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.     

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.     

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.     

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.     

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。     

Long‐Range Nanoparticle Surface‐Energy‐Transfer Ruler for Monitoring Photothermal Therapy Response

A recent gold nanotechnology‐driven approach opens up a new possibility for the destruction of cancer cells through photothermal therapy. Ultimately, photothermal therapy may enter into clinical therapy and, as a result, there is an urgent need for techniques to monitor the tumor response to therapy. Driven by this need, a nanoparticle surface‐energy‐transfer (NSET) approach to monitor the photothermal therapy process by measuring a simple fluorescence intensity change is reported. The fluorescence intensity change is due to the light‐controlled photothermal release of single‐stranded DNA/RNA via dehybridization during the therapy process. Time‐dependent results show that just by measuring the fluorescence intensity change, the photothermal therapy response during the therapy process can be monitored. The possible mechanism and operating principle of the NSET assay are discussed. Ultimately, this NSET assay could have enormous potential applications in rapid, on‐site monitoring of the photothermal therapy process, which is critical to providing effective treatment of cancer and multidrug‐resistant bacterial infections.     

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.     

Experimental Investigations of Fracture Process in Concrete by Means of X‐ray Micro‐computed Tomography

The paper describes investigation results on fracture in notched concrete beams under quasi‐static three‐point bending by the X‐ray micro‐computed tomography. The two‐dimensional (2D) and three‐dimensional image procedures were used. Attention was paid to width, length, height and shape of cracks along beam depth. In addition, the displacements on the surface of concrete beams during the deformation process were measured with the 2D digital image correlation technique in order to detect strain localisation before a discrete crack occurred. The 2D fracture patterns in beams were numerically simulated with the finite‐element method using an isotropic damage constitutive model enhanced by a characteristic length of micro‐structure. Concrete was modelled as a random heterogeneous four‐phase material composed of aggregate, cement matrix, interfacial transitional zones and air voids. The advantages of the X‐ray micro‐computed tomography were outlined.     

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。     

Long Electron–Hole Diffusion Length in High‐Quality Lead‐Free Double Perovskite Films

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead‐based perovskite solar cells. Here, the first double perovskite (Cs₂AgBiBr₆) solar cells using the planar structure are demonstrated. The prepared Cs₂AgBiBr₆ films are composed of high‐crystal‐quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high‐quality double perovskite films show long electron–hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO₂ exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.     

一种新颖的光回波损耗测量方法

介绍了一种新颖的光回波损耗测量方法.使用该方法测量光纤器件的回波损耗不但具有小的测量不确定度,而且有效地解决了光回波损耗参数的量值溯源问题.