Precast Concrete Hollow Core Slabs Research Articles

Test and analysis on in-plane shear behavior of precast concrete hollow core slabs

Precast concrete hollow core (PHC) slabs are mostly used in industrial and civil buildings because of the low weight and high efficiency of construction. The previous researches are mainly focused on the out-of-plane bending and shear behavior of PHC slabs. However, the in-plane shear behavior of the PHC slabs with high-strength tendons is not fully understood yet. In this regard, to investigate the in-plane shear behavior of PHC slabs, shear tests were conducted on a new long PHC slab with high-strength tendons and a new spliced slab in the present paper. The spliced slab was composed of two PHC short slabs with U-type bars, post-pouring concrete and anchorage bars. Subsequently, finite element models were established and validated by the test. Parametric analysis was conducted on the spliced slab to study the effect of the reinforcement ratio of tendons, concrete strength, tendon prestress, position of loading point, etc. Furthermore, based on the deep beam theory of single generalized displacement, the modified theoretical formulas on the in-plane initial stiffness of the specimens were illustrated. The results show that the mechanical behavior of the spliced slab was better than that of the long slab, including shear capacity, energy dissipation capacity and ductility. The initial stiffness and shear capacity of the spliced slab improved obviously with the reduction of the distance between the supports. The difference between in-plane initial stiffness of the PHC slab obtained from the modified theoretical formulas and that obtained from experiment are within 5%, which means the proposed formula can predict the in-plane initial stiffness of the PHC slabs well. This may provide a reference for the application of the long PHC and spliced slabs in engineering.

Fire resistance and associated failure modes of axially restrained hollow core slabs

When a severe fire occurs in the central part of a large hollow core slab floor, the thermal expansion of the affected floor members will be highly restrained by the stiffness of the surrounding structure. The present paper presents the development of a numerical model to evaluate the structural performance of precast concrete hollow core slabs exposed to fire, taking into account the interaction with the surrounding structure and how it affects the fire resistance and failure mode. The performance of the model is validated against fire tests reported in the literature. Subsequently, the model is employed to evaluate the influence of various degrees of restraint on the fire resistance of three hollow core slab sections, with depths ranging from 200 mm to 500 mm. The results indicate that restrained thermal expansion can significantly enhance the fire resistance of concrete hollow core slabs with low span-to-depth ratios. Conversely, in hollow core slabs with a high span-to-depth ratio, the restraint provided by the surroundings can considerably impair the fire performance. Additionally, it is shown that the failure mode of hollow core slabs can change from ductile bending failure to a brittle shear failure mode in the presence of axially restraining boundary conditions.

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.

A NOVEL STRATEGY FOR SLIM‐FLOOR FIRE PROTECTION

AbstractThis paper presents a novel slim‐floor configuration which consist of an I‐section steel profile welded to a wider bottom steel plate with an insulation layer of 5mm thick placed in between the two. The insulation layer is made up of a flexible and non‐combustible material, which protects the I‐section profile from the temperature rise through its bottom surface. Besides, this insulation layer remains protected from external agents with no need of maintenance because it is placed in the cavity created on purpose between the bottom plate and the lower flange of the steel profile thank to the weld between them. This new slim‐floor configuration is currently right‐protected by the Spanish Office of Patents and Trademarks with the reference number P201930438 and has been tested in the Universitat Politècnica de Valencia testing facilities to assess its thermal behaviour in fire. Specifically, the specimen was tested using an electric furnace where the slim‐floor is placed at the top surface. Therefore, the composite beam is exposed to the heat source only from the bottom surface, as it is the case in real fire scenarios. Moreover, the experimental campaign includes also a common slim‐floor specimen, without the insulation layer, in order to compare their thermal behaviour. The steel parts used in all the test specimens were grade S355, having a nominal yield strength of 355 MPa, the nominal compressive strength (cylindrical) of the cast in‐situ concrete was 30 MPa and the precast concrete hollow core slabs were manufactured with 45 MPa nominal compressive strength (cylindrical). In turn, the embedded reinforcing bars were grade S500, with 500 MPa nominal yield strength. The first experimental results have shown that the temperature gap between the bottom steel plate and the lower flange of the steel profile can reach up to 200°C. This impressive thermal gap delays the temperature rise along the I‐section steel profile which produces a significant improvement on the composite section fire resistant time, up to 90 or 120 minutes. Additionally, it should be highlighted that the observed improvement on the cross‐section thermal behaviour has been achieved using a novel configuration which no need maintenance in contrast with the common external protection systems using intumescent coating or other fireproofing materials.

Structural reliability of hollow core slabs considering compressive membrane action

Compressive membrane action can considerably improve the load bearing capacity of concrete slabs and beams in case of excessive loaded due to an accidental event. Currently, only limited research has been focusing on compressive membrane action in prestressed concrete elements, or on concrete elements with large cavities, such as precast concrete hollow core slabs. Therefore, a novel real-scale test setup has been developed in order to assess this effect in precast hollow core slabs, and how it can enhance the load-carrying capacity in accidental events. In parallel with these tests, a numerical finite element model has been developed in order to perform a more detailed structural analysis of this phenomenon, and to study the influence of various input parameters. The details of this test setup are briefly explained, and some relevant experimental test results are provided. Considering the experimental findings and validated numerical model, this contribution aims to quantify the influence of compressive membrane action on the structural reliability of precast concrete hollow core slabs. To this end, probabilistic models for the most important material and geometric variables are gathered, and the structural reliability is assessed using Latin Hypercube sampling. Overall, the results indicate that considering the formation of compressive membrane action strongly influences the variability of the ultimate load-carrying capacity of precast concrete hollow core slabs.

Flexural behavior of precast concrete hollow-core slabs with high-strength tendons

Precast concrete hollow core (PHC) slabs have been widely used in brick and concrete structures of rural buildings due to their economic and rapid construction in China. A new PHC slab with high-strength tendons and spliced slab made of the short slabs were proposed in this paper. To investigate the flexural behavior of the slabs, full-scale tests were conducted on three PHC slabs, including one long slab and two spliced slabs with side beams. In detail, the spliced slab was assembled by two short slabs through U-type bars, anchorage bars, and post-pouring concrete. The mechanical properties of specimens were evaluated including the failure modes, strain of concrete and bars, flexural capacity and deflection. The tests results shows that the flexural performance of the spliced slab is close to that of the long slab. Subsequently, finite element models were established and validated by the test data. Based on the validated models, an additional specimen made up of long slab and side beams, was studied to further verify the reliability of the spliced slab. And parametric analysis were carried out on the spliced slab to investigate the influence of prestress of tendons, diameter of tendons, concrete strength, etc. Moreover, the cracking and failure moment of the slabs were calculated by the formulas of GB50010 and ACI318, respectively. The calculated results were compared with the test and simulation results. The results proves that the prediction values calculated by the codes on the flexural capacity of the spliced slab is not conservative. Therefore, considering a reduction factor to correct the results calculated by the codes is necessary.

Experimental study of shear transfer in slim floor systems using precast concrete hollow core slabs and steel beam with web circular opening

Abstract Steel-concrete slim flooring system using precast concrete hollow core slabs and steel beam with web openings is an innovative construction system designed to combine the high bending resistance of both precast prestressed hollow core slabs and steel beam with web openings. This system can provide floor systems with a minimum constructional depth in comparison with ordinary composite floors. The aim of this study was to evaluate in an exploratory way the shear transferring mechanism between the steel beam with circular web opening and the precast hollow-concrete slab. The shear connection is formed by in-situ concrete passes through the web openings and infill the voids of the precast slabs. One push-out test was conducted to investigate the shear transferring mechanism of shear connection and the experimental results were compared to analytical methods. The shear resistance of the shear connection was predicted with good accurate by analytical methods.

Experimental and Numerical Investigations on Fire-Resistance Performance of Precast Concrete Hollow-Core Slabs

In this study, full-scale fire tests and finite element (FE) analyses are conducted to investigate the fire resistance performance of hollow-core slabs (HCSs) manufactured using the extrusion method. The deflection of the HCS specimens and the temperature distribution in the section according to the fire exposure time are measured and analyzed comprehensively, and the test results are compared with the FE analysis results. In addition, parametric analyses are conducted on 21 cases with the HCS depth, span length, hollow ratio in a section, cover thickness of concrete, and load ratio (i.e., the ratio of the external load to the ultimate load) as variables, based on which the fire resistance performance of the HCS according to each variable is investigated. The analysis results show that the load ratio is a key factor governing the fire resistance behavior of HCSs, whereas the effects of the cover thickness of concrete and the hollow ratio in a section are relatively slight within the range of variables examined in this study.

Out‐of‐plane flexural behavior of full precast concrete hollow‐core slabs with lateral joints

AbstractThis paper proposed a new lateral joint to connect precast concrete hollow‐core slabs (PCHSs) and increase their structural assembly efficiency. The main sensitive parameters are the longitudinal ribs and the number of proposed joints. Out‐of‐plane bending performance tests were conducted on four PCHSs with lateral joints (PCHS‐LJs) to study the effects of the main sensitive parameters on the mechanical properties and failure modes of the PCHS‐LJs. The test results showed that there is no obvious relative slip in the joints between the precast bottom slab and the upper post‐pouring concrete before PCHS‐LJs reaching the ultimate load. The transverse directions of PCHS‐LJs are the main force bearing directions and the cracks are mainly occurred in the longitudinal direction of the specimens, which is similar to the stress characteristics of one‐way slabs. Moreover, the vertical bearing capacities of PCHS‐LJs are increased when decreasing the number of proposed joints or when adding the longitudinal ribs. For the specimens without the longitudinal ribs, a wide transverse crack appears near the first transverse ribs, whereas the specimens with the longitudinal ribs do not exhibit this phenomenon. Thus, longitudinal ribs should be added to optimize the PCHS‐LJ failure modes. Finally, solid finite element (FE) models of the test specimens were established in MSC. MARC. The analysis results are basically consistent with the experimental results.

Consideration of the Torsional Stiffness in Hollow-Core Slabs’ Design

The paper discusses the principles of precast concrete hollow-core slabs taking into account their spatial work. It is shown that consideration of spatial work makes it possible to determine the forces in individual floor slabs significantly more precise. The fact that strain redistribution between precast floor slabs depends on slabs’ bending and torsional stiffness is shown. The research has been mostly devoted to determination of the bending stiffness with regard to formation of cracks and the change in torsional stiffness, especially considering the presence of normal cracks, which is still unstudied. This paper presents the technique for determining the torsional stiffness of hollow-core slabs with normal cracks. In order to determine the components included in the resolving system of equations, it is proposed to use an approximation method based on the processing of numerical data using spatial finite elements.

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.

Experimental and numerical analysis of the push-out test on shear studs in hollow core slabs

Abstract Prefabricating concrete slabs combined with the speedy erection of steel structures is a good approach to reduce the construction time and optimize construction processes. Composite action between a steel profile and a concrete slab is most commonly established by using headed studs welded to the flange of a steel profile. In this research, an innovative arrangement was proposed for a composite slim floor with the headed stud welded to the steel web inside the hollow core slab. This paper presents the results of an experimental program using push-out tests to determine the shear resistance of the headed studs. In addition, a numerical study of experimental results was conducted to analyse the composite action of headed studs and precast concrete hollow core slabs. The experimental results showed the proposed solution had good behaviour. Theoretical analysis presented safe side predictions of the connector resistance. The finite element model was created, and a satisfactory match with the experimental data was obtained. Parametric FE (Finite Element) analysis was performed by using the variation of the compressive strength of the concrete and yielding strength of the headed studs.

Experimental study on the shear behaviour of precast concrete hollow core slabs with concrete topping

In typical precast construction practice of floor slabs using precast concrete hollow core unit (HCU), in-situ concrete is cast on top of the HCU to obtain smooth and even floor finish. The surface of the HCU is seldom given proper treatment prior to casting the concrete topping. The texture and surface moisture condition of the HCU just before receiving concrete topping may affect the overall strength of the slabs when the concrete topping and the HCU act compositely during service. This paper presents the experimental study on shear-flexure capacity of composite slabs using HCU and concrete topping. Full scale three point load test are carried out on 14 composite slab specimens with different surface roughness and surface condition of the HCU before casting the concrete topping. The surface roughness considered is smooth and rough, while the moisture conditions are dry, ponded and optimum wet. The effect of the longitudinal joint between the HCU panels is also considered. The experimental results are also compared with predicted values using the available equation in Eurocode 2 and an equation published by a previous researcher. The results of the experiment show that the HCU surface condition and longitudinal joint affect the stiffness and shear-flexure strength of the slabs. The optimum HCU surface condition which can produce highest stiffness and shear strength is rough and wet conditions, while the longitudinal joint between HCU panels reduces the slab shear strength. The interfacial horizontal shear is not the factor that governs the strength and behaviour of the slabs. The equation available in Eurocode 2 gives non-conservative prediction of the shear strength. In contrary, the equation published by the previous researcher gives conservative prediction of the shear strength.

Headed steel stud connectors for composite steel beams with precast hollow-core slabs with structural topping

Composite beams of steel and concrete have been studied for a long time. The transfer of longitudinal shear stresses at the interface between the beam and the slab is a key point in obtaining the composite beam behaviour, which is usually achieved by means of shear connectors. In this case, the joint behaviour of the two materials depends on the strength and stiffness of the interface connector. Headed stud connectors for solid concrete slabs are the most common solution to achieve the composite behaviour. However, there is little information on shear connectors associated with precast concrete hollow-core slabs. This study aims to determine, through push-out tests, the shear strength of headed stud connectors associated with precast hollow-core slabs with a structural concrete topping. The analysed hollow-core slabs have two different heights and a minimum structural concrete topping of 40mm. The strength of the in situ concrete infill joints and the rate of transverse reinforcement were varied in the present study. The results were compared to code prescriptions, and a proposition to modify an existing design equation for the ultimate shear capacity of headed studs in composite precast hollow-core slab beams is presented, focusing on the influence of the structural concrete topping.

Effects of cast-in-place concrete topping on flexural response of precast concrete hollow-core slabs

Results of a study focusing on the flexural response of precast prestressed concrete hollow-core slabs with cast-in-place concrete topping are presented. The experimental part of the study included load testing of five precast concrete hollow-core units. The numerically determined flexural response of test specimens was later compared with the experimentally obtained behavior. Results demonstrate that a major composite action is valid between the hollow-core unit and the topping slab under load levels corresponding to uncracked state of the cross section. Existence of a topping slab resulted in improvements in the cracking moment and initial stiffness of hollow-core units. The beneficial effect of topping slab on the ultimate moment capacity was observed to be limited, mainly because of the loss of composite action prior to reaching the ultimate moment capacity. Horizontal shear strength at the interface between hollow-core unit and topping slab was determined (1) through limited number of pushoff load tests and (2) through calculations considering the load level corresponding to initiation of significant relative slip using the basic mechanics of materials approach and the simplified code expression. The measured and computed interface shear strength values were observed to be significantly lower than the horizontal shear strength values specified by the ACI and AASHTO Specifications.

Experimental study on semi-rigid composite joints with steel beams and precast hollowcore slabs

The concept of semi-rigid composite connection has been widely researched in the past; however, most of the researches are limited to composite joints with metal deck flooring and solid concrete slabs. Composite construction incorporating precast concrete hollowcore slabs (HCU) is a recently developed composite floor system for buildings. The research on the structural behaviour of the semi-rigid composite joints with HCU is new and without any previous experimental database. In this paper, eight full-scale tests of beam-to-column semi-rigid composite joints with steel beams and precast hollowcore slabs are reported. The variables are stud spacing, degree of the shear connections, area of the longitudinal reinforcement and slab thickness. The test set-up and instrumentation is described in detail. The experimental behaviour is analysed and based on the test data the structural behaviour of these semi-rigid composite joints is discussed. Based on the experimental data, a simplified method to predict rotation and moment capacity for this type of composite connection is proposed.

Waved Joint for Seismic-Resistant Precast Floor Diaphragms

Floors made of precast concrete hollow-core slabs may be constructed without a structural concrete topping and with transverse tie reinforcement placed only out of the slabs. Longitudinal joints between adjacent precast units are filled with mortar on site. To ensure good diaphragm performance under seismic action, a special joint has been devised, in which the slab sides are profiled as continuous sinusoidal waved shear keys. The research involved experimental investigations and analytical modeling. The features of the joint and the means of production were first worked out. Then, a series of destructive tests were performed, on both short joint samples and full-scale floors, made of extruded prestressed hollow-core slabs without topping, all subjected to large in-plane loading reversals. They showed that, after the adhesion between the grout and precast concrete is overcome, a stable cyclic shear transfer mechanism develops, based on wedge action and friction raised at the wavy slab–mortar interface. Th...

Implementation of the Maturity Method for Zero-Slump Concrete Products

This paper describes the implementation of the maturity method at Prestressed System Incorporated's precast concrete hollow core plant. The objectives of the investigation were: to investigate the methods and materials used to produce precast concrete hollow core slabs; evaluate the practice of making, curing, and testing zero-slump concrete test cylinders; determine the appropriate maturity functions for zero-slump concrete; correlate the maturity method with measured compressive strength to calculate required maturity values that equate to specified hollow core concrete compressive release strengths.

Designing composite steel beams with precast concrete hollow-core slabs

This paper presents a design procedure for steel beams acting compositely with proprietary precast hollow-core slabs. It also contains selected underpinning theoretical analyses that illustrate the rationale behind the design formulae and recommendations. Numerical results for the transfer of horizontal compressive forces through the numerous interfaces and through the load–slip characteristics of the shear studs are presented, i.e. the interfaces between the precast slab and in situ concrete infill, and the in situ infill and steel beam. A method for designing composite beams, including the effective breadth of the precast slab, the degree of shear interaction and prediction of the moment capacity, is given. Load against span graphs for composite beams are provided for use in initial design. A worked example based on the procedures illustrates their application in practice.