasonic pulse velocity (UPV) test is one of the most popular non-destructive techniques used in the assessment of the mechanical properties of concrete or rock materials. In this study, the effects of binder type/dosage, water to cement ratio (w/c) and fines content (<20 μm) of the tailings on ultrasonic pulse velocity (UPV) of cemented paste backfill (CPB) samples were investigated and correlated with the corresponding unconfined compressive strength (UCS) data. A total of 96 CPB samples prepared at different mixture properties were subjected to the UPV and UCS tests at 7, 14, 28 and 56-days of curing periods. UPV and UCS of CPB samples of ordinary Portland cement (CEM I 42.5 R) and sulphate resistant cement (SRC 32.5) initially increased rapidly, but, slowed down after 14 days. However, UPV and UCS of CPB samples of the blast furnace slag cement (CEM III/A 42.5 N) steadily increased between 7 and 56 days. Increasing binder dosage or reducing w/c ratio and fines content (<20 μm) increased the UCS and UPV of CPB samples. UPV was found to be particularly sensitive to fines content. UCS data were correlated with the corresponding UPV data. A linear relation appeared to exist between the UCS and UPV of CPB samples. These findings have demonstrated that the UPV test can be reliably used for the estimation of the strength of CPB samples.     

Elasticity and inelasticity of ceramic samples of graphitelike boron nitride

This paper reports a study of the effect of temperature and strain amplitude on Young’s modulus and ultrasonic damping in ceramic samples of graphitelike boron nitride prepared in the RF and USA by different technologies and using various additions to improve the mechanical properties of the ceramic. It is shown that, in addition to the influence on the effective elasticity modulus, additions improve the sample microplasticity, which apparently increases the strength of the material.     

An ultrasonic method for measuring the elastic constants of wood increment cores bored from living trees

Three stiffness moduli and three shear moduli were determined using an ultrasonic velocity method for 5 mm diameter increment cores bored from living beech trees. The validity of the method was assessed by comparing the results for the increment cores with those for standard specimens loaded in static bending and on cubes tested with ultrasonic transverse waves.Good correlation was found between the elastic constants of the cores and standard specimens. It was noted that the regression equation between the modulus of rupture and the longitudinal stiffness modulus for cores, or between the modulus of rupture for standard specimens and longitudinal velocity in cores, permits one to predict the strength of living tree wood through a simple ultrasonic test. The technique is capable of rapidly detecting the differences between individual characteristics of living trees.     

Application of micromechanics to the characterization of mortar by ultrasound

Mechanical properties of concrete and mortar structures can be estimated by ultrasonic non-destructive testing. When the ultrasonic velocity is known, there are standardized methods based on considering the concrete a homogeneous material. Cement composites, however, are heterogeneous and porous, and have a negative effect on the mechanical properties of structures. This work studies the impact of porosity on mechanical properties by considering concrete a multiphase material. A micromechanical model is applied in which the material is considered to consist of two phases: a solid matrix and pores. From this method, a set of expressions is obtained that relates the acoustic velocity and Young's modulus of mortar. Experimental work is based on non-destructive and destructive procedures over mortar samples whose porosity is varied. A comparison is drawn between micromechanical and standard methods, showing positive results for the method here proposed.     

螺栓材料应力与声速、温度关系的测定

基于应力－超声波速度关系方法已用于螺栓轴向应力的测量。而声速与被测材料的许多物理因素有关，如弹性模量，密度与温度。本文介绍了用于分析超声波速度－应力－温度三者关系的实验与计算方法，并给出了三种金属材料的结果。检测的质量。     

Ultrasonic characterization of ceramic fibres at ambient and elevated temperatures

A non-destructive laser-generated ultrasonic inspection system has been developed to evaluate the elastic properties of ceramic fibres. The approach uses a pulsed Nd:YAG laser to excite ultrasonic signals in fibres. The signal is detected by a piezoelectric acoustic emission transducer to obtain the appropriate frequency response suitable for an elastically one-dimensional sample. By using a differential time-of-flight system, a very accurate measure of the velocity can be obtained in the fibre, with a total scatter of less than 0.5%. This approach has been used to investigate the Young's modulus of polycrystalline carbon and boron fibres as a function of stress. Both types of fibres were found to have a Young's modulus increase as greater applied loads were imposed. The carbon and boron fibres, along with silicon carbide fibres, were evaluated at elevated temperatures up to 700 °C. The carbon fibres were found to have an immediate decrease in the Young's modulus as the temperature was increased, due to oxidation of the carbon. The Young's modulus of the boron fibres decreased only at temperatures higher than 200 °C, probably the result of a microstructural transformation or relaxation. The silicon carbide fibres were found to have no significant change in the elastic properties up to 700 °C. The ultrasonic technique was also applied to polycrystalline alumina fibres and fibre tows between ambient temperature and 1200 °C in a specially designed furnace. Using this technique, it was possible to distinguish the changes in the elasticity of the alumina fibres as they were processed into -alumina. The change in the Young's modulus was readily apparent during phase transformations to -alumina. In addition, the ultrasonic velocity can be used to infer information concerning any coatings that were applied to the alumina fibres. This can be used to aid in the quantification of the coating thickness and uniformity. The application of the ultrasonic inspection system has demonstrated the ability to determine rapidly and non-destructively the elastic properties in ceramic fibres. The information gained from the measurements can be used as a quality assurance technique, or can be modified to be a real-time process control/process monitoring system.