Measuring the Size‐Independent Fracture Energy of Concrete

Abstract: The fracture energy of concrete is an important parameter in the analysis of the mechanical behaviour of concrete structures, so it is considered as a material property. However, the most universal test method for measuring the specific fracture energy of concrete (RILEM work‐of‐fracture) has been a subject of intense debate among researchers. The values have been found to vary with the size and shape of the test specimen. In this study, an experimental comparative analysis of the size‐independent fracture energy obtained by two main methods has been carried out. One of these is based on the local fracture energy model of Hu et al. The second is based on the curtailment of the tail of the P–δ curve by Elices et al. Therefore, the relationship between both methods is highlighted. It is shown that both methods give almost identical results.     

Optical Sensor Developments for Measuring the Surface Strains in Prestressed Concrete Members

Abstract: Conventionally, transfer‐length strain measurements are performed using mechanical gauges such as the Whittemore gauge, or demountable mechanical (DEMEC) strain gauges, and others devices using ‘contact’ measuring principles. These methods involve tedious surface preparation, and are also prone to significant human errors and inaccuracies. Furthermore, these mechanical sensors can only detect lateral displacements. This paper presents a new optical sensor of measuring prestress concrete surface strains. It makes use of the laser‐speckle displacement that is detected by cross correlating the associated optical signals from a Charged‐Coupled Device (CCD) sensor. The sensor was designed to be able to measure the surface displacement components without being affected by other surface motions that are generally present during the concrete detensioning process. Experiments were conducted on a compressed concrete beam and a real prestressed concrete member during the manufacturing process. The results from the optical strain sensor showed good consistency with contact measurements made by using both a foil strain gauge and a Whittemore gauge.     

Analytical and Experimental Investigations on the Heat Transfer Properties of Light Concrete

It is well known that the thermal performance of some insulating and building materials is related to the actual operating conditions because the thermal conductivity of such materials is highly dependent on the moisture content. Since the thermal conductivity of liquid water is about 25 times greater than that of air, it is quite easy to understand how even small variations of the moisture content can have a significant impact on thermal performance. For this reason it is important to find a correlation between the moisture content in a specimen and its thermal conductivity. The purpose of this paper is to investigate both experimentally and theoretically the moisture contribution during the measurement of the heat transfer properties in light concrete slabs (autoclaved concrete and concrete lighted with polystyrene pearls) and to correlate its thermal transmissivity with the moisture content.     

The Identification of a Hidden Long‐Term Plastic Damage Stage During Splitting Tensile Loading of Concrete: A Fracture Mechanics Approach

Abstract: A new method for the determination of fracture toughness K_IC, fracture energy G_F and critical crack tip opening displacement CTOD_c of mortars and concretes with maximum size of aggregates up to 8 mm is presented. This was achieved by using a suitable modification of the conventional splitting tensile test. The values that were obtained are in accordance with those of other researchers. On the basis a clear‐cut fracture mechanics analysis, we extract a set of equations that correlates the above quantities with the crack rate. Specific plastic damage stages that accompany the crack growth were thus easily identified. A pre‐peak damage process of a characteristic bandwidth of 0.1–1 μm and length from a few micrometres to a few millimetres appears first. Damage process of this kind can be diffused and dispersed in bulk of the loading component or it can also be localised and formed into an active attractor cracking path through its chain accumulation. Then another damage stage, coming directly through the formation of the well‐known fracture process zone near the crack tip of 10 μm in bandwidth and a few tens of millimetres in length, must be added. Finally, a large‐scale stage involving the influences of composition and texture of stressed components as well as their loading and geometry conditions must be superimposed on the first two. This third damage process comes directly through the reordering and redistribution of the aforementioned features, during the evolution of fracture, giving an effective length that varies from few tens of millimetres to some metres. All these effective lengths could easily be extracted from the calculated concrete fracture quantities.     

Cut‐off Frequencies Induced by the Length of Strain Gauges Measuring Impact Events

Abstract: The finite length of strain gauges may induce filtering effects when measuring impact events. In this study, we are interested in quantifying these effects. Precisely, we determined the cut‐off frequencies of strain gauges cemented on visco‐elastic bars and measuring impact‐induced strain waves. This study shows that the cut‐off frequencies increase with the bar’s wave velocity and decrease with the bar’s diameter. The asymptotic value, corresponding to an infinite bar diameter, is reached rapidly (bar diameter ≈ 15 mm). Moreover, we showed that the mode cut‐off frequencies are more severe (lower) than the gauge length cut‐off frequencies for bar diameters greater than 8 mm.     

Bond of Reinforcement in Concrete Applied to Concrete Quality Control: The Bottle Bond Test

This paper presents the results of an experimental research dealing with bond strength as a parameter for concrete quality control. To this end, a low‐cost testing technique has been developed: the Bottle Bond Test (BBT). Specimens for the BBT are produced by casting concrete into empty plastic bottles (used as moulds) with a reinforcing bar longitudinally centred. The result is a bottle‐shaped concrete specimen with an embedded rebar, which is pulled out to determine bond strength. Different parameters related to this test setup modify bond strength: their effect has been analyzed. An equation to relate the obtained bond strength values to concrete compressive strength is presented. This equation has been validated with real production data from a readymix concrete plant. Its accuracy and therefore the feasibility of BBT for concrete quality control have been verified. Therefore, the BBT can be an alternative to conventional concrete quality based on uniaxial compression tests.     

碳化混凝土再碱化试验效果的机理分析

通过试验研究和机理分析,验证了混凝土电化学再碱化技术对恢复钢筋周围碱性环境以及腐蚀防护的有效性,得出再碱化过程中电化学作用、电渗作用以及扩散作用是同时存在的,再碱化是电化学、电渗和扩散相互结合共同作用的过程.     

Assessment of the Effectiveness of the Embedded Through‐Section Technique for the Shear Strengthening of Reinforced Concrete Beams

Abstract: Embedded Through‐Section (ETS) technique is a relatively recent shear strengthening strategy for reinforced concrete (RC) beams and consists of opening holes across the depth of the beam’s cross‐section, with the desired inclinations, where bars are introduced and are bonded to the concrete substrate with adhesive materials. To assess the effectiveness of this technique, a comprehensive experimental program composed of 14 RC beams was carried out, and the obtained results confirm the feasibility of the ETS method and revealed that: (i) inclined ETS strengthening bars were more effective than vertical ETS bars, and the shear capacity of the beams has increased with the decrease of the spacing between bars; (ii) brittle shear failure was converted in ductile flexural failure, and (iii) the contribution of the ETS strengthening bars for the beam shear resistance was limited by the concrete crushing or due to the yielding of the longitudinal reinforcement. The applicability of the ACI 318 (2008) and Eurocode 2 (2004) standard specifications for shear resistance was examined, and a good agreement between the experimental and analytical results was obtained.     

碳化混凝土再碱化后钢筋混凝土粘结性能研究

碳化混凝土再碱化过程中,由于物理和化学作用,对钢筋与混凝土的粘结性能有影响。采用拉拔试验,考虑到电流密度、再碱化时间、碳酸钠溶液浓度等因素研究了再碱化后钢筋混凝土的粘结强度。采用扫描电镜(SEM)分析了混凝土的微观结构,结果表明,再碱化后,钢筋混凝土的粘结强度得到明显提高;电流、碳酸钠溶液浓度以及再碱化时间对再碱化后的粘结强度有显著影响;混凝土的微观结构发生了改变,孔隙率减小,有利于粘结强度的增加。     

从头算法研究HEDP中质子传导机理

采用软件Hperchem中的从头算法研究HEDP(羟基亚乙基二膦酸)中质子的传递过程。本文选用了STO-3G、3-21G、6-31G、6-31G*、6-31G**为基组来优化HEDP的分子结构,实验结果表明6-31G为最优基组。利用该最优基组优化结构,得到HEDP单重态稳定结构。通过H+与磷酸中P=O、P-O、P-O缔合得到HEDP的双重态稳定结构。分析H~+与上述三种环境中氧原子的结合能,结果表明H+与C-O中氧原子结合时,分子中多个氢原子处于被激活状态,具有较高能量,容易脱离体系形成H~+,这个过程有利于质子的传递。     

Testing Concrete E‐modulus at Very Early Ages Through Several Techniques: An Inter‐laboratory Comparison

The design of concrete structures is based on calculation rules, which often do not take into account the very early age behaviour of the material. However, during this period, structural concrete is subjected to strains due to the hydration process of cement. If these strains are restrained by concrete itself or surrounding boundaries, stresses start to build up that can lead to the formation of cracks. Among the parameters involved in the stress build up, the stiffness evolution is of major importance. This paper reports the use of eight different techniques aimed at stiffness evolution assessment, applied on the same concrete mix, in a round robin experimental test within three laboratories. The observations are compared after having expressed the results at the same equivalent age. Both the loading stress rate and amplitude are observed to have an effect of limited importance on the determination of the quasi‐static elastic modulus, which might be explained by very short term creep. Ultrasonic measurements provide values of E‐modulus that are higher than the values provided by the quasi‐static tests at the time of the concrete setting. Similar mechanisms associated to very short term creep could explain the difference between the quasi‐static and high‐frequency elastic modulus.     

Determination of the bilinear stress‐crack opening curve for normal‐ and high‐strength concrete

An improved version of the method proposed to ACI committee 446 and to RILEM TC 187‐SOC to determine the fracture parameters of concrete is applied in this study to several mixtures of normal and high‐strength concretes. The results are processed with a C++ program developed by the authors to automatise the mathematical operations required to obtain the bilinear softening curve of concrete from the experimental results. Numerical simulations of the tests are also carried out using finite elements with an embedded cohesive crack. The comparison between numerical and experimental results confirms that the experimental and numerical procedures are appropiate for normal‐strength concretes and high‐strength concretes.     

Assessment of Pre‐Stress Losses in Instrumented Pre‐Stressed Concrete Beams Using Stochastic Analysis

Abstract: In this study, stochastic analyses of pre‐stress losses in two pre‐stressed concrete beams are carried out, and the results of the analyses are compared with those obtained from the laboratory experimental investigations. The deterministic models given in ACI 209 (with modelling error) are used for predicting the pre‐stress losses. The compressive strength of concrete, water–cement ratio, coarse aggregate–cement ratio, fine aggregate–cement ratio, cement content, initial stress in steel and relative humidity are considered as random variables. Monte Carlo simulation technique is used to determine the statistical properties of pre‐stress losses at different times. From the results obtained, it is noted that while the relaxation strain can be assumed to follow a normal distribution, the frequency distributions of creep and shrinkage strains are not uni‐modal. A relation for estimating total losses is also proposed in the article. The stochastic analysis approach presented in the article would be useful in the assessment of pre‐stress losses in existing pre‐stressed concrete members in a more rational way.     

A New Drop‐Weight Impact Machine for Studying Fracture Processes in Structural Concrete

Abstract: This paper describes the main characteristics of a new drop‐weight impact machine that has been specifically designed for studying the dynamic mechanical behaviour of structural concrete samples. Such a type of equipment has been used to generate simple and measurable fracture processes under moderate‐to‐fast loading rates, contrary to blast chambers, which produce complicated crack patterns that are difficult to analyse. The machine consists of two main parts, the mechanical structure and the data acquisition system. The former is just a hammer, guided by two robust columns, which can impact the specimen with energy up to 7860 J. The latter consists of piezoelectric force sensors, accelerometers and optical fibre photoelectric sensors plus oscilloscopes and signal conditioners. The paper also presents the results of some preliminary tests on plain‐notched specimens that show the sensitivity of the work of fracture of a high‐strength concrete to the loading rate.     

Development of a Vibrating Wire Strain Gauge for Measuring Small Strains in Concrete Beams

Abstract:  A vibrating wire strain gauge capable of measuring strains in concrete elements to an accuracy of better than 0.5  μ ɛ is presented. This offers some advantages over conventional electrical resistance gauges, the quoted accuracy of which is typically 3  μ ɛ , and which are often considered unsuitable for concrete because of their inability to span cracks. While vibrating wire gauges are potentially more accurate, they are prone to significant errors because of temperature changes. In the purpose-built gauge described here, temperature correction is achieved using an unstrained reference gauge. The vibration data are analysed using a moving-window Fourier transformation in order to identify and remove the geometrically nonlinear portion of the response. The resulting system is accurate, economical and easy to use. The gauges have been used to study the behaviour of cracked concrete specimens. Typical results are presented and discussed.     

供应链企业间知识传导成本的实证研究

从知识的特征、知识供应商、知识接收方、传导双方的差异以及传导的渠道五个层面探讨供应链企业间知识的传导成本,详细分析五个层面的一系列影响因素和提出相关的假设,并通过上海张江和深圳高新技术产业园区供应链企业的调研问卷收集的数据进行多元OLS回归检验,研究发现相关因素显著地解释了其对供应链企业间知识传导绩效的影响.     

Powder Metallurgical Near‐Net‐Shape Fabrication of Porous NiTi Shape Memory Alloys for Use as Long‐Term Implants by the Combination of the Metal Injection Molding Process with the Space‐Holder Technique

A new method was developed for producing highly porous NiTi for use as an implant material. The combination of the space‐holder technique with the metal injection molding process allows a net‐shape fabrication of geometrically complex samples and the possibility of mass production for porous NiTi. Further, the porosity can be easily adjusted with respect to pore size, pore shape, and total porosity. The influence of the surface properties of powder metallurgical NiTi on the biocompatibility was first examined using human mesenchymal stem cells (hMSCs). It was found that pre‐alloyed NiTi powders with an average particle size smaller than 45 μm led to the surface properties most suitable for the adhesion and proliferation of hMSCs. For the production of highly porous NiTi, different space‐holder materials were investigated regarding low C‐ and O‐impurity contents and the reproducibility of the process. NaCl was the most promising space‐holder material compared to PMMA and saccharose and was used in subsequent studies. In these studies, the influence of the total porosity on the mechanical properties of NiTi is investigated in detail. As a result, bone‐like mechanical properties were achieved by the choice of Ni‐rich NiTi powder and a space‐holder content of 50 vol% with a particle size fraction of 355–500 μm. Pseudoelasticity of up to 6% was achieved in compression tests at 37 °C as well as a bone‐like loading stiffness of 6.5 GPa, a sufficient plateau stress σ₂₅ of 261 MPa and a value for σ₅₀ of 415 MPa. The first biological tests of the porous NiTi samples produced by this method showed promising results regarding proliferation and ingrowth of mesenchymal stem cells, also in the pores of the implant material.     

碳化混凝土再碱化控制理论研究的探讨

再碱化技术应用于实践必须解决的一个重要问题是再碱化过程的控制.在综合分析国内外相关研究的基础上,首先,对不同研究者所做的研究进行了剖析,分析了其研究的路线方法;然后,从控制理论研究的不同方面分析展示了其研究进展;最后,深入分析了各研究,揭示了其中不足之处,指出了再碱化控制理论的发展现状,为进一步研究提供参考.