Two-way Slabs Research Articles

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.

Parametric finite element study of novel corrugated steel deck for two-way action in steel-concrete composite floors

Composite construction using steel and concrete is a widely adopted flooring construction technique. Past literature has reported ample research in the development of composite one-way slabs. However, there is a lack of understanding of two-way action in these composite slabs. It has prevented the applicability of two-way steel-concrete composite slabs in floor construction. The performance of a novel deck composed of corrugated hat sections placed orthogonally at equal spacing on a corrugated base sheet has been investigated for achieving two-way action in steel-concrete composite slab construction. Non-linear finite element (FE) models were developed and researched for feasibility in two-way action using the commercial FE package ABAQUS. The decks were analyzed in the construction stage, that is, in the absence of concrete, to isolate the properties of the new steel deck. Various parameters such as depth and spacing of hat section, angle of hat section webs and aspect ratio of the decks were investigated to establish a suitable configuration for two-way action. The effect of the parameters on the load-deflection relationship and distribution of load in two directions were studied to optimize the two-way action. Decks were also analyzed for one-way action to benchmark the change in behavior of the corresponding two-way decks. Obtained numerical results showed that the proposed corrugated deck is efficient in two-way action and could be adopted in two-way steel-concrete composite slabs. The distribution of loads in both directions improved with a decrease in hat-section spacing but increased aspect ratio and depth.

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.

Experimental Investigation on Bending Behavior of Innovative Poplar LVL Floor Diaphragms

Poplar laminated veneer lumber (poplar LVL) is made of fast-growing poplar veneer and structural adhesive, which owns the advantages of sustainability and stable quality. Here an innovative poplar LVL floor diaphragm is presented, mainly made up of orthogonal rib beams fitted together using L-shape steel connectors. The paper mainly deals with an experimental study on the bending behavior of the floor under transverse uniform load. Full-scale testing on eight 3.6 m × 4.8 m specimens shows that the damage phenomena of the floor mainly exhibited as the separation between the rib beams and pulling out from the rib beam for the tapping screw. Though some local damage phenomena appeared before the preset maximum loading level, the load-deflection curves basically kept linear for most of the specimens. Under the service load level of 2.5 kN/m2, the distribution of deflection and strain for the full-length rib beam substantially exhibited the characteristic of a two-way slab. In contrast, for the segmented rib beam, the situation was much more complex. Due to the parametric design of the specimens, testing results illustrated that the rib beam height played the most important role in floor stiffness. Next was the sheathing panel, while the role of segmented rib beam spacing was relatively unremarkable. At last, a revised pseudo-plate method was proposed to evaluate the maximum deflection of the novel floor, which considered the composite action by rigidity factors.

Experimental, analytical, and numerical investigation of punching behaviour of two-way rc slab with multiple openings

Because of its advantages, reinforced concrete (RC) flat slabs have become increasingly popular in recent years. On the other hand, flat slabs have the problem of being vulnerable to punching, which is a dangerous type of collapse. The openings around the column are the most important factor affecting this behavior. The effects of multiple opening positions in two-way reinforced concrete slabs on punching behavior were explored in this study. The factors studied were opening sizes (300 × 300 mm, 500 × 500 mm, and 700 × 700 mm) and opening positions (parallel, diagonal, and adjacent). In this context, ten two-way reinforced concrete (RC) slabs with a length of 2000 × 2000 × 120 mm were made and tested under punching loading from a 200 × 200 mm square column in the slab's middle. The analytical capacity equations presented in TS 500 (TSE, 2000), ACI 318 (ACI, 2019), and Eurocode 2 (CEN, 2002) were used to determine the specimens' ultimate load capabilities. In addition to these studies, finite element models of the slabs were created using ABAQUS finite element software, and comments on how successful the numerical model could be were made by comparing the analysis findings with the experimental results. According to the research, as the openings grew larger, unfavorable results were achieved in ultimate load capacity, initial stiffness, and energy dissipation capacity, with the most unsuccessful results in the test specimens with the adjacent opening position.

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.

Punching behaviour of two-way RC slabs having different multiple opening locations and sizes strengthened with TRM

Flat slabs have been widely used to allow the larger interior areas to be created in architectural plans, reduce the height of the building by allowing more story heights, and save time and money during the manufacturing phase by decreasing the formwork and reinforcing labor. However, the punching mechanism that may occur suddenly and brittle in such slabs creates an important risk that should be considered and avoided during the design phase. Within the scope of this study, the punching behavior of two-way reinforced concrete slabs, with more than one opening with variations in size and position, was examined. In addition, the performance of an innovative strengthening method has been investigated by using anchors in the Textile Reinforced Mortar (TRM) strips to reduce the negative effects of openings on the punching behavior of the slabs. As a result of the experimental study, it was determined that the openings in the slabs negatively affect the punching behavior. Applying the strengthening method reduced these negative effects and increased the punching strength of the reinforced slabs by an average of 67%. The punching capacities of the experimental specimens tested within the horizon of the study were calculated using the equations suggested by Eurocode 2. The obtained results were compared with the experimental results, and the success of Eurocode 2 was evaluated with the experimental variables.

Structural Behavior of Reinforced Lightweight Concrete Slabs

This paper presents an exploration of choosing the optimum density for concrete that achieves the best structural performance for two-way slabs made of concrete with fine aggregate in different proportions less than the ratio used in ordinary concrete to produce different densities, by taking advantage of the idea of fine aggregate concrete which considers as light-weight concrete to achieve lighter concrete with higher structural endurance. The experimental program includes constructing and testing five slabs, four of them made of concrete with different fine aggregate radios 75%, 50%, 25%, and 0% to get different densities (2207, 1792, 1536, 1310 kg/m3) as well as another slab made of normal concrete used as a reference slab with a density of 2414 kg/m3. The outcomes reveal that decreasing the density of the slab from 2414 kg/m3 to 1310 kg/m3 by reduction fine aggregate in concrete from 100% to 0% respectively has more effect on the first crack load than that on the ultimate load of two-way slabs as the first crack load decreases with percentages 16.7%, 33.3%, 38.9%, and 61.1% the while the ultimate load decreases with percentages 7.3%, 21.9%, 46.3%, and 56.1%, respectively as compared to the reference (normal wight concrete slab). Also, decreasing the density of the slab made the cracks form and spread quickly and the slab failure tends towards the brittleness, and the cracks diffused and grew faster and wider.

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.

Research on the Residual Bearing Capacity of a Rib Beam Bridge Carriageway Slab Based on Fatigue Cumulative Damage

In order to study the residual bearing capacity of the carriageway slab of a reinforced concrete rib beam bridge after fatigue damage, a double T-beam carriageway slab model with a high degree of symmetry was designed and fatigue tests were carried out. Based on the fatigue cumulative damage theory, the fatigue residual strength equation of concrete was improved and a constitutive fatigue model of a steel bar and concrete with arbitrary loading times was established. Because the carriageway slabs are highly symmetrical, we used a ratio of length to width and a ratio of width to height to express the dimensional changes in the carriageway slabs. Abaqus general analysis program was used to create a symmetrical T-beams model and analyze the influence of the fatigue load level, the ratio of width to height of the roadway slab, and the ratio of length to width on the remaining bearing capacity of the roadway slab. Finally, the 1stOpt fitting software was employed to fit the roadway slab’s residual bearing capacity evaluation model. The fatigue test results show that the fatigue failure form of the roadway slab is punching failure, and the development law of fatigue damage can be divided into three stages, accounting for 6.25%, 56.25%, and 37.5% of the total fatigue life. The finite element analysis results showed that the length–width ratio of the roadway slab is positively correlated with its relative residual bearing capacity, and the residual bearing capacity of the two-way slab is higher than that of the one-way slab. The fatigue load level and the aspect ratio of width to height are negatively correlated with their relative residual bearing capacity.

Experimental and Numerical Investigation of RC Two-Way Slabs with Middle, Side and Corner Openings

Experimental and Numerical Investigation of RC Two-Way Slabs with Middle, Side and Corner Openings

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.

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.

Analysis of Mechanical Properties of Long-span Reinforced Concrete Composite Slabs under Different Splicing Methods

Based on the long-span slab in an assembled building project, ABAQUS is used to simulate the mechanical properties and failure modes of the slab under the condition of the whole span cast-in-place slab and the two-way rebar truss composite slab (hereinafter referred to as composite slab) with different splicing methods, in which the slabs adopt fixed support and the slab loads are uniformly distributed load. The finite element simulation results show that the integrity and mechanical properties of the whole span cast-in-place slab are not different from those of the parallel splicing two-way composite slab, but they are obviously better than the parallel cross splicing two-way composite slab; In the same slab span, the cast-in-place seam of the composite slab in the middle cracks first due to stress concentration, and with the increase of the uniformly distributed load of the slab, the crack will pass through the cast-in-place seam and develop to the composite slab on both sides, and finally all the concrete at the bottom of the slab will crack and fail. According to the simulation results, the following suggestions are given: if the truss rebar composite slab is adopted for the long-span slab, the two-way composite slab should be adopted, and the cast-in-place seam should be arranged in the direction of less stress, and the seam should not be arranged in both directions.

Prestressed ultra-high performance concrete (UHPC) beams for reusable structural systems: design and testing

With the aim to reduce the environmental footprint of buildings, this paper presents an original structural system concept for two-way slabs in residential and office buildings. The proposed system extends the best practice in terms of modularity, versatility, demountability, reusability, and durability. A finite set of elements can be used to form the main load-bearing system of multiple successively constructed buildings having different and unpredicted layouts and static systems. Ultra-High Performance Concrete (UHPC) was identified as one of the most promising materials for this application because of its high strength, extreme durability and the opportunities it opens for shape optimization and material consumption reduction. In the first part of this contribution, the main features of the new structural system and preliminary design assumptions for UHPC modules are briefly outlined. Then, the authors present and discuss the results of an experimental campaign on prestressed UHPC beams that were tested in bending and shear, under service and ultimate load conditions. Beams with transversal openings in the web were also tested to assess the influence of these openings on the cracking behavior and shear strength of the beams. Experimental results provide useful information for the subsequent modeling of the structural system. Thanks to the presence of fibers, transversal openings in the web only have a limited influence on the response of the beams, thus allowing horizontal technical shafts to pass through the structural thickness of the slab.

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.

Fiber-Type Influence on the Flexural Behavior of RC Two-Way Slabs with an Opening

Combining fiber with concrete mixes has become essential and its widespread use improves the strength of structural concrete elements. This research conducted an experiment into the structural performance of flat slabs with and without a square opening using four types of fiber (hooked-end, straight, corrugated steel fiber and polyolefin fiber) to gain a better understanding of how the variance of fiber type and shape effects the flexural behaviors of two-way slabs. The test program involved (a) testing the properties of hardened concrete, such as compressive properties, modulus of rupture and splitting tensile strength, and (b) testing the flexural behavior of two-way slabs. Ten slabs were divided into five pairs, including two specimens used as reference specimens (with and without openings), and eight other specimens with different types of fibers. Results revealed that the existing fiber in concrete improved the mechanical properties of hardened concrete mix, and the compressive strength test showed higher improvement in specimens with hooked and straight steel fiber. The flexural behavior of reinforced concrete slab was significantly enhanced, and the flexural strength capacity was especially improved for the slabs strengthened with hooked-end and corrugated steel fiber. Polyolefin fiber showed a slight enhancement of mechanical properties and good improvement in flexural capacity. Generally, the highest increments in compressive strength and modulus of rupture were 24.8%, 20% and 11%, and the ultimate load-carrying capacity of slabs was 39%, 13% and 19% for specimens with steel hooked, steel corrugated and polyolefin fibers, respectively, compared with control specimens.

Comparative Study on Slab Deflections

Slabs are one of the most common structural components in RC-framed structures, usually analysed under transverse loading. In the analysis, the thickness of the slab generally governs the serviceability requirement for deflection. The thickness of the slab controls the weight of the building since its contribution is nearly 50% of the total weight of the building. In order to minimize the weight of the building, proper analysis of the slab ensures minimum thickness satisfying deflection criteria. For a given loading, span, and assumed thickness of the slab, analysis will be performed using the material properties such as modulus of elasticity and Poisson’s ratio. Several theories, including the Rankine-Grashoff theory, Timoshenko’s plate theory, Marcus, and other methods have been proposed over the years using the same concept. These methods do not consider the participation of reinforcement in deflection estimation. IS code 456 – 2016 specifies limiting span to depth ratios with suitable modification factors based on percentage of tension reinforcement and stress level in steel for satisfactory serviceability criteria of deflection. In the present study, an attempt was made to compare the slab deflections analytically and experimentally with and without reinforcement contribution for the exactness of proposed theories. It concludes that, in one-way simply supported slabs, theoretical values differ by 45% from experimental values in reinforced concrete (RC) slabs. In two-way simply supported plain concrete (PC) slabs, the Rankine-Grashoff theory is in good agreement with the experimental values. Theoretical values of the two-way slabs when compared with experimental (RC slab) values are in good agreement with plate theory.

Comparing different strategies of minimising embodied carbon in concrete floors

The present climate emergency demands the construction industry to minimise the carbon footprint of concrete buildings. In this paper, the potential different optimisation strategies to reduce ‘cradle-to-gate’ embodied carbon of concrete floors which require different levels of modifications to the conventional design and construction practice were compared. The embodied carbon savings possible from parametrically optimising slab depth and grade of concrete, post-tensioning, considering alternative conventional slab types, and adopting novel thin shell floor systems were quantified for a range of spans. Compared to reinforced concrete flat slabs designed for conventional span/depth ratios, minimising slab depths and considering lower grades of concrete can reduce embodied carbon of flat slabs up to 12%, only with changes to the design methods. By adopting other conventional alternatives available in the present market, post-tensioning can save embodied carbon up to 23% but two-way slabs on beams and hollow-core slabs can save up to 36%. Much higher carbon reductions up to 65% are possible with novel construction methods of thin shell floors that transfer loads through membrane action rather than bending. Hence, the construction industry should approach shape optimised floor construction forms in future while adopting parametric design and considering conventional alternatives in the present to minimise carbon emissions.