Punching Shear Behavior Research Articles

Punching Shear Capacity of Recycled Aggregate Concrete Slabs

This article investigates the punching shear behavior of recycled aggregate concrete (RAC) two-way slabs. Ten 1500 mm × 1500 mm × 100 mm slabs were tested monotonically. Eight slabs were cast with RAC, whereas two control slabs were cast with natural aggregate concrete (NAC). The RAC incorporated coarse recycled concrete aggregate (RCA) at replacement levels of 25%, 50%, 75% and 100%. Two flexural reinforcement ratios (0.8% and 1.5%) were examined. The results show that the normalized punching shear strength of 100% RAC slabs decreased by 6.5% and 9% compared to NAC slabs for ρ = 1.5% and ρ = 0.8%, respectively. Doubling the amount of flexural reinforcement can increase the punching shear capacity of 100% RAC slabs by up to 45%. A punching shear database of 44 RAC slabs from literature and the 8 RAC slabs presented in this study revealed that the punching shear strength of RAC slabs predicted by ACI 318 was conservative, except for slabs with low reinforcement ratios (<0.6%). The punching shear strength predicted by Eurocode 2 gave more conservative results for all levels of RCA replacement and all flexural reinforcement ratios. A yield-line analysis also showed that the failure mode of the RAC slabs was controlled by punching shear.

Punching shear behavior of FRP reinforced concrete slabs under different opening configurations and loading conditions

This study investigates the punching shear behavior of reinforced concrete (RC) flat slabs using theoretical and non-linear finite element analysis techniques (NLFEA). The internal slab-column connection was studied under the combined effect of openings and loading eccentricity. No additional reinforcement was introduced after the opening was made to simulate the effect of opening in an existing structure. Thirty-six slabs were simulated (1800 mm × 1800 mm × 120) mm3 for the length, width, and depth, respectively. The investigated parameters were: (i) reinforcement type (basalt FRP, carbon FRP, and steel); (ii) the existence of opening and its location (with or opposite to the eccentricity side); and (iii) eccentricity value (0, 75, 150, and 225 mm). The slabs were supported at 1500 mm in both directions using eight high-strength bolts and were analyzed up to failure under square column loading of 150 mm side. All simulated slabs failed in punching shear. It was concluded that both the opening and loading eccentricity adversely affected the behavior of the slab, where using FRP bars as an internal reinforcement causes the slab's ductility index to be significantly reduced. Moreover, Increasing the loading eccentricity value adversely affects the slab's behavior. It reduces its ultimate capacity, ultimate deflection, cracking deflection, plastic stiffness, and energy absorption. The type of reinforcement affects the general behavior of the flat slabs. Slabs reinforced with steel reinforcement have the highest cracking and ultimate load, followed by slabs with CFRP, and BFRP reinforcements, respectively. Meanwhile, the relationship is inversed for the ultimate deflection. Simulation results were compared to those obtained from different international design codes (ACI 318–19, EC2, MC 2010, ACI 440, CSA/S806–12, and JSCE), and all overestimated the punching shear capacity of the tested slabs with the JSCE and the ACI 318 the JSCE have approximately the same predictions.

Prediction of punching shear capacity of concrete slabs reinforced with additional middle-thickness rebar mesh

This paper aims to evaluate the influence of using additional middle-thickness rebar mesh (mid-mesh) on improving the punching shear behavior of interior slab-column connection. Sixteen two-way slabs were cast and tested including different flexural, compression, and mid-mesh reinforcement ratios at different levels of concrete strength. The performance of code-based expressions in predicting the punching shear capacity of reinforced concrete flat plates was also ascertained. The results indicated that using additional mid-mesh showed an increase in the punching shear capacity of slabs reached up to 17.7%. The results also indicated that the ACI 318-19 and CSA A23.1-19 codes underestimate the punching shear strength of tested slabs. Eurocode showed the closest predictions of the ultimate shear load compared to the other design codes investigated in this study. The authors also proposed an equation that can help the designers/engineers to accurately predict the punching shear capacity of intermediate slab–column connection with and/or without additional mid-mesh reinforcement.

Experimental and Theoretical Investigation of the Punching Shear Strength of GFRP-Reinforced Two-Way Slabs

This study aims at investigating the punching shear behavior of square RC interior slab–column connections reinforced with glass fiber reinforced polymer (GFRP) rebars. A total of eight specimens are tested under vertical shear until failure. Five of the specimens are reinforced with GFRP rebars and three specimens with conventional steel reinforcement for comparison. The investigated test parameters are flexural reinforcement type (GFRP or steel), column stub dimensions (100 mm × 100 mm or 150 mm × 150 mm), slab dimensions (900 mm × 900 mm or 1200 mm × 1200 mm), and slab thickness (100 mm or 120 mm). The slenderness effect is also taken into account by testing specimens with varying span lengths and thicknesses. The 3D nonlinear finite element method (FEM) is also implemented to validate the experimental findings. Additionally, the accuracy of existing design equations is assessed against the experimental results. The results reveal that GFRP-reinforced specimens yield higher deformation capacity, yet lower punching shear strength, as compared to steel-reinforced counterparts, owing to the lower elastic modulus of GFRP rebars. Although neglected in most studies, the net span-to-depth ratio (L net /d) is a substantial parameter and should be considered in design.

Punching shear behavior of geopolymer concrete two-way flat slabs incorporating a combination of nano silica and steel fibers

This research was conducted by performing center point load tests on the two way flat slab samples to investigate the combined effects of steel fiber (SF) and Nano-silica (NS) on the punching shear and deflection capacities of ten slag-based Geopolymer Concrete (GC). This study consists of two parts; in the first part, the effects of only SFs were investigated, while in the second part the combined effects of NS and SFs on the properties of GC were examined. During the study, two types of hook-end SFs (0.5% and 1%) and only 2% NS were used. The tested slabs showed that the energy absorption capacity/toughness of GC slabs was positively affected by the inclusion of NS and SFs. Also, it was favorably influenced by SFs content and aspect ratio. Furthermore, these slabs were categorized. While, NS and SFs enhanced the punching shear strength of the slabs, the combined influence of NS and SFs exhibited better the punching shear capacity compared to individual utilization. It is clear from the experimental studies that the slab with the combination of NS and SF2 outperformed all of them and showed 131.08% higher strength bearing capacity than the slab without SFs and NS.

Experimental Evaluation of the Punching Shear Strength of Interior Slab-column Connections with Different Shear Reinforcement Details

This research aims to evaluate the effect of using different shear reinforcement details on the punching shear behavior of interior slab column connections. A comprehensive experimental program is conducted on sixteen specimens having the same concrete dimensions of 1100×1100×160 mm where the slab depth is chosen to be less than that stipulated by different design codes. The parameters under examination were the type of shear reinforcement arranged in a cross shape perpendicular to the column edges (single leg, multi-leg, and closed stirrups), the spacing between stirrups (25 and 50 mm), and the extended length covered by the stirrups (300 and 425 mm). Experimental results showed that slabs reinforced with multi-leg or closed stirrups, even for slabs with a thickness of 160 mm, had an increase in the shear capacity by up to 40% depending on the stirrup amount. A noticeable enhancement in ductility was also observed. Slabs reinforced with vertical single-leg stirrups did not exhibit any improvements. A finite element analysis was conducted to further assess the punching shear behavior of the tested specimens. A comparison between the test results and values obtained using design codes such as ACI 318-19 and ECP 203-2018 showed that the two design codes give a rather underestimated prediction of the punching shear capacity. Doi: 10.28991/CEJ-2022-08-09-015 Full Text: PDF

Punching Shear Behavior of Flat Slab Strengthen with Y-Type Perfobond Shear

This study demonstrates the behavior of a flat slab experimentally and numerically with two different types of shear reinforcements. Ten slabs were cast and tested in the experimental part. The samples were divided into two groups; the first consists of steel fiber, and the second has no steel fiber. Two control samples were considered with only flexural reinforcement, while the other eight samples had two types of shear reinforcement; the first was rebar to form a Y-shape, while the other was a steel plate formed as a Y-type perfobond. Shear reinforcement is provided by employing radial and parallel shear reinforcement. The tested samples in the experimental part have been simulated numerically using the Abaqus/CAE program. As a result of the laboratory examination, it was determined that the addition of steel fibers increased the ultimate load by (7.4% to 20.58%) for models with steel fibers compared to models without steel fibers. It was determined that the presence of steel plates in the models increased their capacity by 2.4% for ultimate load and by (6% to 50%) for deflection compared to ultimate load and deflection in models with steel bars. In Abaqus, the models with steel fibers were found to be 12.7%-26.6% stronger than those without them.

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.

Punching Shear Strength of Voided Slab: Literature Review and Evaluation of Design Codes

Despite of the economic advantages provided by the voided slabs, their design against punching shear failure, as suggested by the current building codes, are based on the equations derived from tests on flat slabs. Previous tests on voided slabs showed that the punching shear capacity of such slabs are different from that of flat slabs due to the differences in cross-sections. This paper reviews the previous works conducted to investigate the punching shear behavior of voided slabs. In addition, it evaluates the punching shear design provisions of the current building codes using the test results of voided slabs from literature. It was found that the punching shear design equations of the current building codes are inconsistent when they applied to estimate the punching shear capacities of voided slabs

Punching shear capacity of unbonded post-tensioned slab - Concrete filled steel tube column joints with innovative connections: Experiment and A new prediction model

The punching shear behaviour of reinforced concrete (RC) flat slab - concrete-filled steel tube (CFT) column joints/connections has been diversely investigated in many studies. However, there have been no studies in the literature investigating the punching shear response of the joint made of unbonded post-tensioned concrete (UPC) slabs. This paper investigates experimentally the punching shear resistance and deformation capacity of the UPC slab - CFT column joints using proposed innovative connections. The experimental program was performed on six large-scale specimens, including three connections of UPC slab - CFT column, two connections of RC slab - CFT column and one conventional connection of UPC slab - RC column (control specimen). The test results showed that the UPC slab – CFT column specimens using the proposed innovative connections had higher punching shear resistance (up to 10%), ultimate deformation (up to 39%) and energy absorption capacity (up to 25%) compared to the control UPC slab - RC column specimen. The stiffness in the elastic stage before concrete cracking of the UPC slab – CFT column specimen was slightly larger than that of the control UPC slab - RC column specimen; however, it decreased rapidly in the post-cracking stage and was smaller than that of the control specimen by 13–28%. Changing the shape of the bearing plate from one annular plate to four discrete rectangular plates did not significantly affect the punching shear resistance of the flat slab–column joints but significantly increased the deformation capacity of the joints (by approximately 19%). The paper also proposes semi-empirical formulas to predict the punching shear resistance of RC/UPC slab - CFT column connections. The verification with the experimental data obtained from this study and other previous studies showed that the proposed formulas predicted the punching shear resistance of RC/UPC slab - CFT column connections close to the experimental data and had acceptable reliability.

Distinguished punching shear behaviour of unbonded post-tensioned concrete slab – CFT column vs reinforced concrete slab – CFT column connections

This article evaluates and compares the punching shear behaviour of unbonded prestressed concrete (UPC) slab - concrete-filled steel tube (CFT) column connections/joints with that of reinforced concrete (RC) slab connection – CFT column connections. The experimental program was carried out on five large-sized samples, including three UPC slab - CFT column samples and two RC slab - CFT column samples. Steel plate connection details were utilised to increase the bond between the concrete slab and the CFT column. The experimental results showed that the prestressing tendons significantly increased the pre and post-cracking stiffness of the slab-column connections, thereby, greatly reducing the deflection of the UPC slab - CFT column samples than that of the RC slab - CFT column samples, especially during the serviceability phase (by up to 58%). However, the use of tendons resulted in a significant decrease in the ductility and energy absorption index of the UPC slab - CFT column samples compared to that of the RC slab - CFT column samples (by up to 44% and 41%, respectively). Using prestressing tendons slightly increased the punching shear capacity, and significantly reduced the strain of the connection details such as the vertical ribs and the horizontal bearing plate. At the same time, the tendons helped greatly reduce the strain of the longitudinal rebars in the UPC slabs.
 Keywords: Unbonded post-tensioned concrete (UPC); connection/joint; CFT column; flat slab; connection details; punching shear; experiment.

Residual punching shear capacity of corroded reinforced concrete slabs

Steel reinforcement corrosion is becoming a critical issue due to the amount of old buildings still in operation with a high probability of substantial corrosion in their reinforcements. The strength and serviceability of corroded reinforced concrete (RC) slab is remarkably impacted by corrosion. Therefore, it is vital to understand more about the punching shear mechanism of slabs subjected to corrosion effects in order to make a good prediction regarding residual capacity. In this paper, critical shear crack theory (CSCT) was further developed to predict the residual punching shear strength of corroded RC slabs, considering the deterioration of material properties induced by corrosion. The effectiveness of modified CSCT was verified by comparison with the available experimental results. In addition, a validated finite element model that considers material deterioration and bond degradation was proposed to verify the accuracy of the proposed model. Finally, a parametric study of 240 cases was conducted to investigate the effect of some important factors on the punching shear behaviour of corroded RC slabs based on modified CSCT. The outcomes demonstrated that the residual punching shear capacity predicted by the proposed model is more accurate than existing design codes. The predicted failure modes also showed fairly good agreement with the experimental results.

Punching shear strength of recycled aggregate-steel fibrous concrete slabs with and without strengthening

This paper presents an experimental study on the punching shear behavior of reinforced concrete flat slabs fabricated using recycled coarse aggregate (RCA) in addition to the presence of steel fibers strengthened by carbon fiber reinforced polymer (CFRP). The ratios of replacement natural coarse aggregate by RCA were (0, 35, 55, and 75%) and the volume fraction of the steel fiber used in this study was 1.2%. Two strengthening methods were used, namely, externally bonded reinforcement (EBR) and externally bonded reinforcement on groove (EBROG). 16 square slabs were cast with overall dimensions of 800 mm by 800 mm by 80 mm thickness. The slabs were designed to fail in punching shear only. All the slabs were simply supported on four edges and tested for punching via a vertical load applied through the square central column. The test results showed that the mechanical properties, ultimate loads, and first crack loads decreased with increasing proportion of RCA replacement, while the incorporation of steel fibers improved the compressive strength and ultimate loads by the ranges of 12%–17% and 16%–25%, respectively. Strengthened slabs lead to considerable improvements in the punching behavior of slabs, with an increase in punching load ranging from 22–30% compared to unstrengthened slabs when using the EBR technique, while the increase in the punching loads ranged between 40 and 55% when using EBROG strengthening technique. However, using half amount of CFRP with the EBROG method led to an increase in punching shear of 20–28%. This research indicates the particular improvement offered by the EBROG with regard to the punching shear capacity of fibrous recycled coarse aggregate concrete (RCAC) slabs compared to the EBR method. In addition, the mode of failure of reinforced concrete (RC) slabs changed from one of debonding failure to concrete cover separation when using the EBROG method.

Punching shear capacity of steel-fibre recycled aggregate concrete slab

For this paper, the punching shear behaviour of 18 steel-fibre recycled aggregate concrete (SFRAC) balanced slab–column connections was experimentally investigated and analytically modelled for the following variables: the recycled concrete aggregate replacement level (0, 50 and 100%) and the volume fraction of hooked-end fibres (0, 0.4 and 0.8%). The measured punching shear capacities were practically insensitive to the recycled aggregate replacement levels, although both the inclination of the punching failure surface as well as punching capacities increased with fibre dosage. Relatively, the best performance improvement was obtained at 0.4% fibre dosage wherein the punching capacities and energy absorption increased by 23% and 70%, respectively, as compared to the non-fibrous concrete specimens. Using the experimental results, the authors propose and calibrate a simple strut-and-tie based punching shear predictive model for SFRAC, and undertake a validation using the limited results available in the literature. Among the selected current design codes under appraisal, the most conservative punching shear predictions for the SFRAC specimens were obtained from the Indian code, IS456:2000, whereas relatively more accurate predictions were obtained from the Eurocode 2.

Experimental investigation on punching shear behavior of the embedded column base for CFSTs

In this paper, the punching shear behavior of the embedded column base for the concrete-filled steel tube is experimentally studied. Push-out tests were firstly conducted to obtain the bond strength between the steel tube and the foundation concrete. Punching tests were then performed on ten specimens to reveal the working mechanism of the embedded column base with shear studs. The test results indicated that shear studs changed the load path, enlarging the punching section of the foundation and improving the punching shear resistance of the column base. It was also found that the diameter of the shear studs and the distance between the shear studs could affect the punching shear resistance significantly. Finally, a method was proposed to determine the punching shear resistance of the embedded column base for the concrete-filled steel tube, by adding up the contributions from the different components of the system, including the bond force and shear resistance of studs above the punching section, and the resistance of foundation. The predicted results showed good agreement with the experimental results, with errors not greater than ± 15%.

Eccentric punching tests on column bases – New insights into the inner concrete strain development

In the past, the punching shear behavior of centrically loaded column bases has been predominantly investigated even though eccentric loads frequently occur in engineering practice. To consider this corresponding uneven shear stress distributions around the column, many design approaches define coefficients to increase the acting load. Due to the limited test data for eccentric punching tests, an assessment of the design approaches is not reliably possible. To fill this gap of experimental test results, four centrically loaded and fourteen eccentrically loaded punching tests were conducted to analyze the effects of systematically graded load eccentricities.The tests reveal a significant influence of load eccentricity on the punching shear capacity and the load-bearing behavior. With increasing eccentricity, the crack formation shifts to the higher loaded side going along with an incompletely developed punching cone. Innovative measurements of the strains in the slab allow to analyze the circumferential multi-axial stress state inside the concrete. In contrast to centrically loaded column bases, this compression ring does not fully develop under eccentric loads. Based on these measured tangential strains, an experimental approach for describing the influence of the load eccentricity on the punching shear strength is derived and compared with the design approaches of Eurocode 2, Model Code 2010 and the stable version of the next generation of Eurocode 2. Overall, the results indicate optimization potential for more advanced design provisions of column bases with eccentric loads.

Slab–column connection punching and ductility improvement methods for seismic response of buildings with flat slabs

AbstractReplicating boundary conditions of slab–column connections under seismic loading in a realistic manner is difficult, and different test setups have been used throughout the years. An innovative test setup has been developed at NOVA School of Science and Technology, and it has been used to test various punching shear behavior enhancement methods, including stirrups, headed studs, fiber‐reinforced concrete (FRC), high‐strength concrete (HSC), and post‐installed bolts under similar conditions. The aim of this paper is to compare the results from these different solutions for seismic loading. It is shown that relatively large drift ratios at failure can be achieved in slabs containing shear reinforcement. However, less conventional methods such as FRC and HSC are shown to be promising alternatives. Compared to reference specimens that failed at 1% drift, the other specimens with various enhancement methods achieved significantly higher drifts, ranging from 2.5% to above 6.0%.

Investigation of punching shear behavior of flat slabs with different types and arrangements of shear reinforcement

Punching shear behavior is experimentally investigated through testing a total of thirteen reinforced concrete flat slabs. Four different types of punching shear reinforcement configurations are examined, namely, ordinary closed rectangular stirrups (ST group), rectangular spiral stirrups (RSP group), advanced rectangular spiral stirrups (ARSP group), and circular spiral stirrups (SP group). Each configuration included three different spacings of 100, 150 and 200 mm. In addition, a reference slab specimen is cast without any punching shear reinforcement. The test results indicated that the circular spiral reinforcement scheme has the highest enhancement with regard to the punching shear capacity compared to the other reinforcement schemes. The percentage of enhancement of the SP group compared to the reference specimen ranges between 23% and 30%. For the other three different rectangular schemes, the enhancement of the punching shear strength for the RSP group and the ARSP group is in the range of 15–23% and 16–25%, respectively. Finally, the punching shear capacity of the ST group increased by 9–13% compared to the reference specimen. The shear carrying capacity of the specimens is also calculated using the ACI 318-19 code and Eurocode 2 equations. The results showed that both codes are capable of predicting the shear carrying capacity for all the schemes. Also, although both codes are conservative, the Eurocode 2 gives a better estimate of the shear carrying capacity than the ACI 318-19 code.

PUNCHING SHEAR BEHAVIOR OF A SELF-COMPACTED BUBBLE DECK SLAB SUBJECTED TO FIRE AND WATER COOLED

In this research the punching shear behavior of Self-Compacted bubble RC slab was investigated after burning it in real fire test until reach 300 C° and cooling it by air and water Experiment results show that using self-compacted concrete improves the cracking pattern and punching failure zone due to improvements in the microstructure of the concrete and how it fills voids and makes contact with the plastic ball. The ultimate load increased by (13) % when using SCC with the same fć, and the effect of water cooling was reduced compared to its effect on normal concrete due to SCC's lower permeability, but the effect of spalling made self-compacted concrete more sensitive to high temperatures.

Experimental Study on Punching Shear Behavior of Crushed Clay Brick Lightweight Concrete Flat Plate Slabs

Experimental Study on Punching Shear Behavior of Crushed Clay Brick Lightweight Concrete Flat Plate Slabs