Core Slabs Research Articles

Analytical investigation of the influence of various void shape and spacing on the load-bearing behavior of concrete hollow core slabs

Analytical investigation of the influence of various void shape and spacing on the load-bearing behavior of concrete hollow core slabs

Analysing the Performance of Hollow Core Slabs Strengthened with CFRP

External bonding with CFRP (Carbon Fiber Reinforced Polymers) has been investigated over the last decade, as it represents a viable technique of strengthening existing prestressed concrete structures, including hollow-core slabs (HCS) with non-circular voids. The high performance of these carbon fibers has been validated through a large volume of experimental and numerical research and yet there are a few issues which remain controversial in simulating their behaviour with the finite element modelling. Although the CFRP mechanical properties are provided by the manufacturers, they are not satisfactory for a complete understanding of the analysis and design approach of HCS strengthened with CFRP. The present research is conducted on prestressed HCS with non-circular voids. The strengthening method consisted in the application of the composite material on the slab’s end internal regions of the voids, on a 500mm length: 1 layer and 2 layers. The objective of this study is to emphasize the effect of damage in the CFRP strips and moreover the interface effectiveness on the CFRP strengthened HCS. Damage is predicted using Hashin’s initiation criteria and the cohesive behaviour in the interface is used to analyse the epoxy resin which bonds the CFRP sheets to the hollow-core units. A plastic damage model was used for modelling the concrete, after a parametric study regarding the dilatancy angle and viscosity parameter was conducted for the most appropriate choice of concrete damage plasticity parameters. The overall procedure consists of numerical FE modelling in Abaqus software. Two different modelling possibilities of CFRP-to-concrete interface were studied: a tie constraint connection was first used and secondly the contact bonding was defined with the cohesive behaviour option of the contact interaction property. The results are provided in terms of load-displacement response, equivalent plastic strain and distribution of Von Misses stresses in the CFRP strips.

Mode coupling at avoided crossings in slab waveguides with comparison to optical fibers: tutorial

Avoided crossings are important in many waveguides and resonators. That is particularly the case in modern-day solid-core and air-core optical fibers that often have a complex geometry. The study of mode coupling at avoided crossings often leads to a complicated analysis. In this tutorial, we aim to explain the basic features of avoided crossings in a simple slab waveguide structure so that the modes can be found analytically with simple sinusoidal and exponential forms. We first review coupled-mode theory for the guided mode in a slab waveguide, which has a higher index in the core. We study the effective index of the guided true mode for a five-layer slab waveguide including two core layers with higher indices compared to the indices in the three cladding layers. Then, we study the same structure by using the overlap between approximate modes confined in the two individual core slabs. When the two individual core slabs are not near each other, the avoided crossing using the true modes within the two-slab waveguide agrees well with the results using the overlap between the two approximate modes. We also study coupled-mode theory and avoided crossings for leaky modes in an antiresonant slab waveguide. We obtain good agreement between the results using the true leaky mode and the results using the overlap between approximate modes. We then discuss examples of avoided crossings in solid-core and air-core optical fibers. We describe the similarities and differences between the optical fibers and simple slab waveguides that we have analyzed in detail.

Concrete precast rejects – treatment, characterization, and application in new concrete as recycled aggregate

This article evaluates the feasibility of using concrete waste from precast production as recycled aggregate in concrete. The processing of concrete waste employed a jaw-hammer crusher and a sieve, producing three types of Recycled Concrete Aggregate (RCA). After their characterization, RCA was incorporated in two types of concrete used in the precast factory: a flowing concrete (FC), slump 220 mm, employed in columns and beams; and the second one, a no-slump extruded concrete (NSC), used for hollow core slabs. X-Ray diffraction, thermogravimetric analysis and scanning electronic microscopy showed phases from the hydrated cement paste and the original aggregate. Results of mechanical performance showed that RCA did not influence the compressive strength but influenced other properties such as water absorption and modulus of elasticity. Lastly, it was concluded that RCA obtained in the precast factory showed great potential to be used within the factory production process.

Probabilistic Assessment of Sandwiched Concrete Slabs in Deflection

Abstract: This study presents a probabilistic evaluation of concrete sand-witched hollow core slabs in accordance with the design requirements of BS8110 (1985; 1997) and Eurocode 2 (2008). The First Order Reliability Method (FORM) was used in computing the probability of failure. For the deflection failure, the effect of varying the load ratio and the breadth of slab on the reliability analysis were carried out at the following values of concrete strengths ݂ܿu: 25N/mm2 , 30 N/mm2 , 40 N/mm2 , 50 N/mm2 . The results indicate that deflection characteristics of the slab are directly affected by the concrete strength, loadings, breadth of slab selected. Thus, the deflection of the slab increases with increases in loading, hollow core, breadth of slab and decrease in concrete strength. This shows that the safety index of reinforced sandwiched concrete slab in deflection generally decreases as the load ratio and breadth of slab increase for each combination of concrete strength. Keywords: Probabilistic Analysis, Sandwiched Concrete, Reliability Analysis, Stochastic Model, FORM

Composite action on web-post buckling shear resistance of composite cellular beams with PCHCS and PCHCSCT

This paper aims to study the composite action on web-post buckling (WPB) resistance of composite cellular beams with precast hollow core slab (PCHCS) units and with precast hollow core slab units with concrete topping (PCHCSCT). Geometrical and material non-linear analyses are performed. A parametric study is developed, varying the opening diameter, the web-post width and the shear stud spacing, with single and double row. It was found that the models with double row of shear studs showed global shear resistance greater than or equal to the models with only one row. For models with PCHCS, there was a reduction in global shear resistance in some analyses due to the absence of shear studs in the second opening, close to the support. For PCHCSCT models, there was an increase in the global shear resistance, in comparison with PCHCS models. However, in most of these models, shear stud rupture occurred. Finally, the WPB models were compared with a simplified equation that takes into account the global shear resistance due to buckling, composite action and Vierendeel bending. The average, standard deviation and variation value of the ratio between the analytical and numerical models were 0.833, 12.47% and 1.55%, respectively.

Experimental evaluation of transmission length equations for pre-tensioned hollow core slabs

This paper presents an experimental evaluation of transmission length in hollow core slabs against 18 suggested equations in some codes of practice in addition to some equations from the previous literature. The experimental transmission length was predicted at 95% average mean strain (95% AMS) by measuring changes in strain after wire cutting on the near concrete face using both electric strain gauge and demountable mechanical strain gauges (DEMEC). The ratio between the measured transmission length and the value predicted using codes of practice equations varied from 1.0 to 1.26. On the other hand, the variation was 0.59 to 1.97 for the equations from the previous literature. The experimental results also confirmed the previous findings of non-linear prestress transfer over the transmission zone

Logging evaluation of lamina structure and reservoir quality in shale oil reservoir of Fengcheng Formation in Mahu Sag, China

The Fengcheng Formation in the Mahu Sag of the Junggar Basin is composed of a set of fine-grained sedimentary rocks located under a layer of alkaline lacustrine rocks. There are alternating dolomite lamina, siltstone and organic matter laminae that show strong heterogeneity. The lamina structure determines reservoir quality and oil-bearing potential in shales. Therefore, the identification of lamina structure is a crucial basis for identifying reserves of shale oil. Identified as massive, layered and laminated, three different types of lamina structures are divided according to core observations in terms of the density of the laminae and the overlapping relationships. High-resolution processing is also performed on dynamic image log data to form slab images for recognizing the various types of lamina structures. Compared to core slab photos, it is believed that slab images offer an excellent thin-layer resolution of up to a millimeter in scale and can effectively identify the lamina structure of the shale reservoir. There are no evident beddings on the core of the massive pattern, and the slab image shows a bright or dark block pattern. There are centimeter-level felsic bands of fine silt sand on the thin-layered sedimentary fabric core, and the slice image shows a band-like pattern. A large number of millimeter-level dolomitic laminae are enriched on the core of the laminated type, and the slab image shows millimeter-level light and dark lamina. Slab images were used for the division and identification of lamina structures in a single well. Then, the relationships between the three types of lamina structures and the two-dimensional nuclear magnetic resonance logs and oil test data were recorded. The results show that massive and laminated pores are dominated by intergranular pores with low porosity and movable oil content. Therefore, only low oil flow can be obtained during an oil test. However, the layered type due to centimeter-level silty bands, T1 and T2, is bimodal with high porosity, movable oil content, and high levels of hydrocarbon production. Therefore, the optimal areas of shale reservoirs are closely related to the layered type. The more significant the proportion of layered types in shale reservoirs, the higher the quality of the reservoir. The present study is intended to identify lamina structure-controlled reservoir quality and oil accumulation and to provide insights into the exploration and development of shale oil.

Evaluating the effects of different slab types on static and dynamic characteristics of structures

In this study, the effect of different types of slabs on dynamic characteristics of structures under the lateral loading was investigated. For this purpose, four different types of slabs namely, beamed slab, flat slab, one way ribbed (hollow core) slab and waffle slab have been modeled in buildings having 3, 4 and 5 storeys with the same geometric dimensions, in accordance to design and construction requirements (TS 500) and Turkish building seismic codes (TBDY, 2018). Seismic analysis calculations of the modeled buildings were done using the equivalent seismic load method. The assumed local soil class was taken from the geotechnical report as ZD. As a result of the analysis, natural periods, base shear forces, maximum horizontal displacements and relative storey drifts of the buildings were compared. Seismic analysis and calculations of the buildings were completed using SAP2000 finite element software.

Effect of Surface Roughness Characteristics on Structural Performance of Hollow Core Slabs.

This study was conducted to evaluate the flexural performance of hollow core slabs (HCS) incorporating the effect of surface roughness. The HCSs are suitable for long span structures due to reduced self-weight. The specimens were HCS with topping concrete and the variables were cross sectional height and surface roughness. The tests were conducted on simply supported beams under four-point loads. The results showed that specimens with interface roughness applied in the lengthwise direction of members exhibited ductile flexural behavior up to peak load than those with interface roughness applied in the member width direction. Their flexural strength was also higher by 1–7% on average, indicating that they are advantageous in improving structural performance.

Prestressed hollow core slabs for topped slim floors – Theory and research of the shear capacity

This research paper discusses the influence of the flexibility of girders in Slim Floor structures on the reduced load capacity of prestressed HC slabs. The current state of knowledge in the field of the transverse behaviour of hollow core slabs is presented. Contradictory information provided in the literature as well as the computational comparative analysis of the existing theoretical models has demonstrated the need for better recognition of the issue of the effect of reinforced concrete topping layer on the load capacity of HC slabs. The paper includes the presentation of experimental tests, the purpose of which was to reproduce the actual work conditions of hollow core slabs depending on their location on the floor and the numerical analyses supplemented these studies. As a result of the conducted tests and analyses, it was found that the increased flexibility of the support beam resulted in an adverse change in the stress state of the hollow core slab, which may lead to the exceedance of the limit state of shear capacity, while laying an additional layer of structural topping on precast hollow core slabs significantly increased the shear capacity of the floor, and thus safety of the structure. The research also indicates that in all the tested topped elements full composite action up to the failure was shown.

Analytical Investigation of the Load-Bearing Behavior of Light Weight Hollow Core Composite Slabs*

Slabs are considered the most important structural member but also the heaviest one. Therefore, in the last few years a new type of slabs was introduced, hollow core slabs with steel decks. On the other hand, the use of light weight concrete that has got a density less than (1850 kg/m3 ) reduces the self-weight of structures considerably. This paper conducts an analytical study of the flexural behavior of five slabs, a solid slab that acts as a reference, a hollow core slab with elliptical voids, a composite slab and two hollow core composite slabs using normal weight and light weight concrete using ANSYS software. All the slabs share the same size (2600 x 1200 x 140) mm. The results show that the use of hollow core slabs increases the load carrying capacity by about 32% while using light weight concrete reduces it by about 34% but it also decreases the self- weight and the deformation considerably. The use of light weight hollow core slabs promises with a wide horizon of privileges and improvements in the construction and research domain

Behaviour of prestressed hollow core slabs strengthened with NSM CFRP strips around openings: A finite element investigation

Prestressed Hollow Core Slabs (PHCSs) have become widely used in the construction industry owing to their economic benefits. During their service life, changes that would require openings to be situated along their spans may arise. A strengthening technique to recover or improve the original serviceability and resistances becomes indispensable. Limited research interest has been given to develop appropriate strengthening methods for the PHCSs with openings. The Near Surface Mounted (NSM) strengthening technique is among the practical and feasible solutions to rehabilitate or enhance the performance of these PHCSs. This paper demonstrates a Finite Element (FE) analysis technique to predict the responses of the PHCSs with openings, either unstrengthened or strengthened with NSM strips. The Concrete Damage Plasticity (CDP) model was successfully implemented to model the non-linear behaviour of concrete. The prestressing transmission length and the bond behaviour between the epoxy adhesive and concrete interfaces were put into practice. Acceptable agreements were found, upon validating the FE analysis results against the experimental data available in the literature. Maximum differences, compared to the experimentally attained results in the ultimate loads and the corresponding deformations of nearly 4% and −8.4%, respectively, were detected. An extensive parametric study was executed to assess the effect of various parameters on the overall behaviour of the examined slabs. The PHCS cross-sectional shape, CFRP reinforcement percentage, average precompression, opening location and size and the concrete compressive strength were taken as parameters. Above all, a gap of knowledge exits regarding the presence of design guidelines that evaluate reductions in the ultimate capacities of the PHCSs associated with the presence of openings at multiple locations along their spans and improvements experienced by employing the NSM strengthening technique. This could be bridged by utilizing the suggested modelling approach and the parametric study results in this research.

Method of manufacturing reinforced concrete floor slabs using non-removable void formers

The article provides a critical analysis of the work on the manufacture of monolithic reinforced concrete, relying on the contour, hollow floor slabs using box-shaped, spherical and tubular hollow-formers. The paper proposes a method of manufacturing monolithic reinforced concrete hollow floor slabs based on the contour using circular tubular polyethylene cardboard holders of rectangular or square shape in plan. The subject of the research is a method of continuous production of reinforced concrete floor slabs with round voids based on the contour using non-removable void formers, their technical and economic efficiency of solutions to reduce the weight of monolithic floor slabs by reducing the resources for their production. The technical and economic feasibility of manufacturing monolithic reinforced concrete floor slabs based on the contour with the use of non-retrievable void formers from round pipes of square and rectangular shape in plan has been substantiated. Materials for the manufacture of void former are proposed. The technical result has been determined - the maximum weight reduction of the proposed hollow core slabs compared to solid slabs.

Ultimate strength prediction of steel–concrete composite cellular beams with PCHCS

This paper aims to predict the ultimate behavior of steel–concrete composite cellular beams using precast hollow core slabs. A finite element model is developed by geometrical non-linear analysis. A parametric study is carried out, considering symmetric and asymmetric sections with precast hollow core slabs. The key parameters such as the web-post width and the opening diameter are varied, as well as the presence of the concrete topping. A total of 120 analyses were performed. The results are compared with typical composite slab models using cast in-situ concrete. For symmetrical sections, considering the hollow core slabs, although some observations occurred with the formation of the plastic mechanism, the predominant failure mode was the web-post buckling. For asymmetric sections, the predominant failure mode was the combination of the plastic mechanism with the web-post buckling accompanied by the shear connector rupture. In both cases, considering symmetrical and asymmetrical sections, excessive cracking was observed in the upper part of the hollow core slab. In cases where the end-post was greater than the web-post, there was damage at the upper region of the hollow core slab/concrete topping, close to the support. The numerical models of composite cellular beams with hollow core slabs, when compared with the models of composite cellular beams with composite slabs, showed greater efficiency in structural behavior. The results imply that the strength of the composite cellular beams was not limited only by the strength of the steel cellular beam, but also, of the slab, due to the resistance to shear stress.

Computation of Core Floor Slabs of Different Length

The purpose of this research is to study what the crosscurrent representation format of core slabs is for samples of different length, by investigating the effect of the above factors on the load-bearing and stress-related properties, as well as the cracking load. The authors examined samples of slabs of different length, freestanding and working in bending, with a cross-section in a natural form, in comparison with samples of similar length in an I-beam shape. The samples were systematically loaded an interval of one kN until the yield point of reinforcement in the tension area was reached. The authors completed a numerical study of core slabs in a nonlinear setting. The researchers designed load-deflection plots when modeling the corresponding lengths of samples with a natural and I-beam form of section. The scholars received and compared results with loading of the cracked condition, midspan deflections with a rupturing load. They found out that the length of the slab affects the calculation results of the shapes issued: the shorter the length, the more highlighted deviation in the load-deflection plots. The authors suggested correction coefficients to perform a refined calculation of slabs for cracking, deflection, and rupturing loads.

Application of the software package in the improvement of floors made of hollow core slabs

An overview of the rationale for the unweighted hollow-core floor slabs use is given. The technological and design features of hollow-core floor slabs are considered. The possibilities of rational use of unweighted hollow-core floor slabs are presented. The calculation of monolithic floor slabs was carried out in the Lira software package on the basis of existing technologies, current standards and the methods for calculating the unweighted hollow-core slabs. The performance of floor slabs with spans equal in two directions is considered. It was found that regardless of the span of the floor slab, the effect of reducing its own weight exceeds the effect of reducing the section rigidity. The total deflection due to the own weight of the floor slab and the payload on it for the hollow core slabs is lower than for solid slabs of the same span. The use of unweighted monolithic slabs provides a reduction in the dead weight of the floor slab up to 35%. Reducing the load on the slab also provides a reduction in the reinforcing steel materials application rate. The use of unweighted monolithic floors by reducing the weight of structures and reducing their deflections will significantly optimize the structural schemes of buildings. The performed numerical experiment makes it possible to establish the boundaries of the effectiveness of the unweighted hollow-core monolithic floor slabs‘ use, upon reaching which the use turns out to be significantly more profitable than the use of traditional beamless floors.

Hollow-core precast and two-way solid slabs: Cost estimation and time duration comparison

Time and cost are important factors affecting the successful completion of the construction building project. This study analyses and examines the cost and time comparison of precast and cast-in-situ slabs of a particular building. Taking into account that slab is one of the important structural members, this study will take the hollow core slab in particular which can define as precast prestressed concrete elements contain an empty void inside of it which mostly used for floor, roof slabs and wall panels. The results demonstrate that the precast slab construction time is extremely faster in comparison with the cast-in-situ slab as it took around 31 of working days and 43 days in total after considering the holidays while the precast slab took around 9 working days only and 13 days in total. The results also show that the cast-in-situ slab is inferior in both cost and time duration. As cast-in-situ cost is 3.76 times higher than the precast slab, and the time duration is 3.31 times longer.

Flexure shear capacity of prestressed hollow core slabs

ABSTRACT: Provided that the anchorage capacity is guaranteed at the supports, the bearing capacity of hollow core slabs depends on the shear capacity of the pretensioned concrete webs, wherein the critical section is in a region between h and 2h from the support. For line loads acting within 2h to 6h from the supports, especially for shallow slabs 150 to 200 mm deep, it is likely to have flexure-shear cracks within the transfer region, wherein the bearing capacity is highly affected by the actual prestressing forces at the critical section. Therefore, one of the major questions pondered by structural engineers is to determine the effective amount of prestressing force that affects the shear resistance mechanism near to the support. According to ABNT NBR 14861:2011, the shear capacity is based on the flexure-shear mechanism, wherein the shear strength is a sum of the tensile concrete strength in the slab webs plus the contribution of the prestressing forces at the critical section, wherein a coefficient of 0.15 is considered. However, in both codes NBR 14861 and NBR 6118 it is required that this coefficient 0.15 should be further multiplied by an additional reduction factor in order to take into account the effect of the transmission length near to the support. Considering the current revision of the NBR 14861, this paper presents a theoretical-experimental comparison from standard shear tests of hollow core slabs with nominal depths from 150 to 200 mm carried out in different research at NETPre-UFSCar. Based on the analytical study of each term of the equation for the flexure-shear capacity, it has been observed that the coefficient 0.15 provides a conservative limit for the contribution of the actual prestressing force. Therefore, there is no need to apply any additional reduction factor in order to guarantee a safe design limit for the shear capacity.

Circular perforated steel yielding demountable shear connector for sustainable precast composite floors

A circular steel-yielding demountable shear connector is proposed. The connector consists of a short perforated circular hollow section (CHS) welded on a base plate which in turn is bolted on the top flange of a steel section. The connector provides longitudinal shear resistance in all directions through yielding of steel and is used in conjunction with precast hollow core slab units. The mechanical behaviour is first studied using a previously validated detailed finite element model simulating horizontal pushout tests, by conducting a series of parametric studies to identify all failure modes of the novel connector. Based on the results of the numerical pushout tests, design equations to predict the strength of the connector are proposed. A composite beam using hollow core slabs and various degrees of shear connection is then simulated to verify the applicability of the circular connector and the implications in the design of composite beams.