Concrete Floor Slabs Research Articles

Simplified method of applying loads to flat slab floor structural model

The article analyses the impact of the live load position on the surface of a reinforced concrete flat slab floor of 32.0 m × 28.8 m. Four variants of a live load position are investigated: located on the entire concrete slab, set in a chessboard pattern, applied by bands and imposed separately in each of the slab panels. Conclusions are drawn upon differences in bending moments, the time of calculation and the size of output files. The problems in the interpretation of results are presented too. A procedure is presented to model the reinforced concrete structures in computational programs. The recommendations of the Eurocodes are presented regarding to load combinations in the Ultimate Limit State (ULS). Convergence analysis of the finite element mesh is carried out to verify the obtained results. The law status on the implementation of the Building Information Modelling (BIM) technology in Poland points out significant time savings in the application of this technology.

The influence of depth of tensile concrete deterioration on the load bearing strength and deflections of corrosion-damaged floor slabs

The paper considers the change of load-bearing strength and deflections of corrosion-damaged precast reinforced concrete floor slabs in the conditions of cyclic freezing-thawing in full-scale climatic conditions. The aim of the study is to determine the influence of the depth of tensile concrete deterioration on the change in load-bearing strength and deflections. Corrosion-damaged precast reinforced concrete floor slabs were selected and experimental investigation was performed. Based on the results of the study, the influence of the depth of deterioration of tensile zone of concrete on the load-bearing strength and deflections of structures was determined. At the same time, as the loads increase, deflections increase and the height of the compressive zone decreases. In the experimental investigation, changes of the load-bearing strength and deflections of precast reinforced concrete slabs were determined, depending on the depth of tensile concrete deterioration. The performed studies allow us to estimate the load-bearing strength and deflection values of corrosion-damaged reinforced concrete slabs depending on the relative value of the concrete deterioration in the tensile zone without performing additional experiments and calculations.

New reinforcement structures of reinforced concrete floor slabs and their technical and economic efficiency

The article deals with the new reinforcement structures of reinforced hollow core slabs, their advantages and disadvantages. Thearticle analyses their technical and economic efficiency. Along with the new construction, the reconstruction and the total renovation of existing buildings are being actively carried out. The given works are produced both for defectselimination of building structures, and for buildings re-profiling, which causes loads increase on the floors. Therefore, issues related to the reinforcement of building structures are getting more and more urgent [1-4]. Here, it is worth paying special attention to the prefabricated reinforced concrete structures. Several types of reinforcement structures for these slabs have been developed and patented [5-15]. Based on these patents, four (4) basic types of reinforcement structures were developed. In the first type, the reinforcement bar is located below the hollowness of the slab. In the second type, the reinforcement bar is located in a slot punched between the bottom of the hollowness and the bed of the slab. In the third type, the reinforcement bar is located under the bed of the slab. In the fourth type, the reinforcement bar is located at a certain distance from the bed of the slab in a monolithic rib, connected with the hollowness of the slab. The technical characteristics of reinforcement were checked and confirmed experimentally on full-scale samples. For the experiment, five series of slabs PC-42.12-6T were manufactured on the basis of the working drawings of the series 1.141-1 at.60, one series for each type of reinforcement and one for testing without reinforcement.

MULTILAYER CONCRETE INDUSTRIAL FLOORING SOLUTIONS ANALYSIS

The subject of the article is the presentation of European experience in the field of construction of industrial concrete floors. For the construction of the concrete slab floor to be prepared subsoil. Land stabilization can include mechanical stabilization, physical stabilization, and chemical stabilization. The most commonly used stabilization is mechanical stabilization. In order to determine the efficiency and effectiveness of the process, the treatment depends on the soil moisture for the soil type, so attention is paid to this material feature. Accordingly, the recipes from the French experience for the concrete stabilization layer are presented. For such prepared substrate the choice of material parameters and properties of concrete mixtures for concrete slabs for the concrete class in the strength of 10 to 35 MPa was presented.

Mechanics of Collapse of WTC Towers Clarified by Recent Column Buckling Tests of Korol and Sivakumaran

The previously formulated model of the gravity-driven collapse of the twin towers of the World Trade Center (WTC) on September 11, 2011 was shown to match all the existing observations, including the video record of the crush-down motion of the top part of tower during the first few seconds, the seismically recorded duration of collapse, the size distribution of particles caused by impact comminution of concrete floor slabs, the loud booms due to near-sonic lateral ejection velocity of air and dust, and precedence of the crush-down collapse mode before the crush-up. Nevertheless, different degrees of ductility, fracturing and end support flexibility of WTC columns could lead to an equally good match of these observations and remained uncertain, due to lack of test data. Recently, Korol and Sivakumaran reported valuable experiments that allow clarifying this uncertainty. They revealed that, under the simplifying assumptions of rigid end supports and unlimited ductility (or no fracturing) of unheated columns, the energy dissipation of the WTC columns would have been at maximum 3.5-times as large as that calculated by the plastic hinge mechanism normally considered for small-deflection buckling. This increase would still allow close match of all the aforementioned observations except for the first two seconds of the video. The proper conclusion from Korol and Sivakumaran’s tests, based on close matching of the video record, is that the fracturing of unheated columns and the flexibility of their end restraints must have significantly reduced the energy dissipation in columns calculated under the assumptions of no fracture and no end restraint flexibility.

Probabilistic analysis of piled earth platform under concrete floor slab

This paper proposes a two-dimensional axisymmetric numerical modelling of an earth platform over a clayey sand improved by stiff vertical piles using a finite-difference continuum approach. The study focuses on the surface settlements and the stress efficacy of the improvement system. Only the soil parameters are considered as random variables and two assumptions (normal distribution or non-normal distribution) are used. The first-order reliability method (FORM) is applied to calculate the reliability index. Response surface methodology (RSM) allows for access to the Hasofer-Lind reliability index and is optimized by the use of a genetic algorithm. The Hasofer-Lind reliability index is calculated for two distinct cases. Firstly, only the geomechanical parameters of the clayey sand are considered as random variables; secondly, the parameters of both the clayey sand and the earth platform are considered. The results show that the reliability of a single soil layer is safer than the reliability of the whole system.

Seismic retrofit of welded steel moment connections with highly composite floor slabs

In the 1994 Northridge earthquake, connection damage initiated from the beam bottom flange was prevalent in welded steel moment frames. The composite action due to the presence of a concrete floor slab was speculated as one of the critical causes of the prevalent bottom flange fracture. Close review of past experimental studies recently conducted by the authors clearly indicated that conventional steel moment connections with highly composite slabs are much more vulnerable to the bottom flange fracture. In this study, three seismic retrofit schemes are presented for welded steel moment connections with highly composite floor slabs typical of existing steel moment frames in Korea. Because top flange modification of existing beams is not feasible due to the presence of a concrete floor slab, beam bottom flange or web modifications by using welded triangular or straight haunch and heavy shear tab were cyclically tested. Test results of this study showed that all the retrofit schemes used are effective in eliminating the detrimental effect caused by high composite action and can ensure excellent connection plastic rotation exceeding 4% rad. Side effects resulting from retrofit as well as design recommendations are also discussed.

Investigation of composite action on seismic performance of weak-axis column-tree moment connections

This paper presents an experimental study on a developed weak-axis column-tree moment connection subjected to horizontal cyclic loading. In order to study possible positive or negative effect of a composite slab on the seismic performance of the proposed connection, six cruciform specimens were designed and constructed in pairs, where one specimen was bare steel joint and the other involved a composite slab. The standard, cover-plate, and material-changed connection details were considered. The test program is described in detail, and then the structural behavior of test specimens is discussed based on the test data. The tests revealed that although designed with 50% degree of shear connection, the concrete slab still caused obvious upward shift of the neutral axis and increase the strain demand on the bottom flange. But it is clear from the comparison of results that the composite joint specimens performed slightly better than the bare steel counterparts in terms of load-displacement hysteretic curves and energy dissipation capacity, as the concrete slab could effectively enhance the stability of the steel beam and delay the deterioration of the strength and stiffness. The cover-plate connection appears to be one of the more promising moment connection schemes which are able to deliver a much higher level of stability and energy dissipation capacity. The maximum magnitude of slab-steel beam interface slip was less than 0.5 mm, indicating that the concrete floor slab and steel beam could work well together under 50% partial shear connection.

Towards net zero energy for older apartment buildings in Brisbane

Abstract: Between 1961 and 1981 the number of Australian apartments more than tripled in response to booming post WW2 immigration, growing local manufacturing industries and the new construction skills of migrants. The Australian Bureau of Statistics records more than 500,000 brick multi-residential dwellings constructed in this period. This cohort is very consistent in construction method, with cavity brickwork external walls, rendered brick internal walls, concrete slab floors, and roofs typically made of an uninsulated low pitch metal sheet. These buildings were built in an age where there were no energy or water efficiency regulations and many remain close to original condition. They are now around 40 years old and are facing the need for replacement of substantial parts of their building envelopes. The uniformity of this building type and the number of dwellings involved make it worth investigating what renovation strategies could be implemented to improve their energy efficiency and indoor comfort conditions. International examples show that substantial energy demand reduction is possible, and net zero energy is an achievable and cost effective target for similar apartment buildings. This paper presents initial results of ongoing research into the current condition and future value of 1960s and 1970s apartment buildings in Brisbane. Modeling tools are employed to assess hygro-thermal behaviour and energy consumption of a typical building, comparing its current and post energy retrofit performance. The feasibility of the most effective retrofit strategies is then evaluated in light of socio economic factors, and conclusions are drawn about the best future for these buildings and their inhabitants.

Timber steel-fibre-reinforced concrete floor slabs subjected to fire

This paper presents a finite element model (FE model) for a numerical analysis of a composite steel-fibre-reinforced concrete (SFRC) slab with unprotected secondary timber beams subjected to fire, taking tensile membrane action into account. The material models were calibrated by material tests and push-out tests in a preliminary study, and then the FE model was validated against the results of a full-scale furnace test. The validated FE model takes into account geometric and material nonlinearity, mechanical damage due to thermal loading, and the nonlinear connection between the SFRC slab and the timber beams. A new material model was created to improve the simulated behaviour of the SFRC, especially in tension. The validated FE model was supplemented by a semi-probabilistic safety concept based on the Eurocode standards, and was applied to define the fire resistance of different variants of timber SFRC flooring subjected to fire with various slab dimensions and design loads. This approach will be used to create a catalogue in which the fire resistance will be defined for specific parameters of the structure, for example dimensions, loads and materials. The research presented in this paper follows up on a previous study, which was presented in 2014.

EXPERIMENTAL STUDY ON H-SHAPED STEEL BEAM WITH CONTINUOUS BRACING SUBJECTED TO END MOMENT

As for H-shaped beams, concrete floor slab and purlins can be considered as bracing if they have enough stiffness and strength. There are many studies about bracing against lateral torsional buckling. However, the effect of lateral bracing cannot be properly considered in the practical design currently. The purpose of this study is to show the effect of the lateral bracing and vertical stiffener experimentally. The test on the lateral buckling of H-shaped steel beam with a steel plate which is a continuous bracing is carried out. Parameters are the position of bracing and presence or absence of vertical stiffener. The steel plate is welded on the compressive flange or the tensile flange. The vertical stiffener may be considered as the bracing for lateral buckling however the effects of them are not clear. The vertical stiffener is set at the position where the lateral deformation is assumed largest. The lateral buckling and local buckling occurs in the test specimens without bracing and with continuous bracing on the tensile flange. The maximum strength of the test specimen with a vertical stiffener is 5% smaller than that of the test specimen without a vertical stiffener. However, the strength degradation of the test specimen with a vertical stiffener is smaller and the load is 90 % more than the maximum load when the rotation angle is 0.07 rad.

COMPARISON OF COOLING AND HEATING REQUIREMENTS BETWEEN BRICK VENEER AND FIBRO CEMENT WALLING SYSTEM

Space cooling and heating in residential sector is significant contributor to energy consumption in Australia. Therefore, it is important to reduce the cooling and heating requirements. The selection of a good walling system helps to save energy by homes. This research compared the thermal efficiency of a modern house (constructed using brick veneer walls with concrete floor slab) with an old house (constructed using fibro cement walls raised timber floor) using the AccuRate simulation tool. A standard house with two living rooms, one kitchen, one laundry and four bedrooms are simulated in a Sydney Suburb in Australia. It was found that modern house showed lower inside temperature variation than the old house all year around. The results also showed that the modern house has a lower energy consumption for space heating and cooling than the old house. The annual energy use for space heating and cooling in both the modern house and old house were 5197 kWh and 15,712 kWh respectively. Moreover, the annual energy costs were found to be $1,403 and $4,242 respectively for modern and old houses. The modern brick veneer house saved about 33 % of energy compared to old old house. When the net present value of the energy cost for f both houses over 50 years is computed, the energy cost of modern house was found to be $25,629 while it of old house is was $77,488 for the old house.

CRITICAL COMPARISON BETWEEN TWO STRENGTHENING TECHNIQUES OF REINFORCED CONCRETE COLUMNS

In last years, in order to better understand the behavior of confined reinforced concrete, a lot of studies have been made. In addition, also a lot of experimental test have been conducted. In particular the attention has been devoted to the columns strengthened with FRP, while other old techniques have been completely forgotten. Herein a comparison between different methodologies for the strengthening of concrete columns is presented. In addition to columns confined with FRP the concrete confined with angles and battens is considered. The comparing between these two methodologies shows that some advantages arise when battens and angles are used. In fact, they can be effectively used, also when the ratio between the two sides of the column section is greater than 2, by means of steel bar passing through the section. In such a case the confining action due to FRP is negligible. In addition, angles can provide strengthening both in tension and in compression, depending on the structural detail adopted at the concrete floor slab.

Strength and Stiffness Analysis by the Finite-Difference Method of a Concrete Floor Slab Reinforced with Composite Rods During a Fire

A strength and stiffness comparative analysis has been made of a concrete slab reinforced with composite-reinforcement rods and a slab reinforced with steel rods. The stress-strain state has been assessed for both versions of reinforcement of the slab. The stress-strain state was determined under the action of only static load and with subsequent application of temperature fields, i.e., under standard-fire conditions. It has been shown that the fire resistance of the slab with a composite reinforcement turns out to be 1.6 higher as far as the bearing capacity is concerned, than the fire resistance of the slab with a steel reinforcement, although the initial deflection due to the action of only static load for the slab reinforced with composite rods exceeds six to seven times the deflection of the slab reinforced with steel rods.

Frequency selection for coda wave interferometry in concrete structures

This study contributes to the establishment of frequency recommendations for use in coda wave interferometry structural health monitoring (SHM) systems for concrete structures. To this end, codas with widely different central frequencies were used to detect boreholes with different diameters in a large concrete floor slab, and to track increasing damage in a small concrete beam subjected to bending loads. SHM results were obtained for damage that can be simulated by drilled holes on the scale of a few mm or microcracks due to bending. These results suggest that signals in the range of 50–150kHz are suitable in large concrete structures where it is necessary to account for the high attenuation of high-frequency signals.

Detecting damage events in concrete using diffuse ultrasound structural health monitoring during strong environmental variations

Diffuse ultrasonic wave measurements used in structural health monitoring applications can detect damage in concrete. However, the accuracy is very susceptible to environmental variations. In this study, a large concrete floor slab was monitored using diffuse wave fields that were generated by continuous-wave transmissions between ultrasonic transducers. The slab was monitored for several weeks while being subjected to changes in environmental conditions. Subsequently, it was damaged using impact hits, resulting in centimeter-scale cracking. The variations caused by the environment masked the effects of the damage in the measurements. To address this issue, the Mahalanobis distance was used to distinguish between the influence of the damage and the influence of the environmental variations. The Mahalanobis model uses amplitude and phase measurements of continuous waves at a set of different frequencies as inputs. A moving window approach was applied to the baseline data set to account for slow trends. This study shows that this technique greatly suppresses most of the variations caused by environmental conditions. All damage events in our data set have been detected.

Monte Carlo simulation of the neutron measurement for the Large Helical Device deuterium experiments

The Large Helical Device (LHD) plans to start deuterium experiments in March 2017. The expected performances of the neutron flux monitor and the vertical neutron camera have been investigated by neutron Monte Carlo simulations using the MCNP-6 Monte-Carlo neutronics code and the cross-section library FENDL-3.0. Three neutron flux monitors using 235U fission chambers are located on the center axis of LHD and at two toroidal locations on the horizontal plane outside the cryostat. The detection efficiency for the neutron emission in the plasma has been evaluated. This is almost insensitive to the neutron source profile and the plasma position. The vertical neutron camera consists of 11 channels of stilbene detectors with a parallel collimator embedded in the concrete floor slab. The response function of the stilbene detector has been evaluated with PHITS code. Based on the response function, the count rate of the stilbene detector is estimated to be ∼6×104 cps for maximum neutron yield shots. Also, the spatial resolution is estimated to be 70mm, which is smaller than the pitch of adjoining channels.

A new route to exposed thermal mass: Sound-absorbing poured concrete

This paper investigates the novel possibility of utilising the structural concrete floor slab as a means of providing additional mid- to low-frequency sound absorption through the addition of cavities on the ceiling side, formed using flexible formwork, acting as Helmholtz resonators. Mid- to low-frequency sound absorption in rooms is typically achieved through the use of perforated plasterboard or a suspended tile-in-grid system. Such an approach can separate the room from the thermal mass of the ceiling above, leading to higher peak temperatures or increased cooling load. Suspended ceilings can also increase the embodied energy of the building and limit the potential for stack effect ventilation by reducing room height. In this work, frequency-dependent absorption coefficients of perforated concrete samples were measured using an impedance tube test. The results were found to agree with a theoretical analysis, suggesting that it would be relatively easy to predict the performance of perforations of other depths and diameters including those targeted at absorbing higher frequencies. Practical application: The measurements indicate that such a slab could be used as a practical replacement for perforated plasterboard or a tile-in-grid system. This would allow the slab to be tuned to the building’s acoustic performance and structural requirements. The approach has the potential to: (1) reduce the embodied and operational energy of buildings; (2) improve thermal comfort and (3) create a better acoustic environment in situations where surface robustness or longevity is important, for example public stairwells, pathways under roads and metro stations.

Flexural capacity of fiber reinforced concrete with a consideration of concrete strength and fiber content

An experimental study was performed to examine the effects of concrete strength and fiber content ratio on the flexural capacity of steel fiber-reinforced concrete. Three fiber volume fractions, 0.25, 0.375, and 0.5%, and three concrete compressive strengths, 25, 35, and 45MPa, were designed for the experiments. The stress and deflection relationship, first peak and post-cracking strength, and energy absorption capacity were evaluated with respect to the variance in the fiber volume fraction and concrete strength. The results showed that the equivalent flexural strength ratio, which is determined from the first peak strength and energy absorption capacity, increased with the increase in the fiber volume fraction but decreased with the increase in the concrete strength. Furthermore, the effects of the concrete strength and fiber content ratio are discussed in a steel fiber-reinforced concrete floor slab. The ultimate flexural capacity also required a consideration of the influence of the content ratio of steel fiber as well as the strength of cement composite matrix.

Design, fabrication and testing of a prototype, thin-vaulted, unreinforced concrete floor

This paper describes the concept, form finding, fabrication and experimental testing of a prototype floor system, derived from principles of shallow arching action, to initiate internal compressive stresses rather than exclusively flexural stresses. This vaulting in a floor system leads to a lightweight structural element, with significant weight savings compared to traditional concrete floor slabs. The form finding process to generate the floor geometry is presented, with a description of the fabrication process, the concrete mix design, material testing and experimental testing. The results from the serviceability and ultimate load testing of the prototype floor are documented in detail. The data showed that the floor unit was both stiff under service load, with maximum vertical deflections less than span/2500, as well as possessing sufficient strength for ultimate loading, carrying 2.5 times the factored design load in a more critical asymmetric loading scenario. A camera setup was used to measure displacements in-line with traditional displacement transducers, to give contour plots of vertical deflections.