Concrete Slabs Research Articles

Residual flexural performance of large-scale ferronickel slag alkali-activated concrete slabs after fire exposure

Background This study explores the flexural behavior of unfired and fired one-way slabs constructed from reinforced ferronickel slag-based alkali-activated concrete (AAC). Despite growing interest in AAC as a sustainable alternative to ordinary Portland cement (OPC), limited research exists on large-scale structural elements, particularly under post-fire conditions. Methods Four slabs (2.1 × 0.9 × 0.18 m3) were tested: two made of AAC and two of OPC concrete with comparable compressive strengths. In each group, one slab served as a reference and was tested under monotonic four-point bending in its original state. The other was exposed to a standard fire curve on the tensioned side, then cooled to ambient conditions before mechanical testing. Additional analyses, including mercury intrusion porosity (MIP) and scanning electron microscopy (SEM), were conducted to evaluate fire-induced microstructural changes in the AAC slabs. Results The AAC slabs demonstrated a load capacity 6% lower than OPC slabs. However, AAC slabs exhibited better post-fire retention of stiffness and ductility. Microstructural analysis revealed increased porosity and microcracking in AAC after fire exposure, which influenced its mechanical performance. Conclusions The results suggest that ferronickel slag-based AAC can provide comparable performance to OPC concrete in flexural applications, with enhanced retention of stiffness and ductility after fire exposure. These findings support the viability of AAC for structural applications while highlighting its resilience under post-fire conditions.

Reliability Assessment of Cracked Reinforced Concrete Slab and Verification using a Full-Scale in-Situ Load Test

Reliability Assessment of Cracked Reinforced Concrete Slab and Verification using a Full-Scale in-Situ Load Test

Mechanical Behavior Research on IHB-CFRP Anchorage Scheme to Reinforce the Flexural Performance of Concrete Beams

This article investigates the use of Carbon Fiber Reinforced Polymer (CFRP) plates with additional anchors to enhance reinforced concrete slab beam structure. A test proposes an improved additional anchorage measure to solve the debonding problem during anchoring. Three point bending load test are carried out to analyze the influence of different anchoring positions on the reinforcement effect. An optimized Improved Hybrid Bonding CFRP (IHB-CFRP) additional anchoring measure is given. The shear friction model proposed in the IHB-CFRP anchoring scheme can simulate the additional adhesion resistance well. The article investigates the concrete crack extension, load–deflection relation, the strain distribution of CFRP plate, CFRP and concrete interface shear stress. It can be seen that the IHB-CFRP anchoring scheme improves the interface bonding ability and bearing capacity of the reinforced specimen.

COMPARAÇÃO ENTRE LAJE DE CONCRETO ARMADO E LAJE PRÉ-FABRICADA TRELIÇADA

This research had the general objective of comparing solid reinforced concrete slabs and prefabricated lattice slabs through a case study with the verification of a building project located in the municipality of Itaperuna/RJ. This is research with a quantitative approach, of an applied, exploratory nature, through a case study. Given the significant savings and operational advantages, such as cost reduction and agility in execution, the prefabricated slab system appears to be a very attractive alternative. However, the final decision must be based on a complete analysis of the project, considering the specific conditions of the work, the active loads and technical and economic feasibility.

Numerical Modelling of the Punching Shear Response of Shearhead Connections between a CFT Column and a Flat Reinforced Concrete Slab

In this article, the punching shear response of shearhead connections between a Reinforced Concrete (RC) flat slab and interior Concrete-Filled steel Tubular (CFT) columns under static load is investigated by using a 3D nonlinear finite element modelling approach. The accuracy of the numerical model is verified against recent experimental results by comparing the results of the load–displacement relationship, failure models and cracking patterns. Based on validated numerical models, parametric investigations are conducted to examine the effects of slab thickness, concrete compressive strength, length of shearhead arm, size of shearhead cross section and flexural reinforcement ratio on the punching shear response of the connections. Then, some recommendations are given for the design of the effective parameters of shearhead connections, particularly focusing on the effective length and cross-section size of shearhear arms. Finally, a design equation considering the effect of the effective length of shearhead arms and flexural reinforcement ratio is proposed to predict the punching shear strength of shearhead connections. The comparison of punching shear strengths predicted by the proposed equation and some design codes shows that the proposed equation can predict the punching shear strength of shearhead connections more accurately than those from ACI 318-14 and Eurocode 2.

Direct shooting method-based optimized design of novel bridge-like pavement structures

Abstract To mitigate the impact of subgrade settlement on pavement in permafrost regions, an innovative prefabricated pavement structure system composed of longitudinal and transverse beams combined with concrete slabs was proposed and designed. The mechanical performance indicators under the most unfavorable conditions were evaluated using ANSYS finite element software. The direct shooting method was employed to systematically optimize the dimensions of the prefabricated pavement structure components. The results indicate that the prefabricated pavement structure exhibits excellent deformation resistance during subgrade settlement. The direct shooting method optimized the prefabricated pavement structure, ensuring structural safety while reducing weight and minimizing construction costs. Furthermore, a correlation analysis was conducted on the dimensions of the structural components affecting maximum deformation, maximum equivalent stress, and overall structural volume. The analysis identified the significant influence of the upper flange plate of the steel beam and the bottom steel plate on the overall mechanical performance and steel consumption of the pavement structure.

Shear resistance of UHPC connection for prefabricated reinforced concrete slabs with shear grooves and dowel rebars

The shear resistance of post-cast ultra-high-performance concrete (UHPC) connections for assembled prefabricated normal strength concrete (NSC) slabs having shear grooves and dowel rebars was investigated. The effects of different factors on the failure mode and shear capacity were investigated by direct shear tests of 28 Z-shaped specimens. The influencing factors included the groove configuration, lap and anchorage length of dowel rebar (the height of UHPC connection), strength of NSC prefabricated slab, and rebar location. The failure modes, UHPC connection shear-slip behavior, dowel rebar strains, and failure mechanism were analyzed. The results indicated that two major failure modes might occur in UHPC connection, including tensile fracture within UHPC connection and shear fracture of UHPC grooves at UHPC-NSC interface. Generally, a larger UHPC connection and smaller groove key width could lead to failure in UHPC-NSC interface, otherwise in UHPC connection. Failure in UHPC connection could exhibit a relatively larger peak bearing capacity than failure in UHPC-NSC interface, but also larger post-peak degradation. In addition, increasing UHPC groove width, UHPC connection height, and prefabricated slab NSC strength could increase the shear capacity. Calculation methods of the peak and post-peak residual shear load-bearing capacity of UHPC connection under different failure modes were also proposed, exhibiting good agreement with the test results.

Hybrid Active Slab with Outer PCM Panels – Geothermal Well to Reduce the Heat Gain of a Building Roof

In hot climates, excessive heat gain through building roofs significantly increases cooling loads and energy consumption. This study aims to reduce roof heat gain by developing an innovative hybrid active slab with phase change material (PCM) panels and a geothermal well (HASP-GW). The HASP-GW system incorporates PCM panels and an active concrete slab embedded with water pipes as heat exchangers. Water at a constant temperature of 22°C, sourced from a geothermal well, flows through these pipes to stabilize indoor temperatures. This research investigates the thermal performance of the HASP-GW with varying PCM transition temperatures and water flow rates using numerical simulations, validated against experimental results. The experimental setup involved two well-insulated test rooms (each 0.85 m × 0.85 m × 1 m); one equipped with the HASP-GW system and the other as a conventional comparison. Results showed that using the HASP-GW with a PCM transition temperature of 38-41°C and a water flow rate of 1 L/min reduced the heat flux on inner surface of the roof, decrement factor, and inner surface temperature by 90.13%, 0.127, and 16.24°C, respectively, compared to a traditional roof. These findings indicate the potential of HASP-GW for energy-efficient roof cooling in hot climates.

Experimental study on seismic behavior of a novel RC slab- SC wall joint based on UHPC and rebar – steel plate lap splices

Steel plate concrete shear walls (SC walls) have been widely used in practical projects because of great mechanical properties. For the application in nuclear power plant containment, the joint of specially shaped reinforced concrete slab and SC walls is difficult to design and construct. By using ultra-high performance concrete (UHPC), a new type of joint with the non-contact lapping method is proposed in this paper, which has advantages of simple construction, superior performance, suitable cost, and wide application range. Five RC-UHPC slab-shear wall joints were designed and tested under reciprocating load to study the effects of structures of lapping steel plate, shear-resisting methods, and interfacial reinforcing methods on the mechanical properties of the joints. The test results showed that the joints adopting the new structures were damaged for the flexural failure of the concrete area, with no damage to the joint zone, while the other specimens failed in the shear failure of the joint zone. It can be demonstrated that the ribs formed by bars and studs on the lapping steel plates are effective measures for non-contact force transferring, and interfacial reinforcing bars can improve the strength of the interface between concrete and UHPC. The force transferring efficiency of the new joints was close to or more than 90%, which indicates that the proposed joint structures can effectively realize the non-contact lapping force transferring. The design formulas and suggestions are proposed based on the analysis of the mechanism of new structures.

Protocol of investigations for the preliminary assessment of the structural safety of highway overpasses

This paper presents the results of a protocol of investigations that have been carried out on 15 overpasses, located along the highway A18 in Sicily (south of Italy) and built about fifty years ago, with the aim of achieving a preliminary evaluation of the structural safety of these overpasses. The investigated overpasses are characterized by a three-span deck consisting of a reinforced concrete slab and prestressed concrete girders. The girders of the central span are simply supported by two Gerber saddles located on cantilever girders at the two ends of the same span. The safety level of the overpasses is investigated by means of different activities that include visual inspection and computation of the class of attention of the overpasses according to the Italian Bridge Guidelines for the classification and the risk management, safety evaluation and monitoring of existing bridges, laboratory and in situ tests on materials, pachometric, georadar and video-endoscopic tests on structural details, and static load tests of the overpasses. The results of the static load tests have also been used to assess the accuracy of two numerical models of the overpasses. These models are elastic but characterised by different levels of accuracy and computational burden: while the first model is the well-established and simple Courbon-Albenga method, the second is a finite element model characterised by a more refined simulation of the response of slabs, longitudinal girders and crossbeams. The results of the investigations have been critically analysed and discussed to underline the ability of the protocol to reveal possible deficiencies of the overpasses and thus identify overpasses that are liable to structural interventions or more accurate and expensive investigations.

Shear behavior of FRP-UHPC composite beams enhanced by FRP shear key: Experimental study and theoretical analysis

To investigate the shear behavior of FRP (fiber reinforced polymers)-UHPC (ultra-high performance concrete) composite beams, four-point bending tests were conducted on seven FRP-UHPC specimens and two FRP-NSC (normal strength concrete) specimens, having different width and depth of concrete flange as well as FRP shear key (FSK) spacing. The slip between FRP profiles and concrete flange was controlled by employing FSK and epoxy resin bonded hybrid connection. The failure pattern, load-deflection/strain curves, and sliding response of composite beams were analyzed to study the influence of concrete type, FSK spacing, width and thickness of concrete slab. The results indicate that FRP-UHPC composite beams exhibited shear failure, while FRP-NSC composite beams experienced bending-shear failure. The composite beams demonstrated shear-lag effect, which became more pronounced with the increasing of the concrete slab width. The use of UHPC, reducing FSK spacing, and increasing the size of cross-section of concrete flange can effectively enhance the shear performance and reduce interface sliding. Formulae were developed to predict the shear capacity and deflection, considering shear deformation. The results predicted by the formulae developed match well with the experimental results.

Punching shear performance of reinforced flat slabs considering in-plane restraints under and after a fire

Two full-scale test specimens, a reinforced concrete reference slab (denoted as S1) and a polypropylene fibre (PPF) concrete slab (S2), with a PPF length of 3 mm and PPF dosage of 1.0 kg/m3, were designed to investigate the effects of the in-plane restraints and PPF admixture on the punching shear response of flat slabs under and after a fire. The results indicated that both S1 and S2 showed a clear trend toward punching shear failure at approximately 210 min after ignition, but S2 had smaller crack widths and a greater limit displacement owing to the PPF admixture. Because of the restrained thermal expansion, many radial cracks first appeared around the punching shear cones on the top surfaces of S1 and S2 and then gradually extended across nearly the entire top surfaces of both test slabs, including their four corners. Meanwhile, both test slabs experienced cracks, which expanded and then retracted under a fire owing to in-plane restraints. In the residual load-carrying capacity tests, both S1 and S2 experienced larger ultimate destructive loads than the unrestrained slabs; thus, the in-plane restraints significantly improved their residual bearing capacities. In addition, S2 experienced a lower residual load and larger plastic deformation owing to the PPF admixture, which exhibited certain ductile failure characteristics.

Experimental and numerical investigation of a cold-formed steel system used to restore old buildings floor

This paper presents a novel configuration of built-up cold-formed steel (CFS) flooring system in the shape of a box section. A new technique is applied to produce the components of the flooring system, which are fastened by self-drilling screws. This box section consists of a cast-in-situ concrete slab, trapezoidal steel decking, two sigma section, steel plate and stiffening equal angles. The main objectives of this system is to enable rapid construction and decrease the time, requirements, and cost. As a result, the proposed system is designed to use the decking in a longitudinal direction. Many old buildings have sturdy structures but their floors were ruined due to being fabricated from timber. This flooring system will be implemented to increase their quality of life and be reused. The loading experiments of four specimens were carried out. The failure modes of the CFS flooring system, load-deflection relation curves, longitudinal strain distribution at different heights were obtained. The experimental results show that the flooring system has high stiffness and flexural performance and can reach ultimate strength without local buckling failure. The failure occurs due to distortion at the end supports. Then, the capacity of the flooring system was calculated theoretically. Then, the practical and theoretical results were compared. The calculated results agree well with the test results. A three-dimensional finite element model is also established to investigate structural performance of the proposed system.

Finite Element Analysis of Flexural Performance of Cut Carbon Fiber Reinforced Concrete Hollow Slab

As a kind of light wall material installed on the surface of the main structure, hollow hanging wall board is widely used in prefabricated buildings. Ordinary concrete hollow board can no longer meet the needs of actual engineering. In order to study the influence of the length and content of carbon fiber on the bending performance of concrete hollow board, the optimal length and content of fiber in hollow board are determined. ABAQUS finite element software is used to simulate the whole process of concrete hollow slab with different fiber length and dosage, and the load displacement curve is analyzed. The results show that the bending performance of concrete slabs increases and then decreases with the increase of fiber length and content. When the fiber length is 10mm and the content is 0.24%, the bending performance of concrete hollow slabs is the strongest, and the ultimate load is increased by 27.90%, showing better deformation resistance under the same load.

Experimental study of static and fatigue push-out test on headed stud shear connectors in UHPC composite steel beams

Steel-concrete composite girder bridges utilize shear connectors to connect concrete deck slabs and steel girders. In current practice, the headed shear stud connector is the most used due to its reliability and ease of installation. Several studies are available on the static and fatigue behavior of welded-headed shear studs embedded in normal-strength concrete. However, relatively little is known about the static and fatigue behavior of headed shear connectors in composite beams when shear studs are embedded in ultra-high-performance concrete (UHPC). This study reports a series of pushout results carried out on headed shear connectors embedded in UHPC at pre-fatigue and post-fatigue stages. Six pushout specimens were first tested under static load to collapse. The precast normal-strength concrete slabs in the pushout specimens were reinforced with glass fiber-reinforced polymer (GFRP) bars to promote sustainable construction. This practice, commonly used in Canada, aims to eliminate the corrosion of steel bars caused by de-icing salts used during winter. The results from these tests were used to determine the applied fatigue stress range of another six identical pushout specimens, which were subjected to constant amplitude fatigue loading over 2 million cycles. Results show that the fatigue cycles applied to the specimens prior to testing them to collapse did not alter the failure mode or the crack pattern of the tested specimens. Furthermore, the studied pushout specimens considered 6 different shapes of shear pockets in which the arrangement and spacing of studs were varied. It was found that the studs in a square pocket form yielded a shear capacity of 13.9 % and 16.7 % greater than those for similar studs arranged in a single row-oriented longitudinally and transversally, respectively. The accuracy of the various design formulas specified in design codes in estimating the shear resistance of the tested specimens, as well as empirical formulas proposed to develop load-slip curves of tested specimens, is studied. It was found that the European and Japanese codes significantly underestimated the stud shear capacity, whereas the Canadian code provides a more accurate estimation.

Experimental and numerical studies on flexural resistance of composite slab with non-dismantle bottom formwork steel-bars truss deck

Composite slab with non-dismantle bottom formwork steel-bars truss deck is a new type of reinforced concrete composite slab. This design features a flat bottom deck that serves both as construction formwork and as protective layer for the stressed bottom chord. Fine stone concrete, used as the material for the non-dismantle bottom formwork, adheres well to the cast-in-situ ordinary concrete topping. A layer of steel wire mesh embedded within the bottom deck effectively mitigates cracking. The objective of this research is to evaluate the flexural capacity of the composite slab with non-dismantle bottom formwork steel-bars truss deck under four-point bending loading. Subsequently, the study analyzes various factors, including failure mode, load mid-span deflection curves, respective loads and strain response of the specimens. The result shows that the peak load for the 110 mm thick slab reached 12.48 kN. Furthermore, the finite element (FE) model was established to verify the test results and to conduct a parametric analysis on the strength of top and bottom chords, braces and the height of the truss and slab. The result shows that the chord and truss height are critical factors influencing the flexural capacity of the slab. This study serves as a valuable reference for the design of composite slab with non-dismantle bottom formwork steel-bars truss deck and lays a foundation for their practical application.

Fundamental issues of the strength of reinforced concrete slabs during punching

Introduction. The calculation model of the strength of slabs during punching, adopted in domestic regulatory documents, contains a number of contradictions to the physical side of the process. So it is known from experience that even before the moment of destruction, many cracks develop in concrete. Therefore, its tensile strength, which is the basis of the current model, is exhausted long before the destruction of the structure. The current model does not take into account the longitudinal reinforcement of the slabs, which is one of the main factors in ensuring strength during penetration. There are similar disadvantages in foreign models. The accuracy of normative calculation methods, both domestic and foreign, is low. Aim. Creation of a method for calculating of the slabs for punching, which correctly takes into account all known features of the structure and has high accuracy. Materials and methods. The method of calculating of the strength of slabs for penetration is considered, which is based on the condition of equilibrium of moments of external and internal forces, in contrast to the normative method, which considers the equilibrium of projections of all forces on the vertical axis. Results. A calculation method has been developed that does not contradict the known experimental data on the operation of slabs during punching. It was possible to correctly take into account the work of the longitudinal reinforcement of the slab. The high accuracy of the proposed calculation method is shown. Conclusions. The conducted research allows us to take a fresh look at the essence of the punching process. The mechanism of destruction, which has fundamental differences, is considered. The proposed calculation method allows us to obtain results comparable in accuracy with the best numerical models.

Out-of-plane shear behavior of BFRP-reinforced geopolymer concrete one-way slabs: Experimental and numerical investigations

Sustainability and durability in construction are vital considerations that involve using environmentally friendly materials and implementing efficient design and construction techniques to minimize the environmental impact. This research study investigated the structural behavior of one-way solid slabs incorporating basalt fiber-reinforced polymer (BFRP) reinforcement and geopolymer concrete to develop an environmentally sustainable and durable structural member. Six full-scale BFRP-reinforced geopolymer concrete one-way slab specimens were prepared and subjected to four-point loading tests to assess their out-of-plane shear performances. The considered experimental parameters were compressive strength of concrete (i.e., 30, 40 and 50 MPa) and longitudinal tensile reinforcement ratios (i.e., 1.20% and 2.18%). The structural performance of the slabs was carefully investigated in terms of crack patterns, failure modes, load-deflection relationships and load-strain relationships. The obtained shear resistances from tests were compared with predicted results from the Codes of Practice and design guidelines. A two-dimensional (2D) finite element (FE) analysis was conducted, considering the bond-slip relationship between BFRP rebar and geopolymer concrete. It was found that all tested specimens were governed by shear failure and had similar shear resistance for the chosen values of considered parameters. The JSCE shear design method provided the closest prediction of shear resistance of BFRP-reinforced geopolymer concrete one-way slab. The developed FE models predicted the out-of-plane shear performance with reasonable accuracy and revealed that the slip distribution of the BFRP rebar presents a sawtooth shape owing to concrete cracking.

Assessment of the Load-Bearing Capacity of the Old Railway Bridge With Encased Steel Beams

The subject of this study is the inventory and assessment of the load-bearing capacity of a railway bridge. The analysis was conducted on a bridge with a concrete slab reinforced by steel beams as the load-bearing element. The structure is located in the village of Bukowina, Warmian-Masurian Voivodeship, Poland. The aim of the study is to assess the load-bearing capacity of the bridge. The structure’s geometry was measured, and its concrete strength class determined. This article presents the results of stress limitation calculations for the tested bridge. Three calculation procedures based on two approaches—a concrete-encased beam and a bending section reinforced with rigid inserts — were applied. The analysis demonstrated that, despite the unesthetic appearance of the bridge, it meets the stress limitation requirements. Though over a hundred years old, the structure remains suitable for use today.

Robust mechanical models for shear and punching strength of concrete slabs

AbstractThe mechanical models in this paper are based on the nominal concrete tensile strength that is considered to be the average strength along a 150 mm long brittle failure crack. Fracture Mechanics principles are applied for addressing the size effect of brittle failures; increasing member size leads to decreasing failure stress. The shear force is assumed to be carried by the compression chord of the slab and by aggregate interlock along part of the initial shear crack. Shear failure occurs when the aggregate interlock reaches its failure force that is assumed to be constant for effective depths larger than 1.0 m. The slab outside the initial shear crack around the column in a flat slab is assumed to be suspended into the stable inverted pyramid above the column. Punching failure occurs when the suspension force fails. These failure modes are used for validation of the provisions for shear and punching in the new version of Eurocode 2 (EC2). It is found that EC2 still underestimates the size effect and the code also incorrectly claims that punching failure is related to shear failure. Corrections of these two cases are proposed in the paper. A formula for design of shear reinforcement for flat slabs is proposed where iteration is not required. The paradox that brittle punching failure can cooperate with flexible shear reinforcement is explained. An additional formula is derived that gives a flat slab such large ductility that brittle punching failure can be considered as eliminated.