Multi-layered pre-cast concrete sandwich panels constitute an already well-established construction technique for industrial, public as well as residential buildings. Due to their efficient and precise production process at a pre-cast plant and quick and easy assembly at the construction site buildings erected in this technique can be constructed in a time-controlled way with high quality. Sandwich panels have also inherently good thermal and acoustic properties what makes them in many cases an attractive housing system. They consist of two concrete outer layers and an insulation layer placed in between. Mechanical connectors in various forms, usually steel trusses or anchors hold all the layers together.
The project aimed development of sandwich panels with enhanced durability, thermal insulation properties and reduced amount of cement used for their production. These goals could be achieved by replacing the traditional steel reinforcement of the concrete layers by reinforcing bars out of Glass Fibre Reinforced Polymers (GFRP). Since they have better durability and do not corrode like steel, the thickness of the protective concrete surrounding them could be reduced. Thus, the thickness of the whole panels could be reduced while the longer life, with less maintenance works needed, is anticipated. Thinner concrete layers of the sandwich panels means less concrete consumption for building constructed with this system.
In addition, the steel connectors were replaced with Glass Fibre Reinforced Polymer bars which as a material much more thermally insulating than steel, reduced thermal bridging effect normally caused by steel connectors and improved thermal properties of the whole system.
The practical application of GFRP components in the system required however, an extensive theoretical and experimental research. The possible reduction of concrete layers thickness had to be investigated by number of tests on bond of the rebars and concrete, so the tensile forces can be safely transmitted from the rebars to the concrete. Further, the mechanical behavior of thin GFRP-reinforced concrete layers had to be tested as a part of sandwich structure, where bending of a panel induced by wind pressure causes complex state of internal forces in the concrete layers. These forces and their influence on performance of sandwich panels had to be investigated in detail.
In the course of ongoing research the impact of quality of the insulation layer on mechanical behavior of sandwich panels was recognized as a key aspect. The stiffer the insulation material is and the better the quality of its bond to the concrete layers is, the better mechanical behavior of the whole panel is and smaller deformations of the structure under loads are expected. These effects have major influence on mechanical performance and functionality of sandwich panels during their service lifetime.
Therefore, various advanced experimental and computational methods have been used to describe and simulate the panels’ behavior. The results of performed tests were used as an input data and a backup for Finite Element Method nonlinear models and an analytical model for calculation of internal forces in layers depending on the quality and damage state of the insulation layer.
The conducted research delivered along with detailed description of theoretical aspects, many practical recommendations for an efficient and optimized design of sandwich panels. Findings about their mechanical behavior have been adjusted to the methods and models well established in structural design codes in Europe, so the results could be easily adopted by European engineers. Application of GFRP products in pre-fabricated concrete elements can be a successful combination leading to more durable, highly efficient and sustainable construction solutions. The ongoing research on innovative sandwich panels is continued at the Technical University of Kaiserslautern.
The research project ‘Thin and Energy-efficient Pre-fabricated FRP Reinforced Concrete Elements’, was conducted in the frame of Marie Curie Initial Training Network endure at the Technical University of Kaiserslautern in Germany. Marcin Haffke, M.Sc.Eng. an Early Stage Researcher under supervision of Prof. Dr.-Ing. Matthias Pahn from the Department of Civil Engineering, Institute of Concrete Structures and Structural Engineering, as a fellow of the Network was working for three years on development of an innovative pre-fabricated sandwich wall system.
Bending test on a GFRP-reinforced sandwich panel