Effect of the surface roughness of fibres on the bonding capacity of the interfacial zone between the fibres and cementitious matrix

Abstract

As fibre-reinforced cementitious composites (FRCC) are multi-scale materials, their structural performance depends on the micro-scale properties of the fibre-matrix bond. However, the development and utilisation of FRCC are restricted due to the limited knowledge of the micro-scale phenomena that influence the bond between the fibres and the cementitious matrix and its response to loading.

The focus of this research was the definition of the properties of the fibre surface and the cement paste surrounding it, which affect the formation and performance of the fibre-matrix bond. The examination involved analysing the effect of the surface roughness of steel fibres on the microstructure of the interfacial transition zone (ITZ) and the degradation of the fibre-matrix bond under repeated loading. The micro-scale properties were explored by employing a different approach to applying the existing experimental techniques to the FRCC.

The utilisation of scanning electron microscopy (SEM) and phase-contrast micro-computed tomography (μCT) enabled the identification of the changes in the distributions of calcium hydroxide, calcium silicate hydrate, pores and unhydrated cement grains within the ITZ. The application of phase-contrast μCT allowed access to the three-dimensional microstructure of the cement paste around the fibre. The importance of the surface roughness of steel fibres for the packing of cementitious grains was examined by estimating the average height and wavelength of surface irregularities using an SEM image analysis and directly measuring the parameters using an atomic force microscope and stylus profilometer. The effect of the fibre surface roughness on its wettability was evaluated through contact angle goniometry. The decrease in the mobility of the water along the fibres that was observed with an increase in fibre surface roughness facilitated the reduction in the porosity near these fibres, which was confirmed using SEM. The resulting mechanical response of the bond between the cement paste and fibres with different types of surface roughness was examined under direct tension cycles with gradually increasing amplitudes.

The outcomes of the fibre pull-out tests indicated that the detected micro-scale changes in the properties of the fibres and the cement paste surrounding them influenced the maximum capacity of the fibre-matrix bond and its deterioration. The development of the residual slip was identified from the beginning of loading with the three stages of evolution: deceleration, steady stage and acceleration.

This study points out that the properties of the fibre surface and the cement paste surrounding it clearly affect the performance of the fibre-matrix bond by introducing novel insights about the fibre-matrix interaction that advance the development and modelling of FRCC.