Lightweight ceiling structures made of layered high-performance concrete

Project data

Titel | Title Leichte Deckentragwerke aus geschichteten Hochleistungsbetonen | Lightweight ceiling structures made of layered high-performance concrete Förderer | Funding Deutsche Forschungsgemeinschaft (DFG) / SPP 1542 Zeitraum | Period 10.2011 – 10.2014 (period 1) 11.2014 – 09.2018 (period 2) Leiter | Project manager Prof. Dr.-Ing. Dr.-Ing. E.h. Manfred Curbach Bearbeiter:in | Contributor Dipl.-Ing. Kristina Farwig, Dipl.-Ing. Michael Frenzel

Report in the annual report 2018

MUSHROOM-SHAPED COLUMN HEADS MADE OF CONCRETE

What does efficient building mean today? Economic efficiency in the construction process is achieved by constant cross-sectional slabs, simple formwork shapes and fast production processes. If, instead, one considers material efficiency, a constant slab cross-section is not necessarily the most sensible choice for two-axis load-bearing flat slabs. For point-supported ceiling slabs, material is particularly required in the area of the column connection in order to absorb the high support moments and the additional transverse forces. On the other hand, less material is required for load transfer in other ceiling areas.

Within the framework of the DFG Priority Programme 1542 “Concrete light”, different slab-column connections were therefore examined analytically and numerically in ANSYS^© 17 in the final phase of the subproject and a selected form was tested experimentally. The volume of the slab cut-outs as well as the existing degree of reinforcement was kept constant and the achieved loads were compared with each other. In order to reduce the calculation and testing effort, the numerical calculation and the experimental verification were carried out on a circular slab section. The numerical investigation showed that even a slight redistribution of the material on the underside of the slab cut-out allows a significantly higher load-bearing capacity and thus the concrete under pressure and the steel under tension can be better utilised. The material efficiency was investigated in the numerical simulations on the basis of differently shaped haunches with different load application angles and gradients.

In the experimental examination of the selected variant, an average load increase of approx. 190 % could be determined from three identically manufactured test specimens and the numerically recognisable failure pattern could be confirmed. While a typical punching failure was observed with the reference plate, a clearly pronounced bending crack pattern and a shift of the concrete pressure failure from the load introduction towards the support edge was observed with the haunched plate. After validation of the numerically determined loads, a further variant was used to remove concrete volume from the slab core while retaining the selected shape. The concrete mass could be reduced by at least 30 % with a load increase of 135 % compared to the flat slab if the material is arranged according to the force flow.

Report in the year book 2017

FROM THE COLUMN TO THE CEILING SLAB

Lightweight ceiling structures made of layered concretes can be used in areas subjected to high bending moments both to reduce the selfweight of the structure and to efficiently use the materials. This thesis has been confirmed in the first and second stage of a subproject in the SPP 1542. In the upper and lower cover layers normal concrete C20/25 was used, while the core layer contained light-weight concrete to ensure the transfer of the shear forces. Questions of interest are: Is it also possible to use light-weight concrete in the support area of the slab to reduce the dead load even more? Could the material be used more efficiently if the shape of the connection of the column to the ceiling slab is adjusted? These questions will be clarified in the last steps of the project. For this, axially symmetric parts of a simple-supported flat ceiling slab have been analytically and numerically analyzed to determine the punching shear resistance of the slab. The material distribution in the section of the slab, as well as the shape at the support area, were subsequently varied. The different design variations, which show on the one hand the lower weight and on the other hand the higher load capacity through the more efficient use of materials, should be verified experimentally.

The experimental setup allowed a realistic depiction of the stress conditions in the slab. The slab was simply supported at sixteen bearing points, while the force was applied in the centre of the slab. The orthogonally placed reinforcement in the bending tension zone of the slab was designed in a way that a punching shear failure occurs before a ductile bending failure. Beside the flat reference slabs, we have tested layered slabs with an equal volume. For the highly stressed compression zone, a concrete C35/45 with a somewhat higher strength in the cover layers was used. In the load transfer area, the reference concrete C20/25 was used, where other areas supporting low loads were filled with light-weight concrete. In the second series of tests, the shape at the bottom side of the slab was optimized to increase the punching load capacity, keeping the same volume as that of the reference slab. As a result, it was possible to sustain the same load, while significantly reducing the material quantities. First results confirm the analytical assumption of reducing the dead load of layered slabs by approximately thirty percent when the form of the material layers follows the direction of the forces.

Report in the year book 2016

Layer by layer: sandwich slabs

Steel-reinforced concrete slabs can be found in nearly every modern building. In comparison to other load-bearing structures, such as beams or columns, their construction requires a large amount of material, and they are structurally inefficient. To optimize the structural per-formance of concrete slabs, cross-sections of two-way spanned slabs were developed with the goal, that nearly everywhere, concrete and steel reinforcement is used efficiently. Such slabs would have a lower dead weight relative to commonly constructed slabs and would con-tribute to the conservation of natural resources.

The focus of the research in 2016 laid on the testing of 2 m × 2 m × 0.1 m large, three-layered slabs, which were simply supported at the corners to simulate the midfield area of a point-supported ceiling. The goals of the tests were to identify different failure modes, to verify that the chosen materials had been used efficiently, and to show that the construction method has the potential to reduce mass relative to conventionally steel reinforced ceilings. A total of 17 slabs in six different configurations were examined. Variations were made in the ratio of rein-forcement as well as the shape of the cross section. The upper and lower cover layers had a thickness of 20 mm resp. 30 mm and were produced with a concrete of strength class C20/25. For the core layer, industrially manufactured cellular lightweight concretes with dry weights of 0.6 kg/dm³, and 0.9 kg/dm³ and a compressive strength between 3 and
7 N/mm² were used. Steel bars with a diameter of 6 mm were placed in the bottom layer, crosswise and parallel to the edges.

The load was introduced at four single points by steel girders. Both expected failure modes – shear and bending failure – were observed in relation to the amount of reinforcement. By inte-grating calibrated calculation models, slabs could be configured so that both failure modes occurred simultaneously, thereby proving the efficiency of the three-layer design.

Furthermore, the core layer of one series of slabs was horizontally graded by placing two different concretes in accordance with the flow of force. A low strength concrete was placed at the slab’s centre and a higher strength one at the supports. The load-bearing behaviour of these slabs was nearly identical to that of the regular one-layer reference slab, allowing for a 15 % reduction in weight.

Report in the year book 2015

Lightweight ceiling structures

The project “Lightweight ceiling structures made of layered high-performance concrete”, as part of the Priority Programme SPP 1542, has continued in the second funding phase that started in autumn 2014. While the focus in the first funding phase was on one-way spanning, lightweight and layered ceiling slabs, two-way, and particularly point supported flat slabs, are now being investigated.

By casting three layers of various concretes according to their functionality, the self-weight of ceiling slabs, and thus the associated consumption of natural resources, are reduced. The high-strength exterior layers absorb stresses due to bending, and the lower strength, lightweight core layer, the shear forces. In order to show possible savings in material and weight, the load bearing capacity of three point-supported multi-span flat slabs, with a span of 7 m, were numerically calculated. For the analysis, a concrete class C25/30 for the outer layers and a lightweight concrete LC8/9 with a dry bulk density of 0.78 kg/dm³ for the core layer were used. All three structures fulfilled the required serviceability and ultimate limit state checks considering the self-weight, a dead load of 1.5 kN/m², a live load of 5 kN/m² with a height of 30 cm, and an amount of reinforcing steel of 0.51 tonnes per  7 m × 7 m slab. As a reference, the material requirements of a conventional flat solid slab was used as a baseline. In comparison, a haunched solid slab with a thickness of 10 cm at midspan and 40 cm above the supports (and 8 % more reinforcing steel), carried equal loads with 48 % less concrete. The savings in weight and steel are higher (52 % and 8 % respectively) by using a three layered cross-section, with a thickness of 10 cm at midspan and of 48 cm at the support.

The theoretical results are to be confirmed by testing. For this purpose, as a first step, sandwich elements with a size of 2 m × 2 m × 0.1 m are manufactured and will be loaded soon. They will be supported along four edges, representing the midspan region of an equivalent point supported flat slab. In the future, a one-point supported flat and haunched slabs, with a thickness of 20 cm, will be casted and tested to investigate punching failure at the support area of the slab.

Report in the year book 2014

Lightweight and efficient structures

Within the DFG-priority program 1542, we investigate lightweight, efficient load bearing, three-layer slabs with the goal of reducing self-weight and associated consumption of natural resources. This may be achieved when the compressive and tensile forces resulting from flexural bending are carried by a higher strength concrete and reinforcement located at the top layers, while the comparatively low forces at the beam’s core causing shear are taken by lightweight, less sustainable materials.

The focus of this year’s research was to develop and validate a suitable computational model to determine a force-flow-oriented and efficient load bearing member shape. The ceiling slab model, used in this research, was divided into a finite number of sections which allowed a flexible discretization. The required thicknesses of each layer were calculated separately for each section using internal forces, displacements and stresses that were determined by known structural calculation methods. For the shape-finding process, different targets were defined, such as minimizing the member’s weight or volume, optimizing the performance factor for different failure modes or reducing the consumption of natural resources. The implementation of the method was carried out in a conventional spreadsheet program.

Furthermore, full-size layered and shape-optimized elements were calculated, manufactured and tested to determine and to demonstrate the suitability of the computational model, as well as the practical feasibility and the associated weight, resource and resistance gains. The testing of the specimens included three 7 m long, single span beams with cantilevers to simulate an endless continuous beam. One conventional beam with parallel faces made of a regular concrete C20/25 was tested under concentrated load, and two beams with a shape optimized layered cross section made of regular concrete C20/25 and foam lightweight concrete were tested under a concentrated and a uniform load. All three beams had an identical concrete volume and amount of reinforcement per cross section in the middle span. Comparing the results of the optimized test specimens with the results of the reference slab, based on the ratio of self-weight versus ultimate bearing capacity, an approximate 10 % gain under concentrated load and an approximate 100 % gain under uniform load was reached beyond the results obtained in previous evaluations.

Report in the year book 2013

Lightweight Ceiling Structures

​As part of the DFG Priority Programme 1542 lightweight, three-ply sandwich slabs with a high load bearing capacity are examined. It is the aim to use the applied materials as effective and resource-efficient as possible. This can be achieved by deliberately combining materials based on their respective loading. Accordingly, high compressive forces are to be absorbed by higher strength concrete and tensile forces by the (steel) reinforcement. Areas which are subjected to less stress can be fitted with particularly lightweight materials with a low load bearing capacity. Sandwich-structured three-ply components are perfect for this purpose as the more solid top layers absorb the bending and the (lightweight) core layer the shear. The investigated slabs contain no shear reinforcement for an easy handling on the construction site.

While last year’s research was focused on theoretical examinations, this year’s emphasis was on the production and examination of layered slabs to demonstrate the efficiency of their load bearing capacity and their practical applicability. We chose lightweight concrete with expanded clay aggregates for particularly weight optimized components. Additionally, lightweight and very resource-efficient sandwich elements were made of regular and foamed concrete. The slabs were manufactured in cooperation with BCS Natur- und Spezialbaustoffe GmbH Dresden. Due to their long-standing experience, the mixtures developed in the laboratory could be produced under industrial conditions. All in all, we produced 2 × 18 plate strips with a width of 50 cm and a height of 10 cm or 20 cm. Their length varied between 150 and 250 cm. In addition, three reference slabs were made of regular concrete without layering.

All slabs were loaded to the point of failure in 4-point bending tests in Otto-Mohr Laboratory. We observed different failure modes depending on the respective slab geometry. Flexural tensile failure is characterized by the ripping of the reinforcement in the centre of the slab. In the case of shear failure an oblique crack develops at the edge of the slab. Joint failure is characterized by the separation of the layers. So far, the analysis has suggested that the chosen materials were used efficiently and that the layered slabs’ load bearing capacity is sufficient. In comparison to concrete slabs of regular weight, sandwich elements manufactured by lightweight materials can reduce the weight up to 50 %.

Report in the year book 2012

Lightweight Ceiling Structures

This research project is part of the DFG priority program 1542 „Leicht Bauen mit Beton – Grundlagen für das Bauen der Zukunft mit bionischen und mathematischen Entwurfsprinzipien“. Its aim is the optimization of ceiling slabs: the common solid slab shall be replaced by a ceiling slab which is lightweight and resource-efficient. This can be achieved by combining various materials based on their respective loading and suitability. High tensile loads can be absorbed by a reinforcement and high compressive forces e.g. by high strength concrete. Areas with lower tensile and compressive stress are assigned to types of concrete which are less load-bearing but also less heavy. This setup results in a sandwich-structured ceiling construction with optimized force flow. The properties and load carrying behaviour of this type of construction shall be researched in depth in this project. The initial research is focussed on single-span and continuous slabs.

With the aim of demonstrating possible ways to save weight and material, four different single-span slabs were first of all theoretically investigated. Slab 1 is a common solid slab made of C 25/30. Slab 2, which is also solid, will be made of structural lightweight concrete LC 25/28. In addition, two sandwich slabs (no. 3 & 4) – each of which is made of a top layer of LC 25/28 and a core layer of infra-lightweight concrete (ILC) with a low load carrying capacity – were examined. Slabs 3 and 4 differ in their outer shape: with parallel chords and with optimized force flow. The generally applicable predefined boundary conditions were a maximum deflection of 20 mm, a service load of 4.4 kN/m and a maximum failure load of 11.4 kN/m. Furthermore, the slabs were expected to show ductile behaviour at ultimate load level.

The diagram shows the numerically determined relation between bending moment and middle deflection. The slab height h, the amount of reinforcement as and the resulting self-weight of the slab g, which are required to fulfil the boundary conditions, have been added. The sandwich slab with optimized force flow (no. 4) displayed the greatest potential for savings. In this case, the required amount of reinforcement as well as the self-weight only amount to approximately half of the weight of the reference slab which was made of regular concrete. Shortly these theoretical results shall be experimentally confirmed through the application of various materials for the lightweight core layer.