The influence of concrete maturity on the pull-out behaviour of steel fibres at early-ages

Steel Fibre Reinforced Concrete (SFRC) is being used for the production of in-situ and prefabricated structural elements. The optimization of production and installation processes often require an accurate understanding of the early-age development of the mechanical performance of the hardening SFRC. However, there is limited data available describing the development of the fibre-matrix bond during the first weeks of maturing, and its relation to the mechanical properties of the hardened concrete matrix is not well understood. Single fibre pull-out tests were performed for straight and hooked end fibres embedded in concrete at ages between 3 and 14 days, to investigate the development of the bond strength over time. The compressive strength and elastic modulus were measured to correlate the pull-out performance with the mechanical properties of the concrete. The experiments showed an increase of the fibre-matrix bond over time at early ages. The maximum pull-out load for the straight fibres increased during the beginning of the hydration, corresponding to the development of the adhesive bond. Whereas, the mechanical bond of the hooked end fibres developed substantially over the first days of curing but decelerated gradually over the time investigated. This study provides insight of a correlation between pull-out behaviour of single fibres and the mechanical properties of the concrete matrix. The pull-out strength of steel fibres developed with time similarly to the strength and stiffness of the matrix; presenting a rapid development at early ages which decreases after the first week of curing, for the mix-design investigated.