Behavior Of Hollow Core Slabs Research Articles

Fire behaviour of hollow core slabs

The objective of our research was to develop an optimum concrete mix for fire resistance, that is usable for hollow core slab prefabrication. Small-scale and full-scale tests were done, and the results were used to calibrate a numerical model that can be used to determine the temperature and load capacity reduction of steel reinforcement. With the help of the calibrated model, the effects of geometric variables were examined. The thickness of the ridge between the cavities has no effect on the change in the maximum temperature of the protected side in the examined 30-minute range, but the minimum temperature decreased.

Assessment of the torsional behaviour of hollow core slabs

There are many applications in buildings in which precast pre-stressed hollow-core (HC) slabs are subjected to shear, torsion, or combined shear and torsion. Nonetheless, extruded HC units contain no transverse reinforcement, being inherently vulnerable to brittle failure modes due to shear and torsional actions. In previous work by the authors, a finite element (FE) modelling approach for HC units failing in shear was developed and validated against experimental test data. This paper aims to extend the applicability of the proposed FE approach to help improve the understanding of the torsional behavior of HC units. For this purpose, the developed model is further validated against experimental data available in the literature and then used to predict the torsional capacity of New Zealand-specific 200 mm deep HC units. Results suggest that the FE model is capable of predicting the capacity of HC slabs with and without eccentricity of the applied vertical load. Finally, the numerical results are used to evaluate the performance of available simplified analysis approaches for assessing the torsional capacity of HC units, which are found to be non-conservative if used with expected material properties.

Capacity and failure modes of restrained hollow core slabs taking into account compressive membrane action

AbstractThe formation of compressive membrane action in restrained precast concrete hollow core slabs is investigated, and how it affects the load bearing capacity and failure mode under excessive loading. To this end, attention is oriented to a recently completed experimental campaign specifically aimed at quantifying this phenomenon in real‐scale hollow core slabs. The outcomes of this experimental campaign are then analyzed using a validated finite element model, to further study the effects of compressive membrane action on the structural behavior of hollow core slabs. From the numerical evaluations, it can be concluded that the ultimate capacity and failure mode of restrained hollow core slabs strongly depend on the restraint conditions, as well as the span‐to‐depth ratio. The results also indicate that in some cases the failure is caused by crushing of concrete in the top flange, which is typically not observed in case of simply supported boundary conditions. Additionally, the influence of the inherent uncertainties associated with material properties and geometry on the ultimate capacity and failure mode of restrained hollow core slabs is investigated. Probabilistic model evaluations are performed for a wide range of boundary conditions. The results indicate a very high influence of the uncertainty on the concrete tensile strength on the membrane action in precast hollow core slabs.

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.

Fire behavior of shallow prestressed hollow core slabs from computational modeling

Abstract Prestressed hollow core slabs are one of the structural systems whose use has increased the most in recent years in Brazil due to its efficiency and versatility. They can be used in many types of structural systems, such as masonry, precast concrete, cast-in-place concrete and steel structures. However, there are few analytical models to evaluate the fire behavior of hollow core slabs. In a simplified way, the fire resistance is evaluated indirectly through the minimum distance of the surface in contact with fire to the reinforcement axis. In this paper, some numerical models in finite element software were developed to analyze the variation of temperature with fire exposure time of shallow hollow core slabs, focusing on the presence of voids in the transversal section of the slab. The 500 °C isotherm method applied to 20 cm high slabs confirmed the Standard Fire Resistance obtained from the tabular method. However, when applied to shallow prestressed hollow core slabs that are 16 cm high, the 500 °C isotherm method indicated that the Standard Fire Resistance of these slabs is lower than values obtained from tabular methods.

Behavior of Hollow Core Slabs Reinforced with GFRP Prestressing Bars

An experimental program was designed to study the behavior of full-scale hollow core slabs prestressed with Glass Fiber-Reinforced Polymer (GFRP) bars in the concrete laboratory at the college of engineering, Mataria, Helwan University, Cairo, Egypt. The hollow core slabs were load-tested under uniformly distributed load. The GFRP bars were manufactured from locally available materials with a 10 mm nominal diameter. To improve the bond properties between the bars and concrete, GFRP threads were axially wrapped around the bars manually. The mechanical properties of the bars were investigated in the laboratory. The bars average ultimate tensile strength (fu) and elastic modulus (E) were 1000 MPa and 46 GPa, respectively. Four full-scale concrete hollow core slab specimens with characteristic strength of 80 MPa were constructed and solely reinforced with a single prestressed GFRP bar. Each slab specimen represented one complete vent with a width of 140 mm, 150 mm thickness and 4000 mm total length. These specimens were simply supported during the experiments where the GFRP bar was placed at the centerline of the vent near the soffit. The Bars were prestressed to different stress levels, namely (10, 20, 30 and 40%) of their ultimate tensile strength (fu). All slab specimens were load-tested under uniformly distributed load. The deflection, strain and crack pattern were investigated during load-testing. From the obtained results, it was observed that the optimum prestressing level was 20% of the ultimate tensile strength of the bar for both the moment carrying capacity and the deformation.

Hollow Core Slabs on Winkler Foundation

This research dealt with the linear elastic behavior of hollow core slabs resting on a linear Winkler type foundation. A finite difference method was used to model the slabs as wide beams and the foundation as elastic springs. The finite element method was also used to model the problem using ABAQUS 6.10 software program. A comparison between the method proposed in this paper and methods from previous studies was made to check the accuracy of the solutions. Several important parameters were incorporated in the analysis, viz. the hollow core size and shape, subgrade reaction and slab depth, to trace their effects on deflections, bending moments and shear forces. A computer program coded in Fortran was developed for the analysis of hollow core slabs on an elastic foundation. It was found that the maximum difference in deflection between the present study and the exact solution was 3% for the finite difference and 7% for the finite element method.

Precast concrete hollow core slabs exposed to elevated temperatures in terms of shear deteriorations – review article

Hollow core units were developed in the 1950s when long-line prestressing techniques evolved. Ever since then extensive studies followed concerning this particular field for concrete structures. Design methods of hollow core slabs have no requirements for transverse reinforcements which make it more prone to shear failure especially at elevated temperatures. It is understandable that the behaviour of hollow core slabs under elevated temperatures is more complex than that of solid slabs. The longitudinal wholes cause discontinuity in thermal transfer even though webs are effective parts transferring thermal loads to the unexposed parts of the structural element. Even if several influencing parameters have been investigated in order to evaluate their influence on different failure mechanisms of the hollow core slabs at elevated temperatures, still further efforts are needed. This paper presents a review for influencing parameters, failure modes and code provisions for hollow core slabs at elevated temperatures.

Analytical and numerical studies on hollow core slabs strengthened with hybrid FRP and overlay techniques

The objective of this study is to understand the behaviour of hollow core slabs strengthened with FRP and hybrid techniques through numerical and analytical studies. Different strengthening techniques considered in this study are (i) External Bonding (EB) of Carbon Fiber Reinforced Polymer (CFRP) laminates, (ii) Near Surface Mounting (NSM) of CFRP laminates, (iii) Bonded Overlay (BO) using concrete layer, and (iv) hybrid strengthening which is a combination of bonded overlay and NSM or EB. In the numerical studies, three-dimensional Finite Element (FE) models of hollow core slabs were developed considering material and geometrical nonlinearities, and a phased nonlinear analysis was carried out. The analytical calculations were carried out using Response-2000 program which is based on Modified Compression Field Theory (MCFT). Both the numerical and analytical models predicted the behaviour in agreement with experimental results. Parametric studies indicated that increase in the bonded overlay thickness increases the peak load capacity without reducing the displacement ductility. The increase in FRP strengthening ratio increased the capacity but reduced the displacement ductility. The hybrid strengthening technique was found to increase the capacity of the hollow core slabs by more than 100% without compromise in ductility when compared to their individual strengthening schemes.

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.

Effect of carbon fibre-reinforced polymers on structural behaviour of hollow-core slabs

This paper presents an experimental study on the effect of carbon fibre-reinforced polymers (CFRP) on the flexural and shear capacity of precast, prestressed, hollow-core slabs. The average cubic compressive strength of the concrete used for casting the hollow-core slab is 45 MPa. Depending upon testing type and CFRP pattern, slab specimens are divided into eight groups, including two groups comprising non-strengthened/control slabs. The remaining six groups comprise CFRP strengthened slabs with six different patterns, three for flexural testing and another three for shear testing. The effects of CFRP strengthening on flexural and shear strength are studied and compared to the control slabs. The effects of CFRP strips on crack pattern, failure behaviour, deflection and energy dissipation of hollow-core slab units are also discussed. It is concluded that CFRP can enhance the flexural and shear capacity of hollow-core units by up to 40%. Similarly, energy dissipation is found to be increased for CFRP strengthened slabs with relatively fewer cracks and smaller deflection as compared with their counterparts.

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.

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.