A new constructive and technological approach was developed for the efficient production of large- dimensioned, curved freeform formworks, which allow the manufacturing of single and double-curved textile reinforced concrete elements. The approach is based on a flexible, multi-layered formwork system, which consists of glass-fibre reinforced plastic (GFRP). Using the unusual structural behavior caused by anisotropy, these GFRP formwork elements permit a specific adjustment of defined curvature. The system design of the developed GFRP formwork and the concrete-lightweight-elements with stabilized spacer fabric was examined exhaustively. Prototypical curved freeform surfaces with different curvature radii were designed, numerically computed and produced. Furthermore, the fabric’s contour accuracy of the fabric was verified, and its integration was adjusted to loads.
Research in the fields of innovative concrete structures with high potential for lightweight design, and of textile reinforcement for special applications has been object of intensive scientific and application-oriented efforts for a couple of years [
A crucial technological objective of textile reinforced concrete elements is the development of complex solid preform-structures. These are produced by processing flat structures through appropriate cutting [
Currently, the only possible shapes of shell structures are that of domes, hyperbolic paraboloids and conoids. Their production furthermore entails a considerable amount of material and high costs [
Close examinations of the production of double-curved concrete-lightweight-elements based on flexible formwork systems made from GFRP are yet to come. The focus of this research work is on the numerical calculation and experimental verification of flexible, anisotropic GFRP formworks and the production of prototypical double-curved concrete-lightweight-elements with integrated stabilizing spacer fabric.
The unidirectional (UD) reinforced fabric UT-E500 by GURIT Holding AG was used for the production of the GFRP formwork. UT-E500 consists of aluminoborosilicate (“E glass”) and has an area density of 500 g/sqm. For the thermosetting resin matrix, the epoxy resin Epilox® T19-27 by LEUNA-Harze GmbH was used. By means of manual laminating, UD single layers were made from the UD fabric and the epoxy resin. They had a fibre volume content of 30%. Within this UD single layer, the independent parameters: longitudinal (E1) and transverse (E2) moduli of elasticity, Poisson’s ratio (v12), shear modulus (G12) and the coefficients of linear thermal expansion α1 and α2 between +20˚C and +120˚C were determined experimentally.
The determined basic parameters of the unidirectional single layer (
After the production of the GFRP bond, the thermosetting matrix was cured in a heating cabinet over a period of 6 hours at a constant temperature of 120˚C. After curing, the GFRP bond were cooled to room temperature (20˚C). Due to this difference in temperature of −100 K, residual stress caused a small curvature of the anisotropic layer structure. Afterwards, high curvatures were caused by external preloading (up to a material load of R = 0.95 after CUNTZE criterion), utilizing the coupling effects that result from the GFRP bond (
The calculations of the coupling effects caused by anisotropy were conducted analytically with the Classical Laminate Theory (CLT) and the First Order Shear Deformation Theory (FSDT). For the experimental verifica-
E1 GPa | E2 GPa | ν12 - | G12 GPa | α1(20/120) 10−6·K−1 | α2(20/120) 10−6·K−1 |
---|---|---|---|---|---|
23.7 | 6.4 | 0.3 | 1.6 | 7 | 130 |
tion of the anisotropic coupling effects calculated beforehand, selected GFRP bonds segments were produced and tested in the institute’s own structural test bench with the ABD-testing device (
The results were used during the further procedure for the determination of the functional relations of curvatures, process parameters and geometrical parameters. Based on that, the analysis and identification of single and double curved basic forms was conducted. Their combination resulted in a maximum of defined freeform surfaces.
The experimental analysis of the curves was conducted with the aid of the optical forming-analysis-systems ARGUS and ARAMIS by GOM (
For the production of the textile reinforced concrete elements, the textile 3D-fabric “SITgrid” by V. Fraas Solutions in Textile GmbH was used. It is made from alkali-resistant glass (AR glass), with 2400 tex in warp and weft (
The development of the fine grained concrete was focused on the workability of the fresh concrete as well as the
durability and good bonding between concrete matrix and textile reinforcement.
Component | Content in kg/m3 | Mass fraction in % |
---|---|---|
White cement CEM I 52.5 R | 500 | 21.39 |
Amorphous aluminosilicate | 150 | 6.42 |
Dolomite sand 0.5/1.0 (×50 = 0.71 mm) | 1270 | 54.32 |
Dolomite filler (×50 = 70 µm) | 150 | 6.42 |
Water | 240 | 10.27 |
AR-glass fibres (12 mm, integral) | 18 | 0.77 |
superplasticizers | 10 | 0.43 |
Selected representative freeform surfaces, double-curved with different radii of curvature, were produced in the production tests for the system structure GFRP formworks/concrete-lightweight-elements. The adjustment of the curves is carried out via the new flexible GFRP formwork elements. The basic proceeding when conducting the production tests comprised the production of the flexible formwork with reference curvature or preload curvature. After the production of the formwork, the spacer (
The theoretical major curves (for calculation approach cf. [
The highest anisotropy was found with a relative 0˚-layer content of about 50 percent. This caused the largest
curvature around both axes (
Due to external preload forces, the tensile load caused an increase of curvature around the 2-axis. At the same time, curvature around the 1-axis decreased, due to traction.
In contrast to that, there are greater differences between the calculated and experimentally verified major curves around the 2-axis (
Technique and technology from textile manufacturing had an influence on shape through coating with a thermosetting resin system. Mechanical characteristics were adapted to the variety of shapes and drapery. This resulted in an exact contour adapting (
Further production studies aimed on an additional stabilization with the cold-setting epoxy resin Indufloor-IB1240 by using of the capillary effect of the pile threads. However, it was not possible to prove an increase in stiffness of the 3D-fabric, due to the insufficient capillary effect of the spacer threads (
can be recognized on the sample “pile thread without epoxy resin contact”, which had no common quantitative characteristic peaks with the sample “epoxy resin”. As opposed to this, the sample “pile thread with epoxy resin contact” had a high quantitative and qualitative peak alignment with the sample “epoxy resin” (
Of special importance during the production tests was to ensure the evenness of the concrete layer thickness, consistently good surface quality, sufficient stability of the GFRP formwork and to avoid critical cracks both in the concrete and in the formwork system. Also essential were good sheeting qualities (
The aim of this research project was to develop a flexible, multi-layered formwork system made from glass-fibre reinforced plastic, which allows for a specific adjustment of defined curvature states, utilizing the structural behavior influenced by anisotropy. The adjustment of the coupling effects, which are induced by anisotropy, was calculated in advance analytically by means of the extended laminate theory and numerically by means of the Finite Element Method. A good correspondence of the respective results for the representative shell structures was proved. An experimental verification of these intrinsic coupling phenomena has been conducted with specifically produced textile reinforced concrete-lightweight-elements. Based on the yielded results, ideal layer
Characteristic | Fresh concrete | Hardened concrete |
---|---|---|
geometric bulk density | 2.32 g/cm³ | 2.24 g/cm³ |
air content | 2.5 Vol.-% | - |
linear shrinkage | 0.71 mm/m | |
compressive strength | - | 109.3 MPa |
3-point bending tensile strength | - | 14.74 MPa |
CDF-Test | - | M 28 = 1172 g/m² Ru, 28 = 100% |
constructions for the key curvature states and their variation range could be set. Beyond the efficient production of curved concrete-lightweight-elements, GFRP formworks employ excellent concrete qualities on highest classes of face concrete. This contributes to the generation of new forms of architecture and buildings. The intensively conducted numerical, technological and experimental tests show that combinations of concrete and stabilizing spacer fabrics permit the implementation of single and double curved, multi-axially loaded surface structures. Furthermore, the flexible GFRP formwork design allows not only a location-independent implementation of freeform surfaces following the principle “form follows form” but also results in thin-walled and thus extremely light concrete shell structures.
This work was supported by the Priority Program SPP 1542 of the German Research Foundation (DFG). The authors would like to acknowledge with gratitude the foundation’s financial support.