Reinforced Concrete Flat Slabs Research Articles

Enhancing the punching performance of two-way RC flat slabs using different configurations of embedded aluminum sections: Experimental program and numerical analysis

Punching occurs in flat slabs around the column as a result of increased column loads, inadequate steel reinforcement, and weak concrete strength. The goal of the current research is to improve the punching resistance of two-way reinforced concrete (RC) flat slabs using internal strengthening with aluminum sections. An experimental program consisting of ten RC flat slabs was prepared and tested to show the effects of three different strengthening configurations (horizontal and vertical, diagonal, or hybrid system consisting of horizontal, vertical, and diagonal sections) on the structural characteristics of RC flat slabs. Also, the effect of the thickness of the strengthening aluminum sections (1.0, 1.5, and 2.0 mm) on the ultimate load, load-mid span deflection responses, stiffness, and absorbed energy of RC flat slabs was investigated. According to the results, the three slabs in the group with the vertical and horizontal aluminum sections had the highest improvements in ultimate loads among all tested slabs of 50.9%, 57.1%, and 68.9% for aluminum thicknesses of 1.0, 1.5, and 2.0 mm, respectively, compared to the control slab. Moreover, a three-dimensional (3D) non-linear finite element model (FEM) was constructed using ABAQUS software. The proposed model was validated using the experimental records. Then, the validated model was used to investigate the effects of the interspacing between the aluminum sections on the ultimate load, load-deflection responses, stiffness, and absorbed energy of two-way RC flat slabs.

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.

Columns Cross-Section Shape Effect on Punching Shear in Reinforced Concrete Flat Slabs

Punching shear is one of the possible failures that must be taken into consideration in designing concrete slabs and foundations. Several factors control the occurrence of this failure mechanism, such as slab thickness, concentrated loads, and the cross-section area of the supporting columns. The cross-section shape of columns may also affect the punching shear in flat slabs. Therefore, this research was specified to study the effect of the columns' cross-section shape on the punching shear in flat slabs by conducting a simulation analysis for two multi-story concrete buildings with a flat slab but with different structural stability systems, building layout, and column distribution, where five cross-section shapes (square, rectangular, circular, hexagonal, and octagonal) were tested. ETABS 2016 software was adopted for modeling and analysis to identify the optimum column shape leading to minimizing punching shear. Although the cross-section area and the reinforcement area of the columns were fixed while changing the models’ column shapes, it was found, in both buildings, that hexagonal columns performed better than others under punching shear and provided the lowest punching shear ratio. In contrast, circular columns gave the lowest resistance. Octagonal, square, and rectangular columns, respectively, follow the hexagonal columns in resisting punching shear.

Performance of Reinforced Concrete Flat Slabs Having GFRP Gratings

A thorough study of the literature indicated that there is not much relevant research on the application of GFRP gratings. Otherwise, extensive study has been done on FRP bars, laminates, sheets, strips, and rods. This study proposes a novel approach to improve the punching shear resistance of flat slab column connections by inserting GFRP gratings across the slab thickness. The results of seven specimens tested under vertical static loading are presented, considering the influence of the grating’s characteristics. All specimens were tested as simply supported slabs under one point of static loading. Experimental results, including crack patterns, slab deflection, concrete compressive strain, tensile steel strain, GFRP grating strain, and failure load, were recorded using extensive electric instrumentation. Test results revealed an enhancement in the failure load ranging between 9.03% and 27.67% for the specimens provided with the GFRP grating. In addition, a nonlinear finite element numerical model analysis was carried out using the ANSYS 15 program. Correlational studies based on the load-deflection response and crack patterns were utilized, resulting in a good agreement between numerical simulations and experimental results with differences ranging from 1.0% to 8.0%.

Predicting the punching shear capacity of corroded reinforced concrete flat slab

Reinforced concrete (RC) flat slab not only provides benefits in terms of floor-to-floor height, aesthetics but also facilitates the installation of formwork, reducing construction time for the project. However, this type of floor structure is prone to punching failure at the slab-column connection, which can lead to the collapse of the structure. Corrosion of the steel reinforcement at the connection of the slab-column makes the behavior more complex. Predicting the load-bearing capacity in this zone needs to account for the factors caused by corrosion. This paper presents the results of evaluating the punching shear strength of corroded RC flat slabs through 123 experimental databases from previous publication based on three building codes: ACI 318-19, Eurocode 2, and TCVN 5574:2018. The statistical metrics indicate that TCVN 5574:2018 provides the safest prediction results. Additionally, a formula derived from nonlinear regression analysis is proposed to enhance the accuracy in predicting the punching shear strength of the corroded RC flat slabs.

Advanced soft computing techniques for predicting punching shear strength

Accurate prediction of punching shear strength is crucial for ensuring the structural integrity and safety of reinforced concrete (RC) flat slab-column connections. However, estimating punching shear strength in complex configurations, such as RC flat slabs, remains a challenging task due to intricate interactions between various parameters. To address this challenge, this study introduces an innovative approach that combines machine learning techniques with hyperparameter optimization strategies to enhance the precision and efficiency of predicting punching shear strength. Specifically, a comprehensive investigation is conducted to assess the performance of XGBoost optimized by different hyperparameter optimization algorithms such as the forensic-based investigation (FBI), Bayesian optimization (BO), grid search (GS), random search (RS), and default setting (DS). The experimental results of 40 runs on three different datasets confirmed the FBI-XGBoost model as the most appropriate for predicting punching shear strength in terms of efficiency and computation time by achieving the highest RMSE, MAPE, MAE, and R2 values. In addition, the statistical results exposed the infeasibility of using RS for the XGBoost model. While the GS method was able to configure the XGBoost model well, doing so required extensive computational time, while the BO-FBI model achieved acceptable prediction accuracy in a reasonable training time. The FBI-XGBoost model was further compared with empirical methods, including ACI-318-08, BS8110-97, and CEB-FIP-90, based on the predictive-to-actual ratio. FBI-XGBoost was identified as the best model by attaining the ratio value closest to 1 and the lowest standard deviation (1.011 ± 0.108). Overall, the statistical experimental results and hypothesis test values firmly support the FBI-XGBoost model as the most reliable free-formulae method for predicting punching shear strength in RC flat slab-column systems.

The Machine-Learning-Based Prediction of the Punching Shear Capacity of Reinforced Concrete Flat Slabs: An Advanced M5P Model Tree Approach

Reinforced concrete (RC) flat slabs are widely employed in modern construction, and accurately predicting their load-carrying capacity is crucial for ensuring safety and reliability. Existing design methods and empirical equations still exhibit discrepancies in determining the ultimate load capacity of flat slabs. This study aims to develop a robust machine learning model, specifically the M5P model tree, for predicting the punching shear capacity of a RC flat slab without shear reinforcement. A comprehensive dataset of 482 experimentally tested flat slabs without shear reinforcement was gathered through an extensive literature review and utilized for the development of the M5P model. The model takes into account influential parameters, such as slab thickness, longitudinal reinforcement ratios, and concrete strength. The performance of the proposed M5P model was compared with existing design codes and other empirical models. The comparison highlights that the developed M5P model tree provides a more accurate and reliable prediction of the punching shear capacity of RC flat slabs. This study contributes to the advancement of structural engineering knowledge and has the potential to improve the design and safety assessment of concrete flat slab structures.

Verification of deflections and cracking of RC flat slabs with numerical and analytical approaches

The Serviceability Limit States (SLS) of reinforced concrete structures, where cracking and deflection limits are verified in the design process, are directly related to time-dependent properties of concrete. In this study, the effects of creep and shrinkage of concrete, implemented in a Finite Element Model for Non-Linear Analysis, are in-deeply investigated for reinforced concrete flat slabs at SLS and used as basis for the development of an analytical framework. In particular, the paper focuses on: (i) the description of the FE model, (ii) the comparison between the numerical and experimental results of some reinforced concrete (RC) flat slab tests from literature characterized by different mechanical properties, span-to-depth ratios, boundary conditions and loading histories and (iii) the development of an analytical framework for the evaluation of long term-term deflections and crack openings in hogging regions of flat slabs near columns. Based on the findings, useful insights for the evaluation of the long term-term deflections and crack opening in the context of Serviceability Limit States are provided for practical purposes.

Low Velocity Impact Response of Reinforced Concrete Flat Slabs

Background Dimensions of the reference concrete slab are 1950x1950x100 mm subjected to drop-weight impact loading. Objective A comprehensive parametric study was performed to examine the influence of many parameters on the RC slabs. Method From the viewpoint of cost and time savings, a three-dimensional finite element is a very good tool to predict the real behavior of the structural elements. Result and Discussion Results showed that the use of CFRP strips enhance the impact behavior of the slab. Contrarily, the existence of opening led to a dramatic decrease in the dynamic capacity of RC slabs with stress concentration around the openings. Furthermore, changing the shape of the impactor showed a significant effect on the peak impact load as well as the ultimate deflection at impact instant. Conclusion In the scope of this paper, the response of RC slab with top and bottom reinforcements exposed to drop-weight impact loading was inspected. Time histories of impact loads and deflections were presented in detail based on ABAQUS/ Explicit analysis. The findings presented in this paper can be presented as follows: 1. The FE models show a good correlation with the experimental data. Consequently, the proposed finite element models are efficient and economical tool to explorer the effect of many parameters on the performance of RC slabs subjected to drop-weight impact load. 2. The numerical simulation confirmed that using externally bonded CFRP strips has more influence on the peak deflection of the reinforced concrete slab than the recorded impact force. 3. Comparing to the flat shape of the impactor, the hemispherical and curved shape impactor can produce large penetration depth at the impact zone with higher plastic deformations in the concrete slab. However, the flat impactor produced higher deflection at the impact instant. 4. As the radius of the impactor increases, both the duration time and the peak impact force are increased. This is because of the higher contact area was obtained when the flat impactor (infinity radius of curvature) was used as compared to other impactors. 5. Due to decreases in RC slab stiffness, the presence of openings (regardless of their shape) has considerably increased deformations in concrete especially around perimeter of the openings extended to the nearby support. 6. It has been found that the eccentric impact loading causes higher plastic deformations than the concentric one.

Comparison of strengthening methods to improve punching behavior of two-way RC flat slabs

Within the scope of this study, three different strengthening methods have been proposed to increase the punching strength of reinforced concrete (RC) flat slabs. These methods are a shear stud, fiber-reinforced polymer (FRP), and textile-reinforced mortar (TRM). Ten large-scale RC flat slabs were produced to investigate the effectiveness of strengthening methods. One of these was chosen as a reference, one strengthened with shear stud, four strengthened with FRP and the rest with TRM. Reinforcement type (shear stud, FRP, and TRM), strip direction (straight and diagonal), and anchor usage were determined as experimental variables. The ultimate load capacity, initial stiffness, and energy dissipation capacity values were calculated and compared using the load-displacement graphs obtained as a result of the tests of specimens. According to the experimental results obtained, TRM, FRP, and shear stud methods increased the ultimate load capacity by an average of 58%, 52%, and 20%, respectively. These increased values for energy dissipation capacity are 118%, 63%, and 47%, respectively. In the numerical part of the study, the experimental study was simulated with the ABAQUS/Explicit FEM program, and the obtained results were compared with the experimental results. Moreover, the stresses and damage distributions in the specimens were evaluated. As a result of the study, it was seen that the method applied using textile reinforced strips gave the most successful results among three different strengthening techniques applied to improve the behavior of RC slabs under the effect of punching loading.

Punching shear behavior of reinforced concrete flat slab column connection containing recycled eggshell powder

Every day, millions of bio-waste eggshells are discarded in the environment by poultry farms, homeowners, bakeries, and the food industry around the world. The possibility of recycling eggshells as a cement replacement in concrete may solve the disposal issue of eggshells. Also, cement production causes the release of carbon dioxide, which contributes to global warming. Replacing cement with eggshell powder (ESP) reduces the level of cement production and indirectly decreases carbon dioxide emissions into the atmosphere. This paper aims to investigate the structural effect of the ESP content as a cement replacement on the punching shear behavior of reinforced concrete (RC) flat slab column connections. The possibility of ESP reuse in RC flat slab structures tested for punching shear was investigated. Four well instrumented RC flat slab column connections (S1, S2, S3, and S4) were cast using four different concrete mixtures containing 0, 5, 10, and 15% ESP, respectively. The slab panels (620x620x50) mm with a constant square column size of (100x100) mm were simply supported and tested under concentrated column load. The test results of different concrete mixes made with ESP content are reported. The test results of RC flat slab behaviors, including failure mode, load–deflection response, load-corner rise, load–strain curves, load-edge rotation response, punching perimeter, angle of punching, shear load, ductility, and stiffness of tested slab panels with and without ESP are discussed. The experimental punching shear load and punching perimeter were compared with those obtained by code model equations in ACI, BS, and Eurocode. It can be concluded that the experimental shear load and stiffness for the slab containing 10% ESP content increased by 5.85% and 24% than that of the control slab without ESP. Eggshell powder could be applied to a certain extent to replace cement in the RC flat slabs.

Behavior of Lightweight Reinforced Concrete Flat Slabs Subjected to Punching Shear

Behavior of Lightweight Reinforced Concrete Flat Slabs Subjected to Punching Shear

Effect of opening shape, size and location on the punching shear behaviour of RC flat slabs

The current study aims to investigate the effect of opening size, location and shape on the punching shear behaviour of Reinforced Concrete (RC) flat slabs. Ten two-way slabs with a span of 1600 mm and a thickness of 120 mm were tested under a static load applied on a central stub column. The test program was divided into three groups with openings and a control slab without opening. Group I was designed to study the effect of opening size, thus, three slabs with three different opening sizes were tested. The openings dimensions were 200 × 200, 300 × 300 and 400 × 400 mm2. These openings were created beside the column. On the other hand, in group II, three slabs with the same opening size of 300 × 300 mm2 were tested, when the openings were at different locations. The distances between the opening edge and the column face were 100, 200 and 300 mm. To study the effect of opening shape, the slabs of group III having circular openings were tested. The openings’ diameters were 250, 350 and 450 mm, these values were configured to equalize the areas of the square openings of group I. The test results indicated that the slabs’ capacities and stiffness notably decreased with increasing the opening size. However, the percentage of the decrease could be reduced by increasing the distance between the column and the opening and/or using a circular opening. The ultimate experimental punching shear capacities of slabs with openings were compared with those obtained by the equations of ECP-203, ACI-318 and EC2 codes. Based on this comparison, ECP-203 and ACI-318 showed higher factors of safety than EC2.

Application of an analytical method for the design for robustness of RC flat slab buildings

Nowadays the structural engineering community needs reliable and design-oriented methods for the design of low and medium-rise reinforced concrete (RC) buildings with regard to progressive collapse. In this context, the current work aims to validate and apply a new analytical method suitable for the design for robustness of RC buildings characterised by the presence of flat slabs falling in low and medium consequence classes. The analytical method presented in this study is an extension of a method recently proposed by some of the authors for the analysis of two-way slabs in which flexural capacity and punching and post-punching failure criteria are added to reproduce the ultimate behaviour of flat slabs.In this paper, the design for robustness is applied together with the design for ultimate and serviceability limit states in the framework of the partial safety factor method of verification for RC flat slabs. The analytical method is applied as a direct method to evaluate the level of robustness of RC flat slabs to sustain a localised failure simulated by considering an internal column removal scenario.First, the method is validated by comparing the analytical results with both experimental data available in the literature and nonlinear finite element results obtained by adopting a multi-layered shell element approach and the PARC_CL 2.0 crack model. The comparison of the results demonstrates the reliability of the analytical approach in reproducing the ultimate load and ultimate chord rotation of flat slabs with a good approximation by considering the effect of tensile membrane action (TMA).Second, in strategies based on unidentified accidental actions, the analytical method is used to carry out a parametric analysis in the context of an internal column loss scenario. The parametric analysis allows the level of robustness to be evaluated by varying the main geometrical properties that affect the failure mode, resistance and ductility of RC flat slabs.Finally, the paper presents—for a given span and span-to-depth ratio—the required reinforcement ratios, design load in the accidental load combination and ultimate chord rotation in the form of design nomographs useful for engineering.

Novel informational bat-ANN model for predicting punching shear of RC flat slabs without shear reinforcement

While design codes provide guidelines to prevent brittle punching shear failures in flat reinforced concrete (RC) slabs, they are associated with high inaccuracy. This study scrutinizes existing design provisions, highlighting its features and limitations. Sensitivity analysis is then used to identify the influential mechanical and geometric parameters. Subsequently, an artificial neural network coupled with a metaheuristic Bat algorithm (Bat-ANN) is used to develop a hybrid model for estimating punching shear strength. Several statistical metrics revealed that the Bat-ANN model achieved superior predictive accuracy. The novel hybrid model was deployed to assess the influence of key parameters affecting punching shear strength, including the slab effective depth, concrete strength, reinforcement ratio, reinforcement yield strength, and width of the square loaded area. The analysis identified the importance of the flexural reinforcement, which is not typically considered in estimating punching shear strength. Subsequently, using the supervised machine learning method through the EUREQA software, a new regression expression was proposed to estimate the punching shear resistance of flat slabs. This hybrid computational intelligence model could be integrated in future automated design platforms of RC structures.

Financial comparative study between post-tensioned and reinforced concrete flat slab

As for today, post-tensioned slabs gained much popularity and started to be widely used among construction companies all around the world. As we refer to the structural members, we acknowledge that the price of the slab is much higher in comparison to the other structural members like columns, beams, or foundation in accordance to the amount of steel and concrete in it. A comparative study has been done between post-tensioned, and reinforced concrete flat slab to compare how much each slab cost. It describes that since the post-tension slabs are thinner and it provides fewer columns, so the amount of concrete required is less than the required amount in a flat slab. Special steel tendons that are used in post-tensioned slabs will be stretched by a hydraulic jack after the casting of concrete, and these tendons have an effect in reducing the reinforcement steel bars. Although tendons are used only in post-tension slabs, the amount of steel used in it is less compared to flat slabs. Furthermore, the contractor work cost differs from doing the post-tensioned slab and flat slab. The study had been done by comparing the amount of concrete, steel and contractor work cost. The results obtained from the comparative study between post-tension slabs and reinforcement concrete flat slabs indicate that post-tensioned slabs are cheaper.

REINFORCED CONCRETE FLAT SLABS WITH COMMON ARRANGEMENTS OF FIBRE REINFORCED POLYMER SHEETS

Retrofitting of reinforced concrete (RC) flat slabs using fibre reinforced polymer (FRP) composites has become common in recent years. Flexural retrofitting of one-way and two-way RC flat slabs using five common arrays of carbon FRP (CFRP) strips has been investigated in this paper. A comparison of the obtained results provided optimum scheme. Finite element method has been used to conduct the study using a commercial computer program. Results from the numerical analyses suggest that the use of CFRP sheets leads to improved load bearing capacities of slabs, which depends on both the type of array and the widths of FRP strips. The most optimal array of the retrofitting material (considering the effects on both strength and deflection) is the cross-like condition, which increases the capacities for one- and two-way slabs, respectively, by thirty-four percent and thirty percent while decreasing their maximum deflection by ten percent and four percent, respectively.

Punching Shear Strength Prediction for Reinforced Concrete Flat Slabs without Shear Reinforcement

Failure of flat slabs usually occurs by punching shear mode. Current structural codes provide an experience-based design provision for punching shear strength which is often associated with high bias and variance. This paper investigates the effect of adding a horizontal reinforcement mesh at the top of the slab-column connection zone on punching the shear strength of flat slabs. A new equation considering the effect of adding this mesh was proposed to determine the punching shear strength. The proposed equation is based on the Critical Shear Crack Theory combined with the analysis of results extracted from previous experimental and theoretical studies. Moreover, the equation of load-rotation curves for different steel ratios together with the failure criterion curves were evaluated to get the design points. The investigated parameters were the slab thicknesses and dimensions, concrete strengths, size of the supporting column, and steel ratios. The model was validated using a new set of specimens and the results were also compared with the predictions of different international design codes (ACI318, BS8110, AS3600, and Eurocode 2). Statistical analysis provides that the proposed equation can predict the punching shear strength with a level of high accuracy (Mean Square Error =2.5%, Standard Deviation =0.104, Mean=1.0) and over a wide range of reinforcement ratios and compressive strengths of concrete. Most of the predictions were conservative with an underestimation rate of 12%. Doi: 10.28991/CEJ-2022-08-01-013 Full Text: PDF

Punching Shear Behavior of Reinforced Concrete Flat Slabs with Steel Shearheads

Punching Shear Behavior of Reinforced Concrete Flat Slabs with Steel Shearheads

Predicting Embodied Carbon and Cost Effectiveness of Post-Tensioned Slabs Using Novel Hybrid Firefly ANN

Post-tensioning has become a strong contender for manufacturing reinforced concrete (RC) members, especially for flat slabs in large-span structures. Post-tensioned (PT) slabs can lead to considerable material savings while reducing the embodied carbon (embodied CO2), construction time, and life cycle maintenance and repair costs. In this research, a novel hybrid Firefly–Artificial Neural Network (Firefly–ANN) computational intelligence model was developed to estimate the cost effectiveness and embodied CO2 of PT slabs with different design variables. To develop the dataset, several numerical models with various design variables, including the pattern of tendons, slab thickness, mechanical properties of materials, and span of slabs, were developed to investigate the sustainability and economic competitiveness of the derived designs compared to benchmark conventional RC flat slabs. Several performance measures, including punching shear and heel drop vibration induced by human activity, were used as design constraints to satisfy safety and serviceability criteria. The economic competitiveness of PT slabs was more evident in larger spans where the cost and embodied CO2 emissions decreased by 39% and 12%, respectively, in PT slabs with a 12-m span length compared to conventional RC slabs. Sensitivity analysis also confirmed that the cost and embodied CO2 emissions were very sensitive to the slab thickness by 86% and 62%, respectively.