Modeling of the Behavior of Concrete Tension Members Reinforced with FRP Rods

The paper is devoted to the analysis of cracking and deformability of concrete tension members reinforced with fiber-reinforced polymer (FRP) rods. A theoretical nonlinear model, derived from a cracking analysis founded on slip and bond stresses, is adopted for evaluating the crack width, crack spacing, and elongation of tension members. The procedure takes into account the local bond-slip law, experimentally determined by means of pullout tests, and allows us to evaluate the influence of tensile stiffening. The analysis is performed with considering all parameters influencing the behavior of tension members, such as the concrete strength, the kind of FRP rebars, the surface treatment of FRP rebars, and the concrete cover thickness. The theoretical predictions are compared with available experimental results, obtained on cylindrical concrete specimens reinforced with carbon FRP (CFRP) rods, and with predictions of the traditional models usually adopted for design purposes.     

Optimization of Multilayered Composite Structures with Randomly Distributed Mechanical Properties

The variability of mechanical properties commonly encountered in composite structures should be taken into account in the optimal design. The present paper discusses such a possibility by using the fuzzy-set and antioptimization approaches. The main attention is focused on the formulation of optimization problems. The formulation and methods proposed are successfully implemented in the buckling analysis of multilayered composite plates. The problem of optimum stacking sequence for plates is also solved. __________ Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 41, No. 6, pp. 753–760, November–December, 2005.     

Some Probabilistic Models of Reinforced Concrete Structures Deterioration

Some probabilistic models for deterioration of reinforced concrete elements (local problem) and whole structures (global problem) caused by the reinforcing steel corrosion and another external and internal influences are proposed. Such problem is important for estimation how the structure reliability changes in time and for planning structure inspections and repairs.     

钢筋混凝土旧工业厂房结构加固方案优选

当前人们选择旧工业厂房结构加固方案效率低下,且所考虑的因素较为片面,为了全面、高效地解决这一问题,通过综合考虑安全性、经济性、时间性、可行性和适用性等因素,以加固效果显著的工程案例为标准,以全面的18个参数为评价指标,构建了一种基于蛛网面积相似和形状相似的结构加固方案优选模型。该模型中首先运用熵权法确定18个指标权重,并将参数值归一化处理,随后运用Python计算蛛网结构相似度,检索出与目标范例最为相似的源范例。最后以结构加固效果显著的工程为例,验证了该模型能有效减少主观误差,具有较强的实用性,并以此模型优选出了某轧钢厂房结构加固方案。     

Bond of FRP Reinforcement in Concrete - a Challenge

The bond of ordinary steel reinforcement in concrete depends on many factors, such as the pullout resistance, the geometry of a concrete member, the placement of a bar in the member cross section, the cover splitting, the confinement caused by concrete and the surrounding reinforcement, the order of bond-crack appearance, and the bond-stress distribution along the bond length. The bond of FRP reinforcement depends on even a greater number of factors. Moreover, the types of FRP bars are numerous. Their surface is weaker than that of steel bars and may fracture by bond forces. The surface of FRP bars is softer and does not create as high local stress concentrations in bond contact points to concrete as the harder steel bars do. This fact often delays the appearance of cover splitting cracks along the bars. However, the load necessary for developing the crack pattern of ultimate splitting failure in concrete is then very dependent on whether the bar surface is glossy or rough. The FRP reinforcement can also be used for external shear and/or flexural strengthening of existing members. For this application, FRP bars are placed in grooves cut on the surface of the member to be strengthened and are fixed there with a cement mortar or epoxy paste. In such an application, the performance of bond between the FRP rod and the mortar or resin and then between the mortar or resin and concrete is critical for the effectiveness of the technique. The presence of two interfaces increases the number of parameters needed to characterize the global joint behavior and introduces new possible failure modes. The fundament for the bond resistance estimation should be an accepted bond philosophy linked to appropriate models. A system of bond tests should provide necessary coefficients for the models.     

Optimal Design of Hole Reinforcements for Composite Structures

In many technical applications like aerospace and automotive structures, holes in thin-walled composite components are necessary for some reason. It easily happens that the presence of a hole results in a detrimental stress concentration in the vicinity of the hole with a strength degradation and premature failure of the structure, respectively. In order to avoid the aforementioned overloading and to achieve a sufficient strength, in practice, a local reinforcement is employed. In the present study, reinforcements by elliptic doublers,as well as doublers adapted to reinforcement requirements in a layerwise manner, are considered. The increasing demands of a low weight and high strength for modern structures lead to the problem of an optimal reinforcement design. For this purpose, an appropriate optimization model is set up, a structural model is developed to describe the mechanical behavior (displacements, stresses, etc.) of such structures, and the techniques of mathematical structural optimization are used to find an optimal design in a systematic manner. In this study, the finite-element method is applied to the structural analysis. Eventually, an appropriate mathematical optimization algorithm is used to approach the desired design optimum in an iterative way. The implemented procedure works with a good reliability and efficiency and yields optimal reinforcement designs which are very useful for direct engineering applications.     

纤维混凝土中应力传递机制的三维弹性理论分析

对纤维混凝土中应力传递机制问题作了分析研究,将应力和位移看作一组“初等解”和一组“修正解”的叠加,“初等解”即为一般的二维理论所得的解答,而“修正解”应用拉甫（Love)位移函数法求得,计算实例表明,解的收敛性良好,将三维弹性理论解与剪滞法的解比较后,可见两种解存在较明显的差别。     

A Deformation Model for Dispersely Failing Elastoplastic Unidirectionally Reinforced Composites

A calculation model is proposed for unidirectionally reinforced elastoplastic composites capable of gradually accumulating disperse microdamages under loading. The composite is assumed to be a homogeneous transversely isotropic solid. To describe its elastoplastic behavior, an incremental plasticity theory with a nonlinear combined hardening mechanism is invoked. At each point of the solid, its damage is characterized by a centrally symmetric scalar function on a unit sphere. This function is approximated by a fourth-rank tensor, which is used for describing the degradation of the elastic properties of the solid due to the accumulation of disperse microdamages. It is shown how to determine, using the known experimental data, all material constants appearing in the theoretical relations suggested.     

Rational Profiling of Reinforced Rotating Disks

The problem of rational profiling of thermoelastic three-layered reinforced rotating disks of gas turbines is formulated. The conditions of equal-stressed reinforcement of the midlayer and the condition of equal-strength binder of the reinforced layer or external isotropic layers are used as criteria for rational designing. An iterative method is proposed for solving such problems. The problem of rational profiling of disks with radial and radially circumferential reinforcements, on condition that the fibers of the radial family are equally stressed, is solved analytically. The calculation results for B-Al and B-Ti disks, obtained by using both the criteria of rational design, are given for plane-parallel and 3D reinforcements. It is shown that, at an equal load-carrying capacity, the weight of such disks can be 30-60% smaller than that of reinforced disks of constant thickness, or, at an equal weight, they can have a considerably higher load-carrying capacity. The criterion of equal-stressed reinforcement, as a rule, is more efficient than the condition of equal-strength binder.