RC Elements Research Articles

RC slabs strengthened with externally bonded CFRP strips under long-term environmental exposure and sustained loading. Part 2: Laboratory experiments

This investigation consists of two parts (outdoor and laboratory experiments). The first part described RC slabs strengthened with prestressed and non-prestressed CFRP strips; these slabs were loaded and exposed to an outdoor environment for four years. It was found that the elevated temperatures reached in the adhesive did not significantly affect the service behaviour of non-prestressed EB CFRPs, but could lead to the failure of the prestressed system.In the second part of the discussion, laboratory experiments are presented. Failure tests were performed to determine the ultimate strengths of the slabs. The results demonstrate that long-term exposure (thermal cycles and sustained load) does not affect the load-carrying capacity of the strengthened RC elements. Furthermore, an additional slab with an identical strengthening configuration was tested after being subjected to thermal cycles under a heavy sustained load under laboratory conditions, allowing us to better understand and verify the results obtained during the long-term experiments.

An analytical study on the behaviour of infilled RC frame with opening in infill

This study focuses on Reinforced Concrete infilled frames that are related with the high-rise buildings for analyzing the seismic effects. The high-rise buildings are focusing more on seismic effect due to the failure in masonry infills. The failures in the masonry infills are studied by many authors and the stiffness are also increased to overcome this issue. The stiffness was improved by inserting an elastic material at the interface portion of RC element and Masonry infills. So that the rigidity was reduced at the point of interface and the stiffness were also improved by many researchers. Here in this study the masonry infills are studied by real time manner by placing the window opening in the masonry infills. These effects were also studied by some authors but this study deals with optimizing the size of the windows and location of the window for development of stiffness property. The stiffness behavior is studied when the lateral pressure is introduced to the frame as a static monotonic loading.

Combining the Distribution of Relaxation Times from EIS and Time-Domain Data for Parameterizing Equivalent Circuit Models of Lithium-Ion Batteries

Equivalent circuit models (ECMs) are a widely used modeling approach for lithium-ion batteries in engineering applications. The RC elements, which display the dynamic loss processes of the cell, are usually parameterized by fitting the ECM to experimental data in either the time-domain or the frequency-domain. However, both types of data have limitations with regard to the observable time constants of electrochemical processes. This work proposes a method to combine time-domain and frequency-domain measurement data for parameterization of RC elements by exploiting the full potential of the distribution of relaxation times (DRT). Instead of using only partial information from the DRT to supplement a conventional fitting algorithm, we determine the parameters of an arbitrary number of RC elements directly from the DRT. The difficulties of automated deconvolution of the DRT, including regularization and the choice of an optimal regularization factor, is tackled by using the L-curve criterion for optimized calculation of the DRT via Tikhonov regularization. Three different approaches to merge time- and frequency-domain data are presented, including a novel approach where the DRT is simultaneously calculated from electrochemical impedance spectoscropy (EIS) and pulse relaxation measurements. The parameterized model for a commercial 18650 NCA cell was validated during a validation cycle consisting of constant current and real-world automotive cycling and yields a relative improvement of over 40% compared to a conventional EIS-fitting algorithm.

Erratum for “Dynamic Response of RC Elements Subjected to Combined Loading of Blast and Fragments” by Hezi Y. Grisaro and Avraham N. Dancygier

Erratum for “Dynamic Response of RC Elements Subjected to Combined Loading of Blast and Fragments” by Hezi Y. Grisaro and Avraham N. Dancygier

Assessment of one-way shear design of RC elements subjected to axial tension

The Shear failure of reinforced concrete is brittle with no warning. In addition, tensile forces could widen the concrete cracking and cause premature failure. Thus, worsen the situation, which could have disastrous consequences in many cases as shown in Fig. 1a. There is a lack of agreement between various design code provisions for the case of one-way shear combined tensile forces. A total of 897 concrete elements tested under one-way shear was compiled, which contain 113 elements tested under one-way shear and tension. A review of recent findings and the provisions of current design codes for concrete subjected to one-way shear and tensile forces. In addition, a comparative study was conducted on the strength calculated using design codes. Moreover, the effect of various parameters including but not limited to the tensile axial force, the size, the flexure reinforcement ratio, the concrete strength, and the shear span to depth ratio on the design using selected design codes were examined. Last but not least, concluding remarks were outlined, which could help further the development of current design codes. In particular, design codes that are based on physically sound models were found to be more accurate for shear design with or without tension compared to other design codes. However, there is room for improvement in terms of consistency and accounting for various effective parameters.

Influence of Fiber Addition on the Properties of High-Performance Concrete.

High performance fiber-reinforced concrete (HPFRC) has been frequently investigated in recent years. Plenty of studies have focused on different materials and types of fibers in combination with the concrete matrix. Experimental tests show that fiber dosage improves the energy absorption capacity of concrete and enhances the robustness of concrete elements. Fiber reinforced concrete has also been illustrated to be a material for developing infrastructure sustainability in RC elements like façade plates, columns, beams, or walls. Due to increasing costs of the produced fiber reinforced concrete and to ensure the serviceability limit state of construction elements, there is a demand to analyze the necessary fiber dosage in the concrete composition. It is expected that the surface and length of used fiber in combination with their dosage influence the structure of fresh and hardened concrete. This work presents an investigation of the mechanical parameters of HPFRC with different polymer fiber dosage. Tests were carried out on a mixture with polypropylene and polyvinyl alcohol fiber with dosages of 15, 25, and 35 kg/m3 as well as with control concrete without fiber. Differences were observed in the compressive strength and in the modulus of elasticity as well as in the flexural and splitting tensile strength. The flexural tensile strength test was conducted on two different element shapes: square panel and beam samples. These mechanical properties could lead to recommendations for designers of façade elements made of HPFRC.

A comparative study and a simplified formula for punching shear design of concrete slabs with or without membrane tensile forces

Punching shear failure of concrete slabs is brittle with very little warning, whether with or without membrane tensile forces. Tensile forces decrease the punching shear strength, which if not accounted for properly causes a premature failure. In addition, there is a lack of agreement between various design codes for the case of punching shear, whether with or without membrane tensile forces. The study aims to examine and refine the punching shear design of concrete slabs with or without membrane tensile forces. A state-of-the-art review of six decades of experimental testing of and models for RC elements subjected to punching shear with or without membrane tensile forces was conducted. In addition, the accuracy, consistency, and safety of the design provisions are assessed. Moreover, concluding remarks were outlined, which could be helpful in further development of existing design codes. Last but not least, an existing simplified punching shear strength formula for lightweight concrete slabs was adapted, further modified, proposed, and validated for normal-weight RC slabs with or without membrane tensile forces. The proposed formula is simple, yet accurate and makes physical sense as well as being compatible with the Critical Crack Theory.

Pregled istraživanja djelovanja požara u AB elementima i procjena stanja AB konstrukcije nakon požara

Procedures for estimating damage and remaining bearing capacity of RC structures after fire are presented in the paper. The paper describes the effect of fire on structures, the danger it poses for the loss of human lives, and financial losses it may generate. The damage to structural RC elements of a column, beam, slab, wall, and connection details, is presented in the paper. Although RC elements are generally considered to be fire resistant, their physical and mechanical characteristics are greatly influenced by temperature, while the bearing capacity before and after fire can be determined through a number of different methods that are presented in the paper. An overview of condition assessment for RC structures after fire is proposed, and methodology for taking further action is presented.

Study of the Residual Bond Strength between Corroded Steel Bars and Concrete—A Comparison with the Recommendations of Fib Model Code 2010

As is well known, corrosion of steel reinforcement deteriorates the steel–concrete interface and causes concrete cracking, degrading significantly the bond strength. Several experimental studies have investigated the magnitude of residual bond strength due to corrosion, which affects either the function of corrosion-damaged steel bars or the surface crack width in concrete. As a result, linear and exponential correlation relationships have been proposed in order to predict the bond loss due to corrosion. Based on the results of an ongoing experimental campaign on the degradation of bond strength of RC specimens, combined with comparable outcomes from existing literature, this manuscript summarizes a database, comparing with the recommendations of Model Code 2010, to analyze and interpret the corrosion effect on the bond loss and highlights some points that need improvement in the current regulations. As indicated, the density of transverse reinforcement (stirrups spacing) has intense impact on the resulting bond loss due to corrosion. Hence, in order to quantify this aspect, the present manuscript introduces a discretization of confinement levels of RC elements, depending on the stirrups spacing. Based on this, regression analyses of data were conducted to extract fitting curves of bond loss, taking into account the amount of transverse reinforcement and predictive zones of residual bond strength in relationship to either corrosion penetration or surface crack width. Furthermore, the outcomes demonstrate that the corrosion penetration depth is an appropriate assessment tool to correlate the residual bond strength with the corrosion level, whereas the surface crack width on concrete is not yet an effective index, since there is a plethora of factors affecting the crack width. Due to this, more research is needed to improve the current level of knowledge on the surface crack width and link it with the corrosion damage of the steel bar and the residual bond strength due to corrosion.

Flexural strengthening and repairing of heat damaged RC beams using continuous near-surface mounted CFRP ropes

CFRP ropes are relatively new and cheap materials that can be used in strengthening and rehabilitating RC elements. This research was designed to investigate the experimental behaviour of heat-damaged and unheated RC beams strengthened and retrofitted using near-surface mounted (NSM) carbon fiber polymer (CFRP) ropes. Ten RC beams were tested under a four-point loading test. Five beams were subjected to 700°C for 3 h and were partially damaged by heat while the other remaining beams were unheated. Several strengthening configurations were adopted where CFRP ropes were continuously anchored along the beam length. The flexural strength of the test specimens was then predicted using the ACI-440 guidelines and compared to the experimental results. Test results showed that the continuous strengthening configuration is considerably efficient in increasing the flexural strength of the RC beams. Adopting two layers of continuous CFRP ropes along the beam length, anchored at the tensile reinforcement level, increased the load capacity and stiffness of the unheated beams by 60% and 78%, respectively. It also restored the load capacity of the heat-damaged beams with an additional 47% in its’ load carrying capacity, but it decreased their peak deflection. Test results showed that the current ACI-440 guidelines that are designed for predicting the flexural strength of RC beams strengthened with NSM bars could be applied to predict the flexural strength of RC beams strengthened with NSM CFRP ropes with a suggested modification factor for the effective strain.

Límites de esbeltez basados en prestaciones para vigas de hormigón armado para el control de deformaciones y el control de tensiones en la armadura

Los procedimientos actuales de dimensionamiento a flexión de elementos de hormigón armado, basados ​​en la comprobación de estado límite último (ELU), proporcionan suficiente resistencia para soportar las cargas de diseño con un canto reducido. Sin embargo, las dimensiones estrictas de la sección transversal y la cantidad de armadura calculada para satisfacer el ELU pueden ser insuficientes para evitar deformaciones excesivas que pueden producir daños en los elementos no estructurales, o para garantizar el control de la fisuración y satisfacer así, los requisitos de durabilidad, funcionalidad o estética.Debido a la complejidad asociada a un cálculo riguroso de las deformaciones y el ancho de fisura en elementos de hormigón armado a lo largo de su vida útil, una forma efectiva de garantizar la satisfacción del estado límite de servicio es mediante la limitación de la relación de esbeltez l/d del elemento. En el presente estudio, el concepto de esbeltez límite, utilizado anteriormente para el control de la deformación, se generaliza para incorporar las limitaciones de la abertura de fisura, en el marco del diseño estructural basado en prestaciones. Por ello, se derivan las ecuaciones para la esbeltez límite que incorporan los parámetros principales que influyen en el comportamiento de servicio de los elementos de hormigón armado, que son válidos para secciones transversales rectangulares en forma de I o T y para elementos simplemente apoyados o continuos. La fisuración y los efectos a largo plazo se explican a través de coeficientes simplificados derivados de la mecánica estructural y las observaciones experimentales.Las esbelteces límite propuestas, tanto para una carga aplicada constante como para una deformación en la armadura constante, se comparan con las obtenidos utilizando el procedimiento EC2 para el cálculo de deformaciones, basado en la integración de curvaturas. Se han obtenido buenos resultados en términos de errores y dispersión, lo que demuestra que las esbelteces límite propuestas son una herramienta útil para el diseño basado en prestaciones de estructuras de hormigón armado. El carácter mecánico de la formulación facilita su modificación para situaciones diferentes a las utilizadas en su deducción, por ejemplo, diferentes tipos de carga, vigas parcialmente pretensadas o elementos de hormigón armado con FRP, entre otros.

Reliability-based resistance of RC element subjected to membrane action and their sensitivity to uncertainties

Traditional design of reinforced concrete structures is usually based on small deformation theories whereas in order to assess the ultimate load bearing capacity of RC structures against progressive collapse significant deformations should be taken into account. The alternate load paths facilitated by membrane action which develop under large deformation can activate a significant strength reserve, redistribute the loads and hence limit the damage progression in the structure. Up to date, no uniform approach is available on how to incorporate these membrane effects in the structural design or verification procedures against progressive collapse. In this article, sensitivity analyses were performed to investigate the effect of the stochastic nature of membrane action on the load bearing capacity for three distinct RC elements, i.e. a beam, one-way slab and two-way slab. It was found that variables which are usually not explicitly taken into account by traditional design methods, such as the axial restraint stiffness and ultimate reinforcement strain have a significant influence on the resistance of the considered elements. Subsequently, the design resistance of the elements was assessed using a probabilistic method and the ECOV method. It was found that the ECOV method and the probabilistic method considering a lognormal distribution provide similar results in case one observes a single failure mode for which the variability of the resistance is limited. In case of multiple failure modes and a large variability of the resistance, a mixed lognormal distribution proved to enable a more accurate design resistance estimation.

Experimental study and numerical prediction of the bond response of NSM CFRP laminates in RC elements under sustained loading

This paper presents a study into bond behaviour under sustained loading of Near-Surface Mounted (NSM) Carbon-Fibre Reinforced Polymers (CFRP) concrete joints, through an experimental programme and a numerical analysis. The experimental programme consisted of eight NSM CFRP-concrete pull-out specimens tested under sustained loading and laboratory conditions (20 °C and 55%HR) inside a climatic chamber. Previously, the instantaneous bond behaviour of the NSM CFRP-concrete joint was studied using 18 specimens tested until failure. Furthermore, the tensile behaviour of the adhesive under instantaneous and sustained loading was studied at different loading levels under the same environmental conditions. Finally, a numerical procedure was proposed to predict the bonded joint response under sustained loading. A creep coefficient and a strength reduction factor were applied to the instantaneous bond-slip law to simulate the degrading effect the sustained loading had on the behaviour of the bonded joint. Good agreement between the experimental and numerical results was obtained for the tested parameters, however, further research needs to be carried out in order to extrapolate this methodology to other cases.

Dual feedback IRC ring for chaotic waveform generation

The authors have proposed an inverter resistor capacitor (IRC)-based simple chaotic ring oscillator with dual feedback for the generation of the chaotic waveform. The proposed chaotic system uses three coupled autonomous first-order differential equations and it can be electrically demonstrated using the complementary metal oxide semiconductor (CMOS) inverters with few passive RC elements for the generation of the high-frequency strange attractor. The basic dynamical characteristics and multistability property with complex bifurcation pattern in a wide range of parameters are well observed through theoretical as well as numerical simulation analysis using scaled inverse tangent function. In addition, a simple IRC chaotic model design is investigated using 0.18μm MOS transistor parameters with grounded capacitors and an equivalent CMOS-based scaled inverse tangent function. Moreover, the Cadence post layout simulation gives useful information about the circuit sustainability for IC design with high frequency (MHz), low power dissipation (940 μW) and small chip area (826.3 μm2).

Novel method for the parameterization of a reliable equivalent circuit model for the precise simulation of a battery cell's electric behavior

This article presents a novel approach to extract the equivalent circuit model parameter from a pulse test. The presented results corroborate that this novel heuristic methodology can very accurately fit the voltage response of the battery, while drastically reducing the dependence of the parametrization on set boundary conditions and initial guess values. The method's robustness is verified by fitting the voltage response of a mock battery model with pre-defined parameters. It could be shown that the suggested method is able to precisely extract the set parameters from the simulated voltage. Furthermore, the model values obtained from the test of a lithium ion battery cell correlate with physical parameter and other investigation methods like the calculation of the distribution of relaxation times (DRT). As a result, an accurate equivalent circuit model of the cell was created. The restriction as to universal validity of the empirical equivalent circuit model was significantly reduced by limiting the voltage of the RC element with highest time constant. Finally, a robust battery cell model with only a minor root-mean-square error of 1.30% was obtained. ResultA new method for the precise parameterization of an equivalent circuit model from pulse tests is presented. The robustness of such equivalent circuit models is increased by limiting the voltage of the RC element with the highest time constant. Overall, a precise, reliable model of a lithium-ion battery cell is created.

Assessment of the existing models to evaluate the shear strength contribution of externally bonded frp shear reinforcements

This paper presents an analysis of the performance of different existing formulations to quantify the FRP contribution to the shear strength of RC elements strengthened in shear by externally bonded FRP sheets. A large database of 555 tests has been assembled distinguishing between the shape of the section, the existence of internal transverse reinforcement and the FRP configurations. In general, predictions are more conservative for beams without transverse reinforcement. In addition, in some cases predictions are unsafe for beams with transverse reinforcement, showing a possible interaction with the internal transverse reinforcement which is not considered in the experimental FRP contribution to the shear strength. For wrapped FRP configurations, models generally assumed failure at the bottom corner of the section and results are very conservative in some cases where failure was experimentally observed along the web. For U-shaped and side-bonded configuration, results depend mainly on the assumed bond model and are more accurate than in the previous case, showing for some models unsafe predictions for the continuous FRP system applied in beams with transverse reinforcement.

Steel fibers for replacing minimum reinforcement in beams under torsion

This paper concerns an investigation on six large-scale Steel Fiber Reinforced Concrete (SFRC) beams tested in pure torsion. All beams had longitudinal rebars to facilitate the well-known space truss resisting mechanism. However, in order to promote economic use of the material, the transverse reinforcement (i.e. stirrups/links) was varied in the six large scale beams. The latter contained either no stirrups, or the minimum amount of transverse reinforcement (according to Eurocode 2), or hooked-end steel fibers (25 or 50 kg/m3). Material characterization were also carried out to determine the performance parameters of SFRC. The results of this study show that SFRC with a post-cracking performance class greater than 2c (according to Model Code 2010) is able to completely substitute the minimum reinforcement required for resisting torsion. In fact, the addition of steel fibers contributes to significantly increase the maximum resisting torque and maximum twist when compared to the same specimen without fibers. Moreover, SFRC provides a rather high post-cracking stiffness and a steadier development of the cracking process as compared to classical RC elements. This phenomenon improves beam behavior at serviceability limit state. The experimental results are critically discussed and compared to available analytical models as well as with other tests available into the literature.

The influence of exposed to corrosion length of rebars on fatigue life of RC elements

Deterioration of reinforced concrete is a main factor on estimation of structures' service lifetime. As it is well known, both corrosion of steel reinforcement and earthquake events, have detrimental effects on structural integrity of RC elements. In this study, the fatigue life of corroded reinforcement is investigated. Bare and embedded (in concrete) specimens of rebars are tested in low cycle fatigue conditions after accelerated corrosion experiments using impressed current technique. Corrosion damage, in terms of mass loss, and the mechanical tests of fatigue are taken account in function of the exposed to corrosion length of reinforcement. The outcomes attained from the experimental study indicate higher mass loss values of specimens with short exposed to corrosion length than the corresponding mass loss values of specimens with long exposed to corrosion length at the same tested corrosion time; subsequently resulting in their reduced fatigue lifetime. Extrapolating the abovementioned results on RC elements in marine environment located in seismic prone areas, issues are raised concerning the assessment of structural integrity and the parameters which are taken into account on monitoring of high importance structures.

A numerical study on the plastic rotation capacity of CFRP-confined rectangular RC columns

To perform a nonlinear analysis of building frames in their seismic performance evaluation program, appropriate force-deformation curves of the structural members in linear and nonlinear phases which represent their actual behavior are required. Although these curves have been provided for common existing RC elements prior to retrofitting in available instructions, the codes are silent about appropriate practical models for strengthened elements. In this regard, a comprehensive numerical study is conducted in the Finite Element (FE) software VecTor2 to investigate the effects of various influential parameters on the moment-rotation behavior of CFRP-confined rectangular RC columns. The investigated parameters are cross-sectional dimensions, longitudinal and transverse reinforcement ratios, the level of column axial load, concrete compressive strength, and the effective confinement provided by external CFRP wraps. Then, through the idealization of the obtained moment-rotation curves, the influence of the aforementioned parameters on the plastic rotation capacity of the columns as one of the main required parameters to define the nonlinear behavior of the retrofitted columns is investigated. Accordingly, column axial load intensity, shear force, effective confinement, and column depth are found to be the major effective parameters on the plastic rotation capacity of the columns. Finally, a practical method consistent with ASCE 41-13 is presented to estimate the plastic rotation capacity of CFRP-confined columns.

Voigt circuit representation model for the diffusion-convection impedance of uniformly accessible rotating disk electrode

Abstract The Voigt circuit results from n serially connected RC parallel elements. The Voigt circuit with infinitely many elements is an exact representation model for the diffusion-convection impedance of uniformly accessible rotating disk electrode (RDE). Starting from the Sturm-Liouville problem associated to the initial and boundary value problem for diffusion-convection near RDE, the resistances Rk, capacitances Ck and time constants (τk = RkCk) of the Voigt circuit, at least for its leading RC elements (k = 1, 2, 3), can be approximated accurately as a function of the angular velocity of RDE and the Schmidt number for redox species reacting at the electrode surface. When synthetic impedance data, generated from the initial and boundary value problem, are fitted to the Voigt circuit impedance at an increasing number of RC elements, the estimates of the first (largest) resistance and time constant converge rapidly towards their asymptotic formulations, R1 and τ1, respectively, unlike the estimates of the other Voigt circuit parameters for k = 2, 3, etc. The resistance R1 and the time constant τ1 should give access respectively to the diffusion-convection resistance of RDE and to the Schmidt number for the species involved in the electrochemical reaction with fast electron transfer kinetics and low double-layer charging effect. Possible deviations from the theoretical predictions caused by slow electrochemical kinetics, double-layer charging and possible time-drift of the electrochemical system at low frequency are briefly discussed.