Electrical and Acoustic Emissions in cement mortar beams subjected to mechanical loading up to fracture

The application of mechanical loading on cement-based materials generates weak electrical currents due to the formation and propagation of microfractures. This paper introduces the simultaneous recording of electrical signal emissions know as Pressure Stimulated Currents (PSCs) and Acoustic Emissions (AEs) detected in cement mortar beams of rectangular cross-section subjected to mechanical loading using the Three Point Bending tests. The rate of the applied load during the experiments was constant up to the fracture of the specimen. The characteristics of the AE and PSC were put in contrast in order to better understand the evolution of the microfracture processes up to fracture. Specifically, the recorded PSC and AE were studied in terms of their time series, cumulative energy along with AE rate and AE interevent times that may provide information about the upcoming specimen fracture. Moreover, non-extensive statistical physics modeling was attempted in terms of studying the Tsallis entropy. Specifically, the entropic index q was calculated and its dependence on the applied mechanical load was investigated.     

TWINTEX纤维增强塑料的应力腐蚀开裂

研究了TWINTEX纤维增强塑料在酸中的应力腐蚀,并与玻璃纤维增强塑料的应力腐蚀进行了比较.结果表明,TWINTEX纤维增强塑料具有比玻璃纤维增强塑料好得多的耐应力腐蚀性能,而且这种好的耐应力腐蚀性主要表现在长的裂纹孕育期.聚丙烯纤维良好的耐蚀性是造成长的裂纹孕育期的主要原因.外加载荷的变化对TWINTEX纤维增强塑料的应力腐蚀裂纹扩展速率的影响不如其对玻璃纤维增强塑料的应力腐蚀裂纹扩展速率的影响显著.     

Failure behaviors of reinforced concrete beams subjected to high impact loading

To attain a better understanding of the failure behavior of reinforced concrete (RC) beams under impact load, series of high speed impact experiments were performed using an instrumented drop-weight impact machine. The test program was successful in providing a substantial volume of test data including impact loads, mid-span deflections, crack profiles and strains. These data was analyzed, focusing on the impact load characteristics and the impact behaviors of RC beams. Various characteristic values and their relationships were investigated such as the drop height, the static flexural load-carrying capacity, the input impact energy and the beam response values. Two empirical formulas were proposed to estimate the maximum and residual deflection of the beam based on the static flexural load-carrying capacity and the input impact energy. The applicability of the proposed equations was confirmed by comparison with the experimental results obtained by other researchers.     

Eight-year deformation tests on reinforced concrete beams

In 1981 the author reported on the long-term testing of nine reinforced concrete beams. The beams had then been under load for five years. The present article is a report on the next three years of tests on the same nine beams and on the unloading of the beams. The beams all had a rectangular cross-section 170 mm by 280 mm and were simply supported with a span of 7.5 m. The loading varied from self-weight only to the same plus 18 kN. The deflections were measured for 8 years. The tests also included measurements of the shrinkage of control specimens and of the humidity in the room, where the beams were stored. This report describes the experimental procedure and presents the test results.     

Accelerated corrosion cracking tests of steels in active-passive loading

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 2, pp. 53–57, March–April, 1990.     

Short and long-term cracking behaviour of GFRP reinforced concrete beams

A total of ten simply supported beams reinforced with different amounts of GFRP and steel bars were subjected to two consecutive test phases in order to evaluate their short and long-term cracking behaviour. The beams were initially tested up to service load and subjected to two additional load cycles. Subsequently, the specimens were subjected to two different levels of sustained load for 250 days. The effect of cyclic load during short-term tests resulted in an increase in crack width up to 25% more than the initial value. The sustained load led to an increase in crack width up to 2.9 times larger than that measured under the corresponding short-term load. A similar cracking behaviour was observed when reinforcing solutions with similar stiffness (GFRP or steel bars) were used.Existing models to estimate crack spacing and crack width for FRP and steel reinforced concrete elements, including ACI 440.1R-06, Eurocode 2 and Model Code 2010 are discussed and their performance is assessed against the experimental results. Model Code 2010 was found to yield more accurate predictions of the cracking behaviour of the test specimens under both short-term and long-term loading.     

Lattice modelling of corrosion induced cracking and bond in reinforced concrete

A lattice approach is used to describe the mechanical interaction of a corroding reinforcement bar, the surrounding concrete and the interface between steel reinforcement and concrete. The cross-section of the ribbed reinforcement bar is taken to be circular, assuming that the interaction of the ribs and the surrounding concrete can be captured by a cap-plasticity interface model. The expansive corrosion process is represented by an Eigenstrain in the lattice elements forming the interface between concrete and reinforcement. Several pull-out tests with varying degree of corrosion are analysed. The numerical results are compared with experiments reported in the literature. The influence of the properties of concrete are studied. The proposed lattice approach offers insight into corrosion induced cracking and its influence on bond strength.     

Self-centering of shape memory alloy fiber reinforced cement mortar members subjected to strong cyclic loading

In addition to ductility of the reinforced concrete, it is desirable to have large recoverable deformations (self-centering ability) under strong cyclic loading conditions. Since steel reinforcement cannot regain its shape beyond yielding, the performance deteriorates enormously when subjected to a strong cyclic load. An alternative reinforcement material such as shape memory alloy (SMA) could offer scope for self-centering, thus improving performance especially after a severe loading has occurred. In this study, the load-deformation characteristics of SMA fiber reinforced cement mortar beams under cyclic loading were investigated to assess the self-centering mechanism. The study involves experiments on a beam structure and related analysis for the prediction of self-centering. Apart from the energy dissipation through its ductility, it is shown in this study that shape memory alloy reinforced cement mortar has very good self-centering properties that may be crucial in bringing back the functionality of a structure and prevention of permanent secondary deformations that may lead to catastrophic failure in some structures.     

Matrix cracking in transverse layers of cross-ply beams subjected to bending and its effect on vibration frequencies

The solution for the matrix crack distribution in transverse layers of a cross-ply beam subjected to bending is developed based on the model of Han and Hahn (Compos Sci Technol 35 (1989) 377). This solution accounts for a bimodulus-type of the material response. As a result, it is possible to predict the extent and distribution of matrix cracks as well as the stiffness distribution in the beam subjected to bending, accounting for residual thermal stresses. The results of the solution of the bending problem are employed in the analysis of free and forced vibrations of a cross-ply beam with matrix cracks in transverse layers. The solution differs from the case of vibrations of an intact beam due to a difference in response of damaged layers under tension and compression. The results are shown for the natural frequency of a representative ceramic matrix composite (CMC) beam.     

Laminated beams subjected to transverse load

A simple elastic solution has been developed for predicting the shear stress distribution in a laminated beam subjected to a variety of transverse loading conditions. Photoelastic tests on composite models involving a single bonding layer have been used to validate the theory, and also to comment upon the significance of these results in interpreting the interlaminar shear strength and bending modulus of the simplest form of laminated strip.     

U形CFRP条带混锚加固混凝土梁抗剪试验

为解决纯粘贴U形纤维增强聚合物基复合材料（FRP）加固钢筋混凝土梁中FRP端部容易发生剥离破坏等问题,自主研发了对纤维布条带端部进行自锁锚固的方法和锚板,提出了端锚与粘贴并用的混锚U形条带抗剪加固方法。通过2根未加固梁、1根纯粘贴和2根混锚U形碳纤维增强聚合物基复合材料（CFRP）带抗剪加固梁的对比试验,证实了混锚抗剪加固的有效性：混锚能够对纤维带端部进行可靠锚固,阻止端部剥离破坏的发生,实现纤维拉断破坏,大幅度提高材料强度利用率。混锚加固在抑制混凝土梁斜裂缝开展、延缓箍筋屈服、提高箍筋和CFRP的极限应变以及提高抗剪承载力等多个方面的表现均明显优于纯粘贴加固。     

Reliability of reinforced concrete beams in limit state of cracking— failure rate analysis approach

The reliability of a reinforced concrete beam with respect to the limit state of crack width under monotonically increasing short-time loading conditions is estimated using the Monte Carlo technique and failure rate analysis. Using both methods the reliabilities of 24 beams whose test results are available in the literature and three beams tested in the laboratory have been determined for a limiting crack width of 0.2 mm. A comparison of the reliabilities obtained using failure rate analysis with those obtained directly from Monte Carlo simulation shows good agreement. From failure rate analysis it is found that a two-parameter Weibull distribution can be used to describe the failure life of a reinforced concrete beam with respect to the limit state of crack width. The mean moment to failure of each beam determined from failure rate analysis is compared with the experimental moment to failure.     

玄武岩纤维增强泡沫混凝土的单轴拉伸及准静态压缩性能

为研究玄武岩纤维增强泡沫混凝土的力学性能,共设计了52组试件,讨论了玄武岩纤维体积掺量和纤维长度对各密度试件的拉伸和压缩性能的影响。结果表明：玄武岩纤维可显著提高试件的抗拉峰值应力(最大提升达到737%)和峰值应变(最大提升达到833%),可有效改善中高密度试件的受拉失效模式,使其出现伪应变硬化现象,提升了试件的抗拉承载能力和变形能力。试件抗拉峰值应力和峰值应变随纤维体积掺量增大而增大,随纤维长度增长先增大后降低;另一方面,玄武岩纤维能改变试件的受压破坏模式,使其从纵向劈裂破坏转变为斜向剪切破坏和横向压溃破坏,显著提高了中低密度试件的抗压承载力和吸能能力(最大提升达到328%)。试件的吸能能力随纤维体积掺量增大而增强,随纤维长度增长先提升后降低。     

Relaxation characteristics of cement mortar subjected to tensile strain

Tensile relaxation characteristics are important for the crack resistance of concrete members subjected to restrained contraction, for example bonded overlays and patch repairs. In the experimental research discussed in this paper, relaxation characteristics were measured for mortar specimens subjected to constant strain which corresponded to stresses close to the tensile strength of the mortar. Relaxation was found to relieve a considerable portion of tensile stresses. Ultimate relaxation values ranged from 20 to 45%, depending on w/c ratio and specimen age. As expected, a decrease in specimen age and an increase in w/c ratio resulted in increasing relaxation values. The rate of stress decay was found to be rapid, with approximately 80% of the ultimate relaxation occurring in the first 12?h after loading. A basic equation for the prediction of time development of relaxation is proposed and future research needs are discussed.     

Mechanism of local corrosion in corrosion cracking cracks

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 4, pp. 3–8, July–August, 1990.     

Size effect tests of torsional failure of plain and reinforced concrete beams

The current design code formulae for the torsional failure of plain or longitudinally reinforced beams exhibit no size effect, i.e. the failure of geometrically similar beams of different sizes is supposed to occur at the same nominal stress. Experiments on reduced-scale beams were carried out, and the results confirm that there is a significant size effect, such that the nominal stress at failure decreases as the beam size increases. This is found for both plain and longitudinally reinforced beams. The results are consistent with the recently proposed Bažant's size-effect law. However, the scatter of the results and the scope, and range limitations prevent it from being concluded that the applicability of this law is proven.     

球罐应力腐蚀开裂研究与安全性分析

对球罐应力腐蚀开裂的原因和主要影响因素进行了分析.针对16MnR和SPV50Q球罐用钢,在分析湿硫化氢环境下应力腐蚀开裂形式的基础上,通过改进的WOL预裂纹试样的应力腐蚀开裂试验,对不同球罐用钢、不同硫化氢浓度、不同焊接状态条件下的应力腐蚀开裂进行了研究,并对设备的安全性能进行了分析,进而提出了防止应力腐蚀开裂的对策.     

Concrete cover cracking due to uniform reinforcement corrosion

Service life design (SLD) is an important tool for civil engineers to ensure that the structural integrity and functionality of the structure is not compromised within a given time frame, i.e. the service life. In SLD of reinforced concrete structures, reinforcement corrosion is of major concern and reinforcement de-passivation is a frequently used limit state. The present paper investigates an alternative limit state: corrosion-induced cover cracking. Results from numerical simulations of concrete cover cracking due to reinforcement corrosion are presented. The potential additional service life is calculated using literature data on corrosion rate and Faraday’s law. The parameters varied comprise reinforcement diameter, concrete cover thickness and concrete material properties, viz. concrete tensile strength and ductility (plain concrete and fibre reinforced concrete). Results obtained from the numerical simulations reveal that, depending on the serviceability limit state applied, the service life of a reinforced concrete structure can be increased significantly by allowing minor damage of the cover.