Biaxial Fracture Mechanical Properties of Natural Fibre Concrete

In order to ensure ductile behaviour and to avoid a possible catastrophic collapse of the structure in many concrete construction elements high energy absorption capacity and increased crack resistance capability is requested. Adding different fibres to concrete matrix has been long recognised as a way to enhance the energy absorption capacity of the plane concrete. The addition of fibres to plain concrete matrix substantially enhances its post- cracking response increasing the toughness of the concrete. Concrete is typically reinforced with steel- or synthetic fibres like carbon, glass or aramid. However the wide-spread use of these fibres in concrete is limited by their high material cost, high energy-consuming process by the production and their adverse environmental impact. Natural fibres obtained from different plants like hemp, sisal, jute etc. ensure a new promising, environmentally friendly and cost-effective substitution to expensive steel- and synthetic fibres. Their main advantage is that they are widely available, cheap, biodegradable, recyclable, non-abrasive and there is no concern with health and safety during handling. However, the employment of natural fibres as reinforcement in concrete is rather superficially investigated area. The objective of the proposed research is to experimentally investigate the fracture mechanical properties of natural fibre concrete (NFC) under biaxial (tension-compression) stress state, to evaluate and verify its presumed higher energy absorption capacity and to develop an analytical model for design of it. In the experimental part of the research the uniaxial- (tension) and biaxial (tension-compression) tests will be carried out with the Wedge Splitting Test method. The fracture energy, notch tensile strength, crack resistance, fracture process zone and characteristic length of the material will be determined. The most significant parameters affecting the fracture mechanical properties of the composite, such as the fibres type, length, volumetric ratio and the concrete quality will be varied. A detailed analysis of the influence of each particular parameter as well as their interaction effect will be carried out. The long term performance i.e. durability of natural fibres in concrete matrix will be also comprehensively studied. The influence of the aging process on the fracture mechanical properties of NFC will be tested and evaluated. Additionally, the enhancing effect of the addition of pozzolan, such as fly ash or silica fume, to the concrete matrix on the durability of NFC will be studied. Based on the experimental data, an analytical model describing the behaviour of NFC and explaining the reinforcing mechanism of fibres will be developed. The model will be based on the fracture mechanical concept and it will enable the design of NFC in practice. The research outcomes will provide comprehensive fundamental knowledge about the mechanical and fracture mechanical properties of NFC. It will ensure a breakthrough in this field of material science gapping the existing deficiencies in the scientific knowledge, ensuring finally the possible application and exploitation of this highly attractive, environmentally friendly and sustainable building material.

Concrete is the most widely used man-made material in the world with a yearly consumption estimated at 3 tons per person on the planet. Concrete is a composite material that is strong in compression but weak under tension load. Because of this concrete usually contains reinforcement in form of bars or fibres made from steel, synthetic or glass. However, traditional reinforcement is highly dependent on virgin material resources and is extremely resource- and energy- intensive to manufacture resulting in high pollution and carbon dioxide emission. In this research project an environmentally friendly alternative to traditional fibre reinforced concrete: Natural Fibre Concrete (NFC) has been developed and its physical-, mechanical- and fracture mechanical properties under different load states were determined. The influence of all essential parameters affecting the NFCs properties, such as different types of natural fibres (hemp, flax, seagrass, etc.), the fibres length, volumetric ratio and cementitious matrix composition as well as their interaction was in detail analysed. Additionally, the durability (long-term performance) of the composite under different environmental conditions was studied and the influence of the aging process on the composites properties was tested and evaluated. The degradation of composites properties was delayed by addition of pozzolanic materials (different fly ashes) and by protecting the deterioration of natural fibres in alkaline environment of cementitious matrix by means of fibre impregnation (with linseed oil). The outcomes of the research provided so far unique, comprehensive and fundamental scientific knowledge about the fracture mechanical properties of NFC under biaxial (tension-compression) load. The results ensured a meaningful advancement of the scientific knowledge in field of design of natural fibre cementitious composites advancing the possible application and exploitation of this highly attractive, environmentally friendly and sustainable building material in future.