Prestressed Hollow Core Slabs Research Articles

Experimental evaluation of bonded overlay and NSM GFRP bar strengthening on flexural behavior of precast prestressed hollow core slabs

A hollow core slab is a precast prestressed concrete member with longitudinal cores that reduce its self-weight. Strengthening of prestressed hollow core slabs is required for several reasons to maintain their structural integrity. The main focus of this research is to understand the flexural behavior of hollow core slabs and failure modes with hybrid strengthening techniques including bonded overlay and near surface mounted (NSM) glass fiber reinforced polymer (GFRP) bars. The test variables include different strengthening techniques such as (i) bonded overlay with and without shear keys, (ii) NSM GFRP rebar and (iii) hybrid strengthening combination which includes a thin bonded overlay in the compression region and NSM GFRP bars in the tension region. A total of seven full scale slabs were cast, strengthened and tested until failure. Test results show that bonded overlay increases the flexural strength by 89% without much compromise on the ductility when compared to control slab, whereas NSM GFRP bar strengthening increases the strength by about 100% but with reduced ductility. Hybrid strengthening led to a highest increase in the ultimate strength (about 200%) and ductility when compared to only NSM and only bonded overlay strengthening.

PERFORMANCE OF PRESTRESSED HOLLOWCORE SLABS WITH AND WITHOUT CUTOUTS

PrestressedHollowcore slabs are typicallyprecast using an extruder machine withprestressing strands as the only internal reinforcement. Various building servicesrequire cut-outs to be provided in these precast slabs.Cut-outs/openings in hollowcore slabs can coincide with the location of strands and may lead to their curtailment.An experimental study was carried out to evaluate the effect of cut-outs in hollowcore slabs. Four full scale tests were conducted by consideringtwo shear span to depth ratiosand out of which two slabs had flexural opening. The strength of concrete, prestressing ratio and size of opening was kept constant in all slabs. It was observed that, provision of cut-outs not only reduced the cracking and ultimate strength of slabs, but also altered the failure modeswith change in deformation capacity.

Numerical web-shear strength assessment of precast prestressed hollow core slab units

The shear strength of precast prestressed hollow core (PPHC) slabs is numerically assessed for 200, 265, 320, 370, 400 and 500mm thick units, the cross-sections of which present both circular and non-circular voids. The evolution of shear stress distributions and crack patterns is predicted by detailed nonlinear solid finite element (FE) analyses, matching experimental test data. A comparison is provided between experimental results and analytical estimates obtained by common design Codes (EC2, EN 1168, ACI and CSA), quantifying the inaccuracy of previous proposals which was shown to be particularly evident for deep slab sections with flat webs where the shear stress peak is localized below the centroidal axis. Numerical observations revealed the sensitivity of web-shear failure mechanism and related shear capacity to hollow core shape and related non-circularity of the voids, inherent web width variation along depth and concrete chords above and below the void. In light of these trends, a closed-form expression is proposed to be used as a preliminary-design-stage tool for analytical web-shear strength assessment of these members.

프리스트레스트 중공 슬래브와 현장타설된 토핑콘크리트의 수평전단성능 평가

Prestressed hollow-core (PHC) slabs are structurally-optimized lightweight precast floor members for long-span concrete structures, which are widely used in construction markets. In Korea, the PHC slabs have been often used with cast-in-place (CIP) topping concrete as a composite slab system. However, the PHC slab members produced by extrusion method use concrete having very low slump, and it is very difficult to make sufficient roughness on the surface as well as to provide shear connectors. In this study, a large number of push-off tests was conducted to evaluate interfacial shear strengths between PHC slabs and CIP topping concrete with the key variable of surface roughness. In addition, the horizontal shear strengths specified in the various design codes were evaluated by comparing to the test results that were collected from literature.

Modelización de tableros de obras de paso de luces cortas ejecutados con placa alveolar pretensada

Traditional methods of plane grilled modeling used for the calculation of prestressed hollow core slab bridge decks not provide sufficient accuracy to determine the influence of phenomena such as slabs torsion or shear between the interface slab-toping. A new hybrid modeling approach is proposed. It allows more realistically quantify the resistant mechanism of this type of decks. The application of the method on a typical deck has yielded results that show the good correlation between the numerical predictions and the behavior observed in reality.

Advanced Analysis of Prestressed Hollow Core Concrete Slabs Exposed to Different Fires

This paper discusses the nonlinear structural analysis of full-scale prestressed hollow core one-way spanning concrete slabs in fire conditions. The slabs were simply-supported and reinforced with 9.3 mm nominal diameter seven-wire mono-strand tendons. Nonlinear 3-D finite element models for the analysis of prestressed hollow core concrete slabs at elevated temperatures were developed. The finite element models were based on a nonlinear finite element modelling approach adopted by the author over the last eight years for the analysis of different structures under fire conditions. The mechanical and thermal material nonlinearities of the concrete and prestressing tendons have been carefully inserted into the model. The interface between the tendon and surrounding concrete was also modelled, allowing the tendon to retain its profile shape during the deformation of the prestressed hollow core slab. In addition, gap elements were used to model the thermal behaviour of the hollow cores, which ensures, for the first time, correct temperature distributions in the regions surrounding the cores. The temperature distribution throughout the prestressed hollow core concrete slab, time-deflection behaviour, time-longitudinal expansion and failure modes of the slabs were predicted by the model and verified against test data. A parametric study was conducted to investigate the effects on the global structural behaviour due to the change in the prestressed hollow core slab geometries, hollow core shapes, load ratios during fire and different fire curves. It was shown that the failure of the prestressed hollow core concrete slabs investigated in this study was due to tensile splitting with longitudinal cracks along the slab directly above and inline with the tendons developed between 15–25 minutes from the start of heating. It was also shown that no shear failure was observed. The fire resistances of the prestressed hollow core concrete slabs obtained from the finite element analyses were compared with the design values obtained from the British Standards (BS) and European Code (EC) for concrete structures at elevated temperatures. The comparisons have shown that the BS were generally conservative for the prestressed hollow core concrete slabs investigated in the parametric study, except for the slabs heated using the standard fire curve under a higher load ratio of 0.7, while the EC predictions were conservative for all the slabs investigated in this study.

Numerical analysis of prestressed hollow core slabs under long term loading

This paper presents a numerical analysis of prestressed hollow core slabs under long term loading. The model considers the time dependence of material and rheological properties in order to predict the actual stage of displacements, strains and stresses. It also takes into account load changes. For the analysis, each slab is divided in a finite number of bar elements, in which the cross section is described in concrete elements, parallel to the flexural axis, and prestressed steel elements. For the results evaluation, the effective concrete area is considered. The numerical results are compared with experimental tests performed on two series of prestressed hollow core slabs. Each series had a different geometry, rate and distribution of prestressing strands. Mid-span displacements were evaluated up to 127 days after initial loading. Good correlation was achieved with both series at and below the service load level.

Load Width Effect on The Shear Behavior of Prestressed Hollow Core Slabs

Instead of performing a costly experimental study to find out whether the effect of different factors on the prediction of shear behavior of hollow core slabs, a numerical model can be developed. From the numerical analysis, the effective of load width across the slab width can be derived. This paper present the shear behavior of prestressed hollow core slabs and examined the effect of load width (in absence of concrete topping and filling of cores) on their effective shear behavior. In the present investigation, physical tests and 3-D finite element models were developed using the general-purpose finite element software ANSYS 11.0 to simulate the behavior of tested slabs. The specimens used in this research have a depth of 150mm and the numerical models are calibrated using the data obtained from physical experiments. TheANSYS finite element analysis results are compared with the experimental data of two prestressed hollow core slabs. The comparisons are made for load-deflection curves, failure loads and crack patterns, respectively. The accuracy of the finite element models is assessed by comparison with the experimental results, which are in good agreement.

Influence of reinforcement arrangement in flexural fire behavior of hollow core slabs

This paper describes standard scale fire tests conducted on four precast prestressed hollow core slabs configurations under flexure. The influence of the reinforcement arrangement was investigated using two test parameters: the average axis distance (from 30.1 to 46.7mm), and the type of reinforcement (5mm diameter wires and 9.5mm diameter strands). An ISO 834 standard fire was applied maintaining the load level in a four-point test arrangement constant. The results presented different failure mechanisms, types of cracks, spalling patterns, and fire resistance ratings and it was shown that wires displayed optimal behavior as opposed to strands. A comparison was carried out with the current verification methods provided in Eurocode 2 part 1–2, and showed that verification methods tend to overestimate the calculation of fire resistance when both wire and strands are moved away from the exposed surface.

Failure modes and shear design of prestressed hollow core slabs made of fiber-reinforced concrete

Hollow core slabs (HCS) are usually precast by extrusion and it is not easy to place stirrups; thus, it is difficult to guarantee shear resistance in some cases. This paper describes an experience using fiber-reinforced concrete (FRC) to produce HCS by extrusion to gain shear reinforcement.An experimental program consisting of 26 HCS was developed. Elements were produced and tested in shear according to the following variables: amount of steel fibers (0, 50 and 70kg/m3) and a shear span/depth (a/d) ratio of 2.3–4.4 and 8.6.Different failure modes took place. Some of the main conclusions drawn were that fibers improve quality of the material for shear, HCS with fibers achieved greater loads than HCS without fiber reinforcement and with a more ductile behavior.

Finite element modeling of pre-stressed hollow core slabs

Finite element modeling of pre-stressed hollow core slabs

Modelling the Fire Resistance of Prestressed Concrete Floors using Multi-Spring Connection Elements

The fire resistance of precast prestressed concrete floor systems is heavily influenced by the end connections and the stiffness of the surrounding structure, both of which must be considered in any analysis. Analysing floor slabs with beam or shell elements whose end nodes share the nodes of supporting beams leads to a major problem for precast prestressed flooring systems where the steel tendons terminate at the end of the flooring units, because the approach of sharing nodes of the supporting beam and floor assumes that these tendons are anchored into the supporting beams. In order to solve this problem, a "multi-spring" connection element has been developed. Experimental data available from previous research work has been used to validate the finite element model. It is concluded that the inclusion of the multi-spring connection results in more accurate predictions of prestressed hollowcore slab behaviour in fire conditions.

Study of Shear Capacity in Brazilian Prestressed Hollow Core Slabs

Hollow core slabs have a bold design and can be used in a wide variety of structural systems, performing like floor slabs or wall panels. These slabs are economical due to geometrical and mechanical efficiency of its cross sections, allowing the rationalization of materials consumption (concrete and prestressed strands). The increase of the use of hollow core slabs within the Brazilian construction market has caused a significant boost of production, demanding the assessment of the actual strength capacity of these elements. This paper presents results relative to several hollow core slabs produced in a Brazilian factory, subjected to shear tests according to FIB recommendations. The experimental results were compared with those obtained with the equations from the Guide to Good Practice: Special Design Considerations for Precast Prestressed Hollow Core Floors, published by FIB in 2000.

Belgrade's Delta City shopping mall – design and construction

The structure of Delta City shopping mall in Belgrade consists of two separate structures: the structure of the mall and the structure of the multi-storey open garage. The overall dimensions of the irregular layout of the structure are 210 m × 120 m, with four main levels in the mall and five parking levels in the garage. Because of the different exposure conditions, the structure of the garage is separated from the mall's structure with an expansion joint at all of the levels, except the level of the foundation slab. The mall's framed structure is designed without any expansion joints, except for temporary joints during the construction stage. It consists mainly of reinforced concrete columns cast in place, at typical spans of 8·2 m × 8·2 m, precast reinforced concrete simple beams and precast prestressed hollow core slabs. The moment resistant reinforced concrete frames cast in place are designed mostly at the façade, to provide additional seismic resistance and to control lateral deflections. The main structure of the open multi-storey garage consists of reinforced concrete frames cast in place and precast hollow core slabs, and it is designed for exposure class XD3, with special attention paid to durability requirements. Owing to the mainly precast structure, especially the hollow core slabs as a floor solution, the complete concrete structure of approximately 80 000 m2 was constructed in less then 12 months.

Shear capacity of prestressed hollow core slabs in slim floor constructions

Research in Finland and Germany reveals that the shear strength of hollow core slabs is reduced considerably due to transverse stresses when the slabs are bedded on flexible supports (e.g. slender beams). The aim of the present investigation is to analyse the load bearing behaviour of hollow core slabs supported on shallow steel beams, so-called slim floor constructions. This paper describes four full-scale tests on two-span floor systems and reference tests on single slabs with rigid supports. The observed shear strengths are compared with predicted failure loads according to international code provisions. Finally, recommendations regarding the design and the construction on flexible supports are given.

Finite Element Modeling of Prestressed Hollow Core Slab Strengthened with CFRP Sheets in Flexure and Shear

Precast concrete construction with prestressed precast hollow core slab as floor and roofing is being extensively used in the Gulf region. These one way slabs are sometimes subjected to unforeseen loads with a partition wall in shear zone or as a cantilever. A detailed experimental program involving full-scale load testing of hollow core slabs has been conducted involving testing of virgin and CFRP strengthened slabs in flexure and shear. This paper presents the results of the experimental and numerical evaluation of flexural and shear behavior of the hollow core slabs. The hollow-core slabs were modeled using the nonlinear finite element software DIANA and the load deflection behavior and ultimate capacity were evaluated numerically. For the FE model, concrete was modeled using Drucker-Prager with tension cut-off failure criterion, and Von-Mises plasticity with multilinear isotropic hardening was used to model the prestressing strands. Comparison between the experimental and the numerical results indicated that the finite element model predicted experimentally determined results successfully.

Shear and torsion in prestressed hollow core units: finite element analyses of full-scale tests

The present calculation methods for shear and torsion in prestressed hollow core slabs add stresses from various influences without taking into account deformations and compatibility, the softening of cracking concrete, or restraint at the boundaries, therefore they are most likely conservative. The main purpose of this work is to establish three-dimensional finite element models, which can be used both to analyse the effect of parameters that influence the shear and torsion response and to be included in global models of complete floors. An important aspect was therefore to simplify the models to avoid time-consuming analyses. Coarse meshes with solid elements were combined with beam elements. The established models were validated by simulating a series of full-scale tests conducted on both 200 mm and 400 mm thick hollow core units subjected to various combinations of shear and torsion. In general, although very coarse meshes were used, the finite element analyses of the tests succeeded in describing the overall behaviour, crack pattern, failure mode, and maximum load, with a reasonably good agreement.

Finite element modelling of reinforcement with bond

This paper presents an efficient numerical model for simulating the bond between reinforcing bars and concrete. It is based on the one-dimensional geometry of reinforcing bars, and it considers interface surface properties. In spite of its relative simplicity, it allows the application of the same physical models as in the case of a full surface interface. An arbitrary bond-slip relationship can be implemented within the model. Four sample analyses are presented: reinforcement bar pull-out experiment, an example of a beam collapsing due to shear failure, an analysis of the shear strength of pre-stressed hollow core slabs and an assessment of serviceability of a pre-stressed concrete slab in the newly built ice hockey arena in Prague.

Shear Resistance of PHC Slabs Supported on Beams. II: Analysis

A simple calculation model is proposed for prediction of the shear resistance of prestressed hollow-core (PHC) slabs supported on beams. A simplified principal stress criterion for the web shear failure is introduced. The criterion is based on three stress components. The vertical shear stress τzy and the axial stress σy are calculated as for slabs supported on nonflexible bearings. The transverse shear stress τzx is calculated applying beam theory to a composite beam comprising the beam, joint concrete, a section of the slabs, and the optional topping. The width of the section is an experimental parameter that takes into account the degree of composite interaction between the slabs and the beam. It is assumed to be proportional to the span of the beam. Also presented are the modifications needed when the beams are continuous, the floor is covered with reinforced concrete topping, or the ends of the hollow cores are filled with concrete.

Design of Prestressed Hollow Core Slabs with Reference to Web Shear Failure

A design procedure to determine the capacity against shear failure of webs of prestressed hollow core slabs is presented. The additional shear stresses in webs caused by prestress forces of strands have been taken into account in this procedure. The proposed procedure has been applied to model slabs and also to test specimens. The capability of the proposed procedure to predict the shear failure of web has been demonstrated by the obtained good agreement between the results predicted by FEM and the results predicted by the suggested formula. Agreement was also good between the test observations and the results predicted by the suggested formula. The proposed procedure can more accurately predict the capacity against shear failure of web than the formulas suggested in design codes. The procedure is reasonable and general in nature, and it can be applied to different loading cases, prestress levels, types of cross section, material, and geometric parameters.