The research project with a title: ‘Thin, Energy-efficient Pre-fabricated FRP RC Elements’ is led by the Marcin Haffke, M.Eng. and Jun.-Prof. Dr.-Ing. Matthias Pahn at the University of Kaiserslautern in Germany. At this biggest technical university in the region there is a long history of laboratory experiments and profound understanding of the topic among the academic stuff, which makes that place suitable for the work of further improvements in the field of multi-layered sandwich panels.
As a quick, efficient and resource saving construction method, prefabricated sandwich panels gain popularity for industrial as well as for private applications worldwide. Such members consists usually of two concrete layers and insulation layer in between. Their structure enable quick assemble at the construction site, easy control of heat conductivity properties and wide variety of possibilities for innovative installations. To ensure proper functioning of the whole structure, panels require number of integral components, such as reinforcement of the concrete, connectors of the layers and anchorage systems.
Internal reinforcement of glass fibre reinforced polymers (GFRP) features significant corrosion resistance against conventional steel reinforcement. Use of such in a sandwich panels will enable design of thinner and thereby cheaper structures, due to less concrete volumes needed for the corrosion protection. Such enhancements, however, require answers for many technical questions which are not possible to find from the literature nor experience. Also essential part of the research is the understanding of the properties of GFRP components and of the whole panels, at the time of construction as well as at the end of the service life of the structure. Thus the research project consists of development of a new theoretical approach and series of laboratory tests (Fig.1), so the safe and efficient application of GFRP-reinforced sandwich structures is possible.
Fig.1 - Laboratory flexural tests of sandwich panels
The main objectives of the research project are development of a new sandwich panel system. Multi-layered sandwich panels currently used in the construction practice consist of two concrete steel-reinforced layers and insulation layer in between and are well developed and efficient precast construction. Steel reinforcement, however, causes thermal bridges and thereby increase thermal conductivity of the whole system. Steel also, as a corrosion-prone material, forces to design thicker panels due to higher required concrete covers. Replacement of steel by the GFRP reinforcing bars or grids would enable engineers to design thinner panels. Beside the advantages of possibility to make the layers and the whole thickness of the wall thinner, this cause also a number of research challenges, not yet clarified. Many of them are not so relevant in case of traditional sandwich panels. Problems, which need closer consideration are: limitation of cracks in the facing layer, bond behavior of reinforcing bar in the thin facing layer with lower concrete covers, anchorage of connectors and their behavior in facing layer as well as their long term behavior under changeable shear loading. All above mentioned research needs create vast area of possible research activities. Project should cover the answers for above questions and explain which of them are the most relevant for the whole system, so the construction of new efficient sandwich panels with use of well-known and recognized materials would be possible. Important is also the determination of maximal stresses in reinforcement considering limitation of them due to deterioration of material in time. Further aim is then better recognition in the field of durability properties of GFRP which can affect the design as well as the production process.
In order to achieve the aimed goals of the research project the literature study on the topic of sandwich panels is needed. This should help to recognize potential relevant problems which can occur while designing and constructing of a new sandwich-based buildings. So far collected information on the topic from literature and industry enabled to name the problems mentioned in the previous section.
After answering to questions about particular elements of the system, the concept of calculation should be created. This is being worked out along using Matlab, Excel and other similar software. In course of the research finite element method modelling will be prepared when use of such is proved to be efficient in the project. Pre-calculation or post-calculation of the whole system can be performed during the secondment of the fellow at the University of Minho. For this reason it is reasonable to attend the secondment after the series of sandwich laboratory tests. The aim would be understanding of sandwich load-carrying effect, its influence on the whole member and its limitation. Laboratory large scale tests and FEM modelling should help in establishing of analytical foundation of calculation for new sandwich panels system.
When the particular research problems are recognized, a series of laboratory test will be planned according to the needs. Up to now this happened for the laboratory investigation of bond behavior under static and dynamic load. Test series was conducted by the Early Stage Researchers from University of Kaiserslautern and Politecnico di Milano in March and April 2015.
The motivation for the tests was lacking understanding and insufficient data of the bond behavior of GFRP bars in thin facing layers of sandwich panels. Results (Fig.2) should show what the minimum efficient thickness of covering layer is in terms of safe anchorage. Bond behavior can help to understand the right approach for crack widths limitation. For these reasons series of eccentric pull-out tests were carried out. Comparing to steel, variability of GFRP bond-slippage curves of different specimens of the same parameters is significantly higher. Digital Image Correlation system was used in the test setup to record the displacement of concrete on the side of low concrete cover(Fig.3). From the test however can be seen that not always cracking of concrete cover, even in case of 10 mm, is decisive. It appears that the biggest impact on ultimate bond capacity has the surface finishing of the bars (Table 1).
Fig.2 - Bond-slip curves of GFRP bars 8 mm, concrete compressive strength 31,64 MPa and different concrete covers. High variability is to be seen, which cannot be attributed only to cracking process but to the whole elastic contributions of concrete to the bond stresses dissipation
Fig.3 - DIC results of main displacement of concrete. In the picture map of concrete plasticizing displacement as well as cracks is visible
Table 1 - Example of results from the eccentric pull-out tests
Following conclusions can be drawn from so far carried out tests:
The data reordered were highly variable. This can be attributed to both variability of concrete properties as well as the differences in surface finishing of two different GFRP bar types.
In case of low concrete cover the ultimate bond stresses turned out to be increased by the contribution of less stiff confining concrete. This puts the GFRP bars of small diameters in a good positon for potential use as a reinforcement in thin concrete facing layers. Understanding of bond behavior in such structures is important not only in terms of sufficient bar anchorage but also as essential part of crack developing process. The data obtained from the tests require further evaluation in order to create an efficient guideline for GFRP reinforced thin sandwich layers.
Attendance of meetings of the ENDURE Network and active participation in teaching during FRP Course organized in Ghent, in Belgium January 2016, helped the fellow to get experience in the field and exchange knowledge among other participants. Information gained from the international environment helped to recognize better problematic points of the project and to plan the experimental investigation programme.
After consideration of main problems and weakest points of the system a test series programme was developed. This test series consisted of 8 sandwich beams (Fig.4) The geometry of the specimen, materials and their properties were chosen in such a way to enable testing of the whole system with all of its components and to keep specimen possibly small and handy. The varying parameters were: thickness of the insulation layer, connectors’ pattern and reinforcement ratio of the concrete wythes. Except two reference beams reinforced with steel rebars, all of the specimens were reinforced completely with GFRP reinforcement. The beams were tested in four point bending test up to failure (Fig.5).
Fig.4 - Manufacturing of the sandwich beam specimens
Fig.5 - Four-point bending test of the thin sandwich panel system
The load-carrying behavior was investigated and recorded during the tests in February 2016. As expected many various failure modes occurred. Their relevance for the structure performance and predicted load level at which they occur need to be further analyzed. Shear transfer capacity has appeared to have great influence on the behavior of each component and its reliable description remains one of the major tasks in the project. Tests indicated also main difficulties in production process and enabled to assess the feasibility of precise manufacturing of them in the pre-cast plant conditions.
Big-scale tests already carried out show whether the assumptions based on theoretical investigation prove to have expected relevance for the structure. Based upon the results of the tests performed so far and further literature review in fields of particular open questions will be continued and further tests will be planned if needed. An analysis of the test results will be presented at CICE 2016 in Honk Kong.
Parallel to the tests of the whole sandwich panel system the second test series of pull-out tests of GFRP rebars was conducted with the main focus on the behavior under dynamic loading. This work was performed in cooperation with POLIMI as a part of secondment of ESR Ana Veljkovic. The results obtained from the tests will be presented in journal paper, which will consist of parametric and analytic evaluation of performed pull-out tests.
Another test series on pull-out and push-through tests of connectors in thin concrete wythes is currently under preparation.
The performed tests will deliver required data needed for creating of a check concept for sandwich panel system, so the analytical solution could be easily and practically used while designing of a new structure.
The ongoing research of sandwich wall panels will focus on recognition of particular failure modes of particular components of the system and finding an effective engineering models for verification of their load –carrying capacity. The aim is using models available in literature and after calibration with experimental models, to be able to conduct capacity checks for each expected failure mode.
Analysis of sandwich flexural test series, performed in March 2016, was summarised in a draft paper submitted for CICE 2016 in Hong Kong.
In the flexural tests it has been observed that anchorage of connectors may play important role in mechanical behavior of sandwich panels at the higher loads. Therefore the series of pull-out and push-through tests was prepared and conducted (Figure 6.). Anchorage of connectors in form of vertical bars was tested under uncracked and cracked concrete conditions.
Pull-out tests of sandwich panel connectors embedded in thin concrete wythes
Work in collaboration with ESR Ana Veljkovic on static and fatigue bond of FRP and concrete was carried on and is supposed to be summarised in a scientific journal paper.
During the scientific mission of Prof. Valter Carvelli at the Technische Universitaet Kaiserslautern the fellow participated in preparation and conduction of a fire test series on sandwich test panels. The possibility of taking part in this work was a fruitful experience giving a lot of understanding of the behaviour of sandwich panels in fire situation and general experience with pre-cast wall panels.
The test series on sandwich panels conducted in March 2016 has been evaluated and the failure mechanisms of thin panels have been analyzed. As it was observed the mechanical behavior of each concrete wythe is strongly dependent on the degree of composite action generated by the insulation layer between the two concrete wythes. The results will be presented at CICE 2016 in Hong Kong and later will be used for calibration of the analytical and FEM models. As a complementary tests to the sandwich test series on panels the polystyrene insulation material used was tested in tensile, compressive and shear tests. The material characteristics will later be used to calibrate the analytical model. Although the insulation material has relatively low stiffness comparing to the other components of the sandwich system its contribution is significant in the serviceability load range as it works with its full area. Therefore the concept of implementing of the load-carrying action caused by the insulation into the design plays an important role in the mechanical behavior of sandwich panels.
Fig.7 - Tensile test on sandwich panel insulation material
As a continuation of research on bond of FRP the last part of tests on dynamic bond bahaviour was performed by the fellow together with Ana Veljkovic during her secondment stay at the TU Kaiserslautern in June 2016. The work was presented at the 11th fib International Symposium in Civil Engineering in Tokyo by Ana Veljkovic. The work on a journal paper reporting the test results is in progress. Analytical analysis is to be prepared and the data collected by Digital Image Correlation system are being processed.
Conferences and meetings