Strength and stiffness are the key parameters characterising the bond performance of fibres in concrete. However, a straightforward procedure for estimating the bond parameters of a synthetic macro-fibre does not exist. This study employs pull-out tests to investigate the bond behaviour of synthetic macro-fibres. Two types of macro-fibres available in the market were investigated. A gripping system was developed to protect the fibres from local damage. The experimental campaign consisted of two stages. At the first stage, 32 concrete specimens were manufactured for performing 96 pull-out tests (three fibre samples were embedded in each cube perpendicular to the top surface and two sides). Two types of macro-fibres with either 10 or 20 mm embedment length were tested. The obtained load-displacement diagrams from pull-out tests demonstrate that the bond performance (characterised by the strength and deformation modulus) of the "top" fibres is almost 20% weaker than fibres positioned to the side surfaces. At the second stage, one type of macro-fibre was chosen for further experimentation of the feasibility of improving the bond performance through the use of colloidal silica or steel micro-fibres. This investigation stage employed an additional 36 concrete specimens. The use of steel micro-fibres was found to be an efficient alternative. The success of this solution requires a suitable proportioning of the concrete.
Keywords: bond; colloidal silica; pull-out test; steel micro-fibres; synthetic macro-fibres.