Two-way Reinforced Concrete Slabs Research Articles

Elasto-Plastic Analysis of Two-Way Reinforced Concrete Slabs Strengthened with Carbon Fiber Reinforced Polymer Laminates

This study explores a technique for enhancing the punching strength of reinforced concrete (RC) flat slabs, namely carbon fiber reinforced polymer (CFRP). Four large-scale RC flat slabs were fabricated, to assess the efficacy of this strengthening method. One slab served as a reference and the three other specimens were strengthened with CFRP, as a method of external strengthening. These slabs, featuring identical overall dimensions and flexural steel reinforcement, underwent testing until failure, under the influence of concentrated patch loads. A concrete plastic damage constitutive model (CDP) was developed and employed to examine the strength of two-way RC slabs. Additionally, to enhance the strength of existing RC slabs, carbon fiber reinforced polymer (CFRP) strips are affixed to the tension surface of the sections. The research begins with the calibration of a numerical model, based on data from laboratory tests. The objective of this study is to constrain the plastic behavior of two-way RC slabs reinforced with CFRP, with a focus on establishing an optimal elasto-plastic analysis, aimed at controlling concrete damage plasticity using CFRP, and employing a plastic limit load multiplier. Subsequently, a series of numerical simulations, incorporating different variables, are conducted to investigate shear behavior. The numerical results indicate that an increase in the strengthening ratio has a significant impact on shear strength. Finite element simulations are carried out using Abaqus CAE®/2018.

Fire Resistance Analysis of Two-Way Reinforced Concrete Slabs

This paper presents a fire resistance analysis of two-way reinforced concrete (RC) slabs. The study analyzes the effect of specific parameters—concrete cover thickness, span, and support conditions—on the fire resistance of the slabs. To that end, the slabs were exposed to Standard Fire ISO 834, and the 3D nonlinear numerical analyses were conducted in SAFIR2016. The results of the numerical analyses were evaluated against experimental results reported in the literature. The agreement between the two sets of results was satisfactory throughout the fire test. Nonetheless, to verify the obtained numerical results, all testing-related parameters must comply with the numerical simulation results. This comparison demonstrated the usefulness of numerical simulations in predicting the behavior of structures in fire conditions. In addition to the nonlinear numerical analysis, the fire resistance was calculated using the simplified method and tabulated data described in Eurocode 2 (Part 1.2) to assess the accuracy and reliability of fire safety regulations in the design of two-way slabs and identify significant differences between the design code and numerical analysis. The comparison showed that SAFIR2016 provides more accurate results by considering additional factors, such as tensile membrane forces, which increase the fire resistance of two-way slabs. According to the load-bearing criteria, the two-way slabs have high fire resistance, considerably higher than prescribed in the fire safety regulations, which ignore the positive effect of tensile membrane forces. According to the numerical analysis, the upper reinforcement in the compression areas of the slab's span was considered, which increased the fire resistance of the slabs. In contrast, according to the design codes, the contribution of this reinforcement is neglected. It was indicated that the increased concrete cover improves the fire resistance of the slabs. The vertical displacements increase by increasing the slab span, but according to the load-bearing criteria, all the slabs show fire resistance of over ten hours. In terms of bearing capacity, slabs with various support conditions show fire resistance of longer than ten hours. In terms of deflections, the supporting conditions of the slabs have a significant influence on their behavior. This study provides valuable insights into the fire resistance of two-way RC slabs and highlights the importance of considering specific parameters in the analysis. Doi: 10.28991/CEJ-2023-09-05-05 Full Text: PDF

Impact of Heating on Structural Performance of Voided Two-Way Reinforced Concrete Slabs

Impact of Heating on Structural Performance of Voided Two-Way Reinforced Concrete Slabs

Performance of reinforced concrete flat slab strengthened with CFRP for punching shear

Existing reinforced concrete (RC) slabs need punching shear reinforcement due to design defects, structural damage, increased loading, corroded reinforcement bars, and change in the use or function of the buildings. Strengthening methods such as using carbon fibre reinforced polymer (CFRP) sheets as external reinforcements in the tension area have proven to be useful in increasing the punching shear capacity and ductility of RC members. Therefore, this study aims to investigate the effect of using CFRP strengthening method in two-way RC slabs. Three strengthened RC solid flat slab samples in addition to unstrengthened slab samples were tested and subjected to static load to simulate the punching shear effect at the middle of the slab. Based on the test results, the samples strengthened with CFRP showed better performance in terms of delaying the appearance of the first crack, improving the resistance of shear crack propagation, and enhancing the load-displacement and ductility capacities compared to the control sample. In addition, the failure mode of the strengthened samples with CFRP was improved and the capacity for punching shear was increased. Furthermore, the analytical model to predict the ultimate punching shear strength has good agreement with the requirements of the practical codes, thus it can be used for the design and analysis of CFRP strengthened slab.

Probabilistic assessment of ACI 318 minimum thickness requirements for two-way slabs

ABSTRACT Following the ACI 318 design code and to control deflections, structural engineers must adhere to minimum thickness requirements for reinforced concrete (RC) slabs. However, these requirements do not account for various influencing variables such as span length, aspect ratio and applied live load. This paper assesses the adequacy of the ACI 318 minimum thickness requirements for two-way RC slabs through a probabilistic approach using Monte Carlo simulation (MCS). The probability distribution of the span-to-depth ratio (SDR) was fitted to the Burr distribution, and the goodness-of-fit test was applied to assess the fitting. Then, the effect of span length, aspect ratio, and live load on the maximum allowable SDR was evaluated, considering all material properties and applied loads as random variables. The analyses have shown a high correlation between the studied variables and the SDR. Therefore, multiple regression analysis was carried out, and four prediction models were proposed to conservatively predict the SDR for two-way slabs. The accuracy of the prediction models was statistically tested by comparing calculated to predicted SDR values, and a 10% maximum relative error was exhibited. The proposed prediction models were accurate for all practical ranges of span length, aspect ratio and live load in two-way slabs.

An experimental and numerical investigation of RC slabs externally strengthened by perforated steel plates

In this paper, the flexural behavior of two-way reinforced concrete (RC) slabs externally strengthened by perforated steel plates was studied. The thickness of the strengthening plates, arrangement of perforated holes, area of the opening in the strengthening plate and the type of connection to the RC slab, including expansion bolts and epoxy resins, were investigated as the most important parameters. To achieve this goal, eight two-way RC slab specimens with 1200 mm × 1200 mm × 80 mm dimensions were constructed and strengthened by 900 mm × 900 mm steel plates. The flexural behavior of the specimens was tested while applying a semi-concentrated load at their center. The load–displacement curves, elastic stiffness, load-bearing capacities, ductility ratios, and fracture energies of the specimens were analyzed and compared. To further investigate the flexural behavior of two-way RC slabs, the numerical models of all the experimental specimens constructed in ABAQUS software and the von Mises stress contours were obtained. The results show that the most increase in load-bearing capacities was obtained in non-perforated strengthening plates with 1 and 2 mm thickness with 202 and 134 % with respect to the control specimen. Using just expansion bolts as the connection system most influences the ductility ratio increment equal to 220 %. Among different geometrical arrangements of the holes, the circular perforation increased the fracture energy by 253 % compared to the control specimen. Moreover, the resulting von Mises stress contours of numerical models remarkably agreed with obtained experimental results from the strengthened two-way RC slabs.

Enhancing the flexural load capacity of the reinforced concrete simply supported slabs using damaged tires strips (DTS)

Damaged tires or ended-life tires represent a difficult problem due to their ability to sustain for a long time which are not able to be dissolved easily. Present study focuses on the ability of using the damaged tires strips (DTS) in the field of structural engineering as an innovative reinforcing material used additionally with the main reinforcement. The adopted technique in the present work represents a clean solution to reuse and recycle DTS to increase the ultimate flexural capacity of the reinforced concrete one-way and two-way slabs used in structural systems satisfying clean environment and economic considerations. The tests were conducted upon eight specimens of reinforced concrete one-way slabs (RCOWS) and two specimens of reinforced concrete two-way slabs (RCTWS) reinforced by the DTS as an additional reinforcement. Experimentally obtained results exhibited enhancement for the ultimate flexural load capacity of the RCOWS and RCTWS models reinforced by the DTS in the range of 16–80 and 14.28% respectively, compared to the original reference specimens.

Experimental and numerical investigation of impact behavior of RC slab with different opening size and layout

The present study intends to investigate the dynamic responses and failure modes of two-way reinforced concrete (RC) slabs that have openings in different sizes and locations under the impact load effect. RC slabs may include the openings due to the various architectural imperatives. Stair and elevator openings, ventilation openings, the openings left for the electricity, and the water installations can be given as examples of openings taken place in the RC slabs. Many studies related to RC slabs subjected to the static and the reversed-cyclic dynamic loads have existed in the literature. However, at the end of the detailed literature review conducted, the authors have not encountered a comprehensive study focused on the RC slabs' impact behavior that has openings in different sizes and locations. In the present experimental study's scope, the impact loading was applied to two-way RC slabs with the openings via the drop-weight test setup designed by the authors. The time-histories of acceleration and displacements measured from the RC slabs and impact forces acting on the RC slabs have been recorded during the tests to investigate the effect of opening sizes and locations on the RC slabs' impact performance. Furthermore, a finite element model (FEM) has been developed using ABAQUS software to calculate dynamic response and failure modes of the RC slabs with openings. Good agreement between experimental and numerical results has proven that the proposed FEM could be used to evaluate the impact behavior of the RC slabs that have the openings.

Evaluation of punching shear capacity of two-way RC slabs without transverse reinforcement according to different provisions

Currently, RC flat slabs are being used commonly because of their advantages. Punching shear failure is one of the governing failure modes of RC flat slabs without column capital and drop panels. In this paper, the provisions for predicting the punching shear capacity of two-way reinforced concrete (RC) flat slabs without shear reinforcement including ACI 318-19, Eurocode 2 and TCVN 5574:2018 provisions are reviewed by mean of considering the influences of the main parameters (effective depth, compressive strength of concrete, loaded area, reinforcement ratio). A total of 169 test results collected from the literature were used to compare with the provisions. The aim of this study was to evaluate the level of applicability of predicting the punching shear capacity of two-way RC flat slabs according to these provisions. The comparison results indicated that the Eurocode 2 provision provides the most accurate prediction of punching shear capacity of two-way RC flat slabs.

Finite element analysis of two-way hollow core reinforced concrete slab under punching repeated load

This research provides informationabout the finite element analysis of the structural behavior of an innovative new type of two-wayhollow reinforced concrete (RC) slabs of two-way plastic piping system under the effects of concentratedpunching repeated loads. This new type of two-way hollow slab could be constructedbyreplacing part of the concrete volume by a continuous network of two-way plastic piping systemsto cutportion of the dead loads by reducing the self-weight of the createdslabsas well as offering networks of voids inside the RC slab which could be valuable for passing utility lines. The reliability of elements forms, material characteristics, types of constantsand the convergence study of the proposed finite element model of the new type of two-way hollow RC slabswas confirmed by the outcomes of the numerical study and the experimental results using five different parametric studies. The proposed FE modeshowed satisfactoryaccuracy with a maximum varianceratio of about 0.11in comparison with the ultimate loading capacity of the experimental behavior. The results weredemonstratingthe efficiency of the new methodology of producing two-way hollow RC slabs using the two-way piping system by reducing the self-weight by about 24% with maintaining about 79% of the total strength. Moreover, the reduction in the strength could be eliminated by locating the networks of plastic pipes out of the locations of the maximum stresses, adding micro steel fiber, using high strength concrete or increasing the reinforcement ratio.

Evaluation of the Effects of Orientation and Coverage Areas of FRP Lamination Bonded with Two-Way RC Slabs – A Modular Approach

Many researchers have proposed different schemes of fiber reinforced polymer (FRP) wrapping to strengthen the Reinforced Concrete (RC) slab. However, the ideal system for FRP laminations is not readily available. A modular approach for strengthening of RC slabs (especially two-way slab) needs to be developed with respect to orientations and coverage area of FRP sheets in order to figure out the effective strengthening system. This paper describes the experimental study carried out to explore the effects of fiber reinforced polymers in different orientation and coverage areas to strengthen the structural members. Three systems of laminates have been used to strengthen the RC slabs using two types of fibers, namely carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) composite. The systems are classified with respect to the orientation of FRP sheets such as edge, corner and center wrapping. Each system has three proportions of FRP coverage area in the order of 25%, 35%, and 45%. A set of fifty-four slabs were fabricated and tested at the rate of three specimens for each variant. The performance of the flexural strength of two-way RC slabs with different strengthening systems has been evaluated and compared. It shows that all the systems of strengthening are effectively enhancing the flexural strength of two-way RC slabs. The extent depends on the system orientation and coverage area of the FRP sheet. The higher-order performance is noticed when the FRP lamination has higher contact with the slab area and with closer proximity to the loaded area.

Implication of Yield Lines Theory in Method Three of the ACI Code for Two-Way Reinforced Concrete Slabs

Reinforced concrete slabs are one of the most important and complicated elements of a building. For supported edges slabs, if the ratio of long span to short span is equal or less than two then the slab is considered as two-way slab otherwise is consider as one-way slab. Two-way reinforced concrete slabs are common in use in reinforced concrete buildings due to geometrically arrangement of columns suggested by architects who prefer a symmetric distribution of columns in their plans. Elastic theory is usually used for analysis of concrete slabs. However, for several reasons design methods based on elastic principles are limited in their function. Correspondingly, limit state analysis offers a powerful technique for considering such matter. The Yield Lines Theory, which is one of limit state analysis based on expected failure criteria of slabs. The assumed failure criteria is termed by a pattern of yield lines, along that the reinforcement has yielded and the location of which counts loads and boundary conditions. This paper deals with comparison of Method 3 for two-way slabs that was provided by the ACI Code and exact derivation of this method by the Yield Lines Theory. Total of nine cases of slabs that have been described by method three are studied and evaluated by plastic analysis assumptions of the yield lines theory. The results are summarized in terms of proposed formulae that derived according to the Yield Lines Ttheory, which could be used as an alternative method for design of two-way reinforced concrete slabs in the ACI Code.

Experimental Investigation of Impact Behaviour of RC Slab with Different Reinforcement Ratios

Reinforced concrete (RC) slabs may be exposed to the low-velocity impact load during their service periods. In low-velocity impact scenarios, the effect of strain rates has been remarkably higher than quasi-static loading because the loading duration is very short. Thus, structural responses and failure modes will be different. The present study aims to investigate dynamic response and failure modes of simply supported two-way RC slabs exposed to low-velocity impact load. In the experimental part of this study, nine RC slabs with the dimension of 1,000 × 1,000 × 80 mm were tested. The reinforcement ratio of RC slabs and the input impact energy applied to RC slabs were experimental variables investigated. A drop-weight test setup was utilized to apply impact load to RC slabs. By varying drop-height as 1,000, 1,250 and 1,500 mm, three different impact energies have been applied to RC slabs via a hammer of which weight is 84 kg. The time histories of the accelerations, displacements and impact loads were recorded. The dynamic responses obtained by tests and the failure modes observed has been interpreted in detail. Besides, a finite element model where explicit dynamic analysis is performed has been established for verification of the experimental results. There was observed good accordance between numerical and experimental results. Consequently, it is considered that the present finite element treatment can be used for the evaluation of the dynamic responses and failure modes of RC slabs exposed to low-velocity impact load.

Experimental Strengthening of the Two-way Reinforced Concrete Slabs with High Performance Fiber Reinforced Cement Composites (HPFRCC) Prefabricated Sheets

Reinforced concrete structures are required to be strengthened and retrofitted for various reasons, including errors during design and/or construction, so in most cases, strengthening of structural elements is much more economical than rebuilding the structure. Employing HPFRCC with tensile stiffening behavior has been developed in order to strengthen the concrete structures over the recent few years. In this paper, applying HPFRCC for strengthening two-way reinforced concrete slabs has been investigated. A total of five two-way slabs were constructed and examined to reach their own collapse stage, one of the specimen was as non-strengthened control slab, and the others were strengthened in various forms. The strengthening was carried out in two ways; by installing precast plate in the tensile area and the other by installing precast plate in both tensile and compression area at two different percentages of the fiber. The bending behavior, cracking, yielding and rupture of the experimental specimens were evaluated. The results revealed that the installation of HPFRCC pre-fabricated laminates significantly ameliorated the bending performance of reinforced slabs, so that the ductility, energy absorption value, cracking strength, and initial hardness of the slabs was increased and the crack width was decreased. Therefore, the proposed precast HPFRCC sheets can be employed in order to strengthen the deficient slabs.

Numerical Study on the Performance and Behavior of Two-way Reinforced Concrete Slabs with Different Types of Styropor Blocks

This paper studies the performance of two-way reinforced concrete slabs with different sizesand shapes of cavities numerically under uniform load. Nineteen simply supported slab models were modeled with dimensions of (1060 1060 70) mm. One of them wasthe reference model which was solid slab and the others were with different shapes, sizes and numbers of cavities.The slab models with cavities divided into three groupsaccording to the shape of the cavities, each group contained six slab models, first group contained slab models with square shape of cavities while the second group contained the models with diamond cavities and third group contained models with circular cavities.In each group there were different numbers of cavities (9, 16 and 25), and the thickness was either 2 cm or 4 cm.
 The numerical results show that, when the percentage reduction of models weight was (14 %) which is corresponding to the cavities thickness of (2 cm), for this case the ultimate load capacity reduced by (3.40 - 13.61)%. While when the percentage reduction in weight was (28 %) which is corresponding with the cavities thickness of (2 cm), the ultimate load capacity reduced by (14.97 - 24.49) %.

Strengthening of Two-way RC Slabs with Central Opening

Although creating an opening in an existing Reinforced Concrete (RC) slab is permitted under specific circumstances, it may lead to a weakened slab. To investigate the strengthening of two-way RC slabs with an opening, six full-scale two-way RC slabs with a central opening, in addition to one reference slab without opening, were tested up to failure under monotonic and cyclic loading. These six slabs consisted of one control un-strengthened slab and five strengthened slabs. The strengthening methods used in this study were either Externally Bonded Glass Fiber Reinforced Polymer (EB-GFRP) or embedded extra steel bars at the tension side of the slabs, as proposed by ACI 318-14 (2014). Failure modes, cracking patterns, ultimate loads, and load -deflection relationships of all slabs are reported. The test results show that using GFRP strengthening method could increase the ultimate strength and flexural stiffness of the slabs significantly. Moreover, embedding extra steel bars around the opening as proposed by ACI 318-14 (2014) may not enhance the load-carrying capacity of the slab to the value of the continuous slab. The test results also show that the deflections of the strengthened slabs with opening under service loads are more than that of the slab without opening.

Modelling the Response of Simply-Supported Two-Way Reinforced Concrete Slabs Exposed to Fire

The historically good performance of concrete structures in real fires, and the lack of urgent drivers for the concrete industry to support research on the fire performance of concrete structures, means that research on the full frame response of concrete buildings to fires has received much less attention than for steel-framed structures. However, a credible understanding of, and ability to model, the response of concrete structures under fire exposure is crucial to make further progress in the field of structural fire engineering, and to make best use of the flexibility enabled by performance-based fire codes. This paper presents a computational study on the structural behaviour of reinforced concrete slabs during fire tests undertaken by Zhang et al.[16]. The distribution of stresses in the slabs is discussed, as is the need for further research to better understand structural response during fire. Amongst other factors, the assumed tensile strength of the concrete is crucial to accurately predict response. The results corroborate the existing consensus that concrete slabs in real buildings can, in some cases, withstand fires for longer than expected; this is due to mobilisation of membrane actions, amongst other factors.

Flexural Strengthening of Two-Way RC Slabs with Textile-Reinforced Mortar: Experimental Investigation and Design Equations

The application of textile-reinforced mortar (TRM) as a means of increasing the flexural capacity of two-way reinforced concrete (RC) slabs is experimentally investigated in this study. The parameters examined include the number of TRM layers, the strengthening configuration, the textile fibers material (carbon versus glass), and the role of initial cracking in the slab. For this purpose six large-scale RC slabs were built and tested to failure under monotonic loading distributed at four points. It is concluded that TRM increases substantially the precracking stiffness, the cracking load, the postcracking stiffness, and eventually the flexural capacity of two-way RC slabs, whereas the strengthening configuration plays an important role in the effectiveness of the technique. Simple design equations that provide good estimation of the experimental flexural moment of resistance are proposed.

Effect of opening size and location on the punching shear behavior of interior slab–column connections strengthened with CFRP strips

This research study is aimed at evaluating the effects of carbon fiber reinforced polymer (CFRP) strengthening on the punching shear behavior of two-way reinforced concrete (RC) slabs strengthened with CFRP strips having openings varying in size and location. In the scope of the research study, series of experiments are conducted on eight test specimens of CFRP strengthened two-way square RC slabs having openings with variable locations and sizes. In the experimental study, the specimens (with dimensions of 2000×2000×120mm) are tested by applying an axial load from the top of a square column (200mm×200mm) located at the center of the test specimen. Each test specimen has one opening with the dimensions of 300×300mm or 500×500mm located adjacent or 300mm far from the central column in a parallel or diagonal position. The test specimens are strengthened with four CFRP strips (150mm wide) located under the slab next to the opening of the specimen. Additionally eight U shaped CFRP strips (150mm wide) located at the corners of the opening are used to further retrofit the test specimens. The effects CFRP strengthening and varying size and location of openings on the punching shear behavior of the test specimens are considered by examining the slabs’ bearing capacities and stiffnesses obtained from the results of the experiments in comparison to the results of a former experimental study. Furthermore, the experimental results obtained from the study are used to evaluate the accuracy of the available code equations to predict the punching shear capacity of flat slabs. From the comparison, it is observed that the punching shear capacities of the retrofitted test specimens significantly increased with respect to the punching shear capacities of the bare specimens with opening tested in a former study. Moreover, it is also observed that the several equations proposed by structural codes underestimate the punching shear capacities of the test specimens.

Development of modelling strategies for two-way RC slabs

Analyses of tested two-way reinforced concrete (RC) slabs were carried out with varying modelling choices to develop better modelling strategies. The aim was to study how accurately the response of a slab subjected to bending could be predicted with nonlinear finite element (FE) analysis using three-dimensional (3D) continuum elements, and how the modelling choices might influence the analysis results. The load-carrying capacity, load–deflection response, crack pattern and reaction-force distribution of the two-way slab studied were compared to experimental data available. The influence of several modelling parameters was investigated, including geometric nonlinearity, element properties, concrete model, reinforcement model and boundary condition. The results show the possibility of accurately reflecting the experimental results concerning load-carrying capacity, load–deflection response and crack pattern giving proper modelling choices. Moreover, the reaction force distribution was found to be highly influenced by the stiffness of the supports.