Concrete Hollow Core Slabs Research Articles

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.

The analysis of deflections and cracking resistance in pre-stressed slabs of off-shuttering formation

The paper presents an example of calculations of an assembled reinforced concrete hollow core slab of off-shuttering formation which has been reinforced with pre-stressed wire in its top and bottom areas. The numerical modelling of statically determinate pre-stressed assembled reinforced concrete hollow core slab exposed to short duration uniform loading is shown as a finite element model in PC “LIRA SAPR 2014 R3”. This article contains information about the finite elements used, fixing conditions of the slab and of the order in which links between concrete and steel reinforcement were modelled. The calculation results are presented in the form of isofields of finite elements motion in this calculation model. Table 1 presents research results as a finite element model calculations according to the methodology given in specification documents.

프리스트레싱을 하지 않은 철근콘크리트 일방향 중공슬래브의 전단강도 평가

국내에서는 프리스트레싱을 하지 않은 RC 중공슬래브 공법이 개발되어 현장에 사용되고 있다. 현행 설계기준에 따라 복부 유효폭(<TEX>$b_w$</TEX>)을 바탕으로 계산된 RC중공슬래브의 전단강도는 지나치게 보수적이고, 중공재 특성의 영향을 합리적으로 고려하지 못한다. 이 연구에서는 1방향 RC중공슬래브의 전단성능을 연구하였다. 휨파괴 또는 전단파괴가 발생하도록 설계된 두께 300 mm, 폭 600 mm의 중공슬래브 실험체 10개를 제작하여 2점 가력 단순보 실험을 수행하였다. 실험결과, RC중공슬래브의 전단강도는 중공률이 클수록 전단강도가 감소하였지만, 복부 유효폭(<TEX>$b_w$</TEX>)을 포함한 다른 요인들은 거의 영향을 미치지 못하였다. KCI 2012, ACI 318-11, Eurocode 2, CSA A23.05 등 현행 설계기준에 따라 예측된 중공슬래브 실험체의 전단강도를 실험강도와 비교하였다. 또한 실험 관찰을 바탕으로 이축평면응력이 작용하는 콘크리트 압축대의 전단-인장 파괴이론을 사용하여 RC중공슬래브 실험체의 전단강도를 평가하였다. In Korea, non-prestressed reinforced concrete hollow-core slab construction method has been developed and used in construction fields. Provisions for the shear strength of hollow-core slabs based on the effective width of web (<TEX>$b_w$</TEX>), which are specified in current design codes, are too conservative and cannot consider accurately the effects of properties of hollow cores. In this study, the shear performance of one-way RC hollow-core slabs was investigated. Two-point loading simple beam tests were performed for ten hollow-core slabs with a thickness of 300 mm and a width of 600 mm, designed to fail in shear or flexure. Results showed that the shear strength of hollow-core slab was degraded as the void ratio increased but hardly affected by other factors including the effective width of web. Shear strengths of the hollow-core slabs estimated by KCI 2012, ACI 318-11, Eurocode 2, and CSA A23.05 were compared with the test strengths. In addition, based on the test observations, the shear resistance of the hollow-core slabs was estimated by using the shear-tension failure theory for the concrete compression zone subjected to biaxial plane stresses.

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.

할로우코어 슬래브의 내화성능에 대한 수치해석 연구

Numerical model on precast prestressed concrete (PC) hollowcore slabs using 11.3 mm diameter 7-wire stand was developed based on finite element analysis. In order to validate the modelling, previous experiment results with respect to prestressed solid concrete slabs were used and compared throughout the course of fire exposure. In addition to, the fire performance of hollowcore slabs with different aggregate types, moisture contents and compressive strength of concrete was investigated. As a result, it can be seen that the type of aggregates and moisture contents used in hollowcore slabs can affect the fire performance as well as temperature developments.

Discussion: Transmission length and shear capacity in prestressed concrete hollow core slabs

Discussion: Transmission length and shear capacity in prestressed concrete hollow core slabs

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 Investigation of the Fire Resistance of Thin Hollow-Core Concrete Slabs with Simple Supports

This paper presents an experimental study on the fire resistance of relatively thin hollow-core concrete slabs with simple supports. A series of fire tests were conducted according to ISO 834 standard on nine hollow-core concrete slab specimens with thickness not exceeding 15 mm. The key parameters investigated are the heating duration, slab thickness and load ratio. Based on the test results, it has been found that the fire-resistance ratings of all the tested specimens fall below the specified requirements in the current building regulations of Thailand. The level of concrete spalling on the fire-exposed surface increases with the heating duration. It has also been found that the fire resistance of thicker hollow-core concrete slabs may be lower, even when smaller values of maximum vertical deflection are observed during the heating period. Furthermore, a higher load ratio leads to a more rapid collapse of the tested specimen. The experimental results suggest the necessity of supplementary means of fire protection for these slabs in practical building construction.

Response of precast prestressed concrete hollowcore slabs under fire conditions

This paper presents results from experimental and numerical studies on the behavior of precast prestressed concrete (PC) hollowcore slabs under fire conditions. Six PC hollowcore slabs were tested under simultaneous application of thermal and structural loads. The test variables included type of aggregate in concrete, load level, fire exposure and restraint conditions at the supports. The hollowcore slabs did not experience any fire induced spalling and the failure of all slabs occurred through temperature on unexposed side of slab reaching limiting temperature criteria. Data generated from fire tests show that aggregate type, fire scenario, load level and end support condition have significant effect on the fire performance of PC hollowcore slabs. Further, results from fire tests indicate that typical hollowcore slabs can sustain service loads for at least two hours under standard and design fire conditions. Data generated from fire tests is utilized to validate a numerical model developed for evaluating fire performance of PC hollowcore slabs. Predicted cross-sectional temperatures and deflections of the slab, generated from the numerical model, compare well with measured data from fire tests indicating that the proposed model is capable of tracing the behavior of PC hollowcore slabs under standard and design fire conditions.

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.

Fire performance assessment of HPLWC hollow core slabs through full-scale furnace testing

High-Performance Light-Weight Concrete (HPLWC) is used for many structural applications when superior strength and low self-weight of the structural components are required. Among these applications there are thin floor structures, like hollow core slabs, that require characteristics of lightness, relatively high resistance and superior durability. Although the fire performance of normal strength concrete hollow core slabs has been extensively studied, the behavior of HPLWC hollow core slabs has not been suitably investigated.The paper reports the results of two full-scale furnace tests on HPLWC hollow core slabs. Each of them involved one panel with an applied load and one without load. The evolution of temperature inside the slabs was measured along with the load bearing capacity under fire conditions. During the first test severe spalling occurred in the loaded slab while during the second one, performed on slabs cured for some months under dry conditions, spalling did not occur. Finite elements simulations were also carried out in order to support the interpretation of the experimental results. Experimental and numerical investigations gave insight into the fire performance of HPLWC hollow core slabs and highlighted the influence of dry curing conditions in reducing the spalling and increasing the fire resistance.

Study on the Calculation Method for Flexural Capacity of Pre-Stressed Concrete Hollow-Core Slabs Strengthened with CFRP

In recent years, the carbon fiber reinforced plastics (CFRP) has been widely used in field of structural reinforcement. To investigate the effect of different amount of CFRP strengthening pre-stressed concrete hollow-core slabs and the capacity calculation methods, test data of six slabs reinforced with CFRP at different amount were compared to obtain a available calculation formula of flexural capacity. The results showed that the bearing capacity and ductility of these specimens are significantly improved strengthened with CFRP; the proposed method of calculation is safe, simple, practical and relatively high accuracy, the calculation results coincide well with experimental results, and it has a certain reference value for the calculation of the followed studies and practical projects.

The Importance of Lightweight Concrete Technology Development for IBS Industry in Malaysia Based on Cost Comparison between Concrete Slab and Hollow Core Slab: A Case Study of LRT Car Park Project

There are several issues related to the implementation of new Industrialised Building System (IBS) technology in Malaysia which some of them are the awareness among industry players, and lack of development on the new technology in the industry. The main constraint that has always been discussed by the IBS players is time and cost. To have more reliable product or technology in terms of cost and time, it is important to craft and develop new idea as well as technology in our own country. The objective of this study was to highlight the cost comparison between cast in-situ concrete slab and hollow core slab (lightweight) which it can be a starting point for future research on the development of new lightweight IBS technology in Malaysia to get the most reliable cost with latest technology by using local resources. Informations on costs and work durations are collected and analysed between two floors using two different construction methods (hollow core slab and conventional concrete slab). Hollow core slab (lightweight IBS) has similar or less cost with shorter construction period (30% shorter) for the project.

Transmission length and shear capacity in prestressed concrete hollow core slabs

The transmission length Lt of bonded tendons in prestressed concrete is crucial to the shear-tension capacity VRd,c, in particular in precast hollow core floor slabs that do not have shear reinforcement. Defining and measuring Lt is difficult and variable, and has steadily increased to almost 100 diameters for indented wire in the present-day Eurocode 2 (EC2). Because de-tensioning of hollow core slabs takes place where the bond has already fully developed, it is not certain whether the code equations reflect their manufacturing process. Full-scale tests were used to measure Lt and relate it to the design length lpt as well as to VRd,c with the aim of proposing a modification to the design equation that links lpt to VRd,c. The ratio of Lt to the predicted length was from 0·99 to 1·21 and the ratio of Lt to the EC2 design value was from 0·51 to 0·67. Furthermore, a 26% reduction in Lt at the ‘cut' versus ‘cast' ends of T-beams adds weight to the argument that the code equations do not reflect manufacturing methods. Ultimate test shear capacities varied greatly due to different geometries, but the test/predicted ratio was 1·09 to 1·17 and the test/design ratio was 1·95 to 2·13, practically doubling the code equation. By comparing the shear loads with Lt, if Lt is divided by 1·5, the predicted and tested shear loads are equal. This paper proposes an equation for prestressed hollow core units manufactured using long-line techniques only. The effect of this on a typical slab design is to increase VRd,c by about 10% and reduce the zone of transmission around holes by about 350 mm, which in some circumstances may benefit the failure criteria at holes.

Modeling the response of precast, prestressed concrete hollow-core slabs exposed to fire

Modeling the response of precast, prestressed concrete hollow-core slabs exposed to fire

Numerical Analysis on Changing Regularity of Transverse Load Distributions with Different Crack Lengthens of Plate-Girder Hinge Joint

The finite element software ABAQUS is used to analyze the influence of hinge joint crack on transverse load distribution of prefabricated concrete hollow core slab bridge, including single-side crack and two-side crack. The relationship between hinge joint crack length and load transverse distribution was built by using the Regression Analysis. Equivalent Area Method was adopted to bring out the critical crack length. The paper introduced a correlative method to illustrate single plate phenomenon and suggested the critical hinge joint crack lengthens for prefabricated concrete hollow slab bridge, which provided a technical basis for the quantitative assessment of single plate phenomenon in the technical inspection practice of bridges. In the end, critical crack lengths of hinge joint were determined based on a criterion of the “Single Plate Load Effect”, that is, the critical crack length of single-side crack is 100% of the total span length of the bridge, while that of the two-side cracks is 67%. This result will lead to further research on the failure mechanics of prefabricated hollow core concrete slab beam bridges and their retrofit techniques.

Evaluation of the shear capacity of precast-prestressed hollow core slabs: numerical and experimental comparisons

Since eighties, 400 and 500 mm thick precast-prestressed concrete hollow core slabs, characterized by increasingly optimized cross-sections with non-circular voids, became very common. However, deeper slabs with long spans, which have to resist high line loads acting close to the supports, are subjected to initial web shear cracking and may fail at loads less than those predicted by traditional codes prescriptions. The shear strength capacity of these members without transverse reinforcement is evaluated through a campaign of detailed nonlinear finite element analyses, matching experimental test data collected from past programs. Constitutive models, based on nonlinear fracture mechanisms, are considered to numerically reproduce the experimental response of single span, simply supported, isolated hollow core units, highlighting web-shear failure mechanism, due to short development length and lack of transverse reinforcement. The adopted diffuse smeared fixed cracking constitutive model allows a reliable prediction of shear stress distributions and crack patterns for these members in their inelastic branch. The presence of a variable inclined strut is clearly evident. Peak shear stress is localized at the bottom side of the cross-section, rather than at the level of the centroid. The experienced brittle web-shear failure mechanism is governed by hollow core shapes with circular or non-circular voids, as evidenced by the evolution of the principal tensile strain distributions. Typically, less inclined, more rounded, diagonal crack, controlled by the smooth web width variation along depth, is opposed to a fairly constant variation of the fracture angle inclination, governed by the abrupt and irregular web width drop.

Early carbonation for hollow-core concrete slab curing and carbon dioxide recycling

Early carbonation curing was developed for hollow-core concrete slab production to partially replace heat curing and recycle carbon dioxide recovered from the cement kiln. The cavities in hollow core slab were utilized as a CO2 gas chamber to facilitate a carbonation curing at a low gas pressure. Concrete performance was evaluated by carbon dioxide uptake, compressive strength, pH values and resistance to air permeation. The microstructure was examined by X-ray diffraction patterns, thermogravimetric analysis and scanning electron microscope. It was found that early carbonation curing could reduce the conventional heat curing duration and produce concrete with higher strength and lower permeability due to the precipitation of calcium carbonates on surface. The pH value of the carbonated concrete at the reinforcement could be kept above the corrosion threshold through a subsequent hydration and the delayed ettringite formation owing to heat curing was totally eliminated by early carbonation. The process offers economical, technical and environmental benefits.

Evaluation of EZStrobe Simulation System as a Tool in Productivity Analysis - A Case Study: Precast Concrete Hollow-Core Slab Installation

This research’s objective was to evaluate the EZStrobe simulation system as a tool in productivity analysis by selecting precast concrete hollow-core slab installation work as a case study. The study started from determination of work elements of precast concrete slab installation work, after that the time data for each work element were collected from construction site to meet 95% confidence interval and 5% limit of error. The Multiple-Activity-Chart then was created, using average time required for each work element, to derive for cycle time of installation work. The alternated tool, EZStrobe simulation system, then was introduced to determine for cycle time by creating a model with stochastic time data. The results, in term of basic cycle time and idle time percentages, from simulation had been compared with deterministic method, Multiple-Activity-Chart model, which validated that EZStrobe simulation system could well represent the construction process. After that, more complicated model was created to see the effectiveness of the system by simulating more realistic site conditions e.g. limited space for hollow-core slab truck in site, number of precast concrete slab per truck. The required results; basic cycle time (min), standard cycle time (min), productivity (pieces/day and square meter/day), and unit cost (Baht/piece); were also formulated in this new model. These outcomes were very useful for construction planning, scheduling and unit cost estimation. Furthermore, the EZStrobe simulation system could give the insight understanding of construction processes for work study and improvement. After all evaluation, the researcher found that this non-commercial EZStrobe simulation system was very worth using for productivity analysis and also the user-friendly Microsoft Visio© interfaces were truly enhanced the system performances, especially in creating the models.

Blast performance of reinforced concrete hollow core slabs in combination with fire: Numerical and experimental assessment

This paper deals with the analysis of the blast bearing capacity of reinforced concrete hollow core slabs when they are subjected first to fire and then to a blast load. The paper first analyzes the blast behavior of a hollow core slab without fire, for two charges, one that leads to elastic dynamic response and the other that causes plastic behavior and severe concrete cracking. The same blast analysis is repeated for slabs that had been subjected to fire. The paper is limited to temperatures up to 450°C at the concrete surface. A discussion of the experimental setup for full scale experiments is presented and the experimental results are compared with simplified numerical models solved with the software LS-DYNA. The paper discusses many difficulties in obtaining a reliable numerical model. The software typically permits to analyze transient phenomena such as an explosion, where material properties change very rapidly (on the level of milliseconds). On the other hand, fire does not change material and structural properties that fast. The solution of this difficulty is offered by the preliminary analysis of the changes of material properties due to fire, which allows determining the proper input for the numerical blast analysis by means of LS-DYNA. The most important conclusion of the analysis is that crack patterns and blast load dynamic responses are indeed altered by fire with temperatures up to 450°C; the eigenfrequency is reduced and the maximum dynamic deformation increases. Yet, within the limitations of the assumptions concerning boundary conditions, the examined slabs keep their blast bearing capacity after blast load scenarios up to 1.5kg C4 with at 1m stand off distance.