Over the last decades steel reinforced concrete was the most widely used structural material in construction. Nevertheless, under certain environments, the corrosion of steel reinforcement can lead to considerable deterioration or even collapse of structural elements, requesting expensive repair and strengthening or reconstruction. This deterioration of steel reinforcement has led researchers to focus on alternative reinforcements.
FRP (Fibre Reinforced Polymer) materials offer promising solution given that for many years they are successfully applied in different industries (such as the automobile and sports manufacturing industries) and recently in construction as well. There are structures reinforced with FRP bars that have been in service in aggressive environments, in various parts of the world for more than 15 - 20 years, without any considerable structural degradation.
The widespread adoption of a new type of reinforcement, such as FRP, requires the development of product specification, testing standards and codes of design practice, which is usually a process that can take many years to be completed. One of the fundamental aspects of structural behaviour is the interaction between constituent elements. Bond stress transfer between reinforcement and the surrounding concrete is the basis of the theory and applicability of reinforced concrete, whenever end anchorages are not used.
Development of new codes requires understanding of bond between FRP bars and concrete and the relationship between the bond behaviour and various material parameters. Despite the large amount of studies done aiming to understand the bond of FRP bars, there is still lack of information and debate among researchers about the effect of particular parameters. This is due to the large number of parameters which are affecting the bond behaviour of FRP bars. Owing to various constituent materials, manufacturing processes and surface treatments of FRP reinforcement, both bond performance and failure of bond can be significantly different than that of conventional steel reinforcement.
The aim of this research project is to define these parameters and to study their effects as well as to contribute to the available experimental database.
After an extensive literature review the list of parameters have been prepared and are listed in Chapter 1. Due to the large number of them, it was not possible to address all of them. However, several of parameters have been studied, such as: FRP bar surface, FRP bar diameter, fibre type (modulus of elasticity), elevated and service temperature, accelerated environmental conditions, short fibres added to concrete (FRC) and test type.
FRP bars are manufactured with different surface deformations, such as ribbed, indented, sand coated, helically wrapped or sand coated and helically wrapped in order to enhance the bond strength of FRP bars. Most of these surface types differ significantly from the traditional ribbed surface of steel bars, both in physical and mechanical properties, thus it is expected that the surface type considerable affects the bond behaviour. There are different opinions in literature about the effect of concrete compressive strength, above a certain limit (approximately 30 MPa), on bond behaviour of FRP bars hence further experimental work was considered necessary to improve the knowledge in this area. Similarly, the effect of FRP diameter is not obvious neither. Standards and guidelines (i.e.: CSA S806-12; ACI 440.1R-15) acknowledge the importance of modulus of elasticity on the bond of FRP bars, however there is either no parameter included (ACI 440.1R-15) or a parameter with conservative values (CSA S806-12) to account this effect. The critical temperatures of FRP bars are lower than those of steel, due to softening of the polymer matrix at temperature levels near their glass transition temperature (Tg). The bond mechanism of FRP to concrete relies on the shear transfer through the bar surface, thus greatly depending on the soundness of the bar coating which makes the bond susceptible to damage at elevated temperature. If short fibres are mixed into concrete a better confinement of concrete in compression is provided, hence a more reasonable use of FRP bars in the tension zone can be achieved. It is important to have more information about the influence of short fibres on bond characteristics of embedded FRP reinforcements before widespread application of this novel combination of various construction materials. Finally, different test types are expected to influence to bond behaviour due to the different stress state in the concrete block around the FRP bars.
The experimental results provide the magnitude of the effects of the above presented parameters. Furthermore, the results give valuable information about available parameter calibration and provide directions how could available FRP bars be improved to show better bond behaviour.
Different types of test set-up, FRP bars and failure surfaces