Two-way Concrete Slabs Research Articles

Experimental and Numerical Study of Postfire Strengthening Methods for Fire-Damaged Two-Way Composite-Topped Precast Concrete Slabs

Experimental and Numerical Study of Postfire Strengthening Methods for Fire-Damaged Two-Way Composite-Topped Precast Concrete Slabs

Punching Shear Behavior of Fibrous High Strength Concrete Slabs Reinforced with BFRP Bars

Abstract This study offers test findings from research work conducted to investigate the punching shear behavior of fibrous high strength concrete flat slabs reinforced with Basalt fiber-reinforced polymer (BFRP) bars. Eleven two-way concrete slabs were tested until they failed under central concentrated force. BFRP bars were employed to reinforce ten of the eleven slabs for flexural reinforcement, while steel bars were used to reinforce one slab for comparison. Each slab measured 1000 × 1000 × 70 mm with a 60 × 60 mm steel column at the center of the slab. The flexural-reinforcement type, flexural-reinforcement ratio, and steel fibers content were the test parameters. According to the test findings, raising the BFRP reinforcement ratio improved the slabs’ punching shear capability and load-deflection response. In comparison to slabs with a lower reinforcing ratio, these slabs also showed fewer, narrower cracks. Steel fibers were added, which improved the failure mode from brittle, unexpected failure to ductile failure. The experimental findings are compared to a shear strength equation offered by several concrete design codes, including ACI 440.1R-15, CAN/CSA S806-12, and JSCE.

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.

Research on calculation of unbonded tendon stress increment in prestressed concrete two-way slab during service stage

The finite element analysis model of unbonded prestressed concrete two-way slab supported by four sides and columns was established by using ABAQUS finite element software. The stress growth law of unbonded tendon in prestressed concrete slab during service stage was researched. The results of finite element analysis show that the stress increment of unbonded tendon increases with the increase of the composite reinforcement index, increases with the increase of the tensile strength of the non-prestressed reinforcement, and decreases with the increase of the rotational constraint level of the boundary of the prestressed concrete slab. According to the analysis results, the formulas for calculating the stress increment of unbonded tendons in service stage were established considering the influence of the composite reinforcement index, the four sides support conditions, the relative position of the prestressed tendon in the slab and the steel type of the non-prestressed reinforcement. The unbonded prestressed concrete two-way slabs supported by columns were divided into upper slab belt and mid-span slab belt. The influence of composite reinforcement indexes and steel types of unprestressed reinforcements on the stress increment of unbonded tendons was investigated respectively. The calculation expressions of stress increment of unbonded tendons in service stage were established for upper slab belt and mid-span slab belt.

Failure Mode of Orthotropic Two-way Composite Slab under Concentrated Load

Ordinary reinforced concrete two-way slabs generally suffer from punching failure under concentrated loads, which is a brittle failure and unfavorable to the structures. The composite slabs with steel sheets and PBL (perfobond rib) shear connectors have been applied for reducing the damage to bridge decks from vehicles. Although it is widely used on bridges, the failure mode of the orthotropic two-way composite slabs has rarely been studied when subject to concentrated loads. Accordingly, static loading tests with concentrated forward central load was carried out on 4 two-way composite slabs, in which the effects of the thickness of steel bottom sheets and concrete plates and spacing of PBL shear connectors on mechanical properties were investigated. The test results show that about 12% of the ultimate bearing capacity decreases when the perfobond ribs are spaced farther apart from 180 mm to 240 mm or the thickness of steel bottom sheets is reduced from 10 mm to 8 mm. And the bearing capacity drops by 38.2% when the thickness of concrete plates decreases by 30 mm. The steel-concrete composite slabs exhibit clear bidirectional force characteristics during the loading process, with the main forced direction being along the perfobond ribs, which is related to the stiffness ratio of the two directions. The two-way composite slabs have high bearing capacity and ductility, with obvious signs before failure. The specimens continue to bear the load after local punching failure of concrete near the loading point, which shows the features of both bending and punching failure. Based on the test results, a bending punching failure mode is proposed, which provides a new approach for the calculation of ultimate bearing capacity. An analytical calculation method for ultimate bearing capacity of orthotropic two-way composite slabs based on the failure mode above is derived with yield line theory and plastic theory, which is in excellent agreement with the tested results.

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.

An experimental study of the mechanical behaviour of squat shear walls built with precast concrete two-way hollow slabs

This paper proposes an innovative precast shear wall system built with precast concrete two-way hollow slabs (PCHS shear walls). In the joints of the precast panels of the PCHS shear walls, noncontact lap splices of rebars with vertical and horizontal holes are used to connect adjacent precast panels. These panels contribute to an automated assembly and facilitate pouring concrete in-situ. The mechanical behaviour of PCHS shear walls and the connection performance of vertical joints are examined through pseudo-static loading testing on one cast-in-situ concrete shear wall and four PCHS shear walls. Moreover, the influences of different parameters, including axial compression ratio, aspect ratio and the magnitude of horizontal distribution reinforcements, were analysed. It was found that the "strong bending and weak shear" requirement was achieved for all specimens. For the PCHS shear walls, the typical prominent diagonal cracks and brittle failure were prevented by the shear slippage at vertical joints and vertical cracks. And the vertical joint enabled the PCHS shear walls to exhibit stable load-bearing capacity with extensive deformations. In addition, the displacement ductility coefficient of all PCHS shear walls was over 5.0, and their ultimate drifts were over 1/100. The bearing capacity was improved with the increase of the axial compression ratio or with the decrease of the aspect ratio, but the deformation capacity was weakened.

STUDIES OF INTERIOR SLAB-COLUMN CONNECTIONS STRENGTHENED WITH HEMP FIBER REINFORCED POLYMER SHEETS

With increasing environmental concerns, using natural materials in concrete, such as bio-based fibers, is now becoming popular. Recently, hemp fibers have been utilized in the production of composite materials that have the possibility to replace synthetic fibers. The main objective of the research reported in this paper was to investigate the feasibility of using externally bonded hemp fiber-reinforced polymer (HFRP) fabric sheets as an alternative to synthetic carbon fiber-reinforced polymer (CFRP) sheets for punching shear strengthening of reinforced concrete two-way slabs. The experimental program consisted of testing small-scale, interior slab-column connections, under monotonically increased load until failure. The main test parameters included the slab thickness, type of strengthening material (HFRP or CFRP sheets), the width of HFRP sheets, number of sheets layers, and the location of the HFRP sheets relative to the column face. Test results showed that HFRP strengthening sheets enhanced the structural behavior of slab-column connections.

STUDIES OF INTERIOR SLAB-COLUMN CONNECTIONS STRENGTHENED WITH HEMP FIBER REINFORCED POLYMER SHEETS

With increasing environmental concerns, using natural materials in concrete, such as bio-based fibers, is now becoming popular. Recently, hemp fibers have been utilized in the production of composite materials that have the possibility to replace synthetic fibers. The main objective of the research reported in this paper was to investigate the feasibility of using externally bonded hemp fiber-reinforced polymer (HFRP) fabric sheets as an alternative to synthetic carbon fiber-reinforced polymer (CFRP) sheets for punching shear strengthening of reinforced concrete two-way slabs. The experimental program consisted of testing small-scale, interior slab-column connections, under monotonically increased load until failure. The main test parameters included the slab thickness, type of strengthening material (HFRP or CFRP sheets), the width of HFRP sheets, number of sheets layers, and the location of the HFRP sheets relative to the column face. Test results showed that HFRP strengthening sheets enhanced the structural behavior of slab-column connections.

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.

Performance of Two-Way Concrete Slabs Reinforced with Basalt and Carbon FRP Rebars

Fibre-reinforced polymer (FRP) rebars are being increasingly used to reinforce concrete structures that require long-term resistance to a corrosive environment. This study presents structural performance of large scale two-way concrete slabs reinforced with FRP rebars, and their performances were compared against conventional steel reinforced concrete. Both carbon FRP (CFRP) and basalt FRP (BFRP) were considered as steel replacement. Experimental results showed that the CFRP- and BFRP-RC slabs had approximately 7% and 4% higher cracking moment capacities than the steel-RC slab, respectively. The BFRP-RC slabs experienced a gradual decrease in the load capacity beyond the peak load, whereas the CFRP-RC slabs underwent a sharp decrease in load capacity, similar to the steel-RC slab. The BFRP-RC slabs demonstrated 1.72 times higher ductility than CFRP-RC slabs. The steel-RC slab was found to be safe against punching shear but failed due to flexural bending moment. The FRP-RC slabs were adequately safe against bending moment but failed due to punching shear. At failure load, the steel rebars were found to be yielded; however, the FRP rebars were not ruptured. FRP-RC slabs experienced a higher number of cracks and higher deflection compared to the steel-RC slab. However, FRP-RC slabs exhibited elastic recovery while unloading. Elastic recovery was not observed in the steel-RC slab. Additionally, the analytical load carrying capacity was validated against experimental values to investigate the efficacy of the current available standards (ACI 318-14 and ACI 440.1R-15) to predict the capacity of a two-way slab reinforced with CFRP or BFRP. The experimental load capacity of the CFRP-RC slabs was found to be approximately 1.20 times higher than the theoretical ultimate load capacity. However, the experimental load capacity of the BFRP-RC slabs was 6% lower than their theoretical ultimate load capacity.

Limit Carrying Capacity Calculation of Two-Way Slabs with Three Simply Supported Edges and One Clamped Edge under Fire

Fire-resistance experiments were conducted on two full-scale two-way reinforced concrete slabs with three simply supported edges and one clamped edge. This paper presents the design of the furnace, specimens and clamped-end device, the test plan, and the measuring contents and methods. The cracking and failure characteristics of the tested two-way reinforced concrete slabs were introduced. The temperature field distribution of the concrete and steel in the direction of the section thickness, out-of-plane deflections, in-plane deflections, and angle of the slab edges of the two-way concrete slab under fire conditions were analyzed. Cracks form on the slab surface, which is shaped similar to the bottom of a shallow bowl, with two transverse cracks in the mid-area of the medially clamped side and intensively annular diagonal cracks on the corner side. There were several main transverse cracks in the central area of the slab surface. The results indicate that there was a significant decrease in frequency under fire conditions. Relations between the frequency and the central vertical deflection of the two slabs were analyzed by the regression method. This research generates valuable test data that can be used to validate the numerical models developed by fellow researchers in the field of structural fire engineering. Based on the plate balance method and energy method, calculation formula of the limit carrying capacity calculation formula of reinforced concrete two-way slabs with four different boundaries under fire are given, which are simply supported on three sides and clamped on one side. The influence of membrane effect under large deflection is considered in the formula, and the calculated results are in good agreement with the experimental results.

Behaviour of Geopolymer Concrete Two-Way Slabs Reinforced by FRP Bars After Exposure to Elevated Temperatures

Behaviour of Geopolymer Concrete Two-Way Slabs Reinforced by FRP Bars After Exposure to Elevated Temperatures

Punching shear behaviour of geopolymer concrete two-way slabs reinforced by FRP bars under monotonic and cyclic loadings

This study examines the punching shear and deflection performance of 16 Geopolymer concrete (GC) two-way slabs subjected to monotonic and cyclic loading by considering the reinforcement material, percentage of reinforcement, type of concrete and the concrete grade. The tested specimens indicated that the crack patterns at the failure and failure modes were almost similar regardless of the type of reinforcement or their ratio. Moreover, the slabs reinforced by fibre-reinforced polymer (FRP) bars exhibited a lower punching capacity than those strengthened by steel bars, even for similar reinforcement ratios. In addition, the results showed that upon increasing the concrete strength and reinforcement ratio, a higher punching shear capacity and lower deflections were obtained under cyclic and monotonic loading. In addition, the punching shear performance of GC slabs was found to be better than that of ordinary concrete (OC), even though both were reinforced by the basalt FRP (BFRP) bar. However, the ultimate load capacity of the slabs was reduced as a result of cyclic loading according to the capacity of the same specimen, resulting from static loading. However, the reduction is very low for slabs reinforced with FRP slabs. Further, the slabs reinforced by FRP had a better fatigue performance compared with slabs reinforced by steel bars with respect to cyclic loading. The results of the tests were also used to evaluate the accuracy of the available punching shear capacity equations.

Comparative evaluation of concrete constitutive models in non-linear finite element simulations of slabs with different flexural reinforcement ratios

Two types of failures can be observed in reinforced concrete slabs supported on columns: flexural and punching. Flexural failure occurs due to the formation of a yield-line mechanism, while punching failure occurs when a punching shear crack with a truncated cone shape develops around the connection. However, flexure-driven punching shear failures due to the yielding of the slab’s flexural reinforcement can occur in the immediate vicinity of the column. These failures appear in a similar manner to “pure” punching shear failures with an additional ductility resulting from the flexural reinforcement yielding. Since these flexure-driven punching failures occur in reinforced concrete slabs with low reinforcement ratio, the paper investigates the punching shear response of reinforced concrete slab-column connections with various flexural reinforcement ratios to distinguish the different failure modes that can be observed: punching and flexural-punching. First, the study presents the calibration procedure needed in three-dimensional non-linear finite element analyses (FEA) to simulate the punching shear failure of reinforced concrete two-way flat slabs without transverse reinforcement under static concentric loading. The calibration approaches are developed with reference to two advanced constitutive concrete models available in two different FEA software. Two series of experimental tests are selected from literature: plain concrete specimens under uniaxial and biaxial loadings and slab-column connections under concentric punching. It is shown that the numerical solutions using both concrete models can capture the cracking and failure mode accurately. It is emphasized that the numerical models provide a valuable tool to characterise failure processes and behavioural mechanisms for the simulation of slabs with varied reinforcement ratios. Numerical simulations showed that the failure mode transition within low and high reinforced concrete slabs can be highlighted by flexural-punching and punching shear mechanisms. In addition, the new code predictions of ACI 318-19 for punching shear of slabs with low reinforcement ratios are presented and discussed in comparison to EC2-2004 predictions and the experimental results. Finally, a numerical assessment of slab-column connections with reinforcement ratios varying between 0.2 and 2.2% is conducted to thoroughly investigate the effect of the flexural reinforcement ratio on the punching behaviour.

Numerical Analysis of Crack Propagation in Beam Supported Reinforced Concrete Two-Way Slab

Numerical Analysis of Crack Propagation in Beam Supported Reinforced Concrete Two-Way Slab

Structural behavior of two-way slabs cast with different fiber types and contents

This paper presents an experimental investigation on the effects of incorporating Stainless-Steel Fibers (SSF) on the flexural behavior of reinforced concrete two-way slabs. For this deal, three types of steel fiber (straight, hooked, and corrugated steel fiber) are used. Each type of steel fiber was added in four different volume fractions. Each steel fiber shape was added separately by proportions (0, 0.5, 1, 1.5) % of the total concrete volume. Subsequently, ten 800 × 800 mm × 100 reinforced concrete slabs (three slabs for each type of fibers, one for each proportion, and the remaining one slab was the control) were cast and subjected to flexural tests. The effect of type and amount of SSF on flexural behavior of two-way slabs were studied. That, where 0.5% of steel fibers was added, the straight fibers were acknowledged in comparison to the others whereas the improvement to the stiffness was better and the ultimate load increased by 22.58%. At 1% of steel fibers, the ultimate load increased by (35, 43, and 29) % for straight, hooked, and corrugated fibers respectively which at this addition ratio the hooked fibers were the notable contributor as opposed to other types, and this observation was apparent in improving splitting and flexure strength for hardened concrete. Adding 1.5% of steel fibers to the concrete degrade the workability severely by (83-92)% for all types of steel fibers in parallel with little improvement on slabs behavior is contrary to 1% ratio, thus, the 1% is the recommended addition.

An analytical method for determining the tensile membrane action of RC slab panels

This paper presents a newly developed method for determining the influence of tensile membrane action on the ultimate loading capacity of a two-way RC concrete slab. In this method, more realistic membrane action and two failure criteria of a slab panel are established. The developed method has been validated against the test results and good agreement is achieved. Compared to the tensile membrane action region obtained from the FEM (Vulcan), the assumptions about the focus of the ellipse equation proposed within the present method are reasonable and accurate. The present method can be used for predicting the ultimate loads and failure modes of the two-way simply supported RC slabs at large deflections. The research indicates that the in-plane shear force has considerable effect on the prediction of ultimate loads of the slabs and neglecting the in-plane shear force results the overestimated the concrete corner strains.

Comparative Analysis of Flexural Performance of Recycled Concrete Two-way Composite Slab and Ordinary Concrete Two-way Slab

Based on the full-scale model test of ordinary concrete two-way slab and recycled concrete two-way composite slab with beam column support, the failure mode, stiffness and deflection changes of ordinary concrete two-way slab and recycled concrete two-way composite slab under concentrated load are analyzed, and the similarities and differences of bending performance of the two components under the same conditions are proposed. The results show that: under concentrated load, the failure mode and deflection change trend of recycled concrete two-way composite slab are basically the same as that of ordinary concrete two-way cast-in-place slab. The deflection of recycled concrete two-way composite slab increases greatly after cracking, which is related to the secondary stress. The bending stiffness of recycled concrete two-way composite slab before cracking can be calculated according to the elastic theory.

Calculation Method of Deflection Recycled Concrete Two-way Composite Under Concentrated Load

In this experiment, a full-scale model of recycled concrete two-way laminated slab with a 30% replacement rate of recycled coarse aggregate was used. Through concentrated load loading tests, the mechanical properties of recycled concrete two-way laminated slabs were studied, it includes bearing capacity, crack distribution, stiffness and deflection. According to its stress characteristics and on the basis of the current specifications, the stiffness formula is adjusted, and the deflection value is calculated theoretically. By comparing the measured value with the theoretical calculation value, the modified formula of the deflection of recycled concrete two-way composite slab is put forward.