A Numerical Investigation on the Influence of Steel Fiber Shape and Interface Strength in Reinforced Concrete

Not all steel fiber reinforced concrete composites are equally effective in enhancing structural performance. Their mechanical behaviour strongly depends upon the reinforcement morphology as well as the properties of the interface lying between steel reinforcement and concrete matrix. Using bone-shaped short (BSS) steel fibers, instead of conventional straight short (CSS) steel fibers, to reinforce concrete has demonstrated their potential in improving toughness, ductility and energy absorbing capacity under impact significantly and simultaneously. Accomplishing a strong steel–concrete interface leads to a slight increase in composite strength but simultaneously to a significant decrease in its toughness. Due to the sensitivity of steel reinforced concrete performance on these complex geometric and material parameters, the development of a numerical tool capable of simulating accurately the composite mechanical behaviour and thus leading to optimized design solutions is desirable. The physical problem of the present work involves a typical concrete composite uniformly reinforced with steel fibers subjected to tensional loading. A micromechanical non-linear finite element formulation is utilized in order to predict the load transfer characteristics and the failure process. A linear material behaviour is assumed for the steel component; a non-linear multi-crack material response is used to describe concrete while a mix-mode bilinear behaviour is utilized for the interface providing separation of primary material phases. Numerical results are presented in terms of the global design parameters. The influence of the fiber end shape, the interface strength and the fiber volume fraction on the composite strength and toughness is addressed and consequently optimized design preferences arise.     

Applications of expanded polystyrene (EPS) beads and polyamide-66 in civil engineering,Part One: Lightweight polymeric concrete

EPS concrete is a lightweight material. In this study different sizes of expanded polystyrene (EPS) beads and polyamide-66 yarns were utilized to make a new lightweight concrete. Parts of the results were obtained from our earlier study reported as a novel lightweight concrete essentially having EPS and polyamide-66 yarns. The research showed that largest EPS beads disintegrate along the contact zone and can decrease the adhesion strength. The use of polyamide-66 yarns had a significant influence in crack reduction.     

Measurement of radon emanation in construction materials

Due to the new proposed Israeli approach for radioactivity in construction materials (IS 5098) issued in 2007, the radon emanation of a construction material, a brick in the form of a box (a rectangular parallelepiped), should be checked when its four faces are insulated in order to achieve the same simulation effect when a concrete block is embedded into a wall. A physical model was developed in order to measure the insulation level (quality) of a building product (bulk concrete, building block, tiles, etc.) when its four sides are insulated by various materials. Under the experimental conditions it was found that the insulation level for bulk concrete was 99.5 ± 0.04% which is in good agreement with the requirement of the new Israeli Standard IS 5098. The influence of aging of concrete materials on radon emanation and the emanation ratio, deduced from measurements of insulated and non-insulated materials, was investigated.     

Generation of Rayleigh waves into mortar and concrete samples

The paper deals with a non-destructive method for characterizing the degraded cover of concrete structures using high-frequency ultrasound. In a preliminary study, the authors emphasized on the interest of using higher frequency Rayleigh waves (within the 0.2-1 MHz frequency band) for on-site inspection of concrete structures with subsurface damage. The present study represents a continuation of the previous work and aims at optimizing the generation and reception of Rayleigh waves into mortar and concrete be means of wedge transducers. This is performed experimentally by checking the influence of the wedge material and coupling agent on the surface wave parameters. The selection of the best combination wedge/coupling is performed by searching separately for the best wedge material and the best coupling material. Three wedge materials and five coupling agents were tested. For each setup the five parameters obtained from the surface wave measurement i.e. the frequency band, the maximal available central frequency, the group velocity error and its standard deviation and finally the error in velocity dispersion characteristic were investigated and classed as a function of the wedge material and the coupling agent. The selection criteria were chosen so as to minimize the absorption of both materials, the randomness of measurements and the systematic error of the group velocity and of dispersion characteristic. Among the three tested wedge materials, Teflon was found to be the best. The investigation on the coupling agent shows that the gel type materials are the best solutions. The "thick" materials displaying higher viscosity were found as the worst. The results show also that the use of a thin plastic film combined with the coupling agent even increases the bandwidth and decreases the uncertainty of measurements.     

铝基碳化硼材料中子屏蔽性能的蒙特卡罗模拟

采用蒙特卡罗方法, 运用MCNP4C程序研究了碳化硼含量20%–40%、中子能量200 eV–15 keV、材料厚度0.3–2 cm对B₄C/Al复合材料中子屏蔽性能的影响. 结果表明: 碳化硼含量与中子透射系数呈一次线性下降关系; 同含量的碳化硼, B₄C/Al材料的中子屏蔽效果要大大优于聚乙烯碳化硼材料; 在等厚度条件下, 模拟试样B20等的中子屏蔽效果要优于水、铜、混凝土等常规屏蔽材料; 材料厚度与中子透射系数呈指数下降关系, 且单位厚度的增加对中子透射系数改变很大; 含硼量对热中子透射系数影响很大; 在热中子能区, 中子每单位能量的变化对中子透射系数改变较大; 在慢中子能区, 中子每单位能量的变化对中子透射系数改变很小. 关键词： 中子透射系数 蒙特卡罗 铝基复合材料 碳化硼     

Processing of concretes with a high power CO2 laser

Laser material processing, being a non-contact process, minimizes many of the complexities involved in the decontamination and decommissioning of nuclear facilities. A high power laser beam incident on a concrete surface can produce spalling, glazing or vaporization, depending upon the laser power density and scan speed. This paper presents effect of various laser processing parameters on the efficiency of material removal by surface spalling and glazing. The size of laser beam at constant fluence or energy density had significantly different effect on the spalling process. In thick concrete block cutting the flow or removal of molten material limits the cutting depth. By employing repeated laser glazing followed by mechanical scrubbing process cutting of 150 mm thick concrete block was carried out. Gravitation force was utilized for molten materials to flow out while drilling holes on vertical concrete walls. The dependence of the incident laser beam angle on drilling time was experimentally studied.     

高温高应变率下混凝土材料的损伤演化方程

采用?74 mm大口径分离式霍普金森压杆(SHPB)对不同温度(20、200、400℃)下的C45混凝土材料进行动态力学性能实验,得到了不同温度、不同应变率下混凝土材料的应力-应变曲线。实验结果表明:在20~400℃温度范围内,混凝土材料具有温度硬化和应变率硬化现象。基于上述实验数据给出了损伤变量关于塑性应变的关系式,并通过相关实验数据确定了不同温度、不同应变率下损伤演化方程的材料参数。将该损伤演化方程应用于混凝土材料的本构关系中,预测结果与实验数据具有较好的一致性,证明了所提出的高温、高应变率下混凝土材料损伤演化方程的合理性。     

Analysis of concrete targets with different kinds of reinforcements subjected to blast loading

In this paper we describe an experimental campaign carried out to study and analyse the behaviour of concrete slabs when subjected to blast loading. Four different types of concrete have been tested: normal strength concrete with steel rebar, normal strength concrete with steel rebar retrofitted with Kevlar coating, steel fibre reinforced concrete (SFRC) and polypropylene fibre reinforced concrete (PFRC). The major asset of the experimental setup used is that it allows to subject up to four specimens to the same blast load what, besides being cost effective, makes possible to have a measure of the experimental scatter. The results of SFRC and PFRC concretes have been analysed by using a previously developed material model for the numerical simulation of concrete elements subjected to blast. The experimental campaign and preliminary results of this numerical analysis show how the high strain rates, in spite of improving the mechanical properties of these kinds of fibre reinforced concretes, lead to an embrittlement of the material, which may be dangerous from the point of view of the structural behaviour.     

复合材料弹侵彻混凝土靶的研究

 利用碳纤维复合材料壳体和金属弹头组成的复合弹体,对混凝土靶进行了高速侵彻实验,弹体分别以336、447和517 m/s的速度对强度为30 MPa、厚度为200 mm的混凝土靶进行正侵彻和30°斜侵彻。实验结果表明：碳纤维复合材料壳体具有较高的强度,在高速侵彻靶体的过程中弹体结构能够保持完整,复合材料壳体没有纤维分层和断裂产生。相对于同样结构尺寸的金属弹体（将复合材料壳体替换为密度7.8 g/cm³的金属材料）,复合材料弹填充物的质量分数（18.5%）约为金属弹体的两倍,因此采用轻质高强复合材料替代高密度金属弹身,不仅可以提高弹体装填比、增加比毁伤威力,而且还具有较高的侵彻能力。     

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.