Niels Wiemer, Alexander Wetzel, Bernhard Middendorf
Department of Structural Materials and Construction Chemistry, Institute of Structural Engineering, University of Kassel, Germany
1 Introduction
The main properties of Ultra-high performance concrete (UHPC) are a high compressive strength and a good durability. These are due to the high packing density resulting from a very low water-binder ratio and the use of fine reactive components. With regard to thermal resistance these properties do not lead to good material performance. This is because of the water pressure which could not evaporate due to the dense microstructure. The UHPC fails even with steel fibre reinforcement brittle and abruptly [1]. Considering a standard concrete, there is no failure due to high temperature as long as its moisture content is less than approx.
2.5 wt % [2]. With a high ratio of capillary pores, the water vapour can evaporate directly out of the structure. In comparison to a standard concrete, the internal vapour pressure of UHPC rises faster.
In [3], a UHPC fine grain mixture M3Q (SPP 1182; [4]) was subjected to thermal stress of up to 500°C. The compressive strength of the not adapted mix design of M3Q increased by approx.
16% at a temperature up to 250°C (good post-treatment method). Above 250°C the UHPC is thermally instable.
Aim of the study
The aim of this research was to develop an UHPC for the use of backing plates with a thickness of 13 to 30 mm and optimized for cyclic, thermal loading. The plates are used in large bakeries and have a length/width of up to 2.85/1.25 m. In order to increase the thermal resistance, the UHPC was optimized with regenerated cellulose (CR) fibres. The CR-fibres shrink under thermal stress and leave thin, elongated cavities like capillary pores. The water vapour pressure created by thermal stress can dissipate and counteract the material failure (Fig. 1).
2 Material and methods
The raw materials used for the material development have been selected in cooperation with a manufacturer of baking plates. In the reference mix design of this plant a high w/b ratio ensures the thermal stability. This is associated with low mechanical properties. Therefore, the packing density was optimized and the w/b ratio was reduced. Furthermore, compared to the standard M3Q formulation, quartz sand was replaced by limestone and basalt sand (Table 1). The low thermal expansion of the basalt sand is supposed to have a positive effect on the heat resistance. The risk of a sudden increase in volume due to the transition from alpha to beta quartz was noticed.
For the investigation of the thermal resistance, the compressive strength according to German standard DIN EN 12390-3 on cubes with a length of 50 mm after a temperature load of 500°C as well as a heating and cooling rate of 10 K/min has been determined. Furthermore, the flexural strength of prisms (40 x 40 x 160 mm3) and plates (30 x 30 mm2) was tested.
Figure 1: Picture of UHPC specimens after thermal treatment (500°C). Failure under thermal stress without CR-fibres (back row).
without CR-fibres
0,2 vol.-% CR-fibres
58
3 Results
The results of the compressive strength after thermal stress on cubes showed that a fibre content of 0.2 vol.-% of the dry mix design is sufficient to ensure thermal resistance at 500°C.
The samples were thermally stable up to a temperature of 300°C without CR fibres. Higher compressive strengths as the reference backing plate were determined by modifying the mixture by changing the fibre content (Fig. 2a). In contrast to the results of M3Q, it is noticeable that higher compressive strengths are achieved after heating up to 500°C. A comparison between the mixtures M3 and M4 shows that an increase of the fibre content leads to a reduction of the compressive strength by approx. 18%
after 500 °C. The flexural strength (Fig. 2b) also showed an increase after 500 °C for all mixtures. Only the flexural strength of the M4 mixture decreased, but a higher proportion of CR fibres leads to a approx. 22% higher flexural strength without thermal treatment. The grey area in Fig. 2b showed the flexural strength of the plates. The reference oven plate and the M5 mixture without basalt sand 1-3mm were compared. In comparison, the reference oven plate had a flexural strength that was approx. 50% lower. After the thermal loading the flexural strength of the M13-mixture decreases because of strong bending deformation.
4 Conclusions and outlook
Based on a UHPC mixture, a baking plate was developed which can resist temperatures up to 500 °C. By adapting it to the factory conditions, an HPC results, which will be further developed in the next step by post-treatment and drying conditions.In addition, the durability of the surface will be tested for chemical resistance.
References
[1] Horvath, J.: Beiträge zum Brandverhalten von Hochleistungsbeton. Dissertation, Technische Universität Wien,2003.
[2] Meyer-Ottens, C.: Zur Frage der Abplatzungen an Betonbauteilen aus Normalbeton bei
Brandbeanspruchungen. Schriftreihe des Instituts für Baustoffe, Massivbau und Brandschutz, Heft 23, Braunschweig, 1972.
[3] Scheffler, B.; Wetzel, A.; Sälzer, P.; Middendorf, B.: Thermische Stabilität von UHPC.
Celluloseregeneratfasern zur Steigerung der Stabilität unter zyklischer thermischer Belastung, Beton- und Stalbetonbau Fachaufsatz/Bericht, 2019.
[4] Schmidt, M.; Fehling, E.; Fröhlich, S.; Thiemicke, J, Eds.: Sustainable Building with Ultra-High Performance Concrete, Kassel:University Press, 2014.
Figure 2: a) Results of the compressive strength (7d) compared to the reference mixture and M3Q with fibres (28d/90°C). b) Results of the flexural strength of prisms (7d) and plates in grey background (14d).
a) b)
Table 1: Compounds of the M2 mixture and their amounts in kg/m³ and wt %.
Session B3: Applications I
59 Figure 1: Glass concrete composite Beams, Objective
Experimental Investigations on glued composite beams of glass and UHPC
Hannes Eichler, Jenny Thiemicke, Roland Vollmar, Ekkehard Fehling
Institute of Structural Engineering, Department of Concrete Structures, University of Kassel, Germany
1 Introduction, motivation and objectives
The growing popularity of glass as an architectural element increases the request to use glass as a load-bearing structural component. While most common structures behave in a ductile manner, glass is charactarised by its very brittle failure mode without any plastic deformation (see Fig. 1). In order to design glass structures safer and more predictible, previous research projects have investigated on glass composite systems. [1], [2], [3]
2 Development and construction of glass-composite bending beams Objective and constructional design
By designing hybrid glass - UHPC beams, the mechanical properties of glass and fibre-reinforced UHPC are combined in order to create an improved load-bearing behaviour.
As shown in Figure 2, the main objectives are to increase the beams’ crack load capacity and to provide a residual load capacity after the glass has already cracked. In addition, it should be possible to further increase the load after the first crack event has occurred. In this way, the load-bearing capacity of the composite beams can be separated from the glass quality by offering a reliable capacity against bending tensile failure.
Test specimens
The composed beams had a length of 2000 mm and a height of 261 mm. The flanges of the beams (45 mm x 50 mm) were made of UHPC with a compression strength of 170 MPa. They were reinforced with 9 mm long steel fibres (2% by vol.) as well as a reinforcing bar with a diameter of 8 mm (fy = 500 MPa). The web was formed by triple laminated glass (1950 mm x 190 mm). In each case, two beams were made of conventional float glass as well as of heat strengthened glass. To assemble all components, the glass was glued into a manufactured H-shaped metal profile which could be successfully anchored into the concrete by developing
“concrete dowels”.
3 Experimental investigations and modelling Test setup and test results
The composite beams were tested in 4-point-bending tests at the University of Kassel. Besides the applied load, the vertical deformation in the load application centrelines as well as horizontal deformations in the area of the maximum bending moment on the flanges and on the glass web
60
were measured. The left side of figure 2 shows the load-deformation-curves of both kinds of beams, the rigth side shows the cracked beams after testing.
Figure 2: Test results of heat strenghened and float glass beams (left); cracked beams after testing (right) Modeling with method of slices
As part of an Excel®-based calculation, the cross section of the composite beam is segmented into several lamellas. This model is updated with the material properties from preceding bonding and material tests. Using this iterative and simple model, the most important parameters (crack load, maximum load, residual load capacity) can be successfully verified without the need for complex FE-models. Figure 3 compares the test results of a float glass beam (left) with the subsequent calculation (right). The crack load is based on the bending stiffness of the composite beam, considering the non-linear failure of the concrete element. The residual load-bearing capacity can be completely attributed to the properties of the lower flange and the lever arm of inner forces-independent of the glass quality.
Figure 3: Float glass beams – comparison of test results (left) and modelling (right) 4 Conclusion and outlook
The results of the experimental investigations are:
· the developed bond construction enables the transfer of high bond forces,
· the composite beams of glass and UHPC possesed a high crack load capacity as well as increasing load bearing capacity after cracking of the glass web,
· the steel fibre reinforced UHPC-flanges allow a ductile behaviour of the composite beam and therefore,
· composite beams of glass and UHPC show a signalised failure.
References
[1] Freytag, B.: Die Glas-Beton-Verbundbauweise, Dissertation, Technische Universität Graz, 2002.
[2] Louter, C.: Fragile yet ductile. Structural aspects of reinforced glass beams, Dissertation, Technische Universität Delft, 2011.
[3] Härth-Großgebauer, K.: Beitrag zum Tragverhalten hybrider Träger aus Glas und Kunststoff, Dissertation, Technische Universität Dresden, p. 53 – 69, 2018.
Float Glass
HS Glass
Session A4: Rheology II
61