RC Elements Research Articles

Effect of Beam Depth on Shear Behavior of FRP RC Beams

AbstractThe behavior of shear critical fiber-reinforced-polymer (FRP) RC elements is characterized by the development of comparatively large strains and crack widths, which can be strongly influenc...

Image Post-Processing Method for Quantification of Cracking in RC Precast Beams under Bending

Image processing methods are increasingly used in civil engineering, especially in the maintenance of concrete structures. Current digital cameras and post-processing methods allow verifying qualitatively the state of conservation of wide areas of concrete in dams and bridges. When dealing with building refurbishments and rehabilitation, it is important to verify that existing structural elements fit the requirements of the standards; in the case of structures formed by traditional RC joists, cracking of the bottom-face provides information about the serviceability of these elements. This research proposed and put in practice through experimental tests an image post-processing method for quantification of cracking (five specimens were used and calibrated). Based on a sequence of shots and through a complex step-by-step post-processing, cracks were identified and measured to calibrate this method for real purposes. The method quantifies the crack opening width and spacing by analyzing the bottom-face of the joists through the shots. Measured values of crack spacing are very similar to those predicted by the standards, while the values of crack opening width differ more from theoretical ones due to the scattering of results. However, the proposed method has been proved as suitable and useful for fast inspections of RC elements under bending.

Insight into the tension stiffening behavior of reinforced concrete tension members revealed by finite element modeling

The disasters happened in recent past pointed out the need of design criteria, ensuring adequate safety levels against progressive collapse. The attention was focused on the behavior of composite beam-to-column joint components in the field of large displacements. This article presents the experimental and numerical study enabling the simulation of the RC elements under tension. This has helped in understanding the non-negligible contribution of concrete in tension stiffening response up to failure especially in the case of discontinuous geometry marked in composite structures. The finite element model proposed may be considered a mid-way between smeared and discrete crack modeling approaches.

Primjena 2D digitalne analize slike za mjerenje pomaka i širina pukotina na AB elementima

Primjena 2D digitalne analize slike za mjerenje pomaka i širina pukotina na AB elementima

Simulation of spatially non-uniform frost damage in RC beams under various exposure and confining conditions

This paper aims to investigate the spatially non-uniform frost damage process in RC beams during large numbers of freeze/thaw cycles (FTC). The material-based thermodynamic and poromechanical frost damage models have been up-scaled to the structural level considering the anisotropic stress-strain conditions and the condensed water movement in the crack system. The ice-induced pore pressures are also integrated in the smeared RC element in consideration of the path-dependent nonlinear constitutive laws. The frost damage accumulation processes (referred as the expansive strain) are simulated with different depths of heat and moisture exposure, as well as various arrangements of reinforcing bars. Based on the proposed empirical model which links the expansive strain with the reduction in ultrasonic velocity, the 3D frost damage profiles are compared with 2D ultrasonic velocity data, which show a satisfactory agreement. Finally, the effects of different steel and structural confinements on the frost damage profile are numerically investigated and discussed in detail.

Structural, magnetic and dielectric properties of the novel magnetic spinel compounds ZnCoSnO4 and ZnCoTiO4

The transparent semiconductor Zn2SnO4 with cubic spinel structure and the isostructural Zn2TiO4 have been magnetically doped with Co2+. ZnCoSnO4 and ZnCoTiO4 exhibit ferrimagnetism below TN ≈ 13 K and TN ≈ 17 K. Ferrimagnetic moments are evident in M vs H curves below TN by small hysteresis. Fits to strictly linear Curie-Weiss plots above TN give μeff ≈ 4.86 μB and ≈4.91 μB for ZnCoSnO4 and ZnCoTiO4, above theoretical predictions. Impedance spectroscopy data from sintered ceramic can be fitted with a standard equivalent circuit model based on two RC elements for bulk and GB areas. The relative dielectric permittivity of the bulk is ≈20 and ≈30 for Zn2SnO4 and Zn2TiO4. The semiconducting ZnCoSnO4 and ZnCoTiO4ceramics exhibit bulk resistivity of ≈1 106 Ω cm and ≈1 105 Ω cm at 560 K (287 °C), and bulk activation energies of EA ≈ 1.2 eV and 1.1 eV.

General solution for shear strength estimate of RC elements based on panel response

The capacity of structural systems subjected to shear loads commonly distinguished by discontinuities such as point loads or supports, or abrupt changes of cross section, where complex fields of stresses and strains are generated, is vital information for design. Four structural systems that present stress concentration due to applied shear loads are commonly short walls, deep beams, corbels and beam-column joints. In the present work a model is developed to predict the shear capacity of these elements based on a panel model that considers average strain and stresses in a reinforced concrete orthotropic material, which covers the section of the structural element subjected to stress concentration. In addition, the panel element complies with the longitudinal equilibrium, by equalizing the applied axial load with the internal stresses of the structural element, requiring constitutive material laws for both concrete and steel reinforcement. The original model that has shown good shear strength prediction requires solving the non-linear equation of vertical equilibrium. Thus, this work eliminates the need to solve the iterative problem for the capacity estimation of four possible limit states (failure of concrete in tension and compression, and yielding of longitudinal web and boundary reinforcement). For that, an expression is calibrated for the strain of the model with respect to relevant parameters, for each limit state, that allow the generation of a non-iterative model. The model results in an average predicted capacity over experimental capacity ratio, Vmodel/Vtest, of 1.0 and a COV of 0.25, with similar performance for all four structural systems. When comparing these results with the general model that requires an iterative method, a similar performance is observed, with an average strength ratio and COV of 0.98 and 0.23, respectively. Likewise, in comparison with the ACI 318, the latter shows worse predictions (on average 24% lower) and with greater scatter (on average 28% higher). The expression in AASHTO code presents better correlation than ACI with predictions closer the proposed model.

Size effect on the plastic rotational capacity of RC elements: critical review of EC8

The ductility is an important parameter in the seismic design process of reinforced concrete structures. Under seismic loadings, ductility is required to allow a significant deformation and energy dissipation before the collapse. The plastic design of RC elements under bending loads is based on the formation of plastic hinges. The dissipation is characterised by the rotational capacity of these zones. The non-linearity of these dissipative zones is described by the moment-curvature laws (or moment-rotation curves). The Eurocode 8 (EC8) design code gives empirical formulas to evaluate the moment-rotation curves. The main disadvantage of using this practical design code is that the size effect is not taken into account in evaluating the plastic rotational capacity. In the present paper, a critical review of EC8 regarding the size dependency of the plastic rotational capacity is firstly presented. Furthermore, a performance simplified model is developed to assess the phenomena of size effects and its influence on the ductility behaviour of RC elements under simple bending (without interaction). A comparison with the experimental results from literature shows the ability of the proposed model to describe the size dependency phenomenon. Based on the new developments, news formulas are proposed to improve the EC 8. Finally, a real-life engineering application is carried out in order to highlight the benefits of the EC8 improvements proposed.

Shear transfer capacity of reinforced concrete exposed to fire

Shear transfer capacity of reinforced concrete elements is a function of concrete compressive strength and reinforcement yield strength. Exposure of concrete and steel to elevated temperature reduces their mechanical properties resulting in reduced shear transfer capacity of RC elements. The objective of present study is to find the effect of elevated temperature on shear transfer capacity of reinforced concrete. For this purpose pushoff specimens were casted using normal strength concrete. After curing, specimens were heated to 250°C and 500°C in an electric furnace. Cooled specimens were tested for shear transfer capacity in a universal testing machine. It was found that shear transfer capacity and stiffness (slope of load-slip curve) were reduced when the specimens were heated to 250°C and 500°C. Load level for the initiation of crack slip was found to be decreased as the temperature was increased. A simple analytical approach is also proposed to predict the shear transfer capacity of reinforced concrete after elevated temperature.

Local effects on RC frames induced by AAC masonry infills through FEM simulation of in-plane tests

Unreinforced masonry infills are widely used in many parts of the world and it is common practice for seismic design to use simplified methods that usually do not take into account the interaction between the infill and the structure. Starting from the 1950s, many researchers have investigated the lateral response of masonry infills focusing on several different topics. The scientific interest on masonry infills is continuously raising due to the unsatisfactory seismic response of the infilled frame structures observed during post-event inspections and to the difficulty to contrive a widely scientifically and practical recognized solution. Although some modern codes consider the presence of infills with some specifications to prevent damage in the masonry panels and global and local effects on the structure, an effective evaluation of these detrimental effects has not been achieved yet. Within this paper, a FEM simulation of in-plane pseudo-static cyclic tests on a RC frame specimen infilled with unreinforced Autoclaved Aerated Concrete (AAC) masonry infill has been performed in order to study accurately the influence and the interaction of the infill with the RC structure. The experimental results performed by Calvi and Bolognini (J Earthq Eng 5:153–185, 1999), and Penna and Calvi (Campagna sperimentale su telai in c.a. con tamponamenti in Gasbeton (AAC) con diverse soluzioni di rinforzo” (in Italian), 2006) on one-bay one-storey full scale specimens are taken as reference. Non-linear static analyses using a “meso-modelling” approach have been carried out. The masonry used in the model has been calibrated according to tests of mechanical characterization and to in-plane cyclic tests on load-bearing AAC masonry conducted by Costa et al. (J Earthq Eng 15:1–31, 2011). The analyses performed have allowed to investigate the local effects on the frame and, in particular, the changes in the moment and shear demands on the RC elements due to the presence of the AAC infill in comparison with the ones in the bare structure, and to estimate the thrust and the contact length activated by the infill on the frame.

Behavior of FRP-strengthened RC elements subjected to pure shear

The shear behavior of fiber reinforced polymer strengthened reinforced concrete (FRP-strengthened RC) has been the focus of extensive research studies. However, the mechanism of this complex phenomenon has not been fully clarified. Recent analytical models which were developed for predicting the shear capacity of FRP strengthened RC girders were based on test results of simply supported beam specimens with various shear span-to-depth (a/d) ratios. In such tests no region of the specimen is subjected to uniform stress conditions, Therefore, the results of such tests cannot predict the true pure shear behavior due to non-uniformity of stresses, the presence of flexural and other non-shear related effects such as a/d ratio that cannot be filtered out. Therefore, proper design of shear strengthening using FRP requires testing of elements that are subjected to pure shear case primary before adding other governing effects. This allows a careful investigation and full understanding of the behavior at the element level. In order to accomplish this task, panel testing of representative RC specimens strengthened with FRP sheets were needed. This paper reports the testing of 10 FRP strengthened RC panels subjected to pure shear stress field. The tests were carried out to evaluate the effects of three variables: FRP stiffness, FRP wrapping scheme, and transverse steel reinforcement ratio. The test results showed that these three variables greatly affected the shear behavior due to various types of failure modes associated with FRP strengthening. In addition, it was observed that the magnitude of increased shear capacity associated with the application of FRP sheets depends not only upon the stiffness of FRP, but also on the stiffness of internal shear reinforcement. With the increase of internal steel shear reinforcement, the effectiveness of shear gain due to externally bonded FRP decreases.

Numerical simulation of the probability of corrosion initiation of RC elements made of reinforcing steel with improved corrosion performance

Steel reinforcing bars in concrete are protected from corrosion by a thin oxide film that is created on the steel surface in the highly alkaline environment of the concrete. Corrosion process begins once this oxide film is destroyed, for example, by chloride ions penetrating from the element surface into the concrete and to the steel rebar. Critical chloride concentration is highly variable owing to numerous factors such as the type of reinforcing steel used. The TransChlor® software is used to simulate the time history of chloride ion ingress and corrosion initiation into a typical reinforced concrete structural element with four different types of reinforcing steel, namely, carbon steel, ferritic stainless steel (1.4003) with rolling skin, duplex stainless steel (1.4462) and ferritic stainless steel (1.4003) with pickled surface. A climate model is used to simulate two different exposure conditions of the structural element, namely direct and mist exposure, representatives for winter conditions in mountainous and lowland areas. A probabilistic method is applied to consider uncertainty of material properties in the model. The results show that reinforcing steel bars with enhanced corrosion performance significantly improve the durability of reinforced concrete.

A critical assessment of the compressive behavior of reinforced recycled aggregate concrete columns

This paper presents a critical assessment of the compressive behavior of reinforced recycled aggregate concrete (RRAC) columns. Previous research has demonstrated that recycled aggregate concrete (RAC), manufactured using crushed concrete aggregates obtained from construction and demolition waste, can be a feasible and environmentally friendly alternative to conventional concrete for the use in structural applications. Currently, very limited information is available in the literature about the methods to be used for designing RC elements manufactured with recycled concrete aggregates. With the aim of extending the available experimental data, a fiber section based nonlinear finite element (FE) model was developed so as to provide in-depth insights into the compressive behavior of RRAC columns. First, a comparison between simulation results of the compressive behavior of RRAC columns and available experimental results were made to validate the FE model. Then, using the validated FE model, a parametric study was performed to investigate the effects of different recycled concrete aggregate (RCA) contents, volumetric ratio and yield strength of longitudinal steel reinforcement, and spacing and yield strength of transverse steel reinforcement on the compressive behavior and load capacity of RRAC columns. Finally, based on the limited available experimental database, a grey correlation analysis was used to evaluate the parametric sensitivity of the compressive behavior of RRAC columns. The experimental and numerical investigations demonstrate that the method used for considering the additional water absorption of RCAs in manufacturing RAC has a significant influence on the compressive load capacity of RRAC columns and that the influence of the RCA content on the compressive load capacity of RRAC columns is lower when compared with other important parameters but this effect should not be ignored in modeling the behavior of RRAC columns.

Required tie spacing to prevent inelastic local buckling of longitudinal reinforcements in RC and FRC elements

Compressed reinforcement buckling in concrete columns can reduce ductility in structural elements. In order to avoid this, design codes propose maximum required tie spacing. Nonetheless, they do not incorporate the positive effect of concrete fibers in their formulation, whose capability of delaying buckling has been proved. For this reason, recommendations for maximum required tie spacing for elements made with concrete, with or without steel fibers, and with normal or high strength, are proposed in this article. In order to achieve this, the mixed model proposed by Pereiro-Barceló and Bonet was extended to consider elements made of HSC with and without steel fibers thanks to the results of an experimental campaign of HSC columns with and without fibers, under monotonic loading. In these tests, the buckling critical load in compressed reinforcement was experimentally determined in all the columns. In addition, a comparison of the proposed transverse concrete separation with respect to the recommendations proposed by the main existing codes was made.

A material-performance-based database for FRC and RC elements under shear loading

The shear strength of elements reinforced by fibres is predicted by Codes using formulations generally developed from a limited set of test results. In fact, only few of available test results are combined with a material mechanical characterization, allowing to evaluate and compare the different performances of Fibre Reinforced Concretes (FRC). To address this problem, a material-performance-based shear database for FRC elements and their related reference samples in Reinforced Concrete (RC, with and without web reinforcement) is presented herein, merging the experiences carried out in the last decade at the University of Brescia and at the Universitat Politecnica de Valencia. The database is composed by 171 specimens: 93 in FRC and 78 in RC with or without web reinforcement. For FRC elements, the post-cracking resistance (fR,1 and fR,3) is also given according to EN 14651 standard. The evaluation of the shear database was also carried out, discussing the influence of the different factors affecting the shear strength both in FRC and RC samples. Finally, the two formulations suggested by Model Code 2010 for FRC elements are compared against the database results in order to shed new light on code requirements.

Application of FE approach to deformation analysis of RC elements under direct tension

Heterogeneous structure and, particularly, low resistance to tension stresses leads to different mechanical properties of the concrete in different loading situations. To solve this problem, the tension zone of concrete elements is reinforced. Development of the cracks, however, becomes even more complicated in the presence of bar reinforcement. Direct tension test is the common layout for analyzing mechanical properties of reinforced concrete. This study investigates scatter of the test results related with arrangement of bar reinforcement. It employs results of six elements with square 60x60 mm cross-section reinforced with one or four 5 mm bars. Differently to the common research practice (limited to the average deformation response), this study presents recordings of numerous strain gauges, which allows to monitor/assess evolution of the deformations during the test. A simple procedure for variation assessment of elasticity modulus of the concrete is proposed. The variation analysis reveals different deformation behavior of the concrete in the prisms with different distribution of the reinforcement bars. Application of finite element approach to carefully collected experimental data has revealed the effects, which were neglected during the test results interpretation stage.

Annealing temperature tuned structural and impedance properties of ZnO thin films

Zinc oxide thin films were synthesized using sol-gel spin coating technique. The films were annealed at 250 °C, 350 °C, 450 °C and 600 °C for 1 h. X-ray diffraction pattern of films showed transition from amorphous to crystalline nature with rise in annealing temperature. Field emission scanning electron microscopy of ZnO films showed absence of granular nature for films with annealing temperature 250 °C and 350 °C. High resolution transmission electron microscopy of single grain revealed lattice spacing of 2.8 Å corresponding to (100) plane of hexagonal ZnO. The impedance spectra of ZnO films was analyzed in terms of parallel RC element which indicated high resistive nature for films annealed at 250 °C and 350 °C. The relaxation time was found to be five orders of magnitude higher for films annealed at 250 °C and 350 °C compared to films with annealing temperature 450 °C and 600 °C, indicating the improvement in crystallinity resulted in improved conduction.

Antiferromagnetic behaviour and dielectric relaxation of xBa2FeNbO6–(1-x)LaFeO3 [x = 0.1, 0.3, 0.5

In this work we have investigated the magnetic and dielectric properties of xBa2FeNbO6–(1-x)LaFeO3 [x = 0.1, 0.3, 0.5] prepared by the solid state reaction technique. The Rietveld refinement of the X-ray diffraction pattern of the samples is performed to confirm the phase formation as well as the crystal structure of the materials. The x = 0.1 and 0.5 materials have orthorhombic Pbnm and cubic Fm3m crystal symmetry respectively. Whereas x = 0.3 material has both Pbnm (83.64%) and Fm3m (16.36%) phases. The effect of lattice distortion on the magnetic and dielectric properties has been discussed. The temperature dependent magnetization study reveals the anti-ferromagnetic behaviour of the materials. The magnetic field dependence magnetization curve indicates the canting of Fe3+ spins in these materials like LaFeO3. The value of magnetization has increased with respect to pure LaFeO3. The dielectric relaxation of the materials is analyzed within impedance and electric modulus mechanisms. The value of conductivity and dielectric constant increases with the increase of x value. Two semi-circular arcs in the complex impedance plane plot originate due to the grain and grain-boundary contributions which give rise to two dispersion regions in the frequency dependence conductivity spectra of the materials. The complex impedance plane plot is described by an electrical equivalent circuit consisting of two parallel RC elements connected in series.

Performance of RC Elements With Laced Reinforcement

Performance of RC Elements With Laced Reinforcement

Expert judgment-based fragility assessment of reinforced concrete buildings exposed to fire

•The fire fragility of a generic modern, mid-rise, RC office building is assessed.•Fragility curves for its slabs and columns are constructed by expert elicitation.•The expert elicitation also used to construct a suitable fire damage scale.•The significance of spalling in the two RC elements is identified.